NORTHEASTERN MANASOTA BASIN
Southwest Florida Water Management District
Manasota Basin Board
Seaburn and Robertson, Inc.
Water Resources Consultants
SEABURN AND ROBERTSON, INC.
Water Resources Consultants
offics:cutive Square ALTON F. ROBERTSON, P. E.
5510 Gray Street GERALD E. SEABURN, Ph.D., P.E.
Tampa. Florida 33609
meln.o Bod3184 September 2, 1980
Tampa, Florida 33623
Manasota Basin Board Of The
Southwest Florida Water Management District
5060 U.S. Highway 41, South
Brooksville, Florida 33512
Re: Northeastern Manasota Basin Hydrologic Investigation
SRI Project No. 78152
Seaburn and Robertson, Inc. takes pleasure in transmitting to you our report
entitled "Northeastern Manasota Basin Hydrologic Investigation." This report
summarizes the findings of our investigation to locate favorable areas in the
Northeastern Manatee County for withdrawal of potable groundwater. -This
study was authorized by the Manasota Basin Board and the Southwest Florida
Water Management District on September 29, 1978. The objectives of the study
were to assess the hydrogeology, water quality and ecology of the project
area, to locate suitable groundwater supplies for regional-scale water supplies,
and to predict the impacts expected from withdrawals at these selected sites.
The project area, consisting of about 400 square miles in northeastern Manatee
County is presently devoted largely to agriculture which uses considerable
amounts of irrigation water. Water quality in the Floridan aquifer is the
principal limiting factor for groundwater development for potable water supply.
Sulfate, fluoride, and total dissolved solids exceed drinking water limits in
many parts of the area and these constituents also increase in concentration
with depth in the aquifer.
Four sites were delineated in the project area as suitable areas for large
scale groundwater withdrawals. The amounts of water available from each site
ranged from 12 to 30 mgd. The total amount of water available from all four
sites is about 45 mgd. These four areas were further subdivided into nine
subareas for more detailed study.
Withdrawals at the rates indicated above from these areas are not expected to
have any impact on the surface waters (streams and lakes) nor on the environ-
mentally sensitive parts of the project area because of the extensive and
highly effective confining unit overlying the Floridan aquifer. This confining
unit effectively retards vertical groundwater movement between the surface
water aquifer and the underlying Floridan aquifer.
Manasota Basin Board Of The
Southwest Florida Water Management District
September 2, 1980
It was a distinct pleasure for Seaburn and Robertson, Inc. to conduct this
important water resource planning study and we hope that the results can be
used to guide the rational development of water resources in the Manasota
Basin. We appreciate the support of the Southwest Florida Water Management
District staff as well as the District and Basin Board Members during the
preparation of this report.
SEABURN AND ROBERTSON, INC.
Water Resources Consultants
Alton F. Robertson, P.E.
John E. Darling, Jr.
SEABURN AND ROBERTSON, INC. si
TABLE OF CONTENTS
LETTER OF TRANSMITTAL ii
LIST OF ILLUSTRATIONS vi
LIST OF TABLES x
FACTORS FOR CONVERTING ENGLISH UNITS TO METRIC UNITS xi
EXECUTIVE SUMMARY 1
Geologic Conditions Favorable for Groundwater Development 4
Hydrologic Conditions Favorable to Groundwater Development 5
Water Quality Conditions Favorable for Groundwater Development 7
Purpose and Scope 21
Description of the Project Area 25
Previous Investigations 27
Investigation Approach 31
Well Inventory and Water Quality Sampling 33
Aquifer Performance Testing 33
Soil Borings 33
Geophysical Logging 33
Aerial Photogrammetry 34
Environmental Assessments 34
Other Hydrologic Information 34
General Description 40
Stream Patterns 44
Karst Activity 46
TABLE OF CONTENTS (Continued)
Lithology and Stratigraphy 53
Geologic Conditions Favorable for Groundwater Development 68
Surface Water 73
Aquifer Descriptions 78
Aquifer Withdrawal Tests 79
Potentiometric Heads and Groundwater Flow 84
Water Use 95 .
Hydrologic Conditions Favorable to Groundwater Development 99
WATER QUALITY 104
Water Quality Sampling Program 108
Well Inventory 108
Surface Water Quality Sampling 108
Groundwater Quality Sampling 112
Results of the Water Quality Sampling Program 114
Surface Water Quality Analyses 114
Groundwater Quality Analyses 120
Variations in Groundwater Quality With Depth and Formation 134
Stability of Potable/Non-Potable Groundwater Interface 144
Water Quality Conditions Favorable for Groundwater Development 150
Endangered Biota 165
Environmentally Sensitive Areas 166
TABLE OF CONTENTS (Continued)
LOCATION AND EVALUATION OF POTABLE GROUNDWATER SUPPLY AREAS 173
General Hydrogeologic Constraints 173
Water Quality Constraints 175
Areas Selected 177
Conjunctive Use 183
OPTIMUM GROUNDWATER AVAILABILITY 189
Digital Model 189
Applications of the Model 192
Description of the Modeled Area 192
Results of Simulated Pumping 195
Description of Simulations 196
Simulation Check 204
Groundwater Availability 204
CONCLUSIONS AND RECOMMENDATIONS 211
REFERENCES CITED 217
SELECTED BIBLIOGRAPHY 222
I AQUIFER TEST DATA 232
II SELECTED VEGETATION COMMUNITY MAPS 248
III DIGITAL MODEL MATRICES 256
LIST OF ILLUSTRATIONS
1 Location of Project Area. 23
2 Map of Project Area Showing Location of Major Streams 26
3 Soil Distribution and Associations in the Northeast 39
4 Physiographic Map of the Northeastern Manasota Basin 42
Study Area and Vicinity.
5 Karst Features and Stream Patterns in the Northeastern 45
Manasota Basin Study Area.
6 Major Lineaments Mapped in the Northeastern Manasota Basin 51
7 Geologist Logs Borehole Locations. 56
8 Cross Section Location of Gamma Ray Geophysical Logs. 57
9 Natural Gamma Ray Geophysical Logs in Cross-Section, 58
Northeast Manasota Basin.
10 Structure Contour Lines on Top of the First Dolostone 62
Unit, Hawthorn Formation.
11 Structure Contour Lines on Top of the Sandy Micritic 63
Limestone, Tampa Limestone.
12 Structure Contour Lines on Top of the Calcarenitic Lime- 64
stone, Suwannee Formation.
13 Structure Contour Lines on Top of the Foram Zone, Ocala 65
14 Structure Contour Lines on Top of the "Hard Brown Lime", 66
Avon Park Formation.
15 Location of Precipitation Stations in the Northeast 70
Manasota Basin Vicinity.
LIST OF ILLUSTRATIONS (Continued)
16 Average Monthly Precipitation and Evaporation Northeast 72
17 Location and Extent of Drainage Basins. 74
18 Location of Soils Borings and Aquifer Test Sites, 81
Northeast Manasota Basin.
19 Well Location Maps for Aquifer Test Sites 4 and 5. 82
20 Estimated Water Table Surface, November-December, 1979. 85
21 Estimated Potentiometric Surface of the Hawthorn Formation, 87
22 Potentiometric Map of the Floridan Aquifer, 1934, Central 89
and Southern Florida.
23 Potentiometric Surface of the Floridan Aquifer, Sept., 90
1954 and June, 1955, Manatee County, Florida.
24 Potentiometric Surface, Eastern Manatee County, Florida, 91
25 Potentiometric Surface, Eastern Manatee County, Florida, 92
26 Estimated Potentiometric Head Difference Between the 94
Hawthorn and Suwannee-Ocala-Avon Park Units, 1978.
27 Areas of Most Favorable Aquifer Hydraulic Characteristics. 103
28 Location of Data Collection Sites in Northeast Manasota 109
Basin Study Area.
29 Sulfate Concentrations for Selected Surface Water Sites, 117
1978 and 1979.
30 Fluoride Concentrations for Selected Surface Water Sites, 118
1978 and 1979.
31 Total Dissolved Solids Concentrations for Selected Surface 119
Water Sites, 1978 and 1979.
32 Composite Map of Sulfate Ion Concentrations in the Floridan 125
LIST OF ILLUSTRATIONS (Continued)
33 Sulfate Concentrations in Selected Wells. 127
34 Composite Map of Total Dissolved Solids Concentrations 128
in the Floridan Aquifer, 1978-1979.
35 Total Dissolved Solids Concentrations in Selected Wells. 129
36 Composite Map of Fluoride Ion Concentrations in the 131
Floridan Aquifer, 1978-1979.
37 Fluoride Concentrations in Selected Wells. 132
38 Location of Wells for Water Quality Versus Depth Data. 137
(From Sutcliffe and Joyner 1968)
39 Selected Water Quality Parameter Concentrations Versus 138
40 Selected Water Quality Parameter Concentrations Versus 139
41 Selected Water Quality Parameter Concentrations Versus 140
42 Fence Diagram Showing Stratigraphy and Water Quality, 145
Northeast Manasota Basin.
43 Locations of Cross-Sections for Stratigraphy and Water 146
Quality Fence Diagram.
44 Areas Showing Well Penetration for Potable Groundwater 149
45 Composite Map of Groundwater Quality Conditions. 151
46 Environmentally Sensitive Areas. 170
47 Areas of Favorable Hydrologic Characteristics. 176
48 Areas Suitable for Future Potable Groundwater Supply 180
49 Area of Simulated Drawdown in the Potentiometric Surface 199
of the Floridan Aquifer that Results from Pumping at the
LIST OF ILLUSTRATIONS (Continued)
50 Area of Simulated Drawdown in the Potentiometric Surface 201
of the Floridan Aquifer that Results from Pumping at the
51 Area of Simulated Drawdown in the Potentiometric Surface 202
of the Floridan Aquifer that Results from Pumping at the
52 Area of Simulated Drawdown in the Potentiometric Surface 203
of the Floridan Aquifer that Results from Pumping at the
53 Detailed Sites Suitable for Future Potable Groundwater 205
54 Ratio of Drawdown Versus Pumping Rate at Various Distances. 206
55 Drawdown Produced at Various Distances by a Pumping Well. 207
A Areas of Most Favorable Aquifer Hydraulic Characteristics. 6
B Composite Map of Groundwater Quality Conditions. 8
C Environmentally Sensitive Areas. 10
D Areas Suitable for Future Potable Groundwater Supply 12
E Detailed Sites Suitable for Future Potable Groundwater 13
LIST OF TABLES
1 Soil Association in the Northeast Manasota Basin 41
2 Lithology of Geologic Formations in the Northeast 55
3 Summary of Average Rainfall Amounts for Selected Stations 71
in the Vicinity of the Project Area.
4 Summary of Streamflow Data in the Vicinity of Northeast 75
5 Estimated Average Annual Streamflow from the Project Area. 76
6 Summary of Hydraulic Coefficients, Determined from Aquifer 80
Tests, in the Northeast Manasota Basin.
7 Summary of Estimated Groundwater Use for Agricultural 96
Crops in Manatee County, Florida, 1978.
8 Summary of U.S.G.S. Water Quality Data, Northeast 105
9 Well Inventory Data for Wells Sampled in the Northeast 110
10 Results of Analyses of Groundwater Samples Collected in 115
October, 1978, and March, June and October, 1979, at
Selected Sites in the Northeast Manasota Basin.
11 Results of Chemical Analyses of Groundwater Samples 121
Collected in October, 1978, and March, June and October,
1979 at Selected Sites in the Northeast Manasota Basin.
