HYDRGGEOLOGY, GRQUOT*WATER MOVEMENT,

AND SUBSURFACE STORAGE IN THE FLORIDAN

AQUIFER SYSTEM IN SOUTHERN FLORIDA

REGIONAL AQUIFER-SYSTEM ANALYSIS

\ SOUTH CAROLINA

L.-S.. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1403-G
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Hydrogeology, Ground-Water Movement, and 

Subsurface Storage in the Floridan Aquifer System in

Southern Florida

By FREDERICK W. MEYER

REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1403-G

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1989
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, Jr., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for 

and does not imj" 

.S. Government

descriptive purposes only and does not imply endorsement by the

Library of Congress Cataloging in Publication Data

Meyer, Frederick W.

Hydrogeology, ground-water movement, and subsurface storage in the Floridan aquifer system in southern Florida.

(Regional aquifer-system analysis) (U.S. Geological Survey professional paper ; 1403-G)

Bibliography: p.

Supt.ofDocs.no.: I 19.16:1403-G

1. Water, Underground Florida. 2. Floridan Aquifer. I. Title. II. Series.

GB1025.F6M48 1988 551.49'09759 87-600373

For sale by the Books and Open-File Reports Section, U.S. Geological Survey, 

Federal Center, Box 25425, Denver, CO 80225
FOREWORD

THE REGIONAL AQUIFER-SYSTEM ANALYSIS PROGRAM

The Regional Aquifer-System Analysis (RASA) Program was started in 

1978 following a congressional mandate to develop quantitative appraisals of 

the major ground-water systems of the United States. The RASA Program 

represents a systematic effort to study a number of the Nation's most 

important aquifer systems, which in aggregate underlie much of the country 

and which represent an important component of the Nation's total water 

supply. In general, the boundaries of these studies are identified by the 

hydrologic extent of each system and accordingly transcend the political 

subdivisions to which investigations have often arbitrarily been limited in the 

past. The broad objective for each study is to assemble geologic, hydrologic, 

and geochemical information, to analyze and develop an understanding of the 

system, and to develop predictive capabilities that will contribute to the 

effective management of the system. The use of computer simulation is an 

important element of the RASA studies, both to develop an understanding of 

the natural, undisturbed hydrologic system and the changes brought about in 

it by human activities, and to provide a means of predicting the regional 

effects of future pumping or other stresses.

The final interpretive results of the RASA Program are presented in a series 

of U.S. Geological Survey Professional Papers that describe the geology, 

hydrology, and geochemistry of each regional aquifer system. Each study 

within the RASA Program is assigned a single Professional Paper number, 

and where the volume of interpretive material warrants, separate topical 

chapters that consider the principal elements of the investigation may be 

published. The series of RASA interpretive reports begins with Professional 

Paper 1400 and thereafter will continue in numerical sequence as the interpretive products of subsequent studies become available.

Dallas L. Peck 

Director

CONTENTS

Page

Foreword......................................................................... Ill

Abstract........................................................................... Gl

Introduction...................................................................... 1

Purpose and Scope....................................................... 3

Previous Investigations ................................................ 4

Acknowledgments........................................................ 4

Hydrogeology of Southern Florida........................................ 5

Floridan Aquifer System............................................... 5

Upper Floridan Aquifer.......................................... 7

Middle Confining Unit............................................ 9

Lower Floridan Aquifer.......................................... 10

Hydrogeology at the Alligator Alley

Test Well Site.................................................... 10

Surficial Aquifer System......................................... 11

Intermediate Aquifer System .................................. 11

Floridan Aquifer System......................................... 12

Ground-Water Movement in the Floridan Aquifer System in

Southern Florida.................................................... 19

Ground-Water Movement Based on Natural Isotopes as

Tracers............................................................. 19

Page

Ground-Water Movement in the Floridan Aquifer System Cont. 

Ground-Water Movement Based on Natural Isotopes Cont.

Carbon Isotopes.................................................... G19

Upper Floridan Aquifer.................................... 20

Lower Floridan Aquifer.................................... 23

Uranium Isotopes.................................................. 23

Upper Floridan Aquifer.................................... 28

Lower Floridan Aquifer.................................... 33

Flow Patterns Based on Hydraulic Gradients ................... 33

Upper Floridan Aquifer.......................................... 34

Lower Floridan Aquifer.......................................... 36

Effects of Rising Sea Level on Ground-Water Movement... 37

Upwelling Ground Water as Evidence of Circulation ......... 41

Subsurface Storage in the Floridan Aquifer System in Southern

Florida.................................................................... 43

Oil Field Brine............................................................ 46

Municipal and Industrial Liquid Wastes........................... 48

Freshwater Storage..................................................... 52

Summary and Conclusions................................................... 54

Selected References........................................................... 57

ILLUSTRATIONS

FIGURE 1. Idealized hydrogeologic cross section through peninsular Florida showing concept of cyclic flow of seawater induced by

geothermal heating......................................................................................................................................... G2

2. Map of south Floridan Plateau showing selected wells, hydrogeologic sections, and submarine topography......................... 3

3. Generalized hydrogeologic section A-A' through southern Florida showing isotherms and top of saltwater in the Floridan

aquifer system............................................................................................................................................... 6

4. Map of peninsular Florida showing the potentiometric surface in May 1980 and the area of potable ground water, Upper

Floridan aquifer............................................................................................................................................. 8

5. Seismic reflection profile B-B' in the western Straits of Florida showing submarine karst on the Miami Terrace................. 9

6. Diagram showing fluid temperature and hydrogeologic units in well G-2334 at site 9, Fort Lauderdale.............................. 10

7. Map of south Floridan Plateau showing temperature of saltwater in the Boulder Zone of the Lower Floridan aquifer .......... 12

8, 9. Diagrams showing:

8. Construction features, selected geophysical logs, and stratigraphy for the Alligator Alley test well....................... 13

9. Hydrostratigraphy, distribution of flow, and selected geophysical logs for the Alligator Alley test well.................. 14

10. Graph showing pressure-depth relation for water-level measurements in the Floridan aquifer system at the Alligator Alley

test well....................................................................................................................................................... 18

11. Map of peninsular Florida showing carbon-14 activities and carbon-13 stable isotope ratios in samples of ground water from

the Upper Floridan aquifer.............................................................................................................................. 21

12, 13. Maps of south Floridan Plateau showing:

12. Carbon-14 activities and carbon-13 stable isotope ratios in samples of saltwater from the Boulder Zone of the Lower

Floridan aquifer................................................................................................................................. 24

13. Apparent carbon-14 ages and relative ages of saltwater in the Boulder Zone of the Lower Floridan aquifer............ 25

14. Graph showing relation of uranium-234/uranium-238 alpha-activity ratio and uranium concentration for selected samples of

ground water from the Floridan aquifer system and seawater worldwide ................................................................ 27
VI CONTENTS

FIGURES 15, 16. Maps of peninsular Florida showing:

15. Uranium concentration and predevelopment potentiometric surface, Upper Floridan aquifer ........................ G29

16. Uranium-234/uranium-238 alpha-activity ratio and predevelopment potentiometric surface, Upper Floridan

aquifer...................................................................................................................................... 31

17. Map of south Floridan Plateau showing uranium concentration and uranium-234/uranium-238 alpha-activity ratio in

saltwater, Boulder Zone of the Lower Floridan aquifer................................................................................ 34

18. Graphs showing estimated predevelopment and May 1980 hydraulic gradients along flow lines from Polk City to

Key Largo and Fort Lauderdale, Upper Floridan aquifer............................................................................. 35

19. Hydrograph of water level (saltwater) in well G-2334 at site 9 near Fort Lauderdale, Boulder Zone of the Lower

Floridan aquifer..................................................................................................................................... 37

20, 21. Graphs showing:

20. Sea-level fluctuations during the late Pleistocene and Holocene Epochs..................................................... 38

21. Comparisons of water level in central Florida, vegetation, and transit distance from subsea outcrop with

sea level during the Holocene transgression.................................................................................... 38

22. Hypothetical hydrogeologic sections through the Floridan Plateau for 18,000 years B.P., and present........................ 39

23. Graph showing theoretical velocity distribution during the Holocene transgression, Boulder Zone of the Lower

Floridan aquifer..................................................................................................................................... 40

24. Idealized cross section A-A' through southern Florida showing ground-water circulation and carbon-14 activities,

Floridan aquifer system .......................................................................................................................... 41

25-28. Maps of:

25. South Floridan Plateau showing location of some temperature anomalies in the Upper Floridan aquifer and

isotherms in the Lower Floridan aquifer ........................................................................................ 42

26. Martin County-St. Lucie County area showing temperature of ground water in the Upper and Lower

Floridan aquifers........................................................................................................................ 43

27. Martin County-St. Lucie County area showing dissolved solids in ground water from the Upper Floridan

aquifer...................................................................................................................................... 44

28. Southern Florida showing location of oil fields, municipal and industrial deep-well injection systems, and

freshwater injection pilot projects................................................................................................. 45

29. Graph showing brine and oil production for southern Florida oil fields, 1943-83...................................................... 46

30. Diagram showing hydrogeology and typical construction of oil field brine disposal wells........................................... 48

31. Graph showing annual and cumulative volumes of municipal and industrial liquid wastes injected, 1959-83 ................. 49

32, 33. Diagrams showing hydrogeology and typical construction characteristics of:

32. Municipal wastewater disposal well..................................................................................................... 51

33. Industrial liquid wastes disposal well................................................................................................... 51

TABLES

Page

TABLE 1. Temperatures of salty ground water in selected wells that tap the Boulder Zone of the Lower Floridan aquifer.................... Gil

2. Estimated distribution of flow and fluid conductance for the Floridan aquifer system at the Alligator Alley test well............. 16

3. Measurements of head and pressure in the Floridan aquifer system at the Alligator Alley test well.................................... 16

4. Summary of isotope analyses of selected water samples from the Floridan aquifer system, southeastern Florida, 1971-83 ...... 19

5. Selected uranium isotope data, Floridan aquifer system, central and southern Florida ...................................................... 26

6. Summary of selected uranium isotope data for the Upper Floridan aquifer by location and hydrogeologic environment,

central and southern Florida.............................................................................................................................. 28

7. Estimated transit times along flow lines from the Polk City potentiometric surface high to Key Largo and to Fort Lauderdale

under predevelopment gradients, Upper Floridan aquifer....................................................................................... 36

8. Comparison of pressure head and related data between well G-2334 at site 9 and well G-2296 at site 10, Boulder Zone of the

Lower Floridan aquifer..................................................................................................................................... 36

9. Estimated hydraulic gradients and related hydraulic data based on measured water levels and radiocarbon dating, Boulder

Zone of the Lower Floridan aquifer.................................................................................................................... 37

10. Estimated maximum and minimum velocities, hydraulic gradients, and head loss between sites 9 and 10 during the

Holocene transgression, Lower Floridan aquifer................................................................................................... 40

11. Selected analyses of oil field brine, southern Florida.................................................................................................... 47

12. Summary of brine production and disposal for oil fields in southern Florida, 1943-83 ........................................................ 47

13. Summary of municipal and industrial injection of liquid wastes in southern Florida, 1959-83 .............................................. 49

14. Selected water-quality characteristics of injectant of industrial liquid wastes, local water supply, and native ground water

in the Boulder Zone at site 15, Palm Beach County............................................................................................... 50

15. Selected water-quality characteristics of injectant of secondary-treated wastewater, local water supply, and native ground

water in the Boulder Zone at site 19, Palm Beach County...................................................................................... 50
CONTENTS VII

TABLE 16. Selected water-quality characteristics of injectant of secondary-treated wastewater, local water supply, and native ground

water in the Boulder Zone at site 22, Bade County.............................................................................................. G50

17. Average rate of municipal and industrial liquid wastes injection, 1959-84 ...................................................................... 51

18. Results of Florida Department of Natural Resources and Florida Department of Environmental Regulation injection,

storage, and recovery tests of freshwater at site 24 in Palm Beach County, 1975-76 ................................................. 52

19. Results of U.S. Geological Survey injection, storage, and recovery tests of freshwater at site 25 in Dade County, 1975-80 ... 53

20. Results of U.S. Geological Survey injection, storage, and recovery tests of freshwater at site 26 in Lee County, 1980-82 ..... 53

21. Results of South Florida Water Management District injection, storage, and recovery tests of freshwater at site 27 in

St. Lucie County, 1982-83 ............................................................................................................................... 53

22. Results of U.S. Geological Survey wastewater recovery tests of freshwater at site 14 in Broward County, 1975-77 ............. 54

CONVERSION FACTORS AND ABBREVIATIONS

For the convenience of readers who may prefer to use metric (International System) units, rather than the inch-pound terms used in this 

report, values may be converted by using the following factors:

Multiply inch-pound unit

inch (in)

foot (ft)

foot per day (ft/d)

foot per foot (ft/ft)

foot squared per day (ft2/d)

mile (mi)

mile per foot (mi/ft)

gallon (gal)

gallon per minute (gal/min) 

million gallons (Mgal) 

billion gallons per year (Ggal/yr) 

pound per square inch (lb/in2) 

pound per square inch per foot 

[(Ib/in2)/ft]

By

25.4

0.3048

0.3048

0.3048

0.09290

1.609

5.279

3.785

0.003785

0.00006309

3,785

3,785,000

6.895

22.6214

To obtain metric unit

millimeter (mm)

meter (m)

meter per day (m/d)

meter per meter (m/m)

meter squared per day (m2/d)

kilometer (km)

kilometer per meter (km/ni)

liter (L)

cubic meter (m3)

cubic meter per second (m3/s)

cubic meter (m3)

cubic meter per year (m3/yr)

kilopascal (kPa)

kilopascal per meter (kPa/m)

A barrel (bbl) contains 42.0 U.S. gallons (gal).

Tritium concentrations in picocuries per liter (pCi/L) can be converted to tritium units (TU) as follows (based on half life of 12.43 years):

TU = pCi/L x 0.313 

Temperature in degrees Fahrenheit ( °F) and temperature in degrees Celsius ( °C) are as follows:

°F=1.8x°C +32 

°C=5/9x(°F-32)

Sea level: In this report "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929) a geodetic datum derived from 

a general adjustment of the first-order level nets of both the United States and Canada, formerly called "Mean Sea Level of 1929."

REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

HYDROGEOLOGY, GROUND-WATER MOVEMENT, AND SUBSURFACE

STORAGE IN THE FLORIDAN AQUIFER SYSTEM IN

SOUTHERN FLORIDA

By FREDERICK W. MEYER

ABSTRACT

The Floridan aquifer system of southern Florida is composed chiefly 

of carbonate rocks that range in age from early Miocene to Paleocene. 

The top of the aquifer system in southern Florida generally is at depths 

ranging from 500 to 1,000 feet, and the average thickness is about 3,000 

feet. It is divided into three general hydrogeologic units: (1) the Upper 

Floridan aquifer, (2) the middle confining unit, and (3) the Lower 

Floridan aquifer. The Upper Floridan aquifer contains brackish ground 

water, and the Lower Floridan aquifer contains salty ground water 

that compares chemically to modern seawater. Zones of high permeability are present in the Upper and Lower Floridan aquifers. A thick, 

cavernous dolostone in the Lower Floridan aquifer, called the Boulder 

Zone, is one of the most permeable carbonate units in the world 

(transmissivity of about 2.5 x 107 feet squared per day). Ground-water 

movement in the Upper Floridan aquifer is generally southward from 

the area of highest head in central Florida, eastward to the Straits of 

Florida, and westward to the Gulf of Mexico.

Distributions of natural isotopes of carbon and uranium generally 

confirm hydraulic gradients in the Lower Floridan aquifer. Groundwater movement in the Lower Floridan aquifer is inland from the 

Straits of Florida. The concentration gradients of the carbon and 

uranium isotopes indicate that the source of cold saltwater in the Lower 

Floridan aquifer is seawater that has entered through the karst 

features on the submarine Miami Terrace near Fort Lauderdale. The 

relative ages of the saltwater suggest that the rate of inland movement 

is related in part to rising sea level during the Holocene transgression. 

Isotope, temperature, and salinity anomalies in waters from the Upper 

Floridan aquifer of southern Florida suggest upwelling of saltwater 

from the Lower Floridan aquifer. The results of the study support the 

hypothesis of circulating relatively modern seawater and cast doubt on 

the theory that the saltwater in the Floridan aquifer system probably 

is connate or unflushed seawater from high stands of sea level.

The principal use of the Floridan aquifer system in southern Florida 

is for subsurface storage of liquid waste. The Boulder Zone of the 

Lower Floridan aquifer is extensively used as a receptacle for injected 

treated municipal wastewater, oil field brine, and, to a lesser extent,

Manuscript approved for publication May 6, 1987.

industrial wastewater. Pilot studies indicate a potential for cyclic 

storage of freshwater in the Upper Floridan aquifer in southern 

Florida.

INTRODUCTION

In October 1978, the U.S. Geological Survey began a 

4-year (yr) study of the Floridan aquifer system (formerly called the Tertiary limestone aquifer) of the Southeastern United States, as part of a national ground-water 

resources investigative program called Regional 

Aquifer-System Analysis (RASA). The objectives of the 

Florida RASA project were to describe the hydrogeology, geochemistry, and flow of ground water in the 

aquifer system (Johnston, 1978). Variations in water 

quality, hydraulic head, and water temperature within 

the carbonate rocks that make up the Floridan aquifer 

system in southern Florida suggest that the flow system 

is complex. Movement of fresh and brackish ground 

water through the upper part of the Floridan aquifer 

system has been documented chiefly on the basis of 

measured head and hydraulic gradients. Movement of 

saltwater in the lower part, however, has been the 

subject of speculation because definitive head data are 

lacking.

Kohout (1965) and Kohout and others (1977) hypothesized inland flow of seawater through the lower part of 

the Floridan aquifer system on the basis of a temperature anomaly (reversed geothermal gradient) in several 

deep wells in southern Florida. The driving force, 

Kohout concluded, was geothermal heat which produced 

a convection flow cell wherein cold seawater was heated 

as it flowed inland through the lower part, called the 

Boulder Zone, then moved upward through vertical

Gl
G2 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

WEST

SEA LEVEL

CENTRAL FLORIDA

80°

SINKHOLE | -- STRAITS OF 

GULF OF MEXICO REGION POTENTIOMETRIC SURFACE FLORIDA

CONFINING 

UNIT

(BOULDER ZONE)

HEAT FLOW

TEMPERATURE PROFILE 

(APPROXIMATE BELOW 

4000 FEET)

GEOTHERMAL

60 80 100 150

NOT TO SCALE TEMPERATURE, IN DEGREES FAHRENHEIT

EAST

6,000 -

7,000 -

8l00° EXPLANATION

 44 LINE OF EQUAL SEAWATER TEMPERATURE Interval, in degrees Fahrenheit, is variable

 ^ DIRECTION OF FLOW

FIGURE 1. Idealized hydrogeologic cross section through peninsular Florida showing concept of cyclic flow of seawater 

induced by geothermal heating (proposed by Kohout, 1965). (Location of site 11 shown in fig. 2.)

paths (such as ancient sinkholes) in the overlying confining units and mixed with the seaward-flowing freshwater 

in the upper part (fig. 1). Opposing views were expressed 

by Vernon (1970), who suggested that the temperature 

anomaly was due only to heat conduction, and Sproul 

(1977), who concluded that existing data were insufficient to support either hypothesis.

