• TABLE OF CONTENTS
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 Front Cover
 Front Matter
 Table of Contents
 List of Illustrations
 Main
 Envelope














Title: Types, features, and occurences of sinkholes in the karst of west-central Florida
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Title: Types, features, and occurences of sinkholes in the karst of west-central Florida
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Table of Contents
    Front Cover
        Front cover
    Front Matter
        Page i
        Page ii
    Table of Contents
        Page iii
    List of Illustrations
        Page iv
        Page v
        Page vi
    Main
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Full Text






TYPES, FEATURES, AND OCCURRENCE
OF SINKHOLES IN THE KARST
OF WEST-CENTRAL FLORIDA
Alr ~ r S'<" *


JU 1 993


A r A I'n ,Cersitv Of F!^::
'T-T- -- ...., Eo 0 o A


a I


*.d









U.S. GEOLOGICAL SURVEY
Water-Resources Investigations Report 85-4126


Prepared in cooperation with the


SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT


/ //


- ; )140







TYPES, FEATURES, AND OCCURRENCE OF SINKHOLES

IN THE KARST OF WEST-CENTRAL FLORIDA

By William C. Sinclair, J. W. Stewart, R. L. Knutilla,
A. E. Gilboy, and R. L. Miller


U.S. GEOLOGICAL SURVEY


Water-Resources Investigations Report 85-4126

















Prepared in cooperation with the

SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT


Tallahassee, Florida





UNITED STATES DEPARTMENT OF THE INTERIOR


DONALD PAUL HODEL, Secretary


GEOLOGICAL SURVEY

Dallas L. Peck, Director


For additional information
write to:

District Chief
U.S. Geological Survey
Suite 3015
227 North Bronough Street
Tallahassee, Florida 32301


Copies of this report may be
purchased from:

Open-File Services Section
Western Distribution Branch
U.S. Geological Survey
Box 25425, Federal Center
Denver, Colorado 80225
(Telephone: (3031 236-7476)





CONTENTS

Page
Abstract -----------------------------------------------------------------
Introduction ------------------------------------------------------------- 2
Purpose and scope --------------------------------------------------- 2
Description of area ------------------------------------------------- 3
Climate ------------------------------------------------------------- 9
Previous investigations --------------------------------------------- 9
Geology ------------------------------------------------------------------ 9
Tertiary System ----------------------------------------------------- 11
Quaternary System --------------------------------------------------- 17
Structural setting -------------------------------------------------- 18
Ground-water hydrology --------------------------------------------------- 18
Recharge and discharge areas ---------------------------------------- 20
Thickness of cover material ---------------------------------------- 22
Water-level fluctuations in aquifers -------------------------------- 26
Surficial aquifer ------------------------------------------- 26
Upper Floridan aquifer ---------------------------------------- 26
Head differences between aquifers ------------------------------ 28
Directions of ground-water flow -------------------------------- 28
Karst development -------------------------------------------------------- 33
Lithology and water movement ---------------------------------------- 37
Dissolution of aquifer materials ------------------------------------ 38
Sea levels ---------------------------------------------------------- 42
Warning signs ------------------------------------------------------------ 43
Types and features of sinkholes ------------------------------------------ 43
Sinkholes in areas of bare or thinly covered limestone -------------- 43
Limestone-solution sinkholes ----------------------------------- 45
Limestone-collapse sinkholes ----------------------------------- 45
Sinkholes in areas of thickly covered limestone --------------------- 48
Cover-subsidence sinkholes ------------------------------------- 48
Cover-collapse sinkholes --------------------------------------- 48
Induced sinkholes --------------------------------------------------- 50
Sinkhole collapse related to ground-water withdrawals ---------- 51
Sinkholes related to construction ------------------------------ 55
Occurrence of sinkholes -------------------------------------------------- 57
Sinkhole-type areas ------------------------------------------------- 57
Reported sinkholes in west-central Florida -------------------------- 66
Sinkholes as sources of water supply or ground-water pollution ---------- 76
Summary and conclusions --------------------------------------- -------- 77
References ---------------------------------------------------------------79



ILLUSTRATIONS

Page

Figure 1. Map showing location of study area --------------------------- 4

2. Map showing topography of west-central Florida --------------- 5
3. Topographic sections ---------------------------------------- 6


iii








ILLUSTRATIONS--Continued


Ligure 4. Map showing locations of ridges in west-central Florida ------
5. Map showing locations of geologic sections ------------

6. Geologic sections A-A', B-B', and C-C' ------------- ---

7. Geologic sections D-D' and E-E' ----------------------

8. Geologic sections F-F' and G-G' ---------------- --

9. Map showing structural features in Florida ---- -----

10. Map showing distribution of Upper Floridan aquifer
recharge and discharge under natural conditions,
excluding spring-flow ------- --------------------

11. Map showing generalized thickness of the surficial
deposits overlying the confining bed ---------------------

12. Map showing generalized thickness of the confining bed
overlying the Upper Floridan aquifer --- ---------------

13. Map showing generalized thickness of the unconsolidated
deposits overlying the Upper Floridan aquifer -------------

14. Hydrographs of month-end water levels in surficial aquifer
wells near Lutz and Lake Placid, 1965-84 ---- -----

15. Hydrograph of water levels in the Maddox well near
Bowling Green, 1981-82 ----------------------------------

16. Hydrograph of month-end water levels in the Maddox well
near Bowling Green, 1963-84 --------- ---------------

17. Hydrographs of month-end water levels in two wells in the
Floridan aquifer system, 1961-84------------------- --

18. Map showing the potentiometric surface of the Upper
Floridan aquifer, September 1979------------------

19. Map showing the potentiometric surface of the Upper
Floridan aquifer, May 1979 ------------------

20. Generalized hydrogeologic section A-A' showing flow
patterns ---------------------------------------- --
21. Generalized hydrogeologic section B-B' showing flow
patterns ------ ------------------------------- --

22. Generalized hydrogeologic section C-C' showing flow
patterns ---------------------------------------

23. Diagram of stages in development of a limestone-solution
sinkhole -------------------------------------------

24. Diagram of stages in development of a limestone-collapse
sinkhole ----- ------------------------------------

25. Diagram of stages in development of a cover-subsidence
sinkhole -------------------------------------------







ILLUSTRATIONS--Continued

Page

Figure 26. Diagram of stages in development of a cover-collapse
sinkhole --------------------------------------------------- 52

27. Hydrographs of pumpage at the Section 21 well field
and water levels in shallow and deep observation wells,
1961-66 ---------------------------------------------------- 54

28. Photograph of the bed of former Lake Grady near Tampa
drained by a cover-collapse sinkhole, May 1974 ------------- 56

29. Map showing zones of different sinkhole types -------------- 58

30. Rose diagram showing lineation of sinkholes within the
Section 21 well-field area near Tampa ---------------------- 60
31. Map of Section 21 well-field area near Tampa -------------- 61
32. Map showing bottom topography of Lake Eloise at
Winter Haven ----------------------------------------------- 63

33. Aerial photograph of a sinkhole near Winter Haven ---------- 64
34. Map showing topography and locations of sinkholes,
Polk County ---------------------------------------------- 65
35. Map showing major lineations along which sinkholes have
occurred in Polk County ------------------------------------ 66
36. Cross section of a cover-collapse sinkhole beneath
Deep Lake near Arcadia ------------------------------------- 67

37. Map showing locations of reported sinkholes ---------------- 75



TABLES

Page
Table 1. Hydrogeologic framework ---------------------------------------- 10
2. Principal features of the major types of sinkholes ------------- 44

3. Inventory of reported sinkholes -------------------------------- 68






CONVERSION FACTORS


For use of readers who prefer to use metric units, conversion factors for


terms used in this report are listed


Multiply


below:


To obtain


inch (in.)
foot (ft)
mile (mi)
square inch (in2)
square mile (mi2)
cubic inch (in3)
gallon per minute
(gal/min)
million gallons per day
(Mgal/d)


Temperatures in degrees Fahrenheit
as follows:


0.0348
1.609
645.2
2.590
16.39
0.630


0.0438


millimeter (mm)
meter (m)
kilometer (km)
square millimeter (mm2)
square kilometer (km )
cubic centimeter (cm3)
liter per second (L/s)


cubiS meter per second
(m /s)


(F) can be converted to degrees Celsius (OC)


*F = 1.8C + 32









TYPES, FEATURES, AND OCCURRENCE OF SINKHOLES

IN THE KARST OF WEST-CENTRAL FLORIDA


By William C. Sinclair, J. W. Stewart, R. L. Knutilla,
A. E. Gilboy, and R. L. Miller


ABSTRACT


Sinkholes are a natural and common geologic feature in areas underlain by
limestone and other soluble rocks. Four major types of sinkholes are common
to west-central Florida. They include limestone-solution, limestone-collapse,
cover-subsidence, and cover-collapse sinkholes. The first two occur in areas
where limestone is bare or is thinly covered. The second two occur where there
is a thick cover (30 to 200 feet) of material over limestone.

Limestone-solution sinkholes result from subsidence of overlying materials
that occurs at approximately the same rate as dissolution of the limestone. The
sinkholes reflect a gradual downward movement of the land surface and development
of funnel-shaped depressions. Limestone-collapse sinkholes occur when a solution
cavity grows in size until its roof can no longer support its weight, causing
generally abrupt collapse that is sometimes catastrophic.

Cover-subsidence sinkholes develop as sand in the cover material moves
downward into space created in limestone by dissolution. Resultant sinkholes
develop gradually and are generally only a few feet in diameter. Cover-collapse
sinkholes occur where clay layers that overlie limestone have sufficient cohe-
siveness to bridge the developing cavities in the limestone. Eventual failure
of the bridge results in a cover-collapse sinkhole that may develop suddenly.

Large withdrawals of water for various uses may provide a triggering
mechanism for sinkholes to occur. Loss of water's buoyant support of unconsol-
idated deposits that overlie cavities can cause the materials that bridge the
cavity to fail and sinkholes to appear. Conversely, loading of the land sur-
face by construction, such as impoundments, may cause collapse of materials
that bridge cavities and sinkholes to develop. Impoundments may also provide
continuous sources of recharge water and hasten development of cavities in
limestone.

West-central Florida was divided into seven zones based on geology, land-
scape, and geomorphology and the relationship of these factors to the types of
sinkholes that occur in each zone. The zones are: (1) areas of bare or thin
cover that experience slow developing limestone-solution sinkholes; (2) areas





of thin cover, little recharge, high overland runoff, and few sinkhole occur-
rences; (3) areas of incohesive sand cover of 50 to 150 feet that have high
recharge and generally experience cover-subsidence sinkholes; (4) areas that
have 25 to 100 feet of cover, many sinkhole lakes, and cypress heads and expe-
rience predominantly cover-collapse sinkholes; (5) areas of 25 to 150 feet of
sand cover overlying clay that experience cover-collapse and cover-subsidence
sinkholes; (6) areas with more than 200 feet of cover, numerous lakes and sink-
holes, and high land-surface altitudes that experience numerous cover-subsidence
sinkholes and occasional large-scale cover-collapse sinkholes; and (7) areas
with cover greater than 200 feet that have 100 or more feet of clay with high
bearing strength and low leakance that preclude infiltration of corrosive water
and development of sinkholes; however, some cover-collapse sinkholes do occur.


INTRODUCTION


Sinkholes are a natural and common geologic feature in areas underlain by
limestone and other rock types that are soluble in natural water. Topographi-
cally, sinkholes are usually identified by closed depressions in the land sur-
face. In west-central Florida, sinkholes are formed by solution of near-surface
limestone or by collapse of near-surface materials into underlying solution
cavities. Sinkholes are a part of the erosion process analagous to valleys that
are carved by rivers in areas underlain by insoluble rocks.

Many lakes in central and west-central Florida are the result of sinkhole
collapse. However, when it is realized that these lakes represent sinkholes
that have occurred during the past 2 to 5 million years, the frequency of their
occurrence is not great.

Sinkholes that collapse abruptly occur infrequently under natural condi-
tions. The abrupt collapse-type sinkholes, however, have occurred at increasing
rates during the past few decades, not only in west-central Florida, but through-
out the world. The cause of this increase in sinkhole occurrence can generally
be attributed to activities of man, such as pumping of ground water, dredging
or diversion of surface water, or construction of reservoirs and ponds. Man
imposes abrupt stresses on land that can induce or hasten development of sink-
holes. Induced sinkholes can be a hazard in developed and developing areas of
west-central Florida, and their occurrence is expected to continue.

An understanding of sinkholes and their relation to the hydrologic system
will become increasingly important as stresses on the system intensify and the
likelihood of sinkhole occurrence increases. Thus, in 1979, the U.S. Geological
Survey, in cooperation with the Southwest Florida Water Management District,
began this study to determine hydrogeologic factors that control sinkhole
occurrences in west-central Florida.


Purpose and Scope


The objectives of the study are to describe hydrogeologic factors that
control sinkhole development and to distinguish types of sinkholes common to
west-central Florida. An understanding of natural factors in the hydrologic






and geologic setting, such as topography, drainage patterns and densities,
thickness and type of surficial materials, thickness of confining beds, head
differences between water levels in surficial and artesian aquifers, and
surface-water and ground-water flow patterns, will provide insight to natural
sinkhole development and sinkhole development caused by man's activities. The
information is needed to assist in determining the potential for sinkholes to
occur and their mode of occurrence.

2
The study area includes about 10,000 mi in west-central Florida (fig. 1).
Boundaries of the area are those of the Southwest Florida Water Management
District (SWFWMD). The report provides a map of geomorphic units that are clas-
sified by sinkhole occurrence, density, and type. Also provided are a tabulation
of reported sinkholes, their locations, dates of occurrence, and descriptions,
and a discussion of signs warning of sinkhole development.


This report supplements and amplifies the statewide map report "Sinkhole
type, development, and distribution in Florida," by Sinclair and Stewart (1985).
In particular, the four areas of sinkhole occurrence in the statewide report have
been subdivided herein into seven zones and the category of solution sinkholes
has been subdivided into limestone-solution and limestone-collapse sinkholes.
Zones 1 and 2 herein comprise Area I of the statewide report; zone 3 comprises
Area II; zones 4, 5, and 6 comprise Area III; and zone 7 comprises Area IV of
the statewide report.



Description of Area



West-central Florida is characterized by relatively flat, generally
swampy lowlands in coastal areas and by gently rolling hills in inland areas.
Except for a coastal ridge in central Pinellas County that has altitudes as
much as 100 feet above sea level, coastal areas are less than 50 feet in alti-
tude (fig. 2). Cross sections that illustrate the topography are shown in
figure 3.


In the southern half of the area, the low-lying coastal plain gradually
rises toward the east, butting against sand-covered ridges that have altitudes
of more than 150 feet above sea level (figs. 2 through 4). Within the ridge
area are numerous sinkhole lakes--circular sinkholes that have filled with
water. Surface drainage in the south is relatively well developed.


In the northern half of the area, the low-lying coastal plain also butts
against sand ridges trending in a north-northwesterly direction (fig. 4). East
of the Brooksville Ridge, the topography is subdued. The northern area has
numerous swamps, lakes, and shallow sinkholes. Irregular karst topography and
poorly developed surface drainage occur in the northern areas. From the northern
part of Pasco County northward, surface drainage, except for the Withlacoochee
River, is essentially nonexistent. The only rivers are near the coast and they
discharge water from springs. Virtually all springflow is discharge from the
Upper Floridan aquifer.






SAL a' a


0 20 40 MILES
I I I
0 20 40 60 KILOMETERS




Figure 1.--Location of study area.






82000


EXPLANATION
ALTITUDE OF LAND
SURFACE ABOVE SEA
MA, IN LEVEL, IN FEET



S0-50


__ 50-100


S100-150


GREATER THAN
150
SOUTHWEST FLORIDA
WATER MANAGEMENT
DISTRICT BOUNDARY


280-


270-


0 20 40 MILES
S2I 0 I I K
0 20 40 60 KILOMETERS


Figure 2.--Topography of west-central


29-


GULF

OF

MEXICO


82045'


81015'


Florida.






