Florida's ground water quality monitoring program: background hydrogeochemistry ( FGS: Special publication 34 )

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FLORIDA GEOLOGICAL SURVEY SPECIAL PUBLICATION NO. 34

FLORIDA'S GROUND WATER QUALITY MONITORING PROGRAM

BACKGROUND HYDROGEOCHEMISTRY

ERRATA -

Page iii, right column, line 17: correct spelling is John Jee.

Page 64, Table 1, line 8 under MAJOR IONS, Sulfate should be indicated as having
been sampled for in the Background, VISA and HRS Networks (B, V,
and H);

Page 64, Table 1, line 2 under ORGANICS AND PESTICIDES, Volatile Organic
Carbon (VOC) should be indicated as having been sampled for in the
VISA Network (V);

Page 77, Table 24, last line: delete footnote ("++ Reported as Nitrate as NO,
mg/L."). All SWFWMD Nitrate results are NO,, mg/L as N.

Page 83, Table 33, asterisk (*) should be deleted from "#Exc" headers on all three
tables. Asterisk (*) should be added to the right of "Sand & Gravel"
under "District" header on all three tables;

Page 276, Figure 44d: Figure caption should read: "Distribution of total nitrate (as
N; mg/L) in the surficial aquifer system, SWFWMD."

Page 281, Figure 45d: Figure caption should read: "Distribution of total nitrate (as
N; mg/L) in the intermediate aquifer system, SWFWMD."

Page 286, Figure 46d: Figure caption should read: "Distribution of total nitrate (as
N; mg/L) in the Floridan aquifer system, SWFWMD."

Page 347: Missing figure caption should read: "Figure 58e. Predominant water
types in the Floridan aquifer system, SFWMD."

FLORIDA GEOLOGICAL SURVEY

STATE OF FLORIDA

DEPARTMENT OF NATURAL RESOURCES
Virginia B. Wetherell, Executive Director

DIVISION OF RESOURCE MANAGEMENT
Jeremy Craft, Director

FLORIDA GEOLOGICAL SURVEY
Walter Schmidt, State Geologist and Chief

DEPARTMENT OF ENVIRONMENTAL REGULATION
Carol M. Browner, Secretary

DIVISION OF WATER FACILITIES
Richard M. Harvey, Director

BUREAU OF DRINKING WATER AND GROUND WATER
RESOURCES
Charles C. Aller, Chief

FLORIDA GEOLOGICAL SURVEY SPECIAL PUBLICATION NO. 34

FLORIDA'S GROUND WATER QUALITY MONITORING PROGRAM

BACKGROUND HYDROGEOCHEMISTRY

;"`' EDITED BY
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ISSN 0085-0640

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SPECIAL PUBLICATION NO. 34

LETTER OF TRANSMITTAL

DEPARTMENT
OF
NATURAL RESOURCES

Florida Geological Survey
Tallahassee
October, 1992

LAWTON CHILES
Governor

Governor Lawton Chiles, Chairman
Florida Department of Natural Resources
Tallahassee, Florida 32301

Dear Governor Chiles:

JIM SMITH
Secretary of State

TOM GALLAGHER
State Treasurer

BETTY CASTOR
Commissioner of Education

BOB BUTTERWORTH
Attorney General

GERALD LEWIS
State Comptroller

BOB CRAWFORD
Commissioner of Agriculture

The Florida Geological Survey, Division of Resource Management, Department of
Natural Resources, is publishing, as its Special Publication 34, Florida's Ground Water
Quality Monitoring Program Background Hydrogeochemistry. This publication is the
second in a series which will present the results of the ground water quality network pro-
gram established by the 1983 Water Quality Assurance Act (Florida Statutes, Chapter
403.063). It is primarily a series of maps which provide the background hydrogeo-
chemical parameters present within the principal aquifer systems of Florida. These
results can be used by state and local governments, planners, and developers for land-
use planning, conservation, and protection of Florida's valuable water resources.

Respectfully yours,

VIRGINIA B. WETHERELL
Executive Director

Walter Schmidt, Ph.D., P.G.
State Geologist and Chief
Florida Geological Survey

FLORIDA GEOLOGICAL SURVEY

ACKNOWLEDGEMENTS

This publication is the result of contributions by a number of individuals and
agencies associated with the Florida's Ground Water Quality Monitoring Program. The
list of contributors below recognizes the many geologists, field technicians, computer
specialists, draftsmen, secretaries and student assistants who aided in this effort.

Much of the work that resulted in the maps found in this volume was performed
by the five water management districts and the county cooperators. These agencies
have primary responsibility for maintaining and sampling the Ground Water Quality
Monitoring Program well networks. Ground-water sampling of these networks has been
conducted since 1984. Personnel of these agencies also contributed by providing inter-
pretation of the hydrogeologic framework within their respective areas. Administration of
the program has been provided by personnel of the Ground Water Quality Monitoring
Section of the Department of Environmental Regulation (DER). Several individuals from
Florida's State University System contributed valuable research, training and technical
advice to the program. The U.S. Geological Survey has provided technical assistance
from the early days of the program. The Florida Geological Survey (FGS) has con-
tributed to the program by providing extensive geotechnical assistance and editing, and
by publication of this report.

The editors of this volume each provided valuable and necessary expertise. Rick
Copeland (DER) provided the leadership and central management necessary for the
development and ultimate completion of this large scale project. Gary Maddox (DER)
served as a central focal point for the interaction between DER, the water management
districts and the FGS and provided computer expertise in the management of the mas-
sive data files generated by this effort. Sam Upchurch (University of South Florida) was
the hydrogeochemical guru advising all the scientists involved in completing this
research effort. Jacqueline Lloyd (FGS) and Tom Scott (FGS) edited the maps and text,
supervised map digitization and correction, and compiled the volume for publication.

The following individuals and agencies contributed time, data and valuable exper-
tise in the development of the Background Network and the preparation of this report:

NORTHWEST FLORIDA WATER MANAGEMENT DISTRICT:

Thomas Pratt (Project Manager)
Jeffry R. Wagner
Jay L. Johnson
Brian E. Caldwell
Ross J. Curry

SUWANNEE RIVER WATER MANAGEMENT DISTRICT:

Nolan Col (Program Administrator)
Ron Ceryak (Project Manager)
Libby Schmidt
Willie Ray Hunter
Ben Barber
Martin Gabriel

ST. JOHNS RIVER WATER MANAGEMENT DISTRICT:

Don Boniol (Project Manager)
Dr. David Toth
George Robinson
Scott Edwards

SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT:

Gregg Jones (Project Manager)
Lee Clark
Eric DeHaven
John Gee
Dave Moore
Tom Rauch

SOUTH FLORIDA WATER MANAGEMENT DISTRICT:

Jeffry W. Herr (Project Manager)
Roberto L. Sanchez
Jonathan E. Shaw
Phillip Fairbank
Steven D. Anderson
Carmen Parada
Alison C. Gray
Milton P. Switanek

ALACHUA COUNTY:

Robin Hallbourg (Project Manager)
Jim Trifilio
John Regan
Libby Schmidt

SPECIAL PUBLICATION NO. 34

UNIVERSITY OF SOUTH FLORIDA:

Dr. Sam B. Upchurch
Jian Chen
Aida Bahtita

FLORIDA STATE UNIVERSITY:

Dr. William C. Burnett
Dr. James B. Cowart
Dr. William C. Parker
Dr. William T. Cooper III

UNIVERSITY OF FLORIDA:

Dr. Robert Lindquist

DEPARTMENT OF ENVIRONMENTAL REGULATION:

Rick Copeland (Program Administrator)
Tim Glover
Gary Maddox
Jackye Bonds
Paul Hansard
Jay Silvanima
Cindy Cosper
Mary Geuin
Cynthia Humphreys
Jeff Spicola
Liang Lin
Donna Burmeister
Peter Grasel
Felix Rizk
David Ouellette

FLORIDA GEOLOGICAL SURVEY:

U.S. GEOLOGICAL SURVEY:

Irv Kantrowitz
Walt Aucott
John Vecchioli
Brian Katz
Marian Berndt

Jacqueline M. Lloyd (Program Manager)
Dr. Thomas M. Scott (Program Manager)
Cindy Collier
Jim Jones
Ted Kiper
Elizabeth Doll
Will Evans
Kent Hartong

FLORIDA GEOLOGICAL SURVEY

TABLE OF CONTENTS

PAGE

ACKNOWLEDGEMENTS .............................................................................

Chapter I INTRODUCTION, by Gary L. Maddox.............................................

History and Purpose.............................................................. ...........
Organization and Establishment of the Ground Water Quality
Monitoring Network ...................................................... ........... .........
Background Network................................................................. .............
Usefulness of Background Network Data....................................................
R eferences C ited ........................................................ .........................

Chapter II DATA COLLECTION AND MANAGEMENT METHODS, by...............
Gary L. Maddox

Well Selection and Sampling....................................................................
S am pling P rotocol ....................................................................................
A nalytical M ethods ...................................................................... ......
Data Base Systems .............................................................................
Availability of Data
A availability of D ata ........................ ............ ............ .............. ... .......... ...
Data Validation Procedures.......................................................................
R eferences C ited ....................................................................................

Chapter III HYDROSTRATIGRAPHY, by Thomas M. Scott.........................

Introduction
Intro d uctio n ....................................................................................
Geologic Structures in Relation to Hydrostratigraphy ...................
Aquifer Systems and Confining Units
Surficial aquifer system.................................. ...............
Northwest Florida Water Management District.........
Suwannee River Water Management District............
St. Johns River Water Management District .............
Southwest Florida Water Management District.........
South Florida Water Management District ...............
Intermediate aquifer system and intermediate confining unit
Northwest Florida Water Management District.........
Suwannee River Water Management District............
St. Johns River Water Management District .............
Southwest Florida Water Management District.........
South Florida Water Management District ................
Floridan aquifer system ................................... ..............
Northwest Florida Water Management District.........
Suwannee River Water Management District............
St. Johns River Water Management District .............
Southwest Florida Water Management District.........
South Florida Water Management District ................
R eferences C ited ..................................................................................

PAGE

Chapter IV QUALITY OF WATER IN FLORIDA'S AQUIFER SYSTEMS, by ......
Sam B. Upchurch

Intro d uctio n ....................................................................................
Scope
S c o p e ....................................................................................
C chapter O organization .....................................................................
Comparison of Map and Table Data ..........................................
Variable Description Conventions........................................ ...............
Nature of Data Distributions ................................... ...............
Distribution Descriptors ......................................... .................
Aquifer Controls on Ground-water Chemistry .........................................
Factors That Control Ground-water Chemistry.............................
Precipitation Chemistry.............................................................
Surface Conditions ...................................................................
Soil Type in Recharge Areas.................................... ................
Soil and Aquifer Mineralogy...................................................
Nature of Aquifer System Porosity and Structure..........................
Intergranular Porosity ................................... ..............
Cavernous, Vuggy, and Fracture Porosity ..........................
Aquifer System Flow Path and Residence Time.............................
Mixing with Other Waters in the Aquifer system..........................
Aquifer M icrobiology.................................................. ....................
Definition of Hydrochemical Facies.................................... .................
Previous Works
Surficial Aquifer System................................................................
Intermediate Aquifer System ................................... ..............
Floridan Aquifer System............................................................
G general Descriptors ................................ ..............................................
Temperature ..........................................................................
Im portance.................................................................. .....
Standard or Guidance Criterion ........................................
Distribution in Ground Water............ .............................
Surficial Aquifer System.........................................
Intermediate Aquifer System...................................
Floridan Aquifer System......... ............. .................
Acid-Base Relationships (pH).......................................................
Importance......................................... ...................
Standard or Guidance Criterion .........................................
Distribution in Ground Water..............................................
Surficial Aquifer System .........................................
Intermediate Aquifer System..................................
Floridan Aquifer System......................... .........

SPECIAL PUBLICATION NO. 34

PAGE PAGE

Cations ....................................................................................
Classification ................................................. ...........................
Major Cations.......................................... ........................
M inor Cations.......................................... ........................
Trace Metals .............................................. .....................
Calcium ....................................................................................
Im portance and Sources .................................... .............
Standard or Guidance Criterion ..........................................
Distribution in Ground W ater..............................................
Surficial Aquifer System ............................................
Intermediate Aquifer System ....................................
Floridan Aquifer System ..........................................
Magnesium ....................................................................................
Im portance and Sources .................................... .............
Standard or Guidance Criterion ..........................................
Distribution in Ground W ater ...............................................
Surficial Aquifer System ............................................
Intermediate Aquifer System ....................................
Floridan Aquifer System ........... ............. .............
Sodium ....................................................................................
Im portance and Sources.................................... .............
Standard or Guidance Criterion ..........................................
Distribution in Ground W ater ...............................................
Surficial Aquifer System ............................................
Intermediate Aquifer System ....................................
Floridan Aquifer System ..........................................
Potassium ....................................................................................
Im portance and Sources .................................... .............
Standard or Guidance Criterion ..........................................
Distribution in Ground W ater..............................................
Surficial Aquifer System ............................................
Intermediate Aquifer System ....................................
Floridan Aquifer System ....... ......................................
Iro n ....................................................................................
Importance and Sources .................................... .............
Standard or Guidance Criterion ..........................................
Distribution in Ground W ater........ ...............................
Surficial Aquifer System ............................................
Intermediate Aquifer System ................. .................
Floridan Aquifer System .............. ........... ............
Mercury ....................................................................................
Im portance and Sources .................................... .............
Standard or Guidance Criterion ..........................................

Lead

Distribution in Ground W ater.................................................
Surficial Aquifer System............................................
Intermediate Aquifer System.....................................
Floridan Aquifer System ..............................................

Importance and Sources .......................................... ..........
Standard or Guidance Criterion ..........................................
Distribution in Ground Water....................................
Surficial Aquifer System............................................
Intermediate Aquifer System.....................................
Floridan Aquifer System............................ ...........

A n io n s ....................................................................................
Classification ................................................. ............................
Major Anions ............................................................... ....
M ino r A nio ns ......... ............................................................. ..
Trace Anions .............................................. .....................
Bicarbonate, Carbonate and Alkalinity ............................... .......
Importance and Controls.......................... ....... ......... .....
Data Interpretation ......................................... ................
Standard or Guidance Criterion ..........................................
Distribution in Ground Water..............................................
Surficial Aquifer System ..........................................
Intermediate Aquifer System....................................
Floridan Aquifer System......... ................. ............
S u lfa te ....................................................................................
Importance and Controls.............. ................ ..............
Sources and Sinks of Sulfur ...............................................
Standard or Guidance Criterion ..........................................
Distribution in Ground Water........ ............... ....... ......... ..
Surficial Aquifer System..........................................
Intermediate Aquifer System....................................
Floridan Aquifer System.............................................
C h lo rid e ....................................................................................
Importance and Controls................................. ..............
Standard or Guidance Criterion ..........................................
Distribution in Ground Water......... ..................... ..........
Surficial Aquifer System............................................
Intermediate Aquifer System....................................
Floridan Aquifer System....................... ............
P ho sp hate ....................................................................................
Importance and Controls........................................... ....
Standard or Guidance Criterion ..........................................
Distribution in Ground Water...............................................
Surficial Aquifer System..........................................
Intermediate Aquifer System....................................
Floridan Aquifer System....... ......................................

FLORIDA GEOLOGICAL SURVEY

PAGE PAGE

F luo rid e .....................................................
Importance and Controls ......... ....................... ............
Standard or Guidance Criterion ...........................................
Distribution in Ground W ater..................................................
Surficial Aquifer System.............................................
Intermediate Aquifer System....................................
Floridan Aquifer System........................ ................
N itrate ............................................
Importance and Controls..................................... ...............
Standard or Guidance Criterion ...........................................
Distribution in Ground Water......................... ...........
Surficial Aquifer System .............................................
Intermediate Aquifer System....................................
Floridan Aquifer System......... ..........................
O their C constituents ..................................................................................
Total Dissolved Solids ............................... ..............................
Im portance............................... ....................... ........... .......
Standard or Guidance Criterion ...........................................
Distribution in Ground W ater.................................................
Surficial Aquifer System.............................................
Intermediate Aquifer System................................
Floridan Aquifer System ......................................
Specific Conductance...................................... ............................
Importance ......... ............. .................... ......
Standard or Guidance Criterion ...........................................
Distribution in Ground Water..........................................
Surficial Aquifer System.............................................
Intermediate Aquifer System....................................
Floridan Aquifer System......... .............................
Total Organic Carbon........................................................................
Im portance...... ......... ................ .. .... ...... ..... ..............
Standard or Guidance Criterion ...........................................
Distribution in Ground Water........ ............................ .....
Surficial Aquifer System ............................................
Intermediate Aquifer System.....................................
Floridan Aquifer System......................................
Synthetic Organics............................................... .............................
Definition and Analytes...............................................
Importance and Controls...................... .....................
Standard or Guidance Criterion ...........................................
Distribution in Ground W ater...............................................
Acrylonitrile ...... ........................ ..... .... ............. ...
Benzene............................................ ... ..............
Bromodichloromethane ...........................................
Bromoform ................................................ ...............

Chlorobenzene ................................................... ......
C hlo reform ..................................................................
Chloromethane......................................................
Dibromochloromethane ...........................................
1,2 Dibromoethane............................. ...............
1,2 Dichlorobenzene ....................................... ........
1,3 Dichlorobenzene ....................................... ........
1,4 Dichlorobenzene ....................................... ........
Dichlorodifluoromethane.................... ...............
1,1 Dichloroethane ......................................... .........
1,2 Dichloroethane ......................................... .........
trans-1,2 Dichloroethene.......... ..............
1,2 Dichloropropane................................ ...............
Ethylbenzene .............................................................
Hexachlorobenzene .............................................
Methylene chloride....................................................
P C B -1 0 16 ...................................................................
1,1,2,2 Tetrachloroethane........................................
1,1,1 Trichloroethane ............................................
Tetrachloroethene .................................... ...........
Toluene ........... .................... ............. .....................
Trichloroethene ................................... .............
Trichlorofluoromethane..............................................
Vinyl Chloride........................................................
Pesticides
Importance...................... ............... ................................
Standard or Guidance Criterion ...........................................
Distribution in Ground Water................................................
A ld rin ................................. ................................
A rsenic ..................................................................
a-BHC ..................................................................
B-BHC ..........................................................
2 ,4-D ......... ........................................... .........
4 ,4 '-D D E ............................................ .........
4,4'-DDT ...............................................................
D field rin ...................................................................
Endrin ....................................................................
Methoxychlor........... ................................................
M ire x .................................................... .......................
Hydrochemical Facies and Predominant Water Types .............................
Introd uctio n ................................ ..............................................
Predominant Water Types..............................................................
Uses of Predominant Water Type and Hydrochemical Facies Maps

SPECIAL PUBLICATION NO. 34

PAGE PAGE

W ater Types in Florida Aquifer System s ........................................
Surficial Aquifer System .................................. ...............
Interm ediate Aquifer System ..............................................
Floridan Aquifer System ................................... ..............
Endnotes .................................................... .............................

Chapter V CONCLUSIONS AND RECOMMENDATIONS, by ...........................
Sam B. Upchurch

Introduction ....................................................................................
Goals
Goals ....................................................................................
Evaluation of Health and Use Risks............................... .............
Data Interpretation and Use ........................................... .....................
Recharge Areas.............................................. ............................
Discharge Areas.............................................................. ............
Flow System s..................................................................................
Surface-W ater Features....................................... ......................
Land Uses
Land Uses .................................. ...................................... ..........
General Summary of the Quality of Florida Ground Water......................
General Quality of Florida's Ground W ater .....................................
Siliciclastic Aquifers .............................................. .....................
Carbonate-Rich Siliciclastic Aquifers ..............................................
Limestone and Dolostone Aquifers .................................... ........
Definition of Background W ater Quality.....................................................
Pristine Water......................................................................................
Background W ater.................................... ...................................
High Salinity W ater............................................ .........................
Coastal Intrusion ................................................ ........................
Connate W ater ................................................ .. ................
Deep-Flow-System W ater ......................................... ...............
Interaquifer Transfer..... ......... ...... ............................................
Nature of Anthropogenic Contam nation ................................... ..........
Point-Source Contam nation ..................................... .............
Non-Point Source Contam nation ................................... ..........
Statewide Levels of Contam ination................................... ....................
p H ....................................................................................
Sodium ....................................................................................
Iro n ....................................................................................
M ercury ....................................................................................
Lead ....................................................................................
Sulfate ....................................................................................
Chloride ....................................................................................
Fluoride ....................................................................................
Nitrate ....................................................................................

Total Dissolved Solids............................................ .....................
Synthetic Organics....................................................................
Pesticides
P estic id es ..... ................... ......................................................
Total Organic Carbon.............................. ..................
Need for Additional W ork..............................................................
Management Implications ...................................................................
Comparison to Background .................................... .............
Sensitivity to Contamination .................................... ..............
Effects of Consumptive Use ................................. .............
Long-Term Resource Evaluation .............................................
Need to Continue the Program and the Future.............................
References Cited Chapters IV and V.................................. ............

FLORIDA GEOLOGICAL SURVEY

LIST OF FIGURES

PAGE

1. Background Network Wells Sampled as of March, 1990 ........................

2. Data Collection and Editing Flowchart................................... .................

3. Hydrostratigraphic nomenclature................................. ...........................

4. Structural features of Florida: a) mid-Cenozoic b) pre-Cenozoic.................

5. Comparison of the sodium to chloride mole ratio of precipitation at ............
the Kennedy Space Center, Brevard County, to the mole ratio of sea
water. Data from the National Atmospheric Depositions Program,
National Trends Network.

6. Distribution of temperature in the surficial aquifer system. Data are............
in degrees Celsius. A. NWFWMD, B. SRWMD, C. SJRWMD, D.
SWFWMD, E. SFWMD.

7. Distribution of temperature in the intermediate aquifer system. Data ..........
are in degrees Celsius. A. NWFWMD, B. SRWMD, C. SJRWMD, D.
SWFWMD, E. SFWMD.

8. Distribution of temperature in the Floridan aquifer system. Data are............
in degrees Celsius. A. NWFWMD, B. SRWMD, C. SJRWMD, D.
SWFWMD, E. SFWMD.

9. Distribution of water pH in the surficial aquifer system. Data are in ..............
standard pH units (s.u.). A. NWFWMD, B. SRWMD, C. SJRWMD, D.
SWFWMD, E. SFWMD.

10. Distribution of water pH in the intermediate aquifer system, Data are .........
in standard pH units (s.u.). A. NWFWMD, B. SRWMD, C. SJRWMD,
D.SWFWMD, E. SFWMD.

11. Distribution of water pH in the Floridan aquifer system. Data are in...........
standard pH units (s.u.). A. NWFWMD, B. SRWMD, C. SJRWMD, D.
SWFWMD, E. SFWMD.

12. Distribution of calcium (Ca2', mg/L) in the surficial aquifer system................
A. NWFWMD, B. SRWMD, C. SJRWMD, D.SWFWMD, E. SFWMD.

13. Distribution of calcium (Ca2', mg/L) in the intermediate aquifer...................
system. A. NWFWMD, B. SRWMD, C. SJRWMD, D.SWFWMD, E.
SFWMD.

14. Distribution of calcium (Ca2', mg/L) in the Floridan aquifer system...............
A. NWFWMD, B. SRWMD, C. SJRWMD, D.SWFWMD, E. SFWMD.

15. Distribution of magnesium (Mg2+, mg/L) in the surficial aquifer....................
system. A. NWFWMD, B. SRWMD, C. SJRWMD, D.SWFWMD, E.
SFWMD.

16. Distribution of magnesium (Mg2', mg/L) in the intermediate aquifer .............
system. A. NWFWMD, B. SRWMD, C. SJRWMD, D.SWFWMD, E.
SFWMD.

17. Distribution of magnesium (Mg2', mg/L) in the Floridan aquifer ...................
system. A. NWFWMD, B. SRWMD, C.SJRWMD, D. SWFWMD, E.
SFWMD.

18. Distribution of sodium (Na', mg/L) in the surficial aquifer system................
A. NWFWMD, B. SRWMD, C.SJRWMD, D. SWFWMD, E. SFWMD.

19. Distribution of sodium (Na+, mg/L) in the intermediate aquifer......................
system. A. NWFWMD, B. SRWMD, C.SJRWMD, D. SWFWMD, E.
SFWMD.

20. Distribution of sodium (Na, mg/L) in the Floridan aquifer system.................
A. NWFWMD, B. SRWMD, C.SJRWMD, D. SWFWMD, E. SFWMD.

21. Distribution of potassium (K+, mg/L) in the surficial aquifer system ..............
A. NWFWMD, B. SRWMD, C.SJRWMD, D. SWFWMD, E. SFWMD.

22. Distribution of potassium (K', mg/L) in the intermediate aquifer .................
system. A. NWFWMD, B. SRWMD, C.SJRWMD, D. SWFWMD, E.
SFWMD.

23. Distribution of potassium (K+, mg/L) in the Floridan aquifer system.............
A. NWFWMD, B. SRWMD, C.SJRWMD, D. SWFWMD, E. SFWMD.

24 Eh-pH diagram showing iron stability fields and water samples from..........
the surficial and Floridan aquifer systems of central Florida at 250C.
Modified from Upchurch etal. (1991).

25. Distribution of total iron (Fe2+ and Fe3 mg/L) in the surficial aquifer.............
system. A. NWFWMD, B. SRWMD, C.SJRWMD, D. SWFWMD, E.
SFWMD.

101

106

111

116

121

126

PAGE

131

136

141

146

151

156

166

171

176

181

182

SPECIAL PUBLICATION NO. 34

PAGE

26. Distribution of total iron (Fe2 and Fe3 mg/L) in the intermediate .................
aquifer system. A. NWFWMD, B. SRWMD, C.SJRWMD, D.
SWFWMD, E.SFWMD.

27. Distribution of total iron (Fe2+ and Fe3, mg/L) in the Floridan aquifer...........
system. A. NWFWMD, B. SRWMD, C.SJRWMD, D. SWFWMD, E. SFWMD.

28. Distribution of bicarbonate (HC03; mg/L) and total alkalinity in the..............
surficial aquifer system. A. NWFWMD, B. SRWMD, C. SJRWMD,
D. SWFWMD, E. SFWMD.

29. Distribution of bicarbonate (HC03; mg/L) and total alkalinity in the...............
intermediate aquifer system. A. NWFWMD, B. SRWMD, C. SJRWMD,
D. SWFWMD, E. SFWMD.

30. Distribution of bicarbonate (HCO ; mg/L) and total alkalinity in the...............
Floridan aquifer system. A. NWFWMD, B. SRWMD, C.SJRWMD, D.
SWFWMD, E. SFWMD.

31. Eh-pH diagram showing sulfur stability fields and water samples.................
from the surficial and Floridan aquifer systems of central Florida at
250C. Modified from Upchurch et al. (1991).

32. Distribution of sulfate (SO42-; mg/L) in the surficial aquifer system.................
A. NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

33. Distribution of sulfate (SO2-; mg/L) in the intermediate aquifer....................
system. A. NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E.
SFWMD.

34. Distribution of sulfate (SO4,2; mg/L) in the Floridan aquifer system ...............
A. NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

35. Distribution of chloride (Cl-; mg/L) in the surficial aquifer system. A.............
NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

36. Distribution of chloride (Cl; mg/L) in the intermediate aquifer system..........
A. NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

37. Distribution of chloride (Cl; mg/L) in the Floridan aquifer system. A............
NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

38. Distribution of phosphate (PO43; mg/L) in the surficial aquifer.....................
system.A. NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E.
SFWMD.

187

192

197

202

207

212

39. Distribution of phosphate (P043'; mg/L) in the intermediate aquifer ..............
system. .A. NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E.
SFWMD.

40. Distribution of phosphate (P043-; mg/L) in the Floridan aquifer ....................
system. .A. NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E.
SFWMD.

41. Distribution of fluoride (F; mg/L) in the surficial aquifer system. .A...............
NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

42. Distribution of fluoride (F; mg/L) in the intermediate aquifer system.............
A. NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

43. Distribution of fluoride (F; mg/L) in the Floridan aquifer system. .A...............
NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

44. Distribution of nitrate (NO,; mg/L) in the surficial aquifer system. A..............
NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

45. Distribution of nitrate (NO3; mg/L) in the intermediate aquifer system...........
A. NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

46. Distribution of nitrate (NO3; mg/L) in theFloridan aquifer system...................
A. NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

47. Distribution of total dissolved solids (TDS; mg/L) in the surficial ...................
aquifer system. A. NWFWMD, B. SRWMD, C. SJRWMD, D.
SWFWMD, E. SFWMD.

48. Distribution of total dissolved solids (TDS; mg/L) in the intermediate............
aquifer system. A. NWFWMD, B. SRWMD, C. SJRWMD, D.
SWFWMD, E. SFWMD.

49. Distribution of total dissolved solids (TDS; mg/L) in the Floridan...................
aquifer system. A. NWFWMD, B. SRWMD, C. SJRWMD, D.
SWFWMD, E. SFWMD.

50. Distribution of specific conductance (imhos/cm) in the surficial...................
aquifer system. A. NWFWMD, B. SRWMD, C. SJRWMD, D.
SWFWMD, E. SFWMD.

51. Distribution of specific conductance (gmhos/cm) in the................................
intermediate aquifer system. A. NWFWMD, B. SRWMD, C
SJRWMD, D. SWFWMD, E. SFWMD.

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228

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FLORIDA GEOLOGICAL SURVEY

52. Distribution of specific conductance (pmhos/cm) in the Floridan..................
aquifer system. A. NWFWMD, B. SRWMD, C. SJRWMD, D.
SWFWMD, E. SFWMD.

53. Distribution of total organic carbon (TOC, mg/L) in the surficial...................
aquifer system. A. NWFWMD, B. SRWMD, C. SJRWMD, D.
SWFWMD, E. SFWMD.

54. Distribution of total organic carbon (TOC, mg/L) in the intermediate ...........
aquifer system. A. NWFWMD, B. SRWMD, C. SJRWMD, D.
SWFWMD, E. SFWMD.

55. Distribution of total organic carbon (TOC, mg/L) in the Floridan....................
aquifer system. A. NWFWMD, B. SRWMD, C. SJRWMD, D.
SWFWMD, E. SFWMD.

56. Predominant water types in the surficial aquifer system. A ...........................
NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

57. Predominant water types in the intermediate aquifer system. A....................
NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

58. Predominant water types in the Floridan aquifer system. A...........................
NWFWMD, B. SRWMD, C. SJRWMD, D. SWFWMD, E. SFWMD.

LIST OF TABLES

1. Ground Water Quality Network Monitoring Parameters...............................

2. Florida Primary and Secondary Drinking Water Standards for ......................
Selected Parameters

3. Summary of the composition of precipitation from selected sites ...............
in Florida.

4. Common minerals in Florida aquifer systems and confining beds ................
and their dissolved weathering products.

5. Common minerals in Florida aquifer systems.....................................

6. Summary of temperature distribution (oC), by region and aquifer..................
system.

7. Summary of water pH distribution, by region and aquifer system.................

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318

323

328

333

338

343

8. Concentrations of selected constituents in average sea water, ....................
ranked by abundance.

9. Classification of water hardness. ............................................................

10. Summary of total calcium distribution (Ca2+, mg/L), by region and ..............
aquifer system.

11. Summary of total magnesium distribution (Mg2 mg/L), by region ..............
and aquifer system.

12. Summary of total sodium distribution (Na, mg/L), by region and ...............
aquifer system.

13. Summary of total potassium distribution (K+, mg/L), by region and................
aquifer system.

14. Summary of total iron distribution (Fe2+, Fe3+, mg/L), by region......................
and aquifer system.

15. Summary of total mercury distribution (Hg2+, mg/L), by region......................
and aquifer system.

16. Summary of total lead distribution (Pb2', mg/L), by region and......................
aquifer system.

17. Summary of total bicarbonate distribution (HCO;, mg/L), by ........................
region and aquifer system.

18. Summary of total carbonate distribution (CO32, mg/L), by region..................
and aquifer system.

19. Summary of total bicarbonate alkalinity distribution (mg/L), by region..........
and aquifer system.

20. Summary of total sulfate distribution (SO42-, mg/L), by region and.................
aquifer system.

21. Summary of total chloride distribution (Cl-, mg/L), by region and..................
aquifer system.

22. Summary of total ortho-phosphate distribution (PO43, mg/L), by region......
and aquifer system.

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71

71

72

72

73

73

74

74

75

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SPECIAL PUBLICATION NO. 34

23. Summary of total fluoride distribution (F, mg/L), by region and ...................
aquifer system.

24. Summary of total nitrate distribution (NO3, mg/L), by region and.................
aquifer system.

25. Summary of total dissolved solids distribution (TDS, mg/L), by ...................
region and aquifer system.

26. Summary of specific conductance distribution (imhos/cm), by....................
region and aquifer system.

27. Summary of total organic carbon distribution (TOC, mg/L), by .....................
region and aquifer system.

28. List of synthetic organic analyzed in the Background Network ..................
with guidance concentrations or standards.

29. Summary of total synthetic organic compound concentrations (pg/L), by....
region and aquifer system.

30. Classification of anthropogenic organic according to volatility...................
in water.

31. Classification of synthetic organic mobility in water..................................

32. List of pesticides analyzed in the Background Network, with......................
guidance concentrations or standards.

33. Summary of total pesticide concentrations (gg/L), by region and .................
aquifer system.

34. Some arsenic-based pesticides and their uses..............................................

35. Proportions of major ions within the trilinear-diagram fields on...................
the Predominant Water Type Maps.

36. Some possible criteria for identification of aquifer system.............................
flow system components.

37. Percent of samples that exceeded water quality standards in ....................
Florida aquifer systems.

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PAGE

APPENDICES

APPENDIX 1 Additional Sources of Information..............................................

APPENDIX 2 Ground Water Quality Monitoring Program references:................
List of related reports and publications

APPENDIX 3 Geomorphic features maps and maps showing major .................
rivers.

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FLORIDA GEOLOGICAL SURVEY

Chapter I

INTRODUCTION

Gary L. Maddox

Florida Department of Environmental Regulation
Tallahassee, Florida

Over the past several decades, Florida has
experienced phenomenal population growth, with
approximately 300,000 new residents annually
joining the 13 million who, in 1992, already call the
"Sunshine State" their home. This trend will likely
continue into the foreseeable future. The rapid
influx of people, in addition to exerting acute
pressures on existing social services and the
infrastructural framework of many communities,
has stressed the water resources of the state in
two ways: a sharp increase in the demand for
potable water supplies, and a corresponding
increase in the volume of waste products
generated.

Florida is blessed with the most abundant fresh
ground-water resources of any state (McGuinness,
1963). Plentiful potable water is perhaps Florida's
most important and vulnerable natural resource.
As of 1980, approximately 87% of Florida's public
drinking water supply came from underground
sources (Fernald and Patton, 1984). The remaining
13% came from surface sources, such as rivers
and lakes. In order to achieve potability, these
surface sources generally require more extensive
treatment than most ground-water sources. This
is due in large part to pollutants introduced into
surface waters by human activities. Without
abundant ground water, there would not be
enough clean fresh water to supply the current
population, especially in the high growth areas of
southern and central Florida.

Since all fresh ground water ultimately has a
surface source, any pollution contained in rainfall,
river or lake water can eventually turn up in our
underground drinking water supplies. In Florida
these two sources, ground and surface water, are
intimately connected: most lakes and rivers in the
state are fed at least partially from ground-water
discharge through springs and seeps, and surface
water bodies recharge aquifers. Changes in land
use activity can supply potential contaminants:
rainfall percolating into the subsurface can carry
with it pesticides and herbicides from agricultural
areas, metals and synthetic organic compounds
from urban stormwater runoff, and hydrocarbons

from leaking fuel storage tanks. Past waste
disposal practices, coupled with the increasingly
high volumes of waste generated, have resulted in
movement of significant quantities of pollutants
into portions of the state's aquifer systems. In
many areas, Florida's ground-water resources are
not well protected from surface infiltration of
potential contaminants. Most of the state's
ground-water supplies are derived from shallow
aquifers, which begin at the top of the water table
and extend downward. Often there is no protective
overlying aquitard or aquiclude to attenuate the
downward migration of potential contaminants.
Where present, these protective low permeability
formations are often locally breached by karst
features, such as sinkholes and solution pipes,
which allow surficial waters to rapidly infiltrate
downward, carrying with them any pollutants
picked up along the way. This provides little time
for natural chemical and biological processes to
break down potential contaminants before they
reach the water table and enter aquifer systems.

Unlike visible contamination in surface waters,
the effects of contaminant transport in under-
ground aquifers are not easily observed.
Delineation of subsurface contamination areas can
involve the use of expensive technology, such as
geophysical detection methods and the installation
of monitoring wells. Cleanup of a contaminated
site can easily cost millions of dollars. Even so,
once contaminated, it is virtually impossible to
remove all pollutants from the subsurface
environment using current technology, making
restoration of an aquifer to completely natural
conditions unlikely. It is much easier (and cost-
effective) to prevent, rather than clean up, ground-
water contamination.

Floridians use more water per capital than any
other state. As a result of this use, coupled with
increasing population growth and development,
the state's ground-water supplies are threatened
by excessive overdraft and contamination.
Increased demand means increased drawdown in
the aquifer systems, and this can also cause water
quality degradation. When withdrawal exceeds
recharge, the aquifer systems are essentially being
mined for water. Excessive withdrawal of fresh
water within some areas causes the upwelling of
underlying denser connate water, or lateral
intrusion of seawater. This is particularly
problematic in high volume withdrawal areas along
the coast, such as in the vicinity of urban wellfields.
In these and other susceptible areas, ground-water
withdrawals must be carefully managed in order to
preserve water quality; this is one reason why

Florida's water management districts require
permits for the installation of certain types of wells.
In many areas, increased demand for potable
ground water has resulted in water shortages and
subsequent restrictions on water use.

In some areas of Florida, the amount of
meteoric water entering the aquifer systems greatly
exceeds the amount locally discharged; these
areas are referred to as recharge zones. These
areas are particularly sensitive to land uses which
contribute contaminants to soil or surface waters,
or restrict downward percolation of meteoric
waters. Protecting these areas from large-scale
human development preserves the quality and
amount of water entering the aquifer, and thus the
ground-water supply. Currently, Florida counties
and municipalities are required to address the
issue of protecting areas of high recharge through
the state's Growth Management Act, particularly
within the Natural Ground Water Recharge element
of local Comprehensive Plans. While forcing some
difficult decisions regarding growth management
and land use, everyone ultimately benefits from
the continued availability of abundant, safe
drinking water.

HISTORY AND PURPOSE

Realizing the need to thoroughly study the
effects of man's activities on our aquifer systems
and to protect and more wisely manage our water
resources, the Florida Legislature, in 1983, passed
the Water Quality Assurance Act. This legislation
required the Department of Environmental
Regulation to "establish a ground water quality
monitoring network designed to detect or predict
contamination of the state's ground water
resources" (Florida Statutes, Section 403.063). To
facilitate this effort, the Act required the
Department to work cooperatively with other
federal and state agencies, including Florida's five
water management districts, in the establishment
of the network.

The three basic goals of the statewide Ground
Water Quality Monitoring Program are:

1) To establish the background and
baseline ground-water quality of major
aquifer systems in the state;

2) To detect and predict changes in
ground-water quality resulting from the
effects of various land uses and

potential sources of contamination;

3) To disseminate water quality data
generated by the network to local
governments and to the public.

The purpose of this report is to present the
results of the initial quantification of background
water quality in each of the state's major potable
aquifer systems. Results are presented and
interpreted in light of the influencing factors which
locally and regionally affect ambient ground-water
quality. This initial data will serve as a baseline
from which future sampling results can be
compared. Future sampling of the Network will
indicate the extent to which Florida's regional
ground-water resources are improving or declining
in quality.

ORGANIZATION AND
ESTABLISHMENT OF THE GROUND
WATER QUALITY MONITORING
NETWORK

The Florida Department of Environmental
Regulation (DER) is the lead agency in
establishment of the Ground Water Quality
Monitoring Network, determining goals and
strategies, setting priorities and coordinating the
overall effort. The Department works closely with
the five water management districts (WMD's)
(WMD boundary lines are shown in Appendix 3,
figure 1), and several counties, which carry out
most of the necessary field work and provide local
technical expertise. The Florida Geological Survey
(FGS) and the Water Resources Division of the
U.S. Geological Survey (USGS) provide additional
technical support, as have several studies funded
through the State University System. The Ground
Water Quality Monitoring Network is actually made
up of three principal elements: two major sub-
networks and one survey, each of which has
unique monitoring priorities and goals. These are:

Background Network, designed to help
define background water quality
through a network of over 1600 wells
that tap all major potable aquifers within
the state;

SPECIAL PUBLICATION NO. 34

VISA (Very Intense Study Area)
Network, designed to monitor the
effects of various land uses on ground-
water quality within specific aquifers in
selected areas. The VISA Network
became operational in 1990, and
results will be published in a
subsequent volume;

Private Well Survey, designed to
analyze, on a one-time basis, ground-
water quality from 50 private drinking
water wells in each of Florida's
counties. This data will supplement the
Background Network by providing over
3000 additional sampling points, while
indicating the general quality of water
consumed by private well owners. This
survey is a joint effort between the
Florida Department of Health and
Rehabilitative Services (HRS) and
the DER. This long-term project
began in 1986 and is ongoing.

This publication is a compilation of water quality
data generated by the Background Network of
Florida's Ground Water Quality Monitoring
Program. The data used in this report were
generated between 1984 and 1988. Future data
generated by both the Background and VISA
Networks will be compared to information
contained in this report in an effort to quantify
changes in ground-water quality over time.

BACKGROUND NETWORK

Before changes in ground-water quality can be
detected, a baseline from which to compare future
changes must be determined. Baseline refers to
current regional ground-water quality, determined
from statewide sampling from 1984-1988. This may
or may not be synonymous with the pristine ground-
water quality that existed before measurable human
impact to the aquifer. A well in the Background
Network is designed to monitor an area of the aquifer
which is representative of the general ground-water
quality of the region (for the purposes of this
program, a region generally incorporates an area
greater than or equal to the size of a county, and is
defined by aquifer extent and, if possible, ground-
water basin boundaries). It is not intended to indicate
changes in aquifer chemistry associated with specific
contamination sources; however, widespread
changes in water quality associated with regional
land use patterns (the accumulated effects of many

sources) may be present.

USEFULNESS OF BACKGROUND
NETWORK DATA

Data generated by the Ground Water Quality
Monitoring Program can be used to evaluate
regional ground-water quality. This has numerous
practical applications in both the public and private
sectors.

Local water quality can be compared to regional
background water quality, where changes in
quality are suspected. This data can provide
upgradient information, against which the effects
of a potential contamination source can be
compared. It can also aid in quantifying temporal
changes in ground-water quality brought on by
sweeping land use changes, such as urbanization.
Until background is defined, it is difficult to
determine whether an unusual parameter
concentration measured at a well is the result of
natural or anthropogenic influences.

Background data will be useful for determining
potential health risks to the public resulting from
ground-water consumption. State and local
agencies will find the data useful in land use
planning and zoning decisions, the protection of
public drinking water supplies, and in the
development of state-mandated comprehensive
growth planning. Water management districts can
use the data to evaluate permit applications
regarding water withdrawal and use. Regulatory
agencies will find the data invaluable in
implementing aquifer resource management
strategies, such as wellhead protection, delineation
of recharge and discharge areas, and surface
water/ground water co-management. Mapping of
physical aquifer extents and distribution (Scott et
al, 1991), coupled with knowledge of chemical
aquifer characteristics (this volume) helps to better
define available resources. Future efforts involving
the mapping of potential aquifer vulnerability,
refinement of hydrostratigraphic units, and the
development of data evaluation methods will all
contribute to the body of information which will aid
in the wise use of the state's ground-water
resources.

The private sector will find the data particularly
useful when preparing reports on such issues as
contamination assessments, risk evaluations,
water supply studies and waste disposal designs.
Industry and agricultural interests will benefit by

being able to identify appropriate water supply
sources based on water quality.

The well and water quality data is available to
the public via access to a computer bulletin board,
or by contacting:

Florida Department Of Environmental
Regulation
Bureau Of Drinking Water & Ground Water
Resources
Ground Water Quality Monitoring Section
2600 Blair Stone Road
Tallahassee, Florida 32399-2400

Staff (904) 488-3601 or
SUNCOM 278-3601
FAX (904) 487-3618 or
SUNCOM 277-3618
GWIS BBS (Computer Bulletin Board)
(904) 487-3592 or
SUNCOM 277-3592

REFERENCES CITED

Fernald, E. A. and Patton, D. J., 1984, W a t e r
resources atlas of Florida: Florida State
University Institute of Science and Public
Affairs, Tallahassee, Florida, 291 p.

Florida, State of, 1983, Florida Statutes, Sect io n
403.063 Water Quality Assurance Act,
Chapter 174.2455 Ground water quality
monitoring: 1983 Florida Legislature,
Tallahassee, Florida.

McGuinness, C. L., 1963, The role of ground water
in the national water situation: U.S. Geological
Survey Water-Supply Paper 1800,
Washington, D.C., p. 244-255.

Scott, Thomas M., Lloyd, Jacqueline M. and
Maddox, Gary L. (eds.), 1991, Florida's Ground
Water Quality Monitoring Program Hydro-
geological Framework: Florida Geological
Survey Special Publication No. 32,
Tallahassee, Florida, 97 p.

FLORIDA GEOLOGICAL SURVEY
/

Chapter II

DATA COLLECTION AND
MANAGEMENT METHODS

Gary L. Maddox

Florida Department of
Environmental Regulation
Tallahassee, Florida

WELL SELECTION AND SAMPLING

Prior to selecting monitoring sites for inclusion
in the Background Network, hydrogeologic data
were evaluated in conjunction with land use in-
formation. Most of this information was of a
general nature and was compiled by the state's
five water management districts (Appendix 3,
figure 1). The first volume of this series (Scott et al,
1991) contains a wealth of hydrogeological data
collected during the initial phase of the program.
This information was used to develop regional
monitoring strategies, and aided in the selection of
potential well sites. These wells were selected or
drilled in order to achieve optimum areal and
aquifer distribution.

The second phase of the program entailed
locating existing wells suitable for inclusion in the
monitoring network. An initial inventory of existing
wells meeting these criteria was conducted by the
U.S. Geological Survey and the water management
districts. The following criteria were used to
determine eligibility:

a) Depth of well and cased interval
known;
b) Open hole interval taps only one aquifer
or water-bearing zone;
c) Precise site location known;
d) Well owner cooperative;
e) Future accessibility for sampling
granted;
f) History of the site (prior land use,
previous sampling results) known.

Other non-mandatory, but desirable criteria
included:

a) Site ownership by local, state or federal
agency;
b) Prior water quality data available;

c) Well diameter known;
d) Lithologic and geophysical logs
available;
e) Hydrogeologic information available.

Over 1200 existing wells were initially selected
through this process. Although optimal quality
assurance and quality control could be more fully
realized by drilling all monitoring wells expressly for
use in the network, the associated costs prohibited
such an approach. It was determined that useful
data could be obtained using wells already in
existence, if the selection criteria were strictly
adhered to.

Subsequent to locating existing wells,
correlating well depth with site hydrogeology and
considering land use patterns, locations for
additional monitoring wells were determined. Over
600 new wells were drilled in areas where no
suitable existing wells could be found. Depending
on the hydrostratigraphy at each new site, a single
well or cluster of wells was installed, allowing each
major water-bearing zone to be separately
monitored. Geological information was obtained at
each site during drilling. At many sites, a core from
the uppermost significant confining bed was
obtained for laboratory determination of
permeability. Initial well placement was biased
toward preferential monitoring of the most
important potable aquifer within a region; current
strategy emphasizes the uppermost aquifer system
in an area. This latter philosophy is based on the
notion that surface-introduced chemical changes
(due to land use or meteorological considerations)
would first be detected in the uppermost water-
bearing unit. Figure 1 shows the general location
of currently sampled wells by aquifer system in the
Background Network.

The first sampling of each well in the network
involved the measurement of a comprehensive set
of field, chemical, microbiological, and naturally-
occurring radioactive parameters (Table 1). These
analyses, combined with historical data, can be
used to estimate baseline ground-water quality.
Once current baseline has been determined, data
from future monitoring of the network will be
continually evaluated to determine changes in
water quality over time. This information is
particularly useful for implementation of wellfield
protection measures, water quality monitoring and
land use planning.

After the initial samples are collected and
analyzed, Background Network monitoring wells

are resampled approximately every three to five
years for all network parameters. A subset of
Background Network wells, the Temporal
Variability Subnetwork ("TV Net"), is sampled more
frequently (monthly or quarterly), in order to detect
variations in ground-water quality over time.
Samples collected on a quarterly basis are
analyzed for major ions and field parameters, while
monthly data collection consists of the
measurement of field parameters only (see Table
1). Wells sampled monthly are a subset of the
wells which are sampled quarterly. In addition, a
pilot project is currently underway to define
temporal variability on an even finer scale. Using
dedicated probes and automatic sampling devices
installed in a few wells, the goal of this "optimal
frequency study" is to observe variations in
ground-water quality on a weekly, daily, or hourly
basis. Results from the Temporal Variability
Subnetwork will be published in a future volume.

Development of the Background Network
occurred in the following phases:

-Phase : Data collection, compilation,
and location of existing wells which could
be incorporated into the Background
Network;

-Phase I: Selection and drilling of initial
monitoring wells;

-Phase III: Initial sampling of the Back-
ground Network to determine ground-
water quality spatial trends and define
baseline;

Phase IV: Resampling of wells found to
contain abnormal concentrations of one or
more parameters;

-Phase V: Refinement of the network
through removal of redundant wells and
those found not to monitor representative
background ground-water quality, as well
as drilling of additional wells where
needed;

-Phase VI: Ongoing periodic resampling
to define variations in ground-water quality
over time.

SAMPLING PROTOCOL

The history of sampling of the Background
Network reflects an increasing awareness of the

many difficulties encountered in collecting and
analyzing a representative ground-water sample.
Potential variability introduced by the use of
different sampling personnel, techniques and
equipment, sample transport from the field to the
laboratory, environmental and laboratory
contamination, concurrent use of several analytical
laboratories, and varying methods of reporting
results have all had an effect on the analyses
discussed here. By working closely with personnel
in the field and in the laboratory, and by developing
standardized QA/QC procedures, sampling and
analytical methods have steadily improved over
the history of the program, with the goal of
minimizing the potential variability introduced
throughout the entire process.

Sampling of the Background Network began in
mid-1985. A portion of the existing wells were
sampled using permanently installed pumps. The
remaining existing wells and all new wells were
sampled using teflon bailers, dedicated bladder
pumps or submersible pumps. Some monitoring
wells have been fitted with semi-permanent
internal standpipes, to facilitate purging and water
sample collection. Sample collection protocol
currently follows that established by the U.S.
Environmental Protection Agency (EPA) (EPA,
1982, 1991). The initial sampling episode included
the collection of a comprehensive set of physical,
chemical, biological and radiometric parameters
(Table 1).

Initial sampling of the Background Network
was overseen or performed by each water
management district. All sampling is currently
conducted or supervised by a trained professional.
Annual training of sampling personnel is funded by
DER and provided by the staff of the USGS Ocala
Quality of Water (QW) Service Unit. A rigorous
quality control program has been established (see
ANALYTICAL METHODS section below). Field,
trip and laboratory blanks are submitted on a
routine basis. Initially, all sampling agencies were
required to have an individually-approved quality
assurance/quality control (QA/QC) plan on file with
the Department, and to submit periodic QA/QC
reports. In order to standardize procedures, all
sampling is now performed under a single
"umbrella" QA/QC plan, authored by DER and
signed by each sampling agency (DER, 1991).
Agreement by each agency to use standardized
sample collection methodology further minimizes
sampling variability.

The determination of sampling frequency and

SPECIAL PUBLICATION NO. 34

the parameters to be monitored at each site were
based on several factors, such as network
designation, land use activity and the
hydrogeologic sensitivity of the site. After initial
sampling, several wells were dropped from the
Background Network, based on analytical sample
results which indicated that data from the wells
were not representative of regional background
water quality. In some instances, existing
monitoring wells did not have good hydraulic
connection with the aquifer to be monitored. In
other instances, the quality of water from the well
was impacted by poor well construction.
Additional wells may be added to fill in gaps in
areal or aquifer-specific coverages. This
refinement process is ongoing.

Residual well construction materials and
casing corrosion can significantly increase the
volume of suspended solid material present within
a well. Turbidity analysis can be used to evaluate
well construction integrity for many wells.
Redevelopment is often required in problem wells
which were improperly installed, or have a
tendency to accumulate residual solids, due to
local hydrogeologic conditions (Aller et al., 1989).
Prior to 1989, almost all samples were unfiltered in
the field prior to laboratory analysis. Thus, the
combined contribution of particulate matter,
suspended (colloidal) solids, and the soluble
fraction in each sample were measured with each
analysis. This is representative of well water
quality, but not necessarily aquifer water quality,
since the well itself could possibly be the source
for many of the particulates present in the samples
(Nielsen, 1991). Since data on both aquifer and
well water quality was desired, filtered and
unfiltered samples are now collected for affected
parameters at each site and separately analyzed.

ANALYTICAL METHODS

The initial chemical analyses of Background
Network samples were performed by private
laboratories, with some parameters analyzed by
the water management district laboratories. Due
to the magnitude of the program and the large
initial number of samples, several different
analytical laboratories were used. This caused
concern about consistency and relative
comparability of data from one lab to another. As
a result, all inorganic analyses are currently
performed by one lab, the USGS Ocala QW
Service Unit, and organic are analyzed by the
DER Central Laboratory or its designated overflow
lab. Sample analysis protocol generally follows

methods described by EPA, USGS or DER
(American Public Health Association, 1980;
Fishman et al., 1989; DER, 1981).

To assess laboratory accuracy and precision,
duplicate samples and reference samples are
anonymously submitted, along with trip,
equipment and field blanks. These QA/QC
samples currently constitute over 20% of the total
number of samples analyzed. As a QA/QC check
on procedures and efficiency, the laboratories are
periodically audited by outside agencies. Frequent
meetings are held among the Ground Water
Quality Monitoring Program staff and laboratory
personnel to discuss procedural problems as they
arise.

Table 1 references the standard EPA
laboratory methods initially used to analyze
samples from the Background Network. These
methods have and will change as better equipment
and procedures become available. Major ions,
metals, organic, radiometrics and microbiological
parameters were all included in the analyses. Field
parameters were measured in the field at the time
of sample collection. Table 2 lists the Primary and
Secondary Drinking Water Standards for
parameters sampled in the Background Network.

To assess methodology protocol and the
performance of sampling personnel, periodic field
audits are carried out by members of the DER
program staff. These audits are supplemented by
required QA/QC reports, submitted quarterly by
each sampling agency, detailing real and potential
problems encountered during sample collection.
All agencies performing sampling or analytical ser-
vices for the program are required to have an
approved QA/QC plan on file with the Department,
or to comply with the "umbrella" plan written by
DER.

All water quality results are submitted to DER
in both paper and electronic formats. In addition
to the actual results, these data also include
information on analytical method used, STORET
code1 (EPA, 1984), well, field and laboratory
identification numbers, units, parameter name,
project name, exceedances of existing standards,
and data submitted by field personnel, such as
sampling date and time, and remarks. This
information is for the most part incorporated into
the data bases discussed below.

DATA BASE SYSTEMS

A variety of data base and software systems
have been developed to store, manipulate and
display information related to the Ground Water
Quality Monitoring Program. All water quality
information collected by the program is uploaded
to DER's mainframe "Central Repository", an
archive of statewide environmental data available
to DER and other state agencies. In the near
future, Background Network data will also be
written into an ORACLE database, for use with
DER's ARC/Info geographic information system
(GIS). Contact DER's Bureau of Information
Systems for details on availability of these data
formats.

Currently the most widespread system in use
is the Generalized Well Information System (GWIS),
a micro-computer database and retrieval system
which houses all well and analytical water quality
information generated by the program. It was
developed on an IBM-PC compatible platform, and
consists of two separate data bases: one
containing physical well information and one
containing analytical results. The two are linked by
a USGS-format (latitude/longitude/sequence
number) common well identifier. The data file
format is currently fixed-field length ASCII, but will
be changed to dBase format in the near future.
The system was written in-house, to quickly and
efficiently handle the large volume of data
generated by the network, and is available to the
public. GWIS water quality data retrievals run in
three main steps:

1) Select wells of interest (from physical
well information database, using almost
any combination of constraints);

2) Select parameters of interest (from
sample database, individually or by pre-
defined group);

3) Retrieve data.

The program also allows the user to constrain
retrievals by sampling dates, or to only retrieve
results above or below a given threshold value
(these can correlate with exceedances of assigned
standards). Output formats allow for row-and-
column or tabular report generation and the
calculation of summary statistics on multiple
samples. Various utility programs allow the
calculation of frequency distributions, water types,
and statistical outliers. A direct interface with a

popular off-the-shelf computer-aided drafting
(CAD) package allows the user to plot well
locations and data on a map, or define a region of
interest graphically, for subsequent data retrieval.

AVAILABILITY OF DATA

The GWIS program and data files can be
obtained from DER by contacting our Tallahassee
office at the address or telephone numbers
previously listed. The programs and data are also
available via a computer bulletin board (BBS)
running 24 hours a day, seven days a week, for
anyone with an IBM-compatible personal
computer and a modem. The entire Background
Network database and GWIS programs are
available for remote use or for down-loading. The
bulletin board system can handle 300, 1200 and
2400 baud calls using industry standard
communications parameters of no parity, 8 data
bits, and 1 stop bit (n-8-1). These are the default
settings for most major communications software
packages. To connect, use your computer to call
(904)487-3592 or SUNCOM 277-3592.

DATA VALIDATION PROCEDURES

Before network data is released to the public,
raw laboratory and field data is converted into
dBase format and then run through an extensive
series of automated and manual screening proce-
dures (Figure 2). Error checking programs detect
parameter values outside allowable ranges. Data
entry mistakes are often the cause for these errors.
Charge balance checks are run, usually an
indication of the integrity of laboratory analyses.
Outlier programs use non-parametric estimation to
flag values which, while within allowable ranges,
seem not to fit regional trends. When outliers are
found, sampling procedures are first checked. In
addition to these automated checks, the data are
inspected manually by staff to assure com-
pleteness of each data set and to remove QA data
to another database. QA data includes field, trip
and laboratory blanks submitted at regular
intervals in the sampling process. After in-house
editing and review, provisional data sets are
released to sampling agencies for their review.
Outliers are flagged, and samplers are asked to
provide any data which may explain the
anomalous values. Results are compared to trip
and equipment blanks taken during sampling. In
some instances, the well may be re-sampled, or
laboratory procedures may be investigated. If
circumstances surrounding the collection or
analysis of the sample are suspect, the data may

FLORIDA GEOLOGICAL SURVEY

be left in the database but flagged with conditional
provisions, or removed altogether. Other para-
meter analyses from the same sample may or may
not be affected. If no disqualifying problems are
found, the outlier may not be rejected, and is
included in the release database.

Ground-water quality data are constantly
being received as ongoing sampling projects
continue. The main GWIS distribution databases
are updated three or more times a year.

ENDNOTES

1 STORET is a water quality database
management system established by the U.S.
Environmental Protection Agency. Five-digit
STORET codes are assigned for each parameter
based on methods used during sample collection
and analysis.

REFERENCES CITED

Aller, Linda, Bennett, Truman W., Hackett, Glen,
Petty, Rebecca J., Lehr, Jay H., Sedoris,
Helen, Nielsen, David M., and Denne, Jane E.,
1989, Handbook of suggested practices for
the design and installation of ground-water
monitoring wells: EPA 600/4-89/034, National
Water Well Association, Dublin, Ohio; 398 p.

American Public Health Association, 1980,
Standard methods for the examination of
water and wastewater, 15th edition: American
Public Health Association, Washington, D.C.,
1134 p.

Fishman, Marvin J. and Linda C. Friedman (eds),
1989, Techniques of water-resources
investigations of the U.S. Geological Survey,
Book 5, Chapter Al Methods for
determination of inorganic substances in water
and fluvial sediments, third edition: U.S.
Geological Survey; 545 p.

Florida Department of Environmental Regulation,
1981, Supplement "A" to standard operating
procedures and quality assurance manual:
Florida Department of Environmental
Regulation, Solid Waste Section, Tallahassee,
Florida, 110 p.

Florida Department of Environmental Regulation,
1991, Chapter 17-160, Quality Assurance:
Tallahassee, Florida; 40 p.

Merchant, Randy, 1989, Florida ground water
guidance concentrations: Florida Department
of Environmental Regulation, UIC, Criteria &
Standards Section, Tallahassee, Florida, 14 p.

Nielsen, David M. (ed), 1991, Practical handbook
of ground-water monitoring: Lewis Publishers,
Inc.; 717 p.

Scott, Thomas M., Lloyd, Jacqueline M. and
Maddox, Gary L. (eds.), 1991, Florida's Ground
Water Quality Monitoring Program Hydrogeo-
logical Framework: Florida Geological Survey
Special Publication No. 32, Tallahassee,
Florida, 97 p.

United States Environmental Protection Agency,
1982, Handbook for sampling and sample
preservation of water and wastewater: United
States Environmental Protection Agency EPA-
600/4-82-029, Cincinnati, Ohio, 402 p.

United States Environmental Protection Agency,
1984, Overview of STORET: United States
Environmental Protection Agency,
Washington, D.C., 26 p.

United States Environmental Protection Agency,
1991, Standard operating procedures and
quality assurance manual: United States
Environmental Protection Agency, Region IV,
Athens, Georgia, 203 p.

SPECIAL PUBLICATION NO. 34

Chapter III

HYDROSTRATIGRAPHY

Thomas M. Scott

Florida Geological Survey
Florida Department of Natural Resources
Tallahassee, Florida

INTRODUCTION

Florida's ground-water resources occur in a
complex lateral and vertical sequence of Cenozoic
sediments comprised of both siliciclastics and
carbonates which underlie the entire state.
Hydrostratigraphically, the section consists of
several major aquifer systems defined on lateral
extent, degree of confinement, and hydrologic
parameters of the sediments. The Southeastern
Geological Society's ad hoc Committee on Florida
Hydrostratigraphic Unit Definition (Southeastern
Geological Society (SEGS), 1986), in an attempt to
alleviate many of the nomenclatural problems
surrounding Florida's hydrostratigraphic units,
defined the framework of the various aquifer
systems occurring in the state. Most of the
geologic community have accepted these
definitions and are using the suggested
nomenclature. Aquifers of lesser importance have
been recognized in some areas of the state and
are discussed in the literature on specific areas.
This text will define and characterize only the major
aquifer systems discussed by the SEGS (1986).
These systems include the surficial aquifer system,
the intermediate aquifer system or intermediate
confining unit, and the Floridan aquifer system
including the Claiborne aquifer and the sub-
Floridan confining unit. Figure 3 indicates which
formations form portions of the various aquifer
systems throughout the state. Miller (1986)
provides an excellent, in-depth discussion of the
Floridan aquifer system and the associated
shallower strata. It is recommended that the
reader review Miller's volume for a more detailed
description of the ground-water system in Florida.
Appendix 3, figure 2 delineates the distribution of
aquifer systems in Florida.

References to Florida's geomorphic features
are made in this and succeeding chapters.
Appendix 3, figures 3 to 7 delineate these features
in each district. For further discussion refer to
Scott (1991).

Geologic Structures in Relation
to Hydrostratigraphy

The occurrence, thickness and, to some
extent, the aquifer characteristics are directly
related to the structural features present in a given
area. The major positive features affecting the
various aquifer systems include the Ocala
Platform, Chattahoochee Anticline,Sanford High
and the St. Johns and Brevard Platforms (Figure
4a). The major negative features include the Gulf
Basin, Apalachicola Embayment, Gulf Trough,
Jacksonville Basin, Osceola Low and the
Okeechobee Basin (Figure 4a). These structures
affected the deposition and erosion of the later
Cenozoic sediments. Older structures, including
the Peninsular Arch and the South Florida Basin
(Figure 4b), affected the lower portions of the
Cenozoic section (see Scott (1991) for a discussion
of the structural features in Florida).

The surficial aquifer system is thin to absent on
the positive features. Its thickness increases off
the positive structures reaching maximum thick-
nesses in the Okeechobee, Jacksonville and Gulf
Basins and the Apalachicola Embayment.

The intermediate aquifer system and/or
intermediate confining unit also thins onto the
positive features. Sediments forming these units
are erosionally absent from much of the
Chattahoochee Anticline, Ocala Platform and the
Sanford High. These units thicken off the highs,
reaching the maximum thicknesses in the basinal
areas. As the sediments of the intermediate
aquifer system and confining unit thicken,
permeable beds become more commonly
interbedded with the impermeable strata, resulting
in a more fully developed intermediate aquifer
system.

Eocene and Oligocene carbonate sediments of
the Floridan aquifer system are exposed to thinly
covered on the Ocala Platform and the
Chattahoochee Anticline. These sediments are
covered by a thin intermediate confining unit on
the flanks of the positive features. In these areas,
the carbonates have been exposed to aggressive
ground water, developing an extensive karstic
terrain. In the basinal areas, the carbonate sedi-
ments have not undergone such extensive
dissolution due to the thick protective cover
provided by the intermediate aquifer system and
intermediate confining unit.

AQUIFER SYSTEMS AND
CONFINING UNITS

Surficial aquifer system

The SEGS (1986) defines the surficial aquifer
system as the

"permeable hydrologic unit contiguous
with the land surface that is comprised
principally of unconsolidated to poorly
indurated, (silici)clastic deposits. It also
includes well-indurated carbonate rocks,
other than those of the Floridan aquifer
system where the Floridan is at or near
land surface. Rocks making up the
surficial aquifer system belong to all or
part of the Upper Miocene to Holocene
Series. It contains the water table, and the
water within it is under mainly unconfined
conditions; but beds of low permeability
may cause semi-confined or locally
confined conditions to prevail in its deeper
parts. The lower limit of the surficial
aquifer system coincides with the top of
the laterally extensive and vertically
persistent beds of much lower
permeability."