A major part of the RASA investigation in southern 

Florida involved drilling a test well, G-2296, also 

referred to as the Alligator Alley test well (fig. 2, site 

10). About 44 miles (mi) west of Fort Lauderdale, the 

well was drilled to a depth of 2,811 feet (ft) during 

1980-81 to obtain hydrogeologic and hydrochemical data, 

including selected isotopic analyses, within the Floridan 

aquifer system. Data from the Alligator Alley test well 

indicated that carbon-14 might be useful in determining 

the direction and rate of ground-water movement in the 

lower saltwater part of the aquifer system information 

that would be useful in assessing the long-term effects of 

deep-well injection. Earlier studies by Osmond and others (1968) suggested that uranium isotopes also could be 

used to assess ground-water movement. Supplementary

data were obtained from deep wells that were drilled by 

local utilities at Fort Lauderdale, Miami, and Stuart for 

injection of treated wastewater into the Boulder Zone a 

cavernous, highly transmissive dolostone that underlies 

southeastern Florida at a depth of about 3,000 ft.

Injection of liquid wastes into the saline water of the 

Boulder Zone as a pollution-control measure was started 

in 1943 with the injection of brine at an oil field a few 

miles west of Naples. Subsequently, the practice 

expanded rapidly, and numerous high-capacity, municipally operated, wastewater injection wells are now in use 

along the southeastern coast of Florida (Vernon, 1970; 

Vecchioli and others, 1979; Meyer, 1984). Determination 

of the direction of ground-water movement in the lower 

part of the Floridan aquifer system was, therefore, a 

very necessary and important part of the RASA project 

in southern Florida.

This report is one of nine chapters in U.S. Geological 

Survey Professional Paper 1403 that describe various 

aspects of the geology, hydrology, and geochemistry of 

the Floridan aquifer system. Those chapters most 

related to this report (chapter G) include the summary
INTRODUCTION G3

87

28

A A UNE OF HYDROGEOLOGIC 

 SECTION SHOWN 

IN FIGURE 3

B B UNE OF SEISMIC REFLECTION 

 PROFILE SHOWN IN

 15 WELL AND SITE NUMBER 

IN REPORT

 6600 SUBMARINE TOPOGRAPHY 

(UCHUPI, 1966)-Shows 

depth below sea level. 

Contour interval, in feet, 

Is variable

FLORIDA AND STUDY AREA

FIGURE 2. Selected wells, hydrogeologic sections, and submarine topography, south Floridan Plateau.

(chapter A) by Johnston and Bush (1988) and chapters on 

the hydrogeologic framework (chapter B) by Miller 

(1986), regional ground-water hydraulics (chapter C) by 

Bush and Johnston (in press), and ground-water 

geochemistry (chapter I) by Sprinkle (in press).

PURPOSE AND SCOPE

The purpose of this report is to describe the hydrogeology and, most importantly, the flow in the Floridan 

aquifer system of southern Florida. To accomplish this, 

geologic, isotopic, and hydraulic data that would indicate 

or suggest the rate and direction of the ground-water 

movement were collected during 1980-83 by the U.S.

Geological Survey and others. Included are data on 

hydraulic head, water temperature, dissolved solids 

(salinity), and selected isotopes. The natural isotopes 

used for the study are chiefly carbon-13, carbon-14, 

uranium-234, and uranium-238. Data were primarily 

collected from 14 widely distributed deep wells in southern Florida, of which the Alligator Alley test well (fig. 2, 

site 10) was the most important. Temperature data are 

presented for 16 wells, isotopic data for 8 wells, and 

hydraulic data for 2 wells. Also included are selected 

uranium isotope analyses from 46 wells and 10 springs in 

central and southern Florida that were compiled from 

existing reports and files. The ground water in the lower 

part of the Floridan aquifer system in southern Florida is
G4 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

too saline for most uses; therefore, data that concern the 

use of the aquifer system for storage of liquid wastes and 

for seasonal storage of surplus freshwater are presented.

PREVIOUS INVESTIGATIONS

Regional aspects of the hydrogeology of the Floridan 

aquifer system (also referred to as the "Vicksburg artesian limestones," the "principal artesian formation," the 

"principal artesian aquifer," the "Floridan aquifer," and 

the "Tertiary limestone aquifer system") in southern 

Florida were reported by Sellards and Gunter (1913), 

Stringfield (1936, 1953, 1966), Parker and others (1955), 

Kohout (1965, 1967), Vernon (1970, 1973), and Miller 

(1982a, 1982b, 1986). Hydrogeologic studies of a local 

nature were reported by Schroeder and others (1954), 

Bishop (1956), Lichtler (1960), McCoy (1962), Klein and 

others (1964), Meyer (1971, 1974), Sproul and others 

(1972), Boggess (1974), Sutcliffe (1975, 1979), Grain and 

others (1975), Wilson (1977), Reece and others (1980), 

Smith and others (1982), and Wedderburn and others 

(1982). Potentiometric surface maps, which indicate the 

direction of ground-water movement in the upper part of 

the aquifer system, were compiled by Stringfield (1936), 

Parker and others (1955), Healy (1975a, 1975b, 1982), 

and Johnston and others (1980,1981). The various uses of 

the Floridan aquifer system in southern Florida are 

presented by Garcia-Bengochea (1970), GarciaBengochea and Vernon (1970), Vernon (1970), Vecchioli 

and others (1979), Merritt and others (1983), and Merritt 

(1985).

The use of natural isotopes as tracers in studies of 

ground-water movement in the Floridan aquifer system 

began in the early 1960's with estimates of ground-water 

velocities in the upper part of the aquifer system in 

central Florida by Hanshaw and others (1965), who used 

carbon-14. Results of their study indicated that although 

absolute ages of the ground-water samples may be 

unreliable, the relative ages of the samples are reasonably reliable for estimating rates and direction of groundwater movement. In a later study of the same area, 

Plummer (1977) examined transient changes in water 

chemistry in relation to the earlier carbon-14 ages, and 

new ages (slightly younger) were assigned to the Hanshaw samples on the basis of mass-transfer reaction 

coefficients.

Pearson and Bodden (1975, p. 143) reported the first 

results of carbon-14 analyses of ground water from the 

Floridan aquifer system in southeastern Florida. The 

samples were obtained from the upper and middle parts 

of the aquifer system, and the uncorrected activities of 

5.6 and 4.3 percent of modern carbon (PMC) were 

anomalous compared with Hanshaw's results in central 

Florida (that is, the samples seemed to become younger

with increasing distance from the recharge areas in 

central Florida). Hanshaw and Back (1971), in studies of 

the origin of dolomite in the Floridan aquifer system, 

used stable isotope ratios of carbon-13/carbon-12 (8I3C) 

and oxygen-18/oxygen-16 (5I80) to distinguish between 

marine (primary) dolomite and nonmarine (stoichiometric) dolomite. Tamers and others (1975) related carbon14 ages to ground-water movement in the surficial aquifer of southeastern Florida.

The use of naturally occurring uranium isotopes as 

tracers of ground-water movement in the Floridan aquifer system also began in the early 1960's (Osmond and 

others, 1968). The alpha-activity ratio (AR) of uranium234/uranium-238 and uranium concentrations were used 

to infer circulation patterns (Osmond and others, 1968). 

Cowart and others (1978) compared the alpha-activity 

ratios and uranium concentrations in three samples of 

saltwater from the lower part (Boulder Zone) of the 

Floridan aquifer system in southeastern Florida with 

ratios and concentrations in modern seawater. They 

concluded that the saltwater originated in the Straits of 

Florida, thereby supporting Kohout's hypothesis. Cowart and others (1978) also concluded that low alphaactivity ratios (less than unity) in samples of brackish 

water from the upper part of the system in the Florida 

Keys may indicate the presence of highly transmissive 

zones containing relic waters that are residual of rapid 

circulation from the recharge area when sea level was 

much lower.

ACKNOWLEDGMENTS

J.B. Cowart, Geology Department, Florida State University, Tallahassee, Fla., provided all uranium isotope 

analyses and contributed many ideas on their usefulness 

as ground-water tracers. He also wrote some pages of an 

early draft that was a predecessor to this report. Those 

pages are included in this report in the section on 

isotopes. The author also thanks the following people, 

who provided technical assistance and data for the study: 

J.I. Garcia-Bengochea, C.R. Sproul, and J.D. Lehnen, 

CH2M Hill, Inc., Gainesville, Fla.; R.J. Kerrn, AlsayPippin Corporation, Lake Worth, Fla.; V.P. Amy and 

J.A. Wheatley, Geraghty and Miller, Inc., West Palm 

Beach, Fla.; P.E. Robbinson and P.A. Davis, Hazen and 

Sawyer, Inc., Hollywood, Fla.; J.R. Peralta, General 

Development Utilities, Inc., Miami, Fla.; W.R. Mehner, 

City of Fort Lauderdale, Fla.; Garrett Sloan, MiamiDade Water and Sewer Authority, Miami, Fla.; E.E. 

Mitchel, City of Stuart, Fla.; T.M. Missimer, Missimer 

and Associates, Cape Coral, Fla.; E.L. Fennell and R.T. 

Mooney, Exxon Company, New Orleans, La.; C.W. 

Hendry, C.H. Toodle, and E.G. Lane, Florida Bureau of 

Geology, Tallahassee, Fla.; N. Van Devender and R.L.
HYDROGEOLOGY OF SOUTHERN FLORIDA G5

Nicholas, Shell Exploration Company, New Orleans, 

La.; and R.C. Runvick and D. Jorgensen, U.S. Gypsum 

Company, Chicago, 111.

The author also wishes to acknowledge the cooperation 

and assistance of the members of the South Florida 

Technical Advisory Committee, Florida Department of 

Environmental Regulation (DER): R.M. Duke, J.A. 

Guidry, and J.G. Strasser, DER, West Palm Beach, 

Fla.; L.A. Bell, R.J. Duerling, and D.R. McClaugherty, 

DER, Tallahassee, Fla.; A.E. Coker, U.S. Environmental Protection Agency Region IV, Atlanta, Ga.; Abe 

Kreitman and L.A. Wedderburn, South Florida Water 

Management District, West Palm Beach, Fla.; and Bruce 

Kester, Broward County Environmental Quality Control 

Board, Fort Lauderdale, Fla.

B.H. Lidz, U.S. Geological Survey, Fisher Island 

Station, Miami Beach, Fla., identified planktonic foraminifers in selected samples of drill cuttings. Conrad 

Newman, University of North Carolina, Chapel Hill, 

N.C., provided data on Pleistocene and Holocene sealevel fluctuations. Peter Popenoe, R.P. Freeman-Lynde, 

and M.M. Ball of the U.S. Geological Survey, Woods 

Hole Oceanographic Institution, Woods Hole, Mass., 

provided seismic reflection profiles for the western 

Straits of Florida. Albert Yang, U.S. Geological Survey, 

Denver, Colo., and H.G. Ostlund, Director, Tritium 

Laboratory, Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Coral Gables, Fla., 

supervised the radiocarbon analyses and provided much 

useful discussion on carbon isotopes during the study. 

The carbon-14 analyses of water samples from the deep 

wells at Fort Lauderdale and Port St. Lucie were 

contributed by the city of Fort Lauderdale and General 

Development Utilities, Inc., Miami, Fla., respectively.

HYDROGEOLOGY OF SOUTHERN FLORIDA

Southern Florida is underlain by rocks of Cenozoic age 

to a depth of about 5,000 ft. These rocks are principally 

carbonates (limestone and dolostone), with minor 

amounts of evaporites (gypsum and anhydrite) in the 

lower part and elastics (sand and clay) in the upper part. 

The movement of ground water from inland areas to the 

ocean and vice versa occurs principally through the 

carbonate rocks.

FLORIDAN AQUIFER SYSTEM

Evaporite deposits in the Cedar Keys Formation of 

Paleocene age probably constitute the lower confining 

unit, or base of the active flow system (fig. 3). Overlying 

the evaporites, in ascending order, are limestone and 

dolostones of the Cedar Keys, Oldsmar, and Avon Park

Formations and the Ocala and Suwannee Limestones 

that make up the Floridan aquifer system, part of which 

was once called the Floridan aquifer (Parker and others, 

1955) and all of which was once called the Tertiary 

limestone aquifer system (Johnston and others, 1980). In 

southwest Florida, the lower part of the Tampa Limestone is also included in the Floridan aquifer system.

The Floridan aquifer system is defined in chapter B of 

this Professional Paper (Miller, 1986) as a vertically 

continuous sequence of permeable carbonate rocks of 

Tertiary age that are hydraulically connected in varying 

degrees, and whose permeability is generally several 

orders of magnitude greater than that of the rocks that 

bound the system above and below. In Florida, the 

Floridan aquifer system includes rocks ranging from 

Paleocene to early Miocene age, and locally in southeast 

Georgia, it includes rocks of Late Cretaceous age. Chapter B presents a detailed geologic description of the 

Floridan aquifer system, its component aquifers and 

confining units, and their relation to stratigraphic units. 

Previous definitions of the term "Floridan" and superseded terms are also discussed in chapter B (Miller, 

1986).

Overlying the Floridan are alternating beds of sand, 

clay, marl, and limestone in the Tampa Limestone and 

Hawthorn Formation (both of Miocene age) that contain 

intermediate artesian aquifers and make up the upper 

confining unit for the Floridan aquifer system. In southeastern Florida, clay in the Tamiami Formation of 

Pliocene age is included in the upper confining unit. 

Overlying these deposits are limestones and sands of the 

Tamiami Formation and of undifferentiated Pleistocene 

deposits that make up the surficial aquifer and contain 

unconfined ground water.

Ground water in the Floridan aquifer system in southern Florida is generally too saline for most uses. The 

Lower Floridan aquifer contains ground water that is 

similar in composition to seawater and is chiefly used as 

a receptacle for injected liquid wastes; the Upper Floridan aquifer contains brackish water and is chiefly used as 

a source of limited industrial amd agricultural supply and 

for feed water to desalting plants. Pilot studies indicate 

that the upper part of the Floridan aquifer system in 

southern Florida can be used for seasonal storage of 

surplus freshwater (Merritt and others, 1983). Limestone aquifers in Miocene deposits, as parts of the upper 

confining unit, are important local sources of ground 

water for supply in parts of southwestern Florida. However, the surficial aquifer generally is the major source of 

potable water in southern Florida. In southeastern Florida, the surficial aquifer is called the Biscayne aquifer 

(Parker and others, 1955; Schroeder and others, 1958), 

and in southwestern Florida, it is called the "shallow 

aquifer" (McCoy, 1962). The hydrogeology of sou/chern
G6

A

REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

LEVEL $& fe^^v^.TAI^lfo1 TORK*/^^ &Mbf&

pfSi^^^ ll[p^TS^iil|iiii!pipii^|

V. , ' <"

2,000

2,500

3,000 £

3,500 I

4,000

-3,500

4,000

10 20

10 20 30 KILOMETERS 

SCALE APPROXIMATE

30 MILES 

I

EXPLANATION

SURFICIAL AQUIFER

CONFINING UNITS

TOP OF SEAWATER-UKE SALINITY

LINE OF EQUAL GROUND-WATER 

TEMPERATURE-lnterval 10 degrees 

Fahrenheit

FIGURE 3. Generalized hydrogeologic section A-A' through southern Florida showing isotherms and top of saltwater in the Floridan aquifer system.

of section shown in fig. 2.)

(Line

Florida, as described here, is based largely on data 

collected from an exploratory test well (Alligator Alley 

test well; fig. 2, site 10) that was drilled near the center 

of the Everglades and from test wells that were drilled in 

Collier County by the South Florida Water Management 

District.

In southeastern Florida, the Floridan aquifer system 

includes (from shallowest to deepest) all or part of the 

j&iwannee Limestone of Oligocene age, the Ocala Limestoite of late Eocene age, the Avon Park Formation of 

middle Eocene age, the Oldsmar Formation of early 

Eocene age, and the upper part of the Cedar Keys 

Formation of Paleocene age (fig. 3). In southwestern

Florida, it locally includes the lower part of the Tampa 

Limestone of early Miocene age.

Some investigators place the top of the Floridan 

aquifer system in the lower part of the Hawthorn Formation of middle Miocene age wherever it contains 

permeable limestone hydraulically connected to deeper 

layers (Parker and others, 1955; Stringfield, 1966). 

Using regional criteria based largely on lithologic 

changes in the rocks, Miller (1986) placed the top of the 

Floridan aquifer system at or near the top of the Suwannee Limestone in southwestern Florida and at or near 

the base of the Suwannee Limestone in southeastern 

Florida. The top of the Floridan aquifer system, as used
HYDROGEOLOGY OF SOUTHERN FLORIDA G7

in this report, ranges from about 500 to 1,000 ft in depth. 

The base of the Floridan aquifer system (the lowest 

confining unit) generally coincides with the top of evaporite beds in the Cedar Keys Formation (Miller, 1986), and 

it ranges from about 3,500 to 4,100 ft in depth.

The rocks that make up the Floridan aquifer system 

vary greatly in permeability so that the system resembles a "layer cake" composed of many alternating zones 

of low and high permeability. Crossflow (vertical flow) 

between permeable zones probably occurs through sinkholes and fractures. However, the amount of crossflow is 

probably small compared with the amount of horizontal 

flow. The zones of highest permeability generally are at 

or near unconformities and are generally parallel to 

bedding planes.

The temperature of ground water in the Floridan 

aquifer system in areas near the southeastern coast 

generally decreases with increasing depth; however, 

anomalies frequently occur, probably owing to local 

upwelling through fractures and sinkholes (a phenomenon that is discussed later in the report). Ground-water 

temperatures are generally coolest along the southeast 

coast, where the temperature of seawater in the adjacent 

Straits of Florida is the lowest. Ground-water salinity is 

generally highest in coastal parts of southern Florida and 

in the lower part of the aquifer system owing to inland 

circulation of seawater.

In southern Florida, the Floridan aquifer system can 

generally be divided largely on the basis of the geology, 

hydrochemistry, and hydraulics interpreted from data 

obtained at the Alligator Alley test well (fig. 2, site 

10) into three hydrogeologic units, as follows:

1. The Upper Floridan aquifer, which contains brackish ground water. The specific conductance of the ground 

water ranges from about 2,500 to 25,000 microsiemens 

per centimeter (|xS/cm) at 78 °F (25 °C) and averages 

about 5,000 jxS/cm.

2. The middle confining unit, which contains salty 

ground water. The specific conductance of the ground 

water ranges from about 35,000 to 37,000 |xS/cm and 

averages about 36,000 |xS/cm.

3. The Lower Floridan aquifer, which contains ground 

water that is similar in composition to seawater. The 

specific conductance of the ground water ranges from 

about 43,000 to 50,000 jxS/cm and averages 49,000 

|xS/cm.

UPPER FLORIDAN AQUIFER

The Upper Floridan aquifer in southern Florida chiefly 

consists of permeable zones in the Tampa, Suwannee, 

and Ocala Limestones and in the upper part of the Avon 

Park Formation. On the basis of aquifer tests and a 

regional flow model, the transmissivity is estimated to

range from 10,000 to 60,000 feet squared per day (ft2/d) 

(Bush and Johnston, in press). The contained ground 

water is brackish. The salinity of the ground water 

generally increases with increasing depth and with distance downgradient and southward from central Florida. 