ALTITUDE,
S IN FEET
m r0
< o 0 o o 0
r 0 0 0 0
SiC GULF OF MEXICO
-LAKE TARPON
















-H/LLSBOROUGH
RIVER




BLACKWATER
CREEK




MINED
AREA



LAKES JULIANA
AND MATTIE


ALTITUDE,
IN FEET
So0 0


in
*11
GULF OF MEX/CO r
m
<
f
r


BROOKSVILLE




RIDGE


- WITHLACOOCHEE
RIVER


ALTITUDE,
IN FEET
(,i o um


>-ULF OF MEXICO


BROOKSVILLE


RIDGE
RIDGE


-W/THLACOOCHEE
RIVER







SUMTER
UPLANDS


-
01







I4
1I ],., c
,1 5:
o I s"i


_ AJ~A
QlII


0, 5 10 15 24
1',T 0 O


0 MILES
KILOMETERS


site locations are shown in figures 1 and 4.)


9cr


2001
i 15o


SEA LEVEL


1 50
LL 100
z


SEA LEVEL


00

200 r


150 -


SEA LEVEL


fi


shown in figure 4;





8245' 8200' 8115'
I I I
BROOKSVILLE
IDGE




S MARION




LAKE
:. -: SU TER T.
:::* UP HANDS



WINTER HAVEN
.. RIDGE

S'Lake Jlioana
Pon and Mottie




BRIDGE
SLAKE


M-ANAT E EXPLANATION
MANATEE

LOCATION OF
E mi I TOPOGRAPHIC
IL SECTIONS
DoE OTO SHOWN IN
SARAOTA | FIGURE 3

270- SOUTHWEST FLORIDA
FihLOTTEtg WATER MANAGEMENT
C \iroik DISTRICT BOUNDARY



0 20 40MILES
I I I II
0 20 40 60 KILOMETERS



Figure 4.--Locations of ridges in west-central Florida.
(Modified from White, 1970.)




Climate

The climate of west-central Florida is subtropical and is characterized
by warm, humid summers and mild winters. Some rainfall normally occurs during
each month, but a distinct rainy season extends from June through September and
a low rainfall season extends from October through May. About 60 percent of
the annual rainfall occurs during the rainy season. Winter rainfall is rela-
tively light because west-central Florida is south of the normal southern limit
of winter frontal systems, the causative factor of winter rainfall. Summer
rainfall is derived principally from convectional storms that usually occur in
the afternoon and early evening and from occasional tropical storms. Spatially,
summer rainfall is highly variable; areas only a few miles apart often receive
widely differing amounts of rain.

The average annual temperature at Lakeland, which typifies the area, is
72.7"F. Average monthly temperatures range from 61*F in January to 820F in
July and August. Rainfall at Lakeland averages 48.3 inches annually. The
amount of rainfall received throughout west-central Florida provides an ade-
quate source of recharge water, a primary need for sinkhole development.


Previous Investigations

Numerous reports have been written on the geology and hydrology of west-
central Florida, but few reports deal primarily with the subject of sinkholes.
Some related reports include a report by Back and Hanshaw (1971) that describes
the rates of physical and chemical processes in a carbonate aquifer. Sinclair
(1982) described sinkhole development resulting from ground-water withdrawals
near Tampa. Sinclair (in press) also prepared a general information pamphlet
that describes sinkhole development and their occurrence and probability in
Florida. Miller and others (1981) described the morphology of sinkholes and
lakes in the Withlacoochee River region. Sinclair and Stewart (1985) described
sinkhole features in Florida, including descriptions of geology and principles
of ground-water movement as related to the creation of sinkholes. Stewart
(1968) described the effects of pumping on hydrology .in parts of Hillsborough,
Pinellas, and Pasco Counties.

Other related reports include that by Ryder (1982) who developed a
digital flow model of the area and described its geologic setting. Vernon
(1951) provided geologic information for Citrus and Levy Counties. In a re-
port on artesian water in the Tertiary limestone in the Southeastern States,
Stringfield (1966) provides data on geologic formations and sinkhole occur-
rences. White (1970) described the geomorphology of the Florida peninsula.
Stewart (1982) described ground-water degradation incidents in west-central
Florida, some of which relate to sinkholes.


GEOLOGY

Limestone underlies the Florida peninsula to depths of several thousand
feet. Tertiary limestones and formations (table 1) -ere deposited much as
the limestones of the Bahamas Bank are deposited today. Deposition of each




Table l.--Hydrogeologic framework
IHodified from Wilson and Gerhart, 1982, table 1; Miller, in press, table 3]


Series


Holocene
Pleistocene




Pliocene


Stratigraphic
unit



Surficial sand,
terrace sand,
phosphorite


Undifferetiated
depasita-


General lithology


Predominantly fine sand;
interbedded clay, marl,
shell, limestone, phos-
phorire.

Clayey and pebbly sand;
clay, marl, shell,
phosphatic.


Quaternary





Tertiary


Paleocene


Cedar Keys
Formation


Dolomite and limestone
with beds of anhydrite.


Major
lithologic
unit


Sand


Clastic
posits


Carbonate and
classic de-
-- posits


Miocene Hawthorn Dolomite, sand, clay,
Formation and limestone; silty,
phosphatic.


Tampa Lime- Limestone, sandy, phos-
stone phatic, fossiliferoua;
sand and clay in lower
part in some areas.

Oligocene Suwannee Limestone, sandy lime-
Limestone stone, fossiliferous.

Eocene Ocala Lime- Limestone, chalky, for-
stone aminiferal, dolomitic
near bottom.

Avon Park Limestone and hard brown
Formation dolomite; intergranular
evaporate in lower part
in some areas.

Oldsmar Dolomite and limestone,
Formation containing intergranu-
lar gypsum in most
areas.


Hydrogeologic
unit


Surficial aquifer


Confining
bed




Aquifer


INTERMEDIATE

AQUIFER

AND

CONFINING


UNITS






Confining
bed


SFLORIDAN AQUIFER SYSTEM

Upper Floridan aquifer


Middle confining unit


Lower Floridan aquifer


Sub-Floridan confining
unit


Geologic process


Fluctuations of see level
with consequent high wa-
ter tables and deposition
in low-lying areas alter-
nating with low water
tables and accelerated
weathering of soluble
rocks.






Exposure and weathering
Carbonate deposition

Exposure and weathering






Carbonate deposition







Exposure and weathering
Carbonate deposition


System


Car-
bon-
ate
and
evaporite
deposits


Age esti-
mates of
boundaries,
in milljin
years-






- 2 -


- 24 -


- 38 -














- 55 -


63


/Geologic Times Chart, 1984.
includess all parts of aloosahatee Marl, Boe Valley Formation, Alahua Formation, and T a Formation.
includes all or parts of Caloosahatchee Marl, Bone Valley Formation, Alachua Formation, and Tamiami Formation.





formation was followed by a period of emergence, erosion, and solution that
resulted in development of surface irregularities and solution cavities.
Paleokarst surfaces karstt features that were formed in previous geologic
times) probably are accordant with each of the periods of emergence and ero-
sion that occurred at the end of deposition of the Avon Park Formation, the
Ocala Limestone, and the Suwannee Limestone (table 1).

Clastic sediments that mantle Tertiary limestone were carried from the
Appalachian Mountains and the Piedmont, a plateau at the base of the mountains,
following the Miocene Epoch and were deposited with limestone. As the depo-
sitional environment changed from marine to estuarine and terrestrial in late
Tertiary time, plastic sediments became predominant. Descriptions of the for-
mations, starting with the Avon Park Formation, are given in the following
sections.


Tertiary System


Eocene.--The Avon Park Formation (table 1) is composed of fossiliferous
limestone and dolomite. The limestone is a moderate brown, dark yellow-brown
to dusky yellow-brown, porous and very-fine to medium-grained, and may be crys-
talline or saccharoidal in texture. The formation is very permeable and cavern-
ous where extensive dissolution has occurred. Its thickness ranges from a few
feet in northern areas to more than 800 feet in the south (figs. 5 through 8).
The differences in thickness reflect depositional and tectonic effects and
postdepositional erosion.

The Ocala Limestone is a shallow-water marine limestone composed of
foraminiferal tests, large foraminifera, mollusks, and large echinoids. Lith-
ologically, it is a soft-to-hard, highly fossiliferous limestone that contains
minor amounts of dolomite. The Ocala Limestone crops out in parts of Citrus,
Levy, and Marion Counties. Where the Ocala Limestone is close to land surface,
dissolution of limestone is characterized by caverns and solution pipes. In
Levy, Citrus, Marion, Polk, and Sumter Counties, the top of the Ocala Limestone
is about 90 feet above sea level. It dips to the south and reaches a maximum
thickness of about 600 feet (figs. 5 through 8).

Oligocene.--The Suwannee Limestone is biogenic, predominantly foraminif-
eral test packstone to grainstone. Interbeds may contain quartz sand, and
dolomite is common toward the unit's base from the Tampa Bay area southward.
The upper part may contain thin'chert lenses and be highly macrofossiliferous.

The Suwannee Limestone consists of relatively pure calcium carbonate. It
is exposed locally in southern Citrus and Sumter Counties, most of Hernando
County, parts of Pasco County, and in the northeast corner of Hillsborough
County. The Suwannee Limestone pinches out in Polk County (section C-C',
fig. 6, and sections F-F' and G-G', fig. 8), is absent in Levy County, and
crops out in central Citrus County where it caps the topographic highs to form
part of the central highlands (White, 1970). The Suwannee Limestone is as
much as 300 feet thick in the south (sections D-D' and E-E', fig. 7).









8245'
I


29-


GULF
OF
MEXICO









28-

C


9855


81015'
I


EXPLANATION
A A'
LINE OF SECTION
SHOWN IN FIGURES
6 THROUGH 8
1e20
WELL LOCATION AND
REFERENCE NUMBER



















43X

i HIGHLANDS


0 10 20 MILES


0 10 20 30


KILOMETERS


Figure 5.--Locations of geologic sections.


82000
I


~











UNDIFFERENTIATED
DEPOSITS

OCALA LIMESTONE


EXPLANATION

SWELL LOCATION AND
REFERENCE NUMBER


10 20 30 MILES


I I I I I
0 10 20 30 40


UNDIFFERENTIATED DEPOSITS


VERTICAL EXAGGERATION OF ALL SECTIONS x250


KILOMETERS


Figure 6.--Geologic sections A-A', B-B', and C-C'. (Locations of sections
are shown in figure 5. Modified from Gilboy, 1982.1


A
200 2
ft
100
SEA LEVEL
100


TAMPA LIME














UNDIFFERENTIATED
DEPOSITS


EXPLANATION

n WELL LOCATION AND
REFERENCE NUMBER


O 10 20 30 MILES
0 10 20 30 40 KILOMETERS


OCALA
LIMESTONEE


VERTICAL EXAGGERATION OF SECTIONS x 256


Figure 7.--Geologic sections D-D' and E-E'. (Locations of sections are shown in
figure 5. Modified from Gilboy, 1982.)























































Figure 8.--Geologic sections F-F' and G-G'. (Locations of sections are shown
in figure 5. Modified from Gilboy, 1982.)





15





Miocene.--Miocene sediments are divided into two stratigraphic units, each
unit formed as a result of a marine transgression. The two units, from older to
younger, are the Tampa Limestone and the Hawthorn Formation (table 1).

A shallow marine environment covered most, if not all, of western
peninsular Florida and resulted in deposition of the Tampa Limestone (Carr and
Alverson, 1959). The Tampa Limestone consists of limestone and varying amounts
of quartz sand and clay embedded in a carbonate matrix. The unit is absent in
the north (section A-A', fig. 6), but is as much as 300 feet thick in the south
(section D-D', fig. 7). The formation is differentiated from the overlying
Hawthorn Formation based on a decrease in or absence of phosphorite and an in-
crease in quartz sand within the rock matrix (King and Wright, 1979). The con-
tact between the Hawthorn Formation and Tampa Limestone is commonly marked by
a chert layer at the top of the Tampa Limestone and a weathered, gray, dolomitic
limestone. From approximately Sarasota County to southern Hardee County, the
lower part of the Hawthorn Formation and Tampa Limestone become lithologically
undifferentiable. The Tampa Limestone contains a much higher percentage of
phosphorite and clay in southern areas than in Hillsborough County. The Tampa
Limestone deposits thin to the north and east of the county. The unit is ab-
sent in central Polk County (section C-C', fig. 6) where it grades into a
blue-green clay that is devoid of carbonates.

During middle Miocene time, the sea rose and covered most of peninsular
Florida except the Ocala uplift area and deposited plastic and carbonate sedi-
ments of the Hawthorn Formation. The diverse composition of the Hawthorn
sediments reflects depositional environments that include open marine and
shallow-water coastal marine. A small percentage of the sediments was derived
from fluvial and estuarine processes (Riggs, 1979). Postdepositional reworking
of the deposits left residual sand and clay in some places.

The Hawthorn Formation is absent in the north, thickens to the south,
and attains a maximum thickness of approximately 600 feet in southern De Soto,
Highlands, and Charlotte Counties (figs. 5 through 8). The basal Hawthorn
section is composed of carbonate deposits (usually dolomitic) that contain
varying amounts of interbedded quartz sand, clay, and phosphate. The middle
section consists of interbedded sandy carbonate, clayey sand, and sandy clay.
The upper Hawthorn section is predominantly composed of plastic deposits that
consist of quartz, phosphate sand and pebbles, and light green to a moderately
dark gray clay (Hall, 1983).

The trifold subdivision of the Hawthorn Formation is most apparent in the
south. Elsewhere, one or two of these units may be absent, or the upper unit
may lie directly over the lowermost unit. In the north, the units become less
distinctive and merge to a single unit where a sandy phosphatic clay predomi-
nates, or the formation is absent.

Pliocene.--During early Pliocene time, west-central Florida remained
emergent, whereas south and central Florida were submerged. The shoreline
extended southward through Lake County to Sebring, circled westward through
Arcadia and Sarasota and northwest across the Gulf of Mexico (Cathcart, 1966).
The Alachua and Bone Valley Formations that contain fossils of early Pliocene
vertebrates accumulated as fluvial deltaic deposits. The shell and marl beds,
sandy limestone, and calcareous clay of the Tamiami Formation seem to be the
result of contemporaneous marine deposition. The Alachua Formation consists




of a blue to gray, sandy clay that weathers to yellow-orange or moderate-red
due to the presence of iron oxide. The formation generally contains suffi-
cient clay to give it a distinct plasticity, but it also contains significant
amounts of quartz sand. This unit is localized, and remnants may exist in the
extreme northern and eastern parts of the study area. This unit is typically
found in solution cavities in weathered karst surfaces.
The Bone Valley Formation of Pliocene age consists of fine to coarse quartz
sand, clay, thin chert sections, phosphate nodules, and vertebrate fossil frag-
ments. It is well represented on structural highs in the Lakeland Ridge area.
The Bone Valley Formation extends approximately from Polk and Hillsborough
Counties southward into Manatee and Hardee Counties. Its thickness is generally
less than 20 feet, but it is 60 feet or more thick in eastern Polk and Hardee
Counties (Cathcart and McGreevy, 1959).
The Tamiami Formation is composed principally of white to cream-colored,
sandy limestone that grades downward into clay, silt, and very fine sand beds
of low permeability. The formation contains abundant oysters and littoral de-
posits that attest to a shallow marine environment of deposition. The Tamiami
Formation may be present in the extreme southern parts of the study area
(Hunter and Wise, 1980). The Caloosahatchee Marl overlies the Tamiami Forma-
tion and consists of a thin sequence of interbedded clay, calcareous clay, and
sand that locally contain broken shelly material (Miller, in press). The upper
part of the Caloosahatchee Marl is of Pleistocene age (table 1).