The surficial aquifer system occurs throughout
most of the state. In many areas, it is used for
small yield domestic and agricultural water
supplies. However, in the western panhandle the
surficial aquifer system, referred to as the Sand
and Gravel Aquifer, supplies important amounts of
water for municipal and industrial supplies. In the
southeastern part of the state, the surficial aquifer
system is called the Biscayne Aquifer and provides
enormous quantities of water for the coastal
communities in this area. The surficial aquifer
system is utilized for public water supply in
southern Brevard, Indian River and St. Johns
Counties. Elsewhere in the state, the surficial
aquifer system is of limited importance.
Throughout the extent of the surficial aquifer
system, the thickness varies significantly from a
feather edge to more than 350 feet in southeastern
Florida and 500 feet in the western-most
panhandle (Scott et al., 1991). The top of the
surficial aquifer system is the natural land surface.
The base occurs where impermeable beds of the
intermediate confining unit and aquifer system
begin or, in those areas where the intermediate is
absent, at the top of the Floridan aquifer system
carbonates.

In many areas of the state, the surficial aquifer
system lies on a karstified erosional surface
developed on Eocene to Miocene carbonates.
Karst processes have also affected the surficial
aquifer system by forming collapse features which
filled with surficial aquifer system sediments and
may be in direct hydrologic contact with the
Floridan aquifer system. Karst features also
perforate the surficial aquifer system developing
open sinkholes on the present land surface.

NORTHWEST FLORIDA WATER
MANAGEMENT DISTRICT

The surficial aquifer system in the Northwest
Florida Water Management District (NWFWMD)
occurs over most of the district. It is absent only in
a limited portion of Wakulla, Leon and Jefferson
Counties at the eastern edge of the district along
the western flank of the Ocala Platform. It is thin to
absent on part of the Chattahoochee Anticline in
Jackson and Holmes Counties. Where the surficial
is present it ranges in thickness from less than 10
feet in the east to more than 500 feet in the north-
western corner of the area (Scott et al., 1991).

The siliciclastic sediments comprising the
surficial aquifer system in NWFWMD are part of
the Citronelle and Miccosukee Formations,
"Coarse Clastics" and the undifferentiated
sediments of Pleistocene-Holocene age (Marsh,
1966; Scott, 1991). These sediments are primarily
quartz sands with varying percentages of clay.
Where the clay content becomes great enough to
inhibit the transmission of ground water, localized
impermeable beds may confine water creating
artesian conditions within the surficial aquifer
system. The surficial aquifer system yields greater
quantities of water in the western panhandle where
the Citronelle contains less clay and is thicker than
in those areas where the clayey Miccosukee
occurs.

SUWANNEE RIVER WATER
MANAGEMENT DISTRICT

The surficial aquifer system in the Suwannee
River Water Management District (SRWMD) is
present in several areas of the district. According
to Ceryak (SRWMD, personal communication,
1991), the surficial aquifer system is present in
adjoining portions of southern Madison, eastern
Taylor and western Lafayette Counties, eastern
Suwannee County, much of Columbia, Hamilton
and Union Counties, along the eastern edge of

FLORIDA GEOLOGICAL SURVEY

Bradford County under Trail Ridge and under
Waccasassa Flats in central Gilchrist County.
Sediments equivalent to the surficial aquifer
system are present throughout much of the district
but are not utilized for water resources.
Thicknesses of the surficial aquifer system range
from 10 to 30 feet but may reach 50 to 60 feet
under Trail Ridge (see Scott (1991) for discussion
of the geomorphology of Florida).

The surficial aquifer system sediments in
SRWMD are part of the undifferentiated sediments
and in some areas, the upper Hawthorn Group
sediments. These sediments are quartz sands
with varying amounts of clay and carbonate. In
localized areas the clay content of the sediments
may form confining beds within the surficial
system.

The base of the surficial aquifer system in the
SRWMD occurs at the top of the impermeable
sediments of the Hawthorn Group throughout
much of the district. However, in the eastern
portion of the district, the base may occur within
the sediments of the upper Hawthorn Group. In
other areas, the intermediate confining unit may be
absent and the surficial aquifer system may lie
directly on the carbonates of the Floridan aquifer
system or be absent.

ST. JOHNS RIVER WATER
MANAGEMENT DISTRICT

The surficial aquifer system in the St. Johns
River Water Management District (SJRWMD) is an
important source of potable water in Duval, Clay,
St. Johns, Putnam, Brevard and Indian River
Counties. The coastal counties utilize the surficial
to varying degrees with St. Johns, southern
Brevard and Indian River Counties, using it for
public supply. Eastern Orange and eastern
Alachua Counties also utilize the surficial aquifer
system. In other areas of the district, the surficial
aquifer system may be used for limited domestic
supplies. The surficial aquifer system thickness is
highly variable, ranging from a few feet to in excess
of 100 feet.

Sediments forming the surficial aquifer system
in SJRWMD are lithostratigraphically assigned to
the undifferentiated sediments, Cypresshead and
Nashua Formations, Caloosahatchee Formation-
equivalent shell beds and the Coosawhatchie
Formation of the Hawthorn Group. The
undifferentiated sediments and the Cypresshead

Formation consist of quartz sands with varying
percentages of clay. The Nashua Formation and
Caloosahatchee Formation-equivalent beds are
composed of varying admixtures of quartz sand,
clay, shells and shell debris. The Anastasia
Formation is composed of sand and coquina.
Quartz sands and varying amounts of clay make
up the Coosawhatchie Formation with limestone
becoming prominent in portions of Duval and
Nassau Counties. Locally, the sediments contain
sufficient clay to form impermeable beds creating
artesian conditions in the surficial aquifer system.

The base of the surficial aquifer system in the
SJRWMD occurs at or near the top of the
Hawthorn Group or in the undifferentiated post-
Hawthorn sediments when those sediments are
relatively impermeable.

SOUTHWEST FLORIDA WATER
MANAGEMENT DISTRICT

The surficial aquifer system occurs over much
of the Southwest Florida Water Management
District (SWFWMD). It is of generally limited value
in the northern portions of the district and
increases in importance to the south. SWFWMD
data indicates that the surficial aquifer system is
thin over much of the district (Scott et al., 1991).
Thicknesses range from less than 25 feet in much
of the northern part of the district on the Ocala
Platform to 25 to 50 feet in the southern area and
more than 250 feet under the Lake Wales Ridge.

Surficial aquifer system sediments in
SWFWMD belong to the undifferentiated
sediments in the northern half of the district. In the
southern half of SWFWMD the sediments include
the Tamiami, Caloosahatchee and Fort Thompson
Formations. Along the Lake Wales Ridge, the
surficial aquifer system is comprised of sediments
belonging to the Cypresshead Formation and the
undifferentiated sediments. In a limited area in
central SWFWMD, the Bone Valley Member of the
Peace River Formation, Hawthorn Group forms
part of the surficial aquifer system. The sediments
in these units generally consist of quartz sand with
varying percentages of clay and shell except in the
Bone Valley Member where phosphate forms a
significant proportion of the sediment. Vacher et
al. (1990) characterize the sediments as quartz
sand with less than 10 percent clay over much of
the district. They also show shell content of the
surficial aquifer system increasing toward the
coast and to the south in the southern half of the
district.

The base of the surficial aquifer system occurs
at the top of the impermeable sediments overlying
the carbonates of the Floridan aquifer system in
the northern part of the district. When
impermeable sediments of the Hawthorn Group
are subjacent to the undifferentiated sediments
they form the base of the surficial. The Hawthorn
Group lies subjacent to the Cypresshead
Formation under the Lake Wales Ridge and forms
the base of the system. The Hawthorn Group
sediments also form the base of the surficial
aquifer system in southern SWFWMD where the
Hawthorn underlies the Tamiami, Caloosahatchee
and Fort Thompson Formations.

SOUTH FLORIDA WATER
MANAGEMENT DISTRICT

The surficial aquifer system is widespread in
the South Florida Water Management District
(SFWMD) constituting an important water
resource. Although the surficial aquifer system is
present over much of the district, it is the most
important source of ground water in the
southeastern portion of SFWMD, in Dade, Broward
and Palm Beach Counties. In Lee, Hendry and
Collier Counties, the surficial provides significant
quantities of potable water for domestic and
agricultural uses. Throughout the district, the
surficial aquifer system varies in thickness from a
few feet to more than 400 feet thick.

The sediments comprising the surficial aquifer
system are from several lithostratigraphic units. In
the north-central SFWMD area, the surficial occurs
in the undifferentiated sediments, Cypresshead
Formation and shell beds of the Caloosa-
hatachee/Fort Thompson Formations. In the
western part of SFWMD, sediments of the
Tamiami, Caloosahatchee/Fort Thompson Forma-
tions and the undifferentiated sediments make up
the system. In the eastern area of SFWMD, the
surficial aquifer system, in part referred to as the
Biscayne Aquifer, consists of sediments from the
Anastasia Formation, Miami and Key Largo
Limestones, Fort Thompson Formation, and
Caloosahatchee and Tamiami-equivalent
sediments. In SFWMD, the base of the surficial
system occurs at the first impermeable sediments
in the Hawthorn Group. Occasionally, the upper
Hawthorn Group sediments may form the basal
portion of the surficial.

The lithostratigraphic units forming the surficial
aquifer system consist of a complex array of
facies. The sediments range from quartz sands to

limestones with varying admixtures of shell and
clay. As a result of the variability, the quality of the
surficial aquifer system in SFWMD changes
dramatically from place to place. Numerous
investigations of these sediments have discussed
the variable nature of the aquifer characteristics
(for example, Causaras, 1985; Wedderburn et al.,
1982; Shaw and Trost, 1984; Knapp et al., 1986;
Smith and Adams, 1988).

Intermediate Aquifer System and
Intermediate Confining Unit

The SEGS (1986) defines the intermediate
aquifer system or intermediate confining system as
including

"all rocks that lie between and collectively
retard the exchange of water between the
overlying surficial aquifer system and the
underlying Floridan aquifer system. These
rocks in general consist of fine grained
(silici)clastic deposits interlayered with
carbonate strata belonging to all or parts
of the Miocene and younger Series. In
places poorly-yielding to non-water-
yielding strata mainly occur and there the
term intermediate confining unit applies.
In other places, one or more low to
moderate-yielding aquifers may be
interlayered with relatively impermeable
confining beds; there the term inter-
mediate aquifer system applies. The
aquifers within this system contain water
under confined conditions."

"The top of the intermediate aquifer
system or intermediate confining unit
coincides with the base of the surficial
aquifer system. The base of the inter-
mediate aquifer is at the top of the
vertically persistent permeable carbonate
section that comprises the Floridan aquifer
system, or, in other words, that place in
the section where (silici)clastic layers of
significant thickness are absent and
permeable carbonate rocks are dominant."

The intermediate aquifer system or inter-
mediate confining unit occurs over much of the
state. It is absent from those areas where it was
removed by erosion and the surficial aquifer
system sediments, if present, lie immediately
suprajacent to the carbonates of the Floridan
aquifer system. Springs are a common feature of

SPECIAL PUBLICATION NO. 34

these areas. Surrounding the areas where these
sediments are missing, the intermediate aquifer
system or intermediate confining unit is often
perforated by karst features. Where this condition
exists, the intermediate aquifer system and the
intermediate confining unit allow water to pass
through into the Floridan aquifer system or into the
surficial aquifer system.

The regional significance of the intermediate
aquifer system is quite limited. Statewide, this
section is referred to as the intermediate confining
unit. It serves to confine the Floridan aquifer
system and forms the base of the surficial aquifer
system. The sediments comprising this section
are predominantly siliciclastic (quartz sand, silt and
clay) with varying proportions of carbonates
(limestone and dolostone) present. Much of the
intermediate confining unit was deposited during
the Miocene and Early Pliocene. It is interesting to
note that in some areas Miller (1986) has included
low permeability Oligocene and Eocene
carbonates in contact with the Miocene sediments
as part of the intermediate confining unit.

The top of the intermediate aquifer system or
intermediate confining unit ranges from more than
350 feet below National Geodetic Vertical Datum
(NGVD) to greater than 225 feet above NGVD.
Miller (1986) cites thicknesses of the intermediate
confining unit (his upper confining unit) ranging
from very thin or absent to greater than 1000 feet.

NORTHWEST FLORIDA WATER
MANAGEMENT DISTRICT

The intermediate confining unit occurs over
much of the NWFWMD serving to effectively
confine the Floridan aquifer system. It is thin to
absent over the Chattahoochee Anticline in
portions of Jackson and Holmes Counties. The
intermediate confining unit is also thin to absent in
eastern Wakulla, southeastern Leon and southern
Jefferson Counties. The intermediate confining unit
thickens dramatically under the western end of
NWFWMD in Escambia County and in the
Apalachicola Embayment under Gulf and Franklin
Counties. Thicknesses range from less than 10
feet to greater than 1000 feet.

The ability of the intermediate confining unit to
effectively confine the subjacent Floridan aquifer
system is impaired in those areas where it has
been breached by karst development. These areas
include portions of Jackson, Holmes, Washington,

Walton, Leon and Wakulla Counties (Sinclair and
Stewart, 1985).

Siliciclastic sediments predominate in the
intermediate confining unit in NWFWMD.
Carbonate sediments are present in the sediments
of the Apalachicola Embayment and east of the
Apalachicola River. In western NWFWMD, the
confining unit is the Pensacola Clay which grades
eastward into the Alum Bluff Group. Further east,
generally east of the Apalachicola River, the
Hawthorn Group forms the intermediate confining
unit. Within the Apalachicola Embayment, portions
of the Intracoastal Formation form the inter-
mediate confining unit.

The intermediate aquifer system is generally
not an important water-bearing unit in NWFWMD.
Permeable beds of limited extent are present
locally and may provide limited amounts of water
to small, domestic wells. The intermediate aquifer
system/confining unit acts as an aquifer system
primarily east of the Choctawhatchee River
(Wagner, 1988). The permeable zones utilized for
ground water are siliciclastic and carbonate beds
in the Intracoastal Formation (Barr and Wagner,
1981), the Alum Bluff Group and, to a very limited
extent, the Hawthorn Group.

SUWANNEE RIVER WATER
MANAGEMENT DISTRICT

The intermediate confining unit is present in
SRWMD under the Northern Highlands. This
includes portions or all of Jefferson, Madison,
Hamilton, Suwannee, Columbia, Baker, Bradford,
Union and Alachua Counties. Within this area, the
thickness of the intermediate confining unit may
exceed 300 feet (Scott, 1988) and confined to
semiconfined conditions exist. It is thin to absent
on the Ocala Platform and thickens on its flanks
reaching the greatest thickness in the Jacksonville
Basin to the east of SRWMD. Karst features are
common throughout this area except in the
northeastern part of SRWMD (parts of Baker,
Bradford, Columbia, Hamilton and Union
Counties). Outliers and sinkhole fill consisting of
the sediments of the intermediate confining unit
are common in the areas where the unit is absent.

Siliciclastic sediments dominate the inter-
mediate confining unit in SRWMD. These
sediments most often are part of the Hawthorn
Group or materials that are residual from it
("Alachua Formation").

The intermediate aquifer system is interbedded
with the impermeable beds of the intermediate
confining unit. The intermediate aquifer system is
developed in the sands and carbonates of the
Hawthorn Group (Ceryak et al., 1983). In the
northeastern portion of the District, four discrete
carbonate units have been identified, each of
which is a separate intermediate aquifer. These
aquifers are up to 40 feet thick, and are all
confined, with the possible exception of the basal
Hawthorn carbonate unit, which may be in
hydraulic contact with the uppermost Floridan
aquifer system.

ST. JOHNS RIVER WATER
MANAGEMENT DISTRICT

The intermediate confining unit and inter-
mediate aquifer system occur throughout the
SJRWMD except along the western district
boundary in parts of Marion and Alachua Counties
on the Ocala Platform. The combined confining
unit and aquifer system ranges in thickness from
less than ten feet to more than 500 feet. It is
thickest in the Jacksonville Basin in northeastern
SJRWMD. It thins over the St. Johns Platform,
Sanford High and Brevard Platform in the central
portion of the district then thickens into the
Osceola Low and the Okeechobee Basin in
southern SJRWMD. In the SJRWMD the
Hawthorn Group or undifferentiated post-
Hawthorn sediments, where present, are
considered to form the top of the intermediate
aquifer system.

The intermediate confining unit and
intermediate aquifer system consist primarily of
interbedded siliciclastic and carbonate sediments
of the Hawthorn Group and sand, clay and
limestone of the undifferentiated post-Hawthorn
sediments. The Hawthorn Group sediments are
absent over much of the Sanford High and limited
portions of the St. Johns and Brevard Platforms in
southern Flagler County, much of Volusia County
and northern Brevard County.

Karst conduits breaching the intermediate
aquifer system and intermediate confining unit are
common in much of the SJRWMD. Only in Baker,
Nassau, Duval and parts of Clay and St. Johns
Counties are karst features very few in number and
the intermediate confining unit is not often
breached (Sinclair and Stewart, 1985).

The intermediate aquifer system is utilized as a

public water supply source in Flagler and eastern
Indian River Counties. Elsewhere it is utilized for
limited domestic and agricultural supplies.
Permeable strata in the Hawthorn Group and the
post-Hawthorn undifferentiated sediments often
exhibit rapid lateral and vertical variability resulting
in a limited areal distribution of water-producing
units. The intermediate aquifer system is most
often utilized in Nassau, Duval, Baker, St. Johns
and Clay Counties where the Hawthorn Group or
post-Hawthorn undifferentiated sediments are
thickest, infilling the Jacksonville Basin.

SOUTHWEST FLORIDA WATER
MANAGEMENT DISTRICT

The intermediate confining unit and inter-
mediate aquifer system are present throughout
most of SWFWMD (Buono et al., 1979). Although
the sediments comprising this section are absent
to very thin in the northern half of SWFWMD, they
thicken to more than 650 feet in the southern end
of the district (Buono et al., 1979). In the northern
half of the district, the section is generally the
intermediate confining unit and is thin to absent on
the southern end of the Ocala Platform. In the
southern half of SWFWMD, approximately from
northern Polk and Hillsborough Counties south,
the intermediate confining unit also contains
permeable sediments forming the intermediate
aquifer system. In this area, the sediments thicken
to the south into the Okeechobee Basin (Buono et
al., 1979; Scott, 1988).

Siliciclastic and carbonate sediments of the
Hawthorn Group comprise the majority of the
intermediate confining unit and intermediate
aquifer system in SWFWMD. In addition, some
post-Hawthorn siliciclastics may form a limited
portion of the intermediate confining unit in the
northern half of the district. In the northern portion
of the district, clayey sediments lying on the
Floridan aquifer system carbonates belong in part
in the Hawthorn Group and in part may be re-
worked Hawthorn sediments along with residuum
from dissolution of carbonates.

Breaching of the intermediate confining unit
and the intermediate aquifer system by karst
features is common in the northern half of the
district and along the Lake Wales Ridge in Polk
County (Sinclair and Stewart, 1985). The southern
portion of SWFWMD has limited karst
development (Sinclair and Stewart, 1985) and few
karst conduits penetrate the intermediate confining
unit and intermediate aquifer system.

FLORIDA GEOLOGICAL SURVEY

The intermediate aquifer system is utilized in
the southern half of SWFWMD and becomes most
important at the southern end of the district where
the Floridan aquifer system is deeply buried and
highly mineralized. The permeable strata of the
Hawthorn Group and portions of the Tamiami
Formation form the water-producing horizons
providing variable quantities of ground water
(Sutcliffe, 1975).

SOUTH FLORIDA WATER
MANAGEMENT DISTRICT

The intermediate confining unit and the inter-
mediate aquifer system occur throughout SFWMD.
However, the intermediate aquifer system is
utilized in a limited number of counties along the
western edge of the district. This section ranges in
thickness from approximately 500 feet in the
northern SFWMD area to more than 900 feet in the
southernmost portion of the district (Scott, 1988).
Much of the SFWMD area lies in the Okeechobee
Basin.

Interbedded siliciclastic and carbonate
sediments from the Hawthorn Group form the
intermediate confining unit and intermediate
aquifer system in SFWMD. Previously, some of
the sediments currently included in the inter-
mediate confining unit and intermediate aquifer
system along the west coast were placed in the
Tamiami Formation but are now considered part of
the Hawthorn Group (Missimer, 1978; Wedderburn
et al., 1982; Scott,1988). In the eastern part of the
district, Tamiami-equivalent sediments may form
the top of the intermediate confining unit
(Causaras, 1985).

The importance of the intermediate confining
unit and intermediate aquifer system in the western
part of SFWMD has led to a number of studies in
Charlotte, Lee, Hendry and Collier Counties
(Sutcliffe, 1975; Wedderburn et al., 1982; Knapp et
al., 1986; Smith and Adams, 1988). There are
three principle producing zones within the
intermediate aquifer system in this area, the
"Sandstone aquifer" named by Boggess and
Missimer (1975), the "mid-Hawthorn aquifer" of
Wedderburn et al. (1982) and the "lower-Hawthorn
aquifer" of Knapp et al. (1984). These producing
zones have been very important to the
development of southwestern Florida.

Floridan Aquifer System

The SEGS (1986) defines the Floridan aquifer
system as a

"thick carbonate sequence which includes
all or part of the Paleocene to Early [sic]
Miocene Series and functions regionally as
a water-yielding hydraulic unit. Where
overlain by either the intermediate aquifer
system or the intermediate confining unit,
the Floridan contains water under confined
conditions. Where overlain directly by the
surficial aquifer system, the Floridan may
or may not contain water under confined
conditions..."

"The top of the aquifer system generally
coincides with the absence of significant
thicknesses of (silici)clastics from the
section and with the top of the vertically
persistent permeable carbonate section.
For the most part, the top of the aquifer
system coincides with the top of the
Suwannee Limestone, where present, or
the top of the Ocala Group (Limestone)."

In limited areas the Avon Park Formation forms
the top of the aquifer system. Sediments of the
Arcadia Formation (Hawthorn Group), the Bruce
Creek Limestone, the St. Marks Formation or the
Tampa Member of the Arcadia Formation may
form the top of the Floridan aquifer system (SEGS,
1986).

"The base of the aquifer system in
panhandle Florida is at the gradational
contact with the fine-grained (silici)clastic
rocks belonging to the Middle Eocene
Series. In peninsular Florida, the base
coincides with the appearance of the
regionally persistent sequence of anhydrite
beds that lies near the top of the Cedar
Keys Limestone (Formation)." (SEGS,
1986).

The Floridan aquifer system exhibits extreme
variations in permeability resulting from a
combination of original depositional conditions,
diagenesis, structural features and dissolution of
carbonates and evaporites (Miller, 1986). The
system has been extensively altered by karst
processes in some areas of the state. Disso-
lutional and diagenetic processes have been
extremely important in the development of the

Floridan aquifer system from carbonate sediments
deposited during the Paleocene through Early
Miocene.

The thickness and lithology of the sediments
suprajacent to the Floridan determine the surficial
expression of the karst processes. On the Ocala
Platform from Hillsborough and Polk Counties
north to the state line, then westward into Leon
and Wakulla Counties and on the Chattahoochee
Anticline in Jackson and Washington Counties,
carbonate sediments of the Floridan aquifer
system crop out or are covered by a thin layer of
unconsolidated siliciclastics (Sinclair and Stewart,
1985). In these areas, the carbonates have been
exposed to extensive dissolution by aggressive
ground waters percolating downward from land
surface. Often the karst geomorphology has
reached a relatively mature stage of development
resulting in numerous surface depressions which
often coalesce. The Floridan aquifer system
exhibits well developed cavernous porosity and
conduit flow paths. Most of Florida's major
springs occur in this zone including Wakulla and
Silver Springs.

The carbonates of the Floridan aquifer system
lie beneath a variable thickness of post-Floridan
siliciclastics and carbonates of the intermediate
confining unit, intermediate aquifer system and the
surficial aquifer system on the flanks of the Ocala
Platform and the Chattahoochee Anticline.
Although karst processes have affected the
sediments of the Floridan in these areas, forming
dissolutional conduits and caverns, the karst
topography is not as well developed as in the
areas of thin cover. However, in these areas the
karst features are often of large diameter and
depth due to overburden thickness (Sinclair and
Stewart, 1985).

The Floridan aquifer system lies subjacent to a
thick sequence of post-Floridan sediments in the
Okeechobee Basin, Jacksonville Basin, Gulf
Trough, Apalachicola Embayment and the Gulf
Basin of the western panhandle. In these areas,
the carbonates of the Floridan have apparently not
been subjected to extensive karstification.
However, subsurface investigations of the
limestones indicate some karstic modification of
the sediments during subaerial exposure prior to
the deposition of the sediments of the inter-
mediate confining unit and intermediate aquifer
system (U. Hamms and D. Budd, University of
Colorado, personal communication, 1991).

The elevation of the top of the Floridan aquifer
system varies significantly throughout the state.
The top occurs at elevations in excess of 100 feet
above NGVD on the Ocala Platform and
Chattahoochee Anticline to depths greater than
1100 feet below NGVD in southern Florida and
1500 feet below NGVD in the western-most
panhandle (Miller, 1986). The thickness of the
Floridan varies from less than 100 feet in the
western half of the panhandle to in excess of 3500
feet in southwestern peninsular Florida (Miller,
1986).

The base of the Floridan aquifer system, the
sub-Floridan confining unit, varies stratigraphically
throughout the state. The SEGS (1986) indicates
that the base of the Floridan in the panhandle
occurs in the Middle Eocene approximately at the
top of the Claiborne Group. The base of the
system in the peninsula generally is considered to
occur within or near the top of the Paleocene
Cedar Keys Formation (SEGS, 1986). Miller (1986)
provides a more detailed picture of the variability of
the stratigraphic positioning of the Floridan aquifer
system base but indicates the same general
regional trends.

NORTHWEST FLORIDA WATER
MANAGEMENT DISTRICT

The Floridan aquifer system in NWFWMD
supplies more than 90 percent of the water
demand and is utilized in all the counties in the
district except Escambia and part of Santa Rosa
Counties (Wagner, 1988). It underlies the entire
district but is too saline for potable water in the
western end of the panhandle. The water quality
over a broad area corresponding to the
Apalachicola Embayment and the Gulf Trough and
the coastal zone may be affected by the upcoming
of mineralized waters (Scott et al., 1991).

The top of the Floridan aquifer system in
NWFWMD varies in elevation from 150 feet above
NGVD in Jackson and Holmes Counties to greater
than 1500 feet below NGVD in Escambia County
(Miller, 1986; Scott et al., 1991). The thickness of
the aquifer system ranges from approximately 100
feet thick in portions of Jackson and Holmes
Counties on the Chattahoochee Anticline to more
than 2800 feet thick in Franklin County in the
Apalachicola Embayment (Scott et al., 1991).

In the western part of the district, the Floridan
aquifer system is subdivided into an upper and

SPECIAL PUBLICATION NO. 34

lower aquifer separated by a confining unit, the
Bucatunna Clay. The confining unit thins and
pinches out towards the east in Okaloosa County,
where the Floridan becomes a single aquifer
(Marsh, 1966; Scott et al., 1991).

Carbonate sediments dominate the Floridan
aquifer system with minor occurrences of
siliciclastics. The siliciclastics generally occur
intimately mixed with the carbonates and are more
common in the upper portion of the aquifer
system. Within the district, the Floridan aquifer
system is composed of the Ocala, Marianna,
Suwannee, Chickasawhay, and Bruce Creek
Limestones and the St. Marks and Chattahoochee
Formations.

Stratigraphically, the base of the Floridan
aquifer system varies significantly throughout
NWFWMD. In the Pensacola area, the base
occurs within the Upper Eocene Ocala Limestone
(Miller, 1986). Under the eastern end of the
district, the base falls within the Paleocene Cedar
Keys Formation. The depth to the base of the
Floridan varies from -100 NGVD on the
Chattahoochee Anticline to -3100 feet NGVD in the
Apalachicola Embayment (Miller, 1986).

The Claiborne aquifer has been recognized
within the sub-Floridan confining unit. The total
extent of this aquifer is not known and it is not
often utilized (Allen, 1987). It is composed of
carbonate and siliciclastic sediments of the
Claiborne Group.

The effects of karstification are most intense
on and surrounding the Chattahoochee Anticline in
Jackson, Holmes and Washington Counties and
on the flank of the Ocala Platform in Leon and
Wakulla Counties. In these areas, the aquifer
system has been extensively altered by dissolution
and often has many direct conduits from the
surface into the Floridan. An extensive,
underwater conduit mapping project of the
Woodville Karst Plain by the Woodville Karst Plain
Project (Parker Turner, Florida State University,
personal communication, 1991) is currently
documenting the length and complexity of the
dissolutional features of the area.

SUWANNEE RIVER WATER
MANAGEMENT DISTRICT

The Floridan aquifer system occurs throughout

the SRWMD providing the vast majority of the
water supplies. The top of the Floridan ranges
from greater than 100 feet above NGVD in
Jefferson County to more than 300 feet below
NGVD in Bradford County (Scott et al., 1991). The
thickness ranges from approximately 1100 feet in
northern Jefferson County to 2200 feet in southern
Jefferson County (Miller, 1986). The thicknesses of
the Floridan aquifer system sediments in SRWMD
show the effects of the Apalachicola Embayment
and Gulf Trough in Jefferson County. These
sediments also exhibit the thicker carbonate
sequence deposited in the peninsular area.

Carbonate sediments deposited during the
Paleocene through the Early Miocene comprise the
Floridan aquifer system in SRWMD. The base of
the system occurs near the top of the Paleocene
Cedar Keys Formation (Miller, 1986). Carbonates
of the Oldsmar and Avon Park Formations, the
Ocala and Suwannee Limestones and the St.
Marks Formation comprise the Floridan aquifer
system in the district. The Suwannee Limestone
forms a portion of the Floridan in approximately
one half of the district while the St. Marks
Formation occurs in limited areas. When the
Suwannee Limestone and the St. Marks Formation
are absent, the Ocala Limestone forms the top of
the system. In the southern portion of the district,
the Ocala Limestone is absent due to erosion and
the Avon Park Formation forms the top of the
system.

The top of the sub-Floridan confining unit
generally occurs within the Cedar Keys Formation
throughout SRWMD (Miller, 1986). The positioning
of the permeability barrier shifts locally within the
upper part of the Cedar Keys Formation from the
top of the unit to some distance below the top.
The depth to the sub-Floridan confining unit varies
from -1200 feet NGVD on the Ocala Platform to
-2100 feet on the flank of the Gulf Trough (Miller,
1986).

The sediments of the Floridan aquifer system
throughout SRWMD have been greatly affected by
karstification. Sinkholes are very common in most
areas and numerous springs are scattered across
the district. The only area of minor karstification is
in northern-most Columbia and Baker Counties.

ST. JOHNS RIVER WATER
MANAGEMENT DISTRICT

The Floridan aquifer system is present

throughout the SJRWMD containing potable water
supplies in most areas. Salt water intrusion or
upwelling is a concern in many of the coastal areas
and along the St. Johns River Valley (Scott et al.,
1991).

The top of the Floridan aquifer system in
SJRWMD occurs at the highest elevations on the
flank of the Ocala Platform in Alachua and Marion
Counties. In this area, the uppermost Floridan
sediments range from 50 to more than 100 feet
above NGVD. The upper surface of the system
dips into the Jacksonville Basin, in the northern
part of SJRWMD, where it may be more than -550
feet NGVD. To the south, the top of the Floridan
reaches more than -350 feet NGVD (Scott et al.,
1991). The thickness of the system ranges from
approximately 1500 feet in Baker County
(northwestern SJRWMD) to 2900 feet in southern
Brevard County (Miller, 1986).

Carbonate sediments dominate the Floridan
aquifer system within the district. Siliciclastic
sediments, when present, occur mixed in with
carbonate lithologies and predominantly in the
uppermost portion of the Floridan. The Ocala
Limestone forms the top of the aquifer system over
the majority of the district. In very limited areas of
Volusia and Orange Counties, the Avon Park
Formation occurs at the top of the Floridan.
Sediments of Oligocene age occur at the top of the
aquifer system along the east coast in
southernmost Brevard County and in Indian River
County. Miller (1986) shows small outliers of
Suwannee Limestone at the top of the Floridan in
the northern portion of the district. The majority of
the aquifer system is comprised of the Avon Park
and Oldsmar Formations.

The sub-Floridan confining unit occurs within
the Cedar Keys Formation throughout the district.
The positioning of the base of the Floridan varies
from the top of the Cedar Keys Formation to within
the upper portion of the formation (Miller, 1986).
The top of the sub-Floridan confining unit varies
from -1600 feet NGVD on the flank of the Ocala
Platform to -3200 feet NGVD in the Jacksonville
Basin and the Okeechobee Basin (Miller, 1986).

Karst processes have significantly altered the
carbonates of the Floridan aquifer system in much
of the SJRWMD. Karst features are common in
much of the central and western portions of the
district (Sinclair and Stewart, 1985). The
karstification in these areas is related to dissolution
of the Ocala Limestone. In the southern half of the

district, dissolution of the carbonate fraction of the
Plio-Pleistocene sediments is responsible for the
development of some of the karst features.

SOUTHWEST FLORIDA WATER
MANAGEMENT DISTRICT

The Floridan aquifer system underlies the
entire SWFWMD area and contains plentiful,
potable water supplies throughout most of the
district. Areas of mineralized water along the coast
and in portions of Charlotte and Sarasota Counties
limit the availability of fresh water from the Floridan
in these areas (Scott et al., 1991).

The top of the Floridan aquifer system in the
SWFWMD displays two distinct elevational trends.
The northern two thirds of the district (from central
Polk and Hillsborough Counties northward) is
relatively flat with elevations varying from sea level
to between 100 and 150 feet above NGVD. The
top of the Floridan in the southern one third of the
district dips distinctly to the south dropping from
sea level to more than 750 feet below NGVD along
the southern district boundary (Scott et al., 1991).
These trends are related to the positions of the
Ocala Platform and the northern edge of the
Okeechobee Basin.

The thickness of the aquifer system also
displays distinct trends. The Floridan is more than
1400 feet thick in the northern-most portion of the
district and thins southward across the northern
one third of SWFWMD to approximately 600 feet
thick (Wolansky and Garbode, 1981). From the
thinnest point of the Floridan aquifer system, it
thickens into the Okeechobee Basin southward,
reaching more than 2400 feet thick in the
SWFWMD part of Highlands County (Wolansky
and Garbode, 1981).

As in the rest of the peninsula, carbonate
sediments dominate the Floridan aquifer system in
SWFWMD. Siliciclastic-bearing carbonates and
siliciclastic units in the basal Hawthorn Group may
form the upper portion of the Floridan in part of the
southern portion of SWFWMD. In much of the
district, the Suwannee Limestone forms the top of
the Floridan. In the northern most portion of
SWFWMD, the Ocala Limestone and, in limited
areas, the Avon Park Formation comprise the top
of the aquifer system. The Avon Park and Oldsmar
Formations form the main body of the Floridan in
the district. The sub-Floridan confining unit occurs
in the upper Cedar Keys Formation and varies from

FLORIDA GEOLOGICAL SURVEY

-1900 feet NGVD on the Ocala Platform to -4100
feet NGVD in the Okeechobee Basin (Miller, 1986).

Karstic alteration of the Floridan aquifer
system has occurred throughout much of the
district. In the southern portion of SWFWMD,
where the Hawthorn Group thickens in the
Okeechobee Basin, karst features are not as
abundant (Sinclair and Stewart, 1985). In the
northern two-thirds of the district and along the
Lake Wales Ridge, karst features are quite
common. Surficial karst features in much of
southern SWFWMD are the result of dissolution of
carbonate sediments and shell material in the
Miocene through Pleistocene units.

SOUTH FLORIDA WATER
MANAGEMENT DISTRICT

Potable water supplies within the Floridan
aquifer system in SFWMD are limited to the
northern part of the district. The sediments of the
Floridan occur throughout the district but in many
areas do not contain acceptable quality water.

The top of the Floridan aquifer system occurs
at elevations ranging from sea level in the northern
most edge of the district (Orange County) to
greater than 1100 feet below NGVD in south-
western SFWMD (Miller, 1986). Most of this area
lies in the Okeechobee Basin. The thickness of the
Floridan ranges from less than 2300 feet in Orange
County to more than 3400 feet under parts of Palm
Beach and Martin Counties and more than 3500
feet under western Lee County (Miller, 1986).

A thick sequence of carbonate sediments
containing some beds of siliciclastics and
siliciclastic-rich carbonates form the Floridan
aquifer system in SFWMD. The majority of the
sediments comprising the Floridan are carbonates
with little to no siliciclastics. However, in
southwestern Florida, sand beds have been noted
in the Ocala Limestone (Missimer, personal
communication, 1991). Siliciclastic-bearing
carbonates and a few siliciclastic beds from the
basal Hawthorn Group may form the upper beds of
the Floridan aquifer system in some areas of the
district. In general, the Suwannee Limestone
forms the upper unit of the aquifer system with the
Ocala Limestone and the Avon Park, Oldsmar and
upper Cedar Keys Formations comprising the main
mass of the system. The base of the Floridan
aquifer system, the top of the sub-Floridan
confining unit, occurs within the upper portion of

the Cedar Keys Formation (Miller, 1986). The top
of the sub-Floridan confining unit ranges from
-3000 feet NGVD on the northern edge of the
Okeechobee Basin to -4400 feet NGVD in the
deeper portion of the Okeechobee Basin.

The development of karst features in the
sediments of the Floridan aquifer system in
SFWMD has not been extensive. Throughout
much of the district, the Floridan contains saline
waters and has not been flushed by fresh water.
The Floridan aquifer system is also buried by as
much as 1100 feet of confining beds and other
aquifer systems under much of SFWMD.

REFERENCES CITED

Allen, T.A., 1987, Hydrogeology of the Holmes,
Jackson and Washington Counties area,
Florida: Florida State University (MS thesis),
183 p.

Barr, D. E. and Wagner, J. R., 1981, Recon-
naissance of the ground-water resources of
southwestern Bay County: Northwest Florida
Water Management District Technical
Publication 81-8, 47 p.

Boggess, D. H., and Missimer, T. M., 1975, A re-
connaissance of the hydrogeologic conditions
in the Lehigh Acres and adjacent areas of Lee
County, Florida: United States Geological
Survey Open File Report 75-55. 88 p.

Buono, A., Spechler, R. M., Barr, G. L., and
Wolansky, R. M., 1979, Generalized thickness
of the confining bed overlying the Floridan
aquifer, Southwest Florida Water Management
District: United States Geological Survey
Open File Report 79-1171, map plus text.

Causaras, C. R., 1985, Geology of the surficial
aquifer system, Broward County, Florida:
United States Geological Survey Water-
Resources Investigations Report 84-4068,
map plus text.

Ceryak, R., Knapp, M. S., and Burnson, T., 1983,
The geology and water resources of the upper
Suwannee River Basin, Florida: Florida Bureau
of Geology Report of Investigation 87, 165 p.

Knapp, M. S., Burns, W. S., Sharp, T. S., and Shih,
G., 1984, Preliminary water resource
assessment of the mid and lower Hawthorn
aquifers in western Lee County, Florida: South
Florida Water Management District Technical
Publication 84-10, 106 p.

Knapp, M. S., Burns, W. S., and Sharp, T. S., 1986,
Preliminary assessment of the groundwater
resources of western Collier County, Florida:
South Florida Water Management District,
Technical Publication 86-1, 142 p.

Marsh, O. T., 1966, Geology of Escambia and
Santa Rosa Counties, western Florida
panhandle: Florida Geological Survey Bulletin
46,140 p.

Miller, J. A., 1986, Hydrogeologic framework of the
Floridan aquifer system in Florida and parts of
Georgia, Alabama and South Carolina: United
States Geological Survey Professional Paper
1403-B, 91 p. plus maps.

Missimer, T. M., 1978, The Tamiami Formation-
Hawthorn Formation contact in southwest
Florida: Florida Scientist, v. 41, p. 31-38.

Scott, T. M., 1988, The lithostratigraphy of the
Hawthorn Group (Miocene) of Florida: Florida
Geological Survey Bulletin 59,148 p.

Scott, T. M., Lloyd, J. M., and Maddox, G.
(editors), 1991, Florida's Ground Water Quality
Monitoring Program-Hydrogeological
framework: Florida Geological Survey Special
Publication 32, 97 p.

Scott, T.M., 1991, A geological overview of Florida:
in Scott, T. M., Lloyd, J. M., and Maddox, G.
(editors), 1991, Florida's Ground Water Quality
Monitoring Program Hydrogeological
framework: Florida Geological Survey Special
Publication 32, p. 5-14.

Shaw, J. E. and Trost, S. M., 1984, Hydrogeology
of the Kissimmee planning area, South Florida
Water Management District: South Florida
Water Management District Technical
Publication 84-1, part 1, 235 p.

Sinclair, W. C., and Stewart, J. W., 1985, Sinkhole
type, development and distribution in Florida:
Florida Geological Survey Map Series 110,
scale 50 km to 1 inch.

Smith, K. R., and Adams, K. M., 1988, Ground
water resource assessment of Hendry County,
Florida: South Florida Water Management
District Technical Publication 88-12, 109 p.
plus appendices.

Southeastern Geological Society (SEGS) Ad Hoc
Committee on Florida Hydrostratigraphic Unit
Definition, 1986, Hydrogeological units of
Florida: Florida Geological Survey Special
Publication 28, 8 p.

Sutcliffe, H., Jr., 1975, Appraisal of the water
resources of Charlotte County, Florida: Florida
Bureau of Geology Report of Investigations 78,
53 p.

Vacher, H. L., Jones, G. W., and Stebnisky, R. J.,
1990, The need for lithostratigraphy: How
heterogenous is the surficial aquifer?: in
Allmon, W. and Scott, T. (compilers), Plio-
Pleistocene stratigraphy and paleontology of
South Florida: Southeastern Geological
Society, Guidebook 31, Annual Field trip 1990.

Wagner, J. R., 1988, Fundamental ground water
conditions within the Northwest Florida Water
Management District: Northwest Florida Water
Management District, Public Information
Bulletin 88, 24 p.

Wedderburn, L. A., Knapp, M. S., Waltz, D. P., and
Burns, W. S., 1982, Hydrogeologic recon-
naissance of Lee County, Florida: South
Florida Water Management District Technical
Publication 82-1,192 p. plus appendices.

White, W.A., 1970, Geomorphology of the Florida
peninsula: Florida Bureau of Geology Bulletin
51,164 p.

Wolansky, R. M., and Garbode, J. M., 1981,
Generalized thickness of the Floridan aquifer,
Southwest Florida Water Management District:
United States Geological Survey Open File
Report 80-1288, map plus text.

SPECIAL PUBLICATION NO. 34

Chapter IV

QUALITY OF WATER IN FLORIDA'S
AQUIFER SYSTEMS

Sam B. Upchurch

Department of Geology
University of South Florida
Tampa, Florida

INTRODUCTION

Scope

This chapter discusses the quality of water in
Florida's aquifer systems. The database used is
the Background Network developed by DER and
the water management districts. Some data have
been omitted from the maps and analyses
because they either do not meet quality-assurance
standards in terms of ion balance1 [Note: endnotes
are located at the end of each chapter] or they are
radically different from data collected at nearby
wells and, therefore, cannot be proven to reflect
background water quality.

Three aquifer systems2 are included in the
report: (1) surficial aquifer system, (2) intermediate
aquifer system and intermediate confining unit
(hereafter termed the intermediate aquifer system),
and (3) Floridan aquifer system. The distributions
of these aquifer systems in Florida are discussed in
Chapter III of this volume and in Scott et al. (1991).

The discussion and maps are arranged by
water management district3. Alachua County
participated as a separate entity during parts of the
data collection and analysis phases of the data.
For convenience, the Alachua County data are
included with the SRWMD. The tables contain data
on the number of samples collected, not number
of wells. Surficial aquifer system results are divided
into (1) Sand and Gravel Aquifer, (2) Biscayne
Aquifer, and "Other", which includes all of the
samples not classified as coming from either the
Sand and Gravel or Biscayne Aquifers.

The data presented below can be used in a
number of ways. They can be used as a baseline
for evaluation of the condition of water quality in
the aquifer systems of the state as they existed

between 1985 and 1988, and for monitoring water-
quality changes in the future, which is part of the
intent of the 1983 Florida Water Quality Assurance
Act. While there is not sufficient data for site
specific determinations, the data can also be used
to assess general background conditions for
permitting, risk assessment, and contamination
evaluation. Finally, the data can be used to
evaluate ground-water flow systems, ability of the
aquifer systems to tolerate contamination, and
potential best uses of the waters. These last uses
require some understanding of how ground-water
chemical systems behave. To this end, the
following discussions summarize some of the
chemical behaviors of the aquifer systems, and it
discusses some ways that the aquifer systems
adjust chemically to contamination and mixing of
waters with different compositions.

Throughout this chapter, references to various
geomorphic features and surface-water basins are
made when discussing ground-water quality.
Appendix 3, figures 3 to 12 show the locations of
these features.

Chapter Organization

The discussions of individual analytes in this
report are organized into four major topics:

General Descriptors the variables that
describe the conditions under which the
chemicals occur, specifically temperature
and pH;
Cations including calcium, magnesium,
sodium, potassium, iron, mercury, and
lead;
Anions including bicarbonate and
carbonate, chloride, sulfate, fluoride,
nitrate, and phosphate; and
Other Constituents including total
dissolved solids, conductivity, total organic
carbon, and synthetic (man-made)
organic.

The discussions for each analyte are broken
down into aquifer system and then into regions
(water management districts), as needed.

Comparison of Map and Table Data

Comparison of the analyte-distribution
maps and tables in the following text will indicate
some discrepancies. These discrepancies are a

result of the dual goals of the Background
Network. The maps represent the program's best
interpretations of analyte distribution, while the
tables reflect GWIS database and an uncritical
review of the data contained therein.

The maps were prepared from the best
available data. Outliers and data that failed the
nearest-neighbor criteria were excluded from the
maps. As a result, many of the "bulls eye" contour
lines typical of representations on unevaluated
data are avoided. The map contours represent the
best assessment of data distributions, although
some of the maxima and minima represented in
the tables are not displayed. The maps, therefore,
represent conservative assessments of the
distributions of analytes.

The tables reflect the distribution of data in the
GWIS database at the time of report preparation.
Maximum and minimum values are reported
regardless of whether they fit the nearest-neighbor
and outlier criteria. Unreasonable maxima and
minima are evaluated in the text in the context of
how valid they are. In most cases, they can be
shown to represent well-construction problems or
values that cannot be considered until confirmed
by re-sampling and analysis. The tables give the
reader a "feel" for the range of data in the GWIS
database.

Use of median and quartile population
descriptors avoids problems with inclusion of the
outliers and represents the best possible indi-
cations of water quality. Users are cautioned to
examine the GWIS database and be selective in
use of these data until confirmations are made with
the second round of sampling.

VARIABLE DESCRIPTION
CONVENTIONS

Nature of Data Distributions

Environmental, geochemical data are seldom
normally distributed (Ahrens, 1954a,b). That is,
they normally do not reflect the classic "bell-
shaped" curve for which the standard distribution
descriptors (mean, standard deviation, variance,
etc.) are intended. Geochemical data are typically
characterized by a large number of samples with
low concentration values and a few samples with
high concentrations. This leads to a distribution
that is skewed. That is, the distribution is

asymmetrical, with a long tail that extends towards
the high concentrations. If mean and standard
deviation were used to describe a skewed
distribution, the mean would be high relative to the
most abundant concentrations, thus giving a false
impression of the quality of water in the aquifer
system.

Distribution Descriptors

In order to better represent the distributions of
variables, the median, quartiles, minima, and
maxima are used, where appropriate. The median
is the 50th percentile. Half the samples are below
the median in concentration, the other half above.
The median is used in this report to compare water
quality between aquifer systems and regionally.

The quartiles represent 25 and 75 percent of
the samples. The 25th percentile (lower quartile or
4, Qrtile in the tables) represents the value below
which 25 percent of the samples occur. Con-
versely, the 75th percentile (upper quartile or t
Qrtile on the tables) represents the value above
which only 25 percent of the samples occur. The
maximum and minimum are also given in the
tables, so a complete representation of the
distribution is available. Where a standard or
guidance concentration is available, the number of
samples that exceeded that value is also
represented in the tables.

AQUIFER CONTROLS ON
GROUNDWATER CHEMISTRY

Factors That Control
Ground-Water Chemistry

Before discussing the water chemistry of
Florida aquifer systems it is important to briefly
discuss the factors that affect ground-water
chemistry. These factors include:

Precipitation chemistry,
Surface conditions at the site
of recharge,
Soil type in the recharge area,
Mineralogy and composition of the
aquifer system,
Nature of aquifer system porosity
and structure,
Flow path in the aquifer system

FLORIDA GEOLOGICAL SURVEY
./

Residence time of water in the
aquifer system,
Mixing with other waters in the
aquifer system, and
Aquifer microbiology.

Precipitation Chemistry

The first major factor that affects the chemical
composition of ground water is the chemical
composition of precipitation. Precipitation, which
recharges the aquifer systems, is important as a
source of dissolved chemical species and as an
acid that induces chemical reactions in the aquifer
systems.

Natural rainfall is affected by reactions with
atmospheric gases and particulates and by
proximity to the sea. Natural rainfall is slightly
acidic. It gains acidity by the reaction of water
(H20) with carbon dioxide (CO2) to form carbonic
acid (H2CO3) according to the reactions:

CO, + HO = H2CO3

HCO3 = H, + HCO;.

As shown in reaction 1, carbonic acid
dissociates into hydrogen ion (H*), which is the
source of acidity, and bicarbonate (HCO;). If
rainwater is fully equilibrated with atmospheric
carbon dioxide, the resulting pH4 is approximately
5.5 to 5.7 at 250C. If atmospheric moisture has not
completely equilibrated with atmospheric carbon
dioxide, pH will be somewhat greater than 5.5.
This is the same level of acidity as soda pop, and
this moderately acidic precipitation has been
responsible for rock weathering over geologic time.

"Acid rain" is a problem caused by intro-
duction of sulfur and nitrogen gases into the
atmosphere as fossil fuels are consumed. These
gases react with water in reactions similar to
reaction 1 to form nitric acid (HNOJ) and sulfuric
acid (H2SO4) and further lower the pH of rainfall
(Table 3). Acid rain has been produced in the U.S.
for less than 200 years. Because of the long times
involved with ground-water transport and the high
buffering capacities of limestone- and dolostone-
rich aquifers, only a small amount of Florida's
ground water has been affected by acid rain. The
data presented in this report suggest that the
surficial aquifer system and near-surface portions

of the intermediate and Floridan aquifer systems
have been locally affected by acid rain where
recharge is rapid and buffering capacities are
minimal. The effects of acid rain on aquifer water
quality cannot yet be evaluated because no
suitable background water-quality database for
comparison has existed until this time.

Florida's climate can be classified as a
"maritime climate". The proximity of all parts of the
state to the sea has a profound affect on rainfall
chemistry. Sea spray is generated by the wind and
transported inland as an aerosol. This aerosol is
mixed with precipitation so that Florida rainfall is a
very dilute mixture that has the ionic proportions of
sea water (Figure 5).

Table 3 summarizes the chemical quality of
rainfall in Florida (National Atmospheric Deposition
Program (IR-7)/National Trends Network, 1990).
Note that there is a small amount of all of the major
ground-water chemicals in average rainfall. These
data also show that there is some nitrate and
sulfate presently being introduced as acid rain.

The last column in Table 3 is the deviation
(difference) of the sodium to chloride mole ratio
from that of average sea water. The coastal and
near-coastal stations (Quincy, Kennedy Space
Center, Verna Well Field, and Everglades National
Park) have average deviations from sea-water
composition of five percent or less. More inland
sites (Austin-Cary Forest, Bradford Forest) have
average deviations from sea water compositions of
54 and 20 percent, respectively. The significance
of these larger deviations should not be exag-
gerated, however, as the individual data points
show a significant grouping near the sea-water
ratio. The means are skewed because of a few
data points that may reflect anomalous conditions
or analyses.

Therefore, as a general rule, newly fallen pre-
cipitation, uncontaminated surface runoff, and
uncontaminated soil waters in Florida have the
initial compositions of dilute sea water. Based on
the differences in chloride concentrations, these
waters average about 0.009 percent (1.66 mg/L in
precipitation, 19,350 mg/kg in sea water) sea
water. While the total dissolved solids contents
are low, the ratios of dissolved metals to chloride,
especially sodium to chloride, are nearly constant
and reveal an origin as marine aerosols.

As this dilute sea-water solution evaporates

from the land surface and vadose zone5 soils or is
transpired by plants, the chemicals dissolved in the
water are concentrated. Evaporation also occurs
from the water table, especially where it is shallow
and in porous and permeable aquifers. As will be
shown below, the increase in dissolved solids
content by evapotranspiration is an important
starting point in the evolution of aquifer waters.

Surface Conditions

Surface conditions have a pronounced effect
on ground-water chemistry, especially in the
surficial aquifer system and in unconfined portions
of the Floridan aquifer system. Land use, for
example, can have a dramatic effect, including
introduction of waste heat and contaminants. The
effects of human activities have been avoided,
where possible, in design of the Background
Network. However, natural conditions can also
affect ground-water quality.

Natural surface features that can have
significant effects on ground water include: (1)
lakes, swamps, and marshes, (2) sinkholes and
sinking streams, and (3) proximity to the sea and
tidal influences. Lakes, swamps, and marshes can
serve as sources of natural organic, metals, and
low pH water. Sinkholes, sinking streams, and
other karst features can introduce surface waters
into deeper portions of the aquifer system (Ceryak,
1977). The sea is a source of sodium, chloride and
other constituents, which can enter the ground
water through canals, river mouths, and other
regions where the fresh-water potential is
insufficient to prevent intrusion. Saline water can
also intrude laterally and vertically when fresh-
water potential is reduced by human activity.

Soil Type in Recharge Area

As precipitation percolates into the soil and
aquifer environment, the weak acids react with the
minerals of the soil or rock and with organic. The
uppermost soil zone, where plant growth is active,
is characterized by an accumulation of plant debris
(humus), which is decomposed by soil microbes. If
the soils are aerated, these microbes produce
carbon dioxide (CO), which combines with water
according to reaction 1 to form additional carbonic
acid and further lower pH. In addition, the partly
decomposed organic material often includes
water-soluble fractions, including fulvic acids.
These organic contain abundant hydrogen as
acid radicals. The added carbonic and organic

acids lower the water pH to values that are
commonly less than four. If the soils are wet and
chemically reducing, the microbes produce
organic acids and methane gas (CH4), rather than
carbon dioxide. Under wet, reducing conditions,
microbial destruction of humus is retarded, and
peats and mucks form as soil components.

Therefore, recharge through wet, lowland or
dry, upland soils will affect local ground-water
chemistry differently. The nature of plant cover,
supply of humus, moisture content, and soil
temperature affect both the availability, quantity,
and chemistry of humic substances and the
microbial populations that feed upon these
substances.

Soil and Aquifer Mineralogy

Once water has passed through the humus
zone, it is characteristically acidic, and it can react
with minerals in the soil or rock. The reaction
(modified from Goldich [1938] and assuming that
the reaction is with carbonic acid) can be
generalized as

Mineral + H2CO, (2)
= Cations + HCO + Residue.

Cations6 are the metals found in the soil or
aquifer minerals, and bicarbonate is the dominant
anion. The residue forms if the mineral contains
aluminum or oxidized iron (Fe3+), which are often
relatively immobile in ground and soil waters.

To illustrate these reactions, we can compare
the reaction of acidic soil water with calcite, the
primary mineral in limestone, to a reaction with
potassium feldspar, a common, aluminum-bearing
mineral that is present in small amounts in Florida
quartzose sands. The reaction with calcite is

CaCO3ca4e + H,CO3
SCa2;q + 2HCOaq;

In this reaction, dissolved calcium and bi-
carbonate are produced. There is no residue
because neither aluminum nor iron is present in the

SPECIAL PUBLICATION NO. 34

mineral. In reality, natural limestones usually
contain other minerals that may leave residues
upon rock weathering.

Potassium feldspar (KAISi3Os) reacts with
carbonic acid according to

2KAISi,3O,8 K + 2HCO3 caroaci
+ 9H,O = A12Si20O(OH)4 kaol,, (4)
+ 2Ka + 4H4SiO4a
+ 2HCO- bicanae

In this reaction, aluminum and some silica remain
to form the common soil clay mineral kaolinite
(AISi2Os(OH),). Silica is also mobilized as silicic
acid (H4SiO4), and potassium (K) is a dissolved ion
cation. The H+ in reaction 4 comes from carbonic
acid through reaction 1, so bicarbonate is also
produced. In tropical to subtropical climates,
kaolinite can be further weathered according to

A1lSi,2O(OH)4 Kao,. + 5HO
= A1220.3H20gibbsite + 2H4SiO4.

with release of additional silica as silicic acid.
Gibbsite (AI,20.3H20) is one of several common
aluminum oxyhydroxides found in Florida soils.
Both kaolinite and gibbsite are residues in the
Goldich weathering reaction (reaction 2). Aluminum
can also be mobilized in ground water if the waters
are acidic or organic rich.

Iron and aluminum oxyhydroxides, sulfide and
sulfate minerals, and a few other minerals are
weathered by different processes, which are dis-
cussed with the individual analytes below. Clay
minerals and some of the oxyhydroxides are also
important as sites for ion exchange, which may
also affect ground-water quality.

When water sequentially passes through rocks
with different mineral compositions, the chemistry
of the resulting water reflects the compositions of
all previous contact, not just the rock type from
which the water was sampled. For example, water
from the Floridan aquifer system that has passed
through the confining beds of the Hawthorn Group
contains fluoride, magnesium, silica, and other

constituents that were derived by weathering of
Hawthorn minerals (Lawrence and Upchurch,
1976).

Table 4 lists the compositions of common
minerals found in Florida aquifer systems and
confining beds, and their dissolved weathering
products according to the weathering reaction
(reaction 2). Most of these minerals are weathered
slowly, so an important factor in determining how
much of the weathering product enters the ground
water is the length of time the water is in contact
with a particular rock type (e.g., the residence
time).

Table 5 lists the most common minerals found
in Florida aquifer systems and confining beds. The
surficial aquifer system is composed
predominantly of sand from the south-central part
of the peninsula to the western end of the
panhandle. This sand is primarily composed of the
mineral quartz, which is essentially chemically inert
(Table 4). The surficial aquifer system in coastal
areas contains varying amounts of shell and sand-
to silt-sized calcite and aragonite. The
calcite/aragonite content of the aquifer system
increases to the south, and the Biscayne Aquifer of
southeastern Florida is predominantly carbonate.
The surficial aquifer system contains highly
variable amounts of clay, oxyhydroxides, and
humic material (Table 5), all of which are reactive
and have the ability to sorb metals and some
anions. Iron oxide coatings, which have some
sorption capacity, are common on the sands. In
all, the sand-rich surficial aquifer system has the
ability to sorb moderate amounts of metals and
anions. In addition, the carbonate-rich portions of
the aquifer system have the ability to consume
(buffer) acidity by reactions similar to reaction 3.

The intermediate aquifer system and confining
beds comprise a complex array of materials (Table
5) with important consequences for ground-water
quality. Strata of the Hawthorn Group include
interfingering beds of clay, sand, dolostone,
limestone, and phosphorite (Scott, 1988).

The clays include a predominance of
magnesium- and iron-rich smectite, palygorskite,
and sepiolite (Weaver and Beck, 1977; Reik, 1982;
Scott, 1988). Weathering of these minerals
releases iron, magnesium, and silica, and
produces kaolinite as a residue (Table 4). Clay-rich
horizons often contain opaline material (opal-A and
opal-CT; Jones and Segnit, 1971), which also
releases silica upon weathering. The clays have

very high sorption capacities and can effectively
bind most metals. As a result of chemical
weathering, the Hawthorn Group introduces
numerous constituents into the ground-water
system.

Dolostone and limestone contain the minerals
dolomite and calcite (Table 4). Reactions with
these carbonate minerals (e.g., reaction 3) buffer
the acidity of ground water and release calcium
and magnesium into the system (Table 4).

The primary phosphorite deposits are
composed of carbonate-fluorapatite, while
weathered and reprecipitated deposits contain
carbonate-hydroxylapatite (Table 4). These apatite
minerals contain fluoride, phosphate, and small
amounts of uranium (Altschuler et al., 1958), which
are released upon weathering.

Finally, the Hawthorn contains a number of
trace constituents that may be locally important. It
contains widespread but small quantities of pyrite
(Table 4), which release iron and sulfur as sulfate or
sulfide to ground water. The sulfate may be in the
form of sulfuric acid (H2SO4), and the sulfide may
be as hydrogen sulfide (H2S), which imparts a
"rotten egg" odor to water. Pyrite, and possibly
other sulfide minerals, can release small quantities
of trace metals and arsenic (which is present in
some metal sulfides) upon weathering. The
Hawthorn also contains gypsum, a source of
sulfate (Table 4), at scattered localities7. This
gypsum has not been previously described, nor
has its origin been determined. Rosette-shaped
clusters of gypsum, especially those which have
been replaced by chalcedonic quartz, are probably
a primary deposit indicative of playa lake or
coastal, evaporitic basin conditions. The other
occurrences do not have diagnostic crystal forms,
but the nature of their occurrence suggests that
the gypsum may have formed as a result of pyrite
oxidation.

The mineral assemblage of the Floridan aquifer
system is less complex than the other aquifer
systems (Table 5). The predominant minerals are
calcite in limestone and dolomite in dolostone
(Table 4). Dolomite is widespread. Significant
portions of the Floridan in the SRWMD and
SFWMD are dolomitic. Also, large sections of
Middle Eocene to Paleocene strata (particularly the
Avon Park and Oldsmar Formations) of the
Floridan aquifer system are dolomitic. Where the
lower Hawthorn Group carbonates are part of the
Floridan (Scott et al., 1991) dolostone is likely to be

significant. Calcite predominates in the Suwannee
and Ocala Limestones elsewhere. Water which
has equilibrated with the Floridan aquifer system,
therefore, includes calcium, magnesium, and
bicarbonate as dominant chemical species. The
pH of Floridan aquifer system water is buffered by
dissolution of carbonate minerals and generally
ranges from 7 to 8.

The base of potable water in the Floridan is
variable. The base of the aquifer system, which
varies from the lower Avon Park Formation,
through the Oldsmar Formation, to the top of the
Cedar Keys Formation (Scott et al., 1991) is usually
dolomitic. The base of the aquifer system is
characterized by reduced permeability, partly as a
result of the presence of intergranular gypsum and
anhydrite (Tables 4, 5). Therefore, water that has
come in contact with the base of the aquifer
system contains sulfate as a major constituent.

Nature of Aquifer System
Porosity and Structure

Porosity, the nature and amount of pore space
in an aquifer system, and structure, the distribution
of large-scale fractures, joints, faults and karst
features affect the ability of water to react with
rock materials. This is because the size and
geometry of the pore space controls the amount of
reactive mineral surface area with which the water
comes in contact. Where faults, fractures, and
large caverns exist, contact of water with rock is
minimized and water can travel through an aquifer
system without significant changes in composition.

INTERGRANULAR POROSITY

All of Florida's aquifer systems contain
significant intergranular porosity. Intergranular
porosity dominates in coarse siliciclastic8 aquifers,
such as occur in the sands and gravels of the
surficial and parts of the intermediate aquifer
systems. With intergranular porosity, the pores
through which water passes are the spaces
between sand grains. The passages (pore throats)
between adjacent pores are often small, and sandy
aquifers can be excellent mechanical filters for
microbes, and small particles of humic material,
oxyhydroxides, and clays. When the pore throats
become plugged, permeability is reduced. Fine to
medium sand aquifers have moderate areas of
mineral surface in contact with the water, and,
while quartz is inert, coated sand grains, interstitial
clays, or interstitial oxyhydroxides can chemically
interact with the water.

FLORIDA GEOLOGICAL SURVEY

Where the surficial aquifer system is
composed of quartz sand and gravel, porosity is
intergranular. White, loose, "sugar sands" have
little interstitial material or grain coatings, so they
have little ability to sorb and bind dissolved
chemicals. Brown, coated sands and clayey sands
have moderate sorption capacities which can
improve water quality to a limited extent. As the
clay and carbonate contents of the surficial aquifer
system increase, porosity remains intergranular,
but permeability is reduced. The clays and
carbonates are reactive and the low water/mineral
surface area ratio suggests that chemical
interactions are increased in comparison to pure
sands.

The siliciclastic beds of the Hawthorn Group
constitute part of the intermediate aquifer system.
These sands contain significant intergranular clay
and phosphatic sediments that range from gravel
to clay sizes. The phosphate minerals (carbonate-
fluorapatite and carbonate-hydroxylapatite) are
sources of weathering products (Table 4), such as
dissolved phosphate and fluoride. The clays are
also subject to weathering, and they also have high
sorption capacities, which assist in retarding the
movement of certain metals in ground water
(Upchurch et al., 1991). The clays yield magnesium
and silica upon weathering.

Intergranular porosity in the carbonate-
mineral-rich Biscayne Aquifer and the Floridan
aquifer system is between carbonate grains, which
are chemically reactive. These aquifer systems are
doubly porous, with both primary, intergranular
porosity and secondary, fracture- or karst-related
porosity. Due to the presence of secondary
dissolution features and fractures, the intergranular
porosity of these aquifer systems may be signi-
ficantly less important in terms of water flow than
the larger openings. As a result, these aquifer
systems may contain blocks of rock where water
quality is dominated by the reactions associated
with the high grain-surface areas characteristic of
intergranular flow systems. These blocks are
separated by zones of cavernous or fracture
porosity that have distinctly different water
compositions.

CAVERNOUS, VUGGY,
AND FRACTURE POROSITY

The Biscayne Aquifer, carbonate aquifers of
the intermediate aquifer system and limestone and
dolostone of the Floridan aquifer system are
doubly porous. That is, they contain two types of

porosity at two different scales. Intergranular
porosity is present in most areas, but the
permeability associated with intergranular porosity
is significantly less than that characteristic of
larger, interconnected cavities, such as caverns,
vugs, fossil molds, and fractures. Therefore, unless
a well is located in the middle of a block of rock
that is affected only by intergranular porosity or in
which the larger pores are unconnected, water
quality in the well reflects the more productive,
larger interconnected pore space. When water
passes through larger cavities, the water may not
come in contact with rock, or it may have a short
time in contact with the rock. The water has little
opportunity for chemical interactions with the rock
and it retains its chemical character inherited from
precipitation, soils and human impacts, or earlier
rock contacts. For example, Upchurch and
Lawrence (1984) were able to identify a plume of
surface water within the Floridan aquifer system
that originated from Alligator Lake in Lake City,
Columbia County. The plume extended several
kilometers south of the lake along a prominent
fracture trace.