Ground-water temperatures also generally increase 

downgradient and southward from the recharge area in 

central Florida. However, temperatures along the southeastern coast are lowest (about 70.0 °F) owing to heat 

transfer to the Atlantic Ocean (Straits of Florida) 

(Sproul, 1977, p. 75) and (or) to heat transfer to cooler 

saltwater in the Lower Floridan aquifer (Kohout, 1965). 

Temperature and salinity anomalies that are related to 

upwelling ground water from the Lower Floridan aquifer 

are discussed later in this report.

Water movement is chiefly lateral through highly 

permeable zones of dissolution at or near the top of each 

formation. Ground-water movement in May 1980 was 

generally southward from the area of highest head near 

Polk City in central Florida to the Gulf of Mexico and to 

the Atlantic Ocean (fig. 4). The area of highest freshwater head is herein referred to as the "Polk City high." 

Prior to development (late 1800's or early 1900's), the 

head in south Florida probably was 5 to 10 ft higher than 

at present. As water use increased and wells were drilled 

in the area north of Lake Okeechobee, water levels were 

lowered and a saddle formed in the potentiometric surface, as shown by the close spacing of the 40- to 70-ft 

contours toward the center of the peninsula. Hydraulic 

gradients in southern Florida were reduced, resulting in 

a decrease of natural discharge by submarine springs 

along the southeastern coast and the movement of seawater inland to a new position of equilibrium.

The concave shape of the contours on the 1980 potentiometric surface map along the southeastern coast indicates convergence of flow toward the submerged karst 

on the Miami Terrace between Fort Lauderdale and 

Miami. Ground-water discharge in this area is also 

suggested by computer flow modeling as described by 

Bush and Johnston (in press). The rugged topography of 

the submarine terrace was formed by the collapse of 

solution features (sinkholes) in the underlying limestone. 

A seismic reflection profile (fig. 5) across the Miami 

Terrace shows the pinnacles and troughs associated with 

the submerged karst and the northward-prograding sediments of Miocene through Pleistocene age unconformably overlying the Suwannee Limestone. Currents and 

perhaps upwelling freshwater from submarine springs 

are probably responsible for the lack of sediment on the 

terrace and terrace slope. Malloy and Hurley (1970) 

reported that rock samples from dredge hauls on the 

Miami Terrace by the University of Miami's Institute of 

Marine Science (now the Rosenstiel School of Marine and 

Atmospheric Sciences) indicated that the ocean floor is
G8 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

83°

SARASOTA

EXPLANATION

AREA WHERE WATER GENERALLY MEETS U.S. 

PUBLIC HEALTH SERVICE DRINKING WATER 

STANDARDS

POTENTIOMETRIC CONTOUR-Shows altitude at 

which water level of Upper Floridan aquifer would 

have stood in tightly cased wells (May 1980). 

Dashed where approximately located. Contour 

interval 10 feet. Datum is sea level

FLOW LINE

DEPRESSION CONTOUR REPRESENTS PUMPING 

WELLFIELD

NAPLES

50 MILES

50 KILOMETERS

FIGURE 4. Potentiometric surface of peninsular Florida in May 1980 and the area of potable ground water, Upper Floridan aquifer

(revised from Johnston and others, 1981, and Healy, 1982).
HYDROGEOLOGY OF SOUTHERN FLORIDA G9

B

B'

250

CO 

Q

Z

o

o

LLJ 

CO

LLJ

1

I- 

_l 

LJJ

DC

PROGRADING

SEDIMENTSgf

 -SCALE APPROXIMATE 

0 5 MILES

5 KILOMETERS

AMI TERRACE

LAG DEPOSIT

SUBMERGED KARST S-STOP OF LIMESTONEJii ,V- "rvV***«-^«.*?«a _1»isi««^ »»^_'i^ta.-v.i%_-*i«i._'« v^titr**^^ _f*f^«~~--.r/»&.r*yK!f?

1430 1500 1530 1600 1630 1700

TIME OF DAY

1730 1800 1830

FIGURE 5. Seismic reflection profile B-B' in the western Straits of Florida showing submarine karst on the Miami Terrace. (Line of

section shown in fig. 2.)

composed of fossiliferous phosphatic limestone that contains numerous foraminifers, chiefly Miogypsina sp., 

Globigerina sp., miliolids, and rotalids. Rock samples 

from equivalent depths in artesian wells at Miami and 

Fort Lauderdale contain the same fossil assemblages and 

are identified as the Suwannee Limestone of Oligocene 

age.

Sinkholes on the Miami Terrace are both filled and 

unfilled. Those that are unfilled probably are active 

submarine springs. The large filled sinkhole in the center 

of the seismic profile (fig. 5) is about 2 mi wide and may 

be related to the collapse in the highly cavernous dolostone (the Boulder Zone) in the lower part of the Oldsmar 

Formation. The Boulder Zone is discussed by Miller 

(1986, p. B65-B66). Sinkholes generally are present 

throughout Florida and are prominent in central Florida 

as chains of sinkhole lakes. The sinkholes are chiefly in 

Tertiary limestones along joints or fractures that trend 

generally northwestward to southeastward (and to a 

lesser extent southwestward to northeastward). Sinkholes in southern Florida are virtually obscured because 

they are filled by deposits that are Miocene or younger in

age. Their presence, however, is often indicated by 

drilling and by local salinity or temperature anomalies.

MIDDLE CONFINING UNIT

The middle confining unit of the Floridan aquifer 

system consists chiefly of the lower part of the Avon 

Park Formation but locally includes the upper part of the 

Oldsmar Formation. The unit has relatively low permeability, and it generally separates the Upper Floridan 

aquifer, containing brackish ground water, from the 

Lower Floridan aquifer, containing ground water that 

compares closely to seawater. Hydraulic connection 

between the upper and lower aquifer is inferred from 

sinkholes and fractures that transect the middle confining unit. Ground-water movement in southern Florida is 

estimated to be chiefly upward from the Lower Floridan 

aquifer through the middle confining unit, then horizontally toward the ocean through the Upper Floridan 

aquifer. Salinity varies greatly at the top of the middle 

confining unit as the upward-moving saltwater is blended 

with the seaward-flowing freshwater in the Upper Floridan aquifer. As previously stated, temperature and
G10 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

salinity anomalies in the Upper Floridan aquifer are 

evidence of upwelling saltwater from the lower part of 

the aquifer system.

LOWER FLORIDAN AQUIFER

The Lower Floridan aquifer consists chiefly of the 

Oldsmar Formation and, to a lesser degree, the upper 

part of the Cedar Keys Formation. Ground water in the 

Lower Floridan aquifer compares chemically to modern 

seawater. In the Lower Floridan aquifer are three 

permeable dolostones of the Oldsmar Formation that are 

separated by less permeable limestones. The transmissivity of the lower dolostone (locally called the Boulder 

Zone; Miller, 1986, p. B65-B66) ranges from about 

3.2xlO6 ft2/d (Meyer, 1974) to 24.6xlO6 ft2/d (Singh and 

others, 1983), whereas that for the overlying dolostones 

is probably an order of magnitude less. In southeastern

DEGREES FAHRENHEIT 

50 60 70 80 90

HYDROGEOLOGIC UNITS

LAND SURFACE 

80O1,200-

1,600-

2,000-

2,400-

2,800-

3,200-

3,600 -

4,000-

BISCAYNE AQUFER

UPPER CONFINING UNIT

CONTAINING MINOR ARTESIAN

AQUIFERS OF MIOCENE AGE

P

CO

>

CO

a:

LU

a.

13 

O 

<

z

o

DC 

O

U.

UPPER

FLORIDAN

AQUFER

MIDDLE

CONFINING

UNIT

QC 

LU

U. ~)

O

7"

<

a

rrO

oc

LU

0

_l

UPPER DOLOSTONE

CONFINING UNIT

INTERMEDIATE

DOLOSTONE

CONFINING UNIT

LOWER DOLOSTONE

BOULDER ZONE

CONFINING UNIT

BASE OF FLORIDAN

FIGURE 6. Fluid temperature and hydrogeologic units in well 

G-2334 at site 9, Fort Lauderdale (see fig. 2).

Florida, hydraulic connection between the lower and 

intermediate dolostones is inferred from pumping tests 

and from the presence of sinkholes and fractures; however, hydraulic connection between the intermediate and 

upper dolostones apparently is poor, and locally the 

upper dolostone may be more closely related to the 

middle confining unit than to the Lower Floridan aquifer. 

In southwestern Florida, drilling data suggest that the 

dolostones are hydraulically connected, although head 

data and aquifer tests to confirm this interpretation are 

lacking.

A pronounced temperature anomaly is present in the 

Lower Floridan aquifer, with the lowest measured temperature (50.5 °F) in a deep disposal well (G-2334) at 

Fort Lauderdale (fig. 6). Temperatures increase generally from the Straits of Florida inland toward the center 

of the Floridan Plateau (table 1, fig. 7), and, as previously mentioned, Kohout (1965) hypothesized circulation 

of cold seawater inland from the Straits of Florida 

through the lower part of the Floridan aquifer system 

driven by geothermal heat flow (fig. 1).

Attempts to calculate hydraulic gradients in the Lower 

Floridan aquifer to verify the direction of ground-water 

movement have, thus far, been unsuccessful owing to a 

lack of reliable head data and to transitory effects of tides 

(ocean, Earth, and atmospheric). However, recent measurements of head and carbon-14 activity, which are 

discussed in subsequent sections, in the waters of the 

Boulder Zone at site 9 (fig. 2) in well G-2334 and at site 

10 (fig. 2) in well G-2296 substantiate the Kohout hypothesis.

HYDROGEOLOGY AT THE ALLIGATOR ALLEY 

TEST WELL SITE

A 2,811-ft-deep test well (Well G-2296) was drilled in 

1980 during this RASA study in the Everglades of 

southern Florida along Alligator Alley (Interstate 75) at 

a point between Naples and Fort Lauderdale (fig. 2, site 

10). A steel casing 16-inches (in) in diameter was 

installed with cement grout from land surface to a depth 

of 895 ft, below which a nominal 8-in-diameter hole was 

drilled to a depth of 2,811 ft (fig. 8). A 2-in-diameter steel 

monitor tube with perforations from 811 to 816 ft was 

grouted with cement in the outer annulus. Hydraulic 

packers were used to isolate selected zones in the well to 

collect samples of ground water and measure water 

levels.

The well penetrated the surficial and intermediate 

aquifer systems and extended into the Floridan aquifer 

system (fig. 9). The surficial aquifer system is about 180 

ft thick and is composed chiefly of sandy limestone of the 

Tamiami Formation of Pliocene age. Three artesian 

limestone aquifers and related confining beds are present
HYDROGEOLOGY OF SOUTHERN FLORIDA Gil

in the intermediate aquifer system between 180 and 770 

ft in Miocene deposits. The top of the Floridan aquifer 

system is at 770 ft. The Floridan is confined by the 

overlying Miocene deposits. About 67 percent of the total 

thickness of the Floridan aquifer system was penetrated 

by the well. The formations that make up the Floridan 

aquifer system (from shallowest to deepest) at the well 

site include the Suwannee and Ocala Limestones and the 

Avon Park and Oldsmar Formations. The Floridan aquifer system is composed of several water-bearing zones 

and associated confining units. The well did not penetrate the Cedar Keys Formation, which contains the 

lower confining unit of the aquifer system.

SURFICIAL AQUIFER SYSTEM

The surficial aquifer system is about 180 ft thick; the 

upper 60 ft is composed of unconsolidated shelly, quartz 

sand of Pleistocene age, and the lower 120 ft is composed 

of sandy, shelly limestone of the Tamiami Formation of 

Pliocene age. Phosphate containing uranium, which 

emits high rates of natural gamma rays, marks the top 

and bottom of the Tamiami Formation (fig. 8). The sand 

is chiefly fine grained and yields only small quantities of 

water with high organic content to wells. The sand 

partially confines ground water in the underlying limestone. The limestone in the Tamiami Formation is relatively permeable (fig. 9, water-bearing zone 1) and is 

capable of yielding large quantities of potable freshwater. In 1982, a sample of ground water from a zone 

between 50 and 150 ft in depth in a shallow test well 

(G-2329) near the Alligator Alley test well had a chloride 

concentration of 120 milligrams per liter (mg/L) and a 

specific conductance of about 1,000 fxS/cm (J.E. Fish, 

U.S. Geological Survey, oral commun., 1983).

INTERMEDIATE AQUIFER SYSTEM

The intermediate aquifer system is about 590 ft thick 

and is composed of three confined limestones (fig. 9, 

water-bearing zones 1 through 3) of Miocene age. Waterbearing zone 1, the upper intermediate aquifer, ranges in 

depth from about 220 to 360 ft and is composed of thinly 

bedded, gray, shelly limestone and interbedded sand and 

clay in the upper part of the Hawthorn Formation of 

middle Miocene age. The hydraulic and water-quality 

characteristics were not determined from the test well, 

but the rock type suggests that small quantities of 

brackish water (less than 1,000 gallons per minute; 

gal/min) can be obtained by wells that tap the entire 

thickness. Water-quality data are not locally available, 

but potable water is known to be present in equivalent 

rocks in parts of Charlotte and Lee Counties on the Gulf 

Coast, west of Lake Okeechobee. Zone 1 is confined 

above by a thick and extensive bed of silty, green clay of

TABLE I. Temperatures of salty ground water in selected wells that 

tap the Boulder Zone of the Lower Floridan aquifer 

[WWTP, wastewater-treatment plant. Site locations shown in fig. 2]

Site 

No.

1 

2 

3 

4

5

6

7

8

9

10

11

12

13

14

15

16

Site

No.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Temperature 

County Owner °F

Charlotte Humble-Lowndes-Treadwell No. 1 96.0 

Charlotte Gulf-Stevens No. 1 100.0 

Palm Beach Quaker Oats injection well 4 79.3 

Indian River Hercules injection well 1 89.6

St. Lucie South Port WWTP injection well 1 72.1

Martin Stuart WWTP injection well 1 70.6

Palm Beach West Palm Beach WWTP injection well 2 60.8

Broward

Broward

Broward

Dade

Dade

Dade

Dade

Lee

Collier

Depth

(feet)

1.641

3.24S

2,794

2,735

3,180

3,290

3.250

3.070

2,920

2.811

3.100

3.000

2.944

2,975

2,800

3,000

Margate monitor well 59.0

Fort Lauderdale Port Everglades WWTP 50.5

injection well 1.

Alligator Alley test well 76.1

Gulf -State Lease 340 No. 1 74.0

Kendale Lakes WWTP injection well 1 61.3

Sunset Park WWTP injection well 1 60.5

Miami-Dade Water and Sewer Authority 60.6

South District WWTP injection well 1.

California- Coastal 224B-1 108.8

Sun- Collier No. 1 97.0

Remarks

Slightly above Boulder Zone. Local well CH-57. Kohout and

others (1977, p. 21).

Temperature log appears to be on cool side. Kohout 4111! others

(1977. p. 18).

Temperature log shows 79.3*F fron 2,794 to 3,208 feet. Local

well PB-1142.

Packer test 2,735 to 3,015 feet. Pumped sample. Source, CH..M

Hill. Inc. i

Temperature log shows 72.0'F from 3,180 to 3,424 feet.

Source, CH.H Hill, Inc. Local well STL-254.

Temperature log shows coolest from 3,140 to 3,290 feet.

Source, CH.H Hill, Inc. Local well H-1034.

Pumped sample. Open hole 3,025 to 3,680 feet. Local well

C-2292.

Pumped sample. Open hole 2,457 to 3,301 feet. Local well

C-2292.

Temperature log. Cold seawater 2,920 to 3,430 feet. Local

well C-2332. Source, Geraghty and Killer, Inc.

Temperature log. Bottoa of hole at 2,811 feet. Local well

C-2296.

Temperature log. Kohout and others (1977, p. 20). Local well

C-3236.

Tenperatura log. Source, Florida Bureau of Geology. Zone

3,000 to 3,160 feet. Local well 1-2.

Temperature log. Source, Florida Bureau of Geology. Local

well 1-1.

Temperature log. Source, CH.H Hill, Inc. Zone 2,975 to

3,130 faet. Local well HDSUI-1.

Temperature log. Kohout and others (1977, p. 20).

Temperature log. Kohout and others (1977, p. 20). Local well

C-41S.

late Miocene age and below by a thick and extensive bed 

of green, micaceous clay of middle Miocene age.

Water-bearing zone 2 ranges in depth from about 460 

to 530 ft at the test well site and is composed chiefly of 

sandy, shelly limestone in the lower part of the Hawthorn Formation of middle Miocene age. The hydraulic 

and water-quality characteristics of zone 2 were not 

determined from the test well, but the rock type is 

comparable to an aquifer in north-central Collier County 

(McCoy, 1962, p. 18) that in 1959 produced artesian 

water having a chloride concentration of 985 mg/L and a
G12 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

87° 85° 84° 83° 82° 81' 80° 79"

24

23°

LINE OF EQUAL WATER

TEMPERATURE-Dashed where 

approximately located. Interval 10 

degrees Fahrenheit

WELL AND SITE NUMBER-Upper 

number is temperature In degrees 

Fahrenheit. Lower number Is approximate 

depth of measurement, in feet. <, less than; 

>, greater than

ESTIMATED INLAND LIMIT OF THE

BOULDER ZONE-Queried where uncertain

 660 SUBMARINE TOPOGRAPHY (Uchupi, 1966)-Shows 

depth below sea level. Contour Interval, in feet, 

is variable

50 KILOMETERS

FLORIDA AND STUDY AREA

FIGURE 7. Temperature of saltwater in the Boulder Zone of the Lower Floridan aquifer, south Floridan Plateau.

head about 31 ft above sea level. Zone 2 is confined above 

by the previously described clay and below by a relatively thin (about 10- to 20-ft-thick) bed of calcareous 

clay. The lower confining bed probably is not areally 

extensive and offers only local confinement.

Water-bearing zone 3 ranges in depth from about 540 

to 600 ft and is composed of slightly sandy, shelly 

limestone of the Tampa Limestone of early Miocene age. 

The hydraulic and water-quality characteristics of zone 3 

were not determined from the test well, but the rock 

type suggests that they are similar to those of the 

overlying aquifer (zone 2). Zone 3 is confined above by a 

thin, calcareous clay at the base of zone 2 and below by 

calcareous clay of early Miocene age. The lower bed of 

clay is the principal confining unit above the Floridan

aquifer system and is characterized on the natural 

gamma-ray log by high rates of gamma-ray emissions 

from uraniferous phosphate.