12 Results of Analyses of Downhole Samples, Northeast 135
13 Endangered, Threatened, Rare and Species of Special 167
Concern of Manatee County, Florida, 1980.
14 Results of Simulated Withdrawals in Northeast Manasota 197
FACTORS FOR CONVERTING
ENGLISH UNITS TO METRIC UNITS
Square Miles (smi)
Gallons Per Minute (gpm)
Million Gallons Per Day (mgd)
Cubic Feet Per Second (cfs)
Gallons Per Day Per Foot
Gallons Per Day Per Square
Gallons Per Day Per Cubic
Miles Per Hour (mph)
Feet Per Day (ft/day)
Feet Per Mile (ft/mi)
3.048 x 10-1
4.047 x 10-3
1.233 x 103
6.309 x 10-2
4.381 x 10-1
2.832 x 10-1
8.053 x 10-1
4.057 x 10-2
7.482 x 10-3
Cubic Meters (m3)
Liters Per Second
Liters Per Second
Liters Per Second
Square Meters Per Day (m2/d)
Meters Per Day (m/d)
Liters Per Day Per Cubic
Meter (1/d) /m3
Kilometers Per Hour (kph)
Meters Per Day (m/d)
Meters Per Kilometer (m/k)
NORTHEASTERN MANASOTA BASIN HYDROLOGIC INVESTIGATION
With the addition of the Manasota Basin to the Southwest Florida Water
Management District in 1977, the District staff and the Manasota Basin
Board have begun the development of a Comprehensive Basin Water Management
Program. One element of this program involves the location and evaluation
of potential potable groundwater supply sites.
On September 29, 1978, the Manasota Basin Board and the Southwest Florida
Water Management District authorized Seaburn and Robertson, Inc. to conduct
the Northeastern Manasota Basin Hydrologic Investigation. The purpose of
this investigation was to locate the most favorable areas in northeastern
Manatee County for withdrawal of potable groundwater. Existing private
uses of water for citrus irrigation, phosphate mining, domestic supplies
and others were not to be considered limiting factors to the location of
potable groundwater supplies during this study because the objective was
to locate hydrologically favorable areas.
The project area consists of about 400 square miles in northeastern Manatee
County. Land use in this area is principally agricultural. Vegetable
farming, citrus and cattle production are the major products.
The objective of the investigation was to assess the hydrogeology, water
quality, and ecology of the project area, to locate suitable groundwater
supply areas for regional-scale water supplies, and to predict the impacts
expected from withdrawals at these selected sites. Specific elements
of the investigation were to:
1. Evaluate the optimum quantities and quality of potable ground-
water that could be developed within the project limits.
2. Evaluate the effects of withdrawals on the potentiometric surface
of the Floridan aquifer system, surficial aquifer levels, and
surface water bodies.
3. Evaluate the possibility of saltwater intrusion or water quality
degradation by groundwater withdrawals.
4. Evaluate the possibilities of augmenting groundwater supplies
by artificial recharge from surface water sources where feasible
within the project limits.
5. Evaluate impacts to the hydrologic and environmental systems
resulting from groundwater withdrawals or conjunctive use.
6. Evaluate the areas within the project limits most suited to
future potable groundwater development.
The study was divided into two phases. The preliminary phase consisted
of a review and evaluation of available hydrologic data pertinent to the
project area. The preliminary phase was completed in February, 1979, and
a report entitled "Status Report Northeastern Manasota Basin Hydrologic
Investigation", was submitted to summarize the available data and to indi-
cate the scope of work for the second phase. That second phase, which
consisted of detailed field data collection and evaluation for selected
areas is the subject of this report.
Field data collection included three aquifer pumping tests, geophysical
well logging of ten wells, 81 soils borings, inventory of approximately
400 wells, quarterly water quality sampling of about 60 wells, and environ-
mental mapping of sensitive vegetation communities. Extensive hydrologic
and water quality evaluations were conducted. Computer simulations of
groundwater withdrawals from selected sites within the project area were
Geologic information was collected from the literature, well construction
information, drillers' logs and from geophysical logging of selected wells.
Additional geologic information included the study of surface depressions
and photolineaments (fracture traces). These were assembled to assist in
locating suitable groundwater supply areas.
Available data about aquifer hydraulics were assembled and supplemented
by site-specific aquifer pumping tests conducted during this investigation.
These data were used in simulations by the digital model to evaluate
aquifer response to groundwater pumping. The areal extent and distribu-
tion of the aquifers within the project area were mapped.
Water quality was determined by sampling wells within the project area
and mapping of selected water quality parameters. The water quality was
a significant aspect to locating potable groundwater supply areas. The
vertical and horizontal extent of selected water quality constituents were
An overall environmental assessment of the project area was conducted, and
a more detailed assessment of environmentally "sensitive" vegetation com-
munities was performed for selected areas.
The Pinder-Trescott digital computer program was calibrated for the area
and used to simulate water level response to selected pumping rates at
The most favorable areas for the development of potable groundwater
supplies were determined by studying a series of composite maps. The
maps contain hydrogeologic, water quality and ecologic data and indicate
areas with the most favorable conditions for potable water development
in each of these categories. These maps became a decision matrix and
were used to locate and rank the suitability of the areas according to
the particular category.
The geologic aspects of the project area were found to be of little
consequence in site selection. These aspects are similar throughout the
area and the presence of extensive confining units (dense clays and lime-
stones) will generally enhance groundwater development by isolating the
surficial aquifer and eliminating or reducing the impacts on terrestrial
and aquatic vegetation.
Geologic Conditions Favorable for Groundwater Development
Results of the geologic investigation indicate four general conditions
favorable to development of groundwater. They are:
1. Limited recent sinkhole development,
2. Hydraulically inactive fracture systems that do not allow inter-
change of surface and ground water,
3. Extensive and permeable carbonate rock systems consisting of a
combination of the Suwannee/Ocala/Avon Park units, and
4. Extensive confining strata composed of the Hawthorn/Tampa
units separating the artesian and surficial aquifers.
Tests to determine aquifer hydraulics which were conducted during this
investigation indicate that the most productive section of the Floridan
aquifer is the Suwannee/Ocala/Avon Park units. Overlying units con-
taining surficial sands and clays and the Hawthorn/Tampa formations
are much less productive although limited production can be obtained from
these upper aquifers. Figure A indicates the areas containing the most
favorable aquifer hydraulic characteristics such as high transmissivity
and low leakance. Additionally, an area of apparent rejected recharge
is indicated in the southern part of the project area.
Hydrologic Conditions Favorable to Groundwater Development
There are seven hydrologic conditions favorable to major groundwater develop-
ment in selected parts of the project area.
1. Moderate to high transmissivity associated with the Suwannee/
Ocala/Avon Park aquifer unit,
2. Considerable differences of water levels between the Hawthorn
and Suwannee/Ocala/Avon Park especially near the headwaters of
the Manatee River,
AREA OF FAVORABLE ....
0 | \ PROJECT BOUNDARY
2- N t
SCALE, MILES i BS FROM DOT MAP ULY, 1974)ABURN AND ROBERTSON,IN
SEABURN AND ROBERTSON, INC.
FIGURE A. -AREAS OF MOST FAVORABLE AQUIFER HYDRAULIC CHARACTERISTICS.
3. Discharge (rejected recharge) from the Hawthorn in the vicinity
of Tatum Sawgrass,
4. Availability of surface water for possible dilution of mineralized
5. Availability of Manatee River stream bed for transportation of
groundwater to treatment and distribution facilities,
6. Low leakance from surficial aquifers to underlying producing
7. Extensive quantities of groundwater beneath the project area.
Water Quality Conditions Favorable for Groundwater Development
Water quality conditions were found to be the limiting factor to the
development of potable groundwater supplies. In parts of the project area,
the most productive units of the Floridan aquifer (Suwanee/Ocala/Avon
Park units) contain waters with sulfate, total dissolved solids and
fluoride concentrations exceeding potable water standards. The Hawthorn/
Tampa formations generally contain potable water with the exception that
fluoride concentrations, greater than 1.4 mg/l, are evident in the north-
eastern.part of the project area. Figure B shows the areas of favorable
water quality conditions within the Suwanee/Ocala/Avon Park units of the
Specific water quality conditions favorable to potable water development
are easily determined by comparison to published and accepted standards.
Three general conditions were found to be important:
1. Stability of the potable/non-potable "interface" during
MOST FAVORABLE AREA
F MARGINALLY FAVORABLE
LEAST FAVORABLE AREA
00.51 2 3 ___
SCALE, MILES I BSE FROM DOT MAP (JULY. 1974)
SEABURN AND ROBERTS
FIGURE B. -COMPOSITE MAP OF GROUNDWATER QUALITY CONDITIONS.
2. The degree to which potable standards may be exceeded, and
3. Water quality characteristics were independent of pumping
or seasonal fluctuations in water levels.
An assessment of the ecologic character of the project area was conducted
to locate areas containing sensitive environmental communities and to
identify endangered biota within the project area. Figure C shows the
general extent of environmentally sensitive areas. Detailed vegetation
community mapping was also conducted in selected areas. However, because
of the extensive confining clay and dense limestones underlying the
project area, these sensitive areas will not be impacted by groundwater
withdrawals from the Suwannee/Ocala/Avon Park units.
Possible conjunctive use of surface water and groundwater was evaluated
as a factor for locating potable groundwater supply areas. For the
purpose of this report conjunctive use was defined as the recharge of
groundwater sources from surface water or surficial aquifer sources.
Because of the relatively impermeable confining units existing in the
project area, the use of surface waters for recharge through natural methods,
such as increasing head gradients, is not effective. Artificial methods
of recharge by connector wells, recharge wells or injection facilities is
hydrologically feasible as a water management procedure in the north-
eastern Manasota Basin. These methods, however, will require detailed
site-specific evaluations to determine the most appropriate and cost-
effective method of conjunctive use.
SEE APPENDIX I FOR
SELECTED COMMUNITY MAPS
00.51 C 2 3 4
SCALE, MILES I
FIGURE C. -ENVIRONMENTALLY SENSITIVE AREAS.
Four major areas were designated as areas most favorable for development
of large (regional-scale) quantities of potable groundwater. These areas,
shown on Figure D, were ranked according to the various factors evaluated.
Surrounding the favorable areas are marginally favorable areas which are
limited by various constraints, but principally by water quality condi-
tions where constituents approach or exceed potable water standards.
Outside of the marginally favorable areas the remainder of the project
area is generally unfavorable for reliable, long-term potable groundwater
supplies because of poor water quality conditions within the major pro-
ducing units of the Suwannee/Ocala/Avon Park.
The digital computer model used to evaluate the impacts expected for each
site at selected pumping rates indicated optimum rates that ranged from 12 mgd
to 30 mgd. Optimum potable groundwater availability for the four most
favorable areas is about 45 mgd. Little, if any, impact would result in
the surficial aquifer or surface waters as the result of withdrawing these
quantities from the Floridan aquifer.
The four most favorable areas were further subdivided to exclude environmen-
tally sensitive portions of the larger areas. The locations of the detailed
sites,as shown on Figure E, consider the presence of environmentally
sensitive vegetation communities and the proximity to areas of less favorable
water quality. This initial refinement of the favorable areas was done to
avoid even the remote possibility of impacts to the sensitive areas and the
encroachment of poor water quality, should the confining units not be continuous
within an area or if withdrawals interfere with other adjacent uses. The
AREA DESIGNATION .I
AREA SUITABLE FOR ........ MYAKKA CTY
GROUNDWATER SUPPLY -
I __ PROJECT BO90UNDARY
SCALE, MILES ASE FROM DOT MAP (JULY, 1974)
SEABURN AND ROBERTSON. INC.