Structure of the clay-rich confining beds of the
Hawthorn Group is also important. Vertical
leakage through the confining beds is most
efficient where these beds are breached by
sinkholes or where the clays have developed
blocky fractures.

Aquifer System Flow Path and
Residence Time

The length, depth, and tortuosity of the flow
path a body of water follows in an aquifer system
profoundly affect the quality of water. In general,
shallow, short flow paths, which are characteristic
of the surficial aquifer system, result in low
residence times for chemical reactions to go to
completion. Also, short flow paths result in
contact with a limited number of different aquifer
minerals and less opportunity for chemical
composition to be altered. Consequently, total
dissolved solids contents are less than those
anticipated for longer flow-path systems. By the
same reasoning, short flow-path systems are more
vulnerable to contamination because of lack of (1)
contact with reactive aquifer minerals or (2)
sufficient time for chemical reactions to occur.

If the flow path is long (on the order of tens of
kilometers), reactions between rock and water
become more probable and the total dissolved
solids content of the water increases as a result of
continued rock weathering. Flow paths of the

Floridan aquifer system in central Florida are
characteristically long, and changes in composition
along the flow paths reflect chemical maturation9
as reactions occur.

Residence time is, therefore, the length of time
the water has been in contact with a particular rock
type. Residence time is a function of hydraulic
head, bulk permeability, and flow path length.
Residence time decreases if either head or
permeability increases. If water passes through
rock rapidly, there may not be sufficient time for
the rock to interact with the water. On the other
hand, a long residence time may allow sufficient
time for full chemical equilibration of the water and
rock. In the first instance, the rock will have little
effect on the water; in the second, the effect may
be considerable.

Typical residence times range from days to
thousands of years depending on the nature of the
flow system. Residence times in the surficial
aquifer system range from days to perhaps
hundreds of years. The flow systems are short
(T6th, 1962, 1963), with primary discharge to local
wetlands, lakes, streams, and canals. It is not
possible to predict residence times in the
intermediate aquifer system because of the
complexity of pathways through the lithologically
diverse Hawthorn Group. Water that passes
through sinkholes, fractures, and karst conduits
that penetrate the Hawthorn may have brief
residence times. Conversely, water that passes
along the tortuous pathways in the clay-rich
horizons may have residence times of thousands
to tens of thousands of years. Residence times in
the Floridan aquifer system also cover a wide
range of time. Short times have been recorded for
some sinking streams and resurgences in north
and central Florida (e.g., Ceryak, 1977). These
short residence times are associated with conduit
flow over distances of a few kilometers. In
contrast, Hanshaw et al. (1965) used 14C dating
methods to approximate residence times in the
regional flow system of the Floridan aquifer system
in the west-central part of the state. They found
residence times in excess of 30,000 years for flow
from northern Polk County to coastal Sarasota
County. This long residence time is associated
with ground-water velocities as high as 2 to 8
meters per year (Hanshaw et al, 1965).

Mixing with Other Waters in the
Aquifer System

Mixing of waters in aquifers is common. Most
of the time mixing is of little consequence. When,

however, the mixing is between waters that differ
significantly in chemistry, important changes in
composition and reactivity of the mixture may
result (Runnels, 1969). There are two situations in
Florida where natural mixing (as opposed to mixing
of contaminated water with native water) is known
to be important. These are (1) mixing along the
fresh-water/sea-water transition zone10 in coastal
areas and (2) mixing of fresh and saline waters
near the base of the upper Floridan aquifer system.

In both of these settings the mixtures have
been shown to have increased ability to react with
the host limestones and dolostones. Hanshaw
and Back (1971a,b), Badiozomani (1973), and
Plummer (1975), for example, showed that mixing
in the transition zone can cause dissolution of
calcite and, under certain circumstances,
precipitation of dolomite. The dissolution of calcite
can lead to development of karstic features
(caverns, enlarged fractures, etc.) and formation of
collapse breccias. Precipitation of dolomite in the
space created by dissolution of calcite is thought
by many (Hanshaw and Back, 1971a,b;
Badiozomani, 1973) to be an important
dolomitization mechanism.

Aquifer Microbiology

All aquifers contain microbes (bacteria, fungi,
and other organisms) that play important roles in
the chemistry of the water. To survive, these
microbes require sources of organic carbon,
nitrogen, phosphorus, sulfur, iron, and other
nutrients. Because these nutrients are normally
introduced into the aquifer systems near the land
surface, the microbes are most abundant there,
and they decline in abundance with depth.
Microbial utilization of these nutrients results in a
number of changes in aquifer chemistry. These
changes are discussed in detail with the individual
analytes (below). The most important roles
microbes play in maintaining aquifer chemistry
deal with transformation of nitrogen, iron, and
sulfur species. For example, sulfate-reducing
bacteria transform sulfate (SO42-) to hydrogen
sulfide (H2S) according to

SO + 2C ganic+2H20
microbial

activity
H2S + 2HCO3.

SPECIAL PUBLICATION NO. 34

By this reaction, organic carbon and sulfate
are consumed and hydrogen sulfide and
bicarbonate produced. Since bicarbonate is also a
product of weathering of calcite, addition of
bicarbonate through sulfate reduction will tend to
drive reaction 3 to the left and may induce calcite
precipitation. Thus, microbial activity may result in
a complex array of "spin off" reactions that affect
major and minor element compositions of ground
water.

DEFINITION OF HYDROCHEMICAL
FACIES

The net result of the factors discussed above
is that the composition of ground water reflects its
recharge and flow history. Shortly after recharge
the chemistry may be highly variable, but with
chemical maturation, as a result of increasing
residence time, the compositions become more
uniform. The final compositions reflect interactions
with the major rock types along the flow paths.

Part of this complex hydrochemical history of
ground water can, therefore, be determined by
examining the regional composition and variability
of the water. Broad regions of an aquifer system
that can be shown to contain water with relatively
uniform major-element compositions are repre-
sented by a particular hydrochemical "facies"11.
Methods of identification and mapping of
hydrochemical facies have been described by
Back (1961, 1966). As an example, waters of the
Floridan aquifer system which have been in con-
tact with limestones are likely to belong to the
calcium-bicarbonate facies as a result of limestone
weathering. Water in a quartzose portion of the
surficial aquifer system may belong to a sodium-
chloride facies derived from the marine aerosols
that give precipitation its chemical character.

PREVIOUS WORKS

The aquifer systems of Florida have been
intensely studied for over 50 years. Previous
studies have largely been directed toward aquifer
systems that are heavily used. As a result, large
areas of the state that are characterized by low
populations and aquifer systems with little use
have been neglected. With the exception of the
Biscayne Aquifer and, to some extent, the Sand-
and-Gravel Aquifer, very few studies have been
directed toward the surficial aquifer system. The
intermediate aquifer system has also been
somewhat neglected. In contrast, the Floridan

aquifer system, which is one of the most
productive aquifer systems in the world and the
major source of public supply for much of the
state, has been widely studied.

Surficial Aquifer System

The surficial aquifer system is not a major
source of water in most of the state. This report is
the first statewide synthesis of its ground-water
quality. Many of the individual county and water
management district water-supply reports
published by the Florida Geological Survey, U.S.
Geological Survey, and the water management
districts contain minor amounts of chemical data
on the surficial aquifer system.

The sand and sand-and-shell portions of the
surficial aquifer system in northern and
northeastern Florida are used for public water
supply, notably in St. Johns County. Elsewhere in
eastern NWFWMD, SRWMD, SJRWMD, and
SWFWMD, scattered domestic wells supply
potable water from the surficial aquifer system, but
otherwise its main use has been for irrigation,
waste disposal, and maintenance of surface-water
features. There are a few studies that characterize
water quality in the surficial aquifer system of north
and central Florida. These include Hutchinson
(1978), Causey and Phelps (1978), Hayes (1981),
Duerr and Wolansky (1986), Duerr et al. (1988), and
Upchurch et al. (1991). In general, the emphasis of
these papers has been to simply describe water
quality in the surficial aquifer system. Upchurch et
al. (1991) included reduction-oxidation potentials
and radionuclides in their study of surficial aquifer
system waters in, and near, interaquifer recharge
wells in Polk County.

The Sand and Gravel Aquifer of Escambia
County is a major source of public and private
supply for western NWFWMD. It had been
somewhat neglected until recent years, when the
U.S. Geological Survey began a comprehensive
study of the aquifer, including water quality.
Significant papers on the chemistry of the Sand
and Gravel Aquifer include Katz and Choquette
(1991) and Roaza et al. (1991).

The Biscayne Aquifer has been extensively
studied because it is the major source of potable
water for southeast Florida. Early work on the
physical aspects of the aquifer includes Parker
(1951) and Parker et al. (1955). More recent papers
that include important summaries of Biscayne

Aquifer water quality include Klein and Hull (1978)
and Radell and Katz (1991). The latter report
includes some of the Background Network data. A
major U.S. Geological Survey report on the
chemistry of surficial aquifer system water in
southwest Florida is in review (Berndt and Katz,
pers. comm., 1991). Salt-water intrusion in the
Miami area has been a major concern since the
1950's. Several landmark papers on the salt-water
transition zone were written on the transition zone
in the Miami area. These include Kohout (1960a,b)
and Cooper et al. (1964).

Intermediate Aquifer System

Southwestern Florida (portions of Charlotte,
Lee, and Collier Counties) and portions of Flagler
and Indian River Counties in east-central Florida
are the only areas where the intermediate aquifer
system is a major water-supply aquifer. The
quality of water in the intermediate aquifer system
in this area has received some attention.
Elsewhere, little interest has been shown in water
quality of the system. Duerr and Enos (1991)
discussed the hydrogeology of the intermediate
aquifer system in Hardee and DeSoto Counties.
Duerr and Wolansky (1986) described the
intermediate aquifer system in central Sarasota
County, and Duerr et al. (1988) described the
intermediate aquifer system in southwest Florida.
Wedderburn et al. (1982) described the geologic
framework of the intermediate aquifer system in
Lee County, and they included some water quality
data. Upchurch (1986) used uranium-series
isotopes and major element chemistry to establish
interaquifer connections between the surficial,
intermediate and Floridan aquifer systems in Lee
County.

Floridan Aquifer System

Because of its importance as a water-supply
aquifer, the Floridan aquifer system has been
extensively studied. Important papers that discuss
water-quality data include Stringfield (1966),
Stringfield and LeGrand (1966), Back et al. (1966),
Kaufman and Dion (1967), Back and Hanshaw
(1970, 1971), Plummer (1977), Wilson (1977), Hull
and Irwin (1979), Ceryak et al. (1983), Crane (1986),
Sprinkle (1989), Duerr and Enos (1991), and I.
Jones (1991). A major review of the hydro-
chemistry of the Floridan by Katz (pers. comm.,
1991) using data from the Background Network is
in review by the U.S. Geological Survey.

GENERAL DESCRIPTORS

Temperature

IMPORTANCE

Temperature of ground water is controlled by
climatic conditions, cultural activities, heat flow
from the earth's interior, and chemical reactions in
the aquifer system. Water temperature affects the
nature and rate of chemical reactions and
microbial activity in aquifers. It can also be used to
evaluate the residence time of water in aquifer
systems and depth to which ground water has
moved.

In shallow aquifers, water temperature is
usually controlled by climatic conditions, as
opposed to other possible causes. Water that has
recently entered the aquifer system normally
reflects atmospheric temperature at the time of
recharge. Therefore, temperature can often be
used to identify recently recharged ground water.
If the temperature of the water source is modified
by human activity, including such activities as
industrial processing, power generation, and some
forms of waste-water disposal, temperature can be
an excellent parameter to identify affected water in
the aquifer system. In karst systems, actively
recharging sinkholes can sometimes be identified
because the water they introduce is at a different
temperature from the ambient ground water. As
the water moves away from local recharge areas
and enters local flow, water temperature in shallow
aquifer systems approaches mean annual
atmospheric temperature. Because of the local
nature of shallow aquifer systems, it may be
difficult to correlate temperatures from one well to
another.

In deeper aquifer systems, temperature can be
affected by recharge from shallow environments,
earth heat flow, and chemical reactions. Of these,
heat flow has received the most attention in Florida
(Smith and Griffin, 1977). Rock is an excellent
thermal insulator, so water temperatures change
slowly. As water passes downward into deeper
parts of an aquifer system, it is warmed by heat
generated nearer the earth's interior. In Florida this
warming is slight because of the dynamic
circulation within the deeper aquifer systems
(Smith and Griffin, 1977).

If deep-flow-system water moves upward

FLORIDA GEOLOGICAL SURVEY

without sufficient time to cool, such as might occur
in large conduits near the coastal salt-water
transition zone, warm springs may result. Few
warm springs exist in Florida. The most notable
are Warm Mineral Springs and Little Salt Springs in
Sarasota County (Rosenau et al., 1977). Warm
Mineral Springs has waters ranging from 23 to
37C (Clausen et al., 1975; Rosenau et al., 1977).
In general, therefore, warm waters are attributed to
discharge of the regional flow system and relatively
cold waters to recharge. Deviations from this
large-scale regional pattern are generally caused
by rapid, local recharge, conduit flow, or
contamination.

STANDARD OR GUIDANCE
CRITERION

There is no standard or guidance criterion for
temperature in ground water (Florida Department
of Environmental Regulation, 1989).

DISTRIBUTION IN GROUND WATER

Temperature data from shallow aquifer system
environments (e.g., the surficial aquifer system and
shallow, unconfined portions of the intermediate
and Floridan aquifer systems) are locally variable.
Water from local recharge areas is likely to reflect
conditions at the time of sampling. Temperature of
recently recharged water and of very shallow water
varies with seasons, so these data do not
represent long-term conditions. Temperatures in
deeper portions of the aquifer systems do not vary
significantly with time, and the data are better
indicators of long-term conditions.

Surficial Aquifer System

Since the surficial aquifer system is, by
definition, unconfined or poorly confined, water
temperature often reflects the most-recent
recharge and seasonal temperatures. Conse-
quently, water temperatures within a region vary
considerably. In addition, temperatures vary with
latitude and aquifer system type (Table 6). Median
water temperatures are slightly less in northern
portions of the state, and higher in the south.
Where the aquifer system is sandy and permeable,
water temperature should fluctuate with recharge.
Clayey portions of the aquifer system have less
exchange with the surface and respond to
recharge events less rapidly.

Figure 6 shows the distribution of temperature
in the surficial aquifer system, by district. With the
exception of the surficial aquifer system in the
SFWMD, the data are not contoured due to the
lack of continuity between data points. The
inability to contour these data reflects the localized
nature of the flow systems and temperature
variations over the time of sampling. The variability
of the data, therefore, reflects time of sampling,
local weather conditions, recharge events, and
depth in the aquifer system.

Intermediate Aquifer System

In many areas of the state the intermediate
aquifer system is more isolated from surficial
conditions than the surficial aquifer system.
However, there is still considerable variability in the
data (Table 6; Figure 7). Median temperature in the
intermediate aquifer system is 24.60C statewide,
with a quartile range of about two degrees (Table
6). As with the surficial aquifer system, there is an
increase in water temperature to the south, which
reflects an increase in mean annual atmospheric
temperatures.

Floridan Aquifer System

Temperature data for the Floridan aquifer
system are summarized in Table 6 and Figure 8.
The regional data clearly illustrate flow-system-
related patterns. Water temperatures are cooler
inland, where recharge is likely, and warmer near
the coasts, where discharge after passing along a
deep flow path occurs. Cooler waters are often
present near drainage divides. Note that water
temperatures in excess of 280C occur along the
Peace River lineament (G. Jones, 1991) and in
coastal Sarasota County (Figure 8d). The wells
along the Peace River tap the lower portions of the
aquifer system in an area characterized by
upwelling (Healy, 1975; Kaufman and Dion, 1967;
Lehman, 1978). The coastal wells are in the
coastal upwelling zone in the vicinity of Florida's
warm springs (Warm Mineral Springs, Little Salt
Springs; Clausen et al., 1975; Rosenau et al.,
1977).

Median ground-water temperature in the
Floridan aquifer system is 24.0C. There is an
increase in temperature toward the south, and the
median temperatures by district reflect atmos-
pheric temperatures.

Acid-Base Relationships (pH)

IMPORTANCE

The variable pH reflects the potential for acid-
base reactions in water. As such, it is often treated
as a variable that determines the reactions in the
aquifer system, rather than as the product of those
reactions. The pH of aquifer water is, in fact, a
result of past chemical reactions, and it is also a
measure of the potential for reactions, if chemical
equilibrium between the water and surrounding
rock has not been established. It is included in this
section because of its importance in predicting
reactions that affect both cationic and anionic
constituents discussed below.

The hydrogen-ion concentration in water is
reported as pH, which is defined as the negative
logarithm of the hydrogen-ion activity. Waters with
a pH of 7 are neutral, while values less than 7 are
acidic and those greater than 7 are basic, or
alkaline. Hydrogen ion (H*) is generally the cause
of acidity, and bicarbonate (HCO3 ) is the most
abundant source of alkalinity in natural waters.
Acidity can also be generated by other proton
donors, notably organic acids, and alkalinity can
be created by proton receptors, such as
phosphate (P43-) and nitrate (NO3-).

The pH in aquifer systems is normally
controlled by chemical reactions with the
atmosphere and rock framework. For example,
ground water becomes acidic by dissolving carbon
dioxide gas (CO2). The carbon dioxide is produced
by equilibration with the atmosphere and with
carbon dioxide produced by microbial decay of
humus in the soil. The reaction forms carbonic
acid (H2CO3) by the reactions given in reaction 1.

Equilibration of water with atmospheric CO2,
which has an average partial pressure (gas
concentration) of 10-3., results in a pH in rainfall of
about 5.5. Once the precipitation infiltrates, the
water reacts with the CO2 in the soil atmosphere,
and the pH drops even more. The partial pressure
of CO2 in the soil can be as high as 10-2.0, which is
10 to 50 times the CO2 in the open atmosphere.
The high CO2 partial pressure in soil atmosphere is
a result of CO, production by soil microbes as they
metabolize humus. Dissolved organic acids are
also a by-product of the microbial decay of humus.
Therefore, the pH's of soil waters and shallow,
surficial aquifer system waters are commonly in the
range of 3-5 from the carbonic and organic acids.

The acids may then react with aquifer
minerals, during which acidity is consumed and
alkalinity is produced. Quartz is inert (Table 5) and
has no affect on pH. Carbonate minerals are
highly reactive, and buffer the pH through
consumption of acidity and production of HCO-.
For example, the major mineral in the Floridan
aquifer system is the carbonate mineral calcite
(CaCO3). It reacts with carbonic acid according to
reaction 3. The resulting pH increases to approx-
imately 7.0-7.5, depending on temperature and
CO2 concentrations.

In recharge areas, waters that have not
equilibrated with carbonate minerals tend to be
more acidic due to the presence of carbonic and
organic acids. In Florida, water from mid-flow and
discharge areas has come in contact with
carbonates and other minerals, so pH values tend
to be higher. In other words, pH is an excellent
indicator of the history of reactions of the water
with aquifer minerals.

STANDARD OR GUIDANCE
CRITERION

The guidance criterion for pH in Florida ground
waters is established by the Secondary Drinking
Water Standards (Chapter 17-550.310-320 F.A.C.),
and is legally enforceable under Florida statutes
(Florida Department of Environmental Regulation,
1989). The pH of water must fall within the range
of 6.5 to 8.5 according to the standard.

Water less than 6.5 is likely to be corrosive,
have high iron and high phosphate, and cause
transport of undesirable metals, such as lead.
Above 8.5, the waters may also be corrosive to
certain alloys and boiler scale and turbidity may
result from precipitation of carbonate minerals.

It is unlikely that natural pH values greater than
8.5 will occur in most Florida aquifer systems.
Where pH values of aquifer water are this high, well
construction problems are usually indicated. This
is because drilling fluids and poorly cured cements
and grouts are highly alkaline. Natural, aquifer
water in siliciclastic aquifers is likely to fall below
the minimum of 6.5 due to the carbonic and
organic acid contents.

Table 7 lists the number of samples in which
the standard was not met. Note that 93 percent of
the surficial aquifer system samples in the

SPECIAL PUBLICATION NO. 34

NWFWMD failed to meet the standard, while only
27 percent failed in SFWMD. Statewide, water
samples from the surficial aquifer system failed to
meet the standard 37 percent of the time. Some of
these failures represent the high alkalinities shown
in Table 7 and are a result of well construction
problems. Most, however, fail the standard
because they are low, which is a result of natural
causes. Failure to meet the standard in the
intermediate aquifer system averages 16 percent
of the samples, while 14 percent failed in the
Floridan aquifer system. The lower failure rates in
the intermediate and Floridan aquifer systems
result from buffering with host-rock carbonates.
The high failure rate for samples from the Floridan
aquifer system in the SRWMD results from high
organic acid content of waters from the poorly
confined Coastal Rivers Basin (Taylor, Dixie, and
Lafayette Counties). This problem is discussed
below and in the Total Organic Carbon section.

DISTRIBUTION IN GROUND WATER

Table 7 summarizes the distribution of pH
measurements. The aquifer systems that are
characterized by high carbonate-mineral contents
have median water pH values that are slightly over
7, while siliciclastic aquifer waters have pH values
of 5 to 6, depending on the amount of admixed
carbonate mineral material.

Surficial Aquifer System

The surficial aquifer system in the panhandle
and north-central Florida is predominantly quartz
sand, which is not reactive with carbonic or
organic acids. As a result, pH values are generally
low (Table 7), and the median pH values of surficial
aquifer system water in NWFWMD and SRWMD
are less than 6.0. Elsewhere, median pH values are
somewhat higher because of equilibration of the
waters with carbonate materials, especially calcite
and aragonite, in the aquifer system. Carbonates
are found in the surficial aquifer system near the
coast in all districts, and throughout the south half
of SWFWMD and all of SFWMD. These result in
higher median pH values in these districts. For
example, compare the median pH of the Sand and
Gravel Aquifer of NWFWMD with the pH of the
Biscayne aquifer of SFWMD. Minimum pH values
are in the 3 to 4 range, which reflects waters from
sandy aquifers in which no equilibration with
carbonate minerals has occurred.

Figure 9 illustrates the distribution of pH of

surficial aquifer system waters, by district. Note
that there is considerable local variability, which
reflects variations in well depth, aquifer mineralogy,
and local production of carbonic and organic
acids. In the SJRWMD (Figure 9c) the effect of
coastward increase in shell content of the surficial
aquifer system on pH is particularly well
demonstrated. Inland, pH values are 6.0 or less,
and near the coast the water may exceed 7.5. In
south Florida (Figure 9e), pH is usually in excess of
6.5 due to the high carbonate mineral content of
the Biscayne Aquifer and related rocks. All pH
values above 8.8 in the SFWMD came from newly
constructed wells. These samples may reflect
incomplete removal of well-construction materials
(grout, drilling mud) prior to sampling.

Intermediate Aquifer System

The high range in pH values in the intermediate
aquifer system (Table 7) reflects the mixed
lithology of the Hawthorn Group and related
sediments. Both siliciclastic and limestone and
dolostone horizons serve as aquifers in the
Hawthorn. Carbonate units have higher water pH
values, while siliciclastic units may have low pH's,
if the water has not come in contact with
carbonate minerals. Median pH of waters from the
intermediate aquifer system is 7.3, which reflects
buffering by reactions with carbonate materials in
many portions of the aquifer system.

Figure 10 illustrates the distributions of pH
within the intermediate aquifer system. Note that
there is considerable variability in pH at a local
scale. This reflects the nature of the aquifer
horizons within the intermediate aquifer system.
Carbonate aquifers near the base of the system
are the most productive, and these waters have pH
values near 7 as a result of reactions with
limestone and dolostone. The upper and middle
parts of the system include siliciclastic horizons
that yield somewhat acidic ground waters.

The amount of carbonate material and lateral
continuity of aquifer horizons increase southward
within the Hawthorn. This can be seen by com-
paring Figures 10a and 10b with 10e. The pH data
from the intermediate aquifer system in NWFWMD
and SRWMD (Figures 10a,10b) cannot be
contoured due to high local variability and lack of
stratigraphic continuity between production zones.
The pH values vary by as much as one unit (one
order of magnitude in hydrogen ion activity)
between adjacent wells. In SFWMD, the
intermediate aquifer system becomes more deeply

buried and individual water-producing horizons
become more continuous. With more isolation
from the land surface and more lateral continuity,
pH data are less variable and more continuous. As
a result the data can be contoured.

Floridan Aquifer System

Table 7 depicts the distribution parameters for
pH in the Floridan aquifer system. Median pH's
are uniformly near 7.4 with the exception of a 7.1 in
the SRWMD. pH values range from 4.9 to 12.5 in
the SRWMD. The high range in Floridan aquifer
system water pH in SRWMD is a result of well
construction problems and high recharge of acidic
waters.

The high values (12.5 in SRWMD, 12.2 in
SJRWMD, 10.7 in SWFWMD; Table 7) reflect
alkalinities that are a result of residual drilling fluids
or well cements or grouts that have cured impro-
perly. pH values within the upper and lower quar-
tiles (Table 7) are natural and represent equi-
libration with the carbonates of the Floridan.

The minimum pH of 4.9 in the SRWMD reflects
water that contains carbonic and/or organic acids
that have not yet reacted with the Floridan
carbonate minerals (Lawrence and Upchurch,
1976). This is locally common in recharge areas
characterized by conduit flow. Because of the
large amount of rock surface area to which non-
conduit (intergranular) flow water is exposed,
equilibration of water and rock is much faster in
intergranular-flow than in conduit-flow water and
low pH values are not expected. Low pH values
are widespread in the Coastal Rivers Basin (Taylor,
Dixie, and Lafayette Counties) of the SRWMD,
where the Floridan is poorly confined and the
surface is predominantly swampy and poorly
drained.

Figure 11 illustrates the distribution of
measured pH in the Floridan aquifer system. In
general, there is little variation in pH data, which is
a common occurrence in carbonate-rock aquifers
due to buffering. The patterns of pH in each of the
maps appear to be characterized by closed areas
of high or low pH within an overall distribution of
little or no variation. For the most part, these
closed areas reflect differences in depth of well
penetration and sampling. Shallow wells usually
have somewhat lower pH values and deeper wells
have somewhat elevated pH values. Several local
areas show data with pH values less than 6.5.

These are from shallow wells that are near swamps
in unconfined, or poorly confined, areas. In these
areas, high concentrations of dissolved organic
acids lower pH. Comparison of these areas with
the distribution of total organic carbon suggests a
close correspondence.

There is an area of high pH that extends from
southeast to northwest through Alachua and
southern Columbia Counties in the SRWMD
(Figure 11 b). This high pH water may reflect some-
what "stagnant" flow under the Northern Highlands
physiographic province (Lawrence and Upchurch,
1976, 1982;Upchurch and Lawrence, 1984), which
allows equilibration with the carbonate minerals.
This is a common phenomenon where the Floridan
is highly confined and the hydraulic gradient is low.

Regional flow in the Floridan is such that
waters from coastal discharge regions are likely to
have somewhat higher pH's due to the multiplicity
of reactions that have affected the water along the
flow path. This is illustrated in scattered coastal
zones through the state. However, there is a
surprising amount of low pH water in coastal
areas. Some of this low pH water in coastal areas
reflects local recharge, which may be acidic due to
organic acids or carbonic acid.

Water from areas where the Floridan is
unconfined and near the mouths of rivers, such as
near the mouth of the Suwannee River (Figure
11b), shows low pH values, which appears
inconsistent with the regional discharge pattern.
These low pH waters may reflect local recharge
from the rivers or flux of lower pH waters on the
salt-water transition zone.

Finally, there are minor indications (isolated
wells with low pH water) near some urban areas
that may reflect use of drainage wells. Drainage
wells are utilized in many areas of the state where
urban and suburban development is in karst
terrain. Drainage wells are installed to carry storm-
water runoff into the host aquifer. The effects of
these wells have been studied by Hull and
Yurewicz (1979), Kimrey and Fayard (1982),
Schiner and German (1983), and Bradner (1991).
The sampling plan for the Background Network
was established to avoid urban areas with similar,
known sources of contamination, but suburban
and rural drainage wells were not avoided. Low
pH water near Orlando (Orange County, Figures
11c,e), Live Oak (Suwannee County, Figure 11b),
and elsewhere may reflect this storm-water
disposal practice.

FLORIDA GEOLOGICAL SURVEY

CATIONS

Classification

Cations are positively charged ions that are
generated by loss of electrons. Cations can be
grouped into three categories according to their
abundance in the natural environment.

MAJOR CATIONS

Major cations are the dominant elemental
cations in an aqueous solution. They are usually
present in concentrations in excess of 1.0 mg/L.
The major cations in Florida waters are calcium
(Ca2+), magnesium (Mg2), sodium (Na'), potassium
(K), and strontium (Sr+). With the exception of
strontium, which is often less than 1 mg/L in
Florida, all of the major cations are discussed
below.

MINOR CATIONS

Minor elemental cations occur in concen-
trations of 0.001 to 1.0 mg/L. Important minor
cations include iron (Fe2+, Fe3+), barium (Ba2'), and
manganese (Mn2+, Mn31). Iron is included in this
report because of its importance as a regulated
water-quality constituent. Barium and manganese
have not been included. Ammonium (NH4') is a
trace cation. However, for convenience it is
discussed in the Anion section with its negatively
charged counterpart nitrate (NO3).

are not included because they are rare in natural
Florida ground waters.

The chemical controls on cations are
discussed under the individual constituents.
Common processes that affect cation abundances
include mixing of water masses, mineral
dissolution and precipitation, reduction/oxidation
reactions, cation exchange on clays, oxy-
hydroxides and organic, and chemical com-
plexing.

Calcium

IMPORTANCE AND SOURCES

In many aquifer systems, calcium is the
dominant cation. It is dominant because of
weathering of the calcite (or aragonite) and
dolomite (Table 4), the minerals that constitute
limestone and dolostone, respectively. Calcite and
aragonite are abundant in shelly portions of the
surficial aquifer system. Dolomite and calcite
constitute the carbonate-rock horizons and are
abundant as clasts in the siliciclastic horizons of
the intermediate aquifer system. The Floridan
aquifer system is composed of calcite and
dolomite.

The reaction for weathering of limestone is
given in reaction 3. Dolomite is weathered
according to

CaMg (CO) 2
dolomite

+ 2H+

TRACE METALS

SCa2" + Mg2+ + 2HCO3.

Trace metals include elemental cations that
are characteristically present at concentrations
less than 0.001 mg/L. Trace metals are usually
present in very low concentrations in natural
ground water due to (1) low abundance in aquifer
rock materials, (2) low mineral solubilities, (3) high
probability of adsorption on mineral surfaces and
particulate organic, and (4) precipitation as a
metal oxide or sulfide. If present, trace metals are
usually in the gg/L concentration range. Some of
the important trace metals that occur in aqueous
systems are lead (Pb +), mercury (Hg2'), cadmium
(Cd2+), chromium (Cr6"), and cobalt (Co2'). In
Florida, lead and mercury are of concern due to
the widespread occurrences of these metals in
aquatic organisms. For this reason, lead and
mercury are discussed below. The other metals

In both reactions, calcium is released as a
dissolved cation. Therefore, calcium is expected
to be a widespread and important cation in
carbonate-rich aquifers. Weathering of limestones
and dolostones consumes acidity (reactions 3 and
7), so calcium concentrations are highest in
alkaline waters that are fully equilibrated with the
host rock.

Calcium is also released upon weathering of
gypsum and anhydrite (Table 4). Gypsum is
occasionally found in the Hawthorn Group, and
both gypsum and anhydrite are common at the
base of the Floridan aquifer system, in the Avon
Park and Oldsmar Formations.

Sea water contributes calcium to the aquifer
systems in two ways. Precipitation that contains
marine aerosols introduces minor amounts of
calcium to the land surface. Average precipitation
throughout Florida contains approximately 0.1-0.3
mg/L Cal' (Table 3). Calcium is also important
where mixing with sea-water-derived ground water
along the salt-water transition zone occurs (Table 8).

Weathering of silicates in siliciclastic aquifer
zones is generally an insignificant source of
calcium in Florida ground waters. Calcium-rich
silicates are rare in Florida sands and carbonate
rocks. An important exception results from
weathering of the clays in the intermediate aquifer
system. Smectite (Tables 4, 5) is a major
component in the Hawthorn Group and although it
contains more magnesium than calcium,
weathering can be shown to contribute some
calcium to intermediate and Floridan aquifer
system waters (Lawrence and Upchurch, 1982).

Calcium is removed from the aquifer systems
by mineral precipitation and ion exchange. Calcite
cements and void fillings are common in
sandstones and carbonate rocks throughout the
state (see for example, Vernon (1951) and Puri and
Vernon (1964) for descriptions of calcite-cemented
strata). These cements are a result of evaporation
of calcium-bicarbonate-rich waters or by
degassing of carbon dioxide. However, Jones et
al. (in press) have argued that there is little regional
cementation in the Floridan aquifer system from
mass-balance calculations.

Calcite precipitates as a result of carbon
dioxide degassing according to the reaction
Ca2 +2HCO;

-CaCOsolid + COgas
solid gas

+ H2O.

Carbon dioxide degassing is the common process
for calcite cementation in caves (White, 1988) and
shallow ground-water systems. Little work has
been done on carbon dioxide mobility in Florida
ground waters. Starks (1986) has shown that
degassing of carbon dioxide occurs on the
upward-flow portion of the Floridan aquifer system
near springs on the middle Gulf Coast. This
degassing provides potential for calcite
precipitation in coastal environments. On-going
research at the SWFWMD (Upchurch, Jones, and
DeHaven, 1992, pers. comm.) indicates that
carbon dioxide partial pressures in shallow

Floridan aquifer system water can be used to
identify local recharge areas.

Precipitation of calcite as a result of
evaporative concentration is common in the
surficial aquifer system. Calcite "sand crystals"12
have been found growing in the quartzose, barrier-
island sands of Dade County and in dolomitic silts
in Citrus and Levy Counties. Calcitic nodules are
widespread in the surficial aquifer system and
barrier islands throughout the state. Vadose
pisolites occur in soil-filled caverns at the top of
the Floridan aquifer system in Hernando, Citrus,
and Alachua Counties. Evaporative precipitation
has also been shown to form calcrete crusts on
rocks of the Biscayne Aquifer in south Florida and
the Keys (Multer and Hoffmeister, 1968).

Ion exchange is a widespread phenomenon in
aquifer systems deposited in coastal plain
environments (Foster, 1950). Clays, particularly
smectites, have high ion-exchange capacities. The
ion-exchange reactions of sodium and calcium
with a clay sorption (ion exchange) site can be
characterized as follows

-clay + CaN

Na

= Ca-clay + 2Naq .

If deposited in sea water, these clays are initially
saturated with sorbed sodium, which is loosely
held on clay-mineral surfaces. Calcium and
magnesium have a higher affinity than does
sodium for clay surfaces, so when calcium- or
magnesium-rich ground waters bathe the Na-
clays, ion exchange is likely to occur. Calcium or
magnesium exchange for sodium, and the clay
becomes a Ca-and/or Mg-saturated clay, while the
water is enriched in sodium. The reverse reaction
occurs upon salt-water intrusion into Ca- or Mg-
rich clays. Even though the sorption potential of
sodium is low relative to calcium or magnesium,
the high concentrations of sodium in sea water
cause exchange with a release of calcium and
magnesium and a loss of sodium in the ground
water.

SPECIAL PUBLICATION NO. 34

STANDARD OR GUIDANCE
CRITERION

There is no standard or guidance criterion for
calcium in ground water (Florida Department of
Environmental Regulation, 1989). Calcium is not
considered a hazardous component in potable
water.

The calcium plus magnesium content of water
is called "hardness". Durfor and Becker (1964)
classified waters according to their hardness
(Table 9). Hardness is of concern because calcium
and magnesium interfere with the function of
soaps and certain detergents. Hard waters are also
a problem because they form calcium- and
magnesium-carbonate mineral residues ("scales")
in hot-water heaters, boilers, and humidifiers.
Evaporation of hard waters leads to scale on
swimming pool walls, bathroom and kitchen
fixtures, and dishes. For these reasons, a large
industry selling and supporting water softeners has
evolved. Care should be taken, however, in
consumption of softened water due to increased
sodium content. Additionally, calcium-rich waters
provide dietary calcium. Hardness of waters in the
Floridan aquifer system has been described by
Shampine (1965) and Sprinkle (1982a).

DISTRIBUTION IN GROUND WATER

Calcium concentrations in ground water in
Florida are a direct result of aquifer contact,
residence time, and flow path. Table 10 compares
the calcium contents of aquifer systems, by
district. Comparison of the surficial, intermediate,
and Floridan aquifer systems illustrates the role of
calcite and dolomite dissolution in aquifer
chemistry.

Calcium concentrations are generally higher in
the intermediate and Floridan aquifer systems than
in the surficial aquifer system (Table 10). This is a
result of interaction of the water with limestone and
dolostone horizons in both aquifer systems and
with carbonate clasts in siliciclastic horizons of the
intermediate aquifer system.

Surficial Aquifer System

Calcium concentrations are least in the
surficial aquifer system, especially in NWFWMD
and SRWMD, where the aquifer system is
predominantly siliciclastic in composition. The

surficial aquifer system in the SJRWMD and
SWFWMD includes carbonate minerals in coastal
environments and in the southern half of the
SWFWMD. The surficial aquifer system in SFWMD
is predominantly carbonate.

The distribution of calcium in the surficial
aquifer system is shown in Figure 12. It is difficult
to reconcile some of the calcium concentrations in
the surficial aquifer system with known
compositions of aquifer materials. For example,
the surficial aquifer system in the interior of the
state in north Florida is predominantly a siliciclastic
aquifer system. As such, calcium concentrations
should be relatively low. Most analyses in
NWFWMD and SRWMD are low (<10 mg/L; Figure
12a,b), however a few of the analyses are in
excess of 20 mg/L. These may reflect calcium
derived from (1) cements used in well construction,
(2) weathering of carbonate minerals or rock
fragments reworked into the surficial sands and
gravels from the underlying Hawthorn Group or
residual from the original shell content, (3) fugitive
dust from unpaved roads, quarries, or nearby
construction sites, or (4) application of calcium-rich
soil amendments, such as gypsum, calcite, or
dolomite. All of these sources can locally affect
the chemistry of the surficial aquifer system. Also,
since the samples were not filtered, the presence
of suspended particles may affect some analytical
results.

In coastal areas and the southern half of the
Florida peninsula, the surficial aquifer system
contains shell, marl, and limestone. In these areas,
calcium will naturally be relatively high and local
influences, such as discussed above, will be
masked. Calcium increases toward the coast in
the SJRWMD (Figure 12c) indicating the increased
importance of calcite and aragonite in the surficial
aquifer system near the coast. A similar pattern is
present in northern and central SWFWMD, but
there is also a net southerly increase in calcium
along the southern district boundary (Figure 12d).
This increase in calcium is continued in western
SFWMD (Figure 12e), and reflects increased
importance of shell to the south.

Intermediate Aquifer System

The intermediate aquifer system includes
abundant beds of limestone and dolostone, and
the siliciclastic horizons include fragmental

limestone and dolostone that have been reworked
by waves and currents into and mixed with the
quartz sand and clays. As a result, calcium
content of the water is uniformly high relative to the
surficial aquifer system (cf. Table 10 and Figures
12 and 13).

Calcium concentrations are highly variable
because of the heterogeneous nature of
composition in the Hawthorn Group and wide
range of contact times between rock and water
(Figure 13a,b, and c). There is a minor increase in
calcium content towards the coast in the SJRWMD
(Figure 13c), which reflects salt-water intrusion and
increased carbonate content and water residence
times.

In southwestern SWFWMD (Figure 13d) and
western SFWMD (Figure 13e) there is a second
process operating. Upwelling along the salt-
water/fresh-water transition zone brings deep
Floridan aquifer system water into the intermediate
aquifer system (Upchurch et al., 1991). This water
has some of the longest and deepest flow paths of
any aquifer system water in Florida. As a result of
having traveled along the base of the Floridan,
where it picked up calcium from the dissolution of
gypsum, calcium ispresent in excess of 100 mg/L.

Floridan Aquifer System

The Floridan aquifer system is almost entirely
limestone or dolostone. Waters that are in equi-
librium with the host aquifer rock, therefore, have
high calcium content. Much of the variability shown
in Figure 14 is a result of well depth or position of
highly productive zones in long open-hole wells.
Water from deeper wells often has higher calcium
and sulfate concentrations due to contact with
gypsum and anhydrite at the base of the Floridan,
long flow paths, and long residence times. Wells
that have long reaches of open hole cannot be
quantified as to depth of the water sample. It is
clear that highly productive horizons near the base
of the aquifer system in central and south Florida
(the "boulder zones" of Puri and Winston, 1974)) are
often characterized by high calcium and sulfate.
Due to the high transmissivities of these zones,
wells that tap them are characteristically dominated
by these deeper waters.

Lawrence and Upchurch (1976; 1982) found
somewhat elevated calcium concentrations in the
highly confined Floridan in north-central Florida,
where flow systems are apparently sluggish and

equilibration thereby enhanced. Upchurch and
Lawrence (1976) also suggested that high calcium
in the Floridan aquifer system in the vicinity of the
Cody Escarpment, a zone of high recharge at the
transition between the unconfined Floridan of the
Coastal Lowlands and the highly confined Floridan
of the Northern Highlands in north Florida, is a
result of completing with natural organic. Brown
(1989) confirmed that organic completing en-
hances transport of calcium, but failed to show a
strong spatial correlation with all of the high
calcium regions in the escarpment environment.
The location of these studies is within the >200
mg/L zone in southern Columbia County (Figure
14b).

Back and Hanshaw (1971) studied the
distribution of calcium and degree of saturation of
aquifer water with respect to calcite and dolomite
in central Florida. They found that calcium
concentration and saturation state of the water
with respect to the minerals increase along the
flow paths radiating from the vicinity of the Green
Swamp in northern Polk County (Figure 14d).

The data from SJRWMD (Figure 14c) and
SWFWMD (Figure 14d) support this conclusion,
although the pattern is not as apparent as
indicated by Back and Hanshaw (1971). There is a
minor increase in calcium toward the coast in the
SJRWMD, but most of the higher calcium
concentrations appear to be a result of saline-
water upcoming in central Flagler and Volusia
Counties (Figure 14c). Sea water, which averages
411 mg/kg (Table 8), is locally a potential source of
calcium at the transition zone throughout the state.

Calcium is lowest in recharge areas in central
Pasco County and in the upper Withlacoochee and
Hillsborough River watersheds in the SWFWMD.
The region associated with the Withlacoochee and
Hillsborough Rivers coincides with the margin and
western half of the Green Swamp and suggests
that recharge is most effective on the margin of the
swamp as opposed to its center. A similar
conclusion has been drawn by Swancar and
Hutchinson (1992) from isotopic data. Concen-
trations increase radially from these regions
according to the Back and Hanshaw (1971) model.
The high calcium in the vicinity of the Peace River
(southern Hardee, Desoto, western Sarasota, and
northern Charlotte Counties; Figure 14d) coincides
with upwelling of calcium-sulfate-rich waters at the
salt-water transition zone. G. Jones (1991) has
suggested that this upwelling is enhanced by the
presence of a fracture system along the axis of the

-LUHIUA GEOLOGICAL SURVEY

Peace River and also along the Myakka River,
which coincides with a re-entrant in the transition
zone in coastal Sarasota County. Culbreth (1988)
has documented some of these fracture systems.
The width of the re-entrant, which occupies a zone
a few kilometers in width along the Peace River, is
too wide to correspond to a single fracture. These
fracture traces must reflect a concentration of
multiple fractures or extensive modification of a
fracture zone by rock dissolution. The width of the
re-entrant in the water-quality data is most likely an
artifact of upcoming along the fracture-trace
system and lateral transport of deep, calcium-
sulfate waters in the "boulder zones" in response
to heavy pumpage.

Finally, since the samples were not filtered,
high calcium may be a result of particulate calcite
or dolomite in the samples. Unusually high
calcium concentrations most likely represent
drilling-fluid or particulate contamination in poorly
developed wells.

Magnesium

IMPORTANCE AND SOURCES

Due to chemical similarities, many of the
factors that govern the distribution of calcium in
Florida aquifer systems may also be applied to
magnesium. Magnesium has several sources and
some possible sinks (pathways by which it is
removed from the water).

Mean magnesium concentration in preci-
pitation ranges from 0.07 mg/L in north Florida to
0.2 mg/L in the south (Table 3). Evapotranspiration
may raise the concentration in surface and soil
waters by a factor of ten. Sea water averages
1,290 mg/kg magnesium (Table 8), so the waters in
the transition zone may be magnesium rich.

The Hawthorn Group contains significant
sources of magnesium (Table 5), including
magnesium-rich clays (Weaver and Beck, 1977;
Miller, 1978; Reik, 1982; Strom and Upchurch,
1983, 1985; Scott, 1988), and dolomite (Wilson,
1977; Prasad, 1985; Scott, 1988). Weathering of
any of these minerals adds to the load of
magnesium in the intermediate and Floridan
aquifer systems (Lawrence and Upchurch, 1976,
1982). The Floridan aquifer system also contains
abundant dolomite, especially within the
Suwannee Limestone and Avon Park Formation

(Randazzo and Saroop, 1976; Randazzo et al.,
1977; Randazzo and Hickey, 1978; Randazzo et
al., 1983). The dolomite in the intermediate and
Floridan aquifer systems may be either a source or
sink for magnesium. Finally, particulate dolomite
in the unfiltered samples may cause a strong
correlation of magnesium with dolostone.

One of the most important models for dolomite
formation in Florida is based on chemical
equilibrium conditions that exist in the salt-water
transition zone. Back and Hanshaw (1970),
Hanshaw and Back (1971a,b), and Hanshaw et al.
(1971) first postulated that dolomites may be
forming in coastal portions of the Floridan aquifer
system at the present time. Runnels (1969) offered
an explanation for this phenomenon. His argument
is that mixing of two water masses that are
saturated with respect to calcite will result in a new
water mass that is out of equilibrium with respect
to calcite. In coastal mixing zones, the mixture is
under-saturated with respect to calcite and
limestone dissolution is predicted. Badiozomani
(1973), Plummer (1975), and Wigley and Plummer
(1976) expanded the concept and showed that
mixtures of calcium-bicarbonate-rich Floridan
aquifer system water and sea water, both of which
may be saturated with respect to calcite and
dolomite before mixing, become undersaturated
with respect to calcite in the approximate range of
4 to 45 percent sea water. The water is over
saturated with respect to dolomite in this same
salinity range. Thus, in the landward "half" of the
salt-water transition zone, the equilibrium models
predict that calcite would be either dissolved,
thereby producing karstic porosity, or replaced by
dolomite. Hanshaw and Back (1980) have
documented calcite dissolution in the mixing zone
in the Yucatan, but the possibility of dolomitization
remains controversial. Hardie (1987) has reviewed
the mixing zone model and noted possible flaws.
If the model is possible, dolomitization of
limestones within transition zones along the coast
and at the base of the Floridan may constitute a
significant sink for magnesium.

Dolomite precipitation within the intermediate
and Floridan aquifer systems is a highly debated
topic. Dolomite is less soluble than calcite or
aragonite, and the mixing-zone dolomitization
model is a possible mechanism for magnesium
removal from aquifer water. Randazzo and Saroop
(1976), Randazzo et al. (1977), Randazzo and
Hickey (1978) and Randazzo et al.(1983) have
extensively studied the origins of dolomite in
Florida aquifer systems from core petrography.
These studies concluded that much of the

dolomite in the Avon Park Formation and
Suwannee Limestone can be attributed to depo-
sitional conditions at, or shortly following, the time
of deposition. Prasad (1985) concluded that
dolostones and dolomitic clays and silts in the
Hawthorn resulted from replacement of aragonitic
or calcitic muds at the transition zone. However,
Randazzo and Bloom (1985) and Randazzo and
Cook (1987) found dolomites in the Floridan
aquifer system that can be attributed to
dolomitization in the transition zone. While modern
dolomitization cannot be documented, they found
that modern, transition-zone ground waters are
thermodynamically saturated with respect to
dolomite and dolomite precipitation is predicted.
One of the problems with correlation of existing
dolostone horizons to modern ground-water
chemistry is that Cenozoic sea-level fluctuations
have been sufficiently rapid to prevent formation of
well-defined dolostone horizons or transition-zone
karst at the present positions of the transition
zones (Fanning et al, 1981).

STANDARD OR GUIDANCE
CRITERION

There is no standard or guidance criterion for
magnesium (Florida Department of Environmental
Regulation, 1989). The problems with hardness
that were discussed under Calcium (above) are
valid for magnesium.

DISTRIBUTION IN GROUND WATER

Surficial Aquifer System

The median magnesium concentrations in the
surficial aquifer system (Table 11) are low (<10
mg/L) throughout the state. Maximum values in
SJRWMD, SWFWMD, and SFWMD probably
reflect either intermediate or Floridan aquifer
system water that has been introduced through
irrigation or upward discharge or weathering of
magnesium-rich minerals reworked into the
sediments of the surficial aquifer system from
underlying strata.

Magnesium concentrations are characteris-
tically less than 2 mg/L in north Florida (Figure 15a-
c). High magnesium concentrations in NWFWMD
and SRWMD are in areas where the Hawthorn is
present beneath the surficial aquifer system.
These high magnesium values may reflect wells
that inadvertently tap the intermediate aquifer

system or horizons where Hawthorn sediments
have been reworked into the surficial aquifer
system. The highest magnesium concentrations in
SJRWMD are in an area of coastal intrusion in
Flagler County and upcoming in Seminole and
southeast Orange Counties. In SWFWMD, high
magnesium concentrations are also coastal (Figure
15d), with minor highs in the interior. Highest
magnesium concentrations in SFWMD are in Lee,
Highlands, and Glades Counties. Much of this
region is characterized by clay-rich, shelly sands,
which contain reworked material from the
Hawthorn. The Biscayne Aquifer is low in
magnesium, but a few wells on the western margin
of the aquifer show minor increases in magnesium
(Figure 15e).

Magnesium concentrations in marine aerosols
(Table 3) are less than 0.4 mg/L magnesium, while
magnesium in the surficial aquifer system in much
of the northern part of the state is less than 1 mg/L
(Figure 15). These concentrations are on the same
order of magnitude as precipitation with minor
evaporative concentration. In the southern part of
the state magnesium concentrations are higher
indicating more evaporative concentration,
additions of irrigation waters from deeper aquifer
systems, and possible mixing with residual marine
waters trapped within the Plio-Pleistocene
sediments.

Intermediate Aquifer System

Intermediate aquifer system water charac-
teristically has higher magnesium concentrations
(Table 11) than the surficial or Floridan aquifer
systems due to the presence of magnesium-rich
minerals (clays, dolomite; Table 5). There is an
increase in median and maximum magnesium to
the south, indicating a southward increase in clay
and dolomite content and in aquifer system thick-
ness, water residence times, and permeabilities.

In the NWFWMD, the magnesium increases to
the east (Figure 16a), where the Hawthorn Group
magnesian clay deposits occur (Scott, 1988). The
trend continues into the SRWMD and western
SJRWMD (Figure 16b,c). Concentrations in the
interior of the SWFWMD (Figure 16d) are similar to
those of north Florida. However, magnesium
increases to concentrations in excess of 50 mg/L
in the coastal transition zone and lower Peace
River basin. The concentrations in Lee, Highlands,
and Glades Counties (Figure 16e) are in excess of
25 mg/L as a result of extensive dolostones that
constitute major aquifers in the area.

SPECIAL PUBLICATION NO. 34

Floridan Aquifer System

Median magnesium concentrations in the
Floridan aquifer system fall between those of the
surficial and intermediate aquifer systems (Table
11). This is an artifact of two processes. First,
much of the water in the Floridan aquifer system
has passed through the intermediate aquifer
system and has inherited magnesium from that
system (Lawrence and Upchurch, 1976, 1982).
This magnesium is diluted by directly recharged
waters that are low in magnesium. Second, some
portions of the Floridan aquifer system (notably
near the base of the aquifer system in the Avon
Park Formation and Oldsmar Limestone, near the
top in the Arcadia Formation (Hawthorn Group),
and in the Suwannee Limestone in central
SRWMD) contain dolomite. Maximum magnesium
contents are in coastal transition-zone
environments where sea water (mean magnesium
concentration = 1,290 mg/kg; Table 8) intrudes the
aquifer system.

The distribution of magnesium in the Floridan
aquifer system (Figure 17) reflects several of the
processes mentioned above. Magnesium
increases in a seaward direction in NWFWMD
(Figure 17a). This increase reflects maturation
along the direction of regional flow and interaction
with dolostones within the aquifer system.
Magnesium concentrations are in excess of 20
mg/L in the Apalachicola River delta as a result of
coastal upwelling. The corridor of low magnesium
along the lower Ocklockonee River reflects
displacement of the transition zone by riverine
waters, while the complex pattern in Wakulla
County reflects the complex flow patterns
associated with the large springs there.

The region of high magnesium in the
northeastern part of the SRWMD (Figure 17b), in
the Northern Highlands of Hamilton, Columbia,
Union, Bradford, and Alachua Counties, represents
the influence of weathering of the magnesium-rich
minerals in the Hawthorn, through which Floridan
aquifer system waters pass during recharge. The
large area of high magnesium concentrations in
Taylor, Lafayette, and Madison Counties
corresponds to an area of dolomitic Suwannee
Limestone. High values in coastal environments
reflect the transition zone, which exhibits large re-
entrants up several rivers.

Floridan aquifer system water in the SJRWMD
is generally low in magnesium, with highest
concentrations near the coastal transition zone

(Figure 17c). There is a re-entrant near Daytona
Beach (Volusia County) that reflects intrusion as a
result of well pumpage. There is a similar area with
high magnesium concentrations near Hastings (St.
Johns County). Highs in northern Lake County,
near Lake George, and in southern Seminole
County underlie the St. Johns River. Leve (1983)
has shown that upwelling from the Floridan aquifer
system along faults occurs elsewhere on the St.
Johns River.

There is a trend of increasing magnesium
towards the coast in SWFWMD (Figure 17d). The
transition zone is well delineated by the 10 mg/L
isoline in the northern part of the district. The 10
mg/L isoline crosses the District from west to east
where the Hawthorn Group ceases to be an
effective confining unit. That is, south of the 10
mg/L isoline the Floridan is confined, and north it is
semi- to unconfined. Therefore, confinement, long
flow paths, and proximity of the overlying
Hawthorn result in higher magnesium concent-
rations. There are significant re-entrants that
parallel both the Peace and Myakka River axes.
These re-entrants were attributed to upwelling
along major lineaments by G. Jones (1991).

Data from SFWMD (Figure 17e) are limited and
cannot be used to draw many conclusions. The
absence of data in the central and south portions
of the District reflect poor water quality in the
Floridan aquifer system. Magnesium concen-
trations are high here as reflected by the 100 mg/L
isoline that skirts the northern shore of Lake
Okeechobee.

Sodium

IMPORTANCE AND SOURCES

The primary sources of sodium in Florida
aquifer systems are marine aerosols and mixing
with sea water in the transition zone.
Concentrations of sodium in rainfall average from
0.44 in north Florida to 1.58 mg/L in south Florida
(Table 3). The range of sodium concentrations
measured in precipitation is 0.02 to 29.3 mg/L.
Evapotranspiration of meteoric water can cause
increases in sodium concentrations above those of
the rainfall itself. The mole ratio of sodium to
chloride in sea water is 0.857, and in Florida rainfall
it ranges from 0.85 to 0.92. These ratios persist
throughout the surficial aquifer system in Florida
and reflect the importance of precipitation as a
source of sodium.

Connate, saline waters that are residual from
the Plio-Pleistocene marine transgressions also
constitute a significant source of sodium inland.
Connate waters may be present in isolated pores
within the fresh-water portions of the aquifer
systems. They may also occur at the base of the
Floridan aquifer system in northern and central
Florida and throughout much of the Floridan
aquifer system in southern Florida, where deep
circulation has not been able to sweep the water
out of the system. Minor sea water is also present
as "bubbles" trapped in unconnected primary
porosity that ranges in size from cavities up to a
few centimeters between grains to cavities less
than a millimeter within marine shells and
authigenic minerals; however, the volume of this
water is likely to be low.

Sorption sites on clays deposited in a marine
environment are usually saturated with sodium.
When bathed in calcium- or magnesium-rich
waters, the calcium and magnesium is exchanged
for the sodium in a form of natural "water
softening" (reaction 9; Foster, 1950). This sodium
is not from connate water, although it is left over
from previous marine transgressions. Sodium
released from clays by ion exchange can be
recognized by comparison of sodium and calcium
to chloride concentrations. Since meteoric waters
have a sodium to chloride ratio similar to that of
sea water (mole ratio = 0.857), increases in the
ratio which are accompanied by a decrease in the
calcium to chloride ratio can be attributed to ion
exchange. The large amount of sodium-
bicarbonate water in the aquifer systems (see
Hydrochemical Facies section) indicates that ion
exchange is an important process in Florida.

The reverse exchange reaction occurs where
sea water intrudes into an aquifer system that
contains potable, calcium- or magnesium-rich
waters. Here, calcium- or magnesium-saturated
clays are bathed in sodium-rich solution and the
exchange reaction (reaction 9) goes to the left.
This results in a calcium- or magnesium-chloride
facies, which has been documented in this report
along the inner margin of the transition zone.

An excellent example of widespread ion
exchange occurs in Taylor County, where a near-
surface, Plio-Pleistocene clay (the "San Pedro
clay") appears to be releasing sodium to the
ground water. The mole ratio of sodium to
chloride in adjacent counties, where the clay is not
present, is essentially the same as sea water
(0.857) due to the influence of marine aerosols. In

Taylor County the prevalent ratio is approximately
1.3, indicating that there is approximately fifty
percent more sodium as a result of exchange for
calcium.

Finally, weathering of sodium-rich minerals,
such as feldspars or clays, can be a source of
sodium in ground water. Sodium feldspars (sodic
plagioclase, (Na,Ca)AI(AI,Si)Si208) and sodium-
rich clays (montmorillonite and nontronite; Table 4)
are found in siliciclastic horizons of the surficial
and intermediate aquifer systems. Minor amounts
occur in the Floridan aquifer system. The
weathering reaction is similar to reaction 4, with
release of sodium, other cations, silicic acid, and
bicarbonate (Table 4).

STANDARD OR GUIDANCE
CRITERION

Sodium is regulated under the Primary
Drinking Water Standard (Ch. 17-550.310-320,
F.A.C.; Florida Department of Environmental
Regulation, 1989). The standard is 160 mg/L. The
standard is based on the possibility of adverse
health effects, including heart disease and
hypertension. Table 12 includes the number of
samples that exceeded the standard by district.
The samples that exceed the standard are
characteristically in the coastal transition zone or in
regions of upcoming of deeper waters.

Statewide, four percent of the samples from
the surficial aquifer system exceeded the 160 mg/L
criterion. The proportion of samples that
exceeded the standard ranged from zero in
SRWMD to 16 percent in SJRWMD. The samples
from the SJRWMD are coastal and reflect the
transition zone.

Twenty-three percent of the samples from the
intermediate aquifer system exceeded the
standard. The range in proportion of samples that
exceeded the standard ranged from zero to 45
percent. The proportion that exceeded the
standard was low in all districts except the
SFWMD, where connate waters are common in the
intermediate aquifer system.

Seventeen percent of the samples from the
Floridan aquifer system exceeded the standard.
The proportion ranged from one percent in
NWFWMD to 59 percent in SFWMD. Again,
connate waters in the Floridan in southern Florida

FLORIDA GEOLOGICAL SURVEY

and in coastal regions elsewhere, and the coastal
transition zone account for these high
concentrations.

DISTRIBUTION IN GROUND WATER

The distribution of sodium in Florida's aquifer
systems is summarized in Table 12. Care should
be taken in interpreting these data as the design of
the Background Network includes coastal, sea
water intruded zones, regions affected by connate
water, and areas of upcoming of deeper waters. In
addition, there are no wells in the central
Everglades region of SFWMD. The distribution of
wells is not in proportion to the areal extents of
these regions of the aquifer systems, so neither the
distributions nor the number of samples exceeding
the 160 mg/L standard can be taken to literally
characterize the aquifer systems. The maps
(Figures 18-20) provide a better synthesis of the
conditions present in the aquifer systems.

Note that Table 12 suggests that there is a
general increase in median and maximum sodium
contents of aquifer system waters to the south.
These increases are, in part, a result of the
increased importance of marine aerosols in
precipitation in the peninsula as opposed to the
northern part of the state, especially with respect
to the surficial aquifer system and shallow wells in
the unconfined parts of the Floridan aquifer
system. The large increases in SFWMD are a
result of low hydraulic gradients, which have not
caused complete flushing of the aquifer systems,
especially near the coast, and to human-induced
salt-water intrusion.

Surficial Aquifer System

The median sodium concentration in the
surficial aquifer system statewide is 17.0 mg/L.
The range of values is 0.7 to 3,730 mg/L (Table 12).
The majority (samples within the upper quartile) of
the sodium concentrations are within the range of
marine-aerosol enriched precipitation that has
been concentrated by evapotranspiration. The
higher concentrations reflect wells that are near the
coastal transition zone or that are inland and are
influenced by connate water or introduction of
deeper aquifer system water through irrigation or
other interaquifer transfer.

Examination of Figure 18 reveals the influence
of the coast and these "pockets" of high sodium

inland. There is a weak coastal influence in the
Sand and Gravel Aquifer (Figure 18a) in the
NWFWMD, but highest sodium values near the
coast appear to reflect upcoming of saline water
under pumping stresses. The influence of the
coastal transition zone and, possibly, incomplete
flushing is well displayed in the SJRWMD (Figure
18c). Here, large re-entrants from the coast have
developed where pumpage has induced intrusion.
The data from the southern half of SWFWMD also
indicate a well-developed transition zone (Figure
18d). Note that well-developed re-entrants of
sodium-rich water extend inland along the axes of
the Peace, Myakka, and Little Manatee Rivers. In
SFWMD (Figure 18e) the Biscayne Aquifer
illustrates the coastward increase in sodium. This
is one of the classic and most studied transition
zones in the world (Kohout, 1960a,b; Cooper et al.,
1964). A major management effort of the SFWMD
is to minimize landward intrusion and restore water
quality in the Biscayne Aquifer. High sodium
concentrations inland, especially in southwestern
Glades County, are probably a result of interaquifer
transfer through irrigation and remnant connate
water.

Intermediate Aquifer System

Sodium in the intermediate aquifer system
statewide ranges upward from 1.0 mg/L (Table
12), with a median concentration of 41.0 mg/L. The
wide range in sodium concentration reflects a
diversity of processes, including marine aerosols,
connate water, coastal, saline waters, and
weathering of Hawthorn Group minerals. The
influence of Hawthorn Group weathering has been
discussed by Lawrence and Upchurch (1976,
1982).

The majority of the inland samples (Figure 19),
particularly in the NWFWMD and SRWMD, are
equivalent to the sodium contents of the surficial
aquifer system, and marine aerosols; so they
reflect simple recharge. Near the coast, upwelling
along the transition zone results in higher sodium
concentrations, especially where pumping or the
presence of large rivers cause the lowering of
hydraulic heads.

There is a high sodium area at the south-
western corner of Glades and northeastern corner
of Lee Counties (Figure 19e). This is the only area
in the intermediate aquifer system sample set
where the sodium standard is significantly
exceeded. This high corresponds with the high
noted in the same area in the surficial aquifer

system (Figure 18e). The high is located near the
Caloosahatchee River and coincides with a re-
entrant in the Floridan aquifer system
potentiometric surface (Healy, 1962). This area
appears to be characterized by both natural
upwelling and heavy pumpage, and coincides with
the location of improperly abandoned wells that
were initially installed for petroleum exploration
purposes.

Floridan Aquifer System

Sodium in the Floridan aquifer system is
greatest along the coast, where the salt-
water/fresh-water transition zone is clearly
delineated (Figure 20). Elsewhere, minor sodium
highs appear to be a result of ion exchange in the
overlying Hawthorn clays, connate water, and
sampling of deeper, more mature waters. Table 12
summarizes the sodium-distribution data for the
Floridan throughout the state. The median sodium
concentration for the state is 11.0 mg/L; and the
range in concentrations is from a minimum of 0.2
mg/L to a maximum of 7043 mg/L.

In the NWFWMD (Figure 20a) the transition
zone is well defined, although few of the samples
exceeded the standard. A major re-entrant occurs
in Walton County, which reflects major withdrawals
in Okaloosa and Walton Counties (Wagner et al,
1984). Wagner et al. (1984) attributed the large re-
entrant in southern Bay and Gulf Counties to
pumpage. The re-entrant in Wakulla County is a
result of discharge from the large spring complex
surrounding Wakulla Springs and by pumpage.

Re-entrants in the SRWMD (Figure 20b) reflect
intrusion towards coastal cities and industries. Re-
entrants on the transition zone include one in
Taylor County that is a result of high pumpage near
Perry and a large re-entrant in Levy County that
corresponds with the Waccasassa River and
swamp. Minor sodium highs in Hamilton and
Alachua Counties correspond with areas of
withdrawal near White Springs and Alachua. The
high sodium in Bradford County corresponds to
withdrawals near Starke.

Data from the SJRWMD (Figure 20c) indicate
that the coastal transition zone is well-developed
throughout the District. A major re-entrant exists in
St. Johns County, and smaller, inland features
exist along the St. Johns River. Much of this high
sodium water is believed to be connate in origin
(Boniol, 1992, pers. comm.). Leve (1983) has

attributed similar features to upwelling along fault-
controlled regions of high permeability in the St.
Johns River. The upwellings are, in some areas,
apparently natural and associated with springs. In
other areas the upwellings are associated with
pumpage.