FLORIDAN AQUIFER SYSTEM

The Alligator Alley test well penetrated about 67 

percent of the estimated thickness of the Floridan aquifer system in southern Florida. The top of the Floridan 

aquifer system in this test well is considered to coincide 

with the top of the Suwannee Limestone of Oligocene age 

at 770 ft (fig. 9), on the basis of hydraulic head and water 

chemistry data. Miller (1986), in describing the regional 

hydrogeologic framework of the Floridan, placed the top 

of the Floridan at about 950 ft at this test well on the
WELL CONSTRUCTION 

FEATURES

INCHES DIAMETER. IN INCHES COUNTS PER SECOND

20 16 12 8 4 0 A 8 12 16 20 2 6 10 14 18 0 100 200 300 400 500 600 0

I I I I I I I I I I I I I I I I I I I I |_____|_____|_____|_____|_____1_____| I

RESISTIVITY

OHM-METERS 

10 20 30 40 50 60 70

STRATIGRAPHY

AGE UNIT

LAND 

SURFACE'

m

PERFORATIONS

81V-816' 

ON 3/26/81

SINGLE 

PACKER' 

TEST #4

DUAL 

PACKER { 

TEST #2A

2,000 

2,800 

CEMENT 

800 SX 

CLASS B

WITH 

2% GEL

SINGLE 

- PACKER

TEST #3

~~ NOMINAL

8'HOLE

(SEE CALJPER

FOR ACTUAL

DIAMETER)

SINGLE

^ PACKER

TEST #1

BEGAN 9-21-80 

COMPLETED 11-8-80

I i I 1

COMPOSITE LOGS.

0'-200' 9-22-80

200'-895' 10-3-80

89S'-2811' 3-25-81

COMPOSITE LOGS:

0'-200' 9-22-80 

200'-895' 10-3-80 

895'-2811' 11-13-80

COMPOSITE LOGS:

0'-200' 9-22-80 

200'-895' 10-3-80 

895'-2811' 11-13-80 

_|____I____l

PLEISTOCENE

PLIOCENE

LAND 

SURFACE

 1,200

 1,400

 2,800

EXPLANAT

KH>^H CLAY

DOLO

LIMES

r~^~ FOSS

-~-l (M

LIMES

STRE

C CAVIT

M MICAC

0 ORGA

P PHOS

V VUGG

FIGURE 8. Construction features, selected geophysical logs, and stratigraphy for the Alligator Alley test well (well G-2296 at site 10, fi
G14 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

HYDROSTRATIGRAPHY

UTHOLAGE UNIT OGY

WATERBEARING 

ZONES

DISTRIBUTION 

OF FLOW

SELECTED GEOPHYSICAL LOGS

 _-=r M

SURFACE 

200- 

400 

600- 

800 

1.000 

1,200 

B 1,400-

z

I

fij 1,600 

1,800 

2,000 

2,200 

2.400 

2,600- 

2,600 

annn

PLEISTOCENE

PLIOCENE

OCENE 

MIDDLE

5 -

A Ml

o z 

go

EOCENE

FT THOMPSON FM

TAMIAMI 

FORMATION

! HAWTHORN TAMPA FORMATION 

LIMESTONE .

j LOWER UPP| 

SUWANNEE LIMESTONE

OCALALS.

AVON PARK FORMATION

OLDSMAR FORMATION

~.'. 

J^T

H

-l- i 

TTTTTT

1 "I x|

fifif

WKf

4r4f

 i i

±Hf± 

} /'"/'

i-i i-

-4VL

wv

1 1

wv

TTI I I I I I I

PERCENT OF FLOW

STATIC FLOWMETER. 

COUNTS PER SECOND

BOTTOM OF 16-INCH 

\ CASING

STATIC 

FLOWMETER

\» 

-58%,

"CALCULATED

'ERCEN

FLOW

ESTIMATED 

AVERAGE 

PERCENT 

32% j> FLOW

SOFT CLAY AND 

CUTTINGS FILLING N 

HOLE ZTOS'^BH 1

MEASURED 

1-14-81

(Rl) FLUID RESISTANCE, OHMS 

40 30 |25

APPROXIMATE CONDUCTANCE, MICROSIEMENS X1000

DECREASING SALINITY 

INCREASING TEMPERATURE

DEGREES FAHRENHEIT

- 400 CLAY

(K) CAUPER, INCHES

LOGGED 7-16-83

WELL FLOWING

ABOUT 1000 GALLONS PER MINUTE

LAND 

SURFACE

EXPLANATION

DOLOMITIC 

LIMESTONE

"V-T FOSSILIFEROUS 

- ~^l (MARINE)

***** CRYSTALLINE

 1,600 I 11 a| LIMESTONE 

I ~°- I STREAKS

MARL

CAVITIES

MICACEOUS

ORGANIC

PHOSPHATE

VUGGY

FIGURE 9.-Hydrostratigraphy, distribution of flow, and selected geophysical logs for the Alligator Alley test well (well G-2296 at site 10, fig. 2).

basis of apparent porosity changes within the Suwannee 

Limestone. This discrepancy of placing the top of the 

Floridan aquifer system at two different depths at this 

test well site is due to the difference in the criteria 

defined for the regional framework and for the area 

studies; more refinements are needed for area studies 

than for a regional study. The test well was terminated 

at 2,811 ft in the Oldsmar Formation of early Eocene age. 

According to Miller (1986), the base of the Floridan 

aquifer system (or top of the lower confining unit) is at 

about 3,800 ft in depth in the Cedar Keys Formation of

Paleocene age (fig. 3). The formations that compose the 

system in the test well are (from shallowest to deepest) 

the Suwannee Limestone of late Oligocene age and the 

Ocala Limestone, Avon Park Formation, and Oldsmar 

Formation of Eocene age.

Discrete water-bearing zones in the Floridan aquifer 

system are recognized in the test well by changes in 

permeability, pressure, water quality, and temperature. 

The zones are chiefly related to dissolution of the limestone, and they are generally located at or near unconformities. Permeability contrasts within the aquifer sys-
HYDROGEOLOGY OF SOUTHERN FLORIDA G15

tern suggest that locally, and perhaps regionally, some of 

the major water-bearing zones act as distinct aquifers 

(fig. 9, zones 3 and 13).

Although the 16-in casing penetrated most of the 

Suwannee Limestone, a 2-in monitor tube with perforations from 811 to 816 ft provided data from the part of the 

aquifer that was cased off. The test well flows at about 

1,000 gal/min from the interval between 895 and 2,811 ft 

and produces a blend of saline water (25,500 |xS/cm) that 

compares to a 50-percent mixture of freshwater with 

seawater. Most of the water is produced from two major 

water-bearing zones. A reverse geothermal gradient is 

indicated, and the coolest water temperature (76.1 °F) is 

at the bottom of the well (fig. 9).

Fluid resistance and temperature logs show the cumulative effects of inflow from the water-bearing zones in 

the borehole (fig. 9). Superimposed on the fluid resistance logs is a scale showing the approximate conductance. The fluid resistance and temperature logs of July 

16, 1983, were obtained after the well had flowed sufficiently for the water chemistry to stabilize. Both logs 

indicate several water-bearing zones, but, as previously 

mentioned, two are the most significant a zone at about 

1,030 ft which contributes a significant amount of warm 

(79.2 °F) brackish water and a zone at about 2,560 ft 

which contributes cooler (77.1 °F) saltwater whose specific conductance is comparable to that of modern seawater. According to the fluid resistance log, the cumulative 

conductance of all water-bearing zones was about 26,000 

|xS/cm, which is similar to the average specific conductance of 14 samples.

All temperature logs that were collected during the 

drilling and testing showed a reverse geothermal gradient that is related to the cooler saltwater in an underlying water-bearing zone called the Boulder Zone. The 

coldest temperature is at the bottom of the well, and the 

flow becomes progressively warmer uphole by contributions of warm water from shallower water-bearing 

zones. The cumulative effect of inflowing ground water 

uphole produces a blend that has a temperature of about 

78.8 °F.

The temperature and fluid resistance logs (fig. 9) show 

that between 2,560 and 2,811 ft the temperature 

decreased from about 78.3 °F to 76.7 °F and that, 

concomitantly, there was an increase in resistivity. The 

temperature decrease is probably related to a very slight 

upward flow of cool saltwater from the lowermost waterbearing zone (the Boulder Zone) that probably occurs at 

about 2,900 ft (Meyer, 1974, 1984) or to heat loss to the 

cooler underlying zone.

Water-bearing zones of the Floridan aquifer system in 

the test well were identified primarily from flowmeter, 

fluid resistance, and fluid temperature logs. The percentage of the total flow (the discharge measured at land

surface) was calculated at about 50-ft intervals from 

point velocities on the flowmeter log and from the hole 

diameter as derived from the caliper log. The flowmetercaliper calculations were supplemented by calculations 

based on the contributions (blending) of inflowing water 

from the water-bearing zones. The fluid resistance log of 

July 16, 1983 (fig. 9) and miscellaneous specific conductance measurements of water samples obtained during 

the packer tests and by a thief sampler were used to 

identify and evaluate the quantity and quality of water 

from each zone. Acoustic televiewer photos and borehole 

television surveys were also used to identify the sources. 

The water-bearing zones in the Floridan aquifer system 

as indicated in the borehole are numerous, but 14 were 

identified and evaluated (table 2).

Zones 3 and 13 contributed 66 percent of the total 

borehole flow. Zone 3 was the principal contributor (34 

percent) of brackish ground water (specific conductance 

of about 3,300 |xS/cm) from the Upper Floridan aquifer, 

and zone 13 was the principal contributor (32 percent) of 

salty ground water (specific conductance of 50,000 

|xS/cm) from the Lower Floridan aquifer. The remaining 

34 percent of the total flow was contributed by many less 

permeable zones within the remaining 12 zones. Zones 2 

through 9, the interval from 920 to 1,645 ft, collectively 

contributed about 50 percent of the flow, with composite 

specific conductance at about 5,000 |xS/cm. Zones 10 

through 14, the interval from 1,645 to 2,811 ft, collectively contributed the other 50 percent of the flow, with 

composite specific conductance of 45,500 |xS/cm. Zones 

that contributed little or no water to the well (that is, 

those that contributed 1 percent or less) probably constitute the confining units within the individual aquifer 

systems. Zones 1 through 9, which collectively contributed about 50 percent of the flow, are identified as the 

Upper Floridan aquifer. Zones 10 and 11, which contributed about 13 percent of the flow, are identified as the 

middle confining unit of the Floridan aquifer system. 

Zones 12 through 14, which contributed about 37 percent 

of the total flow, are identified as the Lower Floridan 

aquifer.

Pressure gradients for 11 water-level measurements 

(table 3) were calculated from estimated densities and 

depths. Measurements that have similar densities and 

pressure gradients (for example, measurements 4, 6, 8, 

and 10, table 3) are generally representative of a common 

pressure (flow) system, and measurements that have 

dissimilar densities and pressure gradients (for example, 

measurements 3 and 10) are generally from different 

pressure (flow) systems. The fact that static conditions 

were reached only for measurements 3, 10, and 11 raises 

some doubt about the calculations of total static pressure 

for the other measurements.
G16 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

TABLE 2. Estimated distribution of flow and fluid conductance for 

the Floridan aquifer system at the Alligator Alley test well

[Conductance in (o-S/cm (microsiemens per centimeter). Well located at 

site 10, fig. 2]

TABLE 3. Measurements of head and pressure in the Floridan 

aquifer system at the Alligator Alley test well

[Pressure gradient in pounds per square inch per foot; pressure at 

depth in pounds per square inch. Well located at site 10, fig. 2]

PerZone Depth cent

(feet) of

flow

Cumulative

percent

of

flow 1

Estimated

average

conduc -

tance

Estimated

conductance

load

(percent

of flow

times

MS/cm)

Remarks

Upper Floridan aquifer

1 770-840 0

2 920-940 1

3 1,020-1,034 34

4 1,110-1,154 6

5 1,180-1,192 1

6 1,248-1,256 2

7 1,280-1,310 1

8 1,350-1,370 1

9 1,430-1,645 4

100

99

65

59

58

56

55

54

6,200

4,600

3,300

3,300

2,500

2,500

3,300

3,300

25,000

0

46

1,122

198

25

50

33

33

1,000

Zone cased off. Sampled

from perforated monitor

tube at 811 to 816 feet.

Minor inflow from numerous

cavities.

Major inflow from large

cavities at 1,025 and

1,032 feet.

Major inflow from large

cavities at 1,114, 1,120,

1,125, 1,127, and 1,132

feet.

Minor inflow from small

cavities.

Minor inflow from small

cavities at 1,248 and

1,256 feet.

Minor inflow from small

cavities at 1,284, 1,286,

1,288, 1,304, and 1,308 

feet.

Minor inflow from small

cavities at 1,356, 1,360, 

1,365, and 1,367 feet.

Major inflow from cavities

at 1,642 feet; minor inflow from small cavities

at 1,430, 1,468, 1,476,

1,506, 1,570, 1,578,

1,592, 1,600, 1,606,

1,610, and 1,625 feet.

Measurement

No.

1

2

3

4

5

6

7

8

9

10

11

'Da tun

'Estlm

Measured

head

(feet

above sea

level)

>52.0

>51.1

7.0

>51.5

>50.5

>56.6

>52.4

>57.7

>54.1

58.8

55.7

Depth (feet)'

895-934

895-2.457

2.463-2.811

895-1.428

1.433-1.618

895-1,249

1,254-2.811

895-1.124

1,129-2.811

1.030-1.154

811-816

is land surface, which is 15

ated pressure gradient Is on

'Estimated representative depth is

Estimated

pressure

gradient 2

0.43284

.43518

.44426

.43253

.43323

.43253

.43435

.43253

.43388

.43253

.43314

Estimated

representative

depth'

(feet below

sea level)

879.6

879.6

2,447.6

879.6

1.417.6

879.6

1.238.6

879.6

1.113.6

1.014.6

795.6

Estimated

pressure

at depth

403.23

405.02

1,090.48

402.73

636.02

404.93

560.75

405.41

506.64

464.28

368.73

.4 feet above sea level.

the basis of estimated fluid density and

top of measured depth minus 15.4

gradients for each measurement. The points for

feet.

meas10 1,645-1,840 8

11 1,840-2,200

nfining unit of the Floridan aquifer system

50 35,000 2,800 Major inflow from cavities 

at 1,715 feet; minor 

inflow from cavities at 

1,678, 1,690, 1,739, 

1,754, 1,764, 1,793, 

and 1,809 feet.

42 36,900 1,845 Major inflow from cavities 

at 1,896, 2,070, and 

2,172 feet; minor inflow 

from cavities at 1,856, 

1,874, 1,960, 2,028, and 

2,126 feet.

Lower Floridan aquifer

12 2,200-2,457 5

13 2,457-2,580 32

14 2,580-2,811 <1

37 42,600 2,130

32 50,000 16,000

50,000

Major inflow from cavities 

at 2,250 feet; minor 

inflow from cavities at 

2,228, 2,258, 2,308, and 

2,340 feet.

Major inflow from cavities 

at 2,490 to 2,491, 2,544 

to 2,546, 2,550 to 2,552, 

and 2,560 to 2,562 feet.

Very minor inflow from 

cavities at 2,616, 2,635, 

2,653, 2,672, 2,703, and 

2,715 feet.

urements 1, 2, 4 through 9, and 11 in the upper (brackish) 

part of the Floridan aquifer system (water-bearing zones 

1 through 9) generally fall near or on the line (GBW) 

represented by water at the depth of measurement 10, 

thereby suggesting that they are part of the same flow 

system (although minor variations in respective pressures and pressure gradients suggest the presence of 

local confining units). Pressures at selected depths 

within the body of brackish ground water in the upper 

part of the aquifer system may be approximated by the 

following equation:

P=GBW(D+43.4) (1)

25,282

depth.

For comparison, the pressure versus depth data for 

each measurement is shown in figure 10, a pressuredepth diagram. The plotted data suggest two distinct 

relations, as indicated by lines of brackish water gradient 

(GBW), represented by water at the depth of measurement 10, and saltwater-like gradient (Gsw), represented 

by water at the depth of measurement 3. The lines 

through the points represent the respective pressure

where

P = pressure, in pounds per square inch; 

GBW = pressure gradient of brackish water (0.43253 

pound per square inch (lb/in2) per foot of 

depth), represented by the water at depth of 

measurement 10 (1,030 to 1,154 ft at the 

Alligator Alley test well site); 

D = depth below land surface, in feet; and 

43.4 = head above land surface of the water at depth of

measurement 10 (58.8 ft-15.4 ft=43.4 ft). 

Measurement 3, which represents the deeper seawaterlike zones below a depth of 2,463 ft, plots slightly above 

the downward extension of the line (GBW) that represents the pressure-depth relation for the upper part of
HYDROGEOLOGY OF SOUTHERN FLORIDA G17

the system. Pressures in the deep, saltwater part of the 

Floridan aquifer system may be approximated by the 

following equation:

P=Gsw(D-8.4) (2)

where

GSW = pressure gradient of saltwater (0.44426 lb/in2 

per foot of depth);

8.4 = head below land surface of the water at depth of

measurement 3 (15.4 ft-7 ft=8.4 ft); and 

D = depth below land surface, in feet.

The upward extension of the line Gsw, representing 

the pressure-head relation for the saltwater part, intersects that for the brackish water part at 1,918.5 ft, the 

point of equal pressure. Two interpretations of the data 

are possible: (1) the saltwater and brackish water systems are unrelated and function independently because 

of intervening confining units; and (2) the two systems 

are interconnected and related by buoyancy, and the 

point of intersection (1,918.5 ft) is the approximate 

brackish water-saltwater contact, or interface.

The conductance or resistance of water that entered 

the borehole from all water-bearing zones (fig. 9, table 2) 

while the well was flowing suggests that the base of the 

brackish water part of the system is in zone 9, which 

ranges in depth from 1,430 to 1,645 ft, and that the top of 

the saltwater part is in zone 12, which ranges in depth 

from 2,200 to 2,457 ft. Between the upper brackish water 

zones and the lower saltwater zones are zones 10 and 11, 

which contain mixtures of both much the same as the 

zone of diffusion in unconfined coastal aquifers such as 

the Biscayne aquifer (Cooper and others, 1964, fig. 8).

According to the fluid resistance log of July 16, 1983, 

while the well was flowing (fig. 9) there was no obvious 

indication that the saltwater-brackish water contact 

occurred at 1,918.5 ft, as projected by buoyancy relations 

in figure 10. The fluid resistance logs of November 13, 

1980, and April 13, 1981 (not shown), obtained while the 

well was shut-in (not flowing), suggest that the pressure 

in the upper brackish water part is sufficient to displace 

the saltwater in the borehole to a depth of about 2,250 ft. 

The maximum head for the upper zone was 43.4 ft above 

land surface, or 58.8 ft above sea level, on April 21, 1981, 

when the average density of the 2,250-ft fluid column was 

estimated to be 1.002 grams per milliliter (g/mL) at 

ambient temperature. Theoretically, given sufficient 

time, brackish water from the high-pressure upper zone 

would have completely displaced the saltwater to about 

2,250 ft with a water column of density 0.998 g/mL. The 

brackish water head required for the displacement 

would, however, be about 9.2 ft higher than the maximum measured on April 21, 1981. Therefore, the static 

head in the upper part of the Floridan aquifer system

could be as high as 68 ft above sea level. The discrepancy 

between the heads (measured and displacement) could be 

caused by intraborehole flow (from high-pressure zones 

to low-pressure zones) during shut-in.

The static head for zone 1 (table 3, measurement 11) 

was 55.7 ft above sea level at the ambient density on 

April 24, 1981. Comparisons show that the head in zone 

1 (measurement 11) was about 3.1 ft lower than that in 

zone 3 (measurement 10) at ambient density. At the same 

density, the difference in head would only be 2.1 ft. The 

slight differences in head and in density suggest that 

confining beds separate these zones (at least locally) or 

that the differences are due to significant permeability 

contrasts, which suggests that ground water moves 

faster and more freely through zone 3. The widespread 

occurrence of fractures and sinkholes in the limestones 

that make up the Floridan aquifer system rules out the 

possibility that water-bearing zones within the aquifer 

system are isolated from each other.