FIGURE D.-AREAS SUITABLE FOR FUTURE POTABLE GROUNDWATER
FIGURE E.- DETAILED SITES SUITABLE FOR FUTURE POTABLE GROUNDWATER
SUPPLY DEVELOPMENT. -13-
detailed sites provide the greatest degree of flexibility for the
development and management of potable groundwater supplies in the north-
eastern Manasota Basin.
Detailed wellfield evaluations, design and subsequent well construction,
as recommended in this report, will be necessary to develop the optimum
quantities of groundwater from the favorable areas. Additionally, optimum
potable groundwater development within these areas may require evaluation
of existing and competing groundwater uses in the vicinity of the most
favorable areas at the time of development.
Reliable regional-scale groundwater supplies are reasonably assured from
the favorable areas. Limited, local supplies may be developed from the
less favorable areas within the northeastern Manasota Basin.
Within the northeastern Manasota Basin there are several favorable potable
groundwater supply areas. Four general areas have been delineated in the
north-central and eastern portions of the project area. Nine detailed
sites result from subdivisions of the general areas. Reliable, long-term,
regional potable groundwater supplies can be developed from these areas.
Withdrawal rates from these separate areas range from 12 to 30 million
gallons per day. The optimum quantity for withdrawal for the total area
is estimated to be about 45 million gallons per day. These estimates are
based on pumping from the most productive sections of the Floridan aquifer,
the Suwannee/Ocala/Avon Park units. Optimum quantities of potable ground-
water development from these areas will have to consider existing and
proposed competing uses at the time of development. In addition, detailed
well field design, well construction, and changes in groundwater use in
the project area can affect these estimates. At these rates of withdrawal,
little impact upon environmentally sensitive areas, water quality or the
regional water resources will result.
Aquifer characteristics in the project area indicate that locating well
fields to capture rejected recharge is not a feasible alternative. Arti-
ficial injection of surface waters would be required to achieve substantial
conjunctive use of water in the area.
Groundwater quality conditions are the limiting factor to locating potable
groundwater supply sites in the project area. Concentrations of sulfate,
total dissolved solids and fluoride exceed recommended potable limits in
many parts of the project area. Sulfates and total dissolved solids
concentrations show a close correlation and generally increase with
depth in the aquifer. High fluoride concentrations generally occur in
the Hawthorn/Tampa unit.
The areal variations of water quality in the project area do not appear
to change with seasons or pumping rates. Upconing of poor quality water
in the most favorable areas identified in the report is limited by
impermeable limestone units below the Suwannee/Ocala/Avon Park unit.
The stability of the potable/non-potable interface can be maintained
with the recommended well design and aquifer penetration discussed in
this report. Environmentally sensitive areas within the project area will
not be adversely affected by pumping from the Floridan aquifer.
1. The geologic character of the project area is typical
of west-central Florida; Karst features exist, however,
surface activity (sinkholes) is relatively old and gen-
erally represents conditions in the limestone units
nearest land surface. There is little hydraulic connec-
tion between these surficial deposits and the lower
Floridan aquifer. A relatively continuous confining
strata supports the surficial aquifer across the project
2. The most productive portion of the Floridan aquifer is
the Suwannee/Ocala/Avon Park unit. This unit is approxi-
mately 600 feet thick and about 50 billion gallons of
water are contained in storage. Aquifer characteristics
for this entire producing unit indicate transmissivities
ranging between 300,000 to 870,000 gpd/ft. Wells tapping
this unit typically produce 1,000 to 1,500 gpm. Leakance
through the Hawthorn Formation for recharge to the Suwannee/
Ocala/Avon Park is estimated to be 1 x 10-5 gpd/ft3.
3. Water levels in the aquifer have declined slightly over
recent years and seasonal fluctuations are about 30 feet.
Generally these water levels return to previous wet season
levels. During each year, water level changes in the aquifer
can be considerable, but the net water lost is negligible.
4. No water level changes will occur in the surficial aquifer
as a result of pumping the Suwannee/Ocala/Avon Park unit.
5. The overlying Hawthorn/Tampa unit is less productive than
the Suwannee/Ocala/Avon Park unit and separates the uncon-
fined aquifer from the Floridan. Some limited local sources
of supply are derived from the Hawthorn/Tampa unit. However,
these supplies are not reliable for large scale production.
6. Aquifer characteristics in the project area indicate that
the location of wells to enhance the conjunctive use of
surface waters to naturally recharge groundwater supplies
is not a feasible alternative. Significant head differ-
ence exists between the Suwannee/Ocala/Avon Park unit and
the Hawthorn/Tampa formations in the north and central
parts of the project area to allow for artificial recharge.
7. Groundwater quality is the limiting factor to locating
potable groundwater supply sites in the project area.
Concentrations of sulfate, total dissolved solids and
fluoride exceed recommended potable limits in many parts
of the project area. Sulfates and total dissolved solids
concentrations show a close correlation and generally
increase with depth in the aquifer. High fluoride concen-
trations generally occur in the shallow Hawthorn/Tampa unit.
8. The areal distribution of water quality conditions in the
project area does not appear to change with changes in
water levels. Upconing of poor quality water in the most
favorable areas identified in the report is very unlikely
because of impermeable limestone units below the Suwannee/
Ocala/Avon Park unit. The stability of the potable/non-
potable interface can be maintained by using the recommended
depths of aquifer penetration.
9. Environmentally sensitive (water table dependent) areas
exist within the project area. Pumping from the Floridan
aquifer will not impact these areas because of the confining
unit beneath the surficial aquifer that permits little leakage
from the water table.
10. The most favorable areas for development of reliable
regional potable groundwaters are in the north-central
and eastern parts of the project area. Areas outside the
most favorable areas are limited chiefly by water quality
conditions. The Hawthorn/Tampa formations are limited by
production quantities. Some local supplies may, however,
be developed in these areas, but with less reliability
and at reduced rates of withdrawal.
11. The results of the investigations presented in this report
are regional and based on data collected primarily from
existing wells. Sufficient detail is available to reliably
locate areas favorable for potable groundwater development.
The results of investigations presented in this report are regional and
are based on data collected primarily from existing wells. The studies
are sufficiently detailed to locate areas favorable for potable groundwater
development. Development of potable groundwater should be accomplished
only after site-specific drilling, testing and water quality monitoring
to verify these regional data and evaluations. The following are specific
recommendations resulting from this investigation:
1. Regional-scale potable groundwater supplies should be developed
only in the most favorable areas indicated in this investigation.
The development of potable groundwater supplies outside the most
favorable areas has a greater probability of water quality problems
and cannot be considered to be reliable long-term supplies. Lesser
quantities for local supplies may be developed from the Hawthorn/
Tampa units in other than the most favorable areas, but with
lesser reliability for good water quality.
2. Well field design should be conducted by qualified engineers
or hydrologists familiar with proper procedures using the results
and well construction criteria developed during this investigation.
3. Potable groundwater supply wells located in the favorable areas
should be cased through the Hawthorn/Tampa formation to reduce
fluoride concentrations in the water supply. The Hawthorn/Tampa
formations should be used only to produce small local supplies
of potable water in the project area.
4. Well penetration into the Suwannee/Ocala/Avon Park unit should
generally be limited to the depths that will prevent any possible
5. An annual water quality sampling program should be conducted in
the northeastern Manasota Basin to determine water quality
variations that may develop. Completion reports of all new wells
constructed in the area should contain complete drinking water
6. A detailed water use inventory should be conducted in the
northeastern Manasota Basin in order to thoroughly evaluate
existing use in the most favorable areas.
7. The economic feasibility and site-specific feasibility of
augmentation of surface water by groundwater should be evaluated
for the vicinity of Lake Manatee and in the headwaters of the
Manatee and Little Manatee Rivers.
NORTHEASTERN MANASOTA BASIN HYDROLOGIC INVESTIGATION
Purpose and Scope
The Manasota Basin Board is one of ten Boards that comprise the Southwest
Florida Water Management District. The Manasota Basin consists of Manatee
and Sarasota Counties with the major population centers predominantly
along the coast on the Gulf of Mexico.
Municipal potable water supplies in the Manasota Basin are obtained
primarily from the Manatee River in Manatee County and well fields in
Sarasota County. Coastal groundwater supply sites are limited in both
counties because of high mineral concentrations. Some of these mineralized
waters are used by blending with fresher water or for desalination. To
meet the expanding demands for water, major groundwater supplies will
have to be developed. Groundwater uses inland are primarily for agricultural
irrigation and for phosphate mining anticipated in the northeastern
portion of Manatee County. Concern for the proper development and manage-
ment and the efficient use of the water sources within the Manasota
Basin has prompted the development of Basin-wide water management planning.
This planning effort was undertaken in 1977 to prepare a Comprehensive
Water Management Program. The Comprehensive Program is being developed
by the Southwest Florida Water Management District staff, and selected
elements are being prepared by private consultants under the direction
of the District.
The District and the Manasota Basin Board anticipate that there will
be an increasing competition for available groundwater resources among
agriculture, municipalities and the phosphate mining industry.
Consequently, one goal of the Manasota Basin Comprehensive Water
Management Program is to identify potential potable groundwater supply
sites. To further this goal, the Northeastern Manasota Basin Hydrologic
Investigation was proposed by the Southwest Florida Water Management
District to identify and evaluate potable groundwater sources within
that portion of the Basin.
On September 29, 1978, the Manasota Basin Board of the Southwest Florida
Water Management District authorized Seaburn and Robertson, Inc. to
conduct the Northeastern Manasota Basin Hydrologic Investigation. The
purpose of the investigation was to identify and evaluate available potable
groundwater supply sites within the northeastern portion of the Manasota
Basin, Figure 1. Hydrologic, geologic, water quality, and environmental
studies were conducted to accomplish the purpose of the investigation and
areas were to be chosen where groundwater would meet potable water quality
standards and have the ability to provide a reliable, long-term, major
supply. Existing or proposed groundwater uses were not considered as
limiting factors for the suitability of a particular site.
EXPLANATION RAS T
FIGURE I LOCATION OF PROJECT AREA.
The specific objectives of the investigation were to:
a. Evaluate the optimum quantity and quality of potable groundwater
that could be developed within the project limits.
b. Evaluate the effects of withdrawals on the potentiometric surface
of the Floridan aquifer system, surficial aquifer levels, and
surface water bodies.
c. Evaluate the possibility of saltwater intrusion or water quality
degradation due to groundwater withdrawals.
d. Evaluate the possibilities of augmenting groundwater supplies
by recharge from surface water sources, where feasible within
the project limits.
e. Evaluate impacts to environmental systems, such as terrestrial
and aquatic vegetation, resulting from groundwater withdrawals
or conjunctive use.
f. Indicate areas within the project limits most suited to future
potable groundwater development.
The scope of this study is limited to the project area previously described
even though information from investigations of adjacent areas provided
much useful data. Because of the extent of the project area the evaluations
are directed toward generalized sites rather than toward specific
locations. Within this general scope of work sufficient detail is provided
to guide site specific studies to locate potable supply well fields.
This report summarizes the results of the study and describes the method-
ologies, evaluations and presents a description of the areas found to be
most favorable for development of potable groundwater supplies.