The coastal transition zone is apparent in the
SWFWMD (Figure 20d), with re-entrants in
Manatee County and along the axes of several of
the major rivers. These features have been
previously discussed.

Most of central and south SFWMD (Figure 20e)
has no data. This region is characterized by low
potentiometric heads and little flushing action.
Consequently, Floridan aquifer system water in
the District is saline and unfit for most uses. If
there were data in this region, sodium contours
would indicate high concentrations, with most
areas in excess of the standard. The contoured
data delineate this region of poor quality with
increasing sodium to the south.

Potassium

IMPORTANCE AND SOURCES

Potassium is primarily derived from sea water,
which averages 399 mg/kg (Table 8). Therefore,
coastal regions, where the fresh-water/salt-water
transition zone is present, are expected to contain
the highest potassium concentrations. Elsewhere,
potassium is derived in trace concentrations
(usually less than 0.2 mg/L) from marine aerosols
in precipitation and from minor weathering of clays
and feldspars. Weathering of potassium feldspars
and clays (see reaction 4) is not considered a
dominant process in Florida due to the scarcity of
these minerals in aquifer sediments and slow
weathering reaction rates. Inland, potassium is
rarely present in quantities over a few milligrams
per liter. This is because there is not a great
quantity of potassium-rich sediment in the aquifer
system and because potassium is immo-bilized as
a nutrient by plants and sorbed onto clays.

Potassium is an excellent indicator of the
integrity of newly installed wells because
potassium is a major constituent of drilling fluids
and cements used in well construction. High
potassium in a potable-water well often indicates
that either the well cement is poorly cured, the well
has been poorly developed and drilling fluids are

SPECIAL PUBLICATION NO. 34

still present, or the well cements are deteriorating.

STANDARD OR GUIDANCE
CRITERION

There is no standard or guidance criterion for
potassium. Potassium is an essential nutrient, and
is considered beneficial in low to moderate
quantities.

DISTRIBUTION IN GROUND WATER

Median potassium content in all districts and
aquifer systems is low indicating minor contri-
butions from aerosols and weathering. The values
that fall below the upper quartile (Table 13) are well
within the expected concentrations from
evaporative concentration of precipitation and rock
weathering. Very high concentrations (>100 mg/L
inland, >400 mg/L in coastal areas) are suspect.
These probably represent well construction
problems.

Surficial Aquifer System

The median potassium concentration for the
surficial aquifer system statewide is 1.2 mg/L
(Table 13). There is no significant difference in
potassium concentration medians or quartiles
within the state. Maxima do vary significantly, but
the highest values represent either sea water or
well-construction problems.

Potassium concentrations in excess of 100
mg/L inland may be considered artifacts of well
construction. These highs are found in several
districts (Table 13, Figure 21) and their distribution
appears to be random and uncorrelated to
adjacent wells.

Potassium concentration in sea water is
approximately three percent of the sodium
concentration. Therefore, potassium in meteoric
waters should be in the range 0.1-0.3 mg/L, if
derived from marine aerosols and not
concentrated by evapotranspiration. Most
samples should remain near three percent of the
sodium concentration unless weathering, plant
uptake, or sorption change the partitioning of
sodium and potassium. Most of the samples from
the surficial aquifer system are within or near this
concentration range, or they are in proper
proportions with sodium to indicate meteoric

origin.

The data from SJRWMD and SWFWMD
(Figures 21c,d) illustrate the influence of the
transition zone on potassium. Note that many of
the coastal re-entrants mentioned above are
represented in the potassium data. The high in
central Hardee County (Figure 21d) coincides with
a zone of upcoming saline water in the underlying
Floridan (Dalton, 1978; Lehman, 1978). Dalton
(1978) documented the flux of Floridan aquifer
system water into the surficial aquifer system in
Hardee County and showed that it is a result of
irrigation practices. Other highs in the surficial
aquifer system, therefore, may reflect irrigation
waters pumped from the underlying Floridan
aquifer system. The high potassium on the east
side of the SWFWMD roughly coincides with the
Lake Wales Ridge. Similar highs are not present in
the SFWMD data (Figure 21e), so SWFWMD data
are suspect at present.

Intermediate Aquifer System

The arguments that were given for potassium in
the surficial aquifer system hold for the intermediate
aquifer system. Median potassium concentration
statewide is 4.4 mg/L. Most of the data are well
within expected concentrations for meteoric water,
but high potassium concentrations that result from
weathering of the Hawthorn and possible well
construction problems exist (Figure 22).

Floridan Aquifer System

The distribution of potassium in the Floridan
aquifer system (Table 13; Figure 23) clearly
illustrates the influence of the transition zone.
There is a coastward increase in potassium in the
Floridan aquifer system in the districts (Figure 23).
Re-entrants along the coast were previously
discussed under Sodium. Scattered occurrences
of high potassium concentrations inland probably
reflect residual potassium in newly constructed
wells. Large areas characterized by minor highs in
potassium concentration in the regions where the
Hawthorn Group overlies the Floridan aquifer
system reflect rock weathering in the siliciclastic
section of the Hawthorn (Lawrence and Upchurch,
1976, 1982). Elsewhere, the data are consistent
with marine aerosols accompanied by minor
concentration as a result of evaporation in the near
surface environment.

Iron

IMPORTANCE AND SOURCES

Iron has two valence states, Fe 2 and Fe3*, and
is highly susceptible to reduction/oxidation redoxx)
reactions. Hem (1976) summarized the stability
relationships of iron in sulfur-rich systems. In
general, the sources of iron in ground waters
include (1) oxidation of pyrite (FeS), (2) oxidation of
organic compounds, and (3) dissolution of iron
oxide and silicate minerals (Table 4).

Upchurch et al. (1991) characterized the
surficial and Floridan aquifer systems in central
Florida in terms of reduction/oxidation redoxx)
potentials (Figure 24). They found that the surficial
aquifer system ranges from slightly oxidizing to
slightly reducing. The Floridan aquifer system is
generally reducing, although areas of rapid
recharge are likely to be oxidizing. The waters of
the intermediate aquifer system are similar to the
Floridan in redox potentials. Data from surficial
and Floridan aquifer system water samples from
Polk County were plotted on Eh-pH diagrams for
iron species, and they indicate that Fe2* is generally
the stable form of iron in central Florida ground
waters (Figure 24).

In general, Fe2+ is the stable iron phase in
acidic, reducing waters (Figure 24). Iron should
remain in solution in acidic, oxidizing waters. In
basic, reducing waters, pyrite (FeS2) and siderite
(FeCO3) are stable solids that may precipitate
depending on the sulfide and bicarbonate contents
of the water; whereas, in basic, oxidizing waters,
amorphous ferric hydroxide (Fe(OH)3) should
precipitate.

Ferric hydroxides form colloidal and larger
particles that generally do not travel long distances
in intergranular aquifers. Travel distances depend
on sizes of the colloids and of pore throats. These
colloids have been documented to travel distances
up to a few meters, but not kilometers. The parti-
cles are more likely to travel as suspended
sediment in karst conduits. Ferric hydroxide is the
reddish to yellowish scale or stain that is so
commonly found where iron-rich waters are
utilized. Ferric hydroxide forms rapidly when water
is heated in hot water heaters or aerated in the
vicinity of well pumps, sinks, toilets, and other
environments where oxidation of Fe2, to Fe31 is
possible. In soils and rocks ferric hydroxide slowly
crystallizes as the mineral goethite (Table 4).

Iron is closely associated with bacterial activity
in ground waters. In oxidizing environments,
bacteria induce colloidal ferric hydroxide
precipitation. These iron colloids and the
associated bacteria cause clogging of well screens
and aquifer pore throats. They also result in
violations of color and turbidity standards. For
example, in a water quality survey of the central
Florida phosphate district, Gordon Palm and
Associates (1983) found that 16 percent of the
shallow-well samples from mine areas violated the
water-quality standards for color and 20 percent
violated standards for iron. In deep wells, 13
percent violated the color standard and 10 percent
violated iron standards. Recharge wells utilized by
the phosphate industry are particularly susceptible
to iron, color and turbidity problems. The well
screens plug with bacterial mats and ferric
hydroxides; and dislodging these encrustations
from the well bore during sampling causes color
and iron standards to be violated (Upchurch et al.,
1991).

Ferrous iron (Fe2l) is a minor, but prevalent,
constituent in organic- (humic-substance) rich
waters. Because organic-rich waters include a
source of carbon, microbial activity tends to cause
strongly reducing conditions, which encourage
reduction of ferric iron (Fe3) to the ferrous state
and transport with the water. Ferrous iron is
known to move moderate to long distances in
reducing, karstic aquifers. Given the presence of
sulfide or phosphate, ferrous iron may precipitate
as pyrite (FeS3), vivianite (Fe3(P04)2.8H20), or other
mineral species.

Filtration, mode of sampling, and well
environment may greatly affect reported iron
concentrations. Iron analyses reported in this
study are total iron, and no attempt is made to
differentiate the two oxidation states. Since Fe3l
tends to precipitate as ferric hydroxide, it is
probable that iron concentrations in well-
developed wells are predominantly Fe2l. Samples
from poorly developed wells probably contain both
iron species, especially since the metals samples
were not filtered. Also, iron-bearing well casing
material may "rust" or otherwise prejudice iron
concentrations. For this study, only data from
non-metal cased wells were used, so the sample
density is much reduced over other analytes.

STANDARD OR GUIDANCE
CRITERION

Iron is subject to the Florida Secondary

FLORIDA GEOLOGICAL SURVEY

Drinking Water Standards, and the maximum
allowed concentration is 0.30 mg/L (300 gg/L;
Florida Department of Environmental Regulation,
1989). This is because of the potential for
discoloration and turbidity in waters with excess
iron.

Table 14 summarizes the proportions of
samples that exceeded the standard. Because of
possible contamination from iron and steel casing,
only samples taken from wells with non-metallic
casing are listed in the table. Statewide, 75 percent
of the surficial aquifer system samples exceeded
the standard. The range in the proportion that
exceeded the standard was from 70 to 90 percent.
The proportion that exceeded the standard in the
intermediate aquifer system was 42 percent
statewide, with a range of 14 to 86 percent. The
proportion that exceeded the standard in the
Floridan aquifer system was 49 percent, statewide.
The proportion ranged from zero to 70 percent.
Clearly, there is a high probability that any aquifer
system water sample from Florida will violate the
standard for iron, especially if it is an unfiltered
sample.

DISTRIBUTION IN GROUND WATER

Iron distribution data are summarized in Table
14. Note that median iron concentrations are
characteristically highest in the surficial aquifer
system. This is a result of proximity to sources of
iron in the siliciclastic portion of this aquifer
system, including iron minerals, ferric iron- and
organic-rich soil horizons, and dissolved humic
substances. Median iron concentrations are low in
all of the aquifer systems. High values
(concentrations > 5 mg/L) are probably a result of
using unfiltered samples. These high con-
centrations represent particulate ferric hydroxides
that were washed from the aquifer system under
the turbulent conditions characteristic of well
pumping.

Surficial Aquifer System

Median iron concentrations reported by district
from the surficial aquifer system range from 0.88 to
2.14 mg/L (Table 14). The statewide median is
1.08 mg/L. Normally, the surficial aquifer system is
high in iron because of the presence of organic
and reduction-oxidation reactions that can
mobilize iron.

Lack of physical continuity between sample
sites prevents contouring of data (Figure 25). The
high degree of variability reflects local well
conditions and surface conditions.

Intermediate Aquifer System

The statewide median iron concentration is
0.07 mg/L in the intermediate aquifer system.
There is a high range in median iron concentrations
(<0.05-1.17 mg/L; Table 14) due to the diversity of
environments in the Hawthorn Group. Iron is
abundant in the Hawthorn Group as a constituent
in clays, pyrite, goethite, and related iron
oxyhydroxides. Figure 26 illustrates the distri-
butions of iron in the districts.

Floridan Aquifer System

The distribution of iron in the Floridan aquifer
system (Figure 27) suggests that high iron waters
may occur near the coast. In SRWMD and
northern SWFWMD this reflects swampy
conditions overlying the unconfined Floridan.
Elsewhere, high iron near the coast and along the
re-entrants previously discussed indicates
mobilization of iron in the aquifer systems. Median
concentration of iron, statewide, is 0.21 mg/L.
Comparison of this median with average sea
water, which has an average concentration of 2
mg/kg (Table 8) indicates that iron concentrations
are increased in the wells by a factor of over 50.

Iron is present in waters directly affected by
the Hawthorn Group. There is also a strong
correspondence of iron and total organic carbon in
many areas of the state as a result of iron com-
plexing by humic substances (Young and
Comstock, 1986) (compare Figure 27 and the
distribution of TOC in Figure 55).

It is important to note that small iron anomalies
are evident in the aquifer system near areas where
rivers discharge water through swallow holes into
the Floridan at the Cody Escarpment in SRWMD
(Figure 27b). These anomalies suggest that areas
where rivers flow directly into karst conduits in the
Floridan aquifer system should be closely
monitored because of their sensitivity to rapid
ground-water quality deterioration, if water quality
in the rivers deteriorates.

Mercury

IMPORTANCE AND SOURCES

Mercury is included in this report because of
recent concerns about mercury in surface waters
and aquatic biota. There is considerable debate as
to the source of mercury in surface waters, and
discovery of mercury in the ground-water data
would have been of great assistance in
determining the origin of the mercury in surface
waters. The ground-water data do not indicate any
sources of mercury in the surface waters.

Elemental mercury is stable under Earth surface
conditions. It is slightly soluble in water (- 25 pg/L;
Hem, 1985), but if the water is open to the atmos-
phere, the mercury is sufficiently volatile that much
of it will escape as a gas. Mercury forms chemical
complexes with chloride and hydroxide in high ionic
strength solutions.

Metallic mercury (Hg) can be oxidized to either
Hg22, mercurouss) or Hg2+ mercuricc) valence states.
Both ions can form strong chemical complexes
with humic substances (Jenne, 1970; Jonasson,
1970; Cline and Upchurch, 1973). In addition,
methanogenic bacteria have the capability of
forming methyl mercury (HgCH3') from metallic
mercury in organic-rich environments (Wood et al.,
1968). Methylated mercury is readily soluble in
body tissues resulting in bioaccumulation of
mercury and its entrance into the food chain.

Mercury minerals are unknown in Florida's
sediments. Sea water, which averages 0.03 pg/kg
Hg, is an important natural source of mercury.
Assuming that the average chloride in precipitation
is 1.66 mg/L (cf. Table 3), the equivalent mercury
concentration in rainfall is estimated to be
approximately 0.05 parts per trillion, which is not
detectable. Marine aerosols may, therefore,
transport very small amounts of mercury inland.
Quaternary marine transgressions may constitute
an additional source of natural mercury in Florida.
Mercury forms strong chemical complexes with
sedimentary organic and clays. Marine trans-
gressions place sea water in juxtaposition with
these sorption media, and sorbed or completed
mercury may remain as the sea retreats.

Human activity has undoubtedly contributed to
the availability of mercury in the environment.
Draining peat- and muck-rich sediments may allow
oxidation of the organic, and any sorbed or
chemically completed mercury has the potential

for release into surface waters and the surficial
aquifer system. Atmospheric fallout is another
possible source of mercury. Burning of fossil fuels
(coal, oil) and municipal and industrial wastes has
the potential of introducing variable amounts of
mercury into the atmosphere and, ultimately, onto
the land surface as either dry fallout or
precipitation (Hem, 1985). Finally, many pesticides
that have been widely used contain mercury
compounds (Grier, 1968). Organomercuric
compounds were used as seed grain treatments
prior to the 1960's. Phenylmercury salts and other
mercury compounds have been widely distributed
for bactericides and fungicides. These pesticides
may also constitute a source of mercury in surface
environments. The relative importance of these
sources are unknown for Florida ground waters at
the present time.

Due to the small natural amounts of mercury in
Florida's aquifer systems and the strong affinity of
mercury for sorption and/or chemical completing
with natural organic it is unlikely that mercury
would be naturally present in detectable amounts.
Any mercury detected in the aquifer systems as
part of the Background Network is most likely a
result of human activity, including atmospheric
fallout or agricultural use of mercury-containing
pesticides.

STANDARD OR GUIDANCE
CRITERION

Mercury is subject to the Florida Primary
Drinking Water Standards (Florida Department of
Environmental Regulation, 1989), and the
maximum allowed concentration is 0.002 mg/L (2
pg/L). This is because of the potential for
accumulation in the food chain and serious toxicity
problems in humans. Mercury intake in humans
has been associated with chronic and acute
toxicity, especially mental illness and gastric and
respiratory distress (Grier, 1968).

Table 15 summarizes the samples that were
found to exceed the 2 lg/L standard. Many of
these detections have not yet been confirmed by
resampling. As might be expected, mercury in
excess of the standard is most common in the
surficial aquifer system. Statewide, two percent of
the samples exceeded the standard. The
proportion of samples that exceeded the standard
ranged from zero to ten percent.

It is somewhat surprising that three percent of
the samples from the intermediate aquifer system

SPECIAL PUBLICATION NO. 34

exceeded the standard. Given the reducing con-
ditions and clay and organic content of the
intermediate aquifer system, one would expect
mercury to be immobile. The range in proportions
of samples within the districts is zero to 13
percent. The district with 13 percent of its
intermediate aquifer samples exceeding the
standard is the SJRWMD.

As might be expected, few samples from the
Floridan aquifer system exceeded the mercury
standard. Statewide, the proportion was 0.9
percent, with a range of zero to two percent.

DISTRIBUTION IN GROUND WATER

The concentration data indicate that there is
minimal opportunity for mercury to enter surface
waters from the ground-water system and that the
risk of exposure to humans from state ground
waters is minimal. A few wells in each aquifer
system have detectable amounts of mercury.
Since the samples that did have detectable
quantities of mercury are widely scattered, and
because many of the incidences of detection have
not been confirmed by resampling, maps showing
the locations of these detections are not included
in this report.

Surficial Aquifer System

Mercury concentrations are generally quite low
in the surficial aquifer system (Table 15). Median
concentrations are below detection limits in the
surficial aquifer system. With the exception of data
from the NWFWMD, at least 75 percent of all
samples were below detection limits. The
SJRWMD had a sample with 52 gg/L.

Intermediate Aquifer System

Similar results exist in the intermediate aquifer
systems. The medians are below detection limits,
and only SJRWMD had more than 25 percent of
the samples with detectable concentrations.

Floridan Aquifer System

The median concentration in the Floridan
aquifer system is also below detection limits. At
least 75 percent of the sample sets from all
districts were below detection limits, as well.
Statewide, there were only six samples from the

Floridan aquifer system with mercury in excess of
the standard.

Lead

IMPORTANCE AND SOURCES

Lead minerals are very rare, but trace
quantities of lead are present in feldspars and
other minerals. Average sea water contains 0.03
Rg/kg lead (Table 8), so Plio-Pleistocene marine
transgressions may have resulted in deposition of
trace quantities of lead in peats and mucks, clay
beds, and other favorable sites. As with mercury,
marine aerosols can be expected to transport lead
into the interior, but only in concentrations of
fractions of a part per trillion concentration.
Because of the lack of an obvious source, any
occurrence of lead in Florida ground water is
probably a result of human activities.

Dissolved lead is divalent (Pb2+; Garrels and
Christ, 1964; Hem, 1985) in most natural waters. If
carbonate is present, lead carbonate (cerussite,
PbCO3) is relatively insoluble and precipitation of
cerussite may control the solubility of lead in
carbonate aquifers. Lead sulfate (anglesite, PbSO4)
and lead sulfide (galena, PbS) are also relatively
insoluble and are likely to precipitate in sulfur-rich,
oxidizing and reducing environments, respectively.
None of these minerals have been found in Florida,
but sorption isotherm experiments by Upchurch et
al. (1991) indicate that trace amounts of cerussite
should precipitate where lead-rich waters en-
counter Florida carbonate rocks.

Lead is also strongly bound to organic,
colloidal oxyhydroxides, and clay surfaces by
sorption and/or chemical completing mechanisms
(Hem, 1976; Moore and Ramamoorthy, 1984;
Upchurch et al., 1991). Sorption isotherms
(Upchurch et al., 1991) of lead on surficial aquifer
system quartz sand (a ferric hydroxide-coated
sand), Hawthorn Group clay (montmorillonite,
palygorskite), and Floridan aquifer system
limestone indicate that all three media are able to
fix lead, but the limestone and sand have less
sorption capacity than the Hawthorn clays. In this
experiment, 99.6 percent of the lead was removed
on the clay, while both the sand and the limestone
were responsible for removal of 97.8 percent.

The absence of a strong source and the affinity
of lead for both mineral precipitation and sorption

or completing suggest that ground-water samples
from Florida's aquifer systems should not contain
detectable lead. Lead concentrations in natural
systems are expected to be in the order of 2 gg/L
or less (Hem, 1985).

Human influences, alternatively, do contribute
lead to the land surface. Fallout from fossil fuel
combustion (e.g., leaded gasoline, coal and fuel
oil); disposal of lead-containing wastes (e.g.,
batteries, paints); widespread use of lead solders,
flashings, paints; use of lead weights and solder in
well or water systems; past uses of lead in
pesticides by the agriculture industry; and spent
bullets are among the many potential sources of
lead in Florida environments. Many of the wells
that were used in this study have had lead weights
on water-level recorders installed at one time. This
may be a reason for some of the detections of lead
mentioned below.

STANDARD OR GUIDANCE
CRITERION

Lead is subject to the Florida Primary Drinking
Water Standards (Florida Department of
Environmental Regulation, 1989), and the
maximum allowed concentration is 0.050 mg/L (50
gg/L). Lead compounds are highly toxic to animals
and humans. Accumulation in the food chain has
been documented. Lead intake in humans has
been associated with chronic and acute toxicity,
and children are particularly at risk (Harris, 1968;
Hem, 1985).

Table 16 summarizes the samples found to
exceed the standard in Florida's aquifer systems.
Many of these exceedances have not been
confirmed by resampling. There is a large number
of samples that exceed the standard. The very
high values (>500 gg/L) almost certainly represent
some contamination problem. All samples with
concentrations above detection limits probably
reflect local contamination. Given that local
contamination as a result of land use cannot be
documented for these wells from which the
samples came, a probable source for the lead is
the use of lead weights in the wells. According to
SFWMD staff (J. Herr, 1991, pers. comm.) many of
the high lead concentrations in that district are a
result of use of lead weights on water-level
recorders in the wells. A similar situation appa-
rently exists in the SWFWMD, where intermediate
and confined Floridan aquifer system samples
have reported lead.

While the widespread nature of lead is a matter
of concern at this time, it is important to recall that
the samples were not filtered, many of the wells
have had lead weights on water-level recorders in
the past. Also, most of the analytical results have
not been confirmed by subsequent sampling. If
the high proportion of wells with detectable lead
continues after the second round of sampling and
analysis of the Background Network data, which
will include filtered and unfiltered samples, has
been completed, then it will be necessary to
determine the cause of lead mobilization in
Florida's aquifer systems.

Samples from the surficial aquifer system that
exceeded the standard made up eight percent of
the samples statewide. This is a high number, but
the range of proportions of wells in which lead was
detected varied from zero percent in the SRWMD
to 21 percent in SWFWMD. The proportion of
samples that exceed the standard in the
intermediate aquifer system averages eight
percent statewide, with a range of two to 24
percent by district. Again, this proportion is high
and suspect. Finally, the proportion in the Floridan
aquifer system is nine percent statewide.
NWFWMD had zero percent above the standard,
and SJRWMD and SWFWMD had 19 and 20
percent, respectively.

DISTRIBUTION IN GROUND WATER

The distribution of lead in Florida's aquifer
systems is summarized in Table 16. Several rather
startling trends are apparent in the lead data.

First, lead appears to be widespread, al-
though usually in quantities less than the standard.
The widespread nature of the metal, combined
with the large number of samples that exceeded
the standard, suggest the need for additional study
and a potential problem.

Second, it is somewhat surprising that
detectable lead is present in the carbonate- and
clay-rich aquifer systems. The intermediate aquifer
system is rich in clays and lead should be
adsorbed, rather than mobile. Lack of sample fil-
tration is likely to result in analysis of the sorbed-
lead fraction, however. In the carbonate aquifer
systems (Floridan aquifer system and portions of
the intermediate and surficial aquifer systems),
lead is likely to precipitate as cerussite.
Comparison of filtered to unfiltered samples in the
second Background Network sampling will

FLORIDA GEOLOGICAL SURVEY

indicate the presence of sorbed or precipitated
lead.

Since the samples that did have detectable
quantities of lead are widely scattered, and
because many of the incidences of detection have
not been confirmed by resampling, maps showing
the locations of these detections are not included
in this report.

Surficial Aquifer System

Median lead content of samples from the
surficial aquifer system statewide was 2 tLg/L. This
is a high value, which is subject to concern. The
medians ranged from less than <2 ugg/L in SFWMD
to below 10 gg/L in the NWFWMD and SRWMD to
36 gg/L in the SWFWMD. The upper quartiles
(75th percentiles) for lead concentration in the
SJRWMD exceeded the standard. Maximum
concentrations exceed 1,000 .gg/L lead in two
districts. The three wells in SFWMD with the
highest measured lead concentrations had once
contained either water-level recorders with lead
weights or had casings perforated by bullet holes.

One would expect that the surficial aquifer
system might be the most susceptible to lead
contamination through fallout or local land uses.
The low clay content, low carbonate content, high
acidity, and high organic content of surficial aquifer
system waters can lead to lead mobility. However,
the large proportion of samples with detectable
lead, and the high concentrations detected in
many wells, seems too great, even with these
potential transport conditions.

Intermediate Aquifer System

Median lead concentrations in the intermediate
aquifer system vary from below detection limits in
three districts (Table 16) to <43 gg/L in the
SWFWMD. Maximum and upper quartile concen-
trations are not as high as in the surficial aquifer
system, but they are still high. Given that the
intermediate aquifer system includes the clay- and
organic-rich sediments, as well as carbonate-rock
horizons, the incidence of lead should be lower.
The SWFWMD samples show the highest lead
concentrations, which reflects, in part, use of lead
weights on water-level recorders in some wells.

Floridan Aquifer System

Lead concentrations from the Floridan aquifer
system are roughly equivalent to those in the
intermediate aquifer system. The statewide
median concentration is below detection limits, but
two districts had upper quartile samples above
detection limits, and two are near the standard.

ANIONS

Classification

Anions are negatively charged species in
aqueous solutions. They form by taking on
electrons given up by cations. Unlike the cations,
many of the naturally occurring anions are not ele-
mental (e.g., CI-), they are compound radicals (e.g.,
P043-). Anions can be classified according to their
abundances in the natural environment.

MAJOR ANIONS

Anions that are present in aqueous systems in
concentrations greater than 1.0 mg/L are said to
be major anions. Major anions in ground-water
systems usually include bicarbonate (HCO3),
sulfate (SO42-), and chloride (CI-). These anions are
used to classify hydrochemical facies and are
discussed in the following section.

MINOR ANIONS

Minor anions range in concentration from
0.001 to 1.0 mg/L. The important minor anions in
Florida ground waters include fluoride (F), nitrate
(NO,), sulfide (S2- and HS), and orthophosphate
(P043-). With the exception of sulfide, the minor
anions are discussed in separate sections. Sulfide
data are limited, so the anion is discussed in
conjunction with sulfate.

TRACE ANIONS

Trace anions normally occur in concentrations
less than 0.001 mg/L. Carbonate (C032-), nitrite
(NO2), and organic nitrogen (total Kjeldahl nitrogen
[TKN]) are included in this group and discussed
below. Carbonate is discussed with bicarbonate.
Nitrite and TKN are discussed in conjunction with
nitrate. While ammonium (NH4 ) is a positively
charged cation, it is included with nitrate for

convenience.

Bicarbonate, Carbonate, and
Alkalinity

IMPORTANCE AND CONTROLS

The importance of the carbonate system as an
agent in weathering reactions has been previously
discussed. Bicarbonate and carbonate are dis-
sociation products of the reactions between
carbon dioxide and water. Carbon dioxide and
water react to produce carbonic acid (reaction 1).
Carbonic acid dissociates to bicarbonate and
hydrogen ions (reaction 1), which are represented
by pH. The hydrogen reacts with rock materials in
the weathering reaction (reaction 2), and
bicarbonate remains as the anionic constituent in
the water. Bicarbonate can further dissociate to
carbonate according to the reaction

HCO3 =' H + CO ~

Thus, pH, bicarbonate, and carbonate are closely
related.

Bicarbonate Bicarbonate (HCO,) is the dominant
carbonate species in the pH range of 6.4 to 10.3 at
250C. Bicarbonate is the primary anionic
weathering product (reactions 2 and 3), so it is an
important indicator of chemical maturity in an
aquifer. Because of its role in the weathering
reaction, bicarbonate is usually the dominant anion
in potable ground waters.

Carbonate Carbonate (CO2-) does not become an
important anion in water unless pH values are in
excess of 8 to 8.5. In most natural systems, car-
bonate is not an important anionic constituent, and
concentrations are small compared to bicarbonate.
Depending on the method of analysis, carbonate
concentrations are unlikely to be reported.

In order for carbonate to be a dominant anion
in water, extraordinary conditions that cause high
pH must exist. Such conditions occur in saline,
arid-environment lakes where pH is greater than 9.
Buffering of the pH through rock-water interactions
limits natural carbonate activities in ground-water
systems in Florida.

Cements, grouts, and some drilling fluids used

in well construction are highly alkaline. If the
cements are not properly cured and if wells are not
properly developed, the pH of the resulting well
water may be alkaline and carbonate may be
present. Therefore, in Florida's aquifer systems,
where highly alkaline environments are not
considered normal, carbonate can be used as a
possible check on well development and
construction.

Alkalinity Total alkalinity is determined by
titrating all of the anions in a solution with strong
acid. The variable is, therefore, a measure of the
ability of a water to consume, or buffer, acid, and it
is a measure of the total anionic concentration in the
water that can be titrated with an acid. The anions
that are neutralized by titration with strong acid
include HCO3, C032-, B(OH)4, HSiO4-, HS-, P43-,
and some organic ligands. Total alkalinity
includes all of these acid-neutralizable anions. In
most natural systems, bicarbonate is the major
anion, and concentrations of the other anions are
minor. As such, bicarbonate is the dominant
constituent of alkalinity of the water. If the analysis
of alkalinity includes only bicarbonate and carbo-
nate, then the alkalinity is called the carbonate
alkalinity. The other components can, however,
significantly contribute to total alkalinity in some
waters. Non-carbonate alkalinity is the total
amount of the non-carbonate species that can be
titrated. Of the non-carbonate components listed
above, HSiO,-, HS-, and P043- are likely to be
significant in Florida waters.

The districts varied in how alkalinity and
concentrations of carbonate and bicarbonate were
measured. Some measured bicarbonate and
carbonate, others carbonate alkalinity, and yet
others total alkalinity. In some cases the districts
measured more than one analyte. Consequently,
the tables and maps that follow indicate the nature
of the variable reported.

DATA INTERPRETATION

Three tables are used to indicate the variety of
alkalinity and carbonate-species measurements.
Table 17 reports bicarbonate concentrations, Table
18 gives carbonate concentrations,and Table 19
reports total alkalinity. Note that the method of
reporting total alkalinity includes both calcium
carbonate alkalinity and alkalinity in milliequivalents
per liter13.

Wide ranges in bicarbonate content (Table 17)

SPECIAL PUBLICATION NO. 34

reflect different levels of "chemical maturity" of the
ground waters. In general, high bicarbonate
concentrations indicate that the waters have
undergone significant equilibration with the rock
matrix. Low values indicate that little equilibration
has occurred.

The bicarbonate concentration ranges repor-
ted in Table 17 are indicative of some of the
problems associated with bicarbonate analyses in
aquifer systems. For example, in the surficial
aquifer system, zero bicarbonate concentrations
are possible due to the acidity of waters in
siliciclastic aquifer materials, especially those
characterized by organic acids. High bicarbonate
concentrations reflect shelly siliciclastic or
limestone aquifers in which equilibration with a
carbonate mineral has occurred. The same
relationships are possible, but less likely, in the
intermediate aquifer system. Here, carbonate
clasts are mixed with the siliciclastics and there are
widespread horizons of limestone and dolostone.

Significant aqueous carbonate concentrations
are unusual in natural systems. They are usually
caused by improper well installation or devel-
opment. Low bicarbonate concentrations (Table
17) associated with high pH (Table 7) values may
reflect high alkalinity in the form of carbonate ion
(CO 3). Obvious problem data have been omitted
from this analysis, yet some data remain that
suggest influences from well construction.
Carbonate concentrations are summarized in Table
18. Comparison of the bicarbonate and carbonate
concentrations in the database, or the bicarbonate
and pH values on the maps in this report, should
suffice to differentiate between natural carbonate
and well-related carbonate. In the Floridan aquifer
system, zero bicarbonate concentrations should
not occur. Newly recharged water located in a
large, karst conduit and, therefore, not in contact
with the host carbonate rock may still be acidic
and have low bicarbonate. Mixing and partial
equilibration with the host rock should occur in
most cases.

The bicarbonate content of the aquifer water
can be used as a very rough indicator of the ability
of the aquifer system to tolerate acidic wastes.
Low bicarbonate waters, such as occur in
siliciclastic aquifers, have little or no tolerance,
while high bicarbonate aquifers have some
tolerance.

STANDARD OR GUIDANCE
CRITERION

The carbonate species do not constitute a
health hazard per se; therefore, there are no
standards or guidance criteria for the carbonates.
To an extent, carbonate concentrations are
represented in criteria for pH. There is a maximum
pH criterion of 8.5, which is about the pH where
HCO, concentration begins to decline and CO32-
begins to grow in importance.

DISTRIBUTION IN GROUND WATER

Most Florida ground-water samples have low
to non-detectable carbonate contents (Table 18).
Wells with detectable carbonate, especially those
wells with carbonate contents in excess of 1 mg/L,
are likely to be newer wells that have not been
thoroughly developed or in which cements and
grouts have not yet cured.

Available alkalinity data for the aquifer systems
(Table 19) are inconsistent. Some of the districts
reported alkalinities in milligrams per liter as
CaCO3, others reported it in milliequivalents per
liter, others did not report it at all. In spite of the
inconsistent data, alkalinity is included in this
report because it is a common analyte in ground-
water contamination studies. Note that, where
available, the alkalinity data closely follow the
bicarbonate concentrations, indicating that the
primary source of alkalinity is bicarbonate.

Surficial Aquifer System

Median bicarbonate (Table 17) and total
alkalinity (Table 19) in the surficial aquifer system
increase to the south in response to increasing
calcium-carbonate content of the aquifer system.
Statewide, the median bicarbonate concentration
is 138 mg/L, no carbonate is present, and total
alkalinity is approximately 111 mg/L as CaCO,.

Bicarbonate concentrations are generally low
in the surficial aquifer system in northern Florida
(Figure 28a,b) as a result of lack of carbonate
minerals in the aquifer system. These low bicar-
bonate concentrations are associated with low
pH's and often with high total dissolved carbon.
Comparison of Figure 28 with Figure 9 illustrates
the relationship between low pH and low bicar-
bonate content.

Shell and limestone content of the surficial
aquifer system increases to the south. As a result,

rock weathering becomes more important and
bicarbonate content increases. The southward
gradient is well illustrated in the SWFWMD (Figure
28c). The increase in bicarbonate content toward
the coast and along the Peace River axis is an
artifact of a general increase coastward in ionic
strength of the water associated with increased
shell contents in coastal, Plio-Pleistocene
sediments.

A similar coastward increase in total alkalinity
is weakly shown in the data from the northern
portion of the SFWMD (Figure 28d). Waters along
the Kissimmee River valley and adjacent parts of
the Lake Wales Ridge and along the
Caloosahatchee River axis have low alkalinities,
while waters elsewhere have high alkalinities.
There are no wells in the surficial aquifer system in
the majority of the Everglades and Big Cypress
drainages. Available data indicate that alkalinities
are low in these regions, which are characterized
by organic-rich waters and direct interconnection
between the surficial aquifer system and surface
water. The SFWMD well with the 2,260 mg/L
concentration probably represents contamination
by grout or drilling mud. Subsequent samples from
this well have produced much lower alkalinity
concentrations.

Carbonate was only detected in wells in the
SRWMD (Table 18), where well construction is
known to have been a problem. In SRWMD and
SWFWMD, the median and quartiles are below
detection limits.

Intermediate Aquifer System

The Hawthorn Group and associated strata
contain abundant carbonate material. Dolostone
and limestone beds are widespread and often
constitute major water-producing zones. Silici-
clastic horizons contain carbonate clasts reworked
from these beds. Also, the Hawthorn contains
beds of silt-sized, unconsolidated dolomite (Scott,
1988). These fine-grained "dolosilts" are highly
reactive with ground water. Water in contact with
either source of carbonate gains bicarbonate
alkalinity as a result of weathering. Consequently,
ground waters in the intermediate aquifer system
generally have higher bicarbonate alkalinities than
do waters in the surficial aquifer system. The
median bicarbonate concentration in the
intermediate aquifer system, for example, is 143
mg/L as compared to 138 mg/L in the surficial
aquifer system. This difference is particularly
distinct in north Florida, where the surficial aquifer
system waters contain little bicarbonate (compare

Figures 28 and 29).

Bicarbonate content in the intermediate aquifer
system is highly variable due to the heterogeneous
nature of the Hawthorn Group and related
sediments. Samples with lower bicarbonate
contents are either from karstic zones or from
siliciclastic horizons. Samples with higher
bicarbonate concentrations are from carbonate-
rich beds, including the dolostones, limestones, or
dolosilts.

Carbonate (Table 18), converted to mg/L
bicarbonate gives similar concentrations.
Carbonate was detected in SRWMD and in a few
SWFWMD wells, where well development problems
have been reported.

Floridan Aquifer System

The Floridan is almost entirely limestone or
dolostone, so bicarbonate concentrations are
characteristically high and variability is less than in
the other aquifer systems. Statewide, the median
concentration is 146 mg/L, and the quartile range
is 96 mg/L.

A weak coastward increase in bicarbonate is
present in the Floridan aquifer system throughout
the state (Figure 30a, b, and d). This increase is a
result of chemical maturation along ground-water
flow lines. Chemical maturation within the Floridan
aquifer system is characterized by increases in
ionic strength accompanied by dissolution of
carbonate minerals (Runnels, 1969; Drever, 1988).
Where deeper waters rise to the surface at springs,
this increase in bicarbonate is pronounced. For
example, the large re-entrant in Liberty, Wakulla,
and Franklin Counties (NWFWMD, Figure 30a) is a
result of upwelling of deeper waters on the inner
margin of the transition zone. Note that average
sea water contains about 142 mg/L bicarbonate
(Table 8), so bicarbonate concentrations higher
than 200 mg/L are a result of maturation along the
flow path, not salt-water intrusion.

The pattern of bicarbonate in SRWMD (Figure
30b) is complicated because of the presence of
flow systems east and west of the Suwannee
River. The highest bicarbonate values are near
springs and areas of regional discharge. High
bicarbonate concentrations also exist along the
Cody Escarpment, in Columbia and Alachua
Counties, and in the Northern Highlands, where

FLORIDA GEOLOGICAL SURVEY

flow is thought to be relatively stagnant and
recharge is limited due to the presence of thick
confining strata (Lawrence and Upchurch, 1976).

Total alkalinity in the Floridan aquifer system in
the SJRWMD (Figure 30c) suggests a weak re-
entrant that follows the St. Johns River. Many of the
higher alkalinity concentrations reflect deeper wells.

Bicarbonate concentrations in the SWFWMD
(Figure 30d) reveal two significant patterns. The
data from the northern half of the district show an
irregular distribution of bicarbonate. Part of this
irregular pattern is a result of variations in well
depth, but much of it reflects the unconfined to
poorly confined nature of the Floridan aquifer
system in the area. Local flow systems and
regions of direct discharge result in the irregular
pattern. The data from the southern half of the
district show a simpler distribution pattern due to
the confined nature of the aquifer system.

Data from the SFWMD (Figure 30e) are
insufficient to provide much information. Total
alkalinities are low in the Kissimmee River valley
and Lake Wales Ridge area. The Lake Wales
Ridge is a region of recharge, which suggests that
the low alkalinities are a result of a low level of
chemical maturation. In contrast, Floridan aquifer
system waters near the coast contain higher
alkalinities, which reflect maturation. As indicated
below, there is an abundance of sulfate in Floridan
aquifer system water in coastal and the central and
southern parts of the SFWMD. This sulfate
replaces bicarbonate as a major anion, and there-
by reduces akalinities.

Sulfate

IMPORTANCE AND CONTROLS

Sulfur occurs in several oxidation states,
depending upon the reduction/oxidation potential
of the water. Two are of major importance in
aquifer systems. These are the oxidized form,
sulfate (SO42), in which sulfur has a valence of +6,
and the reduced form, sulfide (S2- or HS), which
has a valence of -2.

SOURCES AND SINKS OF SULFUR

There are many potential sources of sulfur in
Florida's aquifer systems. Sulfate is directly

introduced to the system by marine aerosols
(Table 8) and by acidic precipitation from airborne
sulfur oxides. Deposition of significant quantities
of airborne sulfur oxides is a recent phenomenon
related to the acid-rain problem. Waters that
recharged Florida's aquifer systems prior to the
late 1800's should contain little or no
anthropogenic sulfur. Recently recharged waters
may contain significant amounts of this sulfur.
Modern precipitation contains an average of 1.75
mg/L sulfate statewide (Table 3). The highest
sulfate concentration recorded in the precipitation
data (Table 3) was almost 23 mg/L, which is well
within the range of concentrations recorded from
Florida's various aquifer systems.

Rock weathering is the most important natural
source of sulfur in aquifers. Most rocks contain at
least trace quantities of pyrite (FeS2) and other
metal sulfides. These sulfide minerals can be
oxidized according to the reaction
R-S2 + 3.502 + H20
(11)
SR2+ + 2S02- + 2H+,

where R-S2 is a metal-sulfide compound, such as
pyrite, containing reduced sulfur, and R2+ is a
metal, such as iron, that is simultaneously involved
in the oxidation reaction
R2 + 0.2502 + 2.5H20

(12)
=1R~ (OH), + 2H,.

If the reaction involves pyrite, R2, is Fe2*, which is
oxidized to Fe3,. Thus, oxidation of metal sulfide
minerals results in the production of sulfate and
hydrogen ion, which reduces the pH. Reactions 11
and 12 are the "acid-mine drainage" reactions.
Pyrite is found in all of Florida's aquifer systems
and aquitards. It is especially abundant in clay-rich
horizons of the Hawthorn Group (Table 5).

Dissolution of gypsum and anhydrite at the
base of the Floridan is also an important source of
sulfur as sulfate. The dissolution of gypsum has
been documented by Rightmire et al. (1974) and
Rye et al. (1981). This sulfate is brought upward
along the coastal transition zone, so that much of
the coastal Floridan aquifer system can contain
high sulfate concentrations.

Sulfur is a necessary element for life. Plants
contain sulfur in amino acids and other organic
components. They obtain this sulfur by reducing
dissolved sulfate, and they pass the sulfur on to
heterotrophs, including humans and other animals.
Because sulfur is stored in tissues, decomposition
of organic materials constitutes another possible
source of sulfur as sulfide. Sulfur constitutes
about one percent dry weight in organisms.
Decomposition of humic substances by microbes
involves chemical reduction, so the released sulfur
is usually in the form of sulfide. Oxidation may
follow rapidly.

Sea water is an important source of sulfate in
coastal environments. Average sulfate concen-
tration in sea water is 2,710 mg/kg (Table 8), so the
transition zone should contain a concentration
gradient that increases toward the coast regard-
less of any contributions from the deep flow
system.

Finally, a number of agricultural, waste
disposal, and industrial activities release sulfur
compounds to ground water. Gypsum (Table 4) is
used as a soil amendment to acidify soils. Landfill
leachate can be high in sulfates and/or sulfides.
Many industries, such as fertilizer, battery, and
plating plants, release sulfates, usually as sulfuric
acid. Phosphogypsum disposal at agrichemical
plants in central and north Florida has been shown
to contribute sulfate to ground water in the
immediate vicinity of the gypsum disposal areas
(Miller and Sutcliffe, 1982, 1984). Hutchinson
(1978) included analyses of seven surficial aquifer
system water samples in the Alafia River basin. He
found calcium-sulfate water in two wells that are
distant from the phosphochemical plants. There-
fore, high sulfate concentrations in the general area
of phosphochemical plants may not be a direct
product of those fertilizer plants.

Sulfate is removed from surficial aquifers by
plant metabolism. It can also be removed by
aquifer microbe metabolism, including reduction to
sulfide. Precipitation of sulfate as gypsum can
occur at high ionic strengths, such as occur in
evaporative lakes and desert soils. Although there
is no evidence of sulfate mineral precipitation in
Florida today, sulfate-rich evaporite minerals are
common at the base of the Floridan aquifer system
and locally in the Hawthorn Group. Sulfides are
removed from ground water by oxidation to
sulfate, metal-sulfide mineral precipitation,
degassing of H2Sgas, and microbial fixation.

Sulfide Given prevailing reducing conditions,
sulfide is the thermodynamically favored species
(Figure 31) in most Florida aquifer system
environments. Upchurch et al. (1991) studied the
Eh-pH conditions of surficial and shallow Floridan
aquifer systems in Polk County. They found that
surficial aquifer system water is reducing (-300 to 0
mV) and, given a pH range of 4 to 6, H,S is the
most probable sulfur species. Water samples from
the Floridan aquifer system were influenced
somewhat in their study by interaquifer recharge.
Eh ranged from -250 to 0 mV, and the pH was near
7 (Figure 31). Under those Eh and pH conditions,
the Floridan aquifer system water samples fell near
the intersection of the SO42-, H2S, and HS-stability
fields. It appears that, outside of regions of
immediate recharge, Floridan aquifer system water
is reducing and H2S is the stable sulfur species.
Hydrogen-sulfide odor (the familiar "rotten egg"
odor) is detectable in many wells in all aquifer
systems of the state, supporting the conclusion of
Upchurch et al. (1991).

Much of the H2S in aquifer systems is a
result of sulfate reduction. The conversion of
sulfate to sulfide is usually accomplished by
aquifer or soil microbes, and follows a reaction
such as shown in reaction 6. Note that a source of
organic carbon must be present for sulfate
reduction. Rightmire et al. (1974) and Rye et al.
(1981) used isotopic analysis of sulfur to determine
the origins of sulfate and sulfide in Floridan aquifer
system waters. Sulfide in shallow portions of the
Floridan near recharge areas was shown to be a
result of sulfate reduction.

Unfortunately, sulfide was not determined
throughout the Background Network, so only the
role of sulfate can be documented in this report.
There are some sulfide data from the SWFWMD
that allow a partial understanding of sulfide
concentrations in Florida's aquifer systems.
Sulfide in the surficial aquifer system averages 0.55
mg/L (o- = 1.09 mg/L, range = 0.00-5.50, n = 82).
There are no data for the intermediate aquifer
system. In the Floridan aquifer system, sulfide
averages 0.44 mg/L (o" = 0.63, range = 0.00-2.43,
n = 136). These concentrations are consistent with
concentrations in equilibrium with H2S at the Eh
and pH ranges found by Upchurch et al. (1991).

Sulfate If the water is oxidizing (Figure 31),
sulfides may be oxidized to sulfates. This reaction
can be driven either inorganically or microbially.
The reactions can be characterized by reactions 11
and 12. Note that the product is SO42 plus H,, in
other words a dilute sulfuric acid solution.

SPECIAL PUBLICATION NO. 34

While sulfide oxidation is a widespread
reaction in Florida's aquifer systems, the dominant
source of sulfate is dissolution of gypsum and
anhydrite near the base of the Floridan aquifer
system. These minerals, which are interstitial in
the Eocene Avon Park Formation and lower
horizons, are dissolved into deep flow systems in
the Floridan. The result is that Floridan aquifer
system water that upwells near the coastal
transition zone has an inner "belt" of sulfate-rich
water. Rightmire et al. (1974) and Rye et al. (1981)
showed that dissolution of the sulfate minerals,
rather than oxidation of sulfides, is the dominant
cause of the deeper and coastal sulfate-rich
waters.

Microbial Activity Microbial processes and
chemical kinetics determine the rates of
conversion from one species to the other (Connell
and Patrick, 1968; Rye et al., 1981). The ability of
the microbes to function in an aquifer system is
dependent upon a complex array of conditions
and, if these are not met, the sulfur may not be
altered regardless of Eh and pH. For example, if
there isn't a source of organic carbon (reaction 6),
aquifer microbes may not be able to reduce sulfate
to sulfide. Therefore, metastable sulfur species
may be present in the water samples.

The abundance of sulfate in the deep Floridan
aquifer system is an example of a potentially
metastable sulfur species. The deep Floridan is
chemically reducing, and sulfide is generally the
stable species. While there is some sulfide in deep
Floridan aquifer system waters, microbes appear
to be unable to effectively reduce all of the sulfate
derived by dissolution of gypsum or anhydrite near
the base of the aquifer system. The inability of the
microbes to reduce the sulfate may be a result of a
lack of an organic carbon source (Watrous and
Upchurch, in prep.). Consequently, sulfate persists
for many years and many kilometers along the flow
path.

STANDARD OR GUIDANCE
CRITERION

Even though sulfur is necessary for life,
adverse effects arise if it is present in hazardous
forms or concentrations. Formation of sulfate
through aqueous sulfide oxidation (reactions 11
and 12) or equilibration of sulfur oxides with water
in the atmosphere both result in acidic, sulfate-rich
waters that have the potential of being corrosive
and hazardous to aquatic organisms. Sulfate-rich
waters are potentially toxic to plants and

deposition of high sulfate water on plant foliage
may cause crop losses. Sulfate-rich water has a
laxative effect on humans and may produce
adverse taste in drinking water, as well. As a result
of the latter effects, the Secondary Drinking Water
standard for sulfate has been set at 250 mg/L
(Florida Department of Environmental Regulation,
1989).

Sulfides are much more undesirable than
sulfates in ground water. Hydrogen sulfide gas
(H2Sgas) is a persistent problem in ground water,
although Florida suffers less than some regions.
This gas produces the familiar "rotten egg" odor,
which can be detected in water at dissolved
concentrations of just a few milligrams per liter.
While some persons treat H2S and sulfate-rich
water ("sulfur water" in the vernacular) as a health-
giving resource which one drinks to "clean out" the
alimentary system or in which one bathes for
therapeutic reasons, most consider this common
phenomenon as a liability. In high concentrations,
hydrogen sulfide is irritating to the eyes and lungs.
It is highly toxic as an atmospheric gas. There is no
standard or guidance criterion for sulfide in ground
or drinking water for two reasons: (1) the odor can
be eliminated by degassing and is unpleasant
enough to serve as its own limitation, and (2) there
is usually sufficient aeration and oxidation in public
water supplies that any sulfide is converted to
sulfate, for which there is a standard.

DISTRIBUTION IN GROUND WATER

It is interesting to note that, while maps and
spatial studies of the distribution of sulfate clearly
show an increase with depth and towards the
coast, Table 20 suggests that there is little
difference in sulfate concentrations between the
three aquifer systems. This is in part because (1)
sea water affects all systems equally, (2) there is
significant interaquifer transfer of water in both
upward and downward directions, and (3) the
sample set is somewhat biased toward the more
potable, low sulfate waters. Maximum concen-
trations do suggest that the deep Floridan aquifer
system is prone to higher sulfate concentrations
than are the other aquifer systems.

Surficial Aquifer System

The distribution of sulfate in the surficial
aquifer system (Table 20) reflects local geology
and flow system dynamics.

Median sulfate concentrations (Table 20) are
low in NWFWMD and SRWMD for three reasons:
(1) the surficial aquifer system, especially the Sand
and Gravel Aquifer of Escambia County, is well
flushed, (2) there is limited upward flux of deeper,
sulfate-rich waters, and (3) few wells in the sample
set are coastal. The maps of sulfate distribution
(Figure 32a,b) in these districts show minor coastal
increases in sulfate concentrations. The causes of
high sulfate concentration inland are unknown, but
oxidation of pyrite-containing peats, upward
transfer of intermediate or Floridan aquifer system
water by pumpage, and evaporative concentration
of sulfate-rich precipitation are possible causes.

Median sulfate concentrations in the other
three districts (Table 20) are relatively high because
the surficial aquifer system includes regions of
natural and irrigation-related upward flux of
deeper, sulfate-rich waters and of mixing with sea
water in the coastal transition zone. Atlantic
Coastal Ridge portions of the surficial aquifer
system often contain sulfide-rich organic and
pyrite. These are locally important aquifers in the
SJRWMD and SWFWMD, and they are well
represented in the data set. Zones of local
upcoming are evident in western Indian River
County, Volusia County, and Orange County and
the coastal interface is well demonstrated in
SJRWMD (Figure 32c). Sulfate upcoming is also
present in Hardee and DeSoto Counties, and there
is a large re-entrant along the Peace River axis
(Figure 32d; Kaufman and Dion, 1967). In SFWMD,
the sulfate concentrations in the Atlantic coastal
ridge portions of the surficial aquifer system are
low due to the dynamic circulation in the aquifer
systems. Few sulfate data exist from the
Everglades portion of the SFWMD (Figure 32e).
Andrejko and Upchurch (1978) have shown that
the peats of the Everglades contain sulfur that is
converted to anhydrite during peat fires. This
anhydrite is then dissolved in surface waters,
which, in turn, recharge the surficial aquifer
system. One can conclude, therefore, that sulfate
concentrations in the surficial aquifer system
should be moderately high.

Comparison of the Eh-pH conditions of the
surficial aquifer system (Figure 31) indicates that,
at 25"C, the water falls at or below the boundary
between SO42- and H2S, with the majority of the
samples well within the H2S stability field. The
kinetics of equilibrium reactions between the two
sulfur species are dominated by bacteria, and
metastable species can exist if bacterial action is
inhibited. Connell and Patrick (1968) investigated
the stability of sulfate and sulfide in waterlogged

soils, such as the surficial aquifer system. They
found that microbial sulfate reduction is inhibited
and sulfide formation is minimized at redox
potentials above -150 my and at pH's outside the
range of 6.5 to 8.5. Since optimal conditions for
microbial sulfate reduction may not be present in
the surficial aquifer system (low pH values and Eh
values that tend to be above -150 my), sulfate, not
sulfide, may predominate as a metastable species.

Intermediate Aquifer System

The intermediate aquifer system contains
abundant pyrite, and gypsum has been found in a
number of regions of the state. Low permeabilities
in the clays and the presence of particulate humic
substances limit oxidation of the pyrite, but the
potential for oxidation and aqueous sulfate
production exists. Red, orange, and yellow tints
from ferric hydroxides and goethite in exposures of
the Hawthorn Group and in immediately underlying
limestones along the flanks of the Ocala Platform
document oxidation of pyrite in the past.

With the exception of Flagler and Indian River
Counties (SJRWMD), the intermediate aquifer
system is neither widespread nor widely used in
the northern districts (NWFWMD, SRWMD, and
SJRWMD; Table 20); consequently, few data are
available. Where data are present, it appears that
flow systems are restricted and sulfide oxidation is
limited. Sulfate concentrations are characteris-
tically low and variable (Figure 33a,b, and c).

In contrast, the intermediate aquifer system is
highly utilized in southwestern SWFWMD and in
western SFWMD. Here, the circulation system is
better developed, and connection with the
underlying Floridan aquifer system is present in
many coastal areas (Upchurch, 1986). As a result,
sulfate is widespread and abundant (Table 20). Re-
entrants along the Peace and Myakka River axes
(Figure 33d) reflect upcoming and intrusion along
lineaments that extend through the Hawthorn
Group (G. Jones, 1991). These regions are a result
of natural discharge, exacerbated by pumping.
There is a region of high sulfate that apparently
reflects upcoming in northeast Lee County, as well
(Figure 33e).

Floridan Aquifer System

Sulfate concentrations increase with depth in
the Floridan aquifer system throughout the state.

FLORIDA GEOLOGICAL SURVEY

The data set summarized in Table 20 and Figure
34 has not been stratified by depth, so some of the
higher sulfate concentrations represent deeper
wells. Lowest sulfate concentrations are in
recharge regions where meteoric waters have not
accumulated sulfate. This lack of sulfate results
from (1) residence times that are too brief for pyrite
oxidation, (2) pyrite or other sulfate sources have
either been depleted or were never present, or (3)
pyrite oxidation is not thermodynamically favored.
Highest concentrations are in deep wells where
contact with the underlying gypsum and/or
anhydrite has enriched the water with sulfate.

In general, sulfate concentrations are least in
NWFWMD and SRWMD. Large, coastal re-
entrants occur in Walton County and along the
Apalachicola River in NWFWMD (Figure 34a).
These are a result of natural discharge along the
river and to pumpage. The pattern in SRWMD is
more complicated due to the local flow system
between the Suwannee River and the coast.
Coastal re-entrants exist along many of the coastal
rivers and in the vicinity of the Suwannee River.
High sulfates are also present in the confined,
sluggish flow system under the Northem Highlands
in the northeast part of the SRWMD (Figure 34b;
Lawrence and Upchurch, 1976).

High sulfate concentrations in SJRWMD and
SWFWMD (Table 20) reflect longer flow paths in
the Floridan that contact the gypsum and anhydrite
at the base of the aquifer system and bring that
water near the surface along the inner, coastal-
transition zone. Characteristically, sulfate
concentrations increase towards the coast. There
is a region of high sulfate along the St. Johns River
(Figure 34c), which has been attributed to
upcoming along a fracture or fault by Leve (1983).
Similar re-entrants along the Peace and Myakka
Rivers (Figure 34d) have also been attributed to
upcoming along lineaments by G. Jones (1991).
The coastward increase in sulfate is best illustrated
in the SWFWMD (Figure 34d). Localized areas of
upcoming of sulfate-rich water are found
throughout both districts.

The Floridan aquifer system in the SFWMD is
very poorly flushed and sulfate-rich. The data given
in Table 20 are somewhat biased in that no wells
are represented from the southern portion of the
district (Figure 34e), where sulfate concentrations
are the highest.

Eh-pH relationships (Figure 31) indicate that
either sulfate or sulfide can be stable in the

Floridan aquifer system. Low concentrations of
sulfate in much of the upper Floridan aquifer
system in the center of the state indicate that
sulfate reduction, sorption/precipitation reactions,
and dilution reduce sulfate in comparison with the
surficial aquifer system. The Eh-pH range in the
deep Floridan is suitable for microbial sulfate
reduction according to the criteria of Connell and
Patrick (1968). Limitations of available organic
carbon apparently inhibits sulfate reduction and
accounts for the persistence of sulfate.

Chloride

IMPORTANCE AND CONTROLS

Chloride (Cl-) is a conservative ion. That is, it is
not reactive in most ground-water chemical
systems. It does not participate in sorption,
mineral precipitation, microbial metabolism, or
other processes. The only processes that lower
chloride concentrations in Florida ground water are
dilution and dispersion. The only common
circumstance in which chloride is chemically
removed from an aquifer is mineral precipitation
under intense evaporation, which occurs only in
desert environments, not in Florida. Because it is
not reactive, chloride travels at the rate of the
ground water, so it is an excellent tracer of ground-
water flow.

Chloride can be added to aquifer environ-
ments in three ways. The most widespread
process is addition of chloride in marine aerosols
that enter the ground water through rainfall.
Florida precipitation averages 1.66 mg/L chloride
(Table 3). Mean chloride concentrations in
precipitation range from 0.75 mg/L in north Florida
to 2.81 mg/L in the south. Lawrence and
Upchurch (1982) documented the origin of sodium
and chloride in the Floridan aquifer system in an
unconfined, high recharge area of north Florida as
having been derived from rainfall.

Chloride is the dominant anion in sea water
(Table 8). Sea water averages 19,350 mg/kg
(Table 8), so chloride content of water on the
coastal transition zone can be quite high. Because
of the water quality standard for chloride, the
coastal transition zone is often defined as the 250
mg/L isochlor. While the transition zone is actually
a broad belt, the position of this isochlor, which
defines a surface, has incorrectly led to the
concept of the transition zone as being an
interface, the "salt-water interface".

Connate water14 can be a minor source of
chloride. The potentiometry of the Floridan aquifer
system in south Florida is insufficient to flush the
Floridan aquifer system completely. As a result,
connate waters remain in much of the aquifer
system, where they render the water non-potable.
Connate waters also occur in southern St. Johns,
north-central Flagler, Brevard, and Indian River
Counties. Elsewhere, minor connate waters may
be present in "dead" spaces in the aquifer system.
Dead spaces may include portions of the aquifer
system with poor to nonexistent circulation or low
permeability. Exploration of the Eocene and older
strata below the lower confining beds of the
Floridan aquifer system for deep-well disposal of
wastes and oil and gas exploration indicates the
widespread presence of connate water and brines.
These high chloride waters may upwell locally into
the potable portion of the Floridan if the confining
sequence is inadequate or where withdrawal is too
great.

Once chloride enters the aquifer systems, it is
subject to several factors that can cause increases
in chloride concentrations. Near the land surface,
evaporation and transpiration may increase
chloride content. Evaporation occurs both at the
land surface, in lakes, streams, and other water
bodies, and within permeable and porous aquifers.
Plant roots extract moisture from the vadose,
capillary, and phreatic zones, as well. The net
result is that chloride content increases in near-
surface environments. Comparison of Tables 3 and
21 shows that median chloride concentrations in
surficial aquifer system waters range from four to
over 30 times the chloride concentrations in preci-
pitation within the respective water management
districts. In the same data, the mole ratio of
sodium to chloride remains relatively constant and
near that of sea water (0.87 to 1.25 in preci-
pitation; 0.71 to 1.42 in ground water), so it
appears that much of this increase in chloride
concentration is a result of evaporation or
transpiration.

The observed increases in chloride concen-
tration with depth can be explained by three
hypotheses. One explanation is that connate
water trapped in sealed pore spaces or below the
sub-Floridan confining sequence is added to the
aquifer system water by rock dissolution and/or
opening of pore throats. Another explanation is
that hydration of minerals removes water from the
aquifer system, thus increasing the residual
chloride content. The most plausible explanation
is that the chloride is a result of incomplete
flushing subsequent to the Plio-Pleistocene marine

transgressions.

STANDARD OR GUIDANCE
CRITERION

Chloride is associated with taste and electro-
lytic corrosion problems. As a result, the
Secondary Drinking Water standard for chloride
has been set at 250 mg/L.

DISTRIBUTION IN GROUND WATER

The distribution of chloride statewide reflects
proximity to connate waters and recharge by
meteoric waters. The surficial aquifer system
contains waters with low median chloride
concentrations (Table 21). Median concentrations
in the intermediate aquifer system are high, largely
as a result of low permeability zones in the
Hawthorn Group. Median concentration in the
Floridan is low, but this reflects the bias in
sampling towards potable water masses. The
maximum chloride content recorded is in the
Floridan and, at 20,500 mg/L, this concentration is
slightly greater than average sea water (19,350
mg/kg, Table 8). Thus, the Floridan aquifer system
contains the highest concentrations of chlorides.

Surficial Aquifer System

The distribution of chloride in the surficial
aquifer system (Table 21) closely mirrors
precipitation (Table 3). Median chloride concen-
trations are lowest in north Florida (NWFWMD,
SRWMD) where continental influences on
precipitation are greatest. Peninsular Florida has
highest median concentrations of chloride in both
precipitation and ground water. The difference in
concentration between precipitation and surficial
aquifer system water can be attributed to eva-
poration and transpiration.

Characteristically, chloride concentrations are
lowest inland (Figure 35). Monitor wells in
NWFWMD and SRWMD are generally inland, so
the median and maximum chloride concentrations
are low. The coastal transition zone is reflected in
a few wells in NWFWMD (Figure 35a).

SJRWMD data include a number of wells near
the transition zone, hence the higher median
concentration (Table 21). As Figure 35c indicates,
coastal re-entrants with high chloride concen-

SPECIAL PUBLICATION NO. 34

trations exist in Flagler and, to some degree, St.
Johns Counties. A high chloride zone centered on
western Brevard and Indian River Counties
coincides with the upper St. Johns River and
wetlands to the south. Intense evapotranspiration
accompanied with possible upcoming or upward
transfer of water from deeper aquifer systems
along fractures and through irrigation can account
for this area of high chloride concentration.

The coastal transition zone and associated
salt-water intrusion are well represented in the
SWFWMD (Figure 35d). The high in Hardee
County has been attributed by Dalton (1978) to up-
ward transfer from the Floridan aquifer system by
irrigation. The re-entrant along the Peace River
has been attributed by G. Jones (1991) to
upcoming and intrusion along a major lineament.

Chloride concentrations in the Biscayne
Aquifer (Table 21, Figure 35e) are generally low,
with a small, intrusion-related re-entrant in the
Miami (Dade County) area. Chloride concentrations
in the northem part of the district are also generally
low, but irrigation with deeper, more saline waters
has resulted in a few high chloride zones. Data
from the SFWMD indicate an increase in chloride
toward the Everglades, which reflects the presence
of connate water and upcoming of poorer quality
water from underlying aquifer systems. Reduced
permeability of the surficial aquifer system has
prevented thorough flushing of connate water
beneath the Everglades. In addition, drainage of
wetlands for agriculture has significantly lowered
the water table and induced upcoming.

Intermediate Aquifer System

Median chloride concentrations in the inter-
mediate aquifer system show the same pattern as
in the surficial aquifer system (Table 21). Chloride
concentrations are low in the continental-climate-
dominated northern part of the state and high in
the more maritime climate of the south.

The intermediate aquifer system, which
includes the Hawthorn Group, contains significant
clay deposits (Scott, 1988), which have high
porosity and low permeability. These clays
apparently contain some connate water. As a
result, one would intuitively expect that chloride
concentrations would be somewhat more variable
and the median would be higher than surficial
aquifer system waters. This pattern is supported
by data from the Background Network (Table 21).

The medians and ranges of chloride in the surficial
and intermediate aquifer systems are generally not
significantly different in the northern part of the
state. However, the central and southern portions,
where the Hawthorn is thick, are characterized by
high chloride concentrations. One should not be
surprised, therefore, if somewhat elevated chloride
concentrations are encountered in the intermediate
aquifer system inland.