Quaternary System


Pleistocene and Holocene.--Changes that took place in sea level during the
Pleistocene Epoch within the Quaternary System resulted in significant changes
in geologic formations that make up the Florida peninsula. Pleistocene and
Holocene sediments within west-central Florida consist of unconsolidated to
poorly indurated clastics, dominated by quartz sand and shell (marine mollusks
predominantly) interbedded with clay and marl. Pleistocene strata include
marine and nonmarine beds and may consist of residual lacustrine, fluvial, and
eolian deposits. During the Pleistocene Epoch, parts of the Florida peninsula
remained relatively stationary, although sea level repeatedly fluctuated in
response to glaciation.
Five major stands of low sea level occurred during the Pleistocene Epoch.
During each stand, the shoreline receded further seaward. Erosion, subterranean
solution, and widespread sinkhole development in the calcareous rocks became
prevalent. Subsequent to each, period of low sea level, there were associated
high-stand periods of deposition that formed five terraces and shorelines
(Cooke, 1931). During each of these interglacial periods, coastal features
such as spits, bars, and lagoons were formed along the shoreline (Healy, 1975).
Within the northern part of the study area, parts of Citrus, Levy, and
Marion Counties have only a veneer of Quaternary sediments overlying the Eocene
Ocala Limestone and Avon Park Formation (section A-A', fig. 6, and E-E', fig. 7).
These plastic sediments thin westward toward the Gulf Coast. South of Citrus,
Levy, and Marion Counties, a number of Pleistocene scarps, terraces, and ridges
dot the landscape. The most prominent ridges include the Brooksville, Lakeland,
Winter Haven, Lake Henry, and Lake Wales Ridges (fig. 4).





Thickness of the Quaternary deposits tends to increase from north to south.
Maximum thicknesses are at the ridges. The Lake Wales Ridge contains the great-
est thickness (about 250 feet) of sediments (Bishop, 1956).


Structural Setting


The area's regional structure incorporates sediments that thicken to the
south and southeast. These strata have a gentle homoclinal dip except where
they have been modified by local structural conditions. Three structural fea-
tures of major significance controlled depositional environments of west-central
Florida. They are the Peninsular arch, the Ocala uplift, and the south Florida
shelf (fig. 9). The Peninsular arch (Applin, 1951) is a large anticlinal fold
that affected rock formations throughout the Tertiary System. The arch is the
dominant subsurface structural feature of the northern two-thirds of the Florida
peninsula. The arch (fig. 9) extends from south-central Georgia to near Lake
Okeechobee (Applin and Applin, 1965). Rocks of Tertiary age, predominantly
calcareous and overlying the arch, control karst topography in west-central
Florida.

It is not clear whether the Ocala uplift is a positive tectonic element
or whether Tertiary deposits are draped over a relatively stable basement high.
Possibly a combination of the two factors has resulted in thickening of Tertiary
sediments away from the two anticlinal crests in the northern part of the
peninsula.

The third structural feature, termed the "south Florida shelf" by Applin
and Applin (1965), is a broad, relatively flat area composed of rocks of Early
Cretaceous and Late Jurassic age. The shelf is southwest of the Peninsular
arch and borders the northeast part of the south Florida basin (fig. 9). This
shelf extends nearly 200 miles across the Florida peninsula from the Atlantic
Coast to Charlotte County. The shelf contains some ancient sinkholes, but
there is little current sinkhole activity.

Other structural features may be present that have affected deposition,
erosion, and development of karst features. For example, Altschuler and Young
(1960) considered the Lakeland Ridge (fig. 4) to be an uplifted block and
faults that parallel the east and west sides of the ridge (fig. 9). Vernon
(1951) delineated two regional fracture patterns associated with the Ocala up-
lift. He states that the regional fracturing reflects structural movement
during the late Tertiary System. Frequently, stream orientation and sinkhole
lineation parallel the fracture patterns. Within the study area, the valley
of the Withlacoochee River (fig. 1) reflects this fracture orientation.


GROUND-WATER HYDROLOGY


The hydrologic system in the southern two-thirds of west-central Florida
differs from that in the north. Differences are based on degree of confinement
of the hydrologic system, primarily due to pinching out of Miocene formations
(section E-E', fig. 7, and F-F', fig. 8). From the central area to the south,







810 80


0 50" 100 150 MILES
Q 100 200 KILOMETERS


Figure 9.--Structural features in Florida. (Modified from Applin
and Applin, 1965; Miller, in press.)





Miocene rocks thicken from a few feet to 600 feet, and the hydrologic system
consists of a surficial aquifer, the intermediate aquifer and confining beds,
and the Floridan aquifer system. In the north, where Miocene rocks generally
are absent, the Floridan aquifer system is unconfined, but may be covered by
permeable sand that is 50 or more feet thick.

The principal water-bearing unit is the Floridan aquifer system. As
defined by Miller (in press), the Floridan aquifer system comprises:

"a vertically continuous sequence of carbonate rocks of generally
high permeability that are mostly of middle and late Tertiary age,
that are hydraulically connected in varying degrees, and whose
permeability is, in general, an order to several orders of magni-
tude greater than that of those rocks that bound the system above
and below.
"* * (in peninsular Florida), less-permeable carbonate units
of subregional extent separate the system into two aquifers, herein
called the Upper and Lower Floridan aquifers."

Throughout west-central Florida, the freshwater-bearing part of the Floridan
aquifer system is the Upper Floridan aquifer. The Upper Floridan aquifer is
also the most sinkhole-prone unit in the hydrologic system.


Recharge and Discharge Areas


The rate of recharge to an aquifer can serve as an indicator of the prox-
imity of the aquifer to the surface, the nature of the overlying sediments, and
the relative position of water levels in one aquifer to another. Ryder (1982)
defined areas and rates of recharge to and discharge from the Upper Floridan
aquifer in west-central Florida (fig. 10). The areas range from those of dis-
charge to areas of recharge of 15 inches or more per year. As noted earlier,
the sediments overlying the Upper Floridan aquifer thicken to the south (figs. 7
and 8). The influence of the overlying sediments in retarding recharge to the
Upper Floridan aquifer can be seen in areas south of Hillsborough and Polk
Counties where recharge rates are relatively low.

The areas that have high recharge rates occur in the north and are indic-
ative of well-drained sands that are in contact with the limestone and areas
where permeable limestone is at or near land surface. Areas that have low to
moderate recharge rates are generally covered with a discontinuous to thick
mantle of silt or clay that slows recharge. Also, some areas where recharge
rates are low may be nearly or completely saturated, such as the Green Swamp
(fig. 1), and recharge is retarded because of slow ground-water movement.

Discharge areas occur where the potentiometric surface of the Upper
Floridan aquifer is at or above land surface and flow is upward. Such areas
occur along the coast, at springs and gaining rivers, and along the western
edge of the Green Swamp.

Areas of high recharge and high discharge generally correspond to areas
where solution channels are well developed in the limestone. In high recharge
areas, chemically aggressive water readily dissolves limestone. As the water





8245' 82*00' 81 IS'
I I I



LEVY

EXPLANATION

29- DISCHARGE AREA

RECHARGE 0- 5
INCHES PER YEAR
GULF 0
RECHARGE 5 15
MEXICO I INCHES PER YEAR
I
RECHARGE GREATER
THAN IS INCHES PER
: YEAR

SOUTHWEST FLORIDA
WATER MANAGEMENT
DISTRICT BOUNDARY
















0 20 40 MILES
0 20 40 60 KILOMETERS



Figure 10.--Distribution of Upper Floridan aquifer recharge and discharge
under natural conditions, excluding springflow. (Modified from Ryder,
1982.)





moves downgradient, the rate of dissolution is reduced as retention time within
the aquifer increases. In the northern part of the area, the ground-water flow
system is relatively shallow and springs discharge much of the water soon after
it is recharged. Stewart and Mills (1984), on the basis of dye tests, estimated
ground-water velocities to be as high as 9,200 ft/d in an area north of Tampa.

In areas of low recharge and low discharge, ground-water movement is slug-
gish and retention times may be hundreds or thousands of years. The deep flow
system in the south produces highly mineralized water, but sinkhole development
is slight.


Thickness of Cover Material


Although west-central Florida is underlain by thousands of feet of lime-
stone and dolomite that are susceptible to dissolution by ground water, most of
the area is covered by a mantle of insoluble sand and clay that control the ulti-
mate route of water that percolates into the materials. In areas where limestone
is exposed at land surface, or is thinly covered, rainfall infiltrates directly
to the water table through conduits that have been opened and enlarged by disso-
lution. Where limestone is covered by permeable sand, however thick, rain water
is largely held where it falls and percolates downward through the sand into the
limestone.

Surface runoff is intermittent where rainfall moves directly into the aqui-
fers. Streams and rivers are widely separated, and valleys that were developed
are poorly defined. Conversely, areas where limestone is covered by thick (more
than 100 feet) and impermeable clay layers, downward movement of water is re-
tarded or impeded, and well-defined stream channels receive surface runoff and
drain a high water table through closely spaced networks of tributaries.

The approximate thickness of the surficial deposits and the confining bed
overlying the Upper Floridan aquifer (table 1) are shown in figures 11 and 12,
respectively. The surficial deposits include sand, clayey sand, shell, and
shelly marl above the confining beds. The surficial deposits consist of Holo-
cene sand, Pleistocene marine terrace sand, unconsolidated parts of the
Pleistocene and Pliocene Caloosahatchee Marl, and the Pliocene Alachua and
Bone Valley Formations. Throughout large areas in the north, the total thick-
ness of cover material is less than 25 feet. These areas reportedly have very
few sinkholes, and those that occur generally are shallow and broad and develop
over fairly long periods of time.

The confining bed (fig. 12) is shown as a single unit, although it is
composed of various lithologic deposits--mostly clay., sandy clay, marl, clayey
sand, limestone, and dolomite. Principal stratigraphic components include:
(1) Miocene to Holocene beds of clay, sandy clay, and marl, undifferentiated
with respect to age, that underlie the surficial aquifer; (2) the Miocene
Hawthorn Formation comprised mainly of phosphatic clays and poorly indurated
limestone and dolomite lenses; and (3) the unconsolidated sections of the un-
derlying Miocene Tampa Limestone that occurs in much of Polk, Hardee, De Soto,
and Charlotte Counties and consists of clay and clayey sand. Figure 13 is a
composite of figures 11 and 12 that shows the total thickness of overburden.




82045'
I


82000
I


610151


EXPLANATION
THICKNESS OF
SURFICIAL DEPOSITS
LESS THAN
I I 25 FEET


25 TO 50 FEET

50 TO 100 FEET

GREATER THAN
100 FEET


- SOUTHWEST FLORIDA
WATER MANAGEMENT
DISTRICT BOUNDARY


0 20 40 MILES

0 '20 40 60 KILOMETERS



Figure 11.--Generalized thickness of the surficial deposits overlying the
confining bed. CFrom Wolansky, Spechler, and Buono, 1979.1







82000'
I


81 *5'


O 20 40 MILES
1 & v- -r-
0 20 40 60 KILOMETERS
i


Figure 12.--Generalized thickness of the confining bed overlying the
Upper Floridan aquifer. (Modified from Buono and others, 1979.1


8245'
I


290-












280-


270-


EXPLANATION
THICKNESS OF THE
CONFINING BED
MARION
SI LESS THAN
(25 FEET

S 25 TO O FEET

L\ ~LANG 50 TO 100 FEET

J100 TO 200 FEET

771 GREATER THAN
.- jL |200 FEET

SOUTHWEST FLORIDA
WATER MANAGEMENT
\ DISTRICT BOUNDARY


ii





82o000


81I 15'
I


EXPLANATION
S-THICKNESS OF THE
MARION DEPOSITS

S\LESS THAN
25 FEET

--- 25 TO 50 FEET
LAKE
O 5 TO 100 FEET

S100 TO 200FEET

SI7 GREATER THAN
SV I200 FEET

SOUTHWEST FLORIDA
WATER MANAGEMENT
DISTRICT BOUNDARY


O 20 40 MILES
i I I
0 20 40 60 KILOMETERS


Figure 13.--Generalized thickness of the unconsolidated deposits overlying the
Upper Floridan aquifer. (Modified from Buono and others, 1979; Wolansky,
Spechler, and Buono, 1979; Gilboy, 1982.)


8245'
I


29-


GULF
OF
MEX ICO


280-













27-






In some areas in the north, such as at the Brooksville Ridge, limestone is
covered by a layer of materials (predominantly sand 50 to 100 feet thick) that
are relatively incohesive and permeable. Where the cover is permeable and con-
fining beds are absent, water infiltrates directly to the water table, which
may be in the cover or in the underlying limestone. The areas reportedly have
very few sinkholes.

Cover material in the central area ranges from 25 to 200 feet or more in
thickness. The clay confining bed provides a degree of cohesiveness to the
cover material that bridges developing solution cavities. This area reportedly
has the greatest number of sinkhole occurrences in west-central Florida.

The southern area has a thick confining bed (more than 200 feet) that
limits ground-water circulation and development of solution cavities in the
carbonate rocks. Where cavities do form, the confining sediments have adequate
bearing strength to bridge cavities of moderate size. Carbonate layers also
occur within the plastic confining unit. Where these layers are permeable
enough to permit circulation of ground water, they too are subject to dissolu-
tion and subsequent sinkhole development. However, the area is generally one
of infrequent sinkhole collapse.


Water-Level Fluctuations in Aquifers

Surficial Aquifer


The surficial aquifer is unconfined, that is, its water surface or water
table is under atmospheric pressure. Water levels in the aquifer respond rap-
idly to recharge and discharge and, if the water table is close to land surface,
to evapotranspiration. Fluctuations of water levels in wells that tap the surfi-
cial aquifer are illustrated in figure 14. Seasonal and long-term fluctuations
of water levels vary with location. In general, fluctuations are greatest in
the highland areas and least in discharge areas near the coast.

Recharge to the surficial aquifer occurs throughout the year and is at a
maximum during the wet season (fig. 14). The surficial aquifer is the principal
source of recharge to underlying aquifers throughout much of the study area.
Where the water table in the surficial aquifer is higher than the potentiometric
surface in underlying aquifers, recharge to the lower aquifers occurs. In
coastal and other low-lying areas, the head gradient between the aquifers is
reversed, and flow is upward from the lower aquifers to the surficial aquifer.


Upper Floridan Aquifer


The Upper Floridan aquifer is confined by a clay or sandy clay that
occurs at the base of the surficial aquifer or intermediate aquifer and confin-
ing bed system (table 1), except in the north where limestone is overlain by
sand. Where confined, the potentiometric surface of the Upper Floridan aquifer
may rise above the top of the limestone. Water levels in wells that tap the




60 0




55 5




>W 50
50- 10

DEPTH OF WELL :22 FEET )
w DEPTH OF CASING I8 FEET
co VAN DYKE SHALLOW WELL NEAR LUTZ <
S45 I ---------- ---- -- -------- --- ,5 -
>45 -J
0 15~
< 120 o
12 ,-- -,- ,-,-,-,-,--, ---
0 _J
w -
LL b
z ii
J 2










DEPTH OF WELL : 23 FEET
DEPTH OF CASING : 13 FEET

LAKE GROVES ROAD WELL NEAR LAKE PLACID
10 5 I , ,- . 1 I
1965 '66 '67 '68 '69 '70 '71 '72 '75 '74 '75 '76 '77 '78 '79 '80 '81 '82 '83 1984


Figure 14.--Month-end water levels in surficial aquifer wells near Lutz
and Lake Placid, 1965-84, (General locations of wells are shown in
figure 1.)



confined aquifer rise slowly in response to recharge during the wet season
(fig. 15). The response to rainfall is dampened by the confining beds and over-
lying deposits. Hydrograph analysis of artesian wells is difficult because water
levels are affected within relatively short times by pumping even at great dis-
tances. Water levels in wells thus reflect rises due to cessation of pumping
and recharge and declines due to pumping and natural discharge. Differences of
as much as 40 feet have been recorded between the annual high and low water
I-~

























as much as 40 feet have been recorded between the annual high and low water






levels in western Polk County, a major center of ground-water withdrawal (fig.
16). Fluctuations in the potentiometric surface in wells relatively unaffected
by ground-water withdrawals, except occasionally for irrigation and frost-freeze
protection, are shown in figure 17. The well near Dover is in the confined part
of the aquifer and the well near Lecanto is in the unconfined part.



Head Differences Between Aquifers


Water provides a hydrostatic pressure that exerts a downward force or an
upward buoyant force on material overlying a cavity. The relative levels of
water in the multiple aquifer system, therefore, are an important factor in
sinkhole development.

In the central part of the study area, the water table in the surficial
aquifer is higher than the potentiometric surfaces of underlying aquifers. The
areas of downward head are more widespread during the dry season and there are
greater differences in head between the water table and the potentiometric sur-
face because of large ground-water withdrawals from the underlying aquifers.

In the south, wells completed in the Upper Floridan aquifer flow, indicat-
ing that the area is one of buoyant pressure. In the north, the Upper Floridan
aquifer is unconfined and pressure, as such, does not affect sinkhole develop-
ment. In the north and south, sinkhole development is less active than in the
central area where downward hydrostatic pressures prevail.