Basic data collected during the course of the project and voluminous
compilations of drillers reports, logs, and water quality lab results are
contained in a separate volume entitled "Basic Data Report Northeastern
Manasota Basin Hydrologic Investigation."
Description of the Project Area
The project area, approximately 400 square miles, consists of the eastern
one-half of Manatee County (Figure 2). The area is bounded on the north
by Hillsborough County and on the east by Hardee and DeSoto Counties. The
southern boundary of the area is Highway 780 and the western boundary is
the line between Range 19 and 20 East.
The project area is about 25 miles east of the Gulf of Mexico in an area
referred to as the Central Highlands (White, 1970) and for the most part
reflects a series of eroded marine terraces formed during Pleistocene
geologic time. Land surface elevations within the project area range from
about 40 feet to 140 feet above msl. The highest elevations are in the
northeastern part of the project area.
The predominant soil type in the project area is the Leon-Immokalee, which
is a slowly drained soil and consists of large tracts of Pine-Palmetto
Flatwoods. These soils are extensively used for agriculture, particularly
~- STATE ROADS Is S
STREAMS MYKA C ----
0 05 \ 3 PROJECT BOUNDARY
SCALE, MILES i 3ASE FROM DOT MAP (JULY. 1974)
SEABURN ANO ROBERTSON, INi
FIGURE 2.-MAP OF PROJECT AREA SHOWING LOCATION OF MAJOR
STREAMS AND HIGHWAYS.
for pasture grasses and livestock production, because the land is nearly
flat and water table conditions are favorable. The natural vegetation
most prominent in the project area consists of slash pine, some long-leaf
pine, scrub oaks, saw palmetto, runner oak, wiregrass and some bitter
The major rivers in the project area are the Little Manatee, Manatee and
Myakka Rivers and their tributaries, which eventually discharge into Tampa
Bay or the Gulf of Mexico. Large parts of the area are poorly drained
and contain many ponds, marshes and swamps.
Population in the project area is largely rural with widely scattered
houses and farms. A few small towns are located at Duette, Myakka Head,
and Myakka City. Agriculture, including vegetable farming, livestock
and citrus production, are the major agricultural activities in the area.
Phosphate mining operations are anticipated in the northeastern part of
Manatee County which will considerably alter the current patterns of
agricultural land use.
Since 1958, numerous hydrogeological investigations relevant to the
groundwater resources of the northeastern Manasota Basin have been conducted
in Manatee County and surrounding counties. The reports pertinent to this
study are described below.
H. M. Peek (1958 ab), investigated the geology and groundwater resources of
Manatee County. This investigation concentrated primarily on that portion
of the county west of the project area. A map of the potentiometric surface
for June, 1955 was prepared, along with a map showing areas of artesian
flow. The chemical quality of groundwater in Manatee County was described.
An aquifer test at a site west of the project area was conducted to
determine the transmissivity and storage coefficient of the Floridan
H. M. Peek (1959) reported on the hydrogeology of southwestern Hillsborough
County adjacent to the project area. The report included information on the
transmissivity and storage coefficient of the Floridan aquifer. Additionally,
a map of the potentiometric surface of the Floridan aquifer for May,
1953, was prepared showing areas of artesian flow.
W. J. Carr and D. C. Alverson (1959) described the stratigraphy of the
middle Tertiary-age rocks for part of west-central Florida. These descrip-
tions were correlated with the rocks of Manatee County and were used in
the evaluation of the thickness of geologic units in the project area. The
water resources of Hillsborough County adjacent to the project area were
investigated by C. G. Menke, E. W. Meredith, and W. S. Wetterhall (1961).
Aquifer coefficients for the Floridan aquifer were determined. Potentio-
metric surface maps for the Floridan aquifer were included for the period
September and October, 1958.
C. S. Chen (1965) studied the Paleocene and Eocene stratigraphy of Florida,
including areas within and adjacent to Manatee County. His interpretations
were made from cuttings, cores and electric logs. Isopach-lithofacies
maps, structure maps, and lithologic cross sections were presented.
The groundwater resources of nearby Polk County were investigated by
H. G. Stewart, Jr. (1966). Groundwater movement within Polk County was
described and estimates were made of the amount of groundwater moving
into other adjacent counties. Detailed descriptions of the hydrogeology
of the various rock units were presented. Structural geologic maps and
potentiometric surface maps were included in the report. Pumping tests
were made to determine aquifer hydraulic coefficients.
H. Sutcliffe, Jr., and B. F. Joyner (1968) conducted an exploratory test
well program in the Myakka River Basin. The results of the test drilling
program yielded geologic and water quality information for formations as
deep as the Tampa Limestone. Four of the test holes were in the northeastern
Manasota Basin project area.
An engineering report on artificial recharge through deep wells at the
Florida Power and Light Company Reservior in the northwesternmost portion
of the project area was conducted by Black, Crow and Eidsness, Inc. (1973).
The report described the hydrology and water quality of the deeper geologic
units. Aquifer tests were conducted to determine pertinent hydraulic
An assessment of the groundwater resources of the Verna Wellfield in adjacent
Sarasota County was made by Geraghty and Miller, Inc. (1974a). The report
concluded that the wells producing highest concentrations of sulfate
penetrated the Suwannee Formation and that deeper wells would yield
increasingly high amounts of sulfate.
The groundwater hydrology of the Beker Phosphate Property in eastern
Manatee County was studied by Richard C. Fountain and Associates (Beker
Phosphate Corporation, 1974). The investigations on the site included
subsurface geological studies, water quality sampling, an inventory of
water use, monitor well installation, geophysical logging and an aquifer
test of the lower Ocala and upper Avon Park formations. Aquifer
coefficients were calculated from the test data.
At the W. R. Grace & Company's Four Corners Mine in northeastern Manatee
County and southeastern Hillsborough County, P. E. LaMoreaux & Associates,
Inc. (W. R. Grace and Company, 1975), conducted an analysis of the site
hydrogeology. That work included a well inventory, water quality sampling,
and aquifer testing.
An investigation of the hydrogeology of the Swift Agricultural Chemicals
Corporation's Manatee Mine site was conducted by William F. Guyton &
Associates (1976a-d). Detailed geologic descriptions of the materials
encountered in the boreholes and a comprehensive geophysical logging and
borehole flow study are presented. The report also includes a well
inventory for this area in easternmost Manatee County. Groundwater quality
samples and a test drilling and aquifer testing program provided a substantial
contribution to the data base of the present study. Aquifer coefficients
were calculated from the data generated during the testing. A multi-layer
model to simulate site hydrology was developed and used to predict the
impact of groundwater withdrawals at the site.
The groundwater resources of DeSoto and Hardee Counties were described
in detail by W. E. Wilson (1977a). The report provides an extensive
discussion of long-term and seasonal changes of the potentiometric
surface in the two-county area. Particular emphasis is placed on
groundwater quality maps for selected chemical constituents.
An investigation by W. E. Wilson (1977b) resulted in simulated changes
in groundwater levels that may result from pumping for proposed phosphate
mining in West-Central Florida, including Manatee County, for the periods
1985 and 2000. Included within the report were regional maps of
transmissivity, confining bed thickness, leakance coefficients, storage
coefficient, Florida aquifer thickness maps and simulated potentiometric
The hydrologic effects of withdrawing groundwater from the Floridan
aquifer at the Phillips Petroleum Company phosphate mine in DeSoto
County and southeasternmost Manatee County were investigated by Geraghty
and Miller, Inc. (1978). The studies yielded geologic data, groundwater
quality information, and geophysical logs. Aquifer coefficients were
determined from pumping tests.
The identification and evaluation of areas of possible groundwater supply
requires study of considerable hydrologic, geologic and water quality
information pertinent to the area of interest. Because of the large size
of the project area, approximately 400 square miles, the Northeastern
Manasota Basin Hydrologic Investigation was subdivided into two phases:
a first phase consisting of data collection and review and identification
of additional data needs, and a second phase for further field data
collection, testing, analysis, and preparation of recommendations.
The first phase involved the collection and review of existing hydrologic
data, studies and reports pertinent to the project area. Data collected
and reviewed in the preliminary phase came from previous hydrologic and
engineering studies in the general area as previously discussed. Sources
of recent hydrologic information were the U.S. Geological Survey, the
Manatee County Agricultural Extension Service and the Soil Conservation
Service. The Southwest Florida Water Management District provided numerous
hydrologic reports, Development of Regional Impact reports and data
collected from the District's Regional Observation and Monitor Well Pro-
gram in Manatee County. Other data were available from the Florida Bureau
of Geology. The phase was completed and summarized in February, 1979, in a
report entitled, "Status Report of Northeastern Manasota Basin Hydrologic
Investigation". That report also provided a detailed description of the
required field data collection and testing to be accomplished during the
final phase. This approach resulted in a field data collection, testing
and analysis program consisting of several tasks, each designed to maximize
the use of existing groundwater wells and ongoing District programs.
The second phase of the project progressed through the several tasks of data
collection and analysis, culminating in the development of a ranking system
that consisted of a series of maps and figures of the hydrologic data
available. Areas most favorable for development of potable water supplies
were then indicated by considering these maps together.
Well Inventory and Water Quality Sampling
Over 400 groundwater wells were inventoried in the project area. Well
construction permit data and consumptive use permit information were
used in the inventory. Water quality sampling program was done quarterly
and provided additional data regarding water quality variations over the
area during periods of low and high potentiometric surface level.
Aquifer Performance Testing
Three aquifer tests were conducted at selected irrigation wells within
the project area to obtain additional data about aquifer hydraulic
characteristics. These data supplemented other available information
and were useful in the aquifer simulation model.
Soil borings were performed at six selected locations within the project
area to obtain information about soils, water table elevations and surficial
clay or hardpans. The locations were selected in areas considered for
further hydrologic evaluations. A total of 83 borings were made to depths
of about 40 feet below land surface.
Ten existing irrigation wells were logged using borehole geophysical tech-
niques by Southwest Florida Water Management District during the course of
this study. The logs consisted of caliper, resistivity, gamma activities,
temperature, and fluid velocity. Down-hole water quality samples
were also collected. This task was performed to refine geologic
descriptions and to determine water quality variation with depth
throughout the project area.
Black and white, and false-color infrared photographs were obtained
for the project area. These photographs were used for several purposes
including the well inventory and soils and vegetation mapping. They
were also useful for delineation of drainage basins and mapping of
Available ecologic and biologic data in existing Development of Regional
Impact reports were collected and evaluated. Site-specific mapping of
environmentally "sensitive" communities was completed by field visits to
selected potential groundwater supply areas.
Other Hydrologic Information
Considerable practical hydrologic information was obtained from local
residents, farmers, well drillers, and agricultural specialists during
the investigation. These discussions regarding drilling problems and
practice and groundwater uses provided much empirical data about water
quality, well construction and water level fluctuations.
Following the completion of the field data collection and testing, a
description of the hydrogeologic system was completed and areas for
additional evaluations were determined.
The geologic, hydrologic, water quality and ecologic factors determined to
be favorable and unfavorable for development of groundwater were mapped
to facilitate decisions about overall area selections. The possibility of
conjunctive use of surface water to recharge groundwater sources and
the effect of current water management practices were also considered to
determine the most favorable areas. Overlays of this series of maps were
used then to evaluate specific areas and detailed sites.
Further evaluations of the selected areas were made using these geologic,
hydrologic, water quality, ecologic, conjunctive use and water management
factors and data discussed above. These factors formed the elements of the
ranking matrix with the addition of various quantities of groundwater
withdrawals used to evaluate hydrologic impacts estimated for the selected
areas. Hydrologic impacts were estimated for the selected areas using
various hydrologic models including digital computer programs.