Distributions of chloride in the intermediate
aquifer system in NWFWMD, SRWMD, and
SJRWMD (Figure 36a-c) reflect evaporative
concentration. There is considerable local
variability, and increases toward the coast can be
seen. The causes of the local variability cannot be
identified from the data set, but interaquifer
transfer through irrigation is likely.

The coastal transition zone is well documented
in SWFWMD (Figure 36d). Local re-entrants reflect
salt-water intrusion due to pumpage and lowering
of potentials by other medians. The large re-
entrant along the Peace River axis is also well
documented.

The transition zone appears as a broad surface
with a relatively shallow dip in the SFWMD (Figure
36e). Highs in northern Collier and eastern Lee
Counties are a result of upcoming caused by
natural upward discharge and by pumpage.

Floridan Aquifer System

With the exception of the data from SFWMD,
the Floridan aquifer system is similar in chloride
distribution to the other aquifer systems of the
state (Table 21). Chloride concentrations are
generally low in shallow wells, inland, and in
recharge areas. They are highest near the coast, in
deeper wells, and in areas of pumpage-induced
intrusion.

The transition zone is well defined in the
NWFWMD (Figure 37a). Areas of intrusion include
Escambia County, southern Walton County,
southern Bay County, and isolated spots in
Franklin and Wakulla Counties. All of the re-
entrants west of the Apalachicola River are
associated with well fields and pumpage. The
isolated highs in Franklin and Wakulla Counties are
near small communities and a large spring com-
plex, both of which can lead to intrusion.

The transition zone is relatively narrow in most
of the SRWMD (Figure 37b). Re-entrants exist
along some coastal rivers (Steinhatchee, Aucilla,
Waccasassa), and re-entrants in Taylor and Dixie
Counties can be attributed to pumpage associated
with small towns and industry. Inland highs
include an upcoming at the Ichetucknee Springs
group (the high at the Suwannee-Columbia County
boundary and the Santa Fe River).

The data from SJRWMD (Figure 37c) illustrate
the coastal transition zone, with a re-entrant in
Volusia County. Other high chloride areas in St.
Johns and central Flagler Counties; along the St.
Johns River in Putnam, Volusia, and Seminole
Counties; and in Brevard and Indian River Counties
are thought to be connate water (Stringfield, 1966;
Boniol, 1981, pers. com.). The re-entrant that
extends from the coast in St. Johns County,
through Putnam County, and into Marion County is
probably due to upcoming.

Comparison of the potentiometric map of the
Floridan aquifer system (Figures 35 and 36 in Scott
et al., 1991) with chloride data from SWFWMD
(Figure 37d) and SFWMD (Figure 37e) clearly
illustrates the effects of poor flushing and intrusion
where the hydraulic potentials are low15. The
southern third of SWFWMD and the southern two
thirds of SFWMD have lower potentials. As a
result, chloride content of the aquifer system is
higher and the transition zone is broader and
flatter. In the central and northern thirds of
SWFWMD, hydraulic potentials are high, chloride
concentrations are low, and the transition zone is
steep and narrow. Scattered areas with chloride
concentrations in excess of 10 mg/L in the
northern third of the SWFWMD reflect somewhat
deeper wells. In general, this area is characterized
by recharge, and chloride contents are near those
of precipitation.

Phosphate

IMPORTANCE AND CONTROLS

Phosphate, as reported here, is ortho-
phosphate (P0,3-). Phosphate is of concern
because it is an essential nutrient of all living
things. Because is it an essential nutrient, excess
phosphate can cause run-away plant growth and
eutrophication16 of surface waters. Therefore,
control of phosphate in surface water has been a
national priority since the late 1960's. Control was
established as a priority of the Federal Water

Pollution Control Administration, and its successor
the U.S. Environmental Protection Agency,
because it was felt that phosphate was the limiting
nutrient17 in most waters of the nation. It is
questionable whether phosphate is a limiting
nutrient in surface water in many areas of the state
due to the abundance of apatite-group minerals in
late Tertiary and Quaternary sediments.

The most important sources of phosphate in
Florida are the phosphate-bearing sediments
found throughout the Hawthorn Group. Two
apatite group minerals predominate in the
phosphatic sediments: carbonate-fluorapatite
[Ca,(PO4,CO3)3F, or "francolite"18] and carbonate-
hydroxylapatite [Cas(PO4,CO,3)(OH), or "dahlite"].
Weathering of both minerals introduces phos-
phate into ground and surface waters (Lawrence
and Upchurch, 1982). The widespread occurrence
of these phosphate minerals (see the discussion of
the distribution of the Hawthorn Group in Scott
[1988]) suggests that phosphate is locally, naturally
available throughout much of the state.

Carbonate-fluorapatite is the primary phos-
phate mineral in the Hawthorn Group. It was
precipitated from the Miocene sea throughout the
eastern and southern sides of the Ocala Platform.
Subsequent erosion of the Hawthorn on the crest
of the Ocala Platform and elsewhere, led to
transport of dissolved and particulate phosphate
into contemporary sediments, where the
phosphate accumulated as extremely rich ore
deposits. These deposits are mined in central
Florida (Polk, Hillsborough, Hardee, DeSoto, and
Manatee Counties) and in north Florida (Hamilton
County). Ore-quality deposits occur at depth in St.
Johns and Brevard Counties and other areas. The
deposits that contain these phosphate-rich
horizons constitute portions of the intermediate
aquifer system. Carbonate-fluorapatite was also
reworked during subsequent marine transgres-
sions and regressions into younger, Quaternary
deposits that constitute the surficial aquifer
system.

Carbonate-fluorapatite is a source of several
other environmentally important constituents in
Florida ground water. Carbonate-fluorapatite is
the primary source of fluoride in the aquifer
systems (see Fluoride below). Carbonate-
fluorapatite also contains trace quantities of
uranium (Cathcart, 1956; Altschuler et al., 1958).
The uranium undergoes a series of decay events
that result in radium, radon, and polonium, all of
which have been shown to be problems in

FLORIDA GEOLOGICAL SURVEY

Florida's aquifer systems (e.g., Kaufmann and
Bliss, 1977; Cowart et al., 1978; Burnett et a/.,
1988; Upchurch etal., 1991).

Carbonate-hydroxylapatite is a result of re-
precipitation of phosphate following weathering of
carbonate-fluorapatite. The phosphate ion (PO43-)
is soluble in acidic waters, such as occur in
siliciclastic horizons of the surficial aquifer system.
The ion is insoluble in alkaline aquifers, such as
occur in the Floridan aquifer system. The reactions
are as follows. In the acidic surficial and inter-
mediate aquifer system waters, carbonate-
fluorapatite is dissolved according to

Ca (PO4, C03) 3Fsd + 7.5H+

-5Ca. 1 + 1.5H3PO,a
aq.aq.

+ 1.5HCOa,, + F,

where the phosphate and carbonate are written as
phosphoric and carbonic acids for simplicity and
phosphate and carbonate are present in equal
mole proportions in the apatite. Upon encoun-
tering an alkaline environment, the phosphoric and
carbonic acids are neutralized and carbonate-
hydroxylapatite precipitates according to

5Ca + 1.5H3PO.

+ 1.5H,CO,3a + H20

-Ca, (PO4, CO3) 3(OH)soi,,

+ 8.5H,.

The hydrogen released by the precipitation reac-
tion is consumed by alkalinity and/or dissolution of
calcite (reaction 3). If it is consumed by dissolution
of calcite, carbonate-hydroxylapatite is likely to
replace limestone.

The crest of the Ocala Platform was stripped
of Hawthorn sediments in Late Miocene, Pliocene,
and Quaternary times. While most of the
weathering products of this erosion were swept
into adjacent rivers, estuaries, and the sea, some
of the dissolved phosphate (H3PO4 in reaction 13)
migrated downward into the underlying Floridan
aquifer system. Upon contact with the limestones
of the Floridan, phosphate precipitated according
to reaction 14. Upchurch and Lawrence (1984)

document an area in Columbia County where this
process is taking place today.

Deposition of carbonate-hydroxylapatite in
significant ore bodies occurred in a belt along the
eastern flank of the Brooksville Ridge (Hernando,
Citrus, Marion, Levy Counties) and elsewhere. The
deposits are commonly preserved in paleo-
sinkholes, where the carbonate-hydroxylapatite
lines the sinkholes and partially replaces the
adjacent limestone. Economically important
deposits are termed "hard-rock" phosphate. Hard-
rock phosphate was mined in Citrus, Gilchrist,
Marion, and Levy Counties from the 1890's to the
mid-1960's. The process of carbonate-
hydroxylapatite precipitation continues today in the
Floridan aquifer system, and dissolution of the
resulting hard-rock deposits may constitute an
additional source of phosphate in ground water.

Marine aerosols constitute a small, but
significant, source of phosphate. Based on an
average chloride concentration of 1.66 mg/L in
precipitation (Table 3), and chloride and phosphate
concentrations in sea water of 19,350 mg/kg and
0.05 mg/L (50 gg/kg, Table 8), respectively, the
concentration of phosphate in precipitation should
be 1.6 utg/ml. Measurements of phosphate in
precipitation (Table 3) average 0.03 mg/L
statewide, and suggest that the atmosphere is
enriched four orders of magnitude over the
predicted aerosol concentrations. This is partly
because the monolayer of sea water at the atmos-
phere/water contact (the source of aerosols) is
enriched in phosphate, and partly a result of
organic and particulates in the atmosphere. At
any rate, precipitation is an important source of
phosphate for plants. The concentration of
phosphate in precipitation is similar to that in the
surficial aquifer system.

Other sources of phosphate include inorganic
and organic fertilizers, organic tissues, animal
wastes, human waste effluent, and industrial
effluent. Phosphate is an abundant constituent of
household waste. In the areas of the state
represented in the Background Network, this
waste is usually released to the environment by
means of on-site treatment systems (septic-tanks)
and small, land-application treatment facilities.
Should such systems fail to properly function,
phosphate may enter the aquifer system. Since
septic-tank systems, fertilizer use, and animal
wastes are common in rural areas, especially in
agricultural areas, phosphate is a likely constituent
in near-surface aquifers in the Background
Network.

Phosphate is removed from ground water by
several processes. In carbonate-rich aquifers, the
removal is by precipitation of carbonate-hydroxy-
lapatite (reaction 14). This reaction is effective as a
mechanism for orthophosphate precipitation, and
alkaline waters seldom have detectable phos-
phate as a result. Should phosphate be detected
in ground water, it is safe to conclude that (1) the
phosphate has not yet encountered sufficient
alkalinity to cause precipitation (a common pheno-
menon in karst conduits and siliciclastic aquifer
horizons) or (2) the phosphate is either completed
with a metal or present as some form other than
aqueous orthophosphate.

Phosphate is strongly adsorbed by ferric
hydroxides and certain other colloids in soils and
aquifers. This sorption is an important mechanism
for phosphate removal from ground water,
especially where on-site and land-application
sewage treatment release phosphate to the
environment. Childs et al. (1974) documented
orthophosphate sorption reactions on ferric
hydroxide-coated sand grains and on clays below
the water table in a siliciclastic aquifer. They
showed that phosphate fixation in septic-tank
drain fields is very nearly quantitative, with
concentration factors on the host soils in excess of
1000 times. Rea and Upchurch (1980) studied
orthophosphate fixation on a ferric hydroxide-
coated fine sand in the surficial aquifer system.
They found that minor amounts of ferric hydroxide
(< 3 weight percent) removed large quantities of
phosphate.

STANDARD OR GUIDANCE
CRITERION

There is no standard or criterion for phosphate
in ground water (Florida Department of
Environmental Regulation, 1989). The U.S.
Environmental Protection Agency and the state
both have standards for total phosphorus in
surface waters. These standards vary with
classification of the surface water body. The use of
a standard for surface water, but not for ground
water, is a result of recognition of natural sources
of phosphorus in ground water and the importance
of phosphorus as a limiting nutrient in surface
water.

DISTRIBUTION IN GROUND WATER

The districts measured phosphate in several
ways (Table 22). SRWMD measured ortho-

phosphate in unfiltered samples. NWFWMD and
SJRWMD measured total phosphorus, which
includes minor amounts of condensed and or-
ganic phosphorus, as well as orthophosphate.
SWFWMD analyzed for total phosphate (ortho-
phosphate, condensed phosphates, and other
phosphate compounds), and SFWMD measured
dissolved (filtered) orthophosphate. Differences in
these measurements are not considered
significant.

The distribution of orthophosphate in Florida
ground waters is summarized in Table 22. In
general, median phosphate concentrations follow
chemical controls rather than the distribution of
sources. That is, highest median and maximum
concentrations are in the surficial aquifer system,
where the water is acidic and reaction 13 pre-
dominates. Water from the intermediate aquifer
system, which contains the majority of the
phosphate mineralization, contains less phosphate
because of buffering with alkalinity derived from
the associated carbonates (reaction 14). Water
from the Floridan aquifer system is character-
istically low in phosphate due to the relatively high
alkalinities.

Surficial Aquifer System

Phosphate in the surficial aquifer system can
come from several sources, including natural
weathering of phosphate minerals reworked into
the aquifer sediments from the underlying
Hawthorn Group, leaching of natural phosphates
from organic sediments and decomposing plant
materials, use of agricultural fertilizers, and septic
tank systems and other waste-disposal practices.
The widespread occurrence of phosphate in the
surficial aquifer system suggests that geologic
conditions produce a strong overprintt" on the
aquifer system and that local sources (swamps,
human activities) build on that background over-
print. Because the human sources are local, they
cannot be identified on the maps, and the main
thrust of the following discussion is the geologic
overprint.

Statewide, the median orthophosphate
concentration is 0.06 mg/L and the maximum
recorded is 4.00 mg/L (Table 22). Maxima are
generally lower, however, and indicate that
phosphate mobility is not a widespread problem in
the state.

Phosphate concentrations are highly variable

SPECIAL PUBLICATION NO. 34

and generally low in NWFWMD and SRWMD
(Figure 38a,b). Higher concentrations are found in
siliciclastic portions of the aquifer system where
pH's are low. The sources of the phosphate range
from natural apatite reworked into aquifer
sediments, especially in eastern SRWMD, to
animal and human wastes and fertilizers. In
SJRWMD (Figure 38c) the phosphatic portion of
the Hawthorn Group, especially in St. Johns
County, is accompanied by higher phosphate
concentrations in ground water.

The data from SWFWMD (Figure 38d) indicate
a region of moderately high phosphate concen-
trations (>0.5 mg/L) in southern Polk and Hardee
Counties, where phosphate mining of the
underlying Hawthorn Group is underway. There is
also a belt of moderate phosphate concentrations
(>0.1 mg/L) that parallels the coast and reflects the
inner margin of the transition zone.

The Biscayne Aquifer (Table 22; Figure 38e) is
generally low in phosphate due to the absence of
phosphate-bearing minerals (apatite) and to the
high alkalinity of the water. Where present,
phosphate can be attributed to land uses. In
western SFWMD (Figure 38e), the Hawthorn is
present near land surface, and phosphatic rock
has been reworked into the surficial aquifer system
sediments. This reworked phosphate is accom-
panied by organic, which may also be a source of
phosphorus.

Intermediate Aquifer System

The distribution of phosphate in waters of the
intermediate aquifer system is variable (Figure 39).
Phosphate tends to be below detection limits in
carbonate-rich portions of the system. In
siliciclastic horizons the phosphate ranges up to
2.28 mg/L (Table 22). It is not possible to discern
large patterns in the distribution maps due to the
localized nature of dissolution/precipitation reac-
tions and the sparsity of data.

Floridan Aquifer System

Phosphate concentrations should be at or
below detection limits in the Floridan aquifer
system due to high water alkalinity. As Table 22
indicates, median concentrations are near detec-
tion limits, and they are generally low. The sample
from the SRWMD that has 21.00 mg/L is suspect.
The value is either an incorrect chemical analysis

or the sample, which is unfiltered, contained
particulate apatite. The median concentration in
the Floridan aquifer system is 0.04 mg/L.

Maximum phosphate concentrations in the
Floridan aquifer system are somewhat higher than
expected (Table 22). This is an artifact of the
"plumbing" of the karstic and fractured aquifer
system. Dissolved phosphate requires alkaline
waters in order to precipitate. In recharge areas,
much of the water moves in conduits (fractures or
caverns) under laminar flow conditions. Under
these conditions, equilibration is slow and phos-
phate can persist for some distance within the
Floridan aquifer system. Lawrence and Upchurch
(1976) illustrated an example of dissolved
phosphate persistence in cavernous flow near
Lake City, Columbia County. There, phosphate
enters the Floridan through karst features. Upon
recharge, the dissolved phosphate persists as a
well organized "plume" for several kilometers with
minimal dilution, dispersion, or precipitation.

The pattern of dissolved phosphate in the
Floridan aquifer system in the NWFWMD (Figure
40a) shows several regions of modest phosphate
concentrations. Phosphate is elevated along the
inner transition zone in Okaloosa and Gulf
Counties. Phosphate is also slightly elevated in a
broad belt that originates near the coast in Franklin
and Wakulla Counties and extends westward,
through the center of the district, into Okaloosa
County. This belt roughly coincides with the
Beacon Slope, New Hope Ridge, and southern
Western Highlands, and with the edge of the
Hawthorn Group and equivalent units to the west
(Scott et al., 1991). Lawrence and Upchurch
(1976, 1982) and Upchurch and Lawrence (1984)
have shown that phosphate is introduced with
rechargein a similar context to the east, in the
SRWMD. An area of low phosphate concen-
trations occurs along the northern boundary of the
district. This region includes recharge areas where
confinement is limited and flow along karst
conduits occurs. All of the data from the
NWFWMD are near the detection limit (0.04 mg/L).

A similar pattern is present in the SRWMD
(Figure 40b), where a center of high phosphate
concentrations occurs in southern Columbia and
Union Counties. This center is situated on the
Cody Escarpment, which is characterized by large
sinkholes and sinkhole lakes that drain into the
Floridan aquifer system. It is within this center that
Lawrence and Upchurch (1976, 1982) and
Upchurch and Lawrence (1984) documented re-

charge of phosphate-rich, intermediate and
surficial aquifer system water, and surface water,
into the Floridan. Their studies show that high
phosphate waters in the Floridan aquifer system
represent local recharge and conduit flow. A
similar high occurs in southeastern Madison
County. Highs in Levy and Gilchrist Counties are
on the flanks of the Brooksville Ridge and of Bell
Ridge. Both represent similar situations to the
Cody Escarpment. It is interesting to note that
there is high phosphate in the Floridan aquifer
system along the coast in Levy and Dixie Counties.
The concentrations reported here are too high for
sea water (Table 8), and most probably reflect the
influence of organic-rich surface water from the
coastal swamps.

Phosphate concentrations in the Floridan
aquifer system in the SJRWMD are generally near
detection limits (Figure 40c). Highs exist in north-
western Volusia and in Flagler Counties. The Flagler
County high is apparently related to withdrawals
near Bunnell, an area characterized by extensive
fern cultivation. This higher phosphate water is
probably a result of induced recharge from the
overlying intermediate aquifer system. The Volusia
County high is less easily explained. It is associated
with a minor recharge area (Figure 74, Scott et al.,
1991) and may reflect downward movement of
intermediate aquifer system water as well.

Phosphate in the Floridan aquifer system in the
SWFWMD (Figure 40d) is associated with local
recharge through the Hawthorn Group. The belt
that extends from southeast to northwest in
Sumter and Citrus Counties cannot be readily
explained. This belt is in roughly the same area as
the hard-rock phosphate district, but the belt of
high phosphate and the mining district do not
coincide. Additional work is needed to explain this
distribution. Many of the local highs in
Hillsborough County coincide with karst terrains
(Upchurch and Littlefield, 1988) that are urbanized
or agricultural in use. These phosphate concen-
tration highs, therefore, may reflect the influences
of local use of on-site waste treatment, animal
wastes, and/or crop and lawn fertilization. The re-
entrant that extends up the Hillsborough River
reflects an unconfined part of the aquifer system
that is immediately overlain by swamps and
moderate cultural development.

Phosphate is generally low in the Floridan
aquifer system in SFWMD (Figure 40e). The
Floridan is confined by the Hawthorn Group, and
there appears to be little recharge to the Floridan

aquifer system. Upward hydraulic potentials and
the high alkalinities of the Floridan aquifer system
also result in precipitation of any significant
phosphate introduced to the aquifer system.

Fluoride

IMPORTANCE AND CONTROLS

Like chloride, fluoride (F-) is a halogen anion. It
is somewhat more reactive than chloride, and it
forms dissolved chemical complexes. These com-
plexes remain soluble in Florida ground water, so
fluoride essentially behaves as a conservative ion.

Marine aerosols can contribute small amounts
of fluoride to ground water. Assuming that the con-
centration of chloride in precipitation is 1.66 mg/L
(Table 3) and that the ratio of fluoride to chloride is
constant in sea water (Table 8) and in marine
aerosols, precipitation should contain approximately
0.00022 mg/L F-, which is three orders of magni-
tude less than median concentrations in Florida's
aquifer systems (Table 23).

Most of the fluoride in Florida ground water is
derived from weathering of carbonate-fluorapatite
in the Hawthorn Group (reaction 13). Conse-
quently, the presence of fluoride is an excellent
indicator of waters that have come in contact with
the Hawthorn Group at some time in the past.

Cook et al. (1985) described fluorite (CaF) in
gypsum nodules at the base of the Floridan aquifer
system (Eocene Avon Park Formation) in Hernando
County. The surrounding water was found to be
undersaturated with respect to gypsum and
fluorite, so dissolution of fluorite at the base of the
aquifer system may constitute a second source of
fluoride for the deep flow system. To date, only
one core has been found to contain fluorite, so it is
uncertain as to whether the fluorite constitutes a
major source of fluoride.

Intrusion of sea water can also be a minor
source of fluoride (Table 8). With the exception of
the coastal transition zone, sea water is an unim-
portant source.

The Florida phosphate industry is one of the
nation's most important producers of fluoride and
fluorine products. The fluoride is extracted from
the carbonate-fluorapatite as hydrofluoric acid

FLORIDA GEOLOGICAL SURVEY

(HF). Some of this fluoride is lost to ground water
near the phosphochemical plants, where it may
present a problem. Upchurch et al. (1982)
characterized the effects of fluoride-rich effluent
that had been introduced from a phospho-
chemical plant into sea water in Tampa Bay. The
minerals fluorite (CaF2) and pachnolite
(NaCaAIF6.H20) were found precipitated in a delta
at the outfall. Similar processes may be occurring
in calcium- and sodium-rich ground water near
other agrichemical plants. At the present time,
there is little evidence that this process is wide-
spread or important.

There is little indication that any natural
process is responsible for actively removing
fluoride from ground water in Florida. Most of the
reduction in concentration is a result of dilution and
dispersion.

STANDARD OR GUIDANCE
CRITERION

In small amounts (< 1 mg/L), fluoride is con-
sidered beneficial as a preventative for dental
caries. The fluoride reacts with the apatite in teeth
to form a fluorapatite that is resistant to cavity-
causing microbes. In excess of 2 mg/L, fluoride
begins to cause unsightly darkening and mottling
of the teeth, a condition termed dental fluorosis.
Severe mottling requires concentrations in excess
of 12-14 mg/L. Extreme doses can induce toxicity,
including excess calcification of bones, stiffness,
and, under some circumstances, death.

Standards for fluoride have been set to control
toxicity and fluorosis. The Primary Drinking Water
Standard, which addresses toxicity, is 4 mg/L. The
Secondary Drinking Water Standard, which
addresses fluorosis, is 2 mg/L.

Due to weathering of carbonate-fluorapatite in
the Hawthorn Group, fluoride is widespread in
Florida's aquifer systems. Concentrations that
exceed the Drinking Water Standards are occa-
sionally found, especially in the SWFWMD and
SFWMD (Table 23). Statewide, only 0.1 percent of
the samples in the surficial aquifer system
exceeded the Primary Drinking Water Standard.
Four tenths of a percent of the samples from the
intermediate aquifer system exceeded the
standard, and 0.1 percent exceeded it in the
Floridan aquifer sample set.

In most areas of the state, ground water does
not contain concentrations sufficient to serve as a
preventative for caries, so many public water
supplies have opted to augment natural fluoride for
the sake of public health.

DISTRIBUTION IN GROUND WATER

The median concentrations of fluoride in the
aquifer systems (Table 23) reflect the source.
Fluoride is lowest in the surficial aquifer system,
with a median of 0.17 mg/L. The intermediate and
Floridan aquifer systems contain median
concentrations of 0.39 and 0.20 mg/L respectively.
The intermediate aquifer system is highest in
fluoride concentration because the source
carbonate-fluorapatite is present in the aquifer
system. Concentrations are somewhat lower in
the Floridan because of dilution and dispersion of
waters that have passed through the Hawthorn
sediments, and because the data set includes
waters that have not come in contact with these
sediments.

Surficial Aquifer System

As Table 23 indicates, fluoride concentrations
in the surficial aquifer system are generally lower
than in the other two aquifer systems. The low
concentrations are a result of limitations on the
sources of fluoride. While the relative importance
of the following sources have not been quantified,
the data presented in this report clearly indicate
that some of the fluoride is from precipitation, with
the concentrations enhanced by evapo-
transpiration. Most of the fluoride in the surficial
aquifer system, however, is derived either from
weathering of reworked carbonate-fluorapatite or
from deeper waters that are introduced to the
aquifer system by natural discharge and by
pumpage (Dalton, 1978).

The pattern of fluoride in the surficial aquifer
system in north Florida (Figure 41a,b) is
characteristic of a local flow-system aquifer. Data
are variable, and concentrations are generally low.

The data from SJRWMD (Figure 41c) suggest
a coastal source of fluoride. High fluoride occurs
in the transition zone in Nassau and Duval
Counties, and in St. Johns County. These highs
coincide with regions of well-field development.
The plume of dissolved phosphate that crosses
Flagler County into northwestern Volusia County

(Figure 38) is also represented in the fluoride data.
This suggests that the chemistry of the plume is
somehow influenced by the weathering of
carbonate-fluorapatites in the Hawthorn Group.

Two regions of high fluoride can be seen in
SWFWMD (Figure 41d). One is in central and
southern Polk County, a phosphate mining area.
High fluoride probably occurs in this area because
it is associated with waters derived from within the
phosphatic deposits, not because of mining. The
Hawthorn Group, especially the Bone Valley
Member of the Peace River Formation, in central
and southern Polk County is characterized by
enriched deposits of carbonate-fluorapatite. Some
of this apatite has been reworked into the surficial
aquifer system here, and the uppermost part of the
Bone Valley Member is in direct hydraulic
connection with the sands of the surficial aquifer
system. Thus, this region of high fluoride directly
reflects mineralization and hydraulics of the aquifer
system. The coastal high fluoride zones in
Hillsborough County and in the southwestern part
of the district reflect the transition zone. The outer
part of the transition zone is directly influenced by
sea water, the inner part reflects fluoride-rich
waters upwelling after following long, deep flow
paths and mixing with the sea water.

Fluoride is somewhat high in coastal areas in
the surficial aquifer system in SFWMD (Figure 41e).
The coastal parts of the Biscayne Aquifer contain
over 0.2 mg/L, as do coastal parts of the aquifer
system in Lee and northern Charlotte Counties.
Fluoride is low in the central part of the district.

Intermediate Aquifer System

Given that the intermediate aquifer system in
the eastern panhandle and the peninsula generally
coincides with the phosphatic Hawthorn Group,
one would expect that fluoride concentrations
would be high compared to the other aquifer
systems. This is the case (Table 23), but con-
centration differences are not significant. Fluoride
is highest in SWFWMD and SFWMD, where the
intermediate aquifer system is thick and used as a
potable water source.

Data from NWFWMD, SRWMD, and SJRWMD
(Figure 42a-c) show similar patterns to other
analytes. The concentrations range from below
detection limits to maximum of 1.75 mg/L. The
data do not reveal systematic patterns because of
the complex, local nature of production zones.

High fluoride concentrations are evident in
SWFWMD and northwest SFWMD (Figure 42d,e)
near the coastal transition zone. Note especially
the well-defined re-entrant along the Peace River
lineament (Figure 42d).

Floridan Aquifer System

While there are no known significant sources
of fluoride in the Floridan aquifer system, water
that has passed through the Hawthorn Group
contains fluoride (Lawrence and Upchurch, 1982),
as does water in the coastal transition zone. The
median concentration in the Floridan aquifer
system is 0.2 mg/L (Table 23). The maximum
concentration found was 6.9 mg/L in the
NWFWMD. The location of this high is near the
coast in Gulf County. The concentration is higher
than expected for sea water (Table 8).

The distribution of fluoride in NWFWMD
(Figure 43a) shows a strong coastward gradient. A
large re-entrant in Gulf and Bay Counties can be
attributed to pumpage and discharge along the
Apalachicola River. To the east, the pattern in
Wakulla and Liberty, and Franklin Counties is
irregular, probably in response to local flow
systems associated with the extensive conduit
flow system there.

High fluoride concentrations occur along the
erosional margin of the Hawthorn Group at the
Cody Escarpment and in the Northern Highlands in
the SRWMD (Figure 43b). These reflect recharge
of intermediate and surficial aquifer system waters
in the large sinkhole complexes that characterize
the escarpment. Other areas of elevated fluoride
concentration exist in Hamilton County. The
eastern area is near a phosphate-mining and
agrichemical complex. The western one is
associated with the recharge of intermediate
aquifer system waters near the Alapaha River
(Ceryak, 1977). High fluoride concentrations in
Taylor and Lafayette Counties appear to be
associated with the San Pedro clays which overlie
the partly confined Floridan aquifer system.
Coastal transition zone fluoride is also evident.

Fluoride concentrations are generally low in
the Floridan aquifer system in SJRWMD (Figure
43c). High concentrations exist in Flagler and
Volusia Counties. There are minor indications of
the coastal transition zone. Since most of the
coastal communities in Flagler County utilize the
intermediate aquifer system, the transition zone in

SPECIAL PUBLICATION NO. 34

the Floridan is not well represented.

The Floridan aquifer system in SWFWMD can
be divided into domains based on fluoride (Figure
43d). The northern half of the District contains low
fluoride concentrations. This region is unconfined
and there is little, or no, Hawthorn to act as a
source. The coastal transition zone is well
developed in this area. The southern half of the
map clearly indicates that fluoride, derived from
waters passing through the Hawthorn, slowly
increases in concentration along the flow paths.
With the exception of the re-entrant along the
Hillsborough River, the 0.2 mg/L isoline closely
approximates the northern edge of the Hawthorn
Group. Fluoride uniformly increases to the south
and west from this isoline. The coastal transition
zone is delineated by a steepened gradient near
the coast and by a re-entrant along the Peace
River lineament.

Little can be said about fluoride in SFWMD
(Figure 43e). There are indications that fluoride
content increases to the south, into the non-
potable portions of the aquifer system where
connate water predominates. There is a high in Lee
County, where pumpage and free-flowing wells
induce upcoming of deeper waters along the
transition zone.

Nitrate

IMPORTANCE AND CONTROLS

Nitrate (NO3) is one member of a sequence of
related nitrogen compounds that includes nitrogen
gas (N2), nitrogen dioxide gas (NO2) and other
oxides, ammonia and ammonium (NH3, NH4,),
nitrite (NO2,), a number of other inorganic
compounds, and many organic. The gaseous
phases exist in the atmosphere and in soil
atmospheres, but are not of importance in the
saturated zones of aquifers. Ammonia gas also
escapes into the atmosphere. Ammonia is present
in ground water as the ammonium ion (NH,:)
because of prevalent pH and reduction-oxidation
potentials. The complex organic compounds can
occur as soluble organic molecules and as par-
ticulates. Concentrations of dissolved, organic-
nitrogen compounds, including amino acids and
proteins, are reported as Total Kjeldahl Nitrogen
(TKN) in samples from aqueous systems and soils.

Organic nitrogen, ammonium, nitrite, and

nitrate are the compounds considered important in
ground-water systems. These compounds are
related through a sequence of reduction and
oxidation reactions as indicated below

Organic Nitrogen (TKN)
- NH, -- NO,- NO;

Oxidation --
--- Reduction -

with oxidation being the normal sequence in
ground-water systems. The reduction/oxidation
transformations between the compounds indicated
in reaction 15 can be driven by inorganic process-
es, but the primary mechanisms for the reactions
are microbial.

The largest reservoir of nitrogen is the atmos-
phere, which is 78.93 percent nitrogen, mostly as
N, gas. NH3 and NO3- occur naturally in the
atmosphere as a result of releases by terrestrial
plants (Stallard and Edmond, 1981). Atmospheric
nitrogen is also converted to NO,1 by lightning.
Modern precipitation contains increased nitrogen
oxides as a result of combustion of fossil and
modern organic fuels. The oxides of nitrogen are
then converted by oxidation and hydrolysis to nitric
acid (HNO3), which dissociates to HI and NO3.
Consequently, precipitation is a source of nitrate
and ammonium derived from both natural and
anthropogenic causes. Nitrate in precipitation in
Florida ranges from 0.00 to 10.32 mg/L (Table 3),
and the statewide mean is 0.97 mg/L. Ammonium
ion ranges from 0.00 to 17.12 (Table 3) and the
mean is 0.17 mg/L.

Clearly, conversion of nitrogen compounds in
the atmosphere followed by precipitation intro-
duces nitrogen to the ground-water system.
Modern rainfall, however, cannot be used as an
argument for high nitrogen in most aquifers. This
is because of the long time intervals involved in
ground-water flow. Waters in surficial environ-
ments, including the surficial aquifer system and
shallow, unconfined portions of the Floridan
aquifer system, may be affected by high-nitrogen
precipitation. Deeper waters were recharged as
meteoric waters before the advent of the industrial
revolution. These older waters entered the aquifer
system with some natural nitrogen content, but at
much lower concentrations than the present.

Certain microbes can fix nitrogen gas in soils.
These microbes, in conjunction with plants such as

the legumes, directly convert nitrogen into tissues
and nitrogenous by-products. Plants require nitrate
as a major nutrient, and they are responsible for
removal of much of the nitrate that is taken from
soils and ground water. Average nitrogen content
of living organisms is 16 percent. These living
tissues contain amino acids and other nitrogen
compounds that can be released back into the
environment upon death or waste elimination.

Animal wastes and decaying plant tissues
release ammonia and ammonium, nitrite, nitrate,
urea0", and a number of nitrogenous organic
molecules. Soil and aquifer microbes metabolize
these according to the reduction-oxidation
potential (reaction 15) of the soils and aquifers.
Under reducing conditions, microbes convert
these compounds to ammonium, and other
reduced nitrogen species. Under oxidizing
conditions, they are converted to nitrate, often with
an intermediate nitrite step.

Therefore, in reducing environments, such as
water-saturated, reducing soils and aquifers, am-
monium may persist and become a part of the
ground-water system. Under these circum-
stances, ammonium can travel considerable
distances before sorption, microbe metabolism,
dilution, or dispersion reduce concentrations to
below detection limits. Ammonium tends to sorb
onto clays and soil particles, so some soil and
aquifer materials mitigate ammonium migration.
Septic-tank systems, land-application waste-
treatment systems, and feed-lot wastes can, under
circumstances of overloading or failure of sorption
systems, cause widespread ammonium con-
tamination.

Oxidizing conditions are necessary for
microbes to produce the complex reactions
required to make the nitrogen useable for plants.
These aerobic microbes convert the ammonium
and complex organic nitrogen molecules to nitrite
and then nitrate. Ammonium and organic-nitrogen
compound concentrations are low in most aquifers
because oxidizing conditions are widespread near
the land surface, where these nitrogen compounds
are generated. Oxidizing conditions occur in
oxygenated soils, vadose environments and
shallow, oxygenated portions of aquifers.

If nitrates are available in small amounts near
the land surface, plants will utilize the nitrates.
There are also microbes that denitrify soils by
conversion of nitrate to nitrogen gas. If nitrate
production from ammonium and more complex

nitrogen compounds is not completed within the
root zone, if the nitrate is unavailable to plants and
denitrifying microbes, or if nitrate is produced in
quantities too great for biological agents to fix,
nitrate migrates with the ground water. With the
exception of plant and microbial activity, there are
few mechanisms for nitrate removal in aquifers.
Once nitrate enters the aquifer and is isolated from
environments where denitrification and plant
fixation occur, nitrate behaves more-or-less con-
servatively and can move long distances in
aquifers.

Ideal, land-based, waste-disposal practices
include sufficient vadose zone and biomass to
convert nitrogen compounds to nitrate and then to
utilize the nitrate. Unfortunately, high water tables,
plugging of soils by particulate matter, under-
design of treatment facilities, crowding of waste-
disposal facilities or animals on too small a tract of
land, and many other factors tend to lead to
failures of natural nitrogen-removal mecha-nisms.
Under such circumstances, nitrate, ammonium,
and other nitrogen compounds may enter the
ground-water system and travel long distances.

Swamps and organic horizons in soils can
contribute natural ammonium and/or nitrates to
aquifers. Under most circumstances, however,
decay of the organic is sufficiently slow that the
nitrogen compounds are utilized within the wetland
and adjacent aquifers. High nitrate and ammo-
nium concentrations in aquifers are more likely to
be caused by inadequate soil and aquifer con-
ditions and contamination by human or animal
wastes.

For microbial decomposition of nitrogenous
compounds to occur, there must be a source of
organic carbon, and other nutrients. The role of
nitrogen-utilizing microbes in deep aquifers has not
been adequately evaluated. It appears that
microbial transformations analogous to sulfate
reduction may occur. Availability of organic
carbon and nitrogen compounds is limited in
deeper portions of the Floridan aquifer system, so
nitrogen-utilizing microbes are probably ineffective
in the same way as are sulfate-reducing microbes.
Our present concepts suggest that the majority of
nitrogen fixation occurs in shallow, oxidizing
aquifers and soils.

The presence of nitrate, and the other nitro-
genous compounds in ground water, is not con-
sidered in Florida to be a result of interaction of
aquifer system water with surrounding rock

FLORIDA GEOLOGICAL SURVEY

materials. Nitrate in ground water is a result of
specific land uses. If the land use is widespread, a
body of nitrate-enriched water that is large enough
to be contoured may result. Otherwise, detection
of nitrate is an isolated phenomenon.

Nitrate contamination of ground water is of
concern in Florida. Numerous areas of the state
have reported nitrate problems. These are asso-
ciated with areas of intense agriculture use and
suburban housing. Some of the areas where
nitrates are of concern include dairies and cattle
ranches in the Suwannee and the Kissimmee River
valleys, crop lands in the northern Everglades, and
suburbs served by on-site waste treatment (septic
tanks) throughout the state.

Finally, Barcelona (1984) has pointed out that
drilling fluids can serve as sources of organic
carbon and nitrogen compounds. Care should be
taken to validate any high nitrate concentrations
reported below, especially if the data come from
newly drilled wells.

STANDARD OR GUIDANCE
CRITERION

The only compound for which there is a stan-
dard or guidance criterion in ground water is
nitrate. Nitrate is subject to the Primary Drinking
Water Standard (Florida Department of Envi-
ronmental Regulation, 1989). The limit under the
Primary standard is 10 mg/L as N, or 44 mg/L as
NO,. There is a health advisory for nitrate at 1 mg
N/L (4.4 mg NO,-/L), as well. The major cause of
concern is methemoglobinemia, an excess of
methemoglobin21, which causes oxygen depri-
vation. This condition is especially hazardous in
infants and young children, where it produces a
condition known as "blue baby syndrome" (Hersh,
1968; Hem, 1985). There are no standards for
ammonium or other nitrogenous decay products in
ground water (Florida Department of Environ-
mental Regulation, 1989).

Table 24 lists the number of samples in which
the 10 mg/L N standard was exceeded. Given the
notoriety of nitrate contamination problems in the
state, the Background Network detected surpris-
ingly little nitrate contamination above the water-
quality standard. Statewide, 0.6 percent of the
samples from the surficial aquifer system
exceeded the standard. No samples from the
intermediate aquifer system exceeded the stan-
dard, and one percent of the samples from the

Floridan aquifer system exceeded it. This does not
mean that there are no problems, only that the
problems are localized.

DISTRIBUTION IN GROUND WATER

With the exception of the SJRWMD all districts
analyzed for nitrate. SJRWMD analyzed its
samples for nitrate plus nitrite19. Unless stated
otherwise, all concentrations given below are
reported as nitrogen.

Surficial Aquifer System

The only nitrogen species widely measured in
the surficial aquifer system in the Background
Network is nitrate. Other analytes have been
included in a few samples. In all cases, most
samples have no detectable nitrogen compounds.
Detection of nitrate, ammonium, and other
compounds is unusual.

Ammonium SFWMD analyzed 577 samples
and found a mean concentration of 0.437 mg/L
(standard deviation = 0.341, range = 0.000 -
1.550). No other surficial aquifer system analyses
are available. Characteristically, if ammonium is
present at all, it should occur near the land surface
and waste sources. Given the high organic
contents of soils and water in the central portion of
the SFWMD, moderate ammonium concentrations
are not unexpected.

TKN Total Kjeldahl Nitrogen (TKN) represents
the nitrogen included in complex, nitrogen-
containing organic and some ammonia. The
SFWMD found an average of 0.775 mg/L (standard
deviation = 0.679 mg/L, range = 0.000 2.660,
number of samples = 20) in the surficial aquifer
system. TKN is closely related to ammonium in
the oxidation/reduction sequence (reaction 15).
Therefore, high TKN should be related to the land
surface and proximity to organic nitrogen sources.

Nitrate Nitrate is relatively widespread in the
surficial aquifer system. This is a result of appli-
cation of fertilizers and wastes on the land surface,
which is the upper boundary of this aquifer system.
Animal wastes are generated in range lands, feed
lots, and dairies. Human wastes also contaminate
the surficial aquifer system in some areas as a
consequence of septic-tank use in rural areas.

The distribution of nitrate is summarized in
Table 24. Nitrates are characteristically at or below
detection limits statewide. The median nitrate con-
centration statewide is below detection limits
(Table 24), and only the Sand and Gravel Aquifer in
the NWFWMD shows significant nitrate con-
centrations.

The distribution of nitrate in the northern
districts (NWFWMD, SRWMD; Figure 44a,b)
illustrates the "point-source" nature of nitrate. The
data are highly variable, and they cannot be cor-
related from point to point. It is important to note
that there are many nitrate "hits", indicating that
nitrate contamination of the surficial aquifer system
is widespread.

There are 64 NO, analyses from the surficial
aquifer system in the SJRWMD (Table 24). The
maximum concentration found was 7.50 mg/L.
The median concentration in the SJRWMD is
<0.01 mg/L. Given data from other districts and
studies, most of the NOx is nitrate, and nitrite is
rare as a constituent. Therefore, most of the con-
centrations reflected in the map (Figure 44c)
represent nitrate. Nitrate in the SJRWMD is at or
below detection limits throughout much of the
district. Moderate nitrate concentrations occur in
agricultural areas along the St. Johns River
corridor.

The SWFWMD and SFWMD nitrate data
(Figure 44d,e) also reflect enrichment of surficial
aquifer system waters under large agricultural
areas. There is some indication of elevated nitrates
in the upper Everglades and along portions of the
Kissimmee River valley.

Intermediate Aquifer System

Because of the confining properties of clay-
rich horizons in the intermediate aquifer system,
one would not expect nitrogen species to be a
problem.

Ammonium and TKN One hundred and fifty-
five samples from the SFWMD have an average
ammonium concentration of 0.32 mg/L (standard
deviation = 0.17 mg/L, range = 0.00-0.78 mg/L).
No other ammonium or TKN data are available.

Nitrate Table 24 illustrates the distribution of
nitrate in the aquifer system. With the exception of
NWFWMD, the median nitrate concentrations are
below detection limits. Maximum nitrate
concentrations are low compared to the other
aquifer systems, and three samples were found to
exceed the 10 mg/L N standard.

Nitrate concentrations are moderately high
throughout the NWFWMD (Figure 45a), but
correlations between the wells are impossible.
Concentrations are variable, indicating local
sources. In contrast, nitrate concentrations in the
intermediate aquifer system in the SRWMD,
SJRWMD, SWFWMD, and SFWMD (Figure 45b-e)
are generally low, with a few widely scattered de-
tections.

Floridan Aquifer System

The distribution of nitrogen species in the
Floridan aquifer system is related to proximity to
the land surface and karst conduits. Lawrence
and Upchurch (1982) attributed nitrates in the
poorly confined Floridan aquifer system near Live
Oak (Suwannee County) to local recharge through
drainage wells and sinkholes and transport in karst
conduits. The waters with nitrates were subject to
relatively rapid infiltration. Elsewhere, nitrogen
species should be absent or rare.

Ammonium There is not enough ammonium
data to draw conclusions.

Nitrate The distribution of nitrate reflects
characteristically low concentrations. With the
exception of the NWFWMD (Table 24), median
nitrate concentrations are at detection limits.
Maximum concentrations are high (>10 mg/L) and
reflect near-surface conditions and flow through
karst conduits.

A belt of moderate nitrate concentrations
occurs in central Okaloosa and Walton Counties
(Figure 46a). This belt is large and includes a
variety of land uses, such as silviculture and a U.S.
Air Force base. A similar belt occurs from
northeastern Bay through Leon Counties. This area
includes a karst terrain, which has been shown by
Lawrence and Upchurch (1982) to be subject to
recharge of nitrate-rich surface waters. Additional
work is needed to determine the reasons for the
extensive nitrate occurrences in the district.

SPECIAL PUBLICATION NO. 34

The most extensive area of nitrate in waters of
the Floridan aquifer system in the SRWMD (Figure
46b) is centered on Suwannee County. This is an
area known to have contributions of nitrates from
agriculture (Upchurch and Lawrence, 1984) and
from surface waters recharged through storm-
water drainage wells (Hull and Yurewicz, 1979).
Lawrence and Upchurch (1982) described the
mechanisms of recharge of ammonium and nitrate
to the Floridan aquifer system in this area. They
found three chemical influences: (1) slowly
recharged waters that were affected by contact
with the Hawthorn Group, (2) high nitrate waters,
which were attributed to rapid infiltration through
sinkholes, and (3) ammonium-rich waters that
rapidly infiltrated through drainage wells and
sinkholes. Other areas of moderate to high nitrate
concentrations with similar origins occur in
portions of Lafayette, Alachua, Gilchrist, and Dixie
Counties. The Floridan is unconfined to poorly
confined in all of the areas indicated, and surface
runoff drains directly into sinkholes that penetrate
the Floridan aquifer system.

Similar arguments can be made for the spotty
distribution of nitrate in waters of the Floridan
aquifer system in the SJRWMD (Figure 46c). High
nitrate concentrations occur under the agricultural
areas that extend across the center of the district
from St. Johns and Flagler Counties to Marion
County. The western and central portions of this
belt have high recharge potentials (Scott et al.,
1991), but the eastern third does not. The sources
of nitrates in the high recharge areas are similar to
those of the SRWMD, while the causes of high
nitrates in the eastern part of the district are less
easily identified. It is possible that recharge is
being induced by pumpage in the eastern area.

Nitrates in the SWFWMD (Figure 46d) also
reflect differences in recharge potential. The
northern half of the district, which is characterized
by high recharge potential, has a spotty pattern of
nitrate concentrations that reflects local land uses.
The Floridan is better confined in the southern half
of the district, and nitrate concentrations are
characteristically lower.

There is little data for the distribution of nitrate
concentrations in the Floridan aquifer system in the
SFWMD (Figure 46e). Most values are at or below
detection limits.

OTHER CONSTITUENTS

The constituents discussed in this section
include the general descriptors of water quality
(Total Dissolved Solids and Specific Conductance)
and the organic chemistry of the state's aquifer
systems. The discussions of organic compounds
in the aquifer systems are divided into three
subjects: Total Organic Carbon, Synthetic
Organics, and Pesticides. Total Organic Carbon is
a measure of the natural organic content of the
water, while Pesticides and Synthetic Organics
reflect anthropogenic compounds.

Total Dissolved Solids

IMPORTANCE

Total dissolved solids (TDS) is a measure of
the total mass of ions dissolved in water. The
procedure for determining total dissolved solids
involves weighing the mass of salts deposited after
the water is evaporated. Volatile materials may be
lost in this procedure, and there is some difficulty
in obtaining a moisture-free environment for weigh-
ing. Consequently, total dissolved solids is, at
best, a general estimator of the total load of chem-
icals dissolved in the water.

The more reactive a rock is, the higher the total
dissolved solids content of waters within that rock
are likely to be. For example, total dissolved solids
are likely to be higher in a limestone aquifer than in
a siliciclastic aquifer. Total dissolved solids also
tends to increase with residence time and as water
progresses along a flow path. An important
consequence of this is that waters in the Floridan
aquifer system that go deep into the aquifer
system and contact the reactive, gypsum- and
anhydrite-bearing lower confining beds may
contain high total dissolved solids due to dissolved
calcium and sulfate (Table 4). Therefore, total
dissolved solids can be used to understand the
chemical maturation and flow history of certain
aquifer systems.

Total dissolved solids in the Floridan aquifer
system have been discussed by Shampine (1975),
Kaufman and Dion (1967, 1968), Hull and Irwin
(1979), Sprinkle (1989), and others. Sprinkle
(1982b) presents a map of the distribution of total
dissolved solids in the Floridan aquifer system.
Sprinkle's map agrees in general with the data pre-
sented below, although the level of detail of his
map is less.

STANDARD OR GUIDANCE
CRITERION

The Florida Secondary Drinking Water
standard for total dissolved solids is 500 mg/L
(F.A.C. CH. 17-550.310-320; Florida Department of
Environmental Regulation, 1989). This standard is
based on a number of concerns. Waters with high
total dissolved solids content have an unpleasant
taste. The high total dissolved solids may result in
development of scale and precipitates in water,
especially in boilers, hot water heaters, and other
heated-water systems. Finally, persons who
consume high total dissolved solids water are at
risk of developing kidney and gall stones.

Table 25 summarizes the samples found to
exceed the 500 mg/L standard. Since the
Background Network includes wells that are
located in the salt-water transition zone, the
number of samples found to exceed the standard
largely reflects deeper wells, that sample the
transition zone near the lower confining beds, and
coastal wells. Statewide, 22 percent of samples
from the surficial aquifer system exceeded the
standard. Most of the samples that exceeded the
standard came from the SFWMD (Table 25), where
upcoming of connate water and coastal intrusion
are widespread. Samples from the intermediate
aquifer system include 37 percent that exceed the
standard. These exceedances are largely located
in southwest SWFWMD and western SFWMD,
where the Hawthorn Group is extensive and
utilized as a water source. The high total dissolved
solids waters are located in coastal areas and
regions of upcoming. Thirty-one percent of the
samples from the Floridan aquifer system
exceeded the standard. These samples are uni-
formly distributed through the districts and reflect
coastal and upcoming areas in the aquifer system.
Given the purposeful location of wells in transition
zones, little significance can be attached to the
high proportion of samples that exceeded the
standard. Examination of the maps discussed
below is a better way of evaluating the total
dissolved solids content of the potable portions of
the aquifer systems.

DISTRIBUTION IN GROUND WATER

The distribution of total dissolved solids in
Florida ground waters is summarized in Table 25.
Note that, while several important trends are
apparent, the data reflect all samples from within a
district. Some districts utilized monitor wells that
are either near the coastal salt-water transition

zone or the base of the aquifer system. These
wells yield high total dissolved solids waters and
bias the summary statistics.

The most significant patterns in total dissolved
solids data (Table 25) reflect equilibration with
carbonates and poor flushing of aquifer systems.
In the surficial aquifer system, total dissolved
solids tends to increase southward, which reflects
the increase in reactive carbonate minerals in the
surficial and intermediate aquifer systems
southward. Total dissolved solids data from the
Floridan aquifer system show similar medians for
all districts except the SFWMD. The high total
dissolved solids concentrations in the SFWMD
reflect low quality of water in the Floridan over
much of the district. This is a result of incomplete
flushing of the aquifer system due to low hydraulic
heads.

Surficial Aquifer System

Figure 47 illustrates the distribution of total
dissolved solids in water of the surficial aquifer
system. Total dissolved solids concentrations are
quite low, indicating minimum weathering of the
siliciclastic host rock materials in NWFWMD and
SRWMD. There is an increase in total dissolved
solids towards the coast and Escambia Bay within
the Sand and Gravel Aquifer (Figure 47a). There
are a few coastal wells that exhibit high total
dissolved solids in SJRWMD (Figure 47c), but most
inland wells have low total dissolved solids waters.
The high total dissolved solids coastal wells are in
areas of both connate water and heavy pumpage,
which may have induced some salt-water
intrusion. Coastal salt-water intrusion is well
documented in SWFWMD (Figure 47d), where the
250 mg/L total dissolved solids isoline in the
surficial aquifer system parallels the coast and
major embayments. The high total dissolved
solids content of waters in the re-entrant along the
Peace River axis result from calcium-sulfate rich
waters that are released to the surficial aquifer
system by irrigation and natural upwelling.

The reverse is somewhat true in SFWMD
(Figure 47e). SFWMD can be divided into three
zones (Figure 47e): the Kissimmee and
Caloosahatchee watersheds, the Everglades and
Big Cypress Swamp, and the Atlantic Coastal
Ridge. In the Kissimmee and Caloosahatchee
watersheds, the total dissolved solids concen-
trations range from below 250 mg/L to over 500
mg/L. Highest total dissolved solids waters seem
to follow the rivers and most likely represent

FLORIDA GEOLOGICAL SURVEY

upwelling and discharge of deeper waters. This
upwelling has been documented in Lee County by
Wedderburn et al. (1982) and Upchurch (1986).
While few data are present from the surficial
aquifer system in the Everglades and Big Cypress
Swamp, total dissolved solids concentrations are
elevated there as well. This is a result of poor
flushing of connate waters and of upcoming
subsequent to draining wetlands to enhance
agriculture. Finally, the Biscayne Aquifer, which
comprises the Atlantic Coastal Ridge, has waters
with total dissolved solids concentrations in the
range of 250 to 500 mg/L. This water is locally
recharged, and it represents the highest quality
ground water in southeast Florida.

Intermediate Aquifer System

Total dissolved solids in the intermediate
aquifer system ranges from 18 mg/L to 6,892 mg/L
(Table 25). This wide range is a result of the
diversity of lithologies represented in the Hawthorn
Group, as well as the influences of the coastal
transition zone. Where water has been in contact
only with siliciclastic materials, the total dissolved
solids content is low. Where it has been in contact
with carbonates and chemically unstable silicates
(clays, opal; Tables 4,5), total dissolved solids
content is high. Highest total dissolved solids
concentrations are in south Florida, where saline
water is present in the aquifer system.

The intermediate aquifer system data cannot
be contoured in north and central Florida due to
the heterogeneity of the aquifer system units
(Figure 48a,b,c). The data can be contoured in
southern SWFWMD and western SFWMD (Figure
48d,e), where the aquifer system is moderately
deep and continuous. Where the intermediate
aquifer system is near the coast, the salt-water
transition zone is characterized by high total
dissolved solids (Figure 48d,e).

In areas where the intermediate aquifer system
contains abundant carbonate horizons, fracture
traces appear to affect water quality. For example,
re-entrants of salty water can be delineated in
Sarasota County (Figure 48d). These coincide with
major lineaments that can be identified from
satellite imagery (Culbreth, 1988). There is also
local upcoming of deeper, high total dissolved
solids water along a lineament which is occupied
by the Peace River (Figure 48d; G. Jones, 1991).
These areas of upcoming follow zones of high
vertical permeability. In some areas the relative
head distribution of the aquifer systems will

encourage upcoming, in other areas pumpage is
the cause.

Floridan Aquifer System

The pattern of total dissolved solids in waters
of the Floridan aquifer system is directly related to
the salt-water transition zone and regional flow
systems (Figure 49). High total dissolved solids
concentrations in the Floridan aquifer system are a
result of long contact times with soluble limestones
and dolostones of the Floridan and mixing with
high total dissolved solids, saline waters at the
base of the aquifer system and at the coast.
Lowest total dissolved solids concentrations are in
the interior, where recharge is prevalent and
residence times in the aquifer system are too short
for effective equilibration with the aquifer system.

The potentiometric surface of the Floridan
aquifer system controls the position of the
transition zone (see the potentiometric maps in
Scott et al., 1991). Where head is high near the
coast, the salt-water transition zone beneath the
land surface is narrow, or it may be offshore. The
transition zone also slopes steeply inland where
potentials are high. Re-entrants occur near river
mouths, where the potential is lowered. These re-
entrants can be seen near the Escambia and
Apalachicola Rivers (Figure 49a), and the Hills-
borough, Manatee, and Peace Rivers (Figure 49d).
Other re-entrants along the coast are related to
intrusion caused by pumpage. Where the potential
is low, such as occurs in the Everglades and Big
Cypress Swamp areas of the SFWMD, the total
dissolved solids content of the Floridan aquifer
system is high, and the coastal transition zone is
broad, with a shallow slope (Figure 49e).

The 500 mg/L isoline defines the extent of
potable water in the Floridan aquifer system. Note
that it crosses the state from Sarasota County
(Figure 49d), north of Lake Okeechobee (Figure
49e), to southern Brevard County (Figure 49c).
South of the 500 mg/L isoline the flow system in
the Floridan aquifer system is weak, and high total
dissolved solids waters have not been flushed
from the aquifer system by fresh-water flow.

Locally, waters high in total dissolved solids
content in the interior of the state (Figure 49) re-
present wells that either are deep enough to reach
high-sulfate concentrations near the base of the
Floridan aquifer system or are in regions of up-
coning as a result of pumpage.

Specific Conductance

IMPORTANCE

Specific conductance is a measure of the
ability of material to conduct electrical currents.
The American Society of Testing and Materials
(1980) has defined specific conductance as the
"reciprocal of the resistance in ohms measured be-
tween opposite faces of a centimeter cube of an
aqueous solution at a specified temperature". The
inverse of the ohm (the measure of electrical resis-
tance) is the mho. Natural waters are moderately
resistive, so specific conductance is measured in
micromhos/cm (imhos/cm). The micromho is
equivalent to the microSiemen (pS) in SI notation.

The ability of water to conduct electricity is
primarily a function of the concentration of
electrical charges in the water and of water
temperature (Miller et al., 1988). When an electrical
potential is applied to water, cations tend to mi-
grate to the cathode, while anions migrate to the
anode. It is this potential for ionic migration that
specific conductance measures. Therefore, spe-
cific conductance is sensitive to the concentrations
and types of ions in the water. Miller et al. (1988)
have discussed the effects of mixtures of ions on
specific conductance. All of the ions previously
discussed contribute to the electrolytic properties
of water.

Specific conductance roughly reflects the
same processes as total dissolved solids, and
there is a good statistical correlation between the
two variables. However, specific conductance is
dependent upon the specific combinations and
concentrations of the electrolytes in the solution,
so two water samples with the same total dis-
solved solids contents may not have the same
conductivities (Miller et al., 1988). As a rule of
thumb, at equal TDS concentrations, the following
anions can be ranked according to ability to
conduct electricity

Cl > SO42- > HCO3-, (
(16)
High Conductivity
Low Conductivity --

where chloride-rich waters are more conductive
than sulfate-rich waters, and so on. Because
specific conductance and total dissolved solids are
correlated, specific conductance can be utilized to
evaluate chemical maturity along a flow path and

salt-water intrusion.

STANDARD OR GUIDANCE
CRITERION

There are no standards or guidance criteria for
specific conductance. Specific conductance, per
se, does not constitute a water-quality hazard to
water users. It is commonly used as a field analyte
for evaluation of gross water quality, so it is
included in this report. Given certain assumptions
about the composition of the water, specific con-
ductance can be correlated with salinity, and many
aquifer salinity measurements given in the literature
are based on specific conductance uncorrected for
water chemical speciation. Consequently, it is not
safe to assume that high specific conductance
waters are necessarily in violation of standards for
important electrolytes for which standards exist,
such as sodium and chloride.

DISTRIBUTION IN GROUND WATER

The distributions of specific conductance data
in Florida ground waters are summarized in Table
26. Since specific conductance is correlated with
total dissolved solids, the arguments concerning
the distribution of dissolved solids data (see Total
Dissolved Solids) hold for specific conductance.

In Florida's aquifer systems, high chloride
waters cause highest specific conductance ano-
malies. For this reason, conductivities tend to
increase towards the coast and estuaries.

High conductivities inland reflect several
factors, including upcoming of deeper, more saline
waters into shallow wells; release of deep Floridan
aquifer system water on the land surface as
irrigation water, which is then recharged to the
surficial aquifer system; increases of chlorides due
to evaporation of meteoric water in unconfined
portions of aquifer systems; and residual (or con-
nate) waters from earlier high sea-level stands that
have yet to be flushed from the aquifer systems.

Surficial Aquifer System

Specific conductance of water from the sur-
ficial aquifer system is characteristically low due to
the low concentrations of electrolytes. Median
conductivities increase southward (Table 26),
indicating an increase in total dissolved solids as

SPECIAL PUBLICATION NO. 34

aquifer carbonate content and influence of salt
water increase. The median specific conductance
for the surficial aquifer system, statewide, is 475
imhos/cm, while the district medians range from
50 pmhos/cm in the NWFWMD to 619 imhos/cm
in the SFWMD.

Increases in specific conductance have been
documented towards the coast and estuaries in
NWFWMD (Figure 50a) and SWFWMD (Figure
50d). Specific conductances in the SRWMD
(Figure 50b) are relatively uniform, indicating that
over its limited extent water from the surficial
aquifer system has a relatively homogeneous
composition. The surficial aquifer system in the
SJRWMD (Figure 50c) has several good examples
of high specific conduc-tance waters inland. Many
of these are asso-ciated with areas of irrigation,
where deeper waters or evaporative concentration
elevate specific conductance. Floridan aquifer
system water is widely used to irrigate pastures
and crops in the area, where it recharges the
surficial aquifer system (Dalton, 1978). Increased
specific conductances in central SFWMD (Figure
50e) are caused by contamination of irrigation
waters from deeper aquifer systems, upcoming of
more miner-alized water, and salt-water intrusion.

Intermediate Aquifer System

Specific conductance in the intermediate
aquifer system is generally higher than in the
surficial aquifer system (Table 26). These specific
conductances are higher in response to increased
anion concentrations that result from chemical
weathering of the host rock. Inland, where
chlorides are of minimal importance, sulfates
produced by the oxidation of pyrite (Table 4)
contribute most significantly to the specific con-
ductance. The southward increase in specific
conductance as a result of increased total
dissolved solids is evident (Table 26).

Figure 51 illustrates the distribution of specific
conductance in the intermediate aquifer system. In
south Florida (Figure 51d,e), where data can be
contoured, it is evident that specific conductance
increases toward coasts and estuaries. This same
pattern is present in the other districts; however, it
is not contourable (Figures 51a,b,c). Upconing
along the Peace River lineament (G. Jones, 1991)
is evident (Figure 51d), as is a zone of high specific
conductance waters along the Caloosahatchee
River (Figure 51 e).

Floridan Aquifer System

Figure 52 illustrates the distribution of specific
conductance in the Floridan aquifer system.
Specific conductance values away from the coast
are low in north and central Florida. Specific con-
ductances tend to increase toward the coast
(Figure 52a), which reflects maturation along long
flow paths and mixing with salt water near the
transition zone.

The Suwannee River divides the SRWMD
(Figure 52b) into two flow systems, east and west
of the river. Both flow systems illustrate the
increase in specific conductance along flow paths.
Lowest conductivities are near surface-water
drainage divides. Highest specific conductance
waters are in the coastal transition zone both in the
western and eastern flow systems, and near the
Suwannee River in the eastern system. High
specific conductances are also found in the
Northern Highlands physiographic province in the
SRWMD. These result from equilibration of the
water with the host rock under nearly stagnant flow
conditions. Note that there are several, isolated,
"plume-like" water masses, such as in Alachua
County. These appear to be related to slightly
elevated chloride levels near karst recharge areas
along the Cody Escarpment (Lawrence and
Upchurch, 1976, 1982).

Specific conductance data from the SJRWMD
(Figure 52c) indicate relatively low specific
conductances inland. There is a large re-entrant
centered on St. Johns and Flagler Counties, which
can be attributed to connate waters and intrusion
under pumping stress. There are also several
centers of high specific conductance along the St.
Johns River which appear to reflect upcoming
along faults and fractures (Leve, 1983).

The influence of coastal salt water is well
illustrated by specific conductance of waters of the
Floridan aquifer system in the SWFWMD (Figure
52d). A strong specific conductance gradient is
shown at the salt-water transition zone. Re-
entrants occur along the axes of the
Withlacoochee, Hillsborough, Manatee, Peace,
and Myakka Rivers.

In south Florida (Figure 53e) the Floridan has
high sulfate, chloride, and bicarbonate contents.
This causes the Floridan water to have
conductivities in excess of 4,000 Imhos/cm south
of the line from Lee to Brevard Counties. Again,

this high anion content is a result of poor flushing
of the aquifer system under low hydraulic head
conditions.

Total Organic Carbon

IMPORTANCE

Total organic carbon (TOC) in aquifers and
monitor wells can have three sources: (1) natural
humic substances, (2) synthetic organic
contaminants, and (3) drilling fluids. Data
presented below (see sections on Synthetic
Organics and Pesticides) indicate that the
concentrations of anthropogenic organic are
orders of magnitude less than the total organic
carbon concentrations. Barcelona (1984) suggests
that the concentrations to be expected from drilling
muds are also much less than the TOC
concentrations reported herein. Therefore, the
TOC concentration data discussed in this section
reflect naturally occurring organic, or humic
substances.

Humic Substances Most of the total organic
carbon reported in ground water is composed of
humic substances humicc and fulvic acids) which
are derived from microbial decay of leaf litter, soil
organic, and soil biota waste products. Humic
substances are complex molecules with a wide
range in molecular weights (Thurman, 1985). Since
they are decomposition products, humic
substances can include a number of different
molecular structures and functional groups, such
as carboxyl, amino, and carbonyl groups. Humic-
substance molecules have flexible structures, large
sizes, and a diversity of functional groups, so they
are effective chemical completing agents,
especially for metals. They are capable of con-
forming to clay surfaces and of flocculation as
particulates in their own right.