Directions of Ground-Water Flow


The configuration of the water table of the surficial aquifer is a subdued
expression of land surface. The water table is higher in the inland ridge areas
than near coastal areas. Regionally, the direction of ground-water movement in
the surficial aquifer is to the west, toward the Gulf of Mexico. However, re-
gional flow patterns are interrupted by local areas of discharge that reduce the
significance of the regional pattern. Discharge from the surficial aquifer occurs
in streams, lakes, and swampy areas.

Movement of water in the Upper Floridan aquifer is better defined really
than movement of water in the surficial aquifer. The potentiometric surface of
the Upper Floridan aquifer shows a pressure gradient from recharge areas (highs)
to discharge areas (lows) (figs. 18 and 19). The topography of the land surface
is reflected only by the discharge areas, such as the valley of the Withlacoochee
River. The potentiometric surface of the Upper Floridan aquifer is highest in
the Green Swamp area in the northern part of Polk County (figs. 18 and 19). The
regional direction of water movement from this and other potentiometric-surface
highs is northwest or southwest. Although the pattern of ground-water movement
is fairly consistent, minor changes are caused by heavy pumping. Water levels
are generally at their annual highs in September (fig. 18) and at their annual
lows in May (fig. 19).























OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY AUG SEPT


LL

44 ,,

48

52


60


.564
-64
2
-J

cx
or~


Figure 15.--Water levels in the Maddox well near Bowling Green, 1981-82.
(General location of well is shown in figure 1.)


1963 '64 '65 '66 '67 '68 '69 '70 '71 '72 '73 '74 '75 '76 '77 '78 '79 '80 '81 '82 '831984
WATER YEARS

Figure 16.--Month-end water levels in the Maddox well near Bowling Green,
1963-84. (General location of well is shown in figure 1.)


't W

50
50


z
55 -

60

0
65

70
(j-
75 z
-r
8so0

85 W

90 3






75


DEPTH OF WELL : 413 FEET
DEPTH OF CASING: 67 FEET
70 --0




65 --5




-60 0
U~U-

s

W 60- I 0 o
0 0

>z
0 55 -15 4
AQUIFER CONFINED
0
W 3j
iLL TAMPA DEEP WELL 15 NEAR DOVER
z 50 I I k I I I I I I I 20
1961 '62 '63 '64 '65 '66 '67 '68 '69 '70 '71 '72 '73 '74 '75 '76 '77 '78 '79 '80 '81 '82 '831984
W
W IL
>z
- 8
I: 8 --T- -1 --I --"- -- ,I..

< DEPTH OF WELL : 335 FEET 59 -
7 DEPTH OF CASING: 288 FEET iL

-60
6 1



5 -

-62

AQUIFER UNCONFINED

NORTH LECANTO DEEP WELL NEAR LECANTO 63
3 I I I m 1 1 I I
1966 '67 '68 '69 '70 '71 '72 '73 ',4 '75 '76 '77 '78 '79 '80 '81 '82 '83 1984


Figure 17.--Month-end water levels in two wells in the Floridan aquifer
system, 1961-84. (General locations of wells are shown in figure 1.)






8200'
I


8115'
I


EXPLANATION
-100-


POTENTIOMETRIC CONTOUR--
SHOWS ALTITUDE AT WHICH
WATER LEVEL WOULD HAVE
STOOD IN A TIGHTLY CASED
WELL. CONTOUR INTERVAL 10
AND 20 FEET DATUM IS
SEA LEVEL
t------
GENERAL DIRECTION OF
GROUND-WATER FLOW

SOUTHWEST FLORIDA
WATER MANAGEMENT
DISTRICT BOUNDARY


0 20 40 MILES
I I. f
0 20 40 60 KILOMETERS



Figure 18.--Potentiometric surface of the Upper Floridan aquifer,
September 1979. O(odified from Yobbi and others, 1979.1


82*45'
I


29-


GULF
OF
MEX/CO







28- I







27 1






27-






82000


81015'
1


EXPLANATION
-100-
POTENTIOMETRIC CONTOUR--
SHOWS ALTITUDE AT WHICH
WATER LEVEL WOULD HAVE
STOOD IN TIGHTLY CASED
WELLS. CONTOUR INTERVAL
10 AND 20 FEET. DATUM IS
SEA LEVEL
^------
GENERAL DIRECTION OF
GROUND-WATER FLOW
A A'
LINE OF SECTION SHOWN
IN FIGURES 20 THROUGH
22


SOUTHWEST FLORIDA
WATER MANAGEMENT
DISTRICT BOUNDARY


O 20 40 MILES

0 20 40 60 KILOMETERS




Figure 19.--Potentiometric surface of the Upper Floridan aquifer, May 1979.
(Modified from Wolansky, Mills, Woodham, and Laughlin, 1979.)


82o45'
I




The unconfined flow system along an east-west hydrogeologic section in
the north is shown in figure 20 (section A-A'). The unconfined or semiconfined
conditions enable a relatively high rate of recharge to occur (fig. 10). Water
enters the Upper Floridan aquifer, travels relatively short distances through
well-developed dissolution channels, and discharges at large springs. Figure
20 shows that the aquifer base, or top of the intergranular evaporites, drops
from about 600 feet below sea level to about 1,800 feet below sea level in the
eastern part of the section. However, water-quality data and a reduction in
permeability and porosity of the lower part of the Avon Park Formation suggests
that the flow is very sluggish in the deeper zones compared to the shallow flow
system (Faulkner, 1973; Ryder, in press).

The east-west hydrogeologic section (B-B') in figure 21 represents the
confined flow system in the southern areas. Within most of the eastern half
of the section, water moves downward from the surficial aquifer through a con-
fining bed and into an intermediate aquifer and confining unit. A relatively
small amount of water returns to the surface in topographically low areas,
such as the Peace River valley. Most water moves downward through a confining
bed and into the Upper Floridan aquifer where flow is westward toward the Gulf
of Mexico. Locally, particularly in the ridge area a few miles north of the
eastern edge of the section, sinkholes may breach the overlying deposits and
penetrate the Upper Floridan aquifer. The sinkholes may be occupied by lakes
or by permeable sands and are sites for relatively high rates of recharge to
the Upper Floridan aquifer. Along the coastal margin and in the Gulf, there
is a reversal of gradient and water flows upward through the confining beds.

In contrast to the hydrogeologic sections in figures 20 and 21, the
north-south section (C-C') in figure 22 is generally perpendicular to flow
paths within the aquifer systems. Flow is generally westward, out of the
plane of the section, except for some local discharge areas near the top of
the intermediate aquifer and the Upper Floridan aquifer. Figure 22 shows from
north to south: (1) thickening of the confining beds and a transition from
generally unconfined to confined conditions in the Upper Floridan aquifer,
(2) thickening of the intermediate aquifer, (3) an increase in importance of
the highly permeable zone in the Avon Park Formation (solution channels in the
Ocala Limestone having less significance), and (4) thickening of the Upper
Floridan aquifer.


KARST DEVELOPMENT


The term karst (Slavic kras) means, literally, a bleak, waterless place.
The area is waterless because the limestone terrane permits precipitation to
drain internally through conduits that have been developed by solution. Solu-
tion development in limestone, as expressed at land surface, gives karst
terrane the unique morphology that distinguishes it from landforms created by
erosion of plastic rocks. Chemical corrosion and internal drainage are the
active processes rather than physical erosion and surface runoff. Monroe (1970)
describes karst as "* * A terrain generally underlain by limestone, in which
the topography is chiefly formed by the dissolving of rock, and which is com-
monly characterized by karren, closed depressions, subterranean drainage, and
caves."








FEET SURFICIAL AQUIFER AQUIFER \ FEET

400- -400
4z I RECHARGE 4
SR oM FROM
00 a g RAINFALL






200- OTENTIOMETRIC 200
SURFACE
V L OF THE
400U PE LORIDAN AQUIFER
200- yTENIOMTI -200
S UPPER FLORIDAN AQUIFER
400- 400

UPPER FLORIDAN AQUIFER
600- -600


800- -800

TOP OF INTERGRANULAR EVAPORATES
1000-/ oo1000
UPPER FLORIDAN AQUIFER

1200 / / 1200
NO-FLOW BOUNDARY

/1400- NO-FLOW BOUNDARY --
40oo- / 1400
LOWER FLORIDAN AQUIFER

1600- / 1600
6000 10 20 MILES 600

0 10 20 30 KILOMETERS
1800// VERTICAL EXAGGERATION X200 -" / 1800
TOP OF EVAPORATE BEDS--

2000 2000


Figure 20.--Generalized hydrogeologic section A-A' showing flow
patterns. (Line of section is shown in figure 19; from Ryder,
in press; site locations are shown in figure 1.)











o POTENTIOMETRIC SURFACE FEET
OF INTERMEDIATE AQUIFER -400
WE lg SURFICIAL
UPPER /F AQUIFER


SA1TWN'TG BE ... NN::NG SEA
t~i~ i ~ "LEVEL
;:'":" """I::""::* :BOUNDARY
DISCONTINUOUS INTERMEDIATE AQUIFER
IO-FLOW_ CLAY BEDS J 3200



1400


UPPER FLORIDAN AQUIFER

ZONE

1000
HNO-FLOW
BOUNDARY
E A UPPER FLORIDAN AQUIFER -1200

1400
v' v' LOWER
NO-FLOW FLORIDAN


o0 10 2 ;0 KILOMETERS
--BOUNDARY AU I FE R 1600

r/ ^^^ EVAPORITES ------^
...." 1800
VERTICAL EXAGGERATION X200 -0 0 20 0KILOMETE0RS





Figure 21.--Generalized hydrogeologic section B-B' showing flow patterns.
(Line of section is shown in figure 19; modified from Ryder, in press;
site locations aresh'own in figure 1.)








POTENTIOMETRIC
SURFACE OF
INTERMEDIATE e


400 J -POTENTIOMETRIC
_0 SURFACE OF
SUPPER FLORIDAN


UPPER FLORIDAN AQUIFER


LOWER FLORIDAN


0 10 20 MILES

0 10 20 30 KILOMETERS



VERTICAL EXAGGERATION X200


UPPER FLORIDAN


AQUIFER


TOP OF INTERGRANULAR EVAPORITE


2000


Figure 22.--Generalized hydrogeologic section C-C' showing flow patterns. (Line of section is shown in
figure 19; modified from Ryder, in press; site locations are shown in figure 1.)


RECHOARGE
RAINFA
RAINFALL


0. 0 0


1000





West-central Florida is not a bleak, waterless place, but many features
of its landscape are due to dissolution of limestone bedrock and are properly
termed karst features. Where limestone is exposed, karst features are obvious.
Throughout most of west-central Florida, however, the limestone is covered with
an overburden of sand and sandy clay. The effects of karst processes on the
underlying limestone become apparent when the unconsolidated sand and clay sub-
side or collapse into solution cavities. Landforms of this nature are desig-
nated "mantled karst"; the karst processes are reflected at land surface. The
term "buried karst" is applied to terrane where the overburden is thick and has
enough bearing strength to lessen subsidence or collapse. In these terranes,
dissolution activity within the underlying limestone may not become apparent
at land surface.

The Florida peninsula is largely developed on Tertiary limestone that
locally has been uplifted and differentially dissolved. The result is a
pattern of physiographic features that have generally low relief. Geologic
structure and stratigraphy are primary controls on karst development. White
(1970) characterized structurally high areas, such as the northern third of
the study area, as "dead zone" karst that contains many steep-walled sinkholes,
abandoned springheads, dry stream courses, intermittent lakes, and dry beds of
former shallow lakes that are now prairies. South of this area, the limestone
bedrock dips downward and is overlain by several hundred feet of plastic strata.
Broad, shallow sinkhole lakes are common in lowland areas and small deep sink-
hole lakes that apparently represent a complex geologic history occur in the
ridge areas.

Karst development in west-central Florida is controlled by lithology and
water movement, dissolution by chemically aggressive water, aquifer material,
and sea levels. Sweeting (1973) described the difference between karst and
nonkarst areas as follows:

"As a result of the solution of the rock, drainage in limestones sinks
into the ground and does not become integrated into surface rivers, whereas
in nonkarst areas the surface water becomes organized and systematized into
valleys to form a connected network. The surface and underground relief fea-
tures in limestones are shaped in a vertical sense. The parts of the surface
where the water sinks into, the ground become isolated from one another, so
that the relief forms appear unconnected and disparate; hollows or pits are
formed where the drainage sinks into the ground, giving the landscape a pitted
character. Thus the landforms of limestone areas and the processes which give
rise to them are so distinctive that they are now known universally as karst
landforms and karst processes."


Lithology and Water Movement


The lithology or physical properties of carbonate rock control the amount
of rock surface that is exposed to chemical corrosion and the way water moves
through the rock. The thickness of rock beds, clay content, texture, grain
size, and brittleness determine whether flow is diffused through the many
primary openings or is channeled through the few secondary openings. Very
few limestones retain their primary openings by the time they are lithified.





Ground-water flow, therefore, is commonly often through secondary openings that
have developed along joints, faults, bedding planes, and erosional surfaces. The
most common of these secondary features are joints that were formed by tensional
or compressional stresses and bedding planes that mark stages of deposition and
were susceptible to chemical corrosion. Faults, especially if surrounded by a
zone of fractured rock, commonly form conduits for water movement. If the rock
is highly pulverized or clays occur along the fault plane, faults may act as
barriers to water flow. Erosional surfaces are the least common of the princi-
pal types of secondary openings, but many are favorable for ground-water flow.

The amount of rock surface exposed to corrosion is small in relation to
the volume of rock. Thus, dissolution of the rock is more apt to develop a few
large conduits. When reefs and reef debris lithify, they usually form rock that
has numerous voids and passages. Flow through this type of rock is diffused,
and the rock surface exposed to corrosion is large in relation to volume. Num-
erous small, interconnected conduits are developed, and the rock generally has
the appearance of Swiss cheese, but major conduits are rare.

When calcareous sediment lithifies, it forms a dense, essentially impervi-
ous rock, and water movement is confined to bedding planes, erosional surfaces,
joints, and other physical features. Soluble materials that comprise the total
surface area of the rock are removed, but insoluble materials remain and may
accumulate in the openings. In addition, material from adjacent areas may be
carried into the solution-formed openings. Dissolution by circulating water is
primarily along conduit walls. The conduits, thus, rapidly enlarge and become
major openings through which water moves freely.


Dissolution of Aquifer Materials


The Floridan aquifer system is composed of approximately 65 percent cal-
cite, 34 percent dolomite, and minor amounts of gypsum (Back and Hanshaw, 1971).
Calcite and dolomite are the dominant minerals and are of major importance in
terms of dissolution and sinkhole development. However, gypsum is highly
soluble and contributes to sinkhole development in some areas.

The geochemistry of calcite, dolomite, and gypsum is relatively straight-
forward. These minerals have relatively simple structures, and processes of
dissolution involve rapid reactions even in dilute solutions. Thus, quantita-
tive rules of chemical equilibrium are particularly effective as a basis for
predicting rates of limestone dissolution and land subsidence.

The dominant factor that determines the ability of recharge water to dis-
solve calcite and dolomite is acidity of the water. Also, temperature, and to
a lesser degree pressure, affect solubility rates. If recharge water has high
ionic strength and low concentrations of calcium, magnesium, and carbonate, its
ability to dissolve these minerals is enhanced.

Rain water, the primary source for recharge water, is naturally acidic.
If rain water is in equilibrium with atmospheric carbon dioxide, it has a pH
of about 5.6 units. The pH of rain water may be lowered by air pollution, thus
enhancing the ability of rain water to react with aquifer materials. However,





air pollution has been of.little significance in terms of dissolution and de-
velopment of sinkholes that occur over geologic time. Acidic gases, such as
sulfur dioxide, hydrogen sulfide, and oxides of nitrogen, may also contribute
to the acidity of rain water.