The following sections of this report describe the project area in terms
of the above factors and present the results of the field data collection,
testing, evaluations and hydrologic analysis of areas for development of
potable groundwater supplies. The latter sections of this report present
the graphical matrix of maps and overlays used to locate and evaluate the
areas and specific sites considered favorable for potable groundwater
This investigation was authorized by the Southwest Florida Water Management
District Governing Board, Bruce A. Samson, Chairman, D.R. Feaster, Executive
Director, and the Manasota Basin Board:
Mr. B.T. Longino, Chairman,
Mr. Gordon D. Hartman, Vice-Chairman,
Mr. Randolph Snell, Secretary,
Mr. J. Lynn Harrison, Member,
Mr. Michael Stuart, Member, and
Mr. John J. Whelan, Member.
Richard V. McLean, Southwest Florida Water Management District Project
Manager, directed the project for the District assisted with data
acquisitions and provided many useful suggestions during the course of
the investigation and report preparation. The late Mr. Fred Roberts
provided considerable assistance during the geophysical logging. The
assistance of the U.S. Geological Survey, the Manatee County Health
Department and the SWFWMD laboratory are also gratefully acknowledged.
Special acknowledgement and appreciation is accorded to the entire agri-
cultural community in Manatee County, without whose cooperation and assistance,
this investigation would have been more difficult. We are especially
grateful to the following ranchers, farmers, and agricultural specialists:
Mr. Joe Cincotta B & C Farms,
Mr. Jim Blackwelder Rutland Ranch,
Mr. Elbert Walker Rutland Ranch
Mr. Jim Luttrel Tropicana Products, Inc.
Mr. Tom Kibler Kibler Ranch,
Mr. Clarence Harrison Scheorder Manatee
Mr. Terry Montgomery Manatee County, Agricultural Extension Services,
Mr. I. H. Stewart U. S. Conservation Service
Mr. Paul Elsberry Elsberry Farms,
Mr. Bill Sexton Kibler Ranch,
Mr. Lee Sirmans Pacific Packing Co.,
Mr. Willie Tedder Pacific Packing Co.,
Mr. John McDonald Cannon Well Drilling,
Mr. Terry Mullins Rainbow Ranch, and
Mr. Harry Cannon Cannon Drilling
The Northeast Manasota Basin Hydrologic Investigation was performed by
hydrologists and geologists of Seaburn and Robertson, Inc. Mr. John E.
Darling, Jr., managed the investigation under the general supervision of
Mr. Alton F. Robertson, P. E. Geohydrologic and geochemical aspects
of the investigation were conducted by Mr. Robert L. Westly and Mr. John
C. Miller, respectively.
Environmental phases of the investigation were performed by Biological
Research Associates under subcontract to Seaburn and Robertson, Inc.
Mr. Richard J. Callahan, Jr., and Mr. William B. Carey conducted the
investigation for Biological Research Associates.
The geology of the study area has been studied and described in previous
investigations. Field reconnaissance conducted to collect geologic
information supplemented the existing data and literature. The geologic
aspects of the area are discussed in detail as they relate to the
potential for production of potable water from the aquifer that are a
part of this geologic framework.
Unconsolidated marine sands and clay are the parent materials from which
the soil of the Northeastern Manasota Basin Project Area were derived
(Caldwell and others, 1958). The soils generally have a sandy surface
layer and are low in organic materials. They have a low supply of
plant nutrients and are generally strongly acid, except in scattered
localities where lime layers are within a few feet of the surface. Soils
located in topographic low areas tend to have a greater organic accumula-
tion, chiefly muck, that is the parent material.
Figure 3 is a map of soil associations in the Northeastern Manasota Basin
Project Area. Soil associations are well-defined groups of soils that
occur in close geographic association. Each delineation on the soil
association map represents a group of soils that occur together in nature
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FIGURE 3.-SOIL DISTRIBUTION AND
MANASOTA BASIN. -39-
ASSOCIATIONS IN THE NORTHEAST
to form a distinctive landscape. A soil association consists of one or more
major soils and at least one minor soil, and is named for the major soils.
Table 1 summarizes the general characteristics of the soil associations
and their flooding potential.
The Soils Association map is useful to predict the impact of large-scale
groundwater withdrawals on wildlife habitats. Soils associations 8, 9
and 10, as shown in Figure 3 and in Table 1 are indicated as "good" for
wetland wildlife. All of the other soils associations are classed as
poor to very poor for wetland wildlife, and only poor to fair for woodland
and openland wildlife (Florida Department of Administration, 1976). Soils
associations 1 and 2 have well drained soils and the greatest depth to
water table. These soils occur chiefly on topographic highs.
Geomorphology is the science concerned with the general configuration of
the Earth's surface; specifically, the study of the classification,
description, nature, origin and development of present land forms and
their relationship to underlying structures. The land forms of the North-
eastern Manasota Basin Project Area are described below and an evaluation
is made regarding the implications of these land forms to groundwater
supplies and to the environmental impacts of groundwater withdrawal.
The Northeastern Manasota Basin Project Area is in the west central
portion of the Mid-Peninsula Zone of the Florida peninsula (White, 1970).
Parts of three major physiographic features are included within the project
area: the Polk Upland; the DeSoto Plain, and the Gulf Coastal Lowlands (Figure 4).
Table 1. Soil Association in the Northeast Manasota Basin Study Area 1/
10 Fresh Water Swamp and
er / Soil Association
2 Pomello-St. Lucei
6 Myakka- Immokalee-8asinger
Moderately well to
soils not subject
/Florida Division of State Planning, Bureau of Comprehensive Planning (1975).
/Reference number refers to areas on Figure 3.
Moderately well drained soils sandy
throughout and poorly drained sandy
soils with weakly cemented sandy
Moderately well drained sandy soils
with weakly cemented sandy subsoil
and excessively drained soils sandy
Poorly and moderately well drained
sandy soils with weakly cemented
Poorly and moderately well drained
sandy soils with weakly cemented
sandy subsoil and poorly drained
soils sandy throughout
Poorly drained sandy soils with
weakly cemented sandy subsoil and
poorly drained soils sandy through-
Poorly drained tandy soils with a
weakly cemented sandy subsoil layer
over loamy subsoil; poorly drained
soils with thin sandy layers over
loamy subsoil; and poorly drained
sandy soils with weekly cemented sandy
Very poorly and poorly drained soils
Very poorly drained soils with thick
sandy layers over loamy subsoil; very
poorly drained sandy soils with loamy
suosoil and poorly drained soils
Very poorly drained soils subject to
Poorly and very
soils subject to
H ILLSSOROUH CO. POLK CO.
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FIGURE 4.-PHYSIOGRAPHIC MAP OF THE NORTHEASTERN
MANASOTA BASIN STUDY AREA AND VICINITY.
The Polk Upland, located in the northeastern portion of the project area
is a relatively flat area with elevations between 100 and 135 feet above
msl. The southern edge is an inconspicuous but persistent scarp, the
toe of which is at 75 to 85 feet elevation, that separates the Polk
Upland from the DeSoto Plain. The origin of this scarp is not well
understood, but most probably it is an erosional marine scarp made by
a Gulf of Mexico shoreline at Wicomico sea level.
The DeSoto Plan extends across the western and southern portions of the
project area with a very flat surface. It is possibly a submarine plain
made during Wicomico time, because there are no beach ridges or other
lineal features to suggest relict shorelines (White, 1970). The
southern edge of the DeSoto Plain is about 10 miles south of the Polk
Upland scarp. The DeSoto Plain is separated from the Gulf Coastal low-
lands by a scarp between 40 to 60 feet elevation.
The southwestermost portion of the project area lies in the Gulf Coastal
lowlands. Tatum Sawgrass and the Myakka River are located in this area.
Superimposed upon the major physiographic features are three principal
streams and their tributaries. The South Fork Little Manatee River
(see Figure 5) is characterized by a relatively deep river valley. Where
the river leaves the Polk Upland, its valley depth is in excess of 60
feet. The Little Manatee River valley tends to make numerous right-
angle turns (Figure 5) which may be indicative of fracture patterns
developed in rock units beneath the river valley.
The Manatee River extends into the project area as a straight channel
for more than 10 miles. As the North Fork of the Manatee River leaves
the Polk Upland, the valley is greater than 50 feet deep. Right-angle
turns are also common in the upper reaches of the Manatee River.
The Myakka River is very different from the previous two rivers. In the
lower 7 miles, before it leaves the project area, it lies in a broader
river plain where it meanders considerably. The valley depth of this
river is only about 35-40 feet. At a point 7 miles upstream from the
southwest edge of the project area the Myakka River turns abruptly
northward for 8 miles, passing through the Long-Ogleby Swamp. Again,
right-angle deviations in the valley are common in the upper reaches.
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FIGURE 5.-KARST FEATURES AND STREAM PATTERNS IN THE NORTH-
EASTERN MANASOTA BASIN STUDY AREA.
Karst is a type of topography that is formed over limestone, dolomite,
or gypsum deposits. The surface is characterized by closed depressions
(dolines), caves, and underground drainage caused by solution of these
underlying deposits. This topography is very common in Florida.
There appear to be three distinct types of closed karst depressions or
dolines in Florida. The first is the collapse sink, which is generally
steep-sided, rocky and large diameter. These form when a cavity in a
soluble rock such as limestone enlarges to a size that can no longer
support the weight of overlying soil and rock.
A second type of closed karst depression is the ravelling sink,
common in Florida, caused by erosion or ravelling of the overburden into
limestone cavities. For such an event to occur, however, the cavernous lime-
stone must be overlain by relatively pervious unconsolidated sediments, the
water table in the overburden must be higher than that of the potentio-
metric water level in the limestone, and the limestone must be breached
into the cavernous zone creating a point of high recharge to the limestone
aquifer from the overlying unconsolidated sediments. The movement of
water from the upper zone carries sediment downward into the cavity which
eventually results in either a small diameter collapse or slow lowering
of the land surface.
A third type of closed karst depression is the solution sink. More common
than the collapse sink, they are not a result of physical disturbance of
the underlying carbonate rock, but are the result of chemical solution of
the rock, generally over a large area, accompanied by removal of the
dissolved material to other areas by moving groundwater. This solutional
activity takes place very slowly and is evident at land surface as large
circular areas that are seasonally wet.
Within the project area the limestones are covered by a considerable thickness
of low permeability materials (as low as 1.1 x 10-6 cm/sec, Ardaman &
Associates, Inc., 1975) that decrease the rate at which near-surface
acidic groundwaters move downward and react with the limestone. The result
is a considerably subdued karst terrain characterized by many small,
shallow, closed depressions filled with muck and wetlands-type vegetation.
These depressions, probably small solution sinks, are a result of the
gradual dissolution of limestone at depth. Estimates by the Florida
Bureau of Geology (1972), indicate that the lowering of the ground surface
due to this type of solution activity in other parts of the state occurs
at the rate of one foot every five to six thousand years. This is probably
the case at present in the project area where all of the dolines are less
than 5 feet deep, although they may be filled with sediment.
To understand the impact that karst activity has had on the ground-
water resources of the project area, all readily visible karst
features were mapped using stereo-paired aerial photographs. All circular
or oval, closed depressions (dolines) were mapped and are shown on Figure 5.