The wide range in molecular weight, diversity
of molecular structures, and large number of
functional groups make characterization of humic
substances difficult. As a result, a common
method of classification is by their response to the
pH of surrounding water (Thurman, 1985). Humic
acids are humic substances that are soluble in
basic solutions and insoluble in acidic solutions
(pH < 2) or ethanol. Humic acids include large,
complex molecules with molecular weights of
2,000 to >5,000 daltons (24,000 to 60,000 a.m.u.;
Thurman, 1985). Because of their low solubility in
acidic solutions, humic acids tend to flocculate in

acidic, siliciclastic soil zones and contribute to the
formation of organic hard pans. In alkaline,
carbonate aquifers humic acids can migrate until
they flocculate or are decomposed. Fulvic acids
are less complex than humic acids (molecular
weights of 500 to 2,000 daltons [6,000 -24,000
a.m.u.; Thurman, 1985]), and they are soluble
under both acid and alkaline conditions. The
smaller fulvic acid molecules do not color water,
while humic acids may. Humins are insoluble in
both acids and bases. The particulates and
colloids that constitute the majority of organic in
soils, especially organic hard pans, and sediments
are humins.

Three processes affect the mobility of organic
acids: (1) microbial decay, (2) pH of the host
water, and (3) total dissolved solids content of the
host water (Thurman, 1985).

Microbial Decay Microbial decay of organic-
rich material is associated with many of the
processes previously discussed. Iron, sulfur, and
nitrogen transformations are associated with soil
microbes, especially bacteria. Reaction 6 is an
example of a microbially driven reaction in which
organic carbon and sulfate are metabolized.
Therefore, bacteria require a source of organic
carbon, as well as the other nutrients. The bacteria
form mats composed of a number of different
species, all of which support each other in a
complex community.

In order for microbial mats to thrive, they
require (1) a solid substrate for attachment and (2)
a constant bath of water rich in organic carbon and
other nutrients. In intergranular aquifers, there is
abundant substrate and particulate organic
materials (particulate humic substances and dead
microbes) are mechanically filtered from the water.
In karstic flow systems, the caverns and karst
conduits provide less substrate per unit volume of
water, so microbial activity may be reduced and
TOC may persist. In karstic aquifers, therefore,
dissolved and particulate organic carbon can travel
some distance before suitable conditions for
microbial decay or mechanical trapping can be
realized. While they are produced in most soils,
humic substances are best preserved in water-
saturated, reducing soils and aquifers where
complete oxidation and aerobic microbial decay
are inhibited.

Transformation Mechanisms and Carbon
Fixation Organic carbon can be fixed in a soil or
aquifer as a particle or as organic tissue, or it can

FLORIDA GEOLOGICAL SURVEY

be released as a gas. The gas can then be lost to
the atmosphere, or it can become involved in
inorganic reactions. Most organic carbon is
consumed by microbial activity with a sequence of
degradation steps leading to the ultimate
production of carbon dioxide (CO,) or methane
(CH,).

Microbes have the capability of converting
organic into CO2 under aerobic conditions22, and
CH4 under anaerobic conditions. Both gases can
pass out of the soil and aquifer by degassing.
Carbon dioxide gas can enter into inorganic
reactions with carbonates as indicated in reactions
1 and 2. Carbon dioxide released by microbes in
the soil contributes to the production of carbonic
acid in soil and aquifer waters. Partial pressures of
CO2 characteristically rise from 10.35 to
approximately 1020 as a result of CO2 production in
soils. This results in a drop in water pH from
around 5.5 to pH values of 3.5-4.5. In aerobic,
intergranular flow systems, microbial decay may
occur over a short distance, in which case the
resulting ground water will become relatively free
of total organic carbon. In anaerobic systems,
microbial activity is somewhat inhibited, and total
organic carbon can persist.

Soluble organic acids can be removed from
ground water by flocculation (Thurman, 1985).
Flocculation of organic acids takes place when
chemically reactive sites on the organic molecule
become saturated with cations. If abundant,
hydrogen ions are sufficient to cause flocculation
of humic acids, which results in a loss of solubility
in acidic solutions. Both organic acid groups can
flocculate if they come in contact with high total
dissolved solids waters, such as occurs when
meteoric waters enter a carbonate-rock aquifer or
sea water.

Transport of Trace Metals Humic and fulvic
acid molecules include numerous sites carbonyll,
carboxyl, amino-, and similar sites) where chemical
completing23 can occur. Because of these sites,
these acids are capable of binding metals and
inducing their transport. For this reason, water
high in total organic carbon is usually high in iron
and trace metals (Young and Comstock, 1986). At
contaminated sites humic and fulvic acids may
cause undesirable movement of large amounts of
metals.

Sources of TOC Most organic material is
derived from the land surface. Swamps and
organic zones in soils are widespread and

contribute total organic carbon to ground water.
Thus, in most places total organic carbon
decreases in concentration with depth (Watrous
and Upchurch, in prep.). This is not to say that
there are not other sources of total organic carbon.
Clays in the Hawthorn Group contain organic that
can be decomposed to produce soluble total
organic carbon. Limestones and dolostones in the
Avon Park Formation contain widespread organic-
rich zones that may also contribute total organic
carbon.

Trace organic contamination of ground water
by synthetic organic is rarely represented in total
organic carbon data because the small concen-
trations that are usually present are below the
detection limits of the total organic carbon
analytical method. None of the total organic
carbon data reported herein can be shown to be
primarily a result of anthropogenic, synthetic or-
ganics.

Many muds used in well drilling contain orga-
nics as emulsifiers, binders, and/or coagulation-
control additives. These can contaminate poorly
developed wells and result in spurious total
organic carbon concentrations (Barcelona, 1984).
The latter is a known problem in the Background
Network data set, and isolated reports of high total
organic carbon must be considered as probable
contamination24 by drilling fluid until confirmed by
further sampling. For example, in the SRWMD the
original data set of 24 surficial aquifer system wells
contained 11 wells (46%) with over 10 mg/L total
organic carbon. When the wells that had known or
suspected drilling-fluid contamination or excess
particulates are removed from the data set, the
number with high total organic carbon drops to 4
out of 15 (27%).

STANDARD OR GUIDANCE
CRITERION

There are no standards or criteria for naturally
occurring organic carbon. Where synthetic
organic constitute all or part of the organic
carbon, individual criteria would be in effect.

Natural organic carbon is not harmful to
humans. It may cause discoloration of water and
may stain porcelain fixtures. Because humic
substances can complex and transport trace

metals, which may be hazardous, organic carbon
may be of secondary concern.

Considerable attention has been given to
organic carbon in public water supplies in
southeast Florida because of the potential for
formation of trihalomethane (or THM) compounds.
When organic-rich water is sterilized by
chlorination, chlorine may substitute for hydrogen
on methane radicals. An example of this reaction
is the formation of trichloromethane (or
chloroform), which has the structure

H

CI C CI.

C I
ClI

The trihalomethanes are known to be carcinogenic,
and considerable effort is expended to avoid
formation of THM's in drinking water.

DISTRIBUTION IN GROUND WATER

The total organic carbon data are among the
most interesting and important data in this report.
No other known study has compiled a data set of
organic carbon in aquifer systems that is as ex-
tensive. It is also of interest that the total organic
carbon levels are so high. Hem (1985) noted that
all natural waters contain organic carbon because
of the intimate relationship of water and living
matter. His review of total organic carbon data
from aquifers indicates, however, that total organic
carbon concentrations can be expected to be less
than 20 mg/L as carbon. Thurman (1985)
summarized the distribution of dissolved humic
substances in filtered ground and surface waters.
He reported the concentration range in ground
water to be 0.03 to 0.10 mg/L as carbon. The
samples described in this section were not filtered,
so one would expect higher concentrations.

Florida total organic carbon data indicate that
organic carbon is very widespread and con-
centrations range as high as 380 mg/L (Table 27).
The statewide median concentrations in mg/L as
carbon are: surficial aquifer system 14.0,
intermediate aquifer system 4.8, and Floridan
aquifer system 2.2 mg/L..All of the medians
exceed Thurman's (1985) expected concentration
range, and they are near the upper level cited by

Hem (1985).

Total organic carbon is distributed throughout
all aquifer systems, but is highest in the surficial
aquifer system (Table 27). Given the regional
extent and continuity of the total organic carbon
concentrations, it is clear that most of the organic
carbon in Florida's aquifer systems is a result of
local recharge of humic substances.

Surficial Aquifer System

The surficial aquifer system should contain the
highest total organic carbon concentrations
because it contains humus in spodic zones and
upper soil horizons, and it is in shallow soils that
most microbial decay of plant material occurs. The
surficial aquifer system median concentration is
14.0 mg/L total organic carbon (Table 27), with the
SFWMD having the highest median total organic
carbon. The highest maximum concentration is
380 mg/L, which must include considerable
colloidal and particulate humic material. Given the
wet and highly vegetated nature of many areas of
the state, one would expect high concentrations of
total organic carbon in the surficial aquifer system.
The median and maximum concentrations increase
to the south, also indicating the prevalence of
organic-rich, wetland environments to the south.

The pattern of total organic carbon in the
surficial aquifer system is largely controlled by
proximity to organic-rich surficial environments,
such as wetlands, rivers, or peat deposits. Total
organic carbon declines with depth in the aquifer
system due to microbial decay and flocculation.

Surficial aquifer system wells that are high in
total organic carbon in NWFWMD and SRWMD
(Figure 53a,b) are closely associated with marshes,
swamps and riverine drainage. High TOC is also
associated with swamps and other surficial
sources of organic in the SJRWMD (Figure 53c).
High total organic carbon occurs in some coastal
wells in all three districts in regions where coastal
swamps and marshes abound. Sediments of the
coastal ridges include peats and disseminated
organic which may also contribute to the total
organic carbon concentrations in the aquifer
system waters. Total organic carbon in the
surficial aquifer system in SFWMD (Figure 53e) is
generally low in the Atlantic Coastal Ridge, where
highly oxidizing environments have reduced the
organic content. Elsewhere, total organic carbon
is high in waters related to the Everglades and Big

SPECIAL PUBLICATION NO. 34

Cypress drainage systems and portions of the
Caloosahatchee and Kissimmee River drainages,
which are characterized by extensive swamps,
marshes, and peat deposits.

Intermediate Aquifer System

Total organic carbon in the intermediate
aquifer system can result from rapid recharge of
surface and surficial aquifer system waters, or it
may be derived from decomposition of organic in
the Hawthorn Group (Miller, 1978). Median total
organic carbon concentration in the system is 4.8
mg/L (Table 27). The highest TOC concentration is
71.0 mg/L in the SFWMD (Table 27).

The high total organic carbon concentrations
throughout the northern and central portions of the
state are generally related to rapid recharge
environments, such as in karst terrains and the
vicinity of drainage well systems. Total organic
carbon in intermediate aquifer system waters of
southern Florida (southern SWFWMD, Figure 54d;
western SFWMD, Figure 54e) are probably not
related to rapid recharge. These high
concentrations, including the highest in the aquifer
system, may be related to disseminated organic
in the Hawthorn Group. Figure 54e indicates that
most of the areas characterized by high total
organic carbon concentrations in the intermediate
aquifer system water are inland from the coastal
salt-water transition zone, so total organic carbon
may also be derived from local recharge of
surficial, organic-rich waters.

Floridan Aquifer System

Total organic carbon in the Floridan aquifer
system (Table 27, Figure 55) ranges from below
detection limits to over 80 mg/L within the state.
While the median total organic carbon concentra-
tion is relatively low (2.2 mg/L) compared to other
Florida aquifer systems, it is high compared to
data from aquifer systems outside of Florida (Hem,
1985; Thurman, 1985).

The highest total organic carbon concen-
trations in northwest Florida are in areas where the
Floridan aquifer system is either unconfined or
characterized by poor confinement in a well-
developed karst terrain. In NWFWMD (Figure 55a),
highest total organic carbon concentrations are in
areas where the Floridan is unconfined and
overlain by thin, organic-rich, sandy soils, such as

in the lower Wacissa and Aucilla drainage basins.
These areas are dominated by fresh-water swamp
communities and poorly drained pine flatwoods,
both of which are excellent sources of humic
substances. Lowest total organic carbon
concentrations are in areas where the Floridan is
well confined by the overlying Hawthorn Group.
High total organic carbon concentrations also
occur in the limestone outcrop belt (Suwannee
Limestone and Ocala Limestone) in Walton,
Holmes, Washington, and Jackson Counties. In
this area the Floridan aquifer system is unconfined
and is characterized by karst drainage.

High total organic carbon concentrations are
found in scattered wells along the karstic Cody
Escarpment in the SRWMD (Figure 55b). High
total organic carbon concentrations have been
noted in the Live Oak and Lake City areas by
Upchurch and Lawrence (1984) and Brown (1989).
Brown found two sources of total organic carbon
in the Lake City area (Columbia County). She
concluded that total organic carbon concen-
trations in the Coastal Lowlands and Cody
Escarpment are a result of direct recharge of the
Floridan by organic-rich surface waters. The
moderately high total organic carbon
concentrations in the Floridan aquifer system
beneath the Northern Highlands, where the aquifer
system is confined by the Hawthorn Group, were
attributed to leaching of organic found in the
Hawthorn (Miller, 1978).

An extensive area of high total organic carbon
occurs in the Coastal Rivers Basin (Figure 55b), the
coastal drainage system west of the Suwannee
River in Taylor, Lafayette and Dixie Counties. This
area is characterized by a thin clay (the "San Pedro
clay") that overlies the Floridan aquifer system. The
Floridan is exposed at a number of sites in the
embayment and the clay is penetrated by many
sinkholes. The land surface is characterized by
extensive woodlands and swamps. The large area
of organic-rich water in the Coastal Rivers Basin is
remarkable because it so accurately defines the
extent of the basin. Total organic carbon in this
basin appears to be controlled by the local flow
system and is most likely a result of local recharge
through swamps and other areas with highly
organic surface waters and organic-rich soils. The
large re-entrant of water with less than 3 mg/L total
organic carbon in Taylor County roughly corre-
sponds to the Fenholloway River and possible
upcoming of deeper Floridan water near Perry and
Foley. Discharge of low total organic carbon water
in the Aucilla and Suwannee basins appears to
limit the lateral extent of the higher total organic

carbon water.

Floridan aquifer system water in the SJRWMD
is comparable to the other northern districts (Table
27), with a median of 3.3 mg/L and a maximum of
29.0 mg/L. The majority of the high total organic
carbon samples came from a large area centered
on Flagler County (Figure 55c). It appears from
other data that this water has a significant connate
component, and the high total organic carbon may
reflect organic in the sedi-ments of the aquifer
system.

Total organic carbon in the SWFWMD (Figure
55d; Table 27) is high relative to the remainder of
the state. The median concentration is 16.8 mg/L
and the maximum concentration is 78.8 mg/L.
High total organic carbon concentrations are
consistently high throughout the district, but the
highest concentrations occur in regions where
wetlands and riverine systems overlie unconfined,
or poorly confined portions of the aquifer system.

Total organic carbon concentrations are
relatively low in the Floridan aquifer system of the
SFWMD (Table 27; Figure 55e). Median con-
centration is 1.9 mg/L, but the maximum
concentration is the highest recorded in the
Floridan at 80.6 mg/L. Subsequent sampling of
this well yielded total organic carbon concentra-
tions of 1.0 and 3.5 mg/L, indicating that the initial,
high concentration may be anomalous. High
concentrations are associated with the Kissimmee
River corridor and with an area in western Lee
County.

Synthetic Organics

DEFINITION AND ANALYTES

Synthetic organic include a list of 142 (Table
28) anthropogenic organic compounds. This list
includes most of the compounds on the U.S.
Environmental Protection Agency priority list.
These compounds range from primary compounds
used in manufacturing and energy industries to
degradation products produced by microbial
decay and hydrolysis. Many of the synthetic
organic have been utilized as pesticides, and
inclusion in Synthetic Organics, as opposed to
Pesticides, is based on common uses. For
additional information on these and other synthetic
organic, refer to Montgomery and Welkom (1990).

Only those compounds that have been
tentatively found in Florida aquifer systems are
discussed below. Many of the occurrences
reported herein are near the detection limit, so they
may represent "false positives". Most of these
occurrences have not been confirmed by re-
sampling.

Table 29 lists the number of samples in which
any of the compounds in Table 28 were found and
the number of samples found to exceed the
standards. The table includes unconfirmed
occurrences. Care should be taken in interpreting
these data because the data are unconfirmed. The
discussions that follow list the compounds
tentatively detected and some important concerns
about each. The proportions of samples that
contain each analyte are given in the text.

No maps are given for this and the following
Pesticides sections because of the uncertainties
encountered in confirmation by resampling, ana-
lytical procedures (many of the positive results are
too near the practical quantitation limit25), and
attribution to causes.

IMPORTANCE AND CONTROLS

The synthetic organic have a diversity of
sources. They range from heavy industry to local,
household uses. Some are solvents, others are
lubricants. None are of natural origins. The den-
sities of the organic vary with respect to water, as
do the solubilities. As a result, they may exist
dissolved in water or, locally, as an non-aqueous
phase. None of the samples in this study are of
free product; all are dissolved in water. Several
factors affect the mobility of synthetic organic.
These include (1) advection, (2) dilution and
dispersion, (3) volatization, (4) density stratification,
(5) dissolution, (6) sorption, and (7) biological
conversion and hydrolysis.

Advection Advection is the term used to
describe transport with the flow of water in the
aquifer system. All constituents in ground water
are subject to advective transport.

Dilution and Dispersion Dilution and
dispersion are caused by individual packets of
water following flow paths of different lengths.
Dilution and dispersion are highly effective in
reducing the concentration of synthetic organic in
intergranular flow systems, but less effective in

FLORIDA GEOLOGICAL SURVEY

conduit flow. Here, flow paths are less tortuous
and dispersion is limited. Dilution within the con-
duit may significantly reduce the concentration,
however.

Volatization Many of the synthetic organic
listed herein are volatile. That is, given proper
conditions, the organic may go to a gaseous state
and pass out of the system. This process is
especially important in unconfined, water-table
aquifers where the gaseous phase can pass
directly into the soil or aquifer atmosphere. Table
30 gives the classification utilized in discussing
volatility below. This classification (Lyman et al.,
1990) is based on the Henry's Law constant, or the
air-water partition coefficient. Henry's Law con-
stant is defined as the ratio of the partial pressure
of the organic vapor in air to the concentration in
water. At 1 atmosphere the Henry's Law constant
is calculated by

P FW
H =
760S

where H = Henry's Law constant (atm m3/mol), P =
partial pressure (mm Hg), S = solubility (mg/L), and
FW = gram formula weight of the compound
(Montgomery and Welkom, 1990).

Density Stratification Many of the compounds
found in Florida waters are liquids with densities that
differ significantly from water. If free product is
present in the aquifer system, it will come to density
equilibrium with the water. If it is less dense than
water, the product floats on the water surface and
forms a Light, Non-Aqueous Phase Liquid (LNAPL).
If it is more dense, the liquid sinks and forms a
Dense, Non-Aqueous Phase Liquid (DNAPL). In
either case, sampling may miss the non-aqueous
phase and detect only that portion of the organic
dissolved in the water. Given the design of the
Background Network sampling plan, it is unlikely
that non-aqueous phase liquids are present at most
of the sample sites.

The compounds in the following discussion are
classified on the basis of their relative density
(specific density) as defined by

Psp p
rw

where psP = the specific density dimensionlesss), ps

= density of the organic (g/mL or g/cm3), and p, =
density of water at 4C (g/mL or g/cm3 ).
Compounds with p, significantly less than 0.7 are
said to be light, while compounds with ps, greater
than 1.3 are said to be dense.

Solubility All of the organic exhibit some
solubility in water. This ability to dissolve into
water encourages transport. The solubilities of the
organic are highly variable, and inversely related
to sorption onto soil or aquifer particulate organic.

Sorption One of the governing principles of
synthetic organic transport is the octanol-water
partition coefficient (Kow). This coefficient is a ratio
of the solubility of a synthetic organic in octanol, a
representative organic solvent, and water. Low Kow
organic are more soluble in water than in organic
solvents. High Kow compounds are more soluble in
organic solvents than in water. In aquifer systems,
one of the possible places where high Kow organic
can be fixed is particulate organic (humins, see
Total Organic Carbon). There is some debate as
to whether the high Kow organic actually dissolve
into the particulate humin (true absorption) or
simply adsorb onto available surface sites. At any
rate, given the presence of humins in the soil or
aquifer, high Kow organic are likely to be fixed on
the particles and thereby retarded from advective
transport. Domenico and Schwartz (1990) present
an excellent discussion of the sorption process.

The octanol-water partition coefficient is
determined by laboratory experiments and data
are readily available for many compounds (see
Montgomery and Welkom, 1990). A more realistic
coefficient for use in ground-water systems is the
organic-carbon partition coefficient (Koc). The
organic-carbon partition coefficient is the ratio of
sorbed chemical per unit mass of humin (as C) to
solubility in water. Many equations have been
developed to relate K,, to K,,ow. For example,
Kenaga and Goring (1980) related the two partition
coefficients by

logKoc = 1.38

+ 0.54 logKow

Other equations are discussed in Freeze and
Cherry (1979) and Domenico and Schwartz (1990).
Care should be taken in choosing which of the
equations (e.g. equation 19) to use because there
is no general agreement as to which is most
appropriate.

In a sense, therefore, solubility in water and
retardation on humins are related and the synthetic
organic can be classified on the basis of either
solubility in water or by Koc. Table 31 gives the
mobility classification of Fetter (1988). In this
classification mobility is classified on the basis of
solubility in water, which is approximately inversely
proportional to the Ko,.

Biological Transformations and Hydrolysis Many
of the synthetic organic can serve as sources of
organic carbon for soil and aquifer microbes. As a
result, some of the chemicals listed in Table 28 are
degradation products, not primary contaminants.
Microbial degradation behaves chemically much
like hydrolysis, with cleaving of radicals and
substitution of water or OH-. The rate of degra-
dation has been likened to radioactive decay as
biodegradation and hydrolysis in ground-water
systems follow an exponential decay rate.

STANDARD OR GUIDANCE
CRITERION

Table 28 gives the standards or guidance
concentrations for the synthetic organic. Only the
hazards or health consequences of analytes de-
tected in the Background Network are discussed
below.

Use of the term "mdl" in Table 28 signifies that
the standard for the organic compound is the
detection or practical quantitation limit. In these
cases, there is no specific standard, but the water
is subject to the "free from" provisions of Florida's
water quality statutes (Ch. 17-550 F.A.C.). Under
this provision, the water must be "free from"
deleterious contaminants, including compounds
that harm biota, use of the water, or humans. Harm
to humans includes toxicity, mutagenicity, carcino-
genicity, or teratogenicity.

Given the large number of samples from
Florida's aquifer systems and the fact that the
detections have not been confirmed, it is
remarkable that so few samples had detectable
synthetic organic. Many of the analytes that were
found to be present were from a single sample, so
the areal distribution of "problems" is highly
limited. Most of the samples are from wells near
urban, industrial, or heavy agriculture areas, which
suggests that it is unlikely that any significant
problem of wide extent exists.

In all cases, the median and upper quartiles of
synthetic organic data (Table 29) are detection
limits. The proportions of samples with detected
synthetic organic are as follows:

Surficial aquifer system

NWFWMD
SRWMD
SJRWMD
SWFWMD
SFWMD

2.8%
0.0%
6.9%
2.4%
8.9%

Statewide 6.6%

Intermediate aquifer system

NWFWMD
SRWMD
SJRWMD
SWFWMD
SFWMD

0.0%
0.0%
0.0%
0.0%
2.8%

Statewide 1.3%

Floridan aquifer system

NWFWMD
SRWMD
SJRWMD
SWFWMD
SFWMD

0.0%
1.3%
4.5%
7.1%
3.8%

Statewide 3.1%

As one might expect, the highest proportions of
samples with detectable synthetic organic are
from the surficial aquifer system and unconfined
portions of the Floridan aquifer system. This indi-
cates the susceptibility of those aquifer systems to
contamination from surficial sources. The
intermediate aquifer system contains little
indication of contamination, which is due to the
confined nature of the water-producing zones in
the aquifer system. Areas at greatest risk include
the unconfined Floridan aquifer system in west-
central Florida (SRWMD, SWFWMD) and in
SJRWMD, and the surficial aquifer system,
especially the Sand-and-Gravel Aquifer in
NWFWMD and the Biscayne Aquifer in SFWMD.

SPECIAL PUBLICATION NO. 34

DISTRIBUTION IN GROUND WATER

Rather than divide this discussion into aquifer
systems and districts, the following discussion
deals with the specific chemicals found or
suspected in the aquifer systems. The chemicals
are in alphabetical order for ease in location, as
opposed to being placed in order of origin or
chemical similarities. Unless stated otherwise, the
Henry's Law constants, Koc's, and uses of the
following chemicals are from Montgomery and
Welkom (1990).

Care should be taken in placing importance on
the occurrences reported below. At this time there
is question as to whether the chemical is actually
in the water or is spurious26. The following list
should be utilized in two ways: (1) to show that,
even at the worst case, there is little background
contamination of Florida's aquifer systems and (2)
to identify chemicals that may be present and
require further study.

As this discussion continues, it will be appar-
ent that many of the compounds detected in
Florida's aquifer systems have been used in the
past as pesticides. Many are no longer licensed for
use in Florida and the Background Network is
detecting residual contamination. Also, note that
most of the chemicals are only moderately volatile,
which is consistent with the residual occurrences.
Very highly volatile compounds (H > 10-2 atm
m3/mol) are suspect because the Background
Network actively avoided sites likely to have active
releases and older releases should have previously
volatized. There is also an absence of highly
mobile or immobile compounds. Immobile com-
pounds do not spread widely, which reduces the
probability of detection. Highly mobile compounds
are diluted and dispersed to concentrations below
detection limits.

Acrylonitrile

Acrylonitrile (C3H3N) is copolymerized with
other organic compounds to produce acrylics,
ABS (acrylonitrile-butadiene-styrene), and other
plastics. It is used as a grain fumigant, and in phar-
maceuticals, antioxidants, dyes, and surfactants. It
is a light compound with moderate volatility (H =
1.1x10-4 atm m3/mol). Based on a Koc of 0.074, the
compound is very mobile. According to literature
cited in Montgomery and Welkom (1990),
acrylonitrile can be photo-oxidized. The practical
quantitation limit, the guidance concentration, is

2.5 [pg/L based on cancer risks (Florida
Department of Environmental Regulation, 1989).

One sample out of 29 from the surficial aquifer
system in the SFWMD contained possible
acrylonitrile. The chemical was not detected in
other districts or aquifer systems.

Benzene

Benzene (CH,) is widely utilized by the energy
and manufacturing industries. It is an additive in
automotive fuels, paints, plastics, and resins. It is
one of the most widely used solvents, and is a
common contaminant in ground water. It is a
common contaminant in plumes from leaky
underground gasoline storage tanks. The Henry's
Law constant for benzene is 5.5x10-3 atm m3/mol,
which categorizes it as highly volatile. Benzene is
very mobile in ground water (Koc = 49.0 100). Soil
microbes can break down benzene to catechol
and other products. The Primary Drinking Water
Standard for benzene is 1 tg/L.

Benzene was detected, but has not yet been
confirmed, in a number of samples around the
state. In the surficial aquifer system eight out of 98
from the NWFWMD contained possible benzene.
One out of 81 from the SWFWMD and 12 out of
575 from the SFWMD also contained possible
benzene. In the intermediate aquifer system, three
out of 52 samples from SWFWMD and two out of
136 from SFWMD contained possible benzene.
Benzene was detected by most districts in the
Floridan aquifer system, as well. The occurrences
were: one out of 101 (NWFWMD), one out of 302
(SRWMD), six of 161 (SWFWMD) and three of 153
(SFWMD).

Bromodichloromethane

Bromodichloromethane (CHBrCI,) is a
component in fire extinguisher fluids and a solvent
for fats, waxes, and resins. It is used as a
degreaser and flame retardant. It is dense and
moderate to highly volatile (H = 2.1x10-4 to 2.4x10-3
atm m3/mol). It is mobile in ground water (Koc =
62). Bromodichloromethane is a trihalomethane,
and the Primary Drinking Water Standard for total
concentration of trihalomethanes is 100 pg/L.

No bromodichloromethane was detected in
the surficial or intermediate aquifer system
samples. One sample in 302 from the Floridan

aquifer system in the SRWMD and one of 100 in
the SWFWMD contained possible bromodi-
chloromethane.

Bromoform

Bromoform (CHBr3) is a dense liquid. It is
moderately volatile (H = 5.3-5.6x10-4 atm m3/mol),
and is mobile in ground water (Ko, = 110 280). It
is used as a solvent for waxes, greases, and oils
and as a component of fire-resistant chemicals.
Bromoform is a trihalomethane, and the Primary
Drinking Water Standard for total concentration of
trihalomethanes is 100 gg/L.

Bromoform was reported from one sample out
of 532 from the surficial aquifer system in the
SFWMD. It was not reported elsewhere.

Chlorobenzene

Chlorobenzene (C,H,CI) is used in the
manufacture of a number of organic chemicals. It
is also utilized as a solvent, insecticide, pesticide,
and heat transfer agent. Chlorobenzene is highly
volatile (H = 3.6-3.9x10-3 atm m3/mol) and
moderately mobile to mobile in ground water (Koc =
48 330). It can be photodegraded to phenol and
chlorophenol under certain circumstances. There
is an Environmental Protection Agency Health
Advisory on chlorobenzene, and it is organoleptic
(it imparts taste or odor problems to water) (Florida
Department of Environmental Regulation, 1989).
As a result the guidance concentration in Florida is
10 gg/L.

Chlorobenzene was detected in the surficial
aquifer system in one sample out of 57 from the
SJRWMD and in 16 out of 652 in the SFWMD. It
was not detected in the intermediate or Floridan
aquifer systems.

Chloroform

Chloroform (CHCI) is a dense liquid. It is highly
volatile (H = 2.9-3.4x10-3 atm m3/mol), and very
mobile in ground water (Koc = 44). Chloroform has
been shown to be microbially degraded in
anaerobic environments to methyl chloride and
other products. It is a member of the
trihalomethane group, and the Primary Drinking
Water Standard for total concentration of
trihalomethanes is 100 pg/L.

Chloroform was widely detected in the
Background Network data set. In the surficial
aquifer system, one sample of 98 in the
NWFWMD, one of 57 from SJRWMD, and three of
632 from SFWMD contained chloroform. None
was detected in the intermediate aquifer system.
One sample of 302 from the Floridan aquifer
system in the SRWMD and two samples of 160
from the SWFWMD contained probable chloro-
form.

Chloromethane

Chloromethane or methyl chloride (CH3CI) is a
highly volatile liquid (H = 6.6-8.8x10-3 atm m3/mol).
The Koc value of 25 estimated by Montgomery and
Welkom (1990) indicates that chloromethane is
very mobile in ground water. U.S. Environmental
Protection Agency draft preliminary protective
concentration limit data (Florida Department of
Environmental Regulation, 1989) have resulted in a
guidance concentration of 3,800 gg/L.

Chloromethane was not detected in the
surficial or intermediate aquifer system. It was
detected in two of 16 samples from the Floridan
aquifer system in the SJRWMD.

Dibromochloromethane

Dibromochloromethane (CHBrCI) is utilized in
the manufacture of fire extinguishing agents and
propellants. It has been used as a refrigerant and
as a pesticide. It is moderate to highly volatile (H =
9.9x10-4 to 7.8x103 atm m3/mol), and mobile in
ground water (Koc = 83). It is a member of the
trihalomethane group, and the Primary Drinking
Water Standard for total concentration of
trihalomethanes is 100 pg/L.

Dibromochloromethane was not detected in
the surficial or intermediate aquifer systems. It
was detected in the Floridan aquifer system in one
sample out of 160 from the SWFWMD.

1,2 Dibromoethane

1,2 Dibromoethane is better known as
ethylene dibromide or EDB. It has the composition
C2H4Br2, and has been widely used in the state of
Florida as a soil fumigant. It is dense and
essentially insoluble in ground water. EDB had
been considered immobile. However, EDB was

FLORIDA GEOLOGICAL SURVEY

detected in a number of wells in Florida in the
1980's and this apparent mobility made EDB a
chemical of major concern. The Drinking Water
Standard for EDB is 0.02 gg/L.

No EDB was detected in the surficial or
intermediate aquifer systems, statewide. Two out
of 55 samples from the Floridan aquifer system in
SWFWMD had possible EDB.

1,2 Dichlorobenzene

1,2 Dichlorobenzene (o-Dichlorobenzene,
CH,CI2) is moderately dense and highly volatile (H
= 1.2-1.9x10-3 atm m3/mol). It has mobilities that
are moderate to low in ground water (Koc = 180 -
1,700). The compound has been detected to be
biodegraded with a number of possible
transformation products. 1,2 Dichlorobenzene is
widely used as a solvent for organic compounds
and nonferrous metals, and as a fumigant and
insecticide, degreaser for hides and wools, metal
polish, among others. The compound imparts
taste and odor to water and there is a U.S.
Environmental Protection Agency Health Advisory for
it (Florida Department of Environmental Regulation,
1989). The guidance criterion is 10 pg/L.

Three samples out of 632 from the surficial
aquifer system in the SFWMD contained possible
1,2 dichlorobenzene. None of the intermediate
aquifer samples contained 1,2 dichlorobenzene.
One sample out of 160 from the Floridan aquifer
system in the SWFWMD and one out of 175 from
the SFWMD contained the compound.

1,3 Dichlorobenzene

1,3 Dichlorobenzene (m-dichlorobenzene,
CH4C12) is highly volatile (H = 2.6-3.6x10-3 atm
m3/mol), and has a mobility that is low to
moderately low in ground water (Koc = 170 1,700).
It is used as a soil fumigant and insecticide, as well
as in organic synthesis. The guidance
concentration, based on taste and odor problems
and a Health Advisory from the Environmental
Protection Agency, is 10 gg/L (Florida Department
of Environmental Regulation, 1989).

Two samples out of 570 from the surficial
aquifer system in the SFWMD contained possible
1,3 dichlorobenzene. No samples from the
intermediate or Floridan aquifer systems contained
the compound.

1,4 Dichlorobenzene

1,4 Dichlorobenzene (p-1,4 dichlorobenzene,
C6H,CI,) is highly volatile (H = 2.7-3.1x103 atm
m3/mol). It can be photodegraded to chlorophenol
or phenol. It is moderately mobile in ground water
(Koc = 160). 1,4 Dichlorobenzene is utilized as a
moth repellent and general insecticide, fumigant,
and germicide; soil fumigant; and disinfectant. The
Primary Drinking Water Standard is 75 gg/L.

Ten samples out of 572 from the surficial
aquifer system in the SFWMD contained possible
1,4 dichlorobenzene. One sample out of 52 from
the intermediate aquifer system in the SWFWMD
and one out of 138 from the SFWMD also
contained possible traces of the chemical. One
sample of 162 from the Floridan aquifer system in
the SWFWMD also contained possible 1,4
dichlorobenzene.

Dichlorodifluoromethane

Dichlorodifluoromethane (Freon-12, CCIF2) is
widely utilized as a refrigerant and aerosol
propellant. It is also used in plastics and as a low
temperature solvent. Dichlorodifluoromethane can
occur as either a gas or a liquid, and the liquid is
very highly volatile (H = 4.3x10-1 to 3 atm m3/mol).
The estimated Koc is 360, which makes it
moderately mobile in ground water. The guidance
concentration in ground water is 1,400 gg/L, based
on the Integrated Risk Information System (Florida
Department of Environmental Regulation, 1989).

One sample out of 116 from the Floridan
aquifer system in the SJRWMD contained possible
dichlorodifluoromethane. Elsewhere, it was not
detected.

1,1 Dichloroethane

1,1 Dichloroethane has the formula C2H,CI2. It
is highly volatile (H = 4.3-5.9x10" atm m3/mol), and
slightly mobile (Koc = 30). It is used as an
extraction solvent; insecticide and fumigant;
preparation for vinyl chloride; finish remover;
solvent for plastics, oils, and fats; and other
applications. Under anaerobic conditions 1,1
dichloroethane can be microbially converted to
vinyl chloride. The guidance concentration is
2,400 gg/L (Florida Department of Environmental
Regulation, 1989), based on toxicant profiles
prepared by the Center for Biomedical and

Toxicological Research at Florida State University.

1,1 Dichloroethane was detected in the
surficial aquifer system in the SWFWMD in one
sample out of 57. Three samples out of 632 in the
SFWMD also may contain the compound. None
was detected in the intermediate aquifer system or
the Floridan aquifer system.

1,2 Dichloroethane

1,2 Dichloroethane (ethylene dichloride,
C2H,CI) is moderately volatile (H = 9.1-9.8x10-4 atm
m3/mol). It is very mobile in ground water (Koc = 14
- 19). It is used as a vinyl chloride solvent; lead
scavenger in certain leaded gasolines; paint and
varnish remover; degreaser; wetting and
penetrating agent; tobacco flavoring; and as a soil
and food fumigant. The Primary Drinking Water
standard for 1,2 dichloroethane is 3 ug/L because
of its carcinogenicity.

No 1,2 dichloroethane was detected in
samples from the surficial or intermediate aquifer
systems. One sample out of 116 from the Floridan
aquifer system in the SJRWMD and one out of 176
from the SFWMD may have contained 1,2
dichloroethane.

trans-1,2 Dichloroethene

The primary uses of trans-1,2dichloroethene
(trans-1,2 dichloroethylene, C2H2CI2) are as
solvents for fats, phenols, and other compounds. It
is an ingredient in perfumes, and is used as a low-
temperature solvent and refrigerant. The
compound is highly volatile (H = 5.3x10-3 to 0.38
atm m3/mol) and mobile in ground water (Koc = 59).
Transformation in methanogenic (anaerobic)
aquifers is to vinyl chloride and other compounds.
The guidance concentration is based on
organoleptic properties (taste and odor) and
recommended protective concentrations sug-
gested by the Center for Biomedical and
Toxicological Research at Florida State University.
The guidance concentration is 4.2 gg/L (Florida
Department of Environmental Regulation, 1989).

Four samples out of 632 from the surficial
aquifer system in the SFWMD contained possible
trans-1,2 dichloroethene. One sample of 302 from
the Floridan aquifer system in the SRWMD also
contained the compound. It was not detected
elsewhere.

1,2 Dichloropropane

1,2 Dichloropropane (C3H,CI2) is a dense
compound. It is highly volatile (H = 2.3-2.9x10-3
atm m3/mol), and it is very mobile in ground water
(Koc = 27 51). It is used as a scavenger in certain
leaded gasolines; metal cleaner and degreaser; soil
fumigant for nematodes; and a solvent for oils,
fats, waxes, and other organic. The guidance
concentration is 1 gg/L, which is the practical
quantitation limit, based on an Environmental
Protection Agency Health Advisory for cancer
risks.

Only one sample out of 81 from the surficial
aquifer system in the SWFWMD contained pos-
sible 1,2 dichloropropane. The chemical was not
detected in samples from any other districts or
aquifer systems.

Ethylbenzene

Ethylbenzene (CH,0) is a highly volatile liquid
(H = 6.4-6.6x10-3 atm m3/mol). It is mobile to
moderately mobile in ground-water systems (Koc =
95 260). Ethylbenzene is an additive to gasoline
products and a widely used solvent. It is utilized in
the manufacture of plastics and other organic.
The guidance concentration is 2 gg/L (Florida
Department of Environmental Regulation, 1989),
which is the practical quantitation limit. The
guidance concentration is based on toxicant
profiles from the Center for Biomedical and
Toxicological Research at Florida State University.

Ethylbenzene was detected in one sample out
of 79 from the surficial aquifer system and two out
of 48 from the intermediate aquifer system in the
SWFWMD. It was also detected in one sample of
151 the Floridan aquifer system in SWFWMD and
in one sample of 154 in SFWMD.

Hexachlorobenzene

Hexachlorobenzene (HCB, CCI,) is used as a
seed fungicide and wood preservative. It is highly
volatile (H = 1.3-1.7x10- atm m3/mol), and, even
though there is a wide range of Koc values (360 -
35,000) in the literature (Montgomery and Welkom,
1990), apparently relatively immobile. The
guidance concentration is based on the practical
quantitation limit and Environmental Protection
Agency cancer risk evaluations (Florida
Department of Environmental Regulation, 1989). It
is set at 10 Rg/L.

SPECIAL PUBLICATION NO. 34

Hexachlorobenzene was possibly detected in
the surficial aquifer system in one sample out of 79
from the SWFWMD. One of nine samples from the
Floridan aquifer system in the SWFWMD also
contained hexachlorobenzene.

Methylene chloride

Methylene chloride (dichloromethane or Freon
30; CH2CI2) is a highly volatile, dense liquid. The
Henry's Law constant is 2.0-3.2x103 atm m3/mol. It
is very mobile in ground water (Koc = 8.7).
Methylene chloride is a widely used solvent. It is
used in paint and varnish removers and as a
degreaser and drying agent. It is also used as a
fumigant and refrigerant. It has been shown to be
converted to methyl chloride, methanol, formic
acid, or formaldehyde by hydrolysis and oxi-
dation/reduction reactions. The guidance
concentration is based on an Environmental
Protection Agency Health Advisory. The guidance
concentration is 5 gg/L.

Two surficial aquifer system samples out of 57
in the SJRWMD and one of 81 from the SWFWMD
contained possible methylene chloride. No
samples from the intermediate aquifer system were
detected to contain methylene chloride. In the
Floridan aquifer system, ten of 116 samples from
the SJRWMD and one of 160 from the SWFWMD
contained methylene chloride.

PCB-1016

PCB-1016 (polychlorinated biphenyl-1016,
Arochlor 1016) is a dense liquid used as an
insulating fluid in electric condensers and as an
additive in high-pressure lubricants. It is volatile (H
= 750 atm/mol fraction) and immobile in ground-
water systems (Ko = 50,000). The guidance
concentration is set at the practical quantitation
limit for all polychlorinated biphenyl compounds,
which is 0.5 ig/L. The guidance concentration is
based on U.S. Environmental Protection Agency
Health Advisories.

PCB-1016 was not detected in the surficial or
intermediate aquifer systems. One sample of
Floridan aquifer system water of two analyzed by
the SJRWMD contained possible PCB-1016.
Given the low sample size (n = 2) and lack of
confirmation by resampling, plus the improbability
of PCB-1016 in the Floridan aquifer system, this
detection of PCB-1016 is considered to be false.

1,1,2,2 Tetrachloroethane

1,1,2,2 Tetrachloroethane (C2HCl4) is dense
and moderately volatile (H 3.8-4.6x10-4 atm
m3/mol). It is very mobile to mobile in ground
water (Koc = 46-118). It serves as a solvent for
chlorinated rubber, and is utilized as a paint, var-
nish, and rust remover; degreaser, drying agent,
and cleaner for metals; denaturant in ethyl alcohol;
insecticide and weed killer; fumigant; and
herbicide. The guidance concentration is 1 gg/L
based on the practical quantitation limit and
recommendations of the Center for Biomedical
and Toxicological Research at Florida State
University.

Two samples out of 57 from the surficial
aquifer system in the SJRWMD contained possible
1,1,2,2 tetrachloroethane. It was not detected
elsewhere in the surficial, intermediate or Floridan
aquifer systems.

1,1,1 Trichloroethane

1,1,1 Trichloroethane has the formula C2H3CI3.
It is dense, highly volatile (H = 1.3-1.8x10-2 atm
m3/mol), and mobile to moderately mobile (Koc =
104-151) in ground water. It is used in organic
syntheses and as a solvent for metal cleaning. It is
also used in textile processing, as a pesticide, and
as an aerosol propellant. The maximum con-
centration standard is 200 gg/L, based on the
Primary Drinking Water Standards.

1,1,1 Trichloroethane was detected in three
out of 57 surficial aquifer system samples in the
SJRWMD. One out of 632 samples from the
surficial aquifer system in SFWMD also contained
possible 1,1,1 trichloroethane. None of the
samples from the intermediate aquifer system
contained 1,1,1 trichloroethane. Three samples out
of 116 from the Floridan aquifer system in
SJRWMD contained the compound.

Tetrachloroethene

Tetrachloroethene (tetrachloroethylene, PERC;
C2014) is a dense liquid used as a cleaning fluid,
degreaser and drying agent, solvent for waxes,
greases, fats, and oils, and in manufacturing of
inks, paint removers, and fluorocarbons. It is very
highly volatile (H = 1.3-1.5x10-2 atm m3/mol). It is
also moderately mobile (Koc = 210 360). The
Primary Drinking Water Standard for tetrachlo-

roethene is 3 gg/L.

Tetrachloroethene was detected in the surficial
aquifer system in two districts, NWFWMD (five of
98 samples) and the SFWMD (four of 632
samples). It was not detected in the intermediate
aquifer system. Two districts detected
tetrachloroethene in the Floridan aquifer system.
These were the SJRWMD (one of 110 samples)
and the SWFWMD (three of 160 samples).

Toluene

Toluene (CH,) is a highly volatile liquid (H =
6.7x10-3 atm m3/mol). It is mobile in ground water
(Koc = 115 -151). Toluene is widely used as a sol-
vent for paints and coatings, gums, resins, rubber,
oils, and vinyl compounds. It is an adhesive
solvent in plastic toys, a diluent in some lacquers
and high octane gasolines. It is a common solvent
in manufacturing processes. Toluene is common
in plumes from leaky underground petroleum
tanks. The guidance concentration for toluene is
24 ug/L, based on taste and odor concerns and
toxicant profiles of the Center for Biomedical and
Toxicological Research at Florida State University
(Florida Department of Environmental Regulation,
1989).

Toluene was detected in a small number of
samples in the Background Network. In the
surficial aquifer system, it was detected in the
NWFWMD (one of 97 samples), SRWMD (one of
20 samples) and SFWMD (12 of 558 samples). It
was detected in the intermediate aquifer system in
the SJRWMD (one of 26 samples) and SWFWMD
(one of 19 samples). In the Floridan aquifer
system, it was detected in the SRWMD (one of 293
samples), SWFWMD (two of 152 samples), and
SFWMD (one of 150 samples). The
preponderance of single detection events in the
district samples suggests that many of these are
questionable.

Trichloroethene

Trichloroethene trichloroethylenee, TCE;
C2HCIs) is a dense liquid. It is highly volatile (H =
9.1x10-3 to 1.7x102 atm m3/mol). It is mobile in
ground water (Koc = 65 130). TCE is used as a
dry-cleaning fluid; degreasing and drying agent;
solvent for fats, oils, and waxes; refrigerant;
fumigant; diluents in paints and adhesives; and
many other uses. It was previously used as a de-

greaser in septic-tank cleaners. The Primary
Drinking Water Standard for TCE is 3 jig/L.

Trichloroethene was detected in samples from
the surficial aquifer system in the SJRWMD (one of
57 samples) and the SFWMD (seven of 632
samples). It was not detected in the intermediate
or Floridan aquifer systems.

Trichlorofluoromethane

Trichlorofluoromethane (Freon 11; CCI1F) is a
dense liquid. It is highly volatile (H = 5.8x10-3 to
1.1x10-1 atm m3/mol) and mobile in ground water
(Koo = 140 160). The primary uses of tri-
chlorofluoromethane are as a propellant and
refrigerant. It is also used as a solvent and a
"blowing agent" in polyurethane foams. The
guidance concentration is 2,400 |ig/L, based on
Environmental Protection Agency draft preliminary
Protective Concentration Limits (Florida
Department of Environmental Regulation, 1989).

Trichlorofluoromethane was detected in the
surficial aquifer system in one sample out of 57
from the SJRWMD. It was not detected in the
intermediate aquifer system. One Floridan aquifer
system sample out of 302 from the SRWMD
possibly contained it. Two samples of 116 from
the SJRWMD also contained possible
trichlorofluoromethane.

Vinyl Chloride

Vinyl chloride (chloroethylene; C2H3CI) is
normally a gas at earth surface temperatures and
pressures. It is available as a liquified compressed
gas. It is highly volatile (H = 2.2x10-2 to 2.8 atm
m3/mol). Montgomery and Welkom (1990)
estimated the Koc to be 2.5, which indicates that it
is very mobile in ground water. Vinyl chloride is a
degradation product of other chlorinated organic
as well as being an ingredient in the manufacture
of polyvinyl chloride and other copolymers. It is
used as an adhesive for plastics, a refrigerant, and
an extraction solvent. The Primary Drinking Water
Standard is 1 utg/L.

One sample out of 57 from the surficial aquifer
system in the SJRWMD contained possible vinyl
chloride. It was not detected in the intermediate
aquifer system. Vinyl chloride was detected in
samples from the Floridan aquifer system in the
SRWMD (one of 302 samples) and SWFWMD (one

FLORIDA GEOLOGICAL SURVEY

of 115 samples).

Pesticides

IMPORTANCE

Pesticides are widely used in Florida by
agriculture, the government, and individuals to
control unwanted plants and insects. Their use is
closely regulated by the state. Pesticide use in the
past has not necessarily been well regulated and
persistent pesticides remain as an environmental
concern.

Pesticides are subject to the same physical
and chemical factors that control the fixation or
movement of Synthetic Organics, and in fact
many of the synthetic organic have been used as
pesticides. Physical factors that affect pesticide
concentrations include advection, dispersion,
dilution, and volatization. Chemical controls
include sorption, decomposition, and biological
transformation. Modern pesticide design and
application criteria emphasize minimization of
exposure to the pesticide. For example, the
pesticide may be approved if it sorbs onto soil
mineral or organic matter, thereby minimizing
mobility. It may also be approved for use in Florida
if it can be shown to be destroyed by photo-
oxidation, biological transformation, or some other
means of neutralization of hazardous effects.

The Background Network samples in the
SWFWMD and SFWMD were scanned for 172
pesticides (Table 32). Samples from the
NWFWMD, SRWMD, and SJRWMD were scanned
for arsenic only, and no organic pesticide analyses
were made in these districts. For details of many
of the organic pesticides, see Montgomery and
Welkom (1990).

STANDARD OR GUIDANCE
CRITERION

Standards and guidance criterion of the com-
mon pesticides are listed in Florida Department of
Environmental Regulation (1989). Others are
subject to the "free from" criteria.

Given the heavy use of pesticides in Florida,
the number of samples in which standards or
guidance concentrations were exceeded is small.
Table 33 summarizes the distribution of pesticides

in the state's aquifer systems. The table reflects
the first sampling and detections have not been
confirmed by resampling. The median and upper
quartiles are at or below detection limits, with one
exception, for all aquifer systems.

As might be expected, the maximum
concentrations are highest in the surficial aquifer
system, where most pesticide application occurs.
Maxima in the intermediate aquifer system are
lowest of the three aquifer systems, the maxima in
the Floridan aquifer system are intermediate
between the two. The high concentrations
detected in the Floridan aquifer system reflect
pesticide application where the Floridan is either
unconfined or poorly confined. The individual
pesticides that are responsible for these maxima
are discussed in detail below.

The proportions of samples that exceed the
standards or guidance concentrations are given
below:

Surficial aquifer system

NWFWMD
SRWMD
SJRWMD
SWFWMD
SFWMD

1.2%
0.0%
0.0%
18.1%
0.3%

Statewide 3.2%

Intermediate aquifer system

NWFWMD
SRWMD
SJRWMD
SWFWMD
SFWMD

0.0%
0.0%
0.0%
41.2%
0.0%

Statewide 6.7%

Floridan aquifer system

NWFWMD
SRWMD
SJRWMD
SWFWMD
SFWMD

0.0%
0.0%
3.8%
21.0%
0.0%

Statewide 4.6%

Recall that arsenic is the only pesticide analyzed in
samples from the NWFWMD, SRWMD, and
SJRWMD. Therefore, the low number of
detections in these districts probably reflects
sampling as much as any other factor. In the
SFWMD, the proportions of samples in which
pesticides were detected and detected to exceed
standards are low and consistent with the
synthetic organic data. The high proportion in the
SWFWMD must be studied further. These
detections have not been confirmed by
resampling. In many cases, these detections
include several pesticides in one sample, so the
number of wells that are believed to be affected is
less than the number of detections.

DISTRIBUTION IN GROUND WATER

As Table 33 indicates, the surficial aquifer
system appears to be more highly impacted by
pesticide application than the other aquifer
systems. The intermediate aquifer system is least
affected, largely because it is the most isolated of
the three aquifer systems. There is no large scale
pattern in the data, except that the maximum con-
centration was detected in the SFWMD, which is a
major cropland area.

The following discussions include only those
pesticides detected or suspected in Florida's
aquifer systems. Samples reported to contain
pesticides, but not confirmed by resampling and
analysis are included. This is to indicate the nature
of probable pesticide contaminants. Recall that,
with the exception of arsenic, only data from the
SWFWMD and SFWMD are summarized below.

Aldrin

Aldrin (C1,HClI) is a widely used insecticide
and fumigant. Pure Aldrin is a solid. It has a low
Henry's Law constant (1.4-5.0x10-6 atm m3/mol)
and is only slightly volatile. In solution, it is
moderately mobile, with a Koc of 407. The
guidance concentration is set at the practical
quantitation limit of 0.05 pg/L, based on toxicant
profiles from the Center for Biomedical and
Toxicological Research at Florida State University
(Florida Department of Environmental Regulation,
1989).

Aldrin was detected in seven samples out of
30 from the surficial aquifer system in the SFWMD.
One sample out of 30 from the intermediate aquifer

system in the SWFWMD contained possible Aldrin.
It was not detected in the Floridan aquifer system.

Arsenic

Unlike the other analytes discussed in this
section, arsenic is not an organic compound,
although it is often formulated into organic. Some
characteristic arsenic-bearing compounds used as
pesticides are listed in Table 34. Arsenic occurs in
two valence states (Ass' and As3), which combine
with oxygen to form arsenates (AsO43+) and
arsenites(AsO;). According to Hem (1985), the
monovalent arsenate anion (H2AsO4) predominates
at pH values of 3 to 7 and positive Eh's. At pH 7 to
11 HAsO42-predominates. Under mildly reducing
conditions arsenite ion (HAsOj,) forms. Arsenates
sorb or co-precipitate with ferric hydroxides and
metal sulfides. Arsenic compounds are involved in
biological transformations, including methylation.
Dimethyl arsenic and methyl arsonic acids
[(CH,),AsOOH and CHAsO(OH),, respectively]
have been synthesized by microbial methylation.

Given the above data from Hem (1985), it
appears that both siliciclastic and carbonate
aquifers may have conditions conducive for
aqueous transport of arsenic compounds.
Organic-rich waters may be characterized by
methylation and transport as an organic complex.

In the 1920's there was a statewide infestation
of the Texas tick. Cattle ranchers were required to
dip their cattle to control the tick. Dipping was
done in thousands of unlined pits filled with arsenic
solutions as pesticides. The locations of most of
these pits have been lost. Today, arsenic remains
in soils and may contaminate the aquifer systems.
Environmental audits are turning these
contaminated sites up, and today's land owners
will have to clean up the sites.

Arsenic is highly toxic, and is regulated under
the Primary Drinking Water Standards at a
maximum concentration limit of 50 pg/L. As
indicated by Table 33, arsenic was detected in the
surficial aquifer system. One sample out of 84
from the surficial aquifer system in NWFWMD and
one surficial aquifer system sample of 324 in the
SFWMD contained possible arsenic. The single
sample from the SFWMD containing 1,100 gg/L is
by far the highest concentration detected in the
data set. This sample is from a well installed to
monitor a closed landfill. Therefore, the sample
may not represent true background conditions in

SPECIAL PUBLICATION NO. 34

the area. Two samples out of 53 from the Floridan
aquifer system in the SJRWMD also contained
possible arsenic.

oa-BHC

o(-BHC (benzene hexachloride-a-isomer, ox-
Lindane, oa-hexachloro-cyclohexane; CHClI) is
not produced in the U.S. and its sale is not allowed
(Montgomery and Welkom, 1990). It has was used
as an insecticide in the past, however. ot-BHC is a
solid. It is slightly volatile in water (H = 5.3x10 6
atm m3/mol) and mobile (Koc = 1,900). a-BHC is
subject to microbial decomposition in aerobic and
anaerobic conditions (Montgomery and Welkom,
1990), although it is slow to react. The guidance
concentration is set at the practical quantitation
limit of 0.05 gg/L due to U.S. Environmental
Protection Agency recommendations to minimize
cancer risks (Florida Department of Environmental
Regulation, 1989).

a-BHC was only detected in the surficial
aquifer system in the SFWMD. There, one sample
out of 29 contained possible traces of the
insecticide.

B-BHC

B-BHC (B-Lindane, B-hexachloro-cyclohexane;
C6H6Cl6) is a solid utilized as an insecticide. B-BHC
has low volatility in water (H = 2.3x107 atm
m3/mol). The chemical exhibits low mobility in
aquifers, as well. The Koc ranges from 2,100 to
3,600. The guidance concentration is set at the
practical quantitation limit of 0.05 gg/L due to U.S.
Environmental Protection Agency recom-
mendations to minimize cancer risks (Florida
Department of Environmental Regulation, 1989).
1-BHC was detected in one of 29 surficial aquifer
system samples in the SFWMD.

2,4-D

2,4-Dichlorophenoxyacetic acid, or 2,4-D, is a
synthetic auxin, or plant hormone-like compound,
used as a selective weed killer. It is slightly mobile
in ground water. 2,4-D is regulated by the Primary
Drinking Water Standards at 100 ig/L (Florida
Department of Environmental Regulation, 1989).
Two samples out of 58 from the surficial aquifer
system and four of 138 from the Floridan aquifer
system in the SWFWMD possibly contained 2,4-D.

4,4'-DDE

1,1 '(Dichloroethenylidene)bis(4-chloro-
benzene), or 4,4'-DDE (C14HC4) is a solid utilized
as an insecticide. It is also a transformation
product of DDT. In water, 4,4'-DDE is slightly
volatile (H = 2.3x10-5 atm m3/mol). It is immobile in
ground water (Koc = 240,000 -1,000,000). It may
degrade in water, and it can be photo-oxidized in
ultraviolet light. The guidance concentration is the
minimum detection level (0.01 gg/L; Florida
Department of Environmental Regulation, 1989).

One out of 29 surficial aquifer system samples
from the SFWMD contained possible 4,4'-DDE.
Two Floridan aquifer system samples out of 134
from the SWFWMD may also have contained the
pesticide.

4,4'-DDT

1,1 '-(2,2,2-Trichloroethylidene)bis[4-chloro-
benzene] or 4,4'-DDT (C14HCls) was formerly used
throughout the world as an insecticide. Use in the
U.S. is now prohibited. It is moderately volatile in
water (H = 3.8-4.9x10-5 atm m3/mol). It is immobile
in ground water (Ko, = 140,000-1,800,000). 4,4'-
DDT can be transformed aerobically and
anaerobically to DDD, DDE and other metabolites.
The guidance concentration is 0.1 gg/L, based on
the practical quantitation limit and Environmental
Protection Agency recommendations to minimize
cancer risks (Florida Department of Environmental
Regulation, 1989). One sample out of 134 from the
Floridan aquifer system in the SWFWMD contained
possible 4,4'-DDT.

Dieldrin

Dieldrin (Ci2HCIeO) is an insecticide. It has a
slight to low volatility in water (H = 3.2x10-5 to 2x10-7
atm mVmol). It is slightly mobile in ground water
(Koc = 12,000 35,000). The guidance concen-
tration is 0.05 gg/L, based on the practical
quantitation limit and Environmental Protection
Agency Health Advisories (Florida Department of
Environmental Regulation, 1989). One surficial
aquifer system sample out of 29 from the SFWMD,
and one out of 134 Floridan aquifer system
samples from the SWFWMD contained possible
dieldrin.

Endrin

Endrin (C,2HClO6) is an insecticide that is
slightly volatile in water (H = 5.0x10-7 atm m3/mol).
It has low mobility in ground water (Koc =1,900).
Endrin is microbially degraded. The Primary
Drinking Water Standard for Endrin is 0.2 gg/L
(Florida Department of Environmental Regulation,
1989). Only one sample out of 29 surficial aquifer
system samples from the SFWMD contained
possible Endrin.

Methoxychlor

Methoxychlor (C,,H,15ClO,) is used to control
mosquito larvae and house flies. It is utilized as a
stock dip to control ectoparasites. It is immobile in
ground water (Koc = 79,000 89,000). Methoxy-
chlor is microbially transformed in aerobic and
anaerobic environments. It is also subject to
hydrolysis. The Primary Drinking Water Standard
for methoxychlor is 100 gg/L (Florida Department
of Environmental Regulation, 1989). Two samples
out of 29 from the surficial aquifer system were
detected to contain methoxychlor in the SFWMD.

Mirex

Mirex (cyclodiene group) was used for fire ant
control. It is no longer used since one of its
breakdown products is dioxin. The guidance
concentration for Mirex is 3.5 Ig/L, based on
toxicant profiles from the Center for Biomedical
and Toxicological Research at Florida State
University (Florida Department of Environmental
Regulation, 1989). One sample out of 134 from the
Floridan aquifer system in the SWFWMD con-
tained Mirex.

HYDROCHEMICAL FACIES AND
PREDOMINANT WATER TYPES

Introduction

As ground water moves along a flow path, it
encounters different rock types with different
mineral assemblages and porosity/permeability
configurations. The residence time of the water in
contact with the rock varies with the nature of the
rock porosity, flow velocity, tortuosity of the flow
path, and hydraulic gradient. The water may also
come in contact and mix with sea water, connate
water, or water that has a different chemistry from

following a different flow path. All of these events
affect the chemistry of the water. The result is that
the water can be classified on the basis of its
chemical composition.

For the purposes of this report, the water-type
classification that is used is modified from Davis
and DeWiest (1966). They utilized two standard
trilinear diagrams one for the dominant cations
and the other for dominant anions in water. Each
diagram was subdivided into fields that represent
different proportions of the ions. These diagrams
are represented on the predominant water type
maps that follow (Figures 56-58). Table 35
explains the proportions of constituents in each
field. The proportions are based on conversion of
the concentrations to milliequivalents per liter
(meq). The cation proportions are based on the
following equation

X, q

-IUU
Na,~ + K, + Ca + Mgr_

where Xo, is the equivalent percent of cation X.
Cation X includes the following cations: sodium
plus potassium, calcium, or magnesium. Xcm, is
the equivalent concentration of X, and Nam~, Km,
Camm, and Mgm, are the equivalent concentrations
of the major cations. The proportions of anions are
calculated in the same fashion. Anion groupings
are: (1) bicarbonate plus carbonate, (2) sulfate, and
(3) chloride.

The arrangement of ions on the trilinear
diagrams is based on logical combinations
expected in ground-water chemistry. For example,
sodium and chloride are paired because of their
common association in marine aerosols and sea
water and calcium and bicarbonate are paired
because they are common weathering products of
limestones and a number of other rock types.

The predominant water type is designated by
the dominant ions present. A water mass that is
predominantly calcium and magnesium (area B on
the trilinear cation diagram) and bicarbonate (area
1 on the anion trilinear diagram), the water is said
to be a calcium-magnesium-bicarbonate water
mass (or Ca-Mg-HCO, water type; B1 symbol on
the maps).

Once the predominant water type is identified

FLORIDA GEOLOGICAL SURVEY

for an aquifer system, hydrochemical faces can be
attributed to areas within that aquifer system that
can be characterized by a single predominant
water type or by a specific mixture of water types.
Hydrochemical facies are interpretational, and
result from assigning a common origin, history, or
composition to a volume of water within an aquifer
system. For example, if a large volume of water
within the Floridan aquifer system has a common
calcium-magnesium-bicarbonate composition, one
could interpret that water mass to reflect a facies
controlled by dissolution of dolomite.

In the following discussions, predominant
water types are attributed to hydrochemical facies
where possible. The symbol used on the pre-
dominant water type maps is given as well.

Predominant Water Types

Some water types are highly unlikely and are
not discussed below. These include magnesium-
bicarbonate, magnesium-chloride, and
magnesium-sulfate waters. Also, many samples
from the Background Network have complex his-
tories and thoroughly mixed compositions. A
number of samples, for example, have a mixed
composition (G7), where all major ions are present
in subequal proportions. These samples are
characteristically transitional between better
defined water types, and they are not discussed
below.

Calcium-Bicarbonate (Al) Calcium-bicarbonate
waters are among the most widespread in Florida.
They are derived from dissolution of calcite or
aragonite in limestone and shelly siliciclastic sedi-
ments.

Calcium-Magnesium-Bicarbonate (B1) -Calcium-
magnesium-bicarbonate waters are either derived
by dissolution of dolomite in dolomitic limestones
and dolostones or mixing of magnesium-rich
waters derived from clay weathering in the
Hawthorn Group with calcium-bicarbonate waters.
Facies attribution is largely based on interpretation
of local geology and hydrology. If the water is from
a dolomitic aquifer, it is attributed to a facies
characterized by dolomite weathering. If it is near
an area of active weathering of the Hawthorn
Group sediments or if the intermediate confining
zones are highly leaky, the facies is said to reflect
weathering of the clays.

Calcium-Sulfate (A3), Calcium-Magnesium-Sulfate
(B3), and Calcium-Magnesium-Bicarbonate-Sulfate
(B2), Calcium-sulfate, calcium-magnesium-sulfate,
and calcium-magnesium-bicarbonate-sulfate
waters are characteristically derived by interaction
with the gypsum and anhydrite at the base of the
Floridan aquifer system. The mixed waters result
from mixing of calcium or calcium-magnesium-
bicarbonate waters with calcium-sulfate waters
derived from dissolution of the gypsum.

Sodium-Chloride (E5) Sodium-chloride waters are
found in two environments. Marine aerosol
dominated waters in the surficial aquifer system
may have a sodium-chloride composition if little or
no reaction with calcite or aragonite has occurred.
Also, Na-CI water masses are common in the salt-
water transition zone.

Sodium-Bicarbonate (El or E6), Calcium-Chloride
(A5 or A6), and Calcium-Magnesium-Chloride (B5
or B6) When cation exchange (see the Calcium
and Sodium sections) occurs sodium-bicarbonate
and calcium-chloride waters may result. Sodium-
bicarbonate waters are most common in Florida.
In areas where Na-HCO, waters predominate,
calcium has exchanged with sodium on the clays.
This phenomenon develops when sodium-
saturated marine clays are bathed in calcium-
bicarbonate waters. Calcium-chloride or calcium-
magnesium-chloride waters are less common.
They result from salt-water intrusion into aquifer
systems that contain calcium or magnesium-
saturated clays.