Comparison of the highest measured head (table 3, 

58.8 ft above sea level) in the well with the 1974 

potentiometric surface map by Healy (1975b) indicates 

that the head extrapolated from the map was about 9 ft 

lower than the measured head at the Alligator Alley test 

well. Potentiometric surface maps by Johnston and others (1980, 1981) were recently modified on the basis of 

the head measured at the Alligator Alley test well. As 

more detailed information on the vertical distribution of 

head in the Floridan aquifer system is obtained from 

other test wells in southern Florida, the mapped configuration of the potentiometric surface can be expected to 

change, particularly in the area between the Alligator 

Alley test well and the potentiometric surface high in 

central Florida.

Flowmeter, fluid resistance, and fluid temperature 

logs indicated that zone 13 contributed a significant 

amount of saltwater to the well during natural flow. 

Prior to the packer tests it was assumed that the static 

head of saltwater in zone 13 was above land surface in 

order to account for the saltwater flow. That assumption 

proved to be incorrect. The pressure-depth diagram (fig. 

10) suggests that at 1,030 ft the static pressure for the 

saltwater column (extension of line Gsw) is lower than 

the static pressure in zone 3. The pressure at 1,030 ft in 

terms of the saltwater gradient (Gsw) would be 453.86 

lb/in2 , and that for the brackish water gradient (GBW) 

would be 464.28 lb/in2 . The fluid pressure in zone 3, 

therefore, would be 10.42 lb/in2 greater than the fluid 

pressure at 1,030 ft in the static column of saltwater 

above zone 13. The difference in static pressure is 

equivalent to about 23.5 ft of saltwater head or about 

24.1 ft of brackish water head. Because the borehole 

provides physical connection between the upper and 

lower zones, the fluid pressure in zone 3 is sufficiently
G18 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

500

1,000

oc 

co 

9 1,500

- 2,000

2,500

3,000

3,500

I I I I

BRACKISH WATER GRADIENT (Gew) = 0.43353 POUNDS PER 

SQUARE INCH PER FOOT

MEASUREMENT 11 - ZONE 1

DEPTH = 811 FEET

PRESSURE = 368.73 POUNDS PER SQUARE INCH

 MEASUREMENTS 1, 2, 4. 6, AND 8

MEASUREMENT 10 - ZONE 3

ESTIMATED DEPTH = 1030 FEET

P = 464.28 POUNDS PER SQUARE INCH

 MEASUREMENT 9 

- MEASUREMENT 7

MEASUREMENT 5

THEORETICAL INTERFACE

OF SALTWATER AND

BRACKISH WATER

D = 1918.5 FEET

P = 848.58 POUNDS

PER SQUARE INCH

MEASUREMENT 3 - ZONE 13'

D = 2463 FEET 

1090.48 POUNDS PER SQUARE INCH

SALTWATER GRADIENT (Gsw) = 0.44426, 

POUNDS PER SQUARE INCH PER FOOT

10

11

12

13

14

CO

4 LJJ 5 §

6 N

o 

cc.

S5

CO.

oc I

LJJ

oc

t

i

<

o

1

200 400 600 800 1,000 

PRESSURE (P), IN POUNDS PER SQUARE INCH

1,200 1,400

FIGURE 10. Pressure-depth relation for water-level measurements in the Floridan aquifer system at the Alligator Alley test well (well

G-2296 at site 10, fig. 2).

greater to displace the saltwater column below the point 

of intersection (about 1,030 ft).

If the brackish water head is reduced by 24.2 ft or 

more (a reduction that would occur when the well is 

permitted to flow naturally), the pressure of the brackish

water column at the intersection (1,030 ft) is exceeded by 

that of the saltwater column, and saltwater will move up 

the borehole from zone 13 to displace and mix with 

brackish water from zone 3. The inflowing brackish 

water from zone 3 effectively dilutes and entrains the
GROUND-WATER MOVEMENT IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G19

saltwater in the upper part of the saltwater column and 

transports the saltwater to the surface as a blend that is 

equivalent to a 50-percent concentration of saltwater. 

This phenomenon is, in some respects, comparable to the 

operation of an airlift pump, and its effects have led to 

misinterpretation of static head distribution in flowing 

wells.

GROUND-WATER MOVEMENT IN THE

FLORIDAN AQUIFER SYSTEM IN

SOUTHERN FLORIDA

GROUND-WATER MOVEMENT BASED ON NATURAL 

ISOTOPES AS TRACERS

Naturally occurring isotopes of carbon and uranium in 

samples of ground water from the Floridan aquifer 

system at nine sites in southeastern Florida were compared with those in modern seawater (table 4) to assess 

their potential as tracers of ground-water movement. 

Carbon isotopes were determined in 20 samples and 

uranium isotopes in 9 samples. Included are data on 

chloride and dissolved solids concentrations, which also 

were compared with concentrations in present-day seawater. Tritium was determined in selected samples to 

evaluate possible contamination of the sample by modern 

water. Oxygen isotopes were determined in 12 samples 

to assess their usefulness as climate indicators.

CARBON ISOTOPES

The radiocarbon dating technique has been an important and accepted research tool in archeology and geology since its inception in 1946 by Libby (1955). However, 

its use in hydrogeology has been dubious because of the 

uncertainties in comparing the carbon-14 in dissolved 

carbon species in ground water with respect to that in 

the water when it was last in contact with the atmospheric reservoir of carbon-14. An understanding of the 

involved chemical processes and the reservoir through 

which the ground water moves is essential to the interpretations and corrections that would apply to the measured carbon-14 in the sample.

Carbon-14 measurements by the U.S. Geological Survey were by liquid scintillation counting of benzene 

which was synthesized from the carbonate in a 30-gallon 

(gal) sample of ground water (Pearson and Bodden, 1975; 

Thatcher and others, 1977); however, measurements by 

the Tritium Laboratory, Rosenstiel School of Marine and 

Atmospheric Sciences, University of Miami, were by gas 

proportional counting of carbon dioxide from a 55-gal 

water sample (Stuiver and Ostlund, 1980). By convention, the measurements were compared with standard 

National Bureau of Standards oxalic acid to determine

TABLE 4. Summary of isotope analyses of selected water samples

from the Floridan aquifer system, southeastern Florida, 1971-83

[Site locations shown in fig. 2. Dashes indicate no data]

County: B, Broward; D, Dade; M, Martin; PB, Palm Beach; STL, St.

Lucie. 3H tritium: Values in picocuries per liter. 

513C: 13C/12C stable isotope ratio of sample with respect to that of the

standard (Pee Dee Belemnita).

R: Relative activity of sample with respect to standard x 100. 

d14C: Millesimal difference with respect to standard. 

D14C: Millesimal difference normalized for isotopic fractionation. 

PMC: Percent of modern carbon (normalized). 

Apparent age: Years before present (1950), in Libby years. 

AR: 234u/238U alpha-activity ratio. 

5 180: 18 0/160 stable isotope ratio of sample with respect to that of

standard mean ocean water. 

Laboratory: GS, U.S. Geological Survey; UM, University of Miami;

FS, Florida State University Geology Department.

County

Local

well

No.

Depth (feet)

Date

of 

sample

n milligrams 

per liter 

Dissolved 

solids

3 H

tritium

S 13 C R

(per (permil) cent)

PB

STL

PB-1141 

STL-255

STL-254

M-1034

PB-965

G-2292

G-2331

G-2333 

G-2296

2,914-3,163

1,610

2,715

1,990

2,532-2,705

1,008-1 

2,532-2

MDUSI-5 

MDUSBZ-1

2,800- 

895- 

2,463- 

1,433- 

895- 

895- 

2 895- 

811- 

2,500- 

2,520- 

1,280- 

1,902- 

2,746- 

1,005- 

2,689-

072

705

3,525

934

2,811

1,618

1,249

1,124

2,811

816

2,811

2,811

1,300

1,922

3,200

1,037

2,960

6/29/76

2/4/83

2/23/83

11/2/82

6/6/74

5/30/77

3/29/74

10/21/81

10/21/81

12/16/82

4/28/83

10/18/80

3/3/81

3/7/81

3/8/81

3/9/81

10/19/81

10/19/81

10/19/81

7/7/83

7/7/71

7/9/71

12/19/79

10/22/81

10/22/81

19,000

1,700

530

20,000

20,000

21,000

20,000

21,000

3,400

20,000

21,000

1,200

19,500

1,800

760

850

1,100

1,600

22,700

19,000

1,960

4,740

19,400

1,400

19,000

36,100

3,240

1,280

35,300

36,500

37,400

35,900

37,200

6,490

37,500

37,500

2,670

38,800

3,640

1,930

2,000

2,400

3,500

36,700

37,800

38,300

2,890

37,900

-3.3 <0.19

+4.3 <.04

-1.3 19.7

-4.2

-2.4

-1.7

-2.4

-3.8

-1.6

-2.6

-2.4

-1.2

-1.8

-3.7

-2.1

-3.9

-5.3

'<.72

3.1

'1.17

65.9

'17.5

'30.3

4.7

5.8

4.7

11.9

7.3

'7.6

38.9

5.6

4.3

6.8 

41.2

19,300 35,800 12 +0.5e

Site 

No.

Local 

well No.

d 14 C 

(per 

mil)

D 14 C 

(per 

mil)

PMC 

(percent)

Apparent

U

(uranium) 

(micrograms

per 

liter)

(per 

mil)

Laboratory

PB-1141 

STL-255

STL-254

M-1034

PB-965

G-2292

G-2331

G-2333 

G-2296

1.24 2.0

MDWSI-5 

MDUSBZ-1

-998.1

-999.6

-803

-992.8

-969

-998.3

-341

-825

-697

-953

-942

-953

-881

-927

-924

-611

-944

-957

-932

-588

-998.2

-999.6

-812.3

-993.1

-970.4

-998.8

-370.8

-833e

-709.8

-955.2

-944.6

-955.1

-886.7

-930.4

-927.2

-628.8

-946.6e

-959e

-934.9

-604.2

<0.18 

<.04 

18.8

<.69

3.0

1.12

62.9 

'16.7e

29 

4.5 

5.5 

4.5

11.3 

7.1

<7.3

37.1 

5.4 

4.1e

6.5 

39.6

>49,900

>55,000

13,400

40,000 

28,200 

36,100

3,700 

14,400

9,900 

24,900 

23,300 

24,900 

17,500 

21,200 

>21,000

8,000 

23,400 

25,700

22,000

7,400

1.26

1.50

1.21

1.15

2.08

1.14

.244

3.04

1.20 2.44

+0.2 

-1.7

-2.2

-.3

-2.3

-2.6

-2.6

-2.2

-1.8

-.2

-2.3 

+1.2

FS

UM

UM

UM.FS

FS

FS

FS

GS

GS

UM.FS

UM.FS

GS

GS

GS

GS

GS

GS

GS

GS

UM,FS

GS

GS

FS

GS

GS

500 1.14 3.30

'Probably contaminated.

2 Probably represents zone 895 to 1,160 feet. 

Estimated from Stuiver and Ostlund, 1980. 

e - estimated.
G20 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

the relative activity (R) for age-dating. The carbon13/carbon-12 stable isotope ratio (813C) was determined 

by mass spectrometry. Also by convention, the measured relative carbon-14 activity (R) was normalized to a 

813C value of -25 °/oo (per mil), the value for wood with 

respect to the standard Pee Dee Belemnita (PDB), to 

compensate for isotopic fractionation. The generally 

accepted equation for isotopic fractionation normalization is

D14C=d14C-2 (S13C+25) (3)

where the normalized activity, D14C, equals the determined activity, d14C, corrected for isotopic fractionation, 

assuming that a carbon-13/carbon-12 change from the 

accepted value for wood (813C=25 °/oo with respect to 

PDB) induces a change in the carbon-14/carbon-12 ratio 

of the sample. A 10 °/oo change in 8 13C value, therefore, 

results in a 20 °/oo change in the carbon-14 millesimal 

difference (d14C).

The percent of modern carbon (PMC), corrected for 

isotopic fractionation normalization or the percentile 

difference, was calculated using the following equation:

(4) 10 10

where s D14C/10 is the standard error. The age of the 

sample is expressed as apparent age owing to the uncertainty about past production of carbon-14 and the origin 

of the carbon in the water. The apparent age is based on 

the Libby half-life of 5,568 yr and was calculated using 

the following equation:

Apparent age (before 1950) = -8033 In (PMC x 10~2) (5)

The measured relative carbon-14 activities, R (that is, 

unconnected for isotopic fractionation normalization), 

ranged from about 0.04 to 65.9 percent, and the normalized activities, PMC, ranged from about 0.04 to 62.9 

percent. The experimental error for the analyses by the 

U.S. Geological Survey ranged from 0.2 to 1.5 percent, 

and that for the Tritium Laboratory, University of 

Miami, ranged from 0.1 to 0.3 percent. The corrections 

for isotopic fractionation normalization had relatively 

insignificant effects on the activities of the samples. No 

corrections for dilution or chemical reaction during transit were applied; therefore, the activities do not represent absolute age. Ages calculated according to equation 

5 are maximum estimates. Corrections for reactions led 

to younger age estimates. However, insufficient data are 

available to correct PMC for reaction effects. Generally, 

the samples of brackish ground water from the Upper 

Floridan aquifer had lowest carbon-14 activities.

UPPER FLORIDAN AQUIFER

The PMC (carbon-14 activity corrected for isotopic 

fractionation normalization) of samples of brackish 

ground water from the Upper Floridan aquifer at sites 5, 

9,10,12, and 14 (table 4) ranged from about 0.04 (a value 

that is near the limit of the dating technique) to 16.7 

percent. The stable isotope ratios (813C) ranged from 

+4.3 to -3.9 °/oo (table 4).

Variations in the PMC with depth are indicated. At 

site 10, location of the Alligator Alley test well, the PMC 

of samples between 895 and 1,618 ft ranged from 4.5 to 

16.7 percent and the carbon-13 stable isotope ratios 

(813C) ranged from -1.6 to -2.6 °/oo. The well was cased 

to a depth of 895 ft (225 ft below the estimated top of the 

Floridan aquifer system), and the open borehole was 

drilled to a depth of 2,811 ft into the limestone and 

dolostone of the Lower Floridan aquifer. Samples from 

various depths in the open hole were obtained chiefly 

with packers. A monitor tube 2 inches in diameter with 

perforations from 811 to 816 ft provided samples of 

ground water from near the top of the Upper Floridan 

aquifer. The chief water-bearing zone in the Upper 

Floridan aquifer at the well site is at about 1,030 ft, at 

the unconformable contact of the Ocala Limestone with 

the overlying Suwannee Limestone. Samples of ground 

water, presumably from the Suwannee-Ocala contact in 

the Upper Floridan aquifer, generally had higher 

carbon-14 activities (except the sea water-like samples 

from the Lower Floridan aquifer), thereby suggesting 

that they are relatively younger in age.

At site 5 (well STL-255), the PMC for samples of 

brackish ground water from depths of about 898 to 1,663 

ft in the Upper Floridan aquifer were virtually zero (0.18 

and 0.04 percent), and the respective carbon-13 stable 

isotope ratios were -3.3 and 4.3 °/oo. The deeper sample 

(1,610 to 1,663 ft), which had a chloride concentration of 

only 530 rng/L, contained an unusually high concentration of carbon-13. The high carbon-13 concentration in 

the deeper sample may indicate dissolution of dolomite 

(Hanshaw and Back, 1971, p. 147) and, therefore, 

greater dilution of carbon-14. The carbon-14 activity of 

the deeper sample would, therefore, be lower chiefly 

because of the dilution. The presence of brackish ground 

water at that depth is further evidence of the variability 

of water chemistry in the aquifer system.

The distribution of carbon-14 and carbon-13 in the 

brackish ground water of the Upper Floridan aquifer is 

apparently very complex and probably is more affected 

by dissolution-precipitation reactions than is the distribution in the Lower Floridan aquifer (saltwater). The 

distribution of carbon-14 activity (corrected for isotopic 

fractionation) and carbon-13 stable isotope ratios (813C) 

in the upper part (from about 900 to 1,200 ft deep) in 

southern Florida is shown in figure 11; also shown is the
GROUND-WATER MOVEMENT IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G21

84°

29"

\

ST. PETERSBURG

250 

 30 -

33.4 

-11.4

EXPLANATION

AREA WHERE UPPER FLORIDAN AQUIFER IS

UNCONFINED OR NEARLY SO 

LINE OF EQUAL CHLORIDE CONCENTRATION OF

250 MILLIGRAMS PER LITER

POTENTIOMETRIC CONTOUR-Shows approximate 

altitude of the potentiometric surface of the Upper 

Floridan aquifer prior to development. Contour 

Interval 10 feet. Datum is sea level

FLOW LINE

FLOW LINE SEGMENT

WELL AND SITE NUMBER-Letters refer to location: 

A, Polk City: B, Fort Meade; C. Wauchida: D. 

Arcadia. Upper number of fraction is percent of 

modern carbon; lower number is carbon-13 stable 

isotope ratio per mi (813C). <, less than. N.A.. 

no analysis

50 MILES

l l

50 KILOMETERS

FIGURE 11. Carbon-14 activities and carbon-13 stable isotope ratios in samples of ground water from the Upper Floridan aquifer,

peninsular Florida.
G22 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

distribution in central Florida, from Hanshaw and others 

(1965) and Plummer (1977). These isotope values are 

plotted on a predevelopment potentiometric surface map 

of the Upper Floridan aquifer to show the relation with 

the Upper Floridan flow system. The 813C values generally increase southward, from 11.4 °/oo at Polk City 

(site A) to -2.4 °/oo at sites 9 and 10 in southern Florida. 

The carbon-14 activity decreases generally southward 

from the recharge area near the Polk City high (site A) in 

central Florida (activity of 33.4 percent) until the activity 

becomes very small (virtually that of dead carbon-14) at 

site 5 east of Lake Okeechobee. Southward from Lake 

Okeechobee, however, the activity increases to about 6.5 

percent at site 14 near Miami.

The cause of the apparent reversal in carbon-14 activity is unclear, but at least two hypotheses are possible. 

The first hypothesis requires the inland flow of seawater 

in the lower part of the Floridan aquifer system, as 

proposed by Kohout (1965) and supported by the isotope 

data in this report. This hypothesis is supported by the 

evidence that carbon-14 activity in water in the Upper 

Floridan aquifer decreases southward from central Florida. The reduction of carbon-14 activity is due to radioactive decay during the residential time and to dilution by 

inorganic carbon from limestone and dolomite in transit 

from central Florida toward the area of Lake 

Okeechobee. Beyond Lake Okeechobee, the southwardflowing water is blended with younger seawater from the 

Lower Floridan aquifer, and ground water in the Upper 

Floridan aquifer becomes brackish. As the ground water 

continues flowing generally southward toward the 

coastal areas, it becomes increasingly salty and contains 

greater amounts of carbon-14 as more seawater is contributed from the Lower Floridan aquifer. Thus, the 

carbon-14 activities in ground water decrease from central Florida to the area around Lake Okeechobee and 

then increase again southward from the lake. This 

hypothesis also is supported by the vertical distribution 

of carbon-14 in the Alligator Alley test well.