The aerial photography used for the karst features mapping was of three
1. 1:24,000 stereo-unpaired prints, black and white, Mark Hurd
Aerial Surveys, Inc., December 10, 1972,
2. 1:80,000 stereo-paired, color IR, 9 x 9 contact prints, Mark
Hurd Aerial Surveys, Inc., December 10, 1972, and
3. 1:24,000 stereo-paired, black and white, 9 x 9 contact prints,
Florida Department of Transportation, December 20, 1977 to
January 10, 1979.
Inspection of the stereo-paired aerial photographs revealed that there
has been no collapse sinkhole activity during the past 10 years. Mr. I.H
Stewart, District Conversationist, Soil Conservation Service, in Manatee
County (personal communication) reports that since his residency in Manatee
County began in 1950, he has not been made aware of any sinkhole activity
of the collapse type in the project area. Most likely, karst activity in
the area was greater in the past. Present higher sea levels or a subsiding
land mass have apparently decreased the head difference in the aquifers, such
that this major cause of karst solutional activity is not as dynamic as it
The small, closed depressions or dolines are not the only karst features
in the project area. Figure 5 shows the location of the Tatum Sawgrass and
Long-Ogleby Swamp. During the geologic past, when head differences between
the water-table aquifer and the Floridan aquifer were greater, these
two wetlands areas were forming as large karst lakes. Sellards (1910)
developed a hypothesis that many Florida lake basins had been formed
by a sequential development of similar sinks, earlier ones being filled
with sediment in the center of the lake, while newer ones developed along
the shoreline. These "Sellards type" lakes are to be found in the project
area as large swampy areas with plugged sinks around the edge. Particularly
good examples are the Tatum Sawgrass, Long-Ogleby Swamp, and the large
shallow depressions northwest of Long-Ogleby Swamp. Examples of
geologically recent sinks that formed out from the edges of the former lakes are
those to the east of Long-Ogleby Swamp. These sinks, about one mile from
the edge of the present swamp (Figure 5) are aligned in a manner that
mimics the shape of the swamp (former lake).
Straight or slightly curving lines, not due to stratigraphic contacts or
man-made features and generally unaffected by topography, are considered
surface manifestations of vertical to near-vertical zones of fracture
concentration in bedrock. Such lines, called fracture traces and lineaments,
are expressed at the surface by aligned surface sags or depressions, gaps in
ridges, soil tonal changes indicating soil moisture variations, springs, seeps,
sinkholes, vegetation alignment or vegetation type and height changes,
straight stream and valley segments, and abrupt valley or lake shore
alignment. It has been shown that these fracture traces (less than one
mile long) observed on aerial photographs are generally underlain by zones
of localized weathering, increased permeability and porosity (Parizek,
R.R., 1976). These features are expressions of bedrock rupture and are
visible even when covered by as much as 300 feet of residual or transported
soils (Wobber, F. J., 1967). A detailed fracture-trace map of the project
area was not prepared because it is likely that the fractured rock causing
surface traces is not the deeper portion of the Floridan aquifer, which is
the major source of groundwater supplies. The rocks containing these
fractures are probably limestones of the Hawthorn and Tampa units.
However, the 1:80,000 scale 9 x 9 stereo-paired color infrared contact
prints were used to map large-scale surface feature lineaments (lineations
greater than one mile long observable on aerial photographs). As mentioned
previously, the three principal streams in the Northeastern Manasota
Basin Project Area tend to change their flow direction rather abruptly. This
is because of geologic controls determined by faulting and extensive fracturing
of the rock. Vernon (1951) first recognized this in Citrus and Levy Coun-
ties, and he prepared a map of northern and central peninsular Florida
that indicates some of these major fracture trends. Paralleling these
major trends are innumerable smaller-scale fractures that also influence
stream orientation. These stream valley trends tend to be in two dominant
directions, northeast-southwest and northwest-southeast. The trends
correspond with those observed in other parts of Florida (Vernon, 1951;
Yon, 1966; and Miller, 1977).
Figure 6 shows the major lineaments that have been detected in the study
area. Some of the lineaments extend as far as 12 miles. And, while a
good number appear as long, straight stream segments, there are others
that cut directly across the land surface. Long trends of sinkholes
often have the same orientation as the streams, and may be controlled
by similar deep fracture zones.
R201 ~ZI! q 22
.. .. ., o.., -
S *.. *. \, ', .., -. 0 ...* .' / <"
,. ". _
0 0.3 -
i. ,-' .' : *. '' --
*. ,. *. : .. *,: y ..
SCALE, MILES I.
"W *" o0 .. .
SI SEASURN AND ROERTSON, !NC.
FIGURE 6.- MAJOR LINEAMENTS MAPPED IN THE NORTHEASTERN -
MANASOTA BASIN STUDY AREA.
e ., "-.. j. ,, :- ',
SA .. ;
"" EINT ", "* _J *, I % ,S ,
S.. ,1 ,' .',
.. ..,~ 'i /- '
S. ,, .- r" .0
EXPLANATION "R A !.
MANASOTA 8ASIN STUDY AREA.
It is expected that groundwater withdrawals in the Northeastern Manasota
Basin Project Area will have minimal impact on sinkhole activity at land
surface. Heavy agricultural pumping in the area for more than 20 years
has caused no new sinkhole activity. Hydrologic conditions have changed
greatly since June, 1955, when as Peek (1958a) reports, artesian flow
conditions existed from the Long-Ogleby Swamp down the Myakka River to and
including the Tatum Sawgrass (Figure 5). Heavy pumping in Manatee County
and adjacent counties appears to have lowered the potentiometric surface
such that in May, 1979, a decline of 20 feet since 1955 had been established
(Wolansky and Others, 1979). Older sinkholes, if containing material of
sufficient permeability, may be locally reactivated. However, the downward
movement of the land surface in these sinkholes at present must be very
slow, despite the pumping, for sinkhole activity in the region has not
Sinkhole formation tends to be favored by the following conditions:
1. The occurrence of permeable limestones in which a cavity system
has been developed through dissolution by groundwaters,
2. A water table higher than the potentiometric surface of the
3. The limestones are overlain by a relatively thin layer of uncon-
solidated sediments, and
4. Overlying sediments are usually well drained and permeable.
In the project area only conditions 1 and 2 are common and for this
reason it is unlikely that sinkhole formation will increase in the study
area as a result of further groundwater withdrawals.
Lithology and Stratigraphy
The lithology and stratigraphy of the Northeast Manasota Basin is described
in this section to indicate the characteristics and location of geologic
units that may affect the development of groundwater supplies. Lithology
was described first; correlated to published regional descriptions and
geologic formation names assigned to particular groups of lithologic
characteristics based on the correlation. The stratigraphy was then devel-
oped by locating the depths to the geologic formations at selected sites
and constructing structure contour maps indicating the top of the formations.
Lithology is the description of physical characteristics of rocks and is
used to assign geologic formation names to groups of rocks with similar
characteristics. Numerous investigators have described the lithology
of rocks in the vicinity of the project area largely by the inspection of
borehole cuttings. Peek (1958a) described the lithology of rocks
beneath western Manatee County and southern Hillsborough County
(Peek, 1959). The lithology of Polk County has.been described by Stewart
(1966). Wilson (1977a) described the lithology of rocks in DeSoto and
Hardee Counties and Bishop (1969) reports on the rocks in Sarasota County.
The lithology beneath the project area is similar to that found in
surrounding areas and is summarized in Table 2. This table was constructed
after evaluating 25 logs of borehole cuttings from wells in eastern Manatee
County analyzed by geologists of the Florida State Bureau of Geology,
the U.S.G.S. (Wilson, 1977a) and William F. Guyton and Associates
(Guyton, 1976b and 1976c). The borehole locations are shown on Figure 7.
Formation names (e.g., Hawthorn) were assigned to the lithologic descriptions
by correlating them to a summary description of all the formations in the
District (MacGill, 1976, unpublished).
Table 2 shows that the project area is underlain by a sequence of sand,
phosphate sands and clay, limestones and dolostones. These units are
typical of Central Florida. The upper sediments are unconsolidated quartz
sands which contain the water table throughout most of the County. Underlying
the upper sediments are the unconsolidated phosphatic sands and clays of
the Bone Valley and Upper Hawthorn Formations. Because uranium associates
with the phosphatic fraction of these formations, they are easily identi-
fied by gamma ray geophysical logs of wells. Figures 8 and 9 show the
locations of geophysical logging and gamma ray logs obtained in the project
area. There are two rock units of relatively high gamma radiation indicated.
The small unit that occurs at about msl is probably related to the phosphatic
sand of the Bone Valley Formation. The zone of relatively lower radioactivity
beneath the Bone Valley is probably the Upper Hawthorn.
Table 2. Lithology of Geologic Formations in
The Northeast Manasota Basin
Average Average Depth
Thickness To Top (feet
Series Formation Lithology (feet) below I.s.)
Recent Quartz Sand: Medium, subangular, non-indurated, 35
with some iron stain and some clay cementation.
Pliocene Bone Valley Phosphate, Sand, Gravel and Clay: Medium quartz
and sand, quartz, sand non-indurated, clay yellow-gray, 90 35
Upper Hawthorn moderately indurated, fissile. Phosphate sand 5
percent of sample.
Lower Dolostone and Phosphate Sand: Some chert, poorly
Hawthorn indurated, phosphate sand 10-20 percent of samples; 225 125
dolostone is microcrystalline.
Tampa Limestone: Micrite with quartz sand, well indurated,
Limestone high percent of allochems, trace of phosphate. 115 350
Quartz sand generally 10-30 percent and sometimes
100 percent of sample.
Oligocene Suwannee Limestone: Calcarenite, trace to no quatrz sand,
Limestone moderately to well indurated, fossils are forams and 275 465
Upper Limestone: Calcarenite, 90% allochems Unknown
Limestone: Chalky, nodular, granular, fragmental,
some oolite, generally very fossiliferous, cream,
Ocala white, some buff; occasional dolomite in lower part, 220 740
Group sucrosic to dense and cherty, yellowish brown to
dark brown and gray.
Dolomite: Slightly calcitic to calcitic, medium to
finely crystalline, euhedral, replacement, sparsely
Upper fossiliferous (small echinoids and relic molds after 172 960
fossils), hard, dark tan to yellow brown with sparse
black specks? Crystalline carbonate.
Eocene Dolomite: Medium to finely (rarely finely to very
finely) crystalline, euhedral, dense, hard, dark
Avon Park 2/ yellow brown, scattered black layering and mot- 95 1130
tling: Crystalline carbonate. A videotape taken
Lower through this unit shows it to be characterized by
numerous caverns (some very large, greater than 10
feet in vertical dimension) with well developed
1/ Description from Wilson, 1977.
2/ Description from Guyton, 1976.
aOP WEOLOGstS LoS
SEE APPENDIX I FOR BORING LOGS
SCALE, MILES ASE FROM OOT MAP (JULY. 1974)
FIGURE 7.- GEOLOGIST LOGS BOREHOLE LOCATIONS.
*SR2 LOCATION AND NUJMK OF WLU. ,.....` ,MYAKK QT
j ,- A^ ..
SR 5 |
00 1 \ E) PROJECT SOUNDARY (
SCALE, MILES 8ASe FROM DOT MAP (JULY, 1974)
SEABURN AND ROBERTSON, INC.
FIGURE 8.- CROSS SECTION LOCATION OF GAMMA RAY GEOPHYSICAL
l 700 -
- I. LOG RATE: 20 COUNTS/
2. TIME CONSTANT: 2
NOTE: LOGS PERFORMED BY
SWFWMD ROMP DIVISION.