The major source of acidity in natural ground water is carbon dioxide
from bacterial decomposition of organic matter in the soil zone. The amount
of carbon dioxide in the soil zone may be several hundred times that in rain
water (Stumm and Morgan, 1970). This results in ground water that has a pH of
about 4 units and, thus, enhances dissolution of limestone. Chemical reactions
that are related to dissolution of carbon dioxide in water, and subsequent dis-
solution of limestone minerals are as follows:

CO CO
2 (gas) 02 (aqueous)
(gaseous carbon dioxide) (aqueous carbon dioxide) (1)


CO + H 0 H CO
CO2 (aqueous) + 20 H2 3
(aqueous carbon dioxide) (water) (carbonic acid)(2)


H2CO3 + CaCO --. Ca" + 2HC03
(carbonic acid) (calcite) (calcium ion) (bicarbonate ion) (


2H2CO3 + CaMg(CO 3) 2 Ca+ + Mg+ + 4HCO3
(carbonic (dolomite) (calcium (magnesium (bicarbonate 4)
acid) ion) ion) ion)


Most dissolved carbon dioxide remains in the unhydrated form CO2 aqand
does not form carbonic acid (H CO ). The ratio of CO ois
650 to 1 (Stumm and Morgan, 190) 2 (aueous)

Because calcite dissolution is the predominant chemical cause of sinkhole
occurrence in the study area, special attention will be given to the calcite
dissolution process. Recharge water that contains carbon dioxide dissolves
calcite to produce two bicarbonate ions and one calcium ion for each carbon
dioxide molecule that reactsV, A small amount of carbonate ions will also be
dissolved until equilibrium 4tii calcite is reached. Because initial concen-
trations of carbon dioxide irrnrecharge water and degree of undersaturation are
generally unknown, another approach to calculate the rate of calcite dissolu-
tion and cavity formation is desirable. Alkalinity provides a measure of the
amount of calcite that has dissolved without knowledge of initial carbon diox-
ide concentrations or the degree of undersaturation. Alkalinity of water from
a limestone aquifer can come 6nly from dissolution of carbonates, such as cal-
cite or dolomite, rather than from carbon dioxide, which is an acid. Although
carbon dioxide permits more alkalinity, all alkalinity comes from the carbonate
rock whether it occurs as two bicarbonate ions because of reaction with carbon
dioxide or as one carbonate ion that is unreacted with carbon dioxide.





The volume of calcite that is dissolved in a pure calcite aquifer is
described by the following equation:

V = 3.70x10-7 x recharge x alkalinity (5)

where
V = volume of dissolved calcite, in cubic inches per square
inch of surface area;
recharge = recharge rate, in inches per year; and
alkalinity = alkalinity, as CaCO3, in recharge water as a result of
dissolution of calcite, in milligrams per liter.

For a pure dolomite aquifer, the coefficient in equation 5 would be 3.21x10-7.

Equation 5 can be used to estimate the rate of cavity formation when used
with average recharge rate, concentration of alkalinity near the bottom of the
zone where sinkhole formation occurs, and an estimate of how concentrated ground-
water flows are in solution channels or fractures as compared to average recharge
rates for an area. For example, if the average annual recharge rats is 5 inches
and the _lkalinity is 200 mg/L as CaCO3, from equation 5: 3.70x10- x 5 x 200 =
3.70x10 in of calcite will dissolve beneath each square inch of land surface
each year.

If the area near a fracture receives 10 times the average recharge b cau5e
of concentration of ground-water flow, 10 times as much calcite (3.70x10 in )
could dissolve each year under each square inch of land surface. This rate of
dissolution would result in development of a 1-foot deep cavity in about 3,200
years [12 + (3.70x10)].

In natural ground water, the solubility of gypsum is independent of pH.
The capacity of ground water to dissolve gypsum is controlled by concentrations
of calcium and sulfate ions, ionic strength, temperature, and pressure. The
dissolution of gypsum is described by the following equation:

CaSO4 *2H20 Ca+ + SO- + 2H20 (
4 2 4 2 (6)
(gypsum) (calcium ion) (sulfate ion) (water)

The dissolution of gypsum can be described by the following equation:

'. _-7
V = 7.73xO10 x recharge x SO4 (7)

where
V volume of gypsum dissolved, in cubic inches per square inch
g of land surface;
recharge recharge rate, iniinches per year; and
SO4 concentration of sulfate in recharge water, in milligrams
per liter.

This equation does not.account for sulfate reduction by bacteria.





Back and Hanshaw (1971) describe a method of estimating from a chemical
analysis for calcium, magnesium, and sulfate the amount of calcite, dolomite,
and gypsum that would have dissolved to produce a 1984 water sample from the
Floridan aquifer system, assuming that saline water has not mixed with recharge
water. Their equations were used to derive an equation that could be used to
estimate the volume of calcite, dolomite, and gypsum that has dissolved. The
equation is as follows:

Vcdg = [(9.213xl0-7 x Ca) + (1.122x10-6 x Mg) + (3.88x10-7 x SO4)] (8)

x recharge

where
Vcdg = volume of calcite, dolomite, and gypsum dissolved, in cubic
S inches per square inch of land surface;

recharge recharge rate, in inches per year; and
Ca, Mg, and SO4 = concentrations, in milligrams per liter.

The equations for dissolution of carbonates and gypsum do not account for
changes in solubility that are caused by changes in temperature and pressure.
Solubility of carbonates is related to solubility of C02, which is inversely
proportional to changes in temperature and directly proportional to changes in
pressure. Gypsum is a mineral whose solubility increases with increasing tem-
perature and pressure.

The following example uses equation 8 and a typical water sample for the
Bartow area to estimate the quantity of material dissolved under each square
inch of land surface per year. The concentrations of calcium, magnesium, and
sulfate in the water sample are 51, 22, and 3.9 mg/L, respectively (Miller and
Sutcliffe, 1982). The annual recharge rate for the area is about 5 inches.
Substituting these values in equation 8 results in the following:


V g = [(9.213x10-7 x 51) + (1.122x10-6 x 22) + (3.88x10-7 x 3.9)] x 5
c,d,g
= 3.66x10-4 in3/yr.

-4 3
Thus, 3.66x10 in of material would 'dissolve under each square inch of land
surface per year. At this rate, it would take about 33,000 years (12 + 3.66x10 )
to dissolve a 1-foot deep cav ty. Because ground-water flow is concentrated in
faults and fractures, howevertecharge and dissolution would also be concen-
trated. If this focused flow resulted in a 10-fold increase in recharge at the
joints and fractures, it would take only about 3,300 years to develop a 1-foot
cavity in such areas.

Sinclair (1982) used a similar analysis to predict the rate that limestone
is removed within a 3-mi cope of depression at a well field in Hillsborough
County. At the 1978 rate of pumpage, 1 foot of limestone would dissolve through-
out the area of influence in about 1,700 years. Thus, the rate of limestone
removal caused by pumping is two or more times greater than the natural rate of
removal indicated for the Bartow area.




Sea Levels


Fluctuations of sea level with respect to the land surface have been an
important factor in the development of karst of west-central Florida. Periods
of submergence and deposition of limestone have alternated with periods of land
exposure and erosion and dissolution of carbonate rocks.
The most recent changes of sea level were in response to climatic changes
during periods of glaciation in the Pleistocene Epoch.. Seawater was retained on
land in continental glaciers and sea level was as much as 400 feet lower than
its present (1984) level. The Gulf Coast of Florida was at least 300 miles west
of its present position. Many of the karst features of the limestone are related
to Pleistocene sea stands and ground-water levels.
During periods of low sea level, ground-water levels were correspondingly
low and water-table conditions probably prevailed in much of the limestone un-
derlying the present peninsula. Perched water tables probably were present in
the surficial sand where the sand was separated from the limestone by clay.
Ground-water circulation and karst development were most active at the surface
of the water table. The lower sea level lowered the base level toward which
surface runoff and ground water would flow and affected rates of recharge,
levels of the water table, and even directions of ground-water flow in some
areas. Conduits and cavern systems likely developed at the base altitude
established by sea level.
The effect of high sea levels was to fill existing karst features with
sand and clay by wave action and coastal currents. Sediments associated with
this inundation were principally silica sand and clay deposits in the northern
areas. Calcareous reefs and reef-associated sediments were deposited from near
the Alafia River southward.
Many beaches and terraces have been defined in west-central Florida, but
recent studies suggest that the terraces and beaches above about 100 feet are
probably pre-Pleistocene or, perhaps, structural in genesis. Along the ridge
areas and where the land surface is above 100 feet in altitude, sinkholes and
sinkhole lakes are well developed, fairly large, and may be in a mature stage
of development. The ridges, for example, probably have not been inundated by
the sea since Miocene time or about 5 million years ago. The many large sink-
hole lakes and internally drained depressions in the central ridges attest to
a long, uninterrupted period of weathering, solution, and subsidence of lime-
stone bedrock.
Most of the topography below an altitude of 100 feet is relatively sub-
dued. The valley of the Green Swamp and the Withlacoochee River (fig. 1), for
example, was inundated several times during the Pleistocene Epoch, Coastal
currents and wave action developed a relatively flat surface on the surficial
fill within that area even th ugh the bedrock surface buried beneath the sand
and clay is very irregular.
Ground-water circulation that occurred during the highest stands of sea
level probably was confined to the ridges and adjacent-areas. Movement of
ground water probably was sluggish because of the relatively flat gradient
and lack of head to move the water. Solution of limestone was greatest at
and immediately below the top of the limestone where most recharge of corro-
sive water occurred.





WARNING SIGNS


Sinkholes induced by man's activities can be expected to increase in
frequency and are particularly hazardous because many occur in populated areas
(Sinclair, in press). Although the occurrence of sinkholes can be abrupt, there
are signs that warn of possible collapse, such as:

1. Slumping or sagging--slanting fence posts or other objects; doors and
windows that fail to close properly.
2. Structural failure--cracks in walls, floors, and pavement; and cracks in
the ground surface.
3. Ponding--ponding of rainfall where it has not ponded previously.
4. Vegetative stress--wilting of small areas of vegetation because of lowered
water table caused by drainage through.a developing sinkhole.
5. Turbidity in well water--turbid water in nearby wells during early stages
of sinkhole development.

Surface erosion by rivers is well understood, visible, and subject to
some control by man, but subsurface erosion is difficult to trace and the pre-
diction of potential collapse is difficult or impossible. Buildings, bridges,
and other structures can generally be designed in a manner that will mitigate
damage. However, reliable methods for the prevention of or prediction of ex-
act time and location of sinkhole occurrence have not been developed. Aerial
photography, geophysical surveys, and test-well drilling are time-consuming
and expensive approaches to detection of cavities at depth, but have not
proven to be consistently effective in detecting cavities.


TYPES AND FEATURES OF SINKHOLES


Types of sinkholes that are common to west-central Florida include:
(1) limestone solution, (2) limestone collapse, (3) cover subsidence, and
(4) cover collapse (Sinclair, in press). Limestone-solution and limestone-
collapse sinkholes usually occur in areas where limestone is bare or thinly
covered, but may also occur where cover materials are thick. Cover-subsidence
and cover-collapse sinkholes usually occur in areas where there is a thick
cover of 'material overlying the limestone. Descriptions of sinkhole types and
factors that affect their occurrence are summarized in table 2.


Sinkholes in Area4 of Bare or Thinly Covered Limestone


Throughout much of the northern part of west-central Florida, cover
material overlying limestone is less than 25 feet thick (fig. 13). Generally,
the cover material is very permeable, and the effect, in terms of solution
development of the limestone, is similar to that of bare limestone exposed to
weathering. Solution of limestone and sinkhole development are greatest at
the surface and in near-surface joints and fractures and generally decrease
with depth.






Table 2.--Principal features of the major types of sinkholes
[From Sinclair, in press]


Cover material


Limestone solution

Limestone bare or
thinly covered.


Sinkhole type

Limestone collapse Cover subsidence


Limestone bare to
deeply buried by
cover material.


Incohesive, perme-
able sand, as much
as 50 feet or more
thick.


Cover collapse

Generally 5 to 100+
feet thick. Incohe-
sive, permeable sand
grading downward to
clayey sand with rel-
atively cohesive,
poorly permeable clay
overlying limestone.


Ground-water Water table generally below top of lime- Water table may be Water table in cover
levels stone, in sand or below top sand perched above
of limestone, and leaking through
poorly permeable
clay. Artesian water
level in limestone
generally below water
table but above lime-
stone surface.
Sinkhole Imperceptibly slow Cavities develop Imperceptibly slow Solution-enlarged
development subsidence of land only at depth be- subsidence of land joints at limestone
surface by removal neath limestone sur- surface. Only very surface are bridged
of limestone in face where they en- small cavities are by overlying cohesive
solution. -- large by upward col- likely to form at clay until bearing
lapse of the roof, limestone surface strength of clay is
layer by layer, un- because incohesive exceeded. Cavity
til the limestone sand moves continu- moves upward by roof
roof is too thin to ously or spasmodic- spelling until it ap-
support itself and ally downward to oc- pears abruptly at
collapses abruptly; cupy space formerly land surface. Sink-
a relatively rare held by dissolved hole size controlled
occurrence in terms limestone, by bearing strength
of landscape devel- of clay, usually re-
opanat. Sinkhole lated to clay thick-
esie controlled by ness.
bearing strength of
2taestone roof and
depth of cavity.


Activities that
commonly trig-
Sger sinkholes


Effects of
activities


Declines in ground-water leels, particularly abrupt declines and
fluctuations due to piping from wells, are the most coman fac-
tors that trigger collapse.
Impoundments, such as reservoirs and holding ponds, may load the
land surface and provide a source of water that percolates down-
ward, eroding cover material into solution cavities. Diversion
of surface drainage by roads or paving of large areas that con-
centrates runoff may also accelerate internal erosion.
Static loads (buildings) and vibratory and harmonic loads (heavy
equipment, trains) may be sufficient to trigger collapse of cavi-
ties at shallow depth.
----------------------------------------------- ---------------------
Slight. Slight. Collapse Snall-scale piping Collapse commonly in-
may be induced where commonly induced. duced.
water table is above
limestone surface.






Limestone-Sblution Sinkholes


In areas where limestone is covered by thin layers (less than 25 feet)
of soil or overburden, solution of the limestone is most active at the lime-
stone surface. The most common type of sinkhole in these areas results from
subsidence that occurs at roughly the same rate as the dissolving of the rock.
Dissolved limestone and insoluble residue are carried along enlarged fractures
as solution of limestone progresses. At many places, actual voids do not form
because subsidence occurs gradually as the limestone dissolves. The result is
a gradual lowering of the land surface and development of depressions that col-
lect more surface runoff as the depression's perimeter expands (fig. 23). This
type of sinkhole commonly forms as a funnel-shaped depression. The slope of its
sides is determined by the rate of subsidence relative to the rate that surface
material is transported into the depression. Surface runoff may carry sand and
clay particles into the depression and form a relatively impermeable clay seal
in its center. If percolation is restricted by the clay seal, a marsh or inter-
mittent lake generally forms in the depression. Limestone solution and subse-
quent land subsidence are common in karst areas. The process produces an
undulating topography throughout much of the northern part of west-central
Florida, particularly in the northern part of the area.



Limestone-Collapse Sinkholes



The most obvious type of sinkhole occurs where a solution cavity expands
in size until the limestone roof collapses (fig. 24). Collapse is generally
abrupt and at times catastrophic. Limestone-collapse sinkholes may occur in
any area of soluble rock regardless of the depth of the rock. However, they
occur most often in areas where limestone is at or near land surface and where
the water table is below the limestone surface. Ground-water circulation is
most vigorous at, and just below,.the water table where solution of limestone
is accelerated. Other causes of accelerated solution at certain depths may be
the occurrence of bedding planes in the limestone or changes in rock composi-
tion that concentrate the flow of water.

Limestone is commonly exposed in the vertical or overhanging walls of col-
lapse sinkholes. The walls arejusually irregular in shape because of joints
and fractures in the rock. Surface drainage and accumulation of sediment will,
eventually, smooth the sides an 'reduce their slopes until they are not distin-
guishable from other types of sinkholes.

Although limestone-collapse sinkholes provide dramatic local topography,
roof collapse is relatively uncommon. Dissolution at the limestone surface
and in near-surface joints is m9re likely the dominant process in landscape
development. Thus, limestone-collapse sinkholes are relatively uncommon in
west-central Florida, except for a coastal strip along Pasco, Hernando, and
Citrus Counties and a small area in Levy and Marion Counties in the extreme
northern part.


























a.
Rainwater percolates through points in limestone to
the water table. Highly transmissive joints dissolve

faster than others.