The second hypothesis assumes that horizontal and 

vertical permeability in the system is highly variable, 

and that velocity differences are the chief cause of the 

apparent reversal in carbon-14 activity. If the permeability is assumed to be greatest along a flow line roughly 

from Polk City in central Florida to site 14 in southern 

Florida, then the time of travel along that line is assumed 

to be the least. Rates of ground-water flow in central 

Florida estimated by Hanshaw and others (1965) and 

modified by Plummer (1977) ranged from about 6 to 32 

feet per year (ft/yr); therefore, the transit time from the 

central Florida recharge area at Polk City (33.4 percent 

of modern carbon) to the Alligator Alley test well (site 

10) in southern Florida (4.5 percent of modern carbon), a

distance of about 150 mi, could range from 132,000 to 

24,750 yr. On the basis of the measured activity at Polk 

City and a rate of 32 ft/yr (the highest rate), the 

carbon-14 activity at site 10 (the Alligator Alley test 

well) in southern Florida would be about 1.5 percent of 

modern carbon, compared with the measured value of 4.5 

percent. Also, the analysis does not consider the chemical reactions that would have occurred in transit and 

reduced the concentration of carbon-14 by dilution to less 

than 1.5 percent. The measured carbon-14 activity at site 

10 (4.5 percent) is probably lower than the actual activity 

as a result of dilution while in transit. The results of this 

sample analysis suggest that either the velocity greatly 

exceeds 32 ft/yr or significant amounts of carbon-14 enter 

the flow system downgradient from Polk City and closer 

to site 10. Possible sources of closer freshwater recharge 

are the sinkhole lakes about 80 mi north-northwest of site 

10 in southern Highlands County (Kohout and Meyer, 

1959). By comparison, the first hypothesis seems to be 

the most plausible because the second hypothesis needs 

two constrained assumptions. However, more data are 

required to test these hypotheses.

The distribution of 813C values suggests that dissolved 

carbon-13 in the ground water of the Upper Floridan 

aquifer increases (values become more positive) southward from the central recharge area to near Lake 

Okeechobee, where the values approximate the value for 

the native rock (dolostone). Hanshaw and Back (1971) 

reported values of -3.1 °/oo for dolomite and -0.4 °/oo 

for calcite in samples of Tertiary carbonate rocks from a 

well near the western shore of Lake Okeechobee. The 

relatively small variation in 813C values in southern 

Florida suggests that locally the water is saturated with 

respect to calcite and dolomite and that in southern 

Florida, the net effect of dissolution-precipitation reactions on the already small carbon-14 activity probably is 

insignificant. The southward increase in 813C (carbon-13 

enrichment) from central Florida to Lake Okeechobee 

would result in substantial dilution of the dissolved 

carbon-14 by inorganic carbon from the limestone and 

dolostone. The net effect of the additional carbon would 

result in relatively less carbon-14 activity, and the age of 

the water samples would appear to be greater.

Attempts to adjust the measured carbon-14 activities 

in the samples from southern Florida, using the chemical 

dilution and isotopic dilution methods suggested by Mook 

(1980, p. 58), yielded adjusted ages that seem to be 

inconsistent with the time-of-travel estimates. No 

attempts were made to use mass-balance and masstransfer calculation schemes, as suggested by Plummer 

(1977) and Wigley and others (1978), to adjust the 

measured carbon-14 activity or 813C values because 

existing data are insufficient to accurately describe the 

hydrochemistry of the flow system.
GROUND-WATER MOVEMENT IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G23

LOWER FLORIDAN AQUIFER

The carbon-14 activities, presented by percent of 

modern carbon (PMC), of samples of salty (salinity 

comparable to seawater) ground water from the Lower 

Floridan aquifer ranged from less than 0.69 to 62.9 

percent (table 4). Of the eight samples, only four are 

considered representative of the native ground water. 

Apparently, four samples were contaminated with salty 

drilling fluid. At site 10, for example, three samples from 

the Lower Floridan aquifer had activities of about 29.0, 

7.3, and 37.1 percent. The sample of October 19, 1981, 

which had an unusually high chloride concentration 

(22,700 mg/L) and the least carbon-14, probably contained residual, artificially produced, salty drilling fluid. 

The artificial drilling fluid was chiefly artesian water 

from the Upper Floridan aquifer and sodium chloride.

Likewise, three samples collected at site 9 yielded 

activities of about 0.69, 1.12, and 62.9 percent. The 

samples of October 21, 1981, and December 16, 1982, 

which had low carbon-14 activities, were from an isolated 

zone in a deep monitor well (G-2331) that apparently had 

not been completely flushed of artificially produced drilling fluid. However, the sample of April 28, 1983, which 

had the highest carbon-14 activity, was pumped at a high 

rate (10,000 gal/min) from a nearby municipal wastewater disposal well (G-2333) and is, therefore, considered 

representative of the native fluid (a conclusion that is 

supported by the concentrations of uranium isotopes).

The normalized carbon-14 activities (PMC) and the 

carbon-13/carbon-12 stable isotope ratios (813C) for salty 

ground water from the Lower Floridan aquifer at sites 5, 

9, 10, and 14 (table 4) are shown with the estimated 

values for modern seawater in figure 12. The carbon-14 

activity in the salty ground water generally decreased 

radially inland from site 9 at Fort Lauderdale, whereas 

the stable isotope ratio (813C) decreased generally southward from site 5. The apparent ages of the four samples 

are shown in figure 13 with the differences in age relative 

to that at site 9 (Fort Lauderdale). Because the deep 

ground-water flow system is virtually closed to atmospheric carbon, except in the Straits of Florida where 

exchange with modern seawater is inferred, the relative 

age difference probably indicates depletion of carbon-14 

primarily by radioactive decay (aging) rather than dilution of the carbon-14 content in the water by dissolutionexchange processes. The apparent southward depletion 

of carbon-13 is probably related to reactions with dolostone, but the relative effect on the activities is insignificant.

The apparent age of the saltwater in the Lower 

Floridan aquifer increases radially inland from site 9, 

suggesting that seawater enters the aquifer chiefly 

through submerged karst in the Straits of Florida, east 

of Fort Lauderdale, and then travels inland through

highly permeable dolostones (chiefly the Boulder Zone). 

The rate of movement between sites 9 and 10 is about 

44.5 mi in about 4,300 yr, or about 54.6 ft/yr, an average 

rate that greatly exceeds Hanshaw and others' (1965) 

estimated range in average rates (7 to 39 ft/yr) for the 

Upper Floridan aquifer in central Florida. Most of this 

difference is probably due to the extremely high transmissivity of the Lower Floridan aquifer (Boulder Zone). 

The rate of ground-water movement between the 

subsea outcrop (karst of the Miami Terrace) in the 

Straits of Florida and site 9 at Fort Lauderdale is about 

10.5 mi in about 3,200 yr, or about 17.3 ft/yr; the rate 

between the subsea outcrop and site 10 is about 55.0 mi 

in about 7,500 yr, or about 38.7 ft/yr. The relative 

increase in rates of movement with distance inland from 

the source suggests that inland movement of seawater 

through the highly permeable zones of the Lower Floridan aquifer could be related in part to the rapid rise of 

sea level during the Holocene transgression. The effects 

of changing sea level on ground-water movement and 

hydraulic gradients in the Lower Floridan aquifer are 

discussed in a later section of this report.

URANIUM ISOTOPES

Dissolved uranium (U) isotopes have been used with 

varying degrees of success to relate samples of ground 

water to aquifers and to deduce ground-water flow 

patterns. Dissolved uranium in ground water from the 

Floridan aquifer system ranges widely, from several 

micrograms per liter (jxg/L) to a trace. Generally, uranium concentrations are relatively high in areas where 

the aquifer is unconfined (oxidizing environment) and 

relatively low in areas where it is confined (reducing 

environment). Also, concentrations are relatively high in 

areas where the ground water represents a mixture of 

seawater, and anomalously high concentrations of dissolved uranium have been found in dolomitized parts of 

the aquifer system. Uranium concentrations in the 

ground water of the karst in west-central Florida (oxidizing environment) are generally one or two orders of 

magnitude higher than concentrations in the ground 

water of central and southern Florida (reducing environment). Concentrations are relatively high in oxidizing 

environments (unconfined aquifer) because the uranyl 

ion (UO^2) forms strong bicarbonate, tricarbonate, and 

phosphate complexes, and concentrations in reducing 

environments (confined aquifer) are relatively low 

because the complex uranium ions are either precipitated 

or adsorbed.

The relative abundance of 234U (uranium-234) to 238U 

(uranium-238) in the ground water is attributed to the 

selected accumulation of 234U in the liquid phase by 

either selective leaching of 234U or by direct-recoil trans-
G24 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

87" 86

28

ESTIMATED

OCEAN

94.0

EXPLANATION

WELL AND SITE NUMBER-Upper 

number of fraction is percent of 

modern carbon normalized (14C), 

lower number is stable isotope 

ratio, per mil (813C)

 3300 SUBMARINE TOPOGRAPHY (UCHUPI 

1966)-Shows depth below sea level 

Contour interval, in feet, is variable

FLORIDA AND STUDY AREA

FIGURE 12. Carbon-14 activities and carbon-13 stable isotope ratios in samples of saltwater from the Boulder Zone of the Lower Floridan aquifer, south

Floridan Plateau.

234i fer of 284Th (thorium-234), which is a za4U precursor 

(Cowart, 1978, p. 713). The relative deficiency of 234U to 

288U in water is attributed to solution of rocks that were 

already deficient in 284U, a result of the above-mentioned 

mechanisms.

The relative abundance of 284U to 238U is theoretically 

about 1.00 (unity at equilibrium) within eight half-lives of 

284U, or about 2 million yr. The ratio of 284U to 238U in a 

sample of ground water is determined by measurements 

of alpha activity, and the results are expressed in terms 

of the alpha-activity ratio (AR). AR values for the 

ground water in the Floridan aquifer system generally 

increase downgradient, and hence with increased time in

transit, as a result of selective leaching, or the alpharecoil phenomenon. For a more complete explanation of 

the theory and analytical methods, the reader is referred 

to Osmond and Cowart (1977, p. 135) and Thatcher and 

others (1977, p. 8).

The uranium concentrations and AR values for 62 

samples of ground water from the Floridan aquifer 

system are presented with the corresponding worldwide 

average values for seawater (Ku and others, 1974) in 

table 5. Eight of the samples represent salty, seawaterlike ground water from the Lower Floridan aquifer 

(chiefly the Boulder Zone), and 54 samples represent 

ground water ranging in salinity from that of freshwater
GROUND-WATER MOVEMENT IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G25

87" 86" 85" 84 83" 82«

LITTLE 

BAHAMA 10,000 BANI<

ESTIMATED OCEAN:

500 

-3200

2000 LINE OF EQUAL DIFFERENCE IN AGE

FROM 3700 YEARS BEFORE PRESENT 

AT WELL 9-Interval 2000 years

WELL AND SITE NUMBER-Upper number 

is apparent age in Libby years before 

present. Lower number is difference in 

age from 3700 years at site 9

SUBMARINE TOPOGRAPHY (Uchupi. 1966)-Shows 

depth below sea level. Contour interval, in feet, 

is variable

FLORIDA AND STUDY AREA

FIGURE 13. Apparent carbon-14 ages and relative ages of saltwater in the Boulder Zone of the Lower Floridan aquifer, south Floridan Plateau.

to that of seawater from the Upper Floridan aquifer. The 

data are shown graphically in figure 14 by geographic 

location and hydrogeologic environment (that is, an 

unconfined aquifer generally represents an oxidizing 

environment and a confined aquifer generally represents 

a reducing environment).

The uranium concentrations for the 62 samples ranged 

from about 7.97 |xg/L in sample 3605 (table 5) to 0.003 

|xg/L in sample 525, and the AR values ranged from 3.9 

in sample 13 to about 0.46 in sample 3773. For the 62 

samples, the average AR value is 1.40 and the average 

uranium concentration is 0.653 |xg/L. The geographichydrologic distribution of the data (fig. 14) suggests that 

(1) a general relation exists between samples of

seawater-like ground water from the confined Lower 

Floridan aquifer (Boulder Zone) in southeastern Florida, 

samples of fresh ground water from the unconfined 

Upper Floridan aquifer in west-central Florida, and 

samples of brackish ground water from the confined 

Upper Floridan aquifer in distal southeastern Florida, 

and (2) a general relation exists between samples of fresh 

ground water from the confined Upper Floridan aquifer 

in central Florida and samples of brackish ground water 

from the confined Upper Floridan aquifer in southern 

Florida.

The first relation suggests that the shared characteristics of the samples are indicative of their close proximity to sources of recharge (that is, direct infiltration of
G26 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

TABLE 5. Selected uranium isotope data, Floridan aquifer system, 

central and southern Florida 

[Do., ditto. Site locations shown in fig. 15] 

Sample No.: A numbering system used by Florida State University

Geology Department. 

Owner or local identifier: WWTP, wastewater treatment plant.

County: B, Broward; CH, Charlotte; CI, Citrus; CO, Collier; D, Dade;

DS, De Soto; H, Hardee; HE, Hernando; HI, Highlands; HL, 

Hillsborough; L, Lake; LE, Lee; M, Marion; MA, Martin; MO, 

Monroe; OK, Okeechobee; OR, Orange; PB, Palm Beach; PC, Pasco; 

PI, Pinellas; PK, Polk; SL, St. Lucie; SR, Sarasota.

AR: ^u/23^ alpha-activity ratio. 

Uranium: Values in micrograms per liter.

u

Sample Owner or local County AR (ura- Reference 

No . identifier nium)

Upper Floridan aquifer

West-central Florida:

91 Rainbow Springs M 1.02 0.15 Cowart (1978, p. 715) 

±0.05 ±0.02 

95 Well SCE 170 M 1.08 0.10 Osmond and others (1974, 

±0.06 ±0.03 p. 1089). 

96 Dunellon well No. 2 M 0.92 0.95 Do. 

±0.06 ±0.03 

99 Silver Springs M 1.03 0.79 Cowart (1978, p. 715) 

±0.03 ±0.03 

105 City of Ocala well M 0.94 1.08 Osmond and others (1974, 

No. 3. ±0.02 ±0.10 p. 1092). 

121 Homosassa Springs CI 0.93 0.58 Cowart (1978, p. 715) 

±0.07 ±0.06 

151 Well CE 30A M 1.47 0.71 Osmond and others (1974, 

±0.06 ±0.03 p. 1092). 

439 Sulphur Springs spa HL 0.88 1.91 Cowart (1978, p. 715) 

±0.04 ±0.12 

913 Week! Wachee Springs HE 0.73 1.16 Do. 

±0 .04 ±0 . 07 

956 Crystal Springs PC 0.72 0.50 Do. 

±0.05 ±0.03 

1118 Bug Springs L 1.01 0.28 Do. 

±0.06 ±0.01

East-central and south-central Florida:

12 Brewster American PK 2.8 0.05 Osmond and others (1974, 

Cyanide Company. ±0.3 ±0.02 p. 1096) 

13 Wauchula City well H 3.9 0.04 Do. 

±0.6 ±0.01 

15 Uinter Haven well PK 1.40 0.03 Osmond and others (1974. 

No. 2. ±0.3 ±0.01 p. 1096) 

39 Carlton (M-186) MA 1.59 0.05 Rydell (1969) 

±0.26 ±0.01 

44 Mulberry City well PK 1.8 0.04 Osmond and others (1974, 

±0.3 ±0.01 p. 1096). 

440 Lakeland well No. 10 PK 2.38 0.015 Osmond and Cowart (1977 

±0.37 ±0.002 p. 139). 

443 Lake Uales well No. 3 PK 1.20 0.038 Unpublished 

±0.18 ±0.004 

444 Avon Park well No. 2 HI 1.31 0.045 Do. 

±0.16 ±0.004 

445 Sebring well No. 3 HI 1.10 0.073 Do. 

±0.08 ±0.084 

448 Big Fine Island LE 2.98 0.004 Osmond and Cowart (1977 

±0.87 ±0.001 p. 139). 

450 Alva LE 1.41 0,016 Do.

481 Hot Springs (Humble- CH 0.94 0.024 Osmond and Cowart (1977 

Lowndes-Treadwell) ±0.07 ±0.002 p. 139); site 1 this 

report.

No. 1. ±0.04 ±0.001 p. 139). 

654 McKay Creek Monitor PI 2.06 0.043 Cowart and others (1978, 

No. 1. ±0.28 ±0.007 p. 166). 

699 Zolfo Springs City H 2.92 0.020 Do. 

well. ±0.67 ±0.006 

703 Northeast of Arcadia DS 1.61 0.044 Do. 

±0 .29 ±0 . 007 

708 Deep Creek ROMP 10 CH 0.94 0.040 Do. 

±0.18 ±0.005 

712 Sun City well No. 5 HL 1.57 0.011 Do. 

±0.35 ±0.002 

719 General Development DS 2.00 0.020 Do. 

Corporation. ±0.24 ±0.002 

727 Uilliamson OK 2.75 0.18 Unpublished 

890 L. H. Avant DS 1.82 0.039 Osmond and Cowart (1977 

±0.24 ±0.004 p. 139).

TABLE 5. Selected uranium isotope data, Floridan aquifer system, 

central and southern Florida Continued 

[Do., ditto. Site locations shown in fig. 15]

u

Sample Owner or local County AR (ura- Reference 

Ho. Identifier ntum)

Upper Floridan aquifer- -Continued

East-central and south-central Florida Continued:

1120 Ueklwa Springs OR 1.46 0.41 Cowart (1978. p. 715) 

±0.10 ±0.03

±0.09 ±0.01 

1124 Juniper Springs M 1.19 0.14 Do. 

±0.08 ±0.01

Southern Florida:

452 Snook Hole Hotel. CO 1.94 0.018 Unpublished

457 Belle Glade (FB-203) PB 0.92 0.051 Cowart and others (1978.

524 Jupiter DNR (PB-747) PB 0.66 0.065 Do. 

±0.03 ±0.003 

525 Peanut Island PB 1.45 0.003 Do. 

(PB-216). ±0.18 ±0.003 

726 Kobe Sound HA 1.06 0.055 Do. 

±0.19 ±0.007 

3771 Alligator Alley test B 1.73 0.071 Site 10 this report, 

well (C-2296). depth ±0.15 ±0.005 

811 to 816 feet. 

3772 Alligator Alley test B 0.92 0.072 Do. 

well (C-2296). depth ±0.13 ±0.008 

895 to 1.150 feet.

Distal southeastern Florida:

454 Hurricane Lodge D 2.12 0.034 Cowart and others (1978, 

(S-1354). ±0.14 ±0.002 p. 166). 

455 Pennekantp (MO-127) HO 0.84 0.147 Do. 

±0.02 ±0.003 

456 Crossnnn (S-524) D 1.47 0.093 Do. 

±0.05 ±0.008 

505 Turkey Point (S-1534). D 0.76 0.455 Osmond and Cowart (1977. 

depth 1.132 to ±0.01 ±0.005 p. 139). 

1.412 feet. 

512 Ocean Reef (MO-133) HO 0.99 0.30 Do. 

±0.03 ±0.02

513 Turkey Point (S-1534). D 1.30 0.24 Osmond and Cowart (1977, 

depth 1.544 to ±0.10 ±0.08 p. 139). 