Sodium-Sulfate (E4) Sodium-sulfate waters have
been found in the surficial aquifer system in central
Florida (Hutchinson, 1978). These waters are
difficult to explain, but may result from addition of
sulfate through oxidation of organic or pyrite to a
sodium-rich water. Upchurch et al. (1991) found
similar water types near phosphogypsum waste
disposal areas in Polk County.

Uses of Predominant Water Type and
Hydrochemical Facies Maps

The maps that follow can be used in a number
of ways, largely related to predicting the outcomes
of water-use options. Grouping the predominant
water type data into really extensive hydro-
chemical facies allows interpolation between data
points and prediction of background water quality
throughout the state. The following are just some
of the benefits from hydrochemical facies analysis.

Water History Since the composition of the water
reflects the sequence of rocks and sediments
through which it has passed and any anthro-
pogenic modifications that have occurred, the
water composition reflects, in a broad way, the
history of the water. While this history is largely of
academic interest, it can allow deduction of flow
paths, vulnerability of aquifer systems to
contamination, and potentials for degradation by
changing the flow paths, especially through
upcoming.

Buffering Capacity Buffering capacity is the ability
of water to neutralize acids or bases. Within the
aquifer systems, buffering capacity involves
interactions with rock as well as water. The
buffering reactions have been discussed pre-
viously (reactions 1-7). When the water is removed
from the aquifer system, its buffering capacity
depends only on reactions within the water.

Calcium- and calcium-magnesium-bicarbonate
water masses have relatively high buffering
capacities. In other words, application of acids and
bases will result in some degree of neutralization.
Sodium-chloride waters in siliciclastic aquifers
have little buffering capacity and cannot tolerate
addition of acids or bases. As an example of the
application of the principle, consider the acid rain
problem. Lakes fed by buffered (calcium-
bicarbonate) ground waters have much higher
tolerances for acidic precipitation than do lakes fed
by sodium-chloride waters in siliciclastic soils.
Similar arguments can be made for waste disposal
impacts on aquifer systems.

Water Use and Treatment Requirements -
Development of ground-water resources for water
supplies requires considerable sensitivity to the
constraints placed on that development by water
quality. The state of Florida has developed a
ground-water classification system that reserves
water utilization to its highest uses. G-l and G-ll
designations are utilized for waters that are potable
and can be used for water-supply aquifer
systems27. The designations G-Ill and G-IV2 are
reserved for aquifer systems that contain non-
potable water that is more compatible with land
uses that may degrade G-l and G-Il waters. While
designation of ground water by this scheme
requires knowledge of the TDS concentration,
number of aquifer systems, and degree of
confinement, knowledge of the hydrochemical
facies can be of assistance as well.

In addition, the chemical composition of the

water will dictate treatment alternatives. Calcium-
and calcium-magnesium-bicarbonate waters are
likely to be hard and require softening to prevent
boiler scale, taste, and soap effectiveness
problems. Low TDS, sodium-chloride waters are
likely to be soft and require little treatment other
than color removal and disinfection. Sulfate-rich
waters may have odor and taste problems from
included sulfides.

Finally, use of underground injection wells for
disposal of storm runoff and waste water is
widespread in Florida (Hull and Yurewicz, 1979;
Kimrey and Fayard, 1982; Schiner and German,
1983; Hickey and Veccioli, 1986; Bradner, 1991).
Knowledge of the general water type into which
injection occurs will allow prediction of reactions
between the host and injection waters. Also,
treated drinking water is stored for later retrieval
(aquifer storage and recovery or ASR) in several
locations throughout the state (Merritt et al., 1983).
The facies maps can be used to identify injection
zones and predict reactions between host and
injection waters. Recovery of brackish waters for
treatment by reverse osmosis is becoming
common in coastal regions of the state. The maps
can assist in locating potential supply wells and the
level of treatment necessary for conversion to
potable water.

Ion Exchange Aquifer systems in many areas of
the state can be shown to have waters that have
been affected by ion exchange. These areas may
be useful for certain types of waste disposal. For
example, it is possible that movement of trace
metals can be more effectively retarded in these
areas. While considerable additional information is
necessary, the maps can assist in locating these
areas.

One engineering aspect of ion exchange has
been extensively studied elsewhere in the country,
but has not been widely applied in Florida is the
Sodium Absorption Ratio. Smectitic clays, such as
are common in the surficial and intermediate
aquifer systems, have the ability to expand and
contract (swell and shrink) depending on the
chemical composition of the surrounding water.
The clays swell in sodium-rich waters and shrink in
calcium- or magnesium-rich solutions. The ability
of clays to shrink or swell is predicted by the
Sodium Absorption Ratio (SAR)

SPECIAL PUBLICATION NO. 34

Naeq
SAR = (21)
Ca, + Mg~,
2

where Nameq,,, Cam, and Mgmeq are the
concentrations in milliequivalents per liter. If the
SAR > 8-10, smectites can be expected to swell
(Bouwer, 1978). It can be assumed that the clays
are in equilibrium with the surrounding water.
Changing the quality of water can induce shrinking
or swelling of the clays. For example, if the clays
are calcium- or magnesium-saturated and high
sodium water is introduced, swelling will occur.
This could occur where septic-tank effluent, landfill
leachate, or other high sodium waters are
introduced to the aquifer systems. Sodium-
saturated clays that are bathed in calcium- or
magnesium-rich water can shrink. This can
happen when sodium-saturated clays in
siliciclastic soils and aquifers are flooded with
calcium-rich, Floridan aquifer system water, such
as might occur when lawns are irrigated or
wastewater is applied. Shrinking or swelling of
clays could, therefore, cause failures of landfill
liners, foundations, and other structures. It can
also dramatically change the permeability of the
soil or aquifer system and reduce its effectiveness
as a water-supply or waste-disposal medium.

Water Types in Florida Aquifer
Systems

SURFICIAL AQUIFER SYSTEM

There is a general change in predominant
water type from north to south. In the north, the
surficial aquifer system is largely siliciclastic, and
shell content is limited to coastal areas. Therefore,
water types are mixed, with sodium-chloride
waters near the coast as a result of the coastal
transition zone and inland as a result of
precipitation of marine aerosols. To the south, the
carbonate content of the aquifer system increases
and the water types become less variable and
dominated by calcium-carbonate water types.

Water from the Sand and Gravel Aquifer in
western NWFWMD (Figure 56a) is predominantly
sodium-chloride in composition. There is calcium-
bicarbonate and sodium-bicarbonate water mixed
with it, indicating a complicated history, different
well depths, and minor carbonate sources. The

sodium-chloride water results from the coastal
transition zone and marine aerosols. There is
insufficient data to speculate on predominant
water type in central and coastal portions of the
district. However, sodium-chloride and calcium-
sulfate waters do occur along the coast. There is a
region centered on Jackson and Gadsden
Counties where calcium-bicarbonate water
predominates. This region is characterized by
exposures of limestones of the Floridan aquifer
system, and surficial aquifer system waters
undoubtedly reflect this influence.

The surficial aquifer system is poorly
developed in the SRWMD (Figure 56b). Waters are
largely calcium-bicarbonate due to influences of
Floridan aquifer system water. Magnesium-
bicarbonate water reflects the magnesium-rich
clays of the Hawthorn Group, and sodium-
bicarbonate waters reflect ion exchange. In
general, surficial aquifer system waters in the
district are calcium-bicarbonate facies derived
from the influences of underlying intermediate and
Floridan aquifer systems.

The surficial aquifer system in the SJRWMD
(Figure 56c) includes some regions of mixed water
type inland. There is a large area characterized by
calcium-bicarbonate facies along the St. Johns
River. This may reflect the upcoming described by
Leve (1983), or shelly horizons in the aquifer
system. Elsewhere sodium-chloride facies pre-
dominate along the river. There is a coastal
sodium-chloride facies, with large re-entrants that
probably reflect connate sea water rather that
active intrusion.

The surficial aquifer system is well developed
in the southern third of the SWFWMD, and it is
spotty in the middle third (Figure 56d). It is poorly
developed to not present in the northern third. In
the central third of the district, the surficial aquifer
system is largely quartz sand. The water types
present are mixed, with a slight predominance of
calcium-bicarbonate over calcium-sulfate and
sodium-chloride. Much of the calcium-
bicarbonate water is Floridan water introduced by
irrigation. The sodium-chloride water is a result of
marine aerosols. The calcium-sulfate water is
problematical. Water near agrichemical plants is
likely to be either sodium-sulfate or calcium-sulfate
in composition. Water some distance from the
chemical plants may reflect atmospheric fallout,
sulfates from oxidation of peats and pyrite, or other
causes. Waters in the southern third of the district
are predominately calcium magnesium-

bicarbonate inland and sodium-chloride near the
coast. The surficial aquifer system is shelly which
results in calcium-bicarbonate waters. Upconing
and irrigation pumpage in many areas of the
southern third result in introduction of deeper,
sulfate- and chloride-rich water.

Figure 56e reflects the surficial aquifer system
water types and facies in the SFWMD. The
Biscayne Aquifer is composed of predominantly
calcite and aragonite. The resulting facies is a
calcium-bicarbonate water. The salt-water
intrusion near Miami (northern Dade County) is well
shown by a re-entrant of sodium-chloride water.
On the west coast the waters are derived from
shelly sands and limestones and belong to the
calcium-bicarbonate facies. Local sodium-
chloride in Collier County may reflect upcoming of
connate water. The Kissimmee River corridor is
predominantly characterized by calcium-
bicarbonate water. The scattered areas of a
sodium-chloride facies are an artifact of the depth
of completion of the surficial aquifer wells. The
wells in the Kissimmee corridor that exhibit a
sodium-chloride facies contain low ionic strength
waters recharged by precipitation.

INTERMEDIATE AQUIFER SYSTEM

Waters of the intermediate aquifer system in
the NWFWMD (Figure 57a) are predominantly
calcium-bicarbonate type. This reflects the
carbonates in the intermediate aquifer system and
the absence of magnesium suggests that calcite
dissolution controls water quality. Water from the
intermediate aquifer system throughout the entire
eastern half of the district area can be said to
belong to a single calcium-bicarbonate facies that
originates from limestone dissolution.

The intermediate aquifer system is very limited
in extent in the SRWMD (Figure 57b). Where it is
present, the predominant water type is calcium- or
calcium-magnesium-bicarbonate, which reflects
the dolostones and limestones of the aquifer
system.

There is not much data on the intermediate
aquifer system in the SJRWMD (Figure 57c).
Where data are present, there is an inland calcium-
bicarbonate facies and a coastal sodium-chloride
faces.

The intermediate aquifer system is extensive

and utilized in southern SWFWMD (Figure 57d). It
is spotty to non-existent in the central third of the
district and absent in the northern portion. Waters
are characteristically calcium-magnesium-
bicarbonate due to the dolostone and magnesium-
saturated clays of the Hawthorn Group. To the
south and west calcium-bicarbonate-sulfate and
calcium-sulfate facies develop as a result of
addition of sulfate during upward flow on the
coastal transition zone. The outer coastal
transition zone is characterized by sodium-chloride
facies derived from sea water.

The intermediate aquifer system is well
developed on the west coast of the SFWMD
(Figure 57e). Elsewhere there is no information as
to facies or water types. The intermediate aquifer
system in Lee County includes sandstone
cemented by calcite and limestone and dolostone
aquifers. There is considerable clay in the aquifer
system. The facies represented show the
"chromatographic" effect in a different way. There
is less sulfate than in the SWFWMD (Figure 57d,
58d), so the sulfate-rich belt is absent. There is,
however, a sodium-bicarbonate belt derived by ion
exchange. The clays of the aquifer system are
sodium-rich and, as calcium-rich water upwells
along the inner transition zone, exchange releases
the sodium and fixes the calcium. There is a re-
entrant along the Caloosahatchee River.

FLORIDAN AQUIFER SYSTEM

Waters in the Floridan aquifer system of the
NWFWMD (Figure 58a) can be subdivided into
three facies. The majority of the district is
characterized by calcium-bicarbonate waters that
reflect dissolution of aquifer calcite. Scattered
magnesium-bicarbonate water masses reflect
dissolution of dolomite. Near the coastal transition
zone the predominant water types are mixed.
Sodium-chloride and sodium-bicarbonate waters
predominate. The sodium-chloride waters result
from sea water on the transition zone. The
sodium-bicarbonate waters are a result of ion
exchange. Where the samples are predominantly
sodium-chloride type, the resulting facies is a sea-
water or sodium-chloride facies. Where several
water types exist because of ion exchange and
local calcium-bicarbonate waters, a mixed facies
results. Near Gulf County there is a magnesium-
bicarbonate facies, which reflects magnesium
enrichment, probably from magnesium-rich clays.

The SRWMD can be subdivided into three
facies. Most of the district (Figure 58b) is

FLORIDA GEOLOGICAL SURVEY

dominated by calcium-bicarbonate water. Local
calcium-magnesium-bicarbonate samples reflect
dissolution of dolomite or additions of magnesium
from the overlying Hawthorn. Most of the facies is
characterized, however, by limestone dissolution.
Near the coast there is a sodium-chloride facies,
which reflects the coastal transition zone. In
central Taylor County there is a calcium-
magnesium-bicarbonate facies which reflects
extensive dolomitization in the Floridan aquifer
system.

Figure 58c shows the facies and predominant
water types for samples from the Floridan aquifer
system in the SJRWMD. There are large re-
entrants in the sodium-chloride facies near the
coast. These reflect connate waters and modern
intrusion. Scattered sodium-bicarbonate samples
indicate ion exchange. The St. Johns River region
includes two areas where the sodium-chloride
facies exists. These reflect upcoming as described
by Leve (1983).

The Floridan aquifer system in the SWFWMD
(Figure 58d) is characterized by three facies.
Inland the water is calcium-magnesium-
bicarbonate from dissolution of dolomite and
calcite in the aquifer system and magnesium from
the Hawthorn Group clays. Along the coast there
is a sodium-chloride facies that reflects the coastal
transition zone. In between, there is a calcium-
sulfate facies that reflects upward flow of sulfate-
rich water along the inner transition zone. This is an
excellent example of the "chromatographic" effect
produced by upwelling of deeper waters along the
coasts of Florida. The inner margin of the
transition zone is sulfate rich because of
dissolution of gypsum and anhydrite at depth in
the aquifer system. The outer belt is sodium-
chloride-rich due to mixing with sea water.

There are no data from the Floridan aquifer
system in most of the SFWMD (Figure 58e).
Samples from the west coast, in Lee County,
suggest a sodium-chloride facies as a result of poor
flushing and/or salt-water intrusion. The northern
and central Kissimmee River corridor is
characterized by calcium-bicarbonate waters, while
the southern portion is characterized by calcium-
sulfate and the sodium-chloride facies. This is the
extension of the "chromatographic" belts discussed
in the previous paragraph. The calcium-sulfate
facies is the inner half of the transition zone, while the
sodium-chloride facies is the outer portion. The
sodium-chloride facies persist into the Everglades
because of little of no flushing of connate waters.

ENDNOTES

1 In order to maintain electrical neutrality of water, the sum of negative charges on anions must equal the
sum of all positive charges on cations. The samples described in this report were not filtered prior to analysis
for metals. Consequently, some of the analyses had excess cationic constituents because particulates were
dissolved during sample preservation. In other words, the analyses do not reflect electrical neutrality, and the
number of positive charges exceeds negative. The ion-balance criterion for validating the analytical results of a
sample is based on the requirement of electrical neutrality. In order to accept an analysis for this report and the
Background Network database, the charge-balance error cannot be more that 30%.

2 The nomenclature used in this report follows that of the Southeastern Geological Society's (SEGS) Ad Hoc
Committee on Florida Hydrostratigraphic Unit Definition (1986). When a local, named hydrostratigraphic horizon
is discussed, the unit is called an aquifer (e.g., Biscayne Aquifer, Sand and Gravel Aquifer). Major, statewide
aquifers, especially those which contain several different aquifer horizons, are termed aquifer systems (e.g.,
surficial aquifer system, intermediate aquifer system).

SFor convenience, the district names are abbreviated in tables and text as follows: Northwest Florida Water
Management District (NWFWMD), Suwannee River Water Management District (SRWMD), St. Johns River Water
Management District (SJRWMD), Southwest Florida Water Management District (SWFWMD), and South Florida
Water Management District (SFWMD).

4pH is a measure of the degree of acidity of waters. Neutral water has a pH of 7, acid waters have pH
values <7, and basics, or alkaline, waters have a pH >7. The pH is defined as pH = -log oaH+ where aH+ is the
activity thermodynamicc concentration) of hydrogen ion. Therefore, a change in concentration of the hydrogen
ions by a factor of 10 results in a change in pH by a factor of 1.

'The vadose zone is the unsaturated zone above the water table. The capillary zone is the partially wetted
zone just above the water table that results from the interaction of surface tension of water and soil or rock
materials. The phreatic zone is the water-saturated soil or rock below the water table or the confining beds of a
confined aquifer.

6Cations have net positive charges, while anions have negative charges.

7 The Pious Museum at the University of South Florida, Geology Department, has gypsum (selenite) rosettes
in its collection from the Hawthorn Group in northern St. Petersburg (Pinellas County) and chalcedonic casts of
selenite rosettes from Tampa (Hillsborough County) and New Port Richey (Pasco County). Single and twinned
gypsum crystals from near Ocala (Marion County) are also in the collection.

8 Siliciclastic sediments are sediments that consist of quartz, silicate minerals and silicate rock fragments
that have been mechanically transported. Characteristic siliciclastic sediment types include quartz sand and
clay beds.

9 Chemical maturation is a term that reflects the changes in chemical composition along a flow path. These
changes typically include increases in total dissolved solids content and changes in specific chemical
composition.

10 The salt-water/fresh-water transition zone is the zone of mixing of discharging fresh water with salty water
near the coast or at the base of the aquifer system. This broad zone is sometimes called the salt-water
interface, although it is a diffuse zone, not an interface. For simplicity, the salt-water/fresh-water transition zone
will henceforth be called the "transition zone".

SPECIAL PUBLICATION NO. 34

11 In this context the term facies is used in the same fashion as in stratigraphy. Facies (from the Latin for
face or appearance of a object) refers to entities with similar attributes which can be used to identify and
distinguish them. Thus, hydrochemical facies represent water masses with similar chemical compositions and
origins.

12 "Sand crystal" are crystal of calcite that have grown in the pore space of a sand matrix and include the
sand grains in their original depositional fabric within the body of the crystal. The crystal lattice is typically
distorted in sand crystals, and crystal faces are convex. In this report, the term sand crystal is used in a more
loose sense to include calcite crystals that have grown around and included any pre-existing material (quartz
sand and silt-sized dolomite).

13 Milliequivalents per liter (meq/L) is a measure of charge concentration in water (Hem, 1985). Milligrams per
liter concentrations are divided by combining the weights of the appropriate ions to obtain milliequivalents per
liter. Conversion tables are provided in Hem (1985). Calcium carbonate alkalinity is the carbonate alkalinity
recalculated as if it were calcium carbonate. To convert mg/L bicarbonate to mg/L calcium carbonate, multiply
the concentration by 0.8202. One meq/L is equal to 0.02 times mg/L as CaCO, (Hem, 1985).

14 Strictly speaking, connate water is water of deposition. That is, it is water trapped in sediments at the time
they were deposited. Since the Floridan Platform has been repeatedly inundated by marine transgressions
during the late Tertiary and Quaternary, one cannot rule out the possibility of sea water trapped in the aquifer
system as a result of more recent inundations. Therefore, connate water is herein defined as sea water trapped
in the aquifer system as a result of any prior transgression.

15 Hydraulic potentials represent the driving forces that cause water to circulate in an aquifer system. Their
spatial distributions are represented by potentiometric-surface maps. Water tends to flow from areas of high
potential (high elevations on the potentiometric surface map) to low. Potentials are low in south Florida, and
there isn't a nearby high to force circulation and flush the aquifer systems.

16 Eutrophication is over enrichment of a water body with food. An eutrophic water body is characterized by
over abundance of autotrophs (green plants, such as algae). Plant and animal diversity are limited due to loss of
oxygen as the plant material decays and to imbalances in the species and abundances of elements of the food
chain.

17 Limiting nutrients are those nutrients that are in shortest supply and, therefore, inhibit primary production
and over population in a water body.

18 Mineral names are those presently accepted by the International Mineralogical Association Commission
on New Minerals and Mineral Names (Fleischer, 1987). Names in quotes have been discredited, but are widely
used within the phosphate industry.

19 The combination of NO3- and NO;- is collectively termed NO, in this report.

20 Urea is an excretory product manufactured in the livers of animals. It is the primary excretory product of
terrestrial animals and the excretory product of metabolism of ammonium, amino acids, and proteins. The

structure of urea is

22 When a chemical system is capable of supporting chemical oxidation, it is said to be aerobic and
assumed to contain an available source of oxygen. In reality, oxidation is possible at reduction/oxidation (Eh)
potentials as low as approximately -200 mV. Below -200mV, chemical reduction occurs, oxygen sources are
absent, and the system is said to be anaerobic.

23 Chemical completing includes the binding of dissolved anions or cations into a soluble molecule.
Complexing occurs in waters with high total dissolved solids contents or in waters with humic substances. An
example of an inorganic complex is CaSO40, which is present in high sulfate and TDS waters. This dissolved
compound removes calcium and sulfate from availability to react with rock and other chemicals. An example of
an organic complex is the formation of lead-humic acid pairs. In basic solutions, this complex is soluble and
can transport lead rather that allowing it to sorb or precipitate.

1" This is especially true if the sample also has high pH, alkalinity (especially as CO32), or high potassium.

25 The Practical Quantitation Limit (PQL) is the minimum limit of detection of a chemical that can be
expected from a laboratory under routine analytical conditions. The U.S. Environmental Protection Agency has
the PQL to be five times the minimum detection limit.

26 Most of the detections of synthetic organic and pesticides discussed herein have not been confirmed by
re-sampling. The term "detected" is used to indicate the uncertainty present as to the presence of these
compounds.

27 G-l ground water is intended for potable uses and single source (the only available) aquifers. TDS is less
than 3,000 mg/L, and Primary and Secondary Drinking Water Standards apply. Zones of discharge are
restricted to domestic waste water and storm-water discharges, and are limited to the lesser of 100 ft. or the
property line.
G-ll ground water is also reserved for potable uses, but the designation is given to areas where multiple,
potable water supply aquifers exist. TDS is less than 10,000 mg/L Primary and Secondary Drinking Water
Standards apply, and zones of discharge are restricted to the lesser of the property line or 100 ft., unless
discharge is beneficial to the aquifer.
G-lll ground water is not potable, with TDS concentrations greater than 10,000 mg/L. The designation is
given to unconfined aquifers, and the zones of discharge are the same as G-ll.
G-IV ground water is reserved for confined, non-potable aquifers. TDS is greater than 10,000 mg/L, and
zones of discharge are allowed on a case-by-case basis.

0

C

H2N

NH2

21 Methemoglobin (ferrihemoglobin) is the equivalent of hemoglobin with the exception that the iron is
oxidized to the ferric state. Methemoglobin is, therefore, incapable of carrying oxygen in the circulatory system.

FLORIDA GEOLOGICAL SURVEY

Chapter V

CONCLUSIONS
AND RECOMMENDATIONS

Sam B. Upchurch

Department of Geology
University of South Florida
Tampa, Florida

INTRODUCTION

The Water Quality Assurance Act of 1983
(Chapter 403.063 Florida Statutes) required the
Florida Department of Environmental Regulation to
assess the quality of water in the aquifer systems
of Florida. This report is the second of a series
that will discuss the aquifer systems of Florida.
The first (Scott et al., 1991) deals with the hydro-
stratigraphic framework of Florida aquifer systems.
This report is an assessment of the quality of water
in Florida's three aquifer systems. Water quality
reports that will follow include (1) a study of the
temporal variability of water quality in the three
aquifer systems, (2) a comparison of background
water quality to the quality of water underlying
areas of specific land uses with the goal of devel-
oping models for predicting changes in ground-
water quality, and (3) a second evaluation of
statewide water quality that will discuss changes in
quality and provide comparisons of concentrations
of metals in filtered and unfiltered samples.

Goals

This report discusses ambient water quality in
Florida's aquifer systems statewide. It was written
to meet four goals:

Evaluation of background ground-
water quality in all of Florida's aquifer
systems,

Development of ground-water-quality
prediction techniques through appli-
cation of hydrochemical faces maps
and fundamental hydrogeochemical
concepts,

Discussion of the factors that affect

regional ground-water quality so that the
user can understand and anticipate
controls on natural ground-water quality in
the state, and

Provide information for under-
standing the chemical consequences
of water use.

The primary goal of this report is to present
and evaluate the quality of water in the state's
aquifer systems. Analytes were selected with two
purposes in mind identification of regional
contamination and establishment of the
hydrogeochemical framework of the aquifer
systems. Many of the analytes described in this
report are not subject to water-quality standards or
guidance criteria, but their inclusion is necessary in
order to understand the geochemical processes
that govern reactions in the aquifer systems. Other
analytes are subject to regulation and their
inclusion is necessary in order to evaluate health
risks and restrictions on ground-water use. Major
and minor constituents, temperature and specific
conductance were selected so that the
geochemical framework of the state's aquifer
systems can be identified. Understanding this
framework allows us to conceptualize how
chemical completing, sorption reactions,
reduction-oxidation reactions, and dissolution-
precipitation reactions affect gross water chemistry
and microchemical reactions in the aquifer
systems. It is these reactions that mitigate
contamination in the aquifer systems and allow us
to utilize soils and aquifer systems for both water
supply and waste disposal. This report, therefore,
attempts to provide a minimum of information that
will allow the user to understand the whys and
hows of water-quality transformations in the
aquifer systems.

The assessment of water quality includes
contour maps where appropriate. These maps
allow interpolation of water quality into areas not
represented by samples. Care should be taken,
however, in extrapolating beyond the data. Use of
hydrochemical faces maps is a better approach
for extrapolation because of the more general
nature of these maps. While specific concen-
trations cannot be determined from the
hydrochemical facies maps, general predictions as
to the chemical constituency and reactions in an
aquifer system can be made. These maps also
allow recognition of regions of salt-water intrusion
and connate water, and they allow prediction of
general water-treatment alternatives for water-

supply development.

The trace constituents (i.e., trace metals, trace
nutrients, synthetic organic, and pesticides) are
typically regulated analytes. Inclusion of these
constituents allows direct assessment of statewide
ground-water quality as affected by human
activity. By comparing the distribution of these
and other regulated constituents to the
hydrogeochemical framework, we can understand
tolerance levels of the aquifer systems to use and
the mechanisms that mitigate introduction of these
anthropogenic constituents into aquifer system
environments.

The concentrations of trace chemicals are
normally highly discontinuous, so they cannot be
contoured. As a result, this assessment is best
used, not to identify specific areas of contam-
ination, some of which have been identified but not
confirmed, but to develop predictive concepts of
the probability of encountering a trace con-
taminant.

The data presented in this report assist greatly
in identification of recharge and discharge areas,
and of flow systems. They provide information that
supports vulnerability evaluation, and they allow
delineation of a number of specific problem areas
that merit further investigation.

Finally, information as to how one can predict
the prospects and outcomes of water uses is
provided. These predictions range from locating
brackish waters for reverse-osmosis treatment to
prediction of the amount of boiler scale that will be
developed by use of hard water to understanding
the ion exchange and sorption reactions that may
affect migration of metals and anthropogenic
chemicals in waste-disposal scenarios.

We caution the user of this report that the
concepts presented are general in nature.
Attempts at using these concepts in a site-specific
context should be undertaken only by those who
thoroughly understand the geochemistry of aquifer
systems. Even though this report summarizes the
most comprehensive ground-water quality survey
ever undertaken in Florida, the sample distribution
is still insufficient for site-specific evaluations.
Neither the authors nor the Departments of
Environmental Regulation or Natural Resources
can take responsibility for misuse of the data
included in this report or the GWIS database from
which the data were drawn.

Evaluation of Health and Use Risks

Major, minor, and trace constituents were
determined according to standard protocols of the
American Public Health Association (1980), U.S.
Environmental Protection Agency (1982) and
Florida Department of Environmental Regulation
(1981). These protocols include use of unfiltered
samples for metals. Since the samples may
contain particulate as well as dissolved metals,
factors other than ambient water quality are
represented in the chemical analyses.

The consequence of use of unfiltered samples
for metals analyses is that the sample reflects
water produced by the well, not necessarily the
chemicals dissolved in aquifer system water.
There are two justifications for use of unfiltered
samples. First, there is a growing body of evidence
to indicate that particulates travel in aquifer
systems. In transport through intergranular
porosity, mechanical filtration reduces the
probability of particulate movement, with colloids
being most likely to move. Conduit flow in
fractured and karstic aquifer systems is conducive
to particulate transport. Second, users with
domestic wells usually do not filter the water
before consumption. Therefore, use of unfiltered
samples in this report constitutes an evaluation of
exposure upon consumption of the water rather
that a simple discussion of natural water
chemistry. A comparison of filtered and unfiltered
samples is underway and will be published as part
of the evaluation of the second statewide
background analysis at a later date.

In many areas of the state, particularly in the
surficial aquifer system, water quality criteria are
exceeded by natural causes. These are pre-
dictable and have been discussed in Chapter IV.

DATA INTERPRETATION AND USE

Data have been interpreted in each section as
to the causes and controls on the distribution of
each chemical. It is possible to use these data in a
much larger context, however. The data constitute
a variety of evidence as to aquifer system flow
paths, recharge and discharge areas, and land
uses. Use of the data to assist in identification of
these features will facilitate development of
knowledgeable growth management, zoning, and
land-use management decisions. This section
notes some of the ways that the data can be
utilized to understand aquifer system behavior and

SPECIAL PUBLICATION NO. 34

impacts of land use on aquifer system water
quality.

Recharge Areas

A number of programs at state and local levels
require identification of aquifer system recharge
areas. Recharge areas are sensitive to ground-
water contamination. Maintenance, or enhance-
ment, of recharge is necessary to insure the long-
term water supply. Recharge areas may also
represent potential for optimal development of
municipal well fields and unique industries that
require low total dissolved solids water, such as
the bottled-water industry.

The water-quality data presented in this report
assist in identification of recharge areas in several
ways (Table 36). Recharge areas are typically
characterized by lack of equilibration of the water
with aquifer system materials and widespread
evidence of surficial conditions, such as atmos-
pheric temperatures, human activities, and
wetlands. While there are many exceptions to the
criteria listed in Table 36, the data do provide a
starting point for identification of recharge areas.

It may also be possible to recognize recharge
areas by vertical continuity of water chemical
quality. If water-quality, for example hydro-
chemical facies, present in the surficial aquifer
system persists downward into the Floridan aquifer
system, there is indication of interconnection. This
interconnection can be the result of downward or
upward flow, so other water quality criteria and
hydraulic head relations (Scott et al., 1991) must
also be considered.

Discharge Areas

The arguments above can be reversed to
identify some regional discharge areas. Table 36
summarizes some of the water quality criteria that
might be useful in identifying discharge areas.
Typically, water-quality data are less variable,
chemicals reflective of surficial conditions more
masked, and water more highly buffered (high
bicarbonate and pH) in discharge areas.

Coastal discharge areas may be represented
by a calcium- or calcium-magnesium-sulfate
hydrochemical facies if the water has followed a
deep flow path and come in contact with gypsum-
and anhydrite-bearing strata at the base of the

Floridan aquifer system.

Flow Systems

Flow systems can be recognized within the
Floridan aquifer system by examining the analyte
maps. Analytes that reflect equilibration with
aquifer system rocks (i.e., total dissolved solids,
pH, calcium, bicarbonate) typically increase along
a flow path. Orthogonals to the concentration
isolines should roughly indicate flow paths.

Some analytes allow identification of specific,
conduit-flow systems. For example, phosphate
may form plumes along karst conduits and indicate
a rapid recharge and local flow system. Fluoride,
nitrate, temperature, total organic carbon, and
sulfate may allow recognition of these more local
flow systems.

Surface-Water Features

The effects of several interesting surface-water
features appear in the data set. For example, there
is evidence in the Floridan aquifer system of
disappearing streams along the Cody Escarpment.
Several large wetland areas were mirrored by high
total organic carbon, low pH, and other analytes.

Lakes, streams, and wetlands affect aquifer
systems through introduction of organic carbon
and nutrients. Several major rivers, notably the
Peace and St. Johns Rivers, follow linear features
that appear to be fractured lineaments. Upconing,
and possibly preferential recharge, along these
features provides clear evidence of the inter-
relationship of surface features and ground water.
These linkages between surface-water features
and regional ground-water quality represent
possible areas of further study, perhaps as Very
Intense Study Area (VISA) projects.

Land Uses

Two of the requirements of the Water Quality
Assurance Act are to detect and predict con-
tamination in Florida's aquifer systems. It is not
possible to detect a significant number of areas of
contamination because of the high costs of an
exhaustive well network. One can, however,
predict water-quality degradation if one can
establish correlations with specific land uses and
ground-water quality. The VISA program is

designed to do this. By comparison of the data
collected in areas specifically selected to represent
a given land use with the Background Network
data, these correlations can be drawn. Results of
the VISA study will be published in the future.

The data presented in this report indicate that
there is a high chance of success for the VISA
studies. A number of regions characterized by
specific land uses do have anomalous ground-
water quality. Some agricultural areas have high
concentrations of nitrate, pesticide, or other
constituents. Synthetic organic were found in
industrial and suburban, as well as agricultural,
areas. Considerably more study is required before
confidence can be placed on the correlations, but
it appears that we may be able to anticipate the
contaminants and a probable level of con-
tamination for a given land use.

GENERAL SUMMARY OF
THE QUALITY OF FLORIDA
GROUND WATER

General Quality of Florida's
Ground Water

In general, the quality of ground water in
Florida is excellent and has been little affected by
humans. However, many local areas of the state's
aquifer systems have been affected by human
activities. Water quality is consistent with the
lithologies of the aquifer systems. Part of the
reason that water quality has not been adversely
affected is the slow rate of recharge and flow in the
aquifer system. Much of the water is simply too
old to have been exposed to human impacts at the
time of recharge.

Siliciclastic Aquifers

In very general terms, siliciclastic aquifers,
such as the Sand and Gravel Aquifer of northwest
Florida and the surficial aquifer system in the
interior of north and central peninsular Florida are
characterized by water quality associated with
precipitation composition. The water usually has
low total dissolved solids content, and is rich in
sodium and chloride as opposed to calcium and
bicarbonate. Water from these aquifers often
contains high total iron and organic carbon
concentrations.

Due to the lack of carbonate minerals in

siliciclastic horizons, buffering capacity is low and
the aquifers are vulnerable to contamination. Clay
minerals, iron and aluminum oxyhydroxides, and
humins range from non-existent to abundant in
siliciclastic horizons. These particles give these
aquifers a wide range in sorptive capacity and
make it difficult to generalize as to ability of
siliciclastic aquifers to tolerate waste loading.
Since much of the flow in these aquifers is
intergranular, mechanical filtration and sorption are
enhanced.

Carbonate-Rich Siliciclastic Aquifers

Carbonate-rich siliciclastic aquifers, such as
the shelly portions of the surficial aquifer system
and the sand- and gravel-rich horizons in the
intermediate aquifer system (especially in the
Hawthorn Group) are intermediate between true
siliciclastic and limestone/ dolostone aquifers. The
presence of carbonate minerals provides buffering
capacity, while abundant clays and organic
provide sorption capacity.

Mechanical filtration is still important because
flow is through intergranular porosity. Siliciclastic
horizons in the Hawthorn Group usually occur be-
tween clay-rich strata which provide both confine-
ment and isolation from anthropogenic chemicals.

Limestone and Dolostone Aquifers

Limestone and dolostone aquifers are doubly
porous. While they exhibit intergranular porosity,
which can be locally important, much of the flow is
through fracture and cavernous porosity. There-
fore, carbonate-rock aquifers have lower
mechanical filtration capabilities than siliciclastic
aquifers, and wastes and particles are capable of
travel through conduit flow for some distance.

Because of the presence of abundant
carbonate minerals, buffering capacities are high
and the waters of these aquifers are usually
alkaline and calcium, magnesium, and bicarbonate
rich. Sorption capacities vary. There is little clay
or organic material in the rock, and much of that
present is isolated from the conduits by low
permeability rock. Detrital clays and other particles
in the conduits may provide sorption capacity.

Floridan aquifer system water contains sur-
prisingly high total organic carbon concentrations.
This high organic carbon may reflect particulate

FLORIDA GEOLOGICAL SURVEY

organic, particularly in the Avon Park Formation,
and some sorption capacity for anthropogenic
organic. There is also a risk of development of
halogenated hydrocarbons, especially trihalo-
methanes, upon chlorination of these waters. In
general, it appears that dilution and dispersion are
more important than sorption once water enters
limestone/dolostone aquifers.

DEFINITION OF BACKGROUND
WATER QUALITY

Pristine Water

Pristine water is water that has not been
affected by human activity. After several hundred
years of human activity in Florida, it is difficult to
assess how much pristine water remains. Surely,
water deep in the Floridan aquifer system and far
away from injection and water-producing wells is
pristine, but much of it may not be potable for
natural causes. Shallow waters may or may not be
pristine.

Since this is the first ground-water quality
survey of its kind, we have little or no data with
which the Background Network data can be
compared. The baseline for comparison began
with the development of the Background Net-
work. Therefore, determinations of degradation of
ground-water quality begin with this data set and
go forward. In a large sense, we are constrained to
monitoring and enforcing changes from the
present status of ground-water quality. This data
set constitutes the baseline against which future
changes will be gauged. Background Network
wells were chosen from or drilled in locations be-
lieved to be minimally affected by human activities.

Background Water

The quality of water today reflects back-
ground, not pristine, conditions. For some of the
state, we can surmise that background water
quality is near pristine conditions. However, large
portions of the state's aquifer systems have been
affected by human activity. Salt-water intrusion,
upcoming of deeper waters, interaquifer transfers,
waste disposal, water withdrawals, land drainage,
and other activities have induced change.
Background water quality is, therefore, a mixture of
human and natural conditions.

High Salinity Water

This report delineates a number of regions
where water with high total dissolved solids
content adversely impairs use. By volume, most of
this water is naturally "contaminated", and the only
human impact is causing that water to move into
potable-water horizons. Suitable uses for this
water include augmentation of potable water
supplies by reverse-osmosis or other treatment,
waste disposal, and certain industrial applications,
such as cooling water. Care must be taken not to
induce migration of this lower quality water into
higher quality waters.

Not all of the high salinity water in Florida
aquifer systems is derived from the modern sea.
Connate water (sea water residual from previous
marine transgressions) is present in many areas. In
addition, sulfate-rich water in deep flow systems
pose a water-quality problem.

Coastal Intrusion

All of Florida's aquifer systems contain salt
water near the coasts. Salt-water intrusion along
the coasts is an historical problem in Florida. The
sampling plan was not constructed to identify the
transition zone and it has not been mapped
everywhere in the state. Likewise, these data do
not allow differentiation of natural intrusion and
intrusion caused by human activities. The data
included in this report do give indication of the
extent of the transition zone, which will help in
planning and management of our ground-water
resources.

In the 1990's we are concerned with global
change, especially global warming. Should global
warming cause a rise of sea level, baseline data
are needed to evaluate the resulting loss of potable
water and salt-water intrusion. These data are
present in this data set. Those data from rural
coastal areas where anthropogenic intrusion is
unlikely will be especially helpful for evaluating the
effects of sea-level rise.

Another important consequence of this study
has been documentation of the slope of the
transition zone and its chemical zonation. The
chemical data clearly indicate that the slope of the
transition zone is directly related to hydraulic head.
The data indicate that regions where the transition-
zone slope is gentle are associated with minimal

flow and low hydraulic potential. Regions where
the transition zone is steep are associated with
more dynamic flow and higher potentials. These
data can be used to assist in locating well fields.
Regions where the transition zone slopes gently
are much more susceptible to intrusion that are
regions with steep transition zones.

Connate Water

Connate water (salt water) is present in all
aquifer systems in regions where hydraulic
potentials are insufficient to flush the aquifer
systems with fresh water. This is particularly a
problem in the Everglades/Big Cypress Swamp
regions of SFWMD and in some portions of the
SJRWMD. Not all of the horizons with connate
waters have been identified in this report. Many
have been recognized, and this experience should
assist in predicting the locations of others.

Deep-Flow-System Water

Waters that recharge the Floridan aquifer
system near the center of the state and that follow
deep flow paths which skim along the top of the
gypsum-and anhydrite-rich, lower confining units
gain significant sulfate concentrations. As a result,
water deep in the Floridan aquifer system and
along the inner (landward) margin of the coastal
transition zone are often sulfate rich. These waters
may include sulfate in excess of the standards due
to natural causes.

The quality of ground water in the Floridan
aquifer system generally decreases with depth.
This drop in water quality results from the
cumulative history of chemical reactions along
lengthy flow paths (Jones et al., in press) and also
from mixing with sulfate-rich waters at the base of
the aquifer system. A number of regions of sulfate-
rich water have been attributed in this report to
upcoming under heavy pumpage stress. In most
cases, the production wells were not completed in
the sulfate-rich waters. Reduction of hydraulic
potentials by pumping has, instead, induced flow
upward along fractures and karst conduits.
Therefore, it is important to anticipate where the
naturally degraded waters are relative to
production wells, karst conduits and fractures, and
pumping stresses or well locations and depths
adjusted accordingly. The experiences described
in this report should assist in identifying conditions
where upcoming of deep water may occur. The
depths of sampling for each well in the Back-

ground Network are contained in the GWIS
database management system. This database can
be used to assist in predicting depths to lower
quality waters.

Interaquifer Transfer

There are many areas of the state where
upcoming and interaquifer transfer bring low quality
(high total dissolved solids) water into aquifer
systems that otherwise would have higher water
quality. Upconing because of natural, upward flow
is common near the coasts and along some
proposed fracture systems, such as along the St.
Johns and Peace River axes. Elsewhere, upcoming
because of pumping stress is a common local
problem. One of the most dramatic artifacts of
human activity documented in this study is the
transfer of calcium-magnesium-bicarbonate water
from the Floridan aquifer system to the surficial
aquifer system through irrigation.

NATURE OF ANTHROPOGENIC
CONTAMINATION

There are numerous, minor exceptions to the
conclusion that human impacts, other than
upcoming and salt-water intrusion, are minimal.
Scattered occurrences of anthropogenic
contaminants were detected, but most are
unconfirmed by re-sampling. It is inappropriate at
this time to conclude that these scattered
detections are real because of the design of the
Background Network, lack of confirmation, and
high probability that some detections are a result
of field or laboratory problems.

Users of the Generalized Well Information
System (GWIS) are advised that caution must be
used when interpreting the data contained therein.
Until such time as contamination is confirmed, the
data must be considered to represent a "worst-
case scenario" for planning and management
purposes.

There are a number of examples of possible
contamination. Nitrate contamination is wide-
spread in some areas, lead and mercury were
found in the sample set, and there is some
indication that synthetic organic and pesticides
are present in limited areas of the state.

SPECIAL PUBLICATION NO. 34

Point-Source Contamination

The Background Network was designed to
avoid known point-source contamination.
Because of the spacing of wells, it cannot be
assumed that single wells that detect anomalous
concentrations of an analyte reflect a point source.
That determination must be made upon further
investigation of activities near the well and of the
lateral extent of the anomaly.

Non-Point Source Contamination

Non-point source contamination is conta-
mination caused by a widespread activity. Typical
non-point sources include application of fertilizers
and pesticides in agriculture, widespread use of
on-site septic systems, and widespread appli-
cation of animal wastes.

The Background Network data clearly indicate
several regions that have been affected by non-
point source contamination. For example, high
nitrate concentrations in the Floridan aquifer
system in Suwannee and Flagler Counties reflect
widely dispersed agricultural practice. This report
suggests the locations of many regions where non-
point source contamination has impaired water
quality.

It must be noted here that, while non-point
sources are suggested in a number of aquifer
systems and locations, the overall water quality of
the state's aquifer systems cannot be shown to
have been significantly impacted by non-point
source contamination.

STATEWIDE LEVELS OF
CONTAMINATION

Table 37 summarizes the proportion of the
total sample set in which samples exceeded
water-quality standards. The data were taken from
preceding sections and the reader is cautioned to
refer to those sections for a sense of the extent of
contamination. In most cases, the proportions are
high, not because the problem is widespread, but
because an area with a problem has several wells.

pH

The second most widespread violation of a
water-quality standard is for pH. The Secondary

Drinking Water standard requires the pH of water
to fall between 6.5 and 8.5 s.u. Thirty-seven
percent of the surficial aquifer system samples
failed these criteria, largely because of low pH's.
These low pH values are generally natural and a
result of carbonic and organic acid content of the
water. Intermediate and Floridan aquifer system
water samples contained 16 and 14 percent,
respectively, that failed the criteria. Many of these
failures are a result of samples that exceeded the
upper pH limit of 8.5. Some of these exceedances
are natural, most appear to reflect high pH values
associated with drilling fluids and cements,
especially in the SRWMD.

Sodium

The Primary Drinking Water Standard for
sodium is 160 mg/L. Only 4 percent of the
samples from the surficial aquifer system
exceeded the standard. However, 23 percent of
samples from the intermediate and 17 percent of
sample from the Floridan aquifer system exceeded
that standard. The high levels of exceedance in
the intermediate and Floridan aquifer systems
reflects salt water in coastal and deep wells. Most
of the intermediate aquifer system wells that
exceeded the standard are located in southwest
Florida (Lee, Charlotte, Collier Counties) where the
intermediate aquifer system is used for public
water supplies. Wells in the Floridan are generally
located in the coastal transition zone.

Iron

The most widespread violation of water quality
standards is for iron. Only wells cased in non-
ferrous materials were used in this analysis, so the
samples reflect dissolved and particulate iron, not
well construction. The Secondary Drinking Water
standard for iron is 0.3 mg/L. Seventy-five percent
of all surficial aquifer system samples exceeded
the standard. Forty-two percent of the
intermediate aquifer system and 49 percent of the
Floridan aquifer system samples violated the
standard. There is no reason to believe that iron
violations are anthropogenic. Iron is a natural con-
stituent, and chemical conditions are conducive to
transport of the iron. Iron sources are widespread
in aquifer systems. It is present as ferric iron in
deeper portions of the aquifer systems and is
highly mobile.

Mercury

The Primary Drinking Water Standard for
mercury is 2 jgg/L. Mercury is of concern in Florida
because of recent discoveries of the metal in the
aquatic food chain. Two percent of surficial
aquifer system samples and three percent of
intermediate aquifer system samples contained
possible mercury in excess of the standard. Only
0.9 percent of Floridan aquifer system samples
contained possible mercury. Occurrences of
mercury are dispersed throughout the state, and
there is no obvious area of concentration. The
surficial and intermediate aquifer systems may be
sources of mercury since both aquifer systems
contain particulate organic. The abundance of
organic carbon in Florida ground water is con-
ducive for transport of mercury. An exceedance
level of four to five percent of the samples seems
high, given the abundance of sorption sites in
Florida aquifer systems. Given that the sample
were unfiltered and detections have not been
confirmed by re-sampling, additional work should
be done to confirm the occurrences and determine
their origins.

Lead

A relatively large number of samples detected
lead in excess of the 50 pg/L Primary Drinking
Water standard. Lack of sample filtration and
confirmation by re-sampling clouds interpretation
of the extent of any lead problem. Eight percent of
samples from the surficial aquifer system, eight
percent of intermediate aquifer system samples,
and nine percent of Floridan aquifer system
samples detected excess lead. There is a good
chance that some of these detections are a result
of well or plumbing materials or of use of lead
weights on water-level recorders. Lead mobility
should be limited through sorption on clays and
organic and precipitation of lead carbonates. The
large proportion of samples with lead suggests a
possible problem, and additional work is needed to
determine if the threat is real or an artifact.

Sulfate

The Secondary Drinking Water standard for
sulfate is 250 mg/L. The proportions of samples
that exceeded the standard are as follows:
surficial aquifer system two percent, inter-
mediate aquifer system 13 percent, and Floridan
aquifer system 13 percent. These exceedances
are typically related to high sulfate waters in

regional discharge zones along the coasts, or to
local upcoming.

Chloride

Chloride is the dominant anion in sea water.
The Secondary Drinking Water Standard is 250
mg/L. Exceedances are related to the coastal
transition zone and to upcoming in areas of pump-
ing stress.

Fluoride

Exceedances of the 4 mg/L Primary Drinking
Water Standard are low. Fluoride is derived from
weathering of Hawthorn Group phosphate
minerals, and is not considered a problem in
Florida.

Nitrate

The Primary Drinking Water Standard for
nitrate, as nitrogen, is 10 mg/L. The proportion of
samples with excess nitrate is low. The exceed-
ances reflect a small percentage of samples (0.6
percent surficial aquifer system, zero percent -
intermediate aquifer system, one percent Floridan
aquifer system) and does not reflect the findings of
this report. While there are few samples that
exceed the standard, there is a health advisory for
nitrate concentrations of 1 mg/L N. Samples with
concentrations in the 1 mg/L range are
widespread. They usually occur in clusters that
reflect local land use, especially agricultural uses.
Nitrates are becoming a subject of concern at the
time of the writing of this report because of the
potential for eutrophication of surface water
bodies. Several coastal springs (i.e. the Kings Bay
spring complex in Citrus County and Lithia and
Buckhorn Springs, Hillsborough County) have
been subjected to significant increases in nitrates
in recent years. A state-funded U.S. Geological
Survey study in the Suwannee River Basin has
shown high nitrate concentrations (up to 140 mg/L)
in monitoring wells associated with dairy
operations. Additional work is needed to

determine the origins of the nitrates and means of
mitigating what appears to be a growing problem.

FLORIDA GEOLOGICAL SURVEY

Total Dissolved Solids

Total dissolved solids content of ground water
reflects the presence of saline waters, such as
occur in the coastal transition zone, at depth in the
Floridan aquifer system, and in poorly flushed,
connate water zones. The exceedances reflect
these naturally "contaminated" regions and regions
of induced degradation through pumpage and land
drainage. The Secondary Drinking Water Standard
is 500 mg/L.

Synthetic Organics

The group of chemicals defined as synthetic
organic in this study includes 142 organic
chemicals. Some of these chemicals are organic
solvents, plastics components, and degradation
products of other chemicals. Many of the
chemicals detected are, or have been, utilized as
pesticides, as well. All are either carcinogenic,
teratogenic, toxic, or mutagenic. Standards vary
with compound and the reader is referred to Table
28 for a list of compound included in this group
and the associated standards. It is important to
note that the presence of these organic has not
been confirmed by re-sampling.

Table 37 lists the percentage of samples in
which exceedances for any of the 142 chemicals
were found. Most of these were not confirmed by
re-sampling. Seven percent of the surficial aquifer
system samples contained possible synthetic
organic in excess of standards. It is not unex-
pected that a proportion of the surficial aquifer
system samples would contain synthetic organic
given that the surficial aquifer system supports
much of the state's agriculture. One and three
percent of the samples from the intermediate and
Floridan aquifer systems, respectively, contain
excess synthetic organic. These proportions of
detections are relatively low and suggest relatively
little human impact on these aquifer systems.
While the level of contamination, statewide, is low,
surveillance must continue given the pandemic use
of synthetic organic.

Pesticides

The 172 chemicals included in the Pesticides
group (Table 32) were analyzed for in the
SWFWMD and SFWMD. Only arsenic was tested
for in NWFWMD, SRWMD, and SJRWMD. Table
37 summarizes the percentage of samples in

which the standards were exceeded. Arsenic is
not a widespread problem, although a few
samples contained excess arsenic in several
districts. The proportions given in Table 37 largely
represent detections in SWFWMD, and to a lesser
extent in SFWMD. Most of these were not verified
by confirmation re-sampling. If these proportions
are representative of the other districts, then a
problem may exist. Additional work, especially
screening of the other districts is needed before
firm conclusions can be drawn.

Total Organic Carbon

There is no standard for total organic carbon,
and it has not been considered a problem in the
past. Within the last few months the U.S.
Environmental Protection Agency has indicated
that it intends to set controls on the use of water
high in organic carbon for public water supplies
because of the widespread occurrence, nationally,
of trihalomethanes and other halogenated
hydrocarbons in finished drinking water. These
compounds result from sanitization of water with
chloride and other strong oxidants in treatment
plants.

Florida aquifer systems have an extraordinary
amount of total organic carbon in water. The
distribution of total organic carbon is summarized
in Table 27.

Need for Additional Work

Additional work needs to be done on several
specific problem areas. First of all, it is necessary
to re-sample and confirm or reject the detections
of contaminants in this first round of sampling.
Filtered and unfiltered samples need to be
compared so that mobility of these suspected con-
taminants can be evaluated.

Nitrate contamination is much more wide-
spread than anticipated. A study of the origins of
this nitrate and means of mitigation of the problem
should be undertaken in an organized way. Lead
may not be a ground-water quality problem (sensu
stricti. Lead in the sample set may result from
choice of wells sampled, not aquifer system con-
ditions. The widespread occurrence of lead must
be further investigated and a determination made
as to the source. Mercury was detected in a
number of samples and, given the current
problems with mercury in surface waters, its origin

and distribution need to be studied. Finally, the
synthetic organic and pesticides found include a
number used in the agricultural industry. These
occurrences need to be confirmed and the origins
of the contaminants identified.

The second round of Background Network
sampling, including filtered and unfiltered metals
samples, will assist in answering some of these
questions. The VISA and Temporal Variability
studies will address others. Specific studies will be
warranted in the future as the water-quality
problems become better defined.

MANAGEMENT IMPLICATIONS

It is important that the data reported in this
publication be utilized in every way possible.
These data are useful for water management,
including monitoring of water-quality change,
location of water-use facilities, and establishing
cause and effect relationships from aquifer system
use. We have attempted to provide some basic
concepts to allow the reader to understand how
aquifer systems work in a chemical sense and to
anticipate the consequences of water-man-
agement options. The following is a summary of
some of the ways that the data can be utilized.

Comparison to Background

The concept of background water quality
allows wise resource management at two scales,
local and regional. At the local scale, all
contamination assessments, environmental impact
studies, environmental audits, and many
consumptive-use applications require devel-
opment of an evaluation of background water
quality for comparison to on-site water-quality
impacts. The person preparing a local study
should not expect to use the data presented herein
to support a site-specific application or evaluation,
but these data can be used to determine what to
expect when background water quality deter-
minations are made. If local background water
quality differs greatly from that predicted by this
report, one should not take alarm, but an
investigation as to the reason for the difference is
appropriate. The water-resources manager can
also utilize the Background Network data in
evaluating permit applications and environmental
assessments. These data provide a basis for
comparison with the local background quality in
the submission and for determination of any de-
gradation of water quality.

On the regional scale, background ground-
water quality takes on another meaning. Water
resources are finite and, in a state with a growing
population, one must anticipate and protect future
water supplies. Data from the Background
Network allow delineation of these future
resources and monitoring of threats to them.
Consultants and water-supply authorities can
utilize the data in well field siting and design.
Managers can evaluate well field sitings in the
regional context and evaluate potentials for
upcoming or other adverse effects.

Sensitivity to Contamination

Recharge areas, aquifer systems with low
buffering, sorption, or microbial degradation
capacities, and aquifer systems with conduit flow
are sensitive to contamination. The data
presented herein identify these conditions and
allow a qualitative judgement of sensitivity to
contamination. More importantly, the data
collected include all major and minor constituents
necessary to model equilibrium, sorption, and
certain types of biological reactions. It may be
possible, therefore, to calculate the carrying
capacity of the aquifer system. In other words,
proper use of the data may allow one to quantify
how much effluent an activity, such as waste
disposal, can release to an aquifer system before
water quality adversely changes.

Effects of Consumptive Use

Coastal intrusion, upcoming or recharge, and
induced lateral flow of contaminants may result
from heavy pumpage of ground water. These data
provide a basis for detecting proximity to low
quality water and, by comparison with regions
where adverse movement of water exists today,
one may be able to anticipate degradation of local
or regional water quality by pumpage.

Long-Term Resource Evaluation

It is clear that the intent of the Legislature
when it adopted the Water Quality Assurance Act
in 1983 was to begin a program that will allow
water-resource managers to closely monitor future
changes in water quality and resulting loss of the
resource. The Background Network provides a
baseline for evaluating the future of our aquifer
systems. It allows us to identify change, but it
does not allow us to predict it with any confidence.

SPECIAL PUBLICATION NO. 34

The VISA and Temporal Variability Networks will
help with our ability to predict, but only continued
monitoring will allow confidence in identification of
change and successful management and
enforcement.

Need to Continue the Program and
the Future

The Ground-Water Quality Monitoring Program
has begun the process of assessing the quality of
ground water in Florida. This report is the first
statewide assessment of all aquifer systems, ever.
Re-sampling of the Background Network has
begun, and this time both filtered and unfiltered
metals analyses will be included. When this survey
is completed, we will be able to compare the data
to the data reported in this document to identify
any changes in water quality. In addition, we will
be able to evaluate the concentrations of dissolved
and particulate metals. This re-sampling will assist
in evaluation of the reality of the detections of
synthetic organic and pesticides. It will also allow
confirmation of the lead and mercury problems
mentioned above.

Sampling of the VISA Network is nearing
completion, and comparisons will be made with
the Background Network to develop predictive
models concerning the impacts of specific land
uses. This comparison will comply with the charge
of the Water Quality Assurance Act to predict
contamination.

Finally, the Temporal Variability Network data
is being analyzed as this report is being written.
This data base indicates that water quality
changes on short-and long-term time frames do
occur. The data will allow one to place the
conclusions of this report into perspective with
respect to both spatial and temporal changes. It
will also assist managers to evaluate the context of
reports that include only one sampling event.

Several additional studies are suggested by
the findings of this report. First, a study of the
mechanisms by which fracture systems affect
water quality should be undertaken. These frac-
tures are widespread, and water-management
policy should include potential for migration of low-
quality water within them. Second, the origin of
the lead and mercury reported in Florida's aquifer
systems should be identified. Third, the origin,
transport, and biochemical transformations of
nitrogen species should be undertaken. There are

several large regions where non-point source
nitrate is abundant. While we may understand the
sources of the nitrate, we must understand how
those sources result in nitrate contamination. If the
synthetic organic and pesticide problems are
confirmed, then additional study will be required to
determine the origins and fates of these com-
pounds. Fourth, efforts should be made to develop
demonstration projects to show how the data can
be used to calculate aquifer system carrying
capacities, identify recharge and discharge areas,
and predict water-quality degradation as a result of
a variety of land uses.

FLORIDA GEOLOGICAL SURVEY

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Watrous, R., and Upchurch, S.B., in preparation,
Distribution of organic carbon in the Floridan
aquifer: To be submitted to the Bulletin of the
Geological Society of America.

Weaver, C.E., and Beck, K.C., 1977, Miocene of
the S.E. United States: A Model for Chemical
Sedimentation in a Peri-Marine Environment:
Amsterdam, Elsevier Scientific Publishing
Company, Developments on Sedimentology,
v. 22, 234 p.

Wedderburn, L.A., Knapp, M.S., Waltz, D.P., and
Burns, W.S., 1982, Hydrogeologic recon-
naissance of Lee County, Florida: South
Florida Water Management District, Technical
Publication 82-part 1 -Text, 192 p.

White, W.B., 1988, Geomorphology and Hydrology
of Karst Terrains: New York, Oxford University
Press, 464 p.

Wilson, W.E., 1977, Ground-water resources of
DeSoto and Hardee Counties, Florida: Florida
Bureau of Geology Report of Investigations
No. 83,102 p.

Wigley, T.M.L., and Plummer, L.N., 1976, Mixing of
carbonate waters: Geochimica et Cosmo-
chimica Acta, v. 42, p. 1117-1139.

FLORIDA GEOLOGICAL SURVEY

Wood, J.M., Kennedy, F.S., and Rosen, C.G.,
1968, Synthesis of methyl-mercury com-
pounds by extracts of a methanogenic bac-
terium: Nature, v. 220, p. 173-174.

Young, T.C., and Comstock, W.G., 1986, Direct
effects and interactions involving iron and
humic acid during formation of colloidal phos-
phorus, in Sly, P.G., (ed.), Sediments and
Water Interactions: New York, Springer-Verlag,
p. 461-470.

TABLE 1

GROUND WATER QUALITY NETWORK MONITORING PARAMETERS

PARAMETER GROUP

Parameter Name

NETWORK

Background VISA HRS Quarterly Monthly

MAJOR IONS

Bicarbonate
Carbonate
Chloride
Cyanide
Fluoride
Nitrate
Phosphate
Sulfate

Q

H Q

406
406
407A, 407B, or 407D
412B, 412C, or 412D
413A, 413B, 413C, or 413E
418C or 418F
424F or 424G
426A or 426C

METALS

Arsenic
Barium
Cadmium
Calcium
Chromium
Copper
Iron
Lead
Magnesium
Manganese
Mercury
Nickel
Potassium
Selenium
Silver
Sodium
Strontium
Zinc

H Q

FIELD PARAMETERS

Conductivity
pH
Eh
Dissolved Oxygen (DO)
Temperature
Water levels
Odor

MICROBIOLOGICAL

Q M
Q M
M
M
Q M
Q M

Fecal Coliform
Total Coliform

303E
303C
303A or 303B
303A or 311C
303A or 303B
303A
303A or 315B
303A or 303B
303A or 319B
303A or 319B
303F
303A or 322B
303A or 322B
303E
303A or 303B
303A or 325B

303A or 303B

212

908C or 909C
908A or 909A

ORGANIC AND PESTICIDES

Total Organic Carbon (TOC)
Volatile Organic Carbon (VOC)
Aldicarb & related compounds
Purgeable Halocarbons
Purgeable Aromatics
Pesticides
PCB's, Chlorinated Pesticides
Pesticides
Organophosphate Pesticides
Mixed Purgeables
Base / Neutral / Acid Extractables
Carbamate Pesticides
Pesticides
Herbicides
Fumigant Pesticides

H
V H

-D
C

I-

O
Z
Z
O

505
EPA 601 and 602, or EPA 624
EPA 531
EPA 601
EPA 602
EPA Alt. 614
EPA Alt. 617
EPA Alt. 619
EPA 622
EPA 624
EPA 625
EPA 632
EPA 644

RADIOMETRICS

Gross Alpha
Gross Beta
Radon
Radium

OTHERS

Total Dissolved Solids (TDS)
Ammonia
Silica

B V
V
V

209B

Methods are from the American Public Health Association's Standard Methods for the Examination of Water and Wastewater, 15th edition
(1980), or from the Florida Department of Environmental Regulation's Supplement "A" to Standard Operating Procedures and Quality
Assurance Manual (1981).

2 Other approved methods with the same or better minimum detection limits, accuracy and precision are also acceptable.

A subset of approximately 100 Background Network wells is being sampled for radon and/or radium.

STANDARD METHOD1,2

TABLE 2

FLORIDA PRIMARY AND SECONDARY DRINKING WATER STANDARDS
FOR SELECTED PARAMETERS (FROM F.A.C. 17-22) ***

PARAMETER GROUP

Parameter Name

MAJOR IONS

MAXIMUM CONTAMINANT LEVEL (jg/L, unless otherwise noted)

Primary DWS

Chloride
Fluoride
Nitrate
Sulfate

METALS

Arsenic
Barium
Cadmium
Chromium
Copper
Iron
Lead
Manganese
Mercury
Selenium
Silver
Sodium
Zinc

4,000
10,000
250,000

50
1,000
10
50

50

2
10
50
160,000

Secondary DWS

250,000
2,000

1,000
300

50

5,000

FIELD PARAMETERS

> 6.5, < 8.5 s.u.
3 T.O.N.**

MICROBIOLOGICAL

Total Coliform

4 col/L (see rule)

ORGANIC

Endrin
Lindane
Methoxychlor
Toxaphene
2,4-D
2,4,5-TP (Silvex)
Tetrachloroethylene
Trichloroethylene
Carbon Tetrachloride
Vinyl Chloride
1,1,1-Trichloroethane
1,2-Dichloroethane
Benzene
Ethylene Dibromide (EDB)
Trihalomethane

RADIOMETRICS

Gross Alpha
Gross Beta
Radium 226, 228

OTHERS

Total Dissolved Solids (TDS)

4
100
5
10
10
3
3
3
1
200
3
1
0.02
100

15 pCi/L
4 mrem/yr
5 pCi/L

500,000

*s.u. = Standard Units
** T.O.N. = Threshold Odor Number
*** Source: Florida Ground Water Guidance Concentrations, Florida Department of Environmental Regulation, February, 1989

Table 3. Summary of the chemical composition of precipitation from selected sites in Florida. Based on data from the
National Atmospheric Deposition Program (IR-7)/National Trends Network (1990).