Sinkhole Intersects the water table. Rate of

dissolution Is greatly reduced and may be les r
than surrounding area.


b.

Differential solution of bedrock Is expressed by a

depression at land surface that funnels water to
the enlarged joints.


Sinkhole is expressed as a shallow sand-filed

depression because of- clay and clayey sand
filling and subsidence of surrounding limestone.


Figure 23.--Stages in development of a limestone-solution sinkhole.
(Arrows indicate direction of water movement.)








/


a. Solution cavity develops along joint or
other plane of weakness at the water
table.


b. Root collapses, most likely at joint
Intersection. Undercutting of cave walls
by diverted ground water.
/ ////


















b. Roof collapses, most likely at joint
Intersection. Undercutting of cave walls
by diverted ground water.


c. Roof collapse reaches land surface.
Undercutting continues.


/'


d. Solf washes into depression and
obscures Its'origin. Breakdown and
cave roof cemented by recrystallized
limestone.


Figure 24.--Stages in development of a limestone-collapse sinkhole.
(From Sinclair, in press. Arrows indicate direction of water
movement.)






Sinkholes in,Areas of Thickly Covered Limestone


The thickness and composition of unconsolidated material that mantles
limestone controls, to a large degree, the shape and size of sinkholes. A
thick layer (more than 50 feet) of dense clay may have sufficient bearing
strength to bridge a limestone cavity of considerable size (more than 50 feet
in diameter). When the clay finally fails, the resulting sinkhole will be
relatively large and will probably form abruptly.

Where limestone is buried beneath a sufficient thickness (more than
100 feet) of unconsolidated material, sinkholes are less common. If the over-
burden consists of incohesive sand, an upward-migrating cavity will be dissi-
pated by a general lessening of density of cover material over a large area,
and the result will be a relatively extensive subsidence of the land surface
that occurs over time. Generally, subsidence of this type may go unnoticed
for several years. If the overburden is clay, its low permeability may impede
downward movement of ground water and retard development of solution cavities
in the underlying limestone. For this reason, areas underlain by thick layers
of relatively impermeable clay generally are not affected by sinkholes.


Cover-Subsidence Sinkholes


In areas where the limestone is covered by materials that are relatively
-incohesive and permeable, sinkholes develop by subsidence (fig. 25). The water
table in the surficial aquifer is above the potentiometric surface of the
Floridan aquifer, and head differences between the aquifers largely determine
the rate of downward movement of water into the underlying limestone. Dissolu-
tion of the limestone is greatest during the early development of a sinkhole and
is smallest after the sinkhole intersects the potentiometric surface (fig. 25d).
Under these conditions, individual grains of sand move downward in sequence,
replacing limestone that has 41ssolved. Areas where sand cover is as much as
50 to 100 feet thick may develop cover-subsidence sinkholes that are only a few
feet in diameter and depth. Their small size and mode of occurrence are due to
Sthe fact that cavities in the limestone cannot develop to appreciable size before
they are filled with sand. the thousands of cypress heads in west-central Florida
occupy depressions formed by cover-subsidence sinkholes.


Cover-Collapse Sinkholes


Throughout much of west-central Florida, the sand cover becomes increas-
ingly clayey with depth and a layer of dense, impermeable clay commonly overlies
the limestone surface. The clay component provides a degree of cohesiveness to
the cover material that allows it to bridge a developing cavity in the limestone.
The result of failure of the bridge is a cover-collapse sinkhole whose dimensions
are related to the size of the cavity and the bearing strength of the clay.
Cover-collapse sinkholes form by the same general mechanism as cover-subsidence
sinkholes. The distinction between the two types of sinkholes is whether the
cover subsides slowly or collapses abruptly. The rate of cover subsidence is
controlled by the degree of cohesion or bearing strength of the cover material.


















THICK SAND
WATER TABLE
POTENTIOMETRIC SURFACE


CONFINING BED


a. Rainwater percolates through inoohesive
deposits to underlying limestone. Highly
transmissive joints dissolve faster than
others,


A V A _r-

c. Sinkhole Intersects the water table an
cypress trees begin to grow. Rate of
dissolution Is reduced because there is
less head difference between the water
table and potentiometric surface and,
thus, less percolation.


b. Differential solution of bedrock is expressed
by a depression at land surface that funnels
water to the enlarged joints.


subsides. A cypress dome forms with <
tree in the center and young trees on
the perimeter.


Figure 25.--Stages in development of a cover-subsidence sinkhole.
(Arrows indicate direction of water movement.)






An example of stages in the development of a small cover-collapse sinkhole
near Tampa, where the limestone is covered by about 4 feet of clay and 30 feet
of sand, is shown in figure 26. The geology is typical of much of west-central
Florida where clay separates the surficial sand from the limestone below. Solu-
tion cavities develop near and at the top of the limestone where acidic water
leaks downward through the clay (fig. 26a). The clay layer overlying the lime-
stone may bridge the cavity for a considerable time by virtue of its cohesive
strength, but eventually the clay will collapse as material continues to fall
from the cavity roof (fig. 26b). The cavity then develops rapidly upward by
piping as the relatively loose sand flows downward. This process is accelerated
by water that percolates through the sand and the breach in the clay to the
limestone cavity. The photograph at the top of figure 26c was taken in 1971,
shortly.after the sinkhole appeared. The top of figure 26d shows the same sink-
hole in 1981. During the 10-year period, the only visible change that occurred
was surface material eroded into the depression.
Where clay fills the limestone cavity, downward movement of water through
Sthe cavity may be diverted because the clay is less permeable than the surround-
ing limestone. Where the clay layer does not completely fill the cavity, down-
ward movement of water from the upper sand is enhanced by disrution of the clay
layer and by increased surface drainage into the expanding depression at land
surface. The sinkhole may then be further enlarged by additional collapse or
subsidence.
The thickness and composition of unconsolidated material that covers the
limestone have an important effect on the shape and size of land-surface col-
lapse. A thick layer (more than 50 feet) of dense clay may have sufficient
strength to bridge a large-diameter cavity whose limestone roof has collapsed
into a solutionally enlarged joint in the limestone. When the clay layer fi-
nally fails, the resulting cover-collapse sinkhole will be relatively large and
will probably form abruptly. The width of the cavity at the limestone surface
may not be as great as that in the clay, or as great as the diameter at land
surface. The limestone cavity may be a deep, small-diameter conduit through
which debris is transported by gravity and ground-water flow. The size of the
sinkhole at land surface fi'proportional to the thickness and bearing strength
of the cover material and the volume of the underlying cavity.
Collapse of cavernous passages may occur at considerable depth (50 to
200 feet) beneath overlying rock formations. The overlying rock layers may
(1) collapse at the same time as the roof of the cave, (2) may bridge the cav-
ity before eventually collapsing, or (3) if the rock is not too brittle, may
sag or slowly settle into the cavity. In the first twe instances, sinkholes
appear abruptly at land surface. If the rock sags or settles, sinkholes form
by gradual subsidence rather than by abrupt collapse. Cover-collapse sink-
holes occur most frequently in.the midsection areas of west-central Florida.
They commonly occur in the rige areas, but have also occurred as a result of
pumping in areas of Hillsborough and Pasco Counties.


Induced Sinkholes
I

Induced sinkholes are caused by man's activities, whereas natural sink-
holes are not related to man's activities (Newton, 1976). Induced sinkholes
consist of two types: those that result from water-level declines caused by






ground-water withdrawals anii those that result from construction. Induced
sinkholes are common in developing areas of west-central Florida. Their
occurrence can be expected to continue as the area develops.


Sinkhole Collapse Related to Ground-Water Withdrawals


The Upper Floridan aquifer is recharged primarily by direct infiltration or
downward leakage from overlying aquifers. This occurs in areas where the water
table or potentiometric surface of the overlying aquifers is higher than the
potentiometric surface of the Upper Floridan aquifer and where confining beds
that separate the aquifers are thin, discontinuous, or breached by sinkholes,
faults, or other openings. In areas where the Upper Floridan aquifer is con-
fined, recharge increases when the aquifer is pumped. The increase in recharge
by leakage is directly proportional to the head differences between aquifers
caused by pumping. That is, the greater the head difference, the greater the
leakage through the confining bed into the Upper Floridan aquifer. Ground-water
withdrawals can accelerate sinkhole development, particularly during dry periods
when water levels in the Upper Floridan aquifer are low. Lowering of water
levels by pumping results in loss of the water's buoyant support of unconsoli-
dated material that overlies cavities in limestone. If the competency of the
overlying layer is exceeded because of the increased weight, the effect is a
collapse of the material into the limestone cavity.

Stewart (1968) documented a sinkhole that developed because of pumpage at
the Section 21 well field north of Tampa (fig. 27). The limestone in the well-
field area is overlain by a dense, relatively impermeable clay layer that aver-
ages about 4 feet in thickness (fig. 26). The clay is overlain by about 30 feet
of sand. The area is a Pleistocefe terrace whose main topographic relief is
that caused by development of sinkholes.

Records of pumpage and water levels in the Section 21 well field are shown
in figure 27. Prior to early 1964, pumpage was about 4 Mgal/d, and the head dif-
ference between the water table and the potentiometric surface was about 10 feet.
By May of 1964, pumpage in the well field had increased to nearly 15 Mgal/d.
The pumping caused the head*difference between water levels in the surficial
aquifer and the Upper Floridan aquifer to increase from about 10 to 15 feet.
Thus, stresses across the confining clay layer increased. In areas where cav-
ities were near critical conditions, that is, approaching the point where their
roofs would collapse under natural conditions, the sudden increase of downward
pressure hastened roof collapA, During the period of pumping from February
through May 1964, 64 new sinkhi4es were reported within a 1-mile radius of the
well field (Sinclair, 1982). -

An example of the effects of large ground-water withdrawals from the
Upper Floridan aquifer on sinkhole development was also documented in a 7-mi
area near Dover (fig. 1) in eastern Hillsborough County (Hall, L. E., and
Metcalfe, S. J., Hydrologists, Hillsborough.County, written commun., 1977).
During the period January 17-24, 1977, air temperatures dropped to about 25*F,
and to protect strawberry crop from freezing, growers spray irrigated through-
out the freeze period. The intensive irrigation caused withdrawals of several































































a. Cavity dissolved in limestone by percolating b. Collapse of overlying clay into cavity by
ground water. spelling.
Time: Centuries. Time: Months years.

SFigure 26.--Stages in development of a cover-collapse

52






























































c. Piping of coheslonless sand Irto cavity.
Time: Hours days.



sinkhole. (From Sinclair, in press.)


d. Modification of sinkhole by surface erosion.
Time: Ten years.












10
0 0 , // /' "



o 4 -.,---1
o ''I' / // /




20 /0


WATER TABLE IN THE --





SDEPTH 15 EET, SCREENED 12-15 FEET
> 8 W
-J





S48



ENTIOETRIC SRFAE O









THE UPPER FLORIDAN AQUIFER


-A I-MILE RAQ(US OF WELLFIELD
2 44 12




z 40 -- E
-. j
r WELL 280702082028.0
W DEPTH 347 FEET, CASED 46 FEE1
_I POTENTIOMETRIC SURFACE OF
fl: THE UPPER FLORIDAN AQUIFER

StewartY 1964-TOTAL OF 64 NEW.)
SINKHOLES COUNTED WITHIN
-A I-MILE RADIUS OF WELLFIELD 1-

32 8. p 24

30
1961 62 63 64 65 1966


Figure 27.--Puspage at the Section 21 well field and water levels
in shallow and deep observation wells, 1961-66. (Modified from
Stewart, 1968.)





million gallons of ground water per day and caused a large decline in the
potentiometric surface throughout the area. The reduction in buoyancy caused
by drawdown in the potentiometric surface, coupled with the decrease in cohesive
strength and the iareased load at the surface caused by the irrigation, trig-
gered the occurrence of 22 sinkholes in the area.

In December 1983, air temperatures at Dover were lower than those in
January 1977 and again the crops were spray irrigated to prevent freezing.
However, the heavy pumping of ground water did not produce a rash of sinkholes
such as those that occurred in 1977. The lack of sinkhole occurrences in 1983
probably was due to the fact that stress on the aquifer system was about the
same as in 1977 and, therefore, was not sufficient to induce development of
new sinkholes. Sinkholes that could result under that level of stress had
occurred during the 1977 event, so the potential for additional sinkholes to
occur was greatly lessened.



Sinkholes Related to Construction


Ponding of water by construction of dams and reservoirs and diversion of
surface water by other activities, such as highway construction, have also been
responsible for triggering the collapse of sinkholes. Impoundment of water in
reservoirs and artificial lakes may contribute to sinkhole development by rais-
ing ground-water levels; loading the land surface, and providing a source of
percolating water that may flow through and erode zones of weakness in carbon-
ate rocks or overlying sediments,

The bed of Lake Grady (fig. 1), a manmade lake in south-central Hills-
borough County, is ahown in figure 28. Two years after the 200-acre lake was
formed, abnormal declines in its level indicated that it was losing water
(Stewart, 1982). Water in neighboring wells that tapped the Upper Floridan
aquifer became turbid about a month before a sinkhole opened and drained the
lake. The sinkhole was subsequently partially excavated and filled with wire
mesh, cement, and clay, The plugged sinkhole was then isolated from the lake
basin by an earthen dike and the lake refilled.

In highway construction, removal of surficial materials and vegetation
may expose openings that connect jpth the underlying limestone or it may change
the natural drainage pattern of 0 rea. Exposed openings provide direct access
for surface water to move downwa into the aquifers And increase dissolution
of the limestone. Changes in the natural drainage pattern generally result in
concentrating surface runoff into ditches or ponds to remove the excess water.
The discharge of large concentrated volumes of water may cause erosion of sur-
ficial materials that overlie cavities in the limestone and hasten development
of sinkholes. Several sinkholes have occurred on highways in the study area,
particularly in Hillsborough andlPolk CountieA. Generally, the sinkholes
ranged in diameter from about 5 to 25 feet and averaged about 15 feet in
depth.















I :r
0(D I








00


t.
0t







I. ID
0r
(D
0 -







0,
(D
a0




rt.
W. a




W.. M



M


Figure 28.--Bed of former Lake Grady near Tampa drained by a cover-collapse sinkhole, May 1974.
(Photograph by J. W. Stewart, U.S. Geological Survey.)


I r r I ~TM'






OCCURRENCE OF SINKHOLES


Sinkhole-Type Areas


The areas shown in figure 29 illustrate zones of different sinkhole type
and their relationship to different types of geology, landscape, and geomor-
phology. Zone 1 consists of a coastal strip below an altitude of about 25 to
30 feet and a small area in the extreme north. The altitude along the coast
marks a relatively long stand of sea level that created beaches from about the
Hillsborough-Pasco County line northward. To the east of these beaches, on
the terrace, the mantle of sand and sandy clay is relatively thin and limestone
bedrock is near land surface. Joint and fracture patterns in the limestone are
evident in the occurrence of linear stream segments in the area. Sinkholes in
zone 1 are generally formed by the collapse of limestone roofs over conduits
that have expanded beyond their roof-support capacity. Because of the very low
ground-water gradient and relatively noncorrosive water, sinkhole development
in zone 1 is not rapid, even in terms of geologic time, except perhaps in the
vicinity of springs. Most sinkholes in zone 1 are limestone-collapse sink-
holes where a limestone roof has failed because of loading of the land surface,
ground vibration, ground-water withdrawals, or other development activities.

Zone 2 is a large area comprising the Green Swamp and the Withlacoochee
and Hillsborough River basins. The area consists of bare to thinly covered
limestone. Because the area is one of dense stream patterns, it suggests that
much of the rainfall that is not lost to evapotranspiration runs off as stream-
flow. Small amounts of rainfall are available for infiltration, and aquifer
recharge, though variable, is generally small (fig. 10). Most of the area was
flooded by high sea levels several times during the Pleistocene Epoch. The
land surface was altered by long-shore currents and wave action so that karst
features within the limestone are filled and relatively inactive. At many
places in zone 2, limestone is at land surface and caves do occur, but for the
most part, the water table is at or near land surface. Ground-water circula-
tion is impeded to a great extent by material of low permeability that filled
the limestone conduits. Thus, sinkhole development in zone 2 is relatively
rare.