1,930 feet. 

521 Everglades National D 1.41 0.093 Cowart and others (1978. 

Park (NP-100). ±0.05 ±0.005 p. 166). 

544 Underwood (S-993) D 1.12 0.33 Unpublished 

±0.05 ±0.04 

545 Hlnleah (C-3061) D 1.51 0.049 Cowart and others (1978. 

±0.06 ±0.03 p. 166). 

2183 MDWSI-5 D 0.53 0.076 Site 14 this report. 

±0.08 ±0.007 

3773 Fort Lauderdale WWTP B 0.46 0.305 Site 9 this report, 

monitor well, depth ±0.03 ±0.014 

1.021 to 1.072 feet. 

3774 Fort Lauderdale WWTP B 0.78 4.70 Do. 

monitor well, depth ±0.02 ±0.30 

1.466 to 1.562 feet.

Lower Floridan aquifer

Southeastern Florida: 

508 Margate WWTP (C-2292) B 1.15 3.42 Cowart and others (1978.

±0.01 10.06 p. 166); site 8 this 

report. 

520 Stuart WVTF (H-1034) KA 1.50 0.45 Cowart and others (1978, 

±0.15 ±0.03 p. 166); site 6 this 

report . 

678 West Pain Beach WWTP PB 1.21 1.42 Cowart and others (1978, 

(FB-965). ±0.06 ±0.04 p. 166); site 7 this 

report. 

935 Quaker Oats IW-3 PB 1.25 1.95 Site 3 this report. 

(PB-1141). ±0.05 »0.12 

2186 Mlaml-Dade WWTP D 1.22 2.50 Site 14 this report. 

(MDUSI-5). ±0.04 ±0.09 

3605 Port St. Lucie WWTP SL 1.26 7.97 Site 5 this report. 

(STL-254). ±0.03 ±0.57 

3680 Fort Lauderdale WWTP B 1.14 3.04 Site 9 this report. 

(C-2333). ±0.04 ±0.14 

3681 Alligator Alley test B 1.20 2.44 Site 10 this report, 

well (C-2296). ±0.03 ±0.07

Worldwide senwater 1.140 3.30 Ku and others (1974. 

±0.016 ±0.14 p. 314).
GROUND-WATER MOVEMENT IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G27

10

cc

UJ

a. 

to

1 

o 

o

0.1

0.01

0.001

3774 

O

A3605

508

SEAWATEFM

^3680 

368,^2186

439 X

A 935

913x

678A

X106 

X96

X99

X151

XKAU-121 

V956 

0505

3773 

O

®491

512 0544 

A

X1118

O513

EXPLANATION

Numbers are sample numbers. Locations of samples 

are shown in figure 15

UPPER FLORIDAN AQUIFER

X UNCONFINED, WEST CENTRAL FLORIDA

® CONFINED, CENTRAL FLORIDA

Q CONFINED, SOUTHERN FLORIDA

O CONFINED, DISTAL SOUTHEASTERN FLORIDA

LOWER FLORIDAN AQUIFER 

A CONFINED, SOUTHEASTERN FLORIDA

(iS) STANDARD MEAN OCEAN WATER

4550 X ®1123 

<8> 1124

®727

,95

21830 3772

Q

0524

o^1

521

Q3771

726

457 Q

44

703

443

654 

O454

FLW-12

Q450 

® 712

Q452

®440

Q 448

D525

234U/238U ALPHA ACTIVITY RATIO

FIGURE 14. Relation of uranium-234/uranium-238 alpha-activity ratio and uranium concentration for selected samples 

of ground water from the Florida aquifer system and seawater worldwide.
G28 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

TABLE 6. Summary of selected uranium isotope data for the Upper 

Floridan aquifer by location and hydrogeologic environment, central and southern Florida

[AR, alpha-activity ratio]

Location

West-central

East -central and

south- central.

Southern

Distal southeastern

Total area

Hydro- No. 

geologic of 

envi- samronment pies

Unconfined 11

Confined 24

Confined 7

Confined 12

54

AR

Range

0.72-1.47

0.94-3.9

0.66-1.94

0.46-2.12

0.46-3.9

Average

0

1

1

1

1

.98

.83

.24

.11

.42

U

(uranium) 

(micrograms 

per liter)

Range

0.10-1.91

0.004-0.41

0.003-0.072

0.034-4.70

0.003-4.70

Average

0

0

0

0

0

.75

.080

.048

.568

.320

freshwater in the karst of west-central Florida and direct 

infiltration of seawater in the Upper and Lower Floridan 

aquifers along the southeastern coast of Florida). The 

second relation suggests that the shared characteristics 

of samples from central and southern Florida result from 

transit of the water from the distant recharge areas (that 

is, the uranium concentration decreases with increased 

distance downgradient, but the AR increases because of 

the alpha-recoil mechanism).

UPPER FLORIDAN AQUIFER

Uranium isotope analyses of 54 samples of ground 

water from the Upper Floridan aquifer (table 5, fig. 14) 

were used to deduce ground-water movement from the 

recharge areas in central Florida and the discharge areas 

in central and southern Florida. The salinities of the 

samples ranged from that of freshwater to that of 

seawater. Ten of the samples were from large freshwater 

springs in central Florida, and 44 were from deep wells. 

Of the 44 samples from deep wells, 4 were from the 

outcrop area (oxidizing environment) for the Upper 

Floridan aquifer in western and central Florida. Of the 

10 springs sampled, 7 were from the outcrop area 

(oxidizing environment) and 3 were from downgradient 

areas where the aquifer is confined (reducing environment). Osmond and others (1974) used isotope dilution 

techniques to estimate ground-water contributions to 

Rainbow Springs (sample 91) and Silver Springs (sample 

99) based on areal variations in uranium concentration 

and AR values. The distribution of dissolved uranium 

and AR values by region and hydrogeologic environment 

is summarized in table 6.

In west-central Florida, the area where the aquifer is 

unconfined, the uranium concentrations in 11 samples of 

ground water ranged from 0.10 to 1.91 (JLg/L, averaging 

0.75 (jLg/L; the AR values ranged from 0.72 to 1.47, 

averaging 0.98. In east-central and south-central Florida, generally the area south and east of the outcrop area 

to the northern half of Lake Okeechobee, the uranium

concentrations in 24 samples of ground water ranged 

from 0.004 to 0.41 jxg/L, averaging 0.080 (xg/L; the AR 

values ranged from 0.94 to 3.9, averaging 1.83. In 

southern Florida, generally the area from the southern 

half of Lake Okeechobee to distal southeastern Florida, 

the uranium concentrations in seven samples ranged 

from 0.003 to 0.072 (xg/L, averaging 0.048 (xg/L; the AR 

values ranged from 0.66 to 1.94, averaging 1.24. In distal 

southeastern Florida (Bade and Monroe Counties), the 

uranium concentrations in 12 samples ranged from 0.034 

to 4.70 (JLg/L, averaging 0.568 (xg/L; the AR values 

ranged from 0.46 to 2.12, averaging 1.11 (table 6).

The general trend indicated by the average values of 

dissolved uranium concentration is decreasing concentration from the outcrop area (unconfined aquifer, oxidizing 

environment) southward to central and southern Florida 

(confined aquifer, reducing environment), then increasing concentration near the coastline in distal southeastern Florida. The general trend in average AR values is 

increasing AR values (234U increasing relative to 238U) 

from the outcrop area southward into central Florida 

(confined aquifer, reducing environment), then decreasing values from central Florida into distal southeastern 

Florida.

The areal distribution of uranium concentration in the 

ground water of the Upper Floridan aquifer (fig. 15) 

suggests that (1) concentrations in the outcrop area 

(unconfined, oxidizing environment) range from 0.1 to 

2.0 (JLg/L (zone 1, fig. 15), (2) concentrations through 

most of south-central and southern Florida (confined, 

reducing environment) range from a trace to about 0.1 

(jLg/L (zone 2), and (3) concentrations chiefly in distal 

southeastern Florida (confined, reducing environment) 

range from about 0.1 to 0.5 jxg/L (zone 3). Sample 3774, 

from the middle confining unit of the Floridan aquifer 

system, has an anomalously high concentration (4.70 

(JLg/L) compared with the other samples in distal southeastern Florida. The indicated general trend in dissolved 

uranium is decreasing concentration southward from the 

area of recharge (outcrop area) in west-central Florida. 

The concentration of uranium decreases as flow conditions change from an unconfined (oxidizing) environment 

(zone 1) to a confined (reducing) environment (zone 2), 

except in east-central Florida, where concentrations in 

downgradient freshwater springs (samples 1120, 1123, 

and 1124) are comparable to those in the outcrop area 

(sample 1118). In south-central and southern Florida, the 

concentrations are mixed but generally less than 0.1 

(jLg/L. The lowest concentrations are on coastlines in 

southern Florida (samples 448 and 525), suggesting 

(subtly at best) that concentrations generally decrease 

southward. Scattered within the broad expanse of low 

concentrations in central and southern Florida (zone 2) 

are areas of anomalously higher concentrations (zone 3).
GROUND-WATER MOVEMENT IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G29

0.1

 452 

0.02

Q 91 

\ 0.15

\

\

VERO BEACH

FORT PIERCE

EXPLANATION

AREA WHERE UPPER FLORIDAN AQUIFER IS

UNCONFINED OR NEARLY SO 

LINE OF EQUAL URANIUM CONCENTRATION OF

0.1 MICROGRAM PER LITER-Queried where

uncertain or inferred value 

ZONE NUMBER AND RANGE IN URANIUM

CONCENTRATION

1 0.1 to 2.0 micrograms per liter

2 less than o. 1 microgram per liter

3 0.1 to 0.5 microgram per liter

POTENTIOMETRIC CONTOUR-Shows approximate 

altitude of the potentiometric surface of the Upper 

Floridan aquifer prior to development. Contour 

interval 10 feet. Datum is sea level

FLOW LINE

WELL-Upper number is sample number. Lower 

number is uranium concentration. In micrograms 

per liter

SPRING-Upper number is sample number. Lower 

number is uranium concentration, in micrograms 

per liter

50 MILES

i I

50 KILOMETERS

FIGURE 15. Uranium concentration and predevelopment potentiometric surface, Upper Floridan aquifer, peninsular Florida.
G30 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

The anomalously higher concentration of uranium in 

sample 727 north of Lake Okeechobee could be related to 

local enrichment from phosphate deposits in the top of 

the aquifer system or perhaps to geochemical processes 

related to dolomitization. The anomalously higher concentrations in sample 491 (table 5) on the Gulf Coast 

south of Tampa Bay and in samples 455, 505, 512, 513, 

544, 3773, and 3774 in distal southeastern Florida are 

perhaps related to local enrichment by geochemical processes related to dolomitization.

The areal distribution of AR values (fig. 16) indicates a 

general trend (with some exceptions) of increasing AR 

values eastward and southward from the outcrop area in 

west-central Florida. The values substantially increase 

as flow conditions change from unconfined (oxidizing) to 

confined (reducing). Of the 11 samples in the outcrop 

area (oxidizing environment), 9 have AR values ranging 

from 0.7 to 1.0 (zone A) and 2 (samples 95 and 151) have 

values that are slightly greater than 1.0 (zone B). Seven 

of the nine samples in zone A are from large freshwater 

springs that rapidly drain relatively large areas of the 

karst, and two (samples 96 and 106) are from wells 

tapping deeper parts of the Upper Floridan aquifer. The 

range in AR values for the springs suggests that groundwater contributions to the springs are chiefly from zone 

A (AR<1), but that some ground water is contributed 

from zone B (ARM), or that the ground water has been 

in transit long enough to be affected by selective leaching, or the alpha-recoil mechanism. For example, contributions of ground water from the northern part of the 

outcrop area (samples 95 and 151) apparently have 

significant effects on the flow from Rainbow Springs 

(sample 91) and Silver Springs (sample 99).

Ground water in the major part of the peninsula is 

confined, and AR values generally range from about 1.1 

to 2.0 (zone B), with locally higher (zone C) and lower 

(zone D) values. Eastward and downgradient from the 

outcrop area in zone B are three freshwater springs 

(samples 1120, 1123, and 1124) with AR values ranging 

from 1.19 to 1.46, which suggest slow transit of the 

ground water from the distant outcrop area. Southward 

and downgradient from the outcrop area, the AR values 

vary widely, but values along the southward-plunging 

nose of the Polk City potentiometric surface high are 

relatively lower as a result of local recharge from sinkhole lakes on the high sand ridge, which forms the major 

axis of the present-day peninsula to a point about 25 mi 

northwest of Lake Okeechobee. Coincident with the sand 

ridge is the ground-water divide that extends to the 

southern terminus of the peninsula and separates flow to 

the east and to the west. In central Florida, there is 

subtle but reasonable evidence that AR values increase 

downgradient (east and west) from this ground-water 

divide.

West and east of the divide in central Florida, AR 

values range from about 2.1 to 3.0 (zone C). The highest 

values chiefly are in the Peace River valley, west of the 

divide, extending southward about 70 mi from the Polk 

City high. Samples in this area were obtained from wells 

that are progressively deeper downdip and southward 

from the outcrop area. The highest AR value (3.9) was 

determined for a sample of water (sample 13) from a 

1,103-ft-deep municipal supply well cased to 404 ft for the 

city of Wauchula. At Zolfo Springs, about 5 mi south of 

Wauchula, a sample of water (sample 699) from a 1,002- 

ft-deep municipal supply well cased to 350 ft yielded an 

AR value of 2.92. At Arcadia, about 22 mi south of 

Wauchula, a sample of water (sample 703) from a 1,410- 

ft-deep supply well cased to 900 ft yielded an AR value of 

1.61. AR values for the three samples decrease southward (downdip), and the lowest value (1.61) is from the 

well that is both cased and drilled the deepest. The 

variations in AR values in the Peace River valley probably are due to vertical variations in water-bearing 

characteristics (permeability and porosity) and to variations in well construction. The relation between depth 

and AR value implies that the higher AR values in the 

upper Peace River valley could be associated with wells 

that receive significant contributions of ground water 

from the top of the aquifer. The higher AR values could 

be related to contributions of water from overlying 

confining beds (hence, longer transit times) by downward leakage during stress from pumping. A similar 

explanation can probably apply to the high AR value 

(2.75) in sample 727 in the easterly flow regimen.

In addition to the above, high AR values were determined for samples from a deep monitor well at McKay 

Creek (sample 654) near St. Petersburg and a deep 

supply well on Big Pine Island (sample 448) near Fort 

Myers. The McKay Creek sample was similar in composition to seawater, and the high AR value (2.06) would be 

related to geochemical mechanisms during inland transit 

of seawater (coastal seawater intrusion). The Big Pine 

Island sample was brackish water (the salinity equivalent to about 4 percent of seawater), and the AR value 

(2.98) could be related to long-term seaward flow of 

freshwater from the principal recharge areas in central 

Florida.

AR values ranging from 0.46 to about 1.0 (zone D) 

were determined for 11 samples from three areas in 

central and southern Florida where the aquifer is confined. As previously stated, values of about 1.0 or less 

are mostly found in unconfined oxidizing environments; 

therefore, the occurrence of values of less than unity in 

the confined reducing environment is considered to be 

anomalous. Samples 481 and 708 near the Peace River 

estuary have identical AR values (0.94) although the 

sources are vastly different. Sample 481 is from a 1,640-
GROUND-WATER MOVEMENT IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G31

AREA WHERE UPPER FLORIDAN AQUIFER IS 

UNCONFINED OR NEARLY SO C

LINE OF EQUAL ALPHA ACTIVITY-Ratio equals 1 or 1

2 as shown. Queried where uncertain or inferred

value 

ZONE LETTER AND RANGE IN ALPHA-ACTIVITY

RATIO

0.7 - 1.0

1.1 - 2.0

2.1 - 4.0

0.5 - 1.0 

POTENTIOMETRIC CONTOUR-Shows approximate

altitude of the potentiometric surface of the Upper

Floridan aquifer prior to development. Contour

interval 10 feet. Datum is sea level 

FLOW LINE

WELL-Upper number is sample number. Lower 

number is 234U/238U alpha-activity ratio

SPRING-Upper number is sample number. Lower 

number is 234U/238U alpha-activity ratio

FIGURE 16. Uranium-234/uranium-238 alpha-activity ratio and predevelopment potentiometric surface, Upper Floridan aquifer,

peninsular Florida.
G32 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

ft-deep flowing well cased to 648 ft producing saltwater 

(chloride concentration of about 18,400 mg/L) at 96.0 °F. 

Sample 708 is from a flowing 917-ft-deep well cased to 

595 ft producing brackish water (chloride concentration 

of about 380 mg/L) at 80.6 °F. The similarity in AR 

values led Osmond and Cowart (1977, p. 145) to conclude 

that both samples are related to local upwelling of deep, 

warm saltwater; however, the salinity and temperature 

data for sample 708 do not support that conclusion. 

Sample 481 probably represents a blend of saltwater 

(seawater-like salinity) from the bottom of the well with 

fresher water from higher water-bearing zones. The AR 

value is lower than that for modern seawater (about 1.14) 

and much lower than that for seawater that had traveled 

significantly far (for comparison see section of report 

concerning AR values in the Lower Floridan aquifer). 

The anomalously high temperature of sample 481 (96.0 

°F) is undoubtedly related to geothermal heating. The 

temperature of the saltwater in the underlying Boulder 

Zone of the Lower Floridan aquifer at the well site 

(sample 481) is about 110 °F (see fig. 7).

Cowart and others (1978, p. 169) hypothesized that the 

low AR value for sample 457 south of Lake Okeechobee 

may be a relic of the late Pleistocene Epoch, when 

oxidized waters (AR^l.O) were transported farther 

downgradient through discrete solution features (zones 

of high transmissivity) during low stands of the sea. 

Samples 524 and 3772 may have similar origins.

The anomalously low AR values in distal southeastern 

Florida (samples 455, 512, 505, 2183, 3773, and 3774) may 

have origins similar to that of sample 457. However, an 

alternate explanation was proposed by Cowart and 

others (1978), one that involves dissolution of previously 

precipitated uranium that was originally deficient 

in 234U. The close proximity of the wells in distal 

southern Florida to the ocean suggests that the low AR 

values there are related to geochemical reactions that 

involve seawater.

According to J.B. Cowart (Florida State University, 

written commun., 1984),

the mechanisms hypothesized to be responsible for uranium 

isotope disequilibrium all assume (on the basis of extensive 

radiochemical data) that 234U is likely to be more mobile (and, 

thus, end up in the liquid phase) than 288U. The reason for the 

increased mobility of 234U is associated with the radiogenic 

origin of the isotope. The formation of 234U results from the 

radioactive decay of 288U so that (1) 234U is likely to be situated 

in a lattice-damaged site (the damage resulting from its formation from 234Th and 234Pa (palladium-234)), or (2) the 234U (or its 

precursors 234Th and 284Pa) is directly propelled into the liquid 

phase surrounding the solid phase by direct alpha recoil. A 234U 

atom in a lattice-damaged site is less tightly bound to the solid 

and is, therefore, more likely to be leached from the solid than 

is the 238U situated in an undamaged site. There is no known

mechanism by which elemental. 238U is more mobile than 

elemental 234U.