Stat. Ca Mg K Na NH4 NO, CI SO4 PO4 pH Cond. Na/CI Ratio
(mg/L(m g(mg/L) (mg/L) (mg/L) (mg) (mg/L) (mg/L) (mg/L) (mg/L) (field) (field) (mole dev.
ratio) from
sea
water

Quincy, Gadsden County

0.06 0.44 0.15 1.05 0.75

0.89 1.84 0.09

179 179 179

0.06 0.10 0.00

5.58 12.78 0.78

4.68

0.41

160

3.57

5.90

17.7 0.90 0.05

15.9

162

4.0

132.5

0.22

179

0.40

2.90

0.22

179

-0.45

2.05

Austin-Cary Forest. Alachua County

1.00 0.96 2.03 0.00

0.88 0.99 1.47 0.00

92 92 92 92

0.07 0.10 0.10 0.00

4.70 8.88 8.88 0.00

Bradford Forest. Bradford County

0.80

2.13

367

0.04

29.30

0.16

0.24

367

0.00

1.92

1.04 1.19 1.96

0.95 3.16 2.13

367 367 367

0.00 0.00 0.00

6.60 52.62 22.80

0.01

0.07

367

0.00

1.19

4.70 16.77 1.05

0.46 12.82 0.64

340 337 366

3.22 2.20 0.31

6.60 99.00 6.17

Kennedy Space Center. Brevard County

0.28 0.20

0.34 0.25

229 229

0.01 0.00

3.28 1.70

1.58

2.03

229

0.09

13.82

0.81

1.50

202

0.07

1.93 17.40 13.31

0.10

0.17

229

0.00

1.05 2.81

3.64

229

0.15

1.18 10.12 12.81 12.81

Verna Well Field. Sarasota County

0.21 1.00 1.39 1.53

0.63 1.15 2.62 1.47

202 202 202 202

0.00 0.00 0.14 0.15

7.30 10.32 24.53 13.64

Everglades National Park. Dade County

0.02

0.07

229

0.00

0.58

0.05

0.39

202

0.00

4.98

4.92 23.15

0.37 14.99

208 201-

3.73 0.80

5.72 85.80

4.85 15.02

0.54 10.70

159 177

3.39 3.00

7.30 85.30

0.20 0.20 1.32

0.31 1.03 1.89

304 304 304

0.01 0.00 0.05

2.66 12.60 15.91

0.13 1.00

0.74 1.80

1373 1373

0.00 0.02

17.4 29.3

0.22

1.12

304

0.00

17.12

0.73

0.85

304

2.31

3.21

304

1.14

1.62

304

0.00 0.12 0.00

8.37 26.89 15.42

State-wide

0.17 0.97

0.61 1.01 2.98

1373 1373 1373

0.00 0.00 0.00

17.2 10.32 52.62

1.75

1.80

1373

0.00

22.8

0.07

0.63

304

0.00

9.98

0.03

0.34

1373

0.00

9.98

4.98 15.98

0.57 13.90

261 269

3.00 3.50

7.27 141.50

4.77 17.58

0.50 13.80

1217 1231

3.0 0.80

7.3 141.5

x = arithmetic median

s = standard deviation

n = number of samples

s

n

Min.

Max.

0.17

179

0.01

1.15

0.08

179

0.01

0.53

0.18

179

0.00

2.12

s

n

Min.

Max.

0.28

0.28

92

0.02

1.33

0.09

0.07

92

0.00

0.39

0.09

0.16

92

0.00

0.95

0.78

0.85

92

0.02

4.87

0.15

0.85

92

0.02

4.87

4.79

0.53

89

3.49

6.70

17.87

12.09

92

3.70

66.70

s

n

Min.

Max.

0.54

1.01

92

-0.73

5.19

0.07

0.22

367

0.00

3.81

0.09

0.10

229

0.00

0.80

s

n

Min.

Max.

0.31

0.42

202

0.02

3.49

0.14

0.25

202

0.01

0.20

0.64

366 U)
m
-0.54 m

5.32
r-
_0
C
r-
0
0.02 >

0.14 0
Z
229

-0.48 O

1.06 "

0.05

0.22

202

-0.34

1.46

0.87

0.14

229

0.37

1.91

0.90

0.22

202

0.51

2.31

x

s

n

Min.

Max.

s

n

Min.

Max.

0.36

0.73

304

0.01

7.68

0.28

0.48

1373

0.01

7.68

0.14

0.25

1373

0.00

4.31

0.88

0.13

304

0.56

2.22

0.96

0.47

1373

0.13

6.17

0.03

0.13

304

-0.30

1.37

0.11

0.47

1372

-0.73

5.32

FLORIDA GEOLOGICAL SURVEY

Table 4. Common minerals in Florida aquifer systems and confining beds and their dissolved weathering
products. Mineral formulae from Fleischer (1987).

Dissolved
weathering
Mineral Composition products
(excl. H,O,
S= plus residual
solids)

Anhydrite CaSO4 Ca2+, SO,42
Aragonite CaCO, Ca2+, HCO,+
Calcite CaCO, Ca2, HCO,3
Carbonate-hydroxylapatite Ca,(PO,,CO),(OH) Ca2+, P043-, HCO,, plus
trace U4.,6+
Carbonate-fluorapatite Ca,(PO,,CO),F Ca2+, PC3-, F-,HCO,-, plus
trace U4',6+
Dolomite CaMg(CO,)2 Ca2, Mg2+, HCO,,
Ferric hydroxide Fe(OH), Fe2,31
Gibbsite (G), G: AI(OH), or Al"'
b6hmite and A1203.3H,O,
diaspore (BD) BD: AIO(OH)
Goethite FeO(OH) Fe2', 3+
Gypsum CaSO4.2HO Ca2+, SO42+
Hematite Fe203 Fe2+, 3
K-feldspar KAISi,30 K', H4SiO4, *
K-mica KAI,(SiAI),0o(OH,F)2 K+, F, H4SiO4, *
Kaolinite AI2Si,20(OH)4 Al, H4SiO4, *
Opal (-A, -CT) SiO2.nHO H4SiO4
Palygorskite (Mg,AI),Si40,O(OH).4H,0 Mg2', A13, H4SiO4, *
Pyrite FeS, Fe2+' 3, SO42
Quartz SiO, Essentially inert
Sepiolite Mg4Si6Oi(OH)2.6HO Mg2, H4SiO4, *
Smectite, v. Montmorillonite (Na,Ca).3(AI,Mg),Si40,O(OH)2.nH20 Na, Ca2", Mg2+,AlP,
H4SiO4, *
Smectite, v. Nontronite NaoFe,(Si,AI),O,,(OH),.nHO Na+, Fe2+, A3+, HSiO4, *

Table 5. Common minerals in Florida aquifer systems. See Table 4 for mineral compositions and weathering
products. Volumetrically or chemically important minerals indicated in bold.

AQUIFERIMINERAL Surficial aquifer Intermediate aquifer Floridan aquifer
FRACTION system system system

Silicate Fraction Quartz Quartz Quartz
Potassium feldspar Potassium feldspar
Potassium mica Potassium mica
Kaolinite Palygorskite
Chlorite Sepiolite
Smectite Smectite
Kaolinite

Carbonate Fraction Calcite Dolomite Calcite
Aragonite Calcite Dolomite
Aragonite (?)

Oxyhydroxides, Ferric hydroxide Pyrite Pyrite
sulfides Goethite Ferric hydroxide Ferric hydroxide
Gibbsite. boehmite, Goethite Goethite
diaspore
Pyrite

Other Humic substances* Carbonate- Gypsum
fluorapatite Anhydrite
Carbonate-
hydroxylapatite
Opal-A
Opal-CT
Gypsum
* Humic substances refer to particulate organic, including organic concentrated in peats and mucks, and
disseminated in other sediments.

SPECIAL PUBLICATION NO. 34

Table 6. Summary of temperature distribution CC), by region and aquifer system.

A. Surficial aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum
NWFWMD 22.0 21.0 23.1 84 18.0 25.1

SRWMD 24.0 22.0 25.0 23 21.0 27.0

SJRWMD 24.0 22.3 25.0 44 19.0 29.0

SWFWMD 25.0 24.0 25.5 99 21.0 31.5

SFWMD 24.8 23.3 26.0 671 18.5 30.0

Statewide 24.4 23.0 25.9 921 18.0 31.5

Sand & Gravel 22.0 21.0 23.1 75 18.0 25.1

Biscayne 25.0 24.0 26.0 313 18.5 30.0

Other 24.2 23.3 25.2 533 19.0 31.5

B. Intermediate aquifer system

District Median 1 Qrtile t Qrtile # Samples Minimum Maximum

NWFWMD 23.2 22.2 24.0 24 21.0 26.0

SRWMD 22.0 21.0 23.0 27 18.0 24.0

SJRWMD 24.0 22.0 25.0 21 18.0 25.5

SWFWMD 25.0 24.0 26.0 60 23.0 30.0

SFWMD 25.1 24.4 25.6 102 22.3 27.5

Statewide 24.6 23.5 25.4 234 18.0 30.0

C. Floridan aquifer system

District Median I Qrtile t Qrtile # Samples Minimum Maximum

NWFWMD 23.0 22.0 24.9 101 19.0 28.9

SRWMD 23.0 21.0 24.0 157 15.0 29.0

SJRWMD 23.0 22.0 24.0 77 18.0 27.0

SWFWMD 25.0 24.0 26.0 191 21.5 30.5

SFWMD 26.3 24.8 27.1 131 22.2 30.5

Statewide 24.0 23.0 25.5 657 15.0 30.5

Table 7. Summary of pH distribution (s.u.), by region and aquifer system.

A. Surficial aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD 4.9 3.8 5.6 84 78 3.0 10.2

SRWMD 5.6 5.1 6.0 25 22 4.5 9.5

SJRWMD 6.6 5.9 7.2 53 24 3.5 9.9

SWFWMD 6.5 5.5 7.2 97 52 3.9 8.6

SFWMD 6.9 6.5 7.2 809 219 3.9 13.2

Statewide 6.8 6.3 7.1 1068 395 3.0 13.2

Sand & Gravel 4.9 3.8 5.6 75 70 3.0 10.2

Biscayne 6.9 6.6 7.2 477 103 5.6 10.5

Other 6.7 6.0 7.1 516 222 3.4 13.2

B. Intermediate aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD 7.2 6.7 7.9 24 7 4.3 9.5

SRWMD 6.5 5.2 6.8 36 21 4.0 9.3

SJRWMD 7.1 7.0 7.5 29 3 5.1 11.3

SWFWMD 7.5 7.3 7.7 56 4 6.7 10.5

SFWMD 7.3 7.0 7.5 94 4 6.1 8.5

Statewide 7.3 6.9 7.6 239 39 4.0 11.3

C. Floridan aquifer system

District Median I Qrtile f Qrtile # Samps # Exc Min Max

NWFWMD 7.5 7.1 7.8 101 3 6.6 8.8

SRWMD 7.1 6.6 7.9 220 63 4.9 12.5

SJRWMD 7.3 7.0 7.7 100 10 6.2 12.2

SWFWMD 7.5 7.3 7.8 172 16 6.0 10.7

SFWMD 7.4 7.1 7.6 125 8 5.6 8.9

Statewide 7.4 7.0 7.8 718 100 4.9 12.5

* Number of samples which exceeded Florida Secondary Drinking Water
Standards for pH (< 6.5 or > 8.5 s.u.).

FLORIDA GEOLOGICAL SURVEY

Table 8. Concentrations of selected constituents in average sea water, ranked by abundance. Data compiled
from various sources by Drever (1988). The predicted concentration in Florida precipitation is determined by
multiplying the mole ratio in sea water times the state-wide average concentration of chloride in precipitation
(1.66 mg/L, Table 3).

Constituent Cone. in Mole ratio Predicted
Sea Water to CI in Cone. in
Sea Water Avg. Florida
(mg/kg) Precipitation
(NOTE 1)
(mg/L)

Chloride 19,350 1.000 1.66
Sodium 10,760 0.857 1.42
Sulfate 2,710 0.052 0.09
Magnesium 1,290 0.097 0.16
Calcium 411 0.019 0.03
Potassium 399 0.019 0.03
Bicarbonate 142 0.0043 0.0071
Fluoride 1.3 0.00013 0.00022
Trace Constituents (pg/kg) (pg/L)
Nitrate 5-2,000 1.5x10-7 0.25- 98
5.9x1 05
Phosphate 1-50 1.9x10-8 0.032 1.6
9.7x10-7
Dissolved organic carbon 300-2,000 NA NA
Iron 2 6.6x1 0- 0.11
Mercury 0.03 2.7x10-10 0.00046
Lead 0.03 2.7x10-10 0.00044

NOTE 1: The predicted concentration in precipitation assumes that the only sources of chemicals in rainfall are
marine aerosols. The predicted concentration is calculated by

X x. = X.... 1.66 = 8.57 x 10- X. ,
19,350
Xpc,p is the predicted concentration in precipitation, in mg/L. X wais the concentration of the chemical in sea
water (mg/kg, Table 8). Chloride concentrations are: 19,350 mg/kg in sea water and 1.66 mg/L in precipitation
(Table 3).

Table 9. Classification of water hardness (from Durfor and Becker, 1964).

Hardness Range Description
(mg/L as CaCOJ

0-60 Soft
61 120 Moderately hard
121-180 Hard
More than 180 Very hard

SPECIAL PUBLICATION NO. 34

Table 10. Summary of total calcium distribution (Ca2+, mg/L), by region and aquifer system.

A. Surficial aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 3.7 1.6 8.8 84 < 0.1 38.0

SRWMD 11.0 5.2 40.0 25 0.4 240.0

SJRWMD 43.0 12.0 94.0 64 < 1.0 857.0

SWFWMD 22.3 7.5 58.8 84 0.2 763.0

SFWMD 98.0 73.3 124.0 610 < 1.0 756.0

Statewide 85.6 27.7 118.0 867 < 0.1 857.0

Sand & Gravel 3.6 1.6 8.9 75 < 0.1 38.0

Biscayne 97.4 79.4 125.0 248 1.4 260.0

Other 85.6 25.6 119.4 544 0.2 857.0

B. Intermediate aquifer system

District Median I Qrtile t Qrtile # Samples Minimum Maximum
NWFWMD 37.5 19.0 60.0 24 3.7 270.0

SRWMD 28.0 9.4 46.0 36 1.5 220.0

SJRWMD 52.0 27.0 133.0 33 1.6 336.0

SWFWMD 61.0 43.6 106.0 52 2.3 397.0

SFWMD 70.5 43.2 100.0 103 2.5 478.0

Statewide 58.0 36.0 100.0 248 1.5 478.0

C. Floridan aquifer system

District Median Qrtile T Qrtile # Samples Minimum Maximum
NWFWMD 34.0 21.0 46.0 101 2.2 143.0

SRWMD 82.0 51.0 150.0 220 0.6 1000.0

SJRWMD 41.0 24.0 64.0 125 4.0 546.0

SWFWMD 68.1 41.7 98.0 165 3.6 639.0

SFWMD 67.2 42.1 94.1 138 5.9 227.0

Statewide 51.9 28.5 84.1 749 0.6 1000.0

Table 11. Summary of total magnesium distribution (Mg+), by region and aquifer system.

A. Surficial aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 0.9 0.6 2.0 84 0.2 20.0

SRWMD 1.6 1.2 3.2 25 0.1 44.0

SJRWMD 3.2 1.7 7.5 64 0.3 138.0

SWFWMD 3.5 1.1 11.5 85 < 0.1 401.0

SFWMD 3.9 2.7 6.5 229 0.1 51.9

Statewide 3.1 1.6 6.4 487 < 0.1 401.0

Sand & Gravel 0.9 0.6 2.0 75 0.2 13.0

Biscayne 3.9 2.7 6.5 229 0.1 51.9
Other 3.0 1.3 8.8 183 < 0.1 401.0

B. Intermediate aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 2.5 1.2 5.4 24 0.4 59.0

SRWMD 8.7 1.0 21.0 37 < 0.1 52.0

SJRWMD 4.8 2.6 14.0 33 0.1 255.0

SWFWMD 24.0 13.9 47.2 52 < 0.1 135.0
SFWMD (f) 26.6 19.4 67.6 103 2.2 465.6

Statewide 17.7 9.5 40.9 249 < 0.1 465.6

C. Floridan aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 6.4 3.0 12.0 101 0.3 60.0

SRWMD 6.3 2.6 19.0 220 < 0.1 430.0

SJRWMD 11.0 5.0 32.0 125 0.4 521.0

SWFWMD 7.0 2.2 23.0 166 0.1 180.0

SFWMD (f) 46.4 20.7 84.7 137 < 0.1 264.2

Statewide 14.6 6.3 33.1 749 < 0.1 521.0

(f) Dissolved (filtered) Magnesium, mg/L.

FLORIDA GEOLOGICAL SURVEY

Table 12. Summary of total sodium distribution (Na, mg/L), by region and aquifer system.

A. Surficial aquifer system

District Median 1 Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD 5.0 3.2 8.2 84 1 1.2 310.0

SRWMD 5.0 2.6 7.0 25 0 0.8 30.0

SJRWMD 17.5 8.0 47.0 64 10 2.0 868.0

SWFWMD 6.4 3.4 15.5 85 2 0.7 3730.0

SFWMD 21.1 11.9 45.2 610 25 1.6 620.0

Statewide 17.0 7.0 39.0 868 38 0.7 3730.0

Sand & Gravel 5.0 3.2 8.6 75 0 1.3 160.0

Biscayne 18.0 11.1 31.0 248 7 2.1 420.0

Other 19.4 7.3 48.3 545 31 0.7 3730.0

B. Intermediate aquifer system

District Median I Qrtile I Qrtile # Samps # Exc Min Max

NWFWMD 4.5 3.3 20.0 24 0 1.0 78.0

SRWMD 4.3 3.7 7.4 37 0 2.3 23.0

SJRWMD 14.0 8.7 31.0 33 5 5.8 2585.0

SWFWMD 31.7 12.9 73.5 52 6 2.9 357.0

SFWMD 108.6 51.8 369.0 103 46 11.4 1264.0

Statewide 41.0 9.6 136.2 249 57 1.0 2585.0

C. Floridan aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD 6.0 2.9 27.0 102 1 0.7 350.0

SRWMD 6.3 3.7 12.0 220 5 0.2 3200.0

SJRWMD 20.0 7.9 80.0 125 22 1.0 7043.0

SWFWMD 7.4 4.3 28.9 165 18 1.8 1450.0

SFWMD 220.5 42.1 490.0 138 81 2.7 2500.0

Statewide 11.0 4.5 84.6 750 127 0.2 7043.0

* Number of samples which exceeded Florida Primary Drinking Water Standards for Sodium (> 160 mg/L).

Table 13. Summary of total potassium distribution (K, mg/L), by region and aquifer system.

A. Surficial aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 1.2 0.7 2.6 84 0.2 31.0

SRWMD 1.5 0.5 3.4 25 < 0.1 19.0

SJRWMD 2.0 0.9 4.4 64 0.2 601.6

SWFWMD 0.8 0.3 2.3 85 < 0.1 29.7

SFWMD 1.3 0.7 2.8 610 < 0.1 159.2

Statewide 1.2 0.7 2.8 868 < 0.1 601.6

Sand & Gravel 1.4 0.7 2.5 75 0.2 31.0

Biscayne 1.1 0.7 2.2 248 < 0.1 69.0

Other 1.3 0.6 3.0 545 < 0.1 601.6

B. Intermediate aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 2.2 0.9 4.2 24 0.4 78.0

SRWMD 0.6 0.5 0.8 37 < 0.1 19.0

SJRWMD 2.3 1.2 8.4 33 0.2 85.0

SWFWMD 2.7 1.3 6.2 52 0.3 22.4

SFWMD 9.6 6.8 19.2 103 0.4 46.9

Statewide 4.4 1.3 11.0 249 < 0.1 85.0

C. Floridan aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 1.6 0.6 4.7 101 0.2 76.0

SRWMD 1.1 0.4 4.2 220 0.1 320.0

SJRWMD 1.7 0.9 4.2 125 0.2 251.0

SWFWMD 1.0 0.4 3.2 166 < 0.1 145.0

SFWMD 9.5 2.5 20.8 138 0.5 99.0

Statewide 1.8 0.7 6.7 750 < 0.1 320.0

SPECIAL PUBLICATION NO. 34

Table 14. Summary of total iron distribution (Fe2++Fe3, mg/L), by region and aquifer system. Data from non-
metal cased wells.

A. Surficial aquifer system

District Median I Qrtile TQrtile # Samps # Exc Min Max

NWFWMD 2.05 0.78 6.75 80 72 0.07 95.00

SRWMD 1.09 0.32 2.85 23 18 < 0.01 18.00

SJRWMD 4.09 0.61 9.73 51 45 0.09 56.21

SWFWMD 2.14 0.32 8.39 39 30 < 0.03 43.90

SFWMD 0.88 0.20 2.58 376 263 < 0.01 41.50

Statewide 1.08 0.24 2.94 569 428 < 0.01 95.00

Sand & Gravel 2.00 0.79 6.30 73 66 0.07 95.00

Biscayne 1.19 0.27 2.46 155 120 <0.01 8.46

Other 0.89 0.24 4.75 341 242 <0.02 56.21

B. Intermediate aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD 0.60 0.23 2.40 22 14 0.02 35.00

SRWMD 1.17 < 0.23 3.10 14 12 < 0.05 15.00

SJRWMD 0.46 0.33 1.72 21 16 < 0.01 4.61

SWFWMD 0.13 0.05 0.54 19 7 < 0.02 12.10

SFWMD < 0.05 < 0.05 0.10 63 9 0.03 26.60

Statewide 0.07 < 0.05 0.43 139 58 <0.01 35.00

C. Floridan aquifer system

District Median 1 Qrtile I Qrtile # Samps # Exc Min Max

NWFWMD 0.23 0.05 0.87 34 16 < 0.01 2.50

SRWMD 0.61 0.17 1.60 135 95 < 0.01 17.00

SJRWMD 0.19 0.06 0.89 48 24 < 0.01 24.92

SWFWMD 0.13 0.05 0.35 70 21 < 0.01 55.70

SFWMD < 0.05 < 0.05 < 0.05 32 0 < 0.02 0.29

Statewide 0.21 < 0.05 1.00 319 156 <0.01 55.70

* Number of samples which exceeded Florida Secondary Drinking Water Standards for Iron (> 0.30 mg/L).

Table 15. Summary of total mercury distribution (Hg2,, ig/L), by region and aquifer system.

A. Surficial aquifer system

District Median I Qrtile I Qrtile # Samps # Exc Min Max

NWFWMD < 0.5 < 0.5 < 0.5 84 8 < 0.5 7.3

SRWMD < 0.2 < 0.2 < 0.2 23 1 < 0.2 3.0

SJRWMD < 0.5 < 0.5 < 0.5 58 2 < 0.5 52.0

SWFWMD < 0.1 < 0.1 < 0.1 67 3 < 0.1 3.1

SFWMD < 0.2 < 0.2 < 0.2 424 0 < 0.1 0.6

Statewide < 0.2 < 0.2 < 0.5 656 14 < 0.1 52.0

Sand & Gravel < 0.5 < 0.5 < 0.5 75 8 < 0.5 7.3

Biscayne < 0.2 < 0.2 < 0.2 333 0 < 0.2 < 1.0

Other < 0.2 < 0.1 < 0.5 248 6 < 0.1 52.0

B. Intermediate aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD < 0.5 < 0.5 < 0.5 37 1 < 0.3 2.2

SRWMD < 0.2 < 0.2 < 0.2 27 0 < 0.2 2.0

SJRWMD < 0.5 < 0.5 < 0.5 32 4 < 0.5 8.0

SWFWMD < 0.1 < 0.1 < 0.1 45 0 < 0.1 1.3

SFWMD < 0.1 < 0.1 < 0.1 10 0 < 0.1 < 0.3

Statewide < 0.5 < 0.1 < 0.5 151 5 < 0.1 8.0

C. Floridan aquifer system

District Median i Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD < 0.5 < 0.5 < 0.5 209 4 < 0.3 6.5

SRWMD < 0.2 < 0.2 < 0.2 157 0 < 0.2 2.0

SJRWMD < 0.5 < 0.5 < 0.5 113 2 < 0.5 4.7

SWFWMD < 0.1 < 0.1 < 0.1 154 0 < 0.1 1.3

SFWMD < 0.1 < 0.1 < 0.1 18 0 < 0.1 0.2

Statewide < 0.5 < 0.1 < 0.5 651 6 < 0.1 6.5
* Number of samples which exceeded Florida Primary Drinking Water Standards for Mercury (> 2.0 ig/L).

FLORIDA GEOLOGICAL SURVEY

Table 16. Summary of total lead distribution (Pb2, pig/L), by region and aquifer system. Data from non-metal
cased wells.

A. Surficial aquifer system

District Median 1 Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD < 10 7 15 80 10 < 1 190

SRWMD < 10 < 10 < 10 23 0 < 10 40

SJRWMD 23 < 10 58 46 18 < 10 4300

SWFWMD 36 < 30 < 50 53 11 < 20 1630

SFWMD < 2 < 1 3 440 9 < 1 173

Statewide 2 < 1 < 10 642 48 < 1 4300
Sand & Gravel < 10 8 16 73 10 < 1 190

Biscayne < 2 < 2 2 218 2 < 1 87

Other 7 1 30 351 36 < 1 4300

B. Intermediate aquifer system

District Median 1 Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD < 10 < 10 < 10 22 1 3 63

SRWMD < 10 < 10 < 10 14 1 < 10 56

SJRWMD < 10 < 10 13 21 2 < 4 192

SWFWMD < 43 < 30 < 50 25 6 < 30 530

SFWMD 1 < 1 5 59 1 < 1 71

Statewide < 10 < 10 24 141 11 < 1 530

C. Floridan aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD < 10 < 10 < 10 35 0 1 50

SRWMD < 10 < 10 < 10 135 4 < 10 100

SJRWMD < 10 < 10 31 47 9 4 260

SWFWMD < 30 < 30 < 36 69 14 < 20 470

SFWMD < 1 < 1 < 1 30 0 < 1 9

Statewide < 10 < 10 25 316 27 < 1 470

* Number of samples which exceeded Florida Primary Drinking Water Standards for Lead (> 50 pg/L).

Table 17. Summary of total bicarbonate distribution (HCO3, mg/L), by region and aquifer system. (n.d. = not
determined)

A. Surficial aquifer system
District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 5 < 1 16 84 < 1 232

SRWMD 27 13 62 25 4 140

SJRWMD n.d. n.d. n.d. n.d. n.d. n.d.

SWFWMD 34 < 5 136 84 < 1 322

SFWMD 263 229 314 169 64 637

Statewide 138 10 260 362 < 1 637
Sand & Gravel 5 < 1 13 75 < 1 134

Biscayne 263 229 314 169 64 637

Other 27 8 101 118 < 1 322

B. Intermediate aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 137 116 183 24 16 1463

SRWMD 90 24 150 36 < 1 490

SJRWMD n.d. n.d. n.d. n.d. n.d. n.d.

SWFWMD 173 118 213 53 4 306

SFWMD n.d. n.d. n.d. n.d. n.d. n.d.

Statewide 143 95 200 113 < 1 1463

C. Floridan aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 158 122 207 101 29 451

SRWMD 150 76 230 220 < 1 770

SJRWMD n.d. n.d. n.d. n.d. n.d. n.d.

SWFWMD 144 116 181 162 3 646

SFWMD n.d. n.d. n.d. n.d. n.d. n.d.

Statewide 146 110 206 483 < 1 770

SPECIAL PUBLICATION NO. 34

Table 18. Summary of total carbonate distribution (CO32-, mg/L), by region and aquifer system. (n.d. = not
determined)

A. Surficial aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD n.d. n.d. n.d. n.d. n.d. n.d.

SRWMD < 1 < 1 < 1 23 < 1 70

SJRWMD n.d. n.d. n.d. n.d. n.d. n.d.

SWFWMD < 1 < 1 < 1 71 < 1 1

SFWMD n.d. n.d. n.d. n.d. n.d. n.d.

Statewide < 1 < 1 < 1 94 < 1 70

Sand & Gravel n.d. n.d. n.d. n.d. n.d. n.d.

Biscayne n.d. n.d. n.d. n.d. n.d. n.d.

Other < 1 < 1 < 1 94 < 1 70

B. Intermediate aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD n.d. n.d. n.d. n.d. n.d. n.d.

SRWMD < 1 < 1 < 1 27 < 1 22

SJRWMD n.d. n.d. n.d. n.d. n.d. n.d.

SWFWMD < 1 < 1 < 1 44 < 1 116

SFWMD n.d. n.d. n.d. n.d. n.d. n.d.

Statewide < 1 < 1 < 1 71 < 1 116

C. Floridan aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD n.d. n.d. n.d. n.d. n.d. n.d.

SRWMD < 1 < 1 < 1 157 < 1 650

SJRWMD n.d. n.d. n.d. n.d. n.d. n.d.

SWFWMD < 1 < 1 < 1 152 < 1 46

SFWMD n.d. n.d. n.d. n.d. n.d. n.d.

Statewide < 1 < 1 < 1 309 < 1 650

Table 19. Summary of total bicarbonate alkalinity distribution (mg/L), by region and aquifer system .(n.d. = not
determined)

A. Surficial aquifer system

District Median I Qrtile t Qrtile # Samples Minimum Maximum

NWFWMD 4 < 1 13 84 < 1 190

SRWMD n.d. n.d. n.d. n.d. n.d. n.d.

** SJRWMD 147 70 238 59 1 508

++ SWFWMD 42 16 97 18 5 251

** SFWMD 251 202 312 581 3 2260

Statewide 111 72 165 742 < 1 2260

Sand & Gravel 4 < 1 11 75 < 1 110

**Biscayne 242 212 294 219 64 637

Other 244 160 315 448 < 1 2260

B. Intermediate aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 113 95 150 24 < 1 1200

SRWMD n.d. n.d. n.d. n.d. n.d. n.d.

** SJRWMD 238 169 290 30 17 561

++ SWFWMD 234 70 259 7 < 1 284

** SFWMD 234 177 271 102 111 445

Statewide 205 128 243 163 < 1 1200

C. Floridan aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 130 100 170 101 24 370

SRWMD n.d. n.d. n.d. n.d. n.d. n.d.

** SJRWMD 145 99 188 103 11 866

** SWFWMD 168 114 215 10 106 530

** SFWMD 130 95 162 138 10 287

Statewide 143 102 184 352 10 866
** Calcium Carbonate Alkalinity, mg/L.
++ Data reported in meq/L.

FLORIDA GEOLOGICAL SURVEY

Table 20. Summary of total sulfate distribution (SO,2-, mg/L), by region and aquifer system.

A. Surficial aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD 3.3 1.5 6.0 84 2 <1.0 380.0

SRWMD 4.3 < 1.0 13.0 23 0 < 0.1 33.0

SJRWMD 12.0 3.0 30.0 61 4 < 1.0 597.0

SWFWMD 8.1 < 1.0 50.6 85 9 < 0.1 1480.0

SFWMD 11.8 < 5.0 24.0 614 4 < 1.0 431.0

Statewide 17.0 7.0 39.0 867 15 < 0.1 1480.0

Sand & Gravel 3.6 1.4 5.5 75 2 0.8 380.0

Biscayne 14.0 < 2.0 26.0 257 0 < 1.0 185.0

Other 10.0 < 5.0 24.5 535 13 < 0.1 1480.0

B. Intermediate aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max
NWFWMD <1.0 <1.0 1.9 38 0 <1.0 49.0

SRWMD <1.0 <1.0 4.7 27 0 <1.0 27.0

SJRWMD <1.0 <1.0 4.0 33 1 <1.0 408.0

SWFWMD 36.9 < 1.5 299.0 56 19 < 0.1 1590.0

SFWMD 52.3 14.4 182.0 97 13 2.0 1754.0

Statewide 5.4 < 1.0 65.5 251 33 < 0.1 1754.0

C. Floridan aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max
NWFWMD < 1.0 <1.0 4.4 147 1 <1.0 310.0

SRWMD 6.7 1.7 16.5 157 2 <1.0 2200.0

SJRWMD 8.5 <1.0 83.5 122 13 <1.0 2040.0

SWFWMD 3.0 < 1.0 63.5 169 33 < 0.1 3102.0

SFWMD 176.4 49.3 308.4 135 46 3.3 713.1

Statewide 5.4 < 1.0 53.0 730 95 < 0.1 3102.0

* Number of samples which exceeded Florida Secondary Drinking Water Standards for Sulfate (> 250 mg/L)..

Table 21. Summary of total chloride distribution (Cl-, mg/L), by region and aquifer system.

A. Surficial aquifer system

District Median 1 Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD 7.0 5.0 11.5 84 1 1.8 410.0

SRWMD 6.0 3.0 8.2 25 0 1.4 32.0
SJRWMD 28.0 13.0 91.0 62 12 4.0 1790.0

SWFWMD 12.9 7.0 37.8 86 3 0.6 8520.0

SFWMD 48.3 26.2 83.0 857 48 < 0.4 1100.0

Statewide 40.5 16.0 74.3 1114 64 < 0.4 8520.0

Sand & Gravel 7.1 5.0 11.0 75 0 2.3 220.0

Biscayne 58.0 34.0 79.0 493 33 4.8 700.0

Other 30.5 13.0 74.7 546 31 < 0.4 8520.0

B. Intermediate aquifer system

District Median I Qrtile t Qrtile # Samps # Exc Min Max

NWFWMD 5.3 3.0 8.9 24 0 1.7 58.0

SRWMD 4.5 3.7 21.0 36 0 3.1 54.0

SJRWMD 18.5 12.5 42.5 32 4 7.0 4480.0

SWFWMD 50.0 13.2 204.0 56 13 2.7 940.0

SFWMD 172.0 61.1 580.0 103 42 15.2 2092.5

Statewide 61.9 18.0 334.5 251 59 1.7 4480.0

C. Floridan aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD 6.3 3.8 23.0 101 2 1.7 300.0

SRWMD 8.9 5.0 19.0 220 4 <1.0 5200.0

SJRWMD 28.0 12.0 203.0 122 27 1.0 16270.0

SWFWMD 11.3 7.3 35.4 169 24 1.7 20500.0

SFWMD 419.6 58.6 922.5 136 84 3.5 3785.0

Statewide 21.0 7.6 276.0 748 141 <1.0 20500.0

* Number of samples which exceeded Florida Secondary Drinking Water Standards for Chloride (> 250 mg/L).

SPECIAL PUBLICATION NO. 34

Table 22. Summary of total ortho-phosphate distribution (PO,43 as P, mg/L), by region and aquifer system.

A. Surficial aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD (P) 0.09 0.05 0.23 84 < 0.05 1.20

SRWMD < 0.10 < 0.10 < 0.10 25 < 0.01 0.20

SJRWMD (P) 0.11 0.05 0.32 64 < 0.01 1.82

SWFWMD (t) 0.07 0.02 0.27 82 < 0.01 1.84

SFWMD (f) 0.01 < 0.01 0.02 357 < 0.01 4.00

Statewide 0.06 0.02 0.17 612 < 0.01 4.00

Sand & Gravel 0.09 0.05 0.22 75 0.01 1.20

Biscayne (f) < 0.01 < 0.01 < 0.01 19 < 0.01 0.06

Other 0.07 < 0.05 0.06 518 < 0.01 4.00

B. Intermediate aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD (P) < 0.05 < 0.01 0.11 29 < 0.01 1.20

SRWMD < 0.10 < 0.01 < 0.10 36 < 0.01 2.00

SJRWMD (P) 0.11 0.03 0.21 33 < 0.01 0.43

SWFWMD (t) 0.11 0.04 0.16 52 < 0.01 1.20

SFWMD (f) < 0.01 < 0.01 < 0.01 103 < 0.01 2.28

Statewide 0.04 0.01 0.10 253 < 0.01 2.28

C. Floridan aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD (P) 0.04 0.02 0.06 116 < 0.01 1.60

SRWMD < 0.10 < 0.10 < 0.10 220 < 0.01 21.00

SJRWMD (P) 0.04 0.01 0.11 122 < 0.01 0.75

SWFWMD (t) 0.10 0.05 0.17 152 < 0.01 0.80

SFWMD (f) < 0.01 < 0.01 < 0.01 115 < 0.01 0.15

Statewide 0.04 0.02 0.07 725 < 0.01 21.00

(P) Total Phosphorus (P), mg/L.
(t) Total Phosphate as PO4, mg/L.
(f) Dissolved (filtered) ortho-phosphate, mg/L.

Table 23. Summary of total fluoride distribution (F-, mg/L), by region and aquifer system.

A. Surficial aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD 0.04 0.03 0.07 84 1 < 0.02 5.90

SRWMD < 0.20 < 0.20 < 0.20 25 0 < 0.02 0.79

SJRWMD < 0.10 < 0.10 0.14 63 0 < 0.01 1.75
SWFWMD 0.10 0.04 0.25 84 0 < 0.01 1.95

SFWMD 0.20 < 0.10 0.30 608 0 0.02 3.73

Statewide 0.17 < 0.10 0.28 864 1 <0.01 5.90

Sand & Gravel 0.04 0.03 0.07 75 1 <0.02 5.90

Biscayne 0.20 0.15 0.25 279 0 0.06 0.93

Other < 0.20 < 0.10 0.31 510 0 < 0.01 3.73

B. Intermediate aquifer system

District Median I Qrtile f Qrtile # Samps # Exc Min Max
NWFWMD 0.19 0.13 0.23 25 0 < 0.05 0.53

SRWMD < 0.20 0.20 0.30 36 0 < 0.01 1.00

SJRWMD 0.12 < 0.10 0.30 33 0 < 0.10 1.75

SWFWMD 0.89 0.30 1.15 53 0 0.07 4.00

SFWMD 0.82 0.43 1.30 103 1 < 0.10 4.78

Statewide 0.39 < 0.20 0.97 250 1 < 0.01 4.78

C. Floridan aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD 0.13 < 0.05 0.37 122 1 < 0.10 6.90

SRWMD < 0.20 < 0.20 < 0.20 220 0 < 0.02 2.50

SJRWMD 0.16 < 0.10 0.26 124 0 < 0.10 1.28

SWFWMD 0.16 0.10 0.36 162 0 0.01 2.32

SFWMD 0.81 0.40 1.26 131 0 < 0.10 3.70

Statewide 0.20 0.12 0.41 759 1 < 0.02 6.90

* Number of samples which exceeded Florida Primary Drinking Water Standards for Fluoride (> 4.00 mg/L).

FLORIDA GEOLOGICAL SURVEY

Table 24. Summary of total nitrate distribution (NO3, mg/L as N), by region and aquifer system.

A. Surficial aquifer system

District Median I Qrtile Qrtile # Samps # Exc Min Max

NWFWMD 0.81 0.29 2.00 84 3 0.07 28.00

SRWMD < 0.05 < 0.05 < 0.05 25 0 < 0.05 1.10

** SJRWMD < 0.01 < 0.01 0.03 64 0 < 0.01 7.50

++ SWFWMD < 0.01 < 0.01 0.18 84 1 < 0.01 52.52

SFWMD < 0.01 < 0.01 < 0.01 571 1 <0.01 44.80

Statewide < 0.01 < 0.01 0.01 828 5 < 0.01 52.52

Sand & Gravel 0.95 0.37 2.30 75 3 0.07 28.00

Biscayne < 0.01 < 0.01 < 0.01 239 1 < 0.01 44.80

Other < 0.05 < 0.01 0.03 514 1 <0.01 52.52

B. Intermediate aquifer system

District Median i Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD 0.11 < 0.01 1.20 35 0 < 0.01 6.70

SRWMD < 0.05 < 0.05 < 0.05 36 0 <0.01 7.10

** SJRWMD < 0.01 < 0.01 < 0.01 33 0 < 0.01 0.03

++ SWFWMD 0.01 < 0.01 0.02 52 0 < 0.01 3.50

SFWMD < 0.01 < 0.01 < 0.01 100 0 < 0.01 0.19

Statewide < 0.01 < 0.01 < 0.05 256 0 < 0.01 7.10

C. Floridan aquifer system

District Median I Qrtile Qrtile # Samps # Exc Min Max

NWFWMD 0.90 < 0.01 1.50 123 7 < 0.01 74.00

SRWMD < 0.05 < 0.05 < 0.05 220 0 < 0.01 8.40

** SJRWMD < 0.01 < 0.01 0.03 123 1 < 0.01 18.40

++ SWFWMD 0.01 < 0.01 0.05 153 0 < 0.01 4.64
SFWMD < 0.01 < 0.01 < 0.01 120 0 < 0.01 1.97

Statewide < 0.01 < 0.01 0.05 739 8 <0.01 74.00

* -Number of samples which exceeded Florida Primary Drinking Water Standards for
Nitrate as N (> 10.00 mg/L) or Nitrate as NO, (> 43.00 mg/L).
** Reported as Nitrate + Nitrite (NO), mg/L.
++ Reported as Nitrate as NO3, mg/L.

Table 25 Summary of total dissolved solids concentrations (TDS,mg/L), by region and aquifer system.

A. Surficial aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD 74 46 125 84 3 15 1000

SRWMD 70 45 110 23 0 27 320

SJRWMD 300 151 472 63 14 63 3821

SWFWMD 187 80 336 83 11 1 17700

SFWMD 388 296 513 656 170 26 2537

Statewide 346 181 474 909 198 1 17700

Sand & Gravel 74 45 110 75 2 15 1000

Biscayne 392 316 468 288 55 108 1712

Other 339 160 512 546 141 1 17700

B. Intermediate aquifer system

District Median I Qrtile I Qrtile # Samps # Exc Min Max

NWFWMD 165 130 260 24 0 36 390

SRWMD 100 57 190 27 0 18 350

SJRWMD 355 241 397 33 6 38 6892

SWFWMD 525 286 943 54 28 40 2340

SFWMD 508 417 1427 103 55 47 4188

Statewide 390 219 871 241 89 18 6892

C. Floridan aquifer system

District Median I Qrtile I Qrtile # Samps # Exc Min Max

NWFWMD 200 160 310 101 10 42 810

SRWMD 220 160 300 157 21 40 10200

SJRWMD 342 183 598 123 37 47 24092

SWFWMD 257 176 656 161 47 55 5990

SFWMD 1138 414 2045 138 97 58 7425

Statewide 277 176 715 680 212 40 24092

* Number of samples which exceeded Florida Secondary Drinking Water Standards for Total Dissolved Solids
(TDS) (> 500 mg/L).

SPECIAL PUBLICATION NO. 34

Table 26. Summary of specific conductance distribution (pmhos/cm), by region and aquifer system.

A. Surficial aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 50 35 88 84 15 1522

SRWMD 90 50 160 25 20 500

SJRWMD 335 140 625 49 40 3900

SWFWMD 255 105 450 100 30 24000

SFWMD 619 450 894 378 41 8281

Statewide 475 138 743 636 15 24030

Sand & Gravel 50 34 85 75 15 747

Biscayne 517 415 587 19 383 687

Other 540 220 805 542 15 24000

B. Intermediate aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 229 193 348 24 41 593

SRWMD 160 80 270 36 25 500

SJRWMD 575 390 650 22 150 8000

SWFWMD 600 410 1200 61 50 3325

SFWMD 947 703 2324 100 245 6920

Statewide 650 319 1500 243 25 8000

C. Floridan aquifer system

District Median I Qrtile I Qrtile # Samples Minimum Maximum
NWFWMD 274 216 470 101 81 1542

SRWMD 310 240 450 220 50 15000

SJRWMD 500 282 899 96 70 14500

SWFWMD 378 255 800 194 100 46000

SFWMD 1787 624 3305 131 120 12204

Statewide 385 251 1000 742 50 46000

Table 27. Summary of total organic carbon distribution (TOC,mg/L), by region and aquifer system.

A. Surficial aquifer system

District Median i Qrtile f Qrtile # Samples Minimum Maximum

NWFWMD 6.8 4.2 9.7 84 1.9 42.4

SRWMD 5.9 < 1.0 17.5 23 < 1.0 50.0

SJRWMD 9.0 4.3 16.9 58 < 0.1 257.3

SWFWMD 11.4 3.4 22.1 82 < 0.1 122.0

SFWMD 17.0 9.5 31.1 548 < 0.1 380.0

Statewide 14.0 7.0 27.0 795 < 0.1 380.0

Sand & Gravel 6.6 4.1 8.4 75 1.9 25.2

Biscayne 14.3 8.4 22.7 258 1.0 73.0

Other 16.9 7.8 36.0 462 < 0.1 380.0

B. Intermediate aquifer system

District Median I Qrtile T Qrtile # Samples Minimum Maximum

NWFWMD 6.1 2.1 8.9 26 < 1.0 31.0

SRWMD < 1.0 < 1.0 2.8 27 < 1.0 12.0

SJRWMD 5.5 3.9 7.7 32 1.4 26.4

SWFWMD 9.8 < 1.0 21.6 52 < 0.1 52.3

SFWMD 6.3 2.0 19.0 91 < 0.1 71.0

Statewide 4.8 < 1.0 13.1 228 < 0.1 71.0

C. Floridan aquifer system

District Median I Qrtile I Qrtile # Samples Minimum Maximum

NWFWMD < 1.0 < 1.0 3.2 178 < 0.1 39.0

SRWMD 2.0 < 1.0 6.2 157 < 1.0 34.0

SJRWMD 3.3 1.5 5.4 111 < 0.5 29.0

SWFWMD 16.8 10.4 27.1 150 < 0.1 78.8

SFWMD 1.9 0.5 3.5 114 < 0.1 80.6

Statewide 2.2 < 1.0 7.9 710 < 0.1 80.6

FLORIDA GEOLOGICAL SURVEY

Table 28. List of synthetic organic analyzed in the Background Network, with guidance concentrations or
standards (Florida Department of Environmental Regulation, 1989).

Parameter Parameter Name Units Guidance
Number Concentration*

11
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120

METHYL BLUE ACTIVE SUBSTANCES
CIS/TRANS-12-DICHLOROETHYLENE
METHYL ISOTHIOCYANATE
CIS/TRANS-12-DICHLOROETHYLENE
1,2-BENZISOTHIAZOLE
PETROLEUM HYDROCARBONS
METHYL-TERT-BUTYL-ETHER
METHYL N-BUTYL KETONE
TOTAL TRIHALOMETHANE
METHYL ISO-BUTYL KETONE
4,6 DINITRO O-CRESOL
1,3-DIBROMO-2-CHLOROPROPANE
P-CHLORO M-CRESOL
HEXACHLOROBUTADIENE
2-METHYL-4,6-DINITROTOLUENE
DICHLORODIFLUOROMETHANE
DIBENZO(A,H)ANTHRACENE
1,3-DICHLOROPROPENE
TRICHLOROFLUOROMETHANE
1,1 DICHLOROETHANE
1,1 DICHLOROPROPANE
1,1 DICHLOROETHENE
HEXACHLOROBENZENE
PCB-1254
TRICHLOROPHENOL ISOMERS
PCB-1242
1,1,1 TRICHLOROETHANE
CIS-1,2-DICHLOROETHENE
1,1,2 TRICHLOROETHANE
METHYL ETHYL KETONE
1,1,2,2 TETRACHLOROETHANE
XYLENE
BENZO(G,H,I)PERYLENE
DICHLOROBENZENE
PCB-1232
4-CHLORO-3-METHYL PHENOL

mg/I
jg/I
Rg/I
gg/1
Rg/I
mg/I
jg/I
gg/I
mg/I
jg/l
ag/I
jg/I
gg/I

jig/I
jag/I
jg/i
jg/I
gg/I
jg/I
jg/I
jg/I
gg/I
gg/I
jg/I
jg/I
jg/I
gg/I
jg/I
pg/I
jg/I
jg/I

gg/I
gg/I
Rg/I
jg/I
jg/I

500
mdl
mdl
mdl
mdl
mdl
mdl
mdl
100
350
50
mdl
3,000
10
mdl
mdl
mdl
mdl
mdl
2,400
mdl
7
mdl
0.5
mdl
0.5
200
mdl
1
170
1
50
mdl
75
0.5
mdl

121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156

Parameter Parameter Name Units Guidance
Number Concentration*

TRICHLOROETHENE
PHENANTHRENE
BENZIDINE
DI-N-BUTYL PHTHALATE
VINYL CHLORIDE
TRIMETHYLBENZENE
BIS(2-ETHYLHEXYL)PHTHALATE
PYRENE
ETHYL BENZENE
TETRACHLOROETHENE
CHLOROTOLUENE
VINYL ACETATE
BIS(2-CHLORO-1-METHYL) ETHER
CIS 1,3 DICHLOROPROPENE
HEXACHLOROETHANE
NAPHTHALENE
PHENOL
TRANS 1,3 DICHLOROPROPENE
1,2-DIBROMOETHANE (EDB)
INDENO(1,2,3-CD)PYRENE
1,2,4 TRIETHYL BENZENE
BENZO(B)THIOPHENE
N-BUTYLBENZENE
N-PROPYLBENZENE
BROMOMETHANE
M XYLENE
CHLOROMETHANE
STYRENE
PCB-1016
BROMODICHLOROMETHANE
TOTAL PCB'S
METHYLENE CHLORIDE
BROMOFORM
N-NITROSODI-N-PROPYL AMINE
CHLOROFORM
N-NITROSODIPHENYLAMINE
i

jg/I
jg/I
jg/i
jig/I
jg/I
jg/I
gg/i

lg/I
pg/I
gg/igA
jg/I
gg/I
jig/I
gg/I

jg/I
jg/I
gg/I

jag/I

jg/I
jg/I
gg/I

jg/i
jg/I
jg/I
jg/I
jag/I
Rg/I
jg/I
gg/i
gg/I
jg/I

jg/I
ig/I
jg/I

3
mdl
mdl
mdl
1
10
mdl
mdl
2
3
mdl
mdl
mdl
1
10
mdl
mdl
1
0.2
10
mdl
mdl
mdl
mdl
20
50
3,800
1
0.5
100
0.5
100
100
10
100
10

SPECIAL PUBLICATION NO. 34

Table 28. (cont.) List of synthetic organic analyzed in the Background Network, with guidance concentrations
or standards (Florida Department of Environmental Regulation, 1989).

Parameter Parameter Name Units Guidance
Number Concentration*

157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191

TOTAL PHENOLS
N-NITROSODIMETHYLAMINE
BENZENE
NITROBENZENE
ACENAPHTHENE
ACROLEIN
ACRYLONITRILE
ANTHRACENE
P XYLENE
BENZO(K)FLUORANTHENE
BENZO(A)PYRENE
D-BHC
BIS(2-CHLOROETHYL)ETHER
BIS(2-CHLOROETHOXY)ME. ETHER
BIS(2-CHLOROISOPROPYL) ETHER
BENZYL BUTYL PHTHALATE
CHLOROBENZENE
CHLOROETHANE
DIETHYLPHTHALATE
2,6-DINITROTOLUENE
PCB-1248
1,2 DIPHENYLHYDRAZINE
2,4,6-TRICHLOROPHENOL
2,4-DINITROPHENOL
2,4-DINITROTOLUENE
2,4-DIMETHYLPHENOL
ETHYLBENZENE
FLUORANTHENE
FLUORENE
PCB-1262
112TRICL.122TRIF ETHANE
ACETONE
2378TETRACHLORODIBENZOPDIOXIN
PCB-1221
PCB-1260

Lig/I
gg/I
gg/I
gg/I
ig/I
,Lg/I
gg/I
Lg/I
Lg/I
gg/I
ig/I
gg/I
gg/I
jig/I
Rg/I
gg/I
gg/I
jg/I
gg/I
gg/I
lg/I
gg/I
lg/I
gg/I
gg/1
jg/I
jg/I
gg/I
gg/I
gg/I
gg/I
gg/I
gig/I
jg/I
jg/I

mdl
20
1
mdl
20
110
2.5
10
50
10
10
0.05
10
10
10
1,400
10
6,300
5,600
mdl
0.5
10
mdl
mdl

192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
228
281
282
283
284
289
297

* mdl = Method Detection Limit

Parameter Parameter Name Units Guidance
Number Concentration*

CARBON TETRACHLORIDE
DIBROMOCHLOROMETHANE
BENZOFURAN
BENZO(A)ANTHRACENE
ACENAPHTHYLENE
1,2 DICHLOROETHANE
BENZO(B)FLUORANTHENE
1,2-DICHLOROBENZENE
4-BROMOPHENYL PHENYL
1,2 DICHLOROPROPANE
DIMETHYLPHTHALATE
TRANS 1,2 DICHLOROETHENE
HEXACHLOROCYCLYPENTANE
1,2,4-TRICHLOROBENZE
O-XYLENE
CARBON DISULFIDE
TOLUENE
4-NITROPHENOL
4-CHLOROPHENYL PHENYL ETHER
3,3'-DICHLOROBENZIDINE
DI-N-OCTYL PHTHALATE
2-NITROPHENOL
2,4-DICHLOROPHENOL
1,3-DICHLOROBENZENE
1,4-DICHLOROBENZENE
2 CHLOROETHYL VINYL ETHER
2-CHLORONAPHTHALENE
2-CHLOROPHENOL
CHRYSENE
CHLOROFORM
TRICHLOROETHYLENE,DISSOLVED
VINYL CHLORIDE
M-XYLENE
PHENANTHRENE,DISSOLVED
PCNB

jg/I
jg/I

gg/1
jg/I
pg/I
jg/I
jg/I
gg/I

Rg/I
jg/i
gig/I
gg/I

jig/I
jg/I
jg/I
gg/I

Rg/i
jg/i

pg/I
gg/I
jg/I
ig/1I
gg/I
jg/I

gig/I
jg/I
jg/I
gg/I
Rg/I
jg/I
gg/I
jig/I
jig/I
jig/I

3
100
mdl
mdl
10
3
10
mdl
mdl
1
mdl
4.2
10
mdl
50
mdl
24
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
1
mdl
mdl
10
100
3
mdl
50
mdl
mdl

FLORIDA GEOLOGICAL SURVEY

Table 29. Summary of total synthetic organic concentrations (gg/L), by region and aquifer system. Most
detections were not confirmed by resampling.

Table 30. Classification of anthropogenic organic according to volatility in water. Modified from
Lyman et al. (1982).

A. Surficial aquifer system

District Median I Qrtile t Qrtile # Samps # Exc Min Max

NWFWMD 0.00 0.00 < 0.00 109 3 0.00 190.00

SRWMD 0.00 0.00 0.00 22 0 0.00 1.00

SJRWMD < 0.50 < 0.50 < 0.50 58 4 < 0.50 128.00

SWFWMD < 1.00 <1.00 < 1.00 83 2 <1.00 6.70

SFWMD 0.00 0.00 < 1.00 392 35 0.00 995.00

Statewide 0.00 0.00 < 1.00 664 44 0.00 995.00

Sand & Gravel 0.00 0.00 < 1.00 93 3 0.00 190.00

Biscayne < 10.00 <1.00 < 10.00 21 2 <1.00 12.00

Other 0.00 0.00 < 1.00 550 39 0.00 995.00

B. Intermediate aquifer system

District Median I Qrtile I Qrtile # Samps # Exc Min Max

NWFWMD 0.00 0.00 <1.00 26 0 0.00 3.50

SRWMD 0.00 0.00 0.00 24 0 0.00 0.00

SJRWMD 0.00 0.00 0.00 27 0 0.00 0.92

SWFWMD <1.00 <1.00 <1.00 52 0 <1.00 1.60

SFWMD <1.00 0.00 <1.00 107 3 0.00 2.10

Statewide 0.00 0.00 <1.00 236 3 0.00 3.50

C. Floridan aquifer system

District Median I Qrtile I Qrtile # Samps # Exc Min Max

NWFWMD 0.00 0.00 0.00 118 0 0.00 2.70

SRWMD 0.00 0.00 0.00 301 4 0.00 14.00

SJRWMD < 0.50 < 0.50 < 0.50 110 5 0.00 20.20

SWFWMD <1.00 <1.00 <1.00 168 12 0.00 70.01

SFWMD 0.00 0.00 0.00 133 5 0.00 3.90

Statewide 0.00 0.00 0.00 830 26 0.00 70.01

Henry's Law Constant Volatility
(atm m /mol)

<10'7 Low

107- 10-5 Slight

105 10-3 Moderate

> 10-3 High

Table 31. Classification of synthetic organic mobility in water. Modified from Fetter (1988).

Mobility Solubility Ko
(mg/L)

Very mobile miscible = 1

Very mobile > 4,000 1 50

Mobile 4,000 =1,000 50 150

Moderately mobile =1,000 = 100 150 500

Low mobility = 100 = 10 500 2,000

Slight mobility = 10 = 0.25 2,000 20,000

Immobile <= 0.25 >20,000

SPECIAL PUBLICATION NO. 34

Table 32. List of pesticides analyzed in the Background Network, as of 1989, with guidance
concentrations or standards.

Parameter Parameter Name Units Guidance
Number Concentration*

CHLOROPICRIN
PROPAZINE
AZINPHOS METHYL
ENDRIN-ALDEHYDE
ENDOSULFAN I
ENDOSULFAN II
ENDOSULFAN-SULFATE
ETHOPROP
ALACHLOR
CHLORPYRIFOS
NORFLURAZON
ISOFENPHOS
DICAMBA
CHLOROTHALONIL
METHYL PARATHION
ISOPHORONE
CARBARYL
METAM-SODIUM
ETHYL PARATHION
KELTHANE
NALED
OXAMYL
DALAPON
TERBUTRYN
DICHLORAN
TRIADEMEFON
METHIOCARB
METHAMIDOPHOS
PROPOXUR
FENAMIPHOS
CHLORPYRIFOS
BENFLURALIN
1,2-DIBROMO-3-CHLOROPROPANE
STROBANE
HEXAZINONE
PENTACHLOROPHENOL
DALAPON

7.3
mdl
mdl
0.1
0.4
0.4
0.3
mdl
1.5
mdl
mdl
mdl
mdl
mdl
mdl
1050
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl

Parameter Parameter Name Units Guidance
Number Concentration*

ATRAZINE
PCNB
PERTHANE
2,4-DB
METALAXYL
LINURON
TERBUTHYLAZINE
DINOSEB
METHOMYL
ALDICARB
SIMAZINE
PROMETON
PROMETRYN
DIQUAT DIBROMIDE (REGLONE)
TRIFLURALIN
PENDIMETHALIN
PERMETHRIN
DIBENZOFURAN
O,P DDE
O,P DDT
4,4'-DDD
DIELDRIN
ALDRIN
ENDRIN
B-BHC
ETHION
CHLORDANE
TOXAPHENE
4,4'-DDT
ot-BHC
f-BHC
O,P DDD
4,4'-DDE
HEPTACHLOR
HEPTACHLOR-EPOXIDE
ISODRIN
CHLOROBENZILATE

FLORIDA GEOLOGICAL SURVEY

Table 32. (cont.) List of pesticides analyzed in the Background Network, as of 1989, with guidance
concentrations or standards.

Parameter Parameter Name Units Guidance
Number Concentration*

295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
373
374
391
392

METHOXYCHLOR
CARBOFURAN
METRIBUZIN
ALDICARB SULFOXIDE
ALDICARB SULFONE
3-HYDROXYCARBOFURAN
DISULFOTON
MIREX
2,4-D
MALATHION
PARATHION ETHYL
2,4,5-TP (SILVEX)
DIAZINON
DCPA
ATRAZINE
DICOFOL
DIURON
LINDANE
PICLORAM
TRITHION
DEMETON
CAPTAIN
CARBOPHENOTHION
GUTHION
TEDION
MEVINPHOS
DIQUAT
TERBUFOS
AMETRYN
BROMACIL
PARAQUAT
TOTAL ARSENIC
ARSENIC,DISSOLVED
BENTAZON, TOTAL
SEVIN,TOTAL

100
36
200
10
40
mdl
mdl
3.5
100
mdl
mdl
10
10
4000
mdl
mdl
10
4
mdl
12
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
mdl
90
30
50
50
mdl
mdl

* mdl = Method Detection Limit

Table 33. Summary of total pesticide concentrations (pg/L), by region and aquifer system. Most detections
were not confirmed by resampling.

A. Surficial aquifer system

District Median I Qrtile I Qrtile # Samps # Exc Min Max

NWFWMD < 2.00 < 2.00 < 2.20 84 1 < 1.00 100.00

SRWMD <10.00 <10.00 <10.00 22 0 <10.00 50.00

SJRWMD <1.00 1.00 4.00 22 0 <1.00 27.00

SWFWMD <1.00 <1.00 <1.00 83 15 <1.00 32.40

SFWMD 0.00 0.00 <1.60 327 1 0.00 1100.00

Statewide < 0.01 0.00 < 0.50 538 17 0.00 1100.00

Sand & Gravel < 2.00 < 2.00 < 2.40 71 1 < 2.00 100.00

Biscayne < 0.03 < 0.03 < 0.03 13 0 0.00 1.40

Other < 1.50 < 0.50 < 2.00 454 16 0.00 1100.00

B. Intermediate aquifer system

District Median I Qrtile I Qrtile # Samps # Exc Min Max

NWFWMD < 2.00 < 2.00 < 2.00 31 0 < 1.00 9.20

SRWMD 0.00 0.00 0.00 24 0 0.00 0.00

SJRWMD 0.00 0.00 1.00 27 0 <1.00 5.00

SWFWMD < 0.01 < 0.01 < 0.01 34 14 0.00 1.80

SFWMD < 1.20 < 0.90 < 1.50 92 0 < 0.01 < 30.00

Statewide < 0.50 0.00 < 0.50 208 14 0.00 9.20

C. Floridan aquifer system

District Median I Qrtile T Qrtile # Samps # Exc Min Max

NWFWMD <2.00 <2.00 <2.00 171 0 < 1.00 14.00

SRWMD <10.00 <10.00 <10.00 299 0 <1.00 30.00

SJRWMD <1.00 <1.00 <1.00 53 2 <1.00 66.00

SWFWMD < 0.01 < 0.01 < 0.01 167 35 0.00 70.01

SFWMD < 1.30 < 0.90 < 1.60 108 0 < 0.70 4.20

Statewide < < 0.01 < 0.01 < 0.50 798 37 0.00 70.01

* Total Arsenic values only (no organic pesticides sampled).

gg/I
lag/I
gg/I
lig/I
gg/I
jIg/I
lgg/I
jg/I
Rg/I
gg/I
jg/I
jg/I
ig/I
lag/I
jg/I
[g/I
ig/I
jg/I
jg/I
gg/I
gg/I
ig/I
gg/I
jg/I
jg/I
gg/I

SPECIAL PUBLICATION NO. 34

Table 34. Some arsenic-based pesticides and their uses (from data in Carapella, 1968).

Pesticide Use

Calcium arsenate Insecticide, herbicide

Lead arsenate Insecticide

Sodium arsenite Herbicide, fungicide,
aquatic weed control,
animal dips for tick control

Sodium arsenate Wood preservative

Disodium methylarsonate Herbicide

Ammonium methane arsonate Herbicide

Table 35. Proportions of major ions within the trilinear-diagram fields on the Predominant Water Type
Maps. Based on the classification of Davis and DeWiest (1966). Percentages are based on total major ion
content, in milliequivalents per liter.

Cation Trilinear Diagram

Cation Percentage
Water Type Calcium Magnesium Sodium Dominant
Ion

A 60-100 0-40 0-40 Ca

B 40-60 40-60 0-20 Mixed
Ca-Mg

C 0-40 60-100 0-40 Mg

D 0-20 20-60 20-60 Mixed
Mg-Na

E 0-40 0-40 60-100 Na

F 40-60 0-20 20-60 Mixed
Ca-Na

G 20-60 20-60 20-60 Mixed
Ca-Mg-Na

Anion Trilinear Diagram

Anion Percentage
Water Type Bicarbonate Sulfate Chloride Dominant
Ion

1 60-100 0-40 0-40 HCO,

2 40-60 40-60 0-20 Mixed
HCO3-SO4

3 0-40 60-100 0-40 SO4

4 0-20 20-60 20-60 Mixed
SO4-CI

5 0-40 0-40 60-100 CI

6 40-60 0-20 20-60 Mixed
HCO3-CI

7 20-60 20-60 20-60 Mixed
HCO3-SO4
-CI

FLORIDA GEOLOGICAL SURVEY

Table 36. Some possible criteria for identification of aquifer flow system components. Assumes that water
comes in contact with carbonate minerals along the flow path.

Analyte Recharge Discharge
Areas Areas

Temperature Locally variable, Low variability,
relatively cool relatively warm

pH Generally acidic, Slightly basic,
locally variable low variability

Calcium, Concentrations Concentrations
magnesium, relatively low, relatively high,
bicarbonate highly variable low variability

Iron Concentrations Concentrations
relatively high may be low

Nitrate May be present Normally absent

Phosphate May be present Normally absent

Synthetic May be present Normally absent
organic,
pesticides

Sulfate Low in most areas, High when deep
possibly high near flow system waters
wetlands discharge in
coastal areas

Sodium:chloride Near that of May differ
ratio sea water greatly from sea
water

Total organic Concentrations often Concentrations often
carbon high low

Table 37. Percent of samples that exceeded water quality standards in Florida aquifers.

1 Summary is for 172 different pesticides in SWFWMD and SFWMD only. Only arsenic was determined
in the other water management districts.

Analyte Surficial Intermediate Floridan
Aquifer Aquifer Aquifer
System System System

pH 37 16 14
Sodium 4 23 17

Iron 75 42 49

Mercury 2 3 0.9

Lead 8 8 9

Sulfate 2 13 13
Chloride 6 24 19

Fluoride 0.1 0.4 0.1

Nitrate 0.6 0 1

Total
Dissolved 22 37 31
Solids

Synthetic 7 1 3
Organics

Pesticides' 3 7 5

SPECIAL PUBLICATION NO. 34

Floridan
aquifer
system

(875 wells)

BACKGROUND NETWORK WELLS

surficial
aquifer
system

Quality Monitoring

1642 wells sampled as of March,

intermediate
aquifer
system

(190 wells)

(577 wells)

Program
1990

A
-N-

it

aI10

Background Network Wells Sampled as of March, 1990

Ground Water

Figure 1.

	Front Cover
	Errata
	Title Page
	Letter of transmittal
	Acknowledgement
	Table of Contents
	List of Figures
	List of Tables
	Introduction
	Data collection and management...
	Hydrostratigraphy
	Quality of water in Florida's aquifer...
	Conclusions and recommendation...
	Reference
	Tables 1-37
	Figures 1-58
	Appendix 1: Additional sources...
	Appendix 2: Ground water quality...
	Appendix 3: Geomorphic features...

MILLISECOND	CLASS.METHOD	MESSAGE
0	sobekcm_page_globals.constructor
0	sobekcm_page_globals.constructor	Application State validated or built
0	sobekcm_database.verify_item_lookup_object
0	sobekcm_page_globals.constructor	Navigation Object created from URI query string
0	sobekcm_database.verify_item_lookup_object
0	sobekcm_page_globals.display_item	Retrieving item or group information
0	sobekcm_page_globals.get_entire_collection_hierarchy	Retrieving hierarchy information
0	sobekcm_assistant.get_entire_collection_hierarchy
0	cached_data_manager.retrieve_item_aggregation
0	cached_data_manager.retrieve_item_aggregation	Found item aggregation on local cache
0	item_aggregation_builder.get_item_aggregation	Found 'all' item aggregation in cache
0	system.web.ui.page.page_load (ufdc.page_load)
0	sobekcm_page_globals.constructor.on_page_load
0	html_echo_mainwriter.add_style_references	Adding style references to HTML
0	html_echo_mainwriter.add_text_to_page	Reading the text from the file and echoing back to the output stream
57	html_echo_mainwriter.add_text_to_page	Finished reading and writing the file