FIGURE 9.- NATURAL GAMMA RAY GEOPHYSICAL LOGS IN CROSS-SECTION, NORTHEAST MANASOTA BASIN.
SEABURN AND ROBERTSON,INC.
SAIIURN AM) ROBERTSONs INC.
-; -- ---------------~-i--a91~cl~,.,,,
The Lower Hawthorn Formation is characterized by a dolostone and phosphatic
sand unit. Most of the unit is indicated by the second, broader zone of
radioactivity on Figure 8. Below the Hawthorn Formation lies the micritic
limestone of the Tampa Limestone. Micritic limestone consists mostly of
chemically precipitated carbonate mud. The Tampa Limestone in the project
area is a micritic limestone with a high percent of carbonate and quartz
grains. The sandy unit of the Tampa Limestone discussed by Wilson (1977)
does not consistently occur in eastern Manatee County. However, geologists
descriptions and geophysical logs indicate that the unit contains sufficient
sand in places to require borehole liners to prevent sloughing of sand into
open boreholes. Hydraulic data from tests performed in the area (Guyton,
1976a) indicate the micritic fraction of the unit probably fills the inter-
connected pore spaces in the rock sufficiently to cause the unit to have a
relatively low transmissivity.
Below the Tampa Limestone lies the calcarenitic limestone of the Suwannee
Limestone Formation. This type of limestone is composed primarily of
detrital calcite particles of sand size. The Suwannee Limestone in the
project area is a consolidated rock containing foram and echinoid fossils
and little or no sand. Hydraulic data (Guyton, 1976a) indicate that inter-
connected pore space is generally open or fractures exist to allow the
Suwannee to function as a producing aquifer unit.
Below the Suwannee Limestone Formation is a sequence of three limestone
units called Ocala Group. The Ocala Group has been described by Vernon
(1951) and Puri (1974). They are the Crystal River (Ocala Limestone),
Williston, and Inglis members, from top to bottom within the Group. The
geologist's logs indicate the upper member (Crystal River) of the Group
to be composed almost entirely of various allochems (limestone fragments,
fossils and fossil fragments). Hydraulic data (Guyton, 1976a) indicate that
the Crystal River and Willison members do not contribute significant
quantities of water to wells but act as semi-confining units between the
upper and lower Floridan aquifer units. The Inglis member apparently
produces sufficient water to identify it hydraulically with the top of
the Avon Park Limestone.
The Avon Park Limestone which underlies the Ocala Group is described in
Table 2 as adopted from a description of cuttings from a phosphate company
test well near Duette, Florida (Guyton, 1976a). Local well drillers suggest
that selected units within this dolomitic formation (locally termed the
"hard brown lime") produce substantial quantities of water to wells through-
out the area. Puri and Winston (1974) report this formation is one of the
most transmissive units in South Florida.
Guyton (1976a) reports that below the Avon Park lie the evaporite units
of the Lake City. No supply wells in the project area penetrate this unit.
Because of its low transmissivity it is considered to be the basal confining
unit of the aquifer system in eastern Manatee County.
Stratigraphy is the description of the arrangement and geographical dis-
tribution of geologic formations. The best description available for the
project area previous to this investigation is given in a series of structure
contour maps (MacGill, 1976, unpublished) which show the tops of the geologic
units, regionally. Because this investigation is directed toward develop-
ment of groundwater supplies, it was important to locate the stratigraphic
units with reasonably distinct and aerially continuous hydrologic character-
istics. MacGill's maps do not necessarily show this in the project area
because little or no weight is given to hydrologic characteristics in the
preparation of the maps.
For this reason, stratigraphy in the project area was described from
evaluations of geologists' logs, drillers' logs, results of a geophysical
logging and aquifer testing program performed during the investigation,
review of various literature and existing stratigraphic maps. Evaluations
were made based on characteristics of the rocks that could be easily
identified with the available data and that represent particular hydraulic
units (i.e., an aquifer or confining unit).
The evaluation resulted in the location of five units within eastern Manatee
County that potentially will effect development of groundwater supplies.
These units are shown by structural contour maps in Figures 10 through 14.
The first unit is a dolostone dolomiticic limestone) which is located near or
at the top of the Hawthorn Formation. The top of this unit forms a
broad shallow trough trending west to east then south across the area (Figure 10).
Data used in constructing the map is limited, therefore, the trough shape
is highly interpretive. This unit is a low yielding artesian aquifer that
actually forms part of the confining system overlying the major producing
FIGURE 10.- STRUCTURE CONTOUR LINES ON TOP OF THE FIRST
DOLOSTONE UNIT, HAWTHORN FORMATION.
LINK SHOWN ALTI'rJ
OP STmIUCTUNE SUMAC
FIGURE II.- STRUCTURE CONTOUR LINES ON TOP OF THE SANDY
MICRITIC LIMESTONE, TAMPA FORMATION.
--.0 L- Ne SHOWnm ALTMUDE
OF STaUCTUL SUJNC
SCALE, MILES i &AS FROM OOT MAP (JULY, 1974)
Su SEABURN AND ROBERTSON, INC.
FIGURE 12.- STRUCTURE CONTOUR LINES ON TOP OF THE CALCARENITIC
LIMESTONE, SUWANNEE FORMATION.
LINE SHOWmN ALTITUDE
00.5 E, M E
FIGURE 13.- STRUCTURE CONTOUR LINES ON TOP OF THE FORAM ZONE,
OCALA GROUP. -65-
-LINE SHOWl ALTITrLE
OF STRUCTURE SUmat
SCALE, MILES N
FIGURE 14.- STRUCTURE CONTOUR LINES ON TOP OF THE HARD BROWN
LIME", AVON PARK FORMATION.
aquifer in the project area (i.e., Suwannee/Ocala/Avon Park). Wells
completed in this unit are used chiefly for domestic supply.
The second stratigraphic unit is a sandy micritic limestone, indicative
of the Tampa Limestone. The top of the unit forms a broad shallow basin
extending southwest into Sarasota County (Figure 11). A closed depression
in the basin lies beneath the Myakka River between State Road 70 and State
Road 64. Wells are not generally completed in this unit because the
common occurance of sand causes open boreholes to collapse. This unit is
the lower portion of the confining system to the Suwannee/Ocala/Avon Park
The third stratigraphic unit located beneath eastern Manatee County is a
calcarenitic limestone unit in the Suwannee Limestone. This unit also
appears to be a broad basin (Figure 12). It is the effective top of the
Floridan aquifer in eastern Manatee County.
Figure 13 shows the top of the fourth stratigraphic unit in the project
area. The geologists' logs indicate this unit may be a marker bed for
the top of the Ocala Group and probably represents the Crystal River
member of the Group. The unit is a broad trough closed toward the north-
east. Guyton (1976a) reports that the Ocala Group acts as a poor semi-
confining unit between the Suwannee and Avon Park Limestone with changes
in water levels heads in one unit being effectively communicated through
the Ocala Group to the other unit.
The "hard brown line" is the fifth stratigraphic unit mapped in the
project area (Figure 14). This unit was mapped using descriptions from
forty driller's logs from wells in the area. It probably represents the
top of the dolomitic unit of the Avon Park Limestone.
Geologic Conditions Favorable for Groundwater Development
The results of the geologic investigation of the project area indicate
that there are four general conditions favorable to development of
groundwater. They are:
1. Limited or no recent sinkhole development,
2. Hydraulically inactive fracture systems that do not readily transmit
surface water vertically to aquifers,
3. Extensive and permeable carbonate rock systems composed of the
combination of the Suwannee-Ocala-Avon Park units, and
4. Extensive overlying confining system composed of the Hawthorn-
Each of these conditions is prevalent throughout the project area. There
are no geologic conditions unfavorable to the development of groundwater
supplies within the project area.
Hydrology is the study of the occurrence, magnitude and distribution of
water in the atmosphere and lithosphere. Movement of water between the
atmosphere and the lithosphere is often depicted as the hydrologic
cycle. In this hydrologic cycle, water occurs as precipitation, evaporation
and transpiration of surface water and groundwater.
The natural hydrologic cycle in the project area has not changed signi-
ficantly for many hundreds of years. Pumping of groundwater for irrigation
has been the major influence to the system imposed during the past two
decades. More recently the construction of two large reservoirs potentially
influences the hydrologic cycle in the area.
National Weather Service maintains three rainfall stations in the vicinty
of the project area from which data were used in this study: Arcadia,
Bartow and Bradenton (Figure 15). Table 3 lists pertinent average rainfall
amounts recorded during the period of record at each station.
FIGURE 15.- LOCATION OF PRECIPITATION STATIONS IN THE NORTHEAST
MANASOTA BASIN VICINITY.
Table 3. Summary of Average Rainfall Amounts for Selected Stations in
the Vicinity of the Project Area.
Period of Rainfall
Record Avg. Annual Maximum Minimum
Station (yrs) (in) (month) (in) (month) (in)
Arcadia 77 55.43 June 9.07 Nov. 1.84
Bartow 93 55.24 July 8.74 Nov. 1.96
Bradenton 15 56.35 Aug. 9.55 Nov. 1.91
The weighted average monthly rainfall is depicted graphically on Figure 16.
The water year is characterized with a dry season occurring in the spring
(March through May) and a summer wet season (June through September).
Summer thunderstorm activity produces about 60 percent of the annual
rainfall. The remaining amounts are generally produced as a result of
winter season frontal activity although rainfall in the fall and winter
Evapotranspiration (ET) is the combined loss of water from evaporation
from the soil and water surfaces and transpiration from vegetation surfaces.
Direct measurements of ET are very difficult and because of this ET is often
computed from meteorological records using a model developed by Penman (1948).
Daily values of ET from 1948 to 1975 were calculated from which monthly totals
and average monthly values were computed. Monthly total ET ranged from
O J F M A M J J A S O N D
WAERAGE PRECIPITATION (THIESSEN METHOD) 19I5-77
9g- TOTAL= 38.72
J F M A M J J A S 0 N D
AVERAGE EVAPORATION (PENMAN EQUATION)
SOURCE* N.O.A.A. SEABURN AND ROBERTSON, INC.
FIGURE 16.- AVERAGE MONTHLY PRECIPITATION AND EVAPORATION,
NORTHEAST MANASOTA BASIN.
high amounts in excess of 6.0 inches in June and July to low amounts less
than 1.0 inch in December and January. Average monthly values of ET in
inches shown below are depicted graphically on Figure 16.
Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.
1.26 1.59 2.86 3.70 5.16 5.17 5.05 4.71 3.60 2.83 1.51 1.15
Surface water in the project area consists of streams, lakes, impoundments
and ponds. There are three major river basins in the project area; Little
Manatee River, Manatee River and Myakka River Basins. Within the project
area each basin drains toward the west and eventually discharges into the
Gulf of Mexico or Charlotte Harbor. Figure 17 shows the drainage boundaries
of each of the river basins and the major tributaries to each river as
well as the location of flow gaging stations in the project area.
Continuous streamflow records have been collected for the Little Manatee
River, Manatee River and Myakka River by the U.S.G.S. at these stations
and pertinent information about these streams is shown in Table 4.
Based on these long term records, the average annual unit discharge for
the project area is about 1.0 cfs/smi. Using this value to estimate the
average annual discharge of all streamflow from the project area results
in an estimated total discharge of 327 cfs (211 mgd). (See Table 5)
s E., MILES
FIGURE 17.-LOCATION AND EXTENT OF DRAINAGE BASINS.