Zone 3 has essentially no surface runoff, although rainfall is plentiful.
Recharge to zone 3 is probably the highest in west-central Florida (fig. 10).
It is also an area of relatively corrosive water in the Upper Floridan aqui-
fer, and corrosion of limestone is taking place relatively rapidly. Zone 3
comprises a sand mantle 50 to 150 feet thick that blankets limestone. The
zone occurs along the Brooksville Ridge and the terrace east of the ridge in
Marion County (fig. 4). Because of the incohesive nature of the sand, sink-
hole development in the area proceeds by way of gradual subsidence rather than
abrupt collapse. As the limestone surface below the sand mantle is dissolved
and removed in solution, sand grains move downward to occupy the created space.
Generally, this movement is transmitted to land surface over a long period of
time. However, piping may occur after a limestone roof collapses, and an open
sinkhole may appear at land surface.

Zone 4 comprises areas in parts of Hillsborough, Pasco, and Pinellas
Counties covered by Pleistocene sand and clay deposits that range in thickness
from 25 to 100 feet. Where the underlying limestone was dissolved, a residuum







82045' 82000' 8115'



EXPLANATION


ZONEl
Bare or thinly covered limestone. Sink-
.: MA\RION hole development is rare, but limestone-
I collapse sinkholes dominate.



S. Bare or thinly covered limestone. Little
recharge. High surface runoff. Sinkhole
LAKE development is rare.
'---
ZONE 3

Incohesive sand cover 50 to 100 fet thick.
High recharge. Few sinkholes, but limestone
solution linkholes dominate.

ZONE 4
Cover is 25 to 100 fast thick consisting of
sand overlying clay. Numerous sinkholes,
sinkhole lakes, and cypress headed. Cover-
collapse sinkholes dominate.



Cover is 25 to 150 feet thick. Sand cover
. is underlain by a thick clay layer. In-
ternal drainage is common. Cover-collapse
and cover-subsidence sinkholes occur.

ZONE 6
Cover is more than 200 feet thick. Alti-
tude exceed 100 feet. Humorous lakes and
sinkholes. Limestone solution is rapid.
Cover-subsidence sinkholes by piping domi-
nIat. Occasional large-scale cover-collapse
sinkholes occur.

ZONE 7

Cover is more than 200 feet thick. Clay
270- f thickness exceeds 200 feet. Sinkhole de-
velopment is rare, but soe cover-collapse
sinkholes occur. Also, dissolution of
shellbeds and marl result in minor cover-
subsidence sinkholes.


O 20 40MILES

O 20 40 60 KILOMETERS





Figure 29.--Zones of different sinkhole types.


MEXICO





of sandy clay was left that grades down to a weathered limestone surface. This
sandy clay forms a confining bed that restricts the downward movement of water
from the surficial aquifer to the Upper Floridan aquifer. Many sinkholes,
sinkhole lakes, and cypress heads occur in zone 4. The cypress heads form in
depressions that intersect the water table and are more obvious indications of
sinkholes than land-surface depressions, such as those that occur in zone 3.
Lakes in zone 4 are commonly irregular in outline because of the coalescence
of many small sinkholes. Figure 30 is a Rose diagram of the composite along
the long axes of sinkholes in the Section 21 well-field area (fig. 31) that is
in zone 4. The small sinkholes that are included in the Rose diagram could
have been oriented in any direction because most small sinkholes in zone 4 are
round. Along fractures, the sinkholes develop laterally and increase in size
as they coalesce. In general, the long axes of many sinkholes in Section 21
fall in the range of 40 to 45 degrees and 110 and 115 degrees. Lineations of
stream channels in the area also correspond somewhat roughly to lineations of
sinkholes and sinkhole features. The sinkholes in zone 4 are generally the
cover-collapse type.

The effects of development on the occurrence of sinkholes have been well
documented in zone 4 because of the number of large well fields in the area
(Stewart, 1968; Sinclair, 1974). Development of sinkholes in this zone, where
the geology is relatively well known, can probably be predicted as well as
anywhere in west-central Florida.

Zone 5 comprises the southern end of the Brooksville Ridge, northwestern
Pinellas County, and an area that extends from about Tampa eastward to the
Lakeland Ridge. Land-surface altitudes in most of the areas are relatively
high, and surficial sands are underlain by a thick section (25 to 100 feet)
of impermeable clay. Much of the rainfall flows through short intermittent
streams to the Gulf or to lakes. The lakes increase in size during the rainy
season and some disappear when intermittent sinkholes develop in lake bottoms
and water drains internally to the Upper Floridan aquifer. This process is
best shown in the southern Brooksville Ridge area because of its high altitude
and rapid ground-water movement. The process is also typical of east-central
Hillsborough County where the limestone is overlain by 25 to 150 feet of sand
and sandy clay. Most streams in the area are perennial. Internal drainage is
common, and sinkholes occur by cover collapse and cover subsidence.

Zone 6 is in the eastern part of the study area and includes parts of
Polk and Highlands Counties. It is a region of ridges often referred to as
the Lake Region. Cover material is greater than 200 feet thick with 100 feet
or more of clay. Ridges in zone 6 include the Winter Haven Ridge, Lake Henry
Ridge, and Lake Wales Ridge (fig. 4). The altitudes of the ridges are 100
feet or more above sea level. These ridges are primarily erosional remnants,
but may be structural as proposed for the Lakeland Ridge. The ridges are
apparently bordered by beaches and terraces of pre-Pleistocene age. Lakes and
sinkholes along the ridges are relatively large and well developed. This may
be because the area has not been innundated since Miocene time.

The ridge area is one of moderate recharge (fig. 10) of relatively corro-
sive water. Because of circulation of corrosive water, solution of bedrock is
as rapid as any area in west-central Florida.





00 200


3400


EXPLANATION


LENGTH IS CUMULATIVE LENGTH OF LONG
AXIS OF SINKS WITHIN A 5 DEGREE ARC.



Figure 30.--Lineation of sinkholes within the Section 21
well-field area near Tampa.


400







600




800

900

I000




1200







-1400





O LARGE CAPACITY WELL ,
OBSERVATION WELL

INDUCED SINKHOLES
1963-64

-tl
CYPRESS HEADS.

O CRENSHAW
LAKKE *

srARVArTON 01
LAKE "
SECTION 21
WELL FIELD SADDLEBAc










W LL 21-10 LA


S SINKHOLE
SSHOWN 2 i


F 0 P




Figure 31.--Section 21 well-field area near Tampa. (Modified from Sinclair, 1982.)





The occurrence of small diameter vertical pipes in the overburden is
illustrated by the bottom contours of Lake Eloise at Winter Haven (fig. 32).
The small circular depressions in the lake bottom at the south end of the lake
are presumed to be relatively recent, small diameter, piping type sinkholes.
Although development of small diameter pipes in the overburden is probably the
most common erosive process in zone 6, large-scale cover-collapse sinkholes
also occur. Figure 33 is an aerial photograph of a sinkhole that occurred in
an orange grove near Winter Haven in the summer of 1981 following a year-long
period of drought. The sinkhole is about 120 feet in diameter and 30 feet deep.
The sinkhole appeared at the same time as several nearby sinkholes and is pre-
sumed to have been caused by limestone collapse rather than by piping in the
overburden. Thus, cover-subsidence sinkholes, largely by piping, dominate in
zone 6, but cover-collapse sinkholes also occur.
Polk County's Department of Public Safety routinely monitors and documents
sinkhole occurrences. Figure 34 is a map of the county that shows locations of
reported sinkholes. Most of the sinkholes are within zone 6. Also shown in
figure 34 is the topography of the county. Sinkholes are concentrated along the
east flank of the Lakeland Ridge that, according to Altschuler and Young (1960),
is an uplifted block. Presumably, the fault traces on either side of the up-
lifted ridge are marked by sinkholes. Analysis of driller's well logs indicates
that the Lakeland Ridge is a bedrock high and that a bedrock low occurs immedi-
ately to the east of it and a less well-defined bedrock low occurs to the west
of it. This fracture area was probably of major importance in accelerating
corrosion of limestone and subsequent collapse of land surface.
Figure 35 shows major lineations along which sinkholes have occurred in
Polk County. Lineations seem to aline with surface indications of underlying
structural features, such as gaps through the Winter Haven and Lake Henry
Ridges. Additional lineations are alined with gaps in the Lake Wales Ridge.
From comparisons of topographic features, structure contours, apparent line-
ations, and surface-water features, it becomes apparent that geology and
geologic structure affect the occurrence of sinkholes.
Zone 7 includes all or parts of eight counties in the lower third of the
study area. The cover material in zone 7 is more than 200 feet thick and con-
sists of cohesive sediments, discontinuous carbonate beds, and clay. Areas,
such as zone 7, where the clay is as much as 200 feet thick may be considered
to be essentially free from sinkhole development. However, shell beds and marl
within the Pleistocene section near land surface and limestone in the Hawthorn
Formation are probably dissolving. Thus, the small circular depressions com-
monly observed on topographic maps of zone 7 are probably cover-subsidence
sinkholes resulting from very small collapse into underlying calcareous units.
Paleokarst features, such as Warm Mineral Spring, Little Salt Spring, and
Deep Lake, occur in the southern part of zone 7 (fig. 1) and are examples of
large solution cavities that formed below the confining clay. In the vicinity
of Deep Lake, the deposits of the Hawthorn Formation are as much as 500 feet
thick (fig. 8). Deep Lake has a maximum depth of about 300 feet. The cross
section of Deep Lake (fig. 36) illustrates a classic example of a cover-
collapse sinkhole and a breakout dome where overlying beds are collapsing into
a large cavity at considerable depth. The debris pile at the base of the lake
is largely clay. Deep Lake developed in response to collapse into deep and
apparently large-scale solution cavities in the Upper Floridan aquifer below
the Hawthorn Formation.






42030' 42000 4130'










I I I








I II
















EXPLANATION
LAKE-BOTTOM CONTOUR--SHOWS
-6- LINE OF EQUAL DEPTH BELOW
LAKE SURFACE. INTERVAL IS 2
FEET. DATUM IS ALTITUDE OF
130 FEET

0 2000 4000 FEET
0 500 1000 METERS




Figure 32.--Bottom topography of Lake Eloise at Winter Haven.
(From Sinclair and Reichenbaugh, 1981.)









Lt rt
I _0

m : 0





~0 0 2
5 10

I--0 0 m~
l-'~0'C 0 2 '' :;-
n




cI CD
roI-..





~%m~ m


0.
:j

o 2 0 '

P- 0 a
0 0
9m m~
0- 0




t- :3,

-0 w m
&I-.




ko m
IVj M~
~* o60

~I1(
r1



I4

Figure 33.--Sinkhole near Winter Haven.






EXPLANATION


-- 0 .

2815'-


0 (

16ce


LOCATION OF SINKHOLE

0.YOL. TOPOGRAPHIC CONTOUR--
SHOWS ALTITUDE OF
LAND SURFACE. CONTOUR
INTERVAL 50 FEET.
DATUM IS SEA LEVEL

F LAKE WALES RIDGE

WINTER HAVEN RIDGE

LAKE HENRY RIDGE

E LAKELAND RIDGE


28000-


27045'-Z



82000' 81015'
Base and land-surface
contours from U.S.
Geological Survey map
of State of Florida, 1968


I
81030'


8145
81o45'


0 5 10 15 MILES
0 I I
O 10 20 KILOMETERS


Figure 34.--Topography and locations of sinkholes, Polk County.




Figures 7, 8, 21, and 22 illustrate the increasing thickness of confining
beds that overlie the Upper Floridan aquifer from north to south. In areas
where the confining beds are about 100 to 150 feet thick, the bearing strength
and leakance of the beds preclude infiltration of corrosive water and develop-
ment of sinkholes. In zone 7 (fig. 29), the thick confining bed provides
sufficient bearing strength to bridge all but the largest cavities that may
develop in the underlying limestone. The low leakance of the confining bed
minimizes recharge and corrosion of limestone.





EXPLANATION


I 00
0 004
-- '
2801'" o
oS
tJ~~ 0o~

ISO
I.n! ~~~t


LOCATION OF SINKHOLES
SALINEMENT OF SINKHOLES
TOPOGRAPHIC CONTOURS--
SHOWS ALTITUDE OF
--/00 LAND SURFACE. CONTOUR
INTERVAL 50 FEET
DATUM IS SEA LEVEL


28000'-








27o45'-


8200' 81015' 81030,


81015'


Base ond land-surface
contours from U.S.
Geological Survey map
of State of Florida, 1968


0 5 10 15 MILES
0 10 20 KILOMETERS


Figure 35.--Major lineations along which sinkholes have occurred
in Polk County.




Reported Sinkholes in West-Central Florida


A listing of 181 sinkholes reported for 1958 and 1968-81, their location,
date of occurrence, and description is given in table 3. The table was compiled
from newspaper accounts, records of the Florida Department of Transportation,
files of the U.S. Geological Survey, files of the Polk County Department of
Public Safety, and files of the Southwest Florida Water Management District.
Figure 37 shows the locations of the sinkholes. The reported information for





0 WATER LEVEL 50

SEA
LEVEL
SURFICIAL DEPOSITS

100

I-
,/- 100
LU
HAWTHORN
Z 200 FORMATION AND
LIMESTONE UNIT I
SOF TAMPA -200 L
U- LIMESTONE, Z
300 UNDIVIDED DEBIS PILEFEET, DEEPEST
.L MEASURED POINT
O
-300 -
,, -J
C',
III '------- ------- --- -- -
Z 400 -SAND AND CLAY
o UNIT OF TAMPA
LIMESTONE 400


500

500


0 100 200 300 400 500 600
DISTANCE, IN FEET


Figure 36.--Cross section of a cover-collapse sinkhole beneath Deep Lake
near Arcadia. (Modified from Wilson, 1977; Sinclair, in press.)





sinkholes is somewhat skewed because of the method of data collection and land-
use factors. For example, very few sinkholes are reported from rural areas
such as farms, limestone quarries, phosphate mines, and forests, whereas many
sinkholes are reported in developed areas. Many of the reported sinkholes are
in Polk County because the county's Department of Public Safety monitors sink-
hole occurrences. Thus, more.sinkholes are reported in Polk County than in
other counties that do not have sinkhole monitoring programs. The list does
not include the many sinkholes caused by ground-water withdrawals for frost-
freeze protection or well-field development because of inadequate documentation
on many of those occurrences.






Table 3.--Inventory of reported sinkholes

[Location: T, township; R, range; S, section. Date of occurrence: M, month; D, day; Y, year.
Description: L, length; W, width; D, depth; B, bearing; Sh, shape; Sl, slope. Length, width,
and depth are in feet; bearing is in degrees from north]


Date of
occurrence


Description


Remarks


T R S M D Y L W D B Sh Sl


34 09 01 74 006 006 003
34 09 01 74 008 008 006
23 04 24 74 006 006 006
18 10 15 74 006 006 006
04 03 21 74 001 001 004
22 04 02 74 008 008 004
22 04 02 74 006 006 006
22 05 23 76 012 012 010
22 04 02 74 003 003 003
31 02 06 78 005 004 006


16S 16E 24 01 02 73 006 006 006


0 Flat
0 High
0 Slope
0 Pipe High
0 Pipe Low
0 Pipe Low
0 Slope
0 Pipe


0 Pipe Low


Very wet.
Very wet.
Depth to rock 3 feet.
Very wet.
Very dry.
Very dry, blasting.
Very dry, blasting.
Very wet, rain.
Very dry, blasting.


Depth to water 12 feet; depth to rock
16 feet.


14 73 002 002 005 0 Pipe Low


002 003
015 005
004 006


0 Pipe


69 002 002 003 0


11 73 002 002 004
01 29 75 010 010 006
02 07 80 004 004 008
07 09 73 002 002 003


0 Pipe


0 Sink Low
0 Pipe Flat


Very dry, traffic.


003 003 003 0 Pipe Flat
008 006
004 003 004


006 006 006
003 003 003


0 Sink Flat
0


Very dry.


2-inch well in center of sink.
Very dry.
A second sinkhole is 10 feet east.


Location


12S
12S
13S
13S
14S
14S
14S
145
14S
0 14S






Table 3.--Inventory of reported sinkholes--Continued


Date of
Location Description
occurrence Rema
T_________ __ __ Remarks
T R S M D Y L W D B Sh S1


18S 22E
18S 22E
19S 17E
19S 17E
19S 17E
19S 20E
19S 20E
19S 20E
195 20E
19S 21E


19S 23E
21S 22E
1 22S 17E
22S 20E


003 003 003
003 003 012
005 005 006
003 003 003
009 009 007
012 011 003
008 006 305
020 015 001


23 03 27 80 004 008 008
08 07 19 74 004 004 004 0


008 008
150 130
035 035


0 Pipe
0 Pipe


Depth to rock 10 feet.

Bulldozer fell in sinkhole.


0 Pipe Flat


Flat


070 350 Sink Flat
015 0


23S 17E 10 06 01 81 015 015 018 0
23S 17E 27 02 14 78 005 004


23S 17E
23S 18E
23S 18E
235 19E


001 010
02 02 78 010 007
09 08 70 010 010
09 01 76 002 001


Flat


0 Pipe


Depth to water 5 feet.
Boatwright and Allman (1975).

Drilling machine fell in sink.


Many small depressions in area.


Water retention, depth to rock 16 feet.


Slope


23S 19E 31 09 19 79 015 010 006


015 015 008 0
005 005 004 04
100 100 4 0
100 100 030 0
050 050 010 0
001 001 004 0


004 003
009 009


24S 16E 34 01 08 80 013 012 010


Slope
Slope
Rolling


0 Pipe Flat


Very wet.


Very dry.
Very wet.


Well in center of sink, poorly cemented.


23S 20E
23S 20E
23S 20E
23S 20E
245 16E
24S 16E
24S L6E
24S L6E




Table 3.--Inventory of reported sinkholes--Continued


Date of
Location ate Description
occurrence Remarks

T R S M D Y L W D B Sh Sl


004 004
050 025
004 004
070 070
012 012


0 Pipe Flat


0
005 0


Slope
Low
Low


Hilly


29 79 005 004 003
15 79 006 003 006
25 79 012 006 010


08 21 78 002 002 004 0 Sink Flat


002 0


006 003
006 010


Depth to water 15 feet, drilling at site.
Very dry, depth to water 4 feet.
Very dry, depth to water 4 feet.
Rain.
Sink began draining small lake.
Very wet, X-drain, depth to rock 5 feet.


Very dry.
Drain sink, sink is in older sink.


Very wet, rain.
Drilling in area.



A second sink 20 feet south.


Flat
Flat


006 004 005
029 021 015
12 79 009 008 005
01 72 008 008 005
21 77 005 004 004
22 79 023 021 010


0 Pipe Low
145 Lakeshore In shore of Lake Drief, drilling nearby.


0 Pipe Flat


Rain.
Very dry.
Earlier sink improperly filled.


25S
25S
25S
25S
25S




Table 3.--Inventory of reported sinkholes--Continued


Date of
occurrence


Description


Remarks


T R S M D Y L W D B Sh S1


0

0 Pipe


003 003


Rain.


13 03 19 80 005 005 003 0 Sink Low
19 02 17 81 002 002 003 0 Flat
16 08 29 73 003 003 003 0 Pipe Flat
27 03 18 81 018 018 006 0 Sink Flat


Irrigated new lawn, pumped 220,000 gal-
lons in month.


062 002
035 030
016 010
010 001


75 007 004 002 360
75 017 005 004 360
80 006 006 0.1
77 012 012 004 0
74 010 010 005 0


003 031


Near South Pasco well field.


Lakeshore
Flat


Sink Flat
Pipe High


Flat
Lakeshore


0 Pipe Flat
0 Flat


Low water table.
Subsidence apparently due to pumping.


Many small sinks in area recently.
Subsidence.
Very dry, Suwannee Limestone.
Very wet, rain, depth to water 15 feet.


Very wet, drilling.

Subsidence during a 2-year period.


Depth to rock 25 feet.


Several small sinks in recent years.
Subsidence.


Location


Rain.
Rain.


27S
27S
27S
27S
27S
27S
27S
27S
27S
27S








Table 3.--Inventory of reported sinkholes--Continued


Date of
Location Description
occurrence Remarks

T R S M D Y L W D B Sh Sl


28S 18E 23 02 23 67


19E 07 11
19E 11- 05


19E 14
19E 17
19E 24


06 80
18 76
69
17 79
78
08 79


006 004 0
003 005
010 004 0 Pipe
002 010 0 Pipe
002 002 0
007 001 360


003 003 027
030 030 025
003 002 004


Undulating


0 Pipe Undulating
0 Flat
073


Depth to water 7 feet, depth to rock 23
feet, construction area.

Several small depressions in area.



Construction in area.
Subsidence, homes built on large filled
depression.

Pumping 2.5 Mgal/d within 0.5 mile.


" 28S
28S
28S
28S
28S


05 11 74
05 11 76
66
03 25 81
08 22 74
81
07 09 74
71
67


0 Pipe
0
0 Pipe
0


0
360 Mult
0


Flat
Slope
Ridge
Undulating
Rolling
Low


High
Swale


Tried to pull well casing 6 feet west.
Sink developed in treated effluent pond.


Very dry, depth to water 12 feet.
Depth to water 25 feet, X-drain.



Construction.
Three sinks developed during drilling.
Rain.


0 Pipe
0 Undulating
0 Sink High


Very wet, depth to water 8 feet.





Table 3.--Inventory of reported sinkholes--Continued


Date of
Location Description
occurrence Remarks

T R S M D Y L W D B Sh Sl


29S 27E 03.
30S 20E 22
30S 20E 36
30S 20E 36
30S 21E 04
30S 21E 31
30S 23E 03
30S 25E 06
30S 25E 06
30S 25E 06
30S 25E 06
30S 25E 06
30S 25E 06
30S 25E 06
30S 25E 06
30S 25E 06
30S 25E 06
30S 25E 07

30S 25E 14
31S 21E 26


08 08
05 01

12 27


74 050 050 0.5
74 003 003 004
74 003 003 0.5
74 016 016 003
74 020 020 0.5
74 020 020 010
74 010 010 003
74 010 010 008
81 130 130 030
81 030 030 035
74 020 020 012
74 025 025 012
75 020 020 025
74 045 040 018
70 005 005 003
81 008 008 006
74 050 027 001
65 175 175 035
79 003 003 010
58 200 200 060
67 025 025 020
68 040 040 015
68 030 030 015
68 120 120 015
67 060 060 060


Low
Low
Low
Hilly
Rolling


Very dry, traffic.
Rain.
Rain.
Rain.
Rain.
Rain.
Very wet, traffic.
Very wet, traffic. 2
Subsidence noted over 0.25-mi area.


Very wet, drilling.


Hilltop Near Lake Grady.


Swale


Lake Grady.


0 Pipe Rolling


0 Sink Low
0


67 050 050 040
66 060 060 060
03 25 74 010 010 025

67 200 200 150
04 07 74 130 100 050


Very wet.


Very wet, rain, depth to water 25 feet,
depth to rock 9 feet.
Depth to rock 100 feet.
Very dry, depth to water 46 feet.








Table 3.--Inventory of reported sinkholes--Continued


Date of
Location DteDescription
occurrence Remarks

T R S M u Y L W D B Sh SI

31S 25E 32 68 200 200 045 0
31S 26E 11 68 060 060 050 0
31S 27E 06 67 225 225 050 0
32S 20E 13 06 15 76 025 015 009 090 Slope
38S 19E 32 07 21 81 008 008 005 0 Flat Depth to water 5 feet.
39S 19E 29 07 22 81 002 001 005 Flat






82045'
I


81015'
I


EXPLANATION
S10
SINKHOLE. NUMBER
INDICATES NUMBER
. OF SINKHOLES AT
SSITE IF MORE THAN
ONE

SOUTHWEST FLORIDA
uMTER WATER MANAGEMENT
DISTRICT BOUNDARY


L^X


290-



GULF
OF
MEX/CO








280-


~j


0 20 40 MILES
i I I I
O 20 40 60 KILOMETERS



Figure 37.--Locations of reported sinkholes.
(Descriptions are given in table 3.)


L.E


*0 60
* A*

*

ATEE
HAROEE


270-


8200'





Sinkhole data from the Department of Transportation are the least skewed
for any particular area. They report 34 sinkholes in Polk County and 33 in
Suwannee County, a smaller, less developed county in northern Florida. Because
Polk County contains many more miles of right-of-way than Suwannee County, it
seems likely that the data may be skewed.

Given the slow progression of geologic events that lead to sinkhole forma-
tion under natural conditions, it is believed that most recently (since 1960)
developed sinkholes were induced by some triggering factor related to man's
activities. Of the sinkholes listed in table 3, 98 have maximum width-depth
dimensions of 10 feet or less. Twenty-seven have widths or lengths of 100 feet
or more. The largest sinkholes have occurred in areas where the overburden is
thick (more than 150 feet). In the Bartow area (Polk County), for example,
where most of the reported sinkholes have occurred, two had widths or lengths
in excess of 150 feet. Of the sinkholes described, about 20 percent had depths
greater than their largest horizontal dimension.

Conclusions cannot be drawn from data on induced sinkholes or recent
sinkholes as to where sinkholes might occur under natural conditions or where
they might occur in undeveloped areas. The sinkhole data in table 3 are listed
to provide a basis for further study and documentation.

Data in table 3 show that sinkholes have occurred during each month of the
year. May had the most reported occurrences (23) and November the least (5).
In general, sinkhole occurrences are higher during the February to May low rain-
fall period when irrigation is high and again in July and August, when surface
loading is increased by heavy rains.


SINKHOLES AS SOURCES OF WATER SUPPLY OR GROUND-WATER POLLUTION


Sinkholes are potential sources of water supply. They are also potential
sources of contamination to existing supplies because of surface inflow into
the sinkholes. As a source of water, Stewart (1977) documented a pumping test
on the Morris Bridge Road sinkhole northeast of Tampa. The test was run for
25 days at an average pumping rate of 4,000 gal/min. Analysis of the data in-
dicated that the sinkhole would be capable of yielding 10,000 gal/min (15 Mgal/d)
on a sustained basis with a maximum drawdown not likely to exceed about 23 feet.
This capacity is equivalent to the production capabilities of some large well
fields in west-central Florida. However, long-term pumping from the sinkhole
probably would cause reduction in flow of a nearby stream.

Tests of Curiosity Sink north of Tampa also indicate suitability of a
sinkhole for water supply (Stewart, 1982). Based on a pumping rate of 5,000
gal/min and a drawdown.of 2 feet, the specific capacity of the sinkhole is
about 2,500 gal/min per foot of drawdown.

The potential for pollution of the Upper Floridan aquifer through sink-
holes or internally drained areas caused by sinkholes in west-central Florida
have also been documented. Analyses of a water sample collected from a sink-
hole northeast of Tampa suggest that water-quality degradation has occurred.
The concentrations of calcium (37 mg/L), bicarbonate (128 mg/L), alkalinity
(105 mg/L as CaCO3), and dissolved solids (144 mg/L) indicate a mixture of





water from surface runoff and from the Upper Floridan aquifer (Stewart, 1977).
A nitrate concentration of 0.77 mg/L as nitrogen also indicates degradation due
to surface-water inflow or introduction of refuse into the sinkhole.
Analyses of water-quality data for Sulphur Springs at Tampa (fig. 4) sug-
gest water-quality degradation from several sinkholes 1 to 2 miles upgradient
of Sulphur Springs (Stewart and Mills, 1984). High ground-water velocities in
the area permit most bacteria in the water from the upgradient sinks to survive
the traveltime needed to reach Sulphur Springs.
Most internally drained areas are devoid of perennial streams, and surface
drainage is internal through sinkholes that are directly or indirectly connected
to the Upper Floridan aquifer. In many areas, overland runoff to internally
drained areas is of poor quality, and because of the direct connection to the
Upper Floridan aquifer without benefit of natural purification, absorption, and
filtration through soils and sands, the aquifer can undergo water-quality
degradation.
Aside from the hazards that sinkholes may cause, many are potential sources
of water supply. However, sinkholes provide direct routes through which surface
water can move into underlying aquifers and cause degradation of ground water.


SUMMARY AND CONCLUSIONS

Sinkholes are a natural and common geologic feature in areas underlain by
limestone and other soluble rocks. Abrupt sinkhole collapse occurs infrequently
under natural conditions. Stresses on the hydrologic system, such as ground-
water withdrawals and construction, have caused an increase in the occurrence
of sinkholes in west-central Florida, a trend that is likely to continue.
The dominant factor that determines the ability of recharge water to dis-
solve limestone is acidity of the water. Rainwater is naturally acidic, but
becomes more acidic as it reacts with organic matter in the soil zone. Based on
equations for dissolution of limestone material and a recharge rate of 5 in/yr,
it is estimated that 3.66x10 in of the material would dissolve under each
square inch of land in the Bartow area each year. At this rate, it would take
33,000 years to dissolve an average 1-foot deep cavity. Because recharge is
generally focused in fractures and joints, however, much less time would be
required to create a 1-foot cavity where flow is concentrated.
Four major types of sinkholes are common to west-central Florida. They
include limestone solution, limestone collapse, cover subsidence, and cover
collapse. The first two occur in areas where limestone is hare or is thinly
covered. The second two occur where there is a thick cover (30 to 200 feet)
of material over limestone.
Limestone-solution sinkholes, the most common type of sinkhole in areas of
thin cover, result from subsidence that occurs at roughly the same rate as dis-
solution of the limestone. The sinkholes reflect a gradual downward movement
of the land surface and development of funnel-shaped depressions. Limestone-
collapse sinkholes occur when a solution cavity grows in size until its roof
can no longer support its weight, causing generally abrupt collapse that can
be catastrophic. The occurrence of limestone-collapse sinkholes is relatively
uncommon in the study area.





Cover-subsidence sinkholes develop as individual grains of sand in the
cover material move downward into space created by dissolution of limestone.
Resultant sinkholes are generally only a few feet in diameter. Cover-collapse
sinkholes occur where clay layers that overlie limestone have sufficient cohe-
siveness to bridge developing cavities in the limestone. Eventual failure of
the bridge results in a cover-collapse sinkhole. The thickness and composition
of materials that cover limestone affect the shape and size of land-surface col-
lapse. A thick layer of clay (more than 150 feet), for example, upon failure
will develop a relatively large sinkhole that forms abruptly.

Large withdrawals of water for water supply, irrigation, or frost protec-
tion may provide a triggering mechanism for sinkhole occurrence. Removal of
water's buoyant support of unconsolidated deposits that overlie cavities can
cause the materials that bridge the cavity to fail and sinkholes to appear.
Such occurrences are common at well fields and irrigation areas in west-central
Florida. Conversely, loading of the land surface by impoundments causes failure
of materials bridging cavities below the impoundment and sinkholes may form. The
impoundments may also provide continuous sources of recharge water that hastens
development of cavities in limestone.

West-central Florida was divided into seven zones based on geology, land-
scape, and geomorphology and how they relate to types of sinkholes that occur
in each zone. The zones are: (1) areas of bare or thin sand cover that have
slowly developing limestone-collapse sinkholes; (2) areas of thin cover, little
recharge, high overland runoff, and few sinkhole occurrences; (3) areas of inco-
hesive sand cover of 50 to 150 feet thick that have high recharge and generally
experience cover-subsidence sinkholes; (4) areas that have 25 to 100 feet of
cover, many sinkhole lakes and cypress heads, and have predominantly cover-
collapse sinkholes; (5) areas of 25 to 150 feet of sand cover overlying thick
clay with both cover-collapse and cover-subsidence sinkholes; (6) areas with
more than 200 feet of cover, numerous lakes and sinkholes, and high land-surface
altitudes with numerous cover-subsidence sinkholes and occasional large-scale
cover-collapse sinkholes; and (7) areas with cover greater than 200 feet with
100 or more feet of clay and where sinkhole occurrences are rare. In areas
where the confining beds are 100 to 150 feet thick, the bearing strength and
leakance of the beds preclude infiltration of corrosive water and development
of sinkholes.

Based on an inventory of 181 reported sinkholes, 98 had maximum widths
or lengths of 10 feet or less and 27 had widths or lengths of 100 feet or more.
Most sinkholes occur during the February to May dry period and during the July
and August wet period. No particular period dominates, however, and no month
is without reported sinkhole occurrences.





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