Table 4.-Summary of Streamflow Data in the Vicinity of Northeast Manatee
at Myakka City
Myakka River nr
nr Myakka Head
L. Manatee River
nr Ft. Lonesome
L. Manatee River
Length of Drainage
Table 5.-Estimated Average Annual Streamflow from the Project Area.
Drainage Estimated Average
Drainage Area Annual Discharge
Basin (sq mi) (cfs)
L. Manatee River 37 37
Manatee River 146 146
Myakka River 144 144
Total 327 327
In addition to the streams, there are numerous lakes, reservoirs, and ponds
within the project area that influence the hydrologic cycle. Small man-
made ponds (less than 1-2 acres) are commonly used for irrigation supplies
and cattle watering throughout the area. Larger ponds and lakes which
occur naturally throughout the area are found in karst depressions and
are usually internal drainage sinks that are hydraulically connected to the
water table aquifer. These lakes and ponds may be dry during part of
the year and may flood to the point of overflowing into other lakes and
stream channels at other times.
Long-Ogleby Swamp and Tatum Sawgrass are low-lying swamps and wetlands in
the southern part of the project area along the Myakka River. These swamps
overflow into the Myakka River and although discharge from these areas is
not known, it is thought to vary directly with water levels in the shallow
Two large man-made reservoirs are located in the project area. Parrish
Lake, shown in Figure 17, is a 4000 acre cooling pond constructed by
Florida Power and Light for the Little ManateePlant (Willow Power Plant).
The pond is located adjacent to the Little Manatee River near Parrish and
about 25 mgd of water is withdraw from the river as make up cooling water
to the pond.
Lake Manatee is a man-made reservoir on the Manatee River (Figure 17).
The 2,000 acre impoundment has a capacity of about 41,000 ac-ft and is
used to supply raw water to the Manatee County Utilities System. The
reservoir is designed to supply 30 mgd, however, a recent investigation,
(Bromwell, 1980), indicates that 50 mgd may be available from the reservoir.
These reservoirs, lakes and ponds are hydraulically connected to the
water-table aquifer and recharge to the deeper aquifer is minimal. The
consistent confining units underlying the water-table aquifer signifi-
cantly retards vertical movement of recharge water into the Floridan aquifer.
Conjunctive use of the surface water and groundwater resources will require
artificial means for accomplishment. Injection of surface water into the
limestone aquifers by means of recharge wells may provide a method of water
storage for later extractions, however, natural enhancement of recharge
as a general matter is not feasible in this area.
Guyton (1976 b) characterized the groundwater system in the project area
near Duette, Florida, as consisting of the following hydrologic units:
1. Shallow aquifer,
2. Hawthorn/Tampa, and
3. Suwannee/Ocala/Avon Park.
He reported yields from the shallow aquifer during aquifer pumping tests
from 28 to 32.5 gpm and transmissivity values from 2,000 to 14,000 gpd/ft.
The major confining unit to the Floridan aquifer lies beneath the shallow
aquifer and includes the Hawthorn and Tampa Formations. Guyton estimated
that the coefficient of leakance for this unit was between 1 x 10"4 and
1 x 10-5 gpd/ft3 in the Duette area.
Below the Hawthorn/Tampa confining unit lies the Suwannee/Ocala/Avon
Park aquifer unit. Water levels in the Suwannee and Avon Park forma-
tions are the same under nonpumping conditions, they diverge when the
Avon Park is pumped. This suggests that the Ocala is less transmissive
and may be a leaky semi-confining unit but that the Suwannee, Ocala,
and Avon Park operate as one aquifer unit over a long period of time.
Transmissivity values were reported by Guyton to be 17,500 and 770,000
gpd/ft in the Suwannee and Avon Park, respectively, near Duette, Florida.
Aquifer Withdrawal Tests
A summary of aquifer hydraulic coefficients reported at a number of sites
in the project area are listed in Table 6 and shown on Figure 18.
Three sites were selected to conduct aquifer pumping. tests to supplement
existing hydraulic data in the project area. Irrigation production wells
open to the Suwannee/Ocala/Avon Park unit were selected. All testing was
conducted in existing wells and interference from irrigation pumping was
unavoidable. For this reason, only two of the tests provided satisfactory
information for further evaluations.
Site 4 consisted of two pumped wells and two monitor wells (Figure 19).
The pumped wells were about 1,050 and 1,180 feet deep and cased to about
200 and 250 feet, respectively. The wells were open from the Hawthorn
into the Avon Park unit. The test was performed using a withdrawal rate
of 3,000 gpm for 3,000 minutes (2.08 days). The data were analyzed by
the Hantush Inflection Point method (Hantush 1960, 1964) and by the Chow
method described by Kruseman and De Ridder (1976). The analyses accounted
Table 6. -Summary of Hydraulic Coefficients, Determined from Aquifer Tests, in the Northeast Manasota Basin.
Test 1/ Transmissivity Storage Leakance Geologic Unit(s)
Site (gpd/ft) Coefficient (gpd/ft ) Open to Well Source
1 17,900 Shallow P. E. LaMoreaux (1975)
170 7.08 x 10-5 0 HAW -do-
170,000 6.4 x 10-4 2.6 x 10-2 TPA/SUW/OC/UAP2 -do-
2 870,000 8.2 x 10-4 SUW/OC/UAP Black, Crow & Eldsness (1973)
3 2,000 14,000 Shallow Guyton & Associates (1976)
Very Low 10-4 to 10-5 HAW -do-
17,500 SUW -do-
1,0003/ OC -do-
770,000 AP -do-
4 332,000 .2 x 10-3 2 x 10-2 HAW/TPA/SUW/OC/UAP4/ Seaburn & Robertson, Inc.
5 340,000 1 x 10-3 -do- -do-
6 6,152 2.12 x 10-2 Shallow Geraghty & Miller, Inc. (1974)
7 2,000 4 x 10-2 Shallow -do-
15,000 2 x 10-2 HAW/TPA -do-
8 1,200,000 4 x 10-4 6 x 10-4 SUW/OC/AP Geraghty & Miller, Inc. (1976)
1/ Reference No. refers to locations on Figure 18.
UAP = Upper Avon Park.
3/ Interpreted from an approximation of permeability resulting from model simulations by Guyton (1976).
4/ Tampa unit may not be open to well.
SOILS BORINGS SERIES T O
APPROXIMATE BORING ...., MYAKKA *T--.
LOCATION j 5 /
/y' LOCATION AND DESIGNATION 3 /" /
OF AQUIFER TEST SITES / / ..
(SEE TABLE 6 FOR TEST / .. --" "" "
RESULTS) .. o
00I I PROJECT BOUNDARY
SCALE, MILES I SA FROM COT MAP (JUL., 1974)1
'SEABURN AND ROBERTSON, INC.
FIGURE 18.-LOCATION OF SOILS BORINGS AND AQUIFER TEST SITES, NORTHEAST
MANASOTA BASIN. -81-
S2 VWTER TABLE
* PUMPED WELL
0 1000 2000
I SEABURN AND ROBERTSON. INC.
FIGURE 19.- WELL LOCATION MAPS FOR AQUIFER TEST SITES 4 AND 5.
AQUIFER TEST SITE 4
OAK KNOLL -*
AQUIFER TEST SITE 5
TO SR 64
2 X 9t
for the varying radii to the pumping wells by a method described by
Wilson (1977). In this analysis the distance from each observation well
to an effective center of pumpage, r, was computed as follows: the
products of the logarithm of the distance to each pumping well and the
discharge of each well were summed and divided by total discharge; r equals
the antilog of that quotient. Transmissivity and storage values are
averages determined from analyzing drawdown and recovery at the monitor
well by both methods. The leakance value was obtained from the analysis
of drawdown data only, using the Hantush inflection point method. Because
of regional pumping interference the recovery data were not sufficient to
permit a leakance value to be calculated. The analyses indicate hydraulic
coefficients for the Hawthorn/Tampa/Suwannee/Ocala/Avon Park unit are:
T = 332,000 gpd/ft (44,382 ft2/day)
S = 2 x 10-3
L = 2 x 10-2 gpd/ft3 (2.7 x 10-3 day-1)
Site 5 consisted of the pumped well and three monitor wells (Figures 18
and 19). The pumped well was about 1,250 feet deep and cased to about
250 feet; open from the Hawthorn into the top of the Avon Park unit.
The wells used to monitor level responses during the test included a
water table well and two wells open to the Floridan aquifer similar to the
pumped well. The test well was pumped at about 1,000 gpm, while withdrawal
and recovery water level data were collected. Background water level
changes during the test were estimated from water level measurements
leading into and following the test. Field data are contained in
The test data were analyzed using the Chow method for leaky aquifers as
described by Kruseman and De Ridder (1976). Because the well is open to
a number of producing units, the transmissivity (T) and storage (S) values
reported below represent values for the entire open hole section. The
analytical results indicate that hydraulic coefficients for the Hawthorn/
Tampa/Suwannee/Ocala/Avon Park unit are:
T = 340,000 gpd/ft (45,451 ft2/day)
S = 1 x 10"3
The data collected was not sufficient to analyze for leakance because of
regional water level trends and some local interference.
Hydraulic data indicate that the Hawthorn Formation is the least trans-
missive while the Avon Park is the most transmissive. Most major wells
in the Northeast Manasota Basin project area, however, are open to the
entire Hawthorn/Tampa/Suwannee/Ocala/Upper Avon Park system. Therefore,
the hydraulic coefficients determined from existing wells describe
the aquifer response to pumping from the deeper high producing portion
of the aquifer.
Potentiometric Heads and Groundwater Flow
The shallow auger boring program indicated that the surficial aquifer water
surface (water table) was generally about four feet below land surface during
November and December throughout the project area. Figure 18 indicates
the locations of the selected soils borings. Figure 20 shows the estimated
altitude of the water table aquifer across the project area determined by
subtracting four feet from the topographic contours. The shallow auger
borings completed during this investigation provided site-specific water
-45-UNE SHOWING ALTITUDE OF WATER
SCALE, MILES I
FIGURE 20.- ESTIMATED WATER TABLE SURFACE, NOVEMBER -DECEMBER, 1979.
table data used to prepare this map and indicated that the water table
ranged from three to five feet below land surface throughout the project
area. Groundwater flow in the shallow aquifer is toward the southwest,
regionally, and toward stream valleys, locally. The variations of
water levels in the shallow aquifer across the project area from the
northeast part to the southwest part is about 75 feet.
A map of the potentiometric surface in the Hawthorn unit was constructed
from water level data reported in driller's logs of wells completed between
1963 and 1979 (Figure 21). The construction of the map assumes that
water levels did not change greatly during the period. This is thought
to be reasonable because the potentiometric surface of the underlying
aquifer has not changed more than annual fluctuations and because of the
permeability of the unit itself. The map is the best available estimate
of Hawthorn water levels throughout the project area.
Regionally, the Floridan aquifer potentiometric surface has been mapped
by several investigators. Because wells constructed in the project
area are usually open to the Suwannee/Ocala/Avon Park, these maps represent
potentiometric surface across that combined aquifer unit. Water levels
in this unit in the vicinity of Duette, Florida are 60 to 90 feet lower
than water levels in the Hawthorn, but similar to water levels in under-
lying units (Guyton, 1976a). During this investigation, water levels
were as much as 40 feet below the level of the Hawthorn across
the project area.
--,0 LINE SHOWING
ALTITUDE OF THE
SCALE, MILES I
FIGURE 21.-ESTIMATED POTENTIOMETRIC SURFACE OF THE HAWTHORN
FORMATION, 1963-1979. -87-