The fact that there are waters in a few places that have a 

deficiency of 234U relative to 238U can be explained using the 

previously discussed isotope fractionation mechanisms. If the 

liquid phase is enriched in 234U, the source of the uranium (the 

solid) must be depleted in 234U relative to 238U. If the environmental geochemical conditions change such that all of the 

uranium in the solid phase (or all of the uranium in part of the 

solid phase) is mobilized, the uranium entrained will be deficient in 234U. Usually, such profound changes in the geochemical environment are associated with near-surface locations 

(water-table changes and erosion) hence the "oxidized unconfined" location of most samples in which the AR is less than 1.

AR's of less than 1 in deep samples pose a different problem. 

One possibility is that the water was transported rapidly to a 

deep and distant location by way of conduits so that the water 

retains aspects of its "oxidized unconfined" origin. The other 

possibility is that changes of a rather profound type have 

occurred in the deep environment.

Cowart and others (1978) suggested that the AR's of less 

than 1 in the Upper Floridan aquifer of southeastern Florida 

(samples 455, 457, 505, 512, and 524) resulted from rapid 

movement of water from the recharge area to south Florida. 

Since that publication, other samples from the Upper Floridan 

aquifer in the area have been analyzed by Cowart (samples 

2183 and 3771 through 3774). Of the samples more recently 

collected, two pairs representing vertical sets of water from the 

Upper Floridan aquifer have been analyzed (samples 3771 and 

3772 from the Alligator Alley test well and samples 3773 and 

3774 from Fort Lauderdale's Port Everglades wastewater 

treatment plant monitor wells). Of the four samples, three have 

AR's of less than 1. The Alligator Alley test well samples have 

AR's of 1.73±0.15 and 0.92±0.13 for depths of 811 to 816 ft and 

895 to 1,150 ft, respectively. The Fort Lauderdale samples 

have AR's of 0.46±0.03 and 0.78±0.02 for depths of 1,021 to 

1,072 ft and 1,466 to 1,562 ft, respectively. The uranium 

concentrations for the Fort Lauderdale samples are 

0.305±0.014 and 4.70±0.30 jxg/L, respectively.

The magnitude of the disparity between the samples in the 

Fort Lauderdale well strongly suggests that the source of the 

<1-AR uranium is relatively nearby. Furthermore, the dilution 

or mixing of the higher concentration sample (sample 3774) 

with any water having an AR of 0.46 or more to produce the 

uranium concentration and isotope ratio of sample 3773 is not 

possible. It is not known whether the other samples in southern 

Florida having AR's of less than 1 result from relict circulation 

or from changes in environment; it is possible they represent 

some combination.

Interestingly, the samples from the Upper Floridan aquifer 

in southern Florida that have AR's of less than 1 and the 

highest uranium concentrations (samples 455, 505, 512, 3773, 

and 3774) are located near the coast. A scenario consistent with 

the results is one in which uranium has been precipitated at a 

geochemical barrier, possibly a redox barrier, the source of the 

dissolved uranium being seawater. With changes in sea level 

and hydraulic head, the precipitated uranium was isolated in a 

reducing environment, wherein by the process of alpha recoil it 

became depleted in 234U. With the advent of geochemical-redox
GROUND-WATER MOVEMENT IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G33

changes, the precipitated uranium became mobile (thus, the 

relatively high concentration), and it was, of course, deficient in 

284U (AR of less than 1). Such a scenario is unlikely for the more 

inland samples (samples 481, 708, 457, and 3772) that have AR's 

of less than 1.

The explanation for the apparent AR anomalies are, 

therefore, many and diverse and could involve several 

processes. Dolomitization, for example, may be important in the selective precipitation of elemental uranium. 

An investigation of high gamma-ray emissions from 

dolostone in the Lower Floridan aquifer at Stuart (fig. 2, 

site 6) in 1974 by borehole spectrometry indicated that 

uranium was the probable source (W.S. Keyes, U.S. 

Geological Survey, written commun., 1974). The low AR 

anomaly in distal southeastern Florida could be linked to 

the mixing of magnesium-rich seawater with brackish 

bicarbonate-type ground water (Hanshaw and Back, 

1971) to yield dolomite that perhaps has excess 234U. 

Lowered temperatures (less than 77 °F) along the southeastern coast may enhance the dolomitization process. In 

other areas, upwelling of warm, magnesium-rich, relatively young seawater from the Boulder Zone may also 

produce uranium-rich dolomite and AR anomalies. Also, 

some of the AR anomalies may be due to experimental 

error since the uranium concentration in the brackish 

artesian water of southern Florida is generally very low.

The trends in dissolved uranium concentration and AR 

values suggest that flow originated from the outcrop area 

(figs. 15, 16); however, predevelopment heads in the 

Upper Floridan aquifer, as indicated by the potentiometric surface map, suggest that flow in the outcrop area is 

chiefly westward toward the Gulf of Mexico. This discrepancy could be related to changes in flow patterns as 

a result of sea-level fluctuations, and perhaps climatic 

changes, during the Holocene transgression. Plummer 

(1977, p. 811) estimated that the ground water at Polk 

City (the highest heads) the northernmost source of 

flow to central and southern Florida is about 3,200 to 

8,000 yr old (water samples contain about 67 percent of 

modern carbon) and that the average southward velocity 

was about 32 ft/yr. Therefore, the present-day concentration gradients are perhaps, to a large degree, a relic of 

antecedent flow patterns when the principal recharge 

area was perhaps 20 mi north of the Polk City potentiometric surface high.

To date, the uranium isotope data for the Upper 

Floridan aquifer are meager and present a complex 

picture of the regional flow system. Data from discrete 

water-bearing zones in the Upper Floridan aquifer are 

necessary prerequisites for correct interpretation, and 

further investigation is needed into the possible causes of 

the apparent anomalies.

LOWER FLORIDAN AQUIFER

Uranium isotope analyses of eight samples of salty 

ground water from the Lower Floridan aquifer (chiefly 

the Boulder Zone) in southeastern Florida were used to 

evaluate the potential for inland circulation of seawater 

from the Straits of Florida (tables 4, 5; fig. 14). A sample 

from a deep monitor well at site 9 (well G-2331) was not 

included because the sample was contaminated with 

drilling fluid. The uranium concentration in the eight 

samples ranged from 0.45 (Jig/L in sample 520 to 7.97 

(jug/L in sample 3605, averaging 2.90 (Jig/L. The AR 

values ranged from 1.14 to 1.50, averaging 1.24. The 

uranium concentrations in six samples are less than that 

of worldwide seawater (U=3.30±0.14 (Jig/L). The anomalously high uranium concentration (7.97 |xg/L) in sample 

3605 is unexplained. The areal distribution of uranium 

(fig. 17) shows that concentrations generally decrease 

radially inland from site 9 in the Fort Lauderdale area, 

where the concentration compares best to that of seawater. The inland concentration gradient suggests that 

uranium is either precipitated or adsorbed during transit 

from a source in the Straits of Florida. A similar flow was 

indicated by the radiocarbon data.

The AR values for the samples from the Lower 

Floridan aquifer generally increase radially inland from 

site 9 in the Fort Lauderdale area, where the AR value 

is identical to that in seawater (fig. 17). The inverse 

gradient suggests that 234U increases relative to 238U, 

probably by the alpha-recoil phenomenon during transit 

inland of seawater from a source area in the Straits of 

Florida, east of Fort Lauderdale. Therefore, the 

increase in AR values is an indication of time in transit 

through the Boulder Zone, and the AR data correspond 

with both the uranium concentration data and the radiocarbon data.

FLOW PATTERNS BASED ON HYDRAULIC GRADIENTS

The maximum hydraulic gradient (dh/dl) along a flow 

path indicates the direction of ground-water movement, 

whereas the rate of movement is dependent on the 

hydraulic gradient and the hydraulic properties of an 

aquifer specifically, hydraulic conductivity and porosity. The average effective linear velocity of ground water 

(v) in a homogeneous and isotropic porous medium is 

explained by Darcy's law, which is expressed by the 

following equation:

K (dh/dl)

(6)

where

v = average effective linear velocity, in feet per day; 

K = hydraulic conductivity, in feet per day;
G34 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

87° Se

28

LINE OF EQUAL URANIUM

CONCENTRATION- Interval 1.0 

microgram per liter

LINE OF EQUAL ALPHA-ACTIVITY RATIOInterval is variable

WELL AND SITE NUMBER-Upper number is 

sample number. Middle number is alphaactivity ratio. Lower number is uranium 

concentration, in micrograms per liter

SUBMARINE TOPOGRAPHY (Uchupi, 1966)-Shows 

depth below sea level. Contour interval, in feet, 

is variable

FLORIDA AND STUDY AREA

FIGURE 17. Uranium concentration and uranium-234/uranium-238 alpha-activity ratio in saltwater, Boulder Zone of the Lower Floridan aquifer, south

Floridan Plateau.

dh/dl = hydraulic gradient (I) or unit change in head per 

unit length of flow line, in feet per foot; and 

<}> = effective porosity.

Where hydraulic conductivity and effective porosity are 

constant, changes in hydraulic gradient indicate relative 

changes in velocity. Velocity can also be determined by 

the equation

v=

Tx I

mx<J) (7)

where

T = transmissivity, in feet squared per day; 

m = aquifer thickness, in feet; and 

I = hydraulic gradient (dh/dl), in feet per foot.

UPPER FLORIDAN AQUIFER

Measurements of head in the Floridan aquifer system 

are chiefly confined to the upper part of the aquifer 

system, where most wells were constructed. Maps of the
GROUND-WATER MOVEMENT IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G35

150

100

so

 SEGMENT SEGMENT

MAY 1980'

 SEGMENT IV50 100 150

MILES FROM POLK CITY

200

FIGURE 18. Estimated predevelopment and May 1980 hydraulic gradients along flow lines from Polk City to Key Largo and Fort Lauderdale,

Upper Floridan aquifer. (Location of segments shown in fig. 11.)

potentiometric surface of the Upper Floridan aquifer 

(see map for 1980, fig. 4) are periodically prepared by the 

U.S. Geological Survey and other government agencies 

from measurements of water levels in many wells 

throughout the State. The maps show the configuration 

of the contours representing the prevailing isopotentials, 

and ground-water movement is downgradient (that is, 

from areas of high potential to areas of low potential) and 

approximately normal to the contours if the aquifer 

system is assumed to be isotropic. Movement of ground 

water in the Upper Floridan aquifer is shown by flow 

lines radiating generally from the area of highest head in 

central Florida near Polk City (fig. 4). South of the Polk 

City potentiometric surface high, the head in the aquifer 

declines gradually along a flow line that divides flow to 

the Atlantic Ocean on the east and the Gulf of Mexico on 

the west.

To gain insight into the rate of ground-water movement southward from the Polk City potentiometric surface high in central Florida, estimates of velocities and 

transit times were calculated for estimated predevelopment hydraulic gradients along selected segments of flow 

lines from Polk City to Key Largo and to Fort Lauderdale (fig. 18).

The estimates are further based on the assumptions 

that (1) predevelopment hydraulic gradients were unaffected by changes in sea level and climate, (2) the 

transmissivity (T) of the Upper Floridan aquifer is 

distributed according to Bush (1982, fig. 6), (3) the 

thickness of the aquifer (m) is 500 ft, and (4) the effective 

porosity (<)>) is 0.30 (table 7) and vertical flow is negligible. Segment I represents the flow along 75 mi from Polk 

City to about the southernmost point of recharge by 

sinkhole lakes in central Florida. Segment II represents
G36 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

TABLE 7 . Estimated transit times along flow lines from the Polk City 

potentiometric surface high to Key Largo and to Fort Lauderdale 

under predevelopment gradients, Upper Floridan aquifer

[Based on aquifer thickness of 500 feet and porosity of 0.30. Location of 

flow segments shown in fig. 11]

Flow

segment

I

II

Head

difference

(feet)

60.5

8.2

'4.1

'4.1

Distance

(miles)

75

72

'36

'36

Transmlsslvlty 

(feet squared

per

Minimum

100,000

'100.000

'10.000

day)

Maximum

250,000

'250.000

'50.000

Velocity 

(feet

per

Minimum

37

'5.2

'0.5

vear)

Maximum

93

'13

'2.6

Transit time

(ye,

Minimum

4.300

'15.000

'73.000

ars)

Maximum

11,000

'36,000

'380.000

88.000 416.000

III

I-III

IV

I. 11. IV

21.8

90.5

16.3

83.0

80

227

I>1

194

10.000 50,000 1.2 6.3 67,000

159,000

10.000 50.000 1.4 7.0 35.000

127.000

352.000

779.000

177.000

604.000

'Segment II divided Into two subsegments.

the flow along 72 mi from the southernmost point of 

recharge to the center of southern Florida. Segment III 

represents the flow along 80 mi from the center of 

southern Florida to Key Largo. Segment IV represents 

the flow along 47 mi from the center of southern Florida 

to Fort Lauderdale.

For estimated predevelopment gradients and an 

assumed average porosity of 0.30, a particle of water 

traveling 227 mi from Polk City to Key Largo (segments 

I through III) would be in transit for a time ranging from 

159,000 to 779,000 yr, depending on the estimated transmissivity. A particle of water traveling 152 mi from the 

southernmost point of recharge in central Florida to Key 

Largo (segments II and III) would be in transit from 

155,000 to 768,000 yr. A particle of water traveling 119 

mi from the southernmost point of recharge to Fort 

Lauderdale (segments II and IV) would be in transit 

from 123,000 to 593,000 yr. Locally, a particle of water 

traveling 47 mi (segment IV) from the center of southern 

Florida (site 10) to Fort Lauderdale (site 9) would be in 

transit from 35,000 to 177,000 yr. Only the transit time 

between sites 10 and 9 (segment IV) is within the 

40,000-yr useful range of carbon-14 dating. Therefore, 

the occurrence of measurable carbon-14 activity in the 

brackish ground water in southern Florida strongly 

suggests that infiltrating freshwater in central Florida is 

not the only source.

LOWER FLORIDAN AQUIFER

Estimates of movement of ground water in the Lower 

Floridan aquifer have chiefly been based on indirect 

evidence because measurements of representative head 

are difficult to obtain. Attempts to calculate hydraulic 

gradients between a few closely spaced wells for which 

data are available have largely been unsuccessful owing

TABLE 8. Comparison of pressure head and related data between 

well G-2334 at site 9 and well G-2296 at site 10, Boulder Zone of 

the Lower Floridan aquifer

[Site locations shown in fig. 2]

Average Dissolved Density2

Well Site temper- solids 1 Salinity (grams per Water Pressure

No. No. ature (milligrams (per cubic level 3 head4

(°F) per liter) mil) centimeter) (feet) (feet)

G-2334 

G-2296

9 

10

62.2 

78.8

37,500 

37,500

36.62 

36.62

1.02683 

1.02427

0.2 2,875.3 

7.0 2,875.1

"Assumed to be the same for both wells. 

2 At prevailing salinity and temperature. 

3 Referred to sea level.

4 At 2,800 feet below sea level; density equals 1.00000 g/cm3 (gram per 

cubic centimeter).

to the extremely high transmissivity of the waterbearing zones in the Lower Floridan aquifer, to the 

effects of short-term tidal-caused fluctuations in water 

levels, and to variations in water density (Meyer, 1974; 

CH2M Hill, Inc., 1981; Singh and others, 1983).

In recognition that the hydraulic gradient for a transmissivity of about 2.5xl07 ft2/d (Singh and others, 1983) 

would be very small, the head in well G-2296 at site 10 

near the center of southern Florida was compared with 

the head in well G-2334 at site 9 in Fort Lauderdale 

about 44.5 mi east of well G-2296 at site 10 (table 8). The 

water level in well G-2296 was measured at 7.0 ft above 

sea level during a packer test on March 4, 1981. It was 

assumed that fluctuations resulting from tides were 

insignificant at well G-2296 because of its great distance 

from the coast. The water level in well G-2334 was 

continuously recorded during May 16-20, 1983, after a 

performance test on May 10, 1983 (fig. 19). Semidiurnal 

tides caused the water level to fluctuate about 0.5 ft, but 

the daily mean water level rose about 0.6 ft during the 

period as the density of the water column decreased 

owing to heating. The water level was about 0.2 ft above 

sea level on May 26, 1981, when a temperature survey 

(log) of the water column was obtained (fig. 6).

The density of the borehole fluid in each well was 

calculated from oceanographic tables (U.S. Navy, 1962) 

based on measurements of salinity (dissolved solids) and 

temperature, and the pressure heads were calculated for 

each well at a common depth of 2,800 ft below sea level 

for a common fluid density of 1.0000 gram per cubic 

centimeter (g/cm3). Salinity was based on the dissolved 

solids residue upon evaporation at 180 °C in a sample of 

water from well G-2334 at site 9. The lack of precision in 

the dissolved solids analyses required an assumption of 

uniform salinity. The average temperature of the borehole fluid (salinity comparable to seawater) in each well 

was determined from temperature logs.

The pressure head at well G-2334, site 9, was about 0.2 

ft higher than that at well G-2296, site 10. The apparent
GROUND-WATER MOVEMENT IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G37

SEA 

LEVEL

Q LLI

t CE

tj ° 

<

FORT LAUDERDALE SITE 9 WELL G-23340W-3)

NOTE:

WATER LEVEL ON MAY 10 AT 2015 HOURS,

1 HOUR AFTER PUMPING CEASED.

WAS MINUS 3 12 FEET

MEASURED 

+ 0.2 FOOT

PROJECTED «SE_

WATER LEVEL RISING IN RESPONSE

TO DECREASING DENSITY (HEATING)

OF THE WATER COLUMN

20 21

MAY 1983

FIGURE 19. Hydrograph of water level (saltwater) in well G-2334 at site 9 near Fort Lauderdale, Boulder Zone of the Lower Floridan aquifer.

(Location of site 9 shown in fig. 2.)

inland hydraulic gradient, I, was estimated by the 

equation

0.2 foot Imile 

"44.5 miles X 5,280 feet

=8.5xlO~7

(8)

The average water velocity, v, was estimated by the 

equation

Txl v= -rx365 (9)

where

v = average water velocity, in feet per year;

T =2.5xl07 ft2/d;

m = 650 ft;

I = 8.5xlO~7 ;and

cj> = porosity, ranging from 0.2 to 0.4. 

Based on the estimated hydraulic gradient (8.5x 10~7), 

the average velocity of ground-water flow through the 

Boulder Zone from site 9 to site 10 ranges between 59.7 

and 29.8 ft/yr for porosities ranging from 0.2 to 0.4, and 

the transit time ranges from 3,900 to 7,900 yr.

A comparison of the estimated hydraulic gradient, 

velocity, and transit time from water-level measurements at sites 9 and 10 with those estimated by radiocarbon dating suggests that the calculated difference in 

head may be too high (table 9, measurement 1). The 

estimated gradients and velocities based on transit time 

from radiocarbon dating suggest a gradual decrease in 

gradient and velocity during the past 7,500 yr. The 

present-day gradient and velocity would probably be 

closer to 3.8xlO~7 and 17 ft/yr, respectively. The difference in head between the wells, located 44.5 mi apart, 

would then be only 0.09 ft a relatively small value 

compared with the range of tidal fluctuations at the coast 

and with the corrections in head for differences in fluid

density. Another possibility is that the ground-water 

velocity calculated by equation 9 is on the basis of 

porous-medium concept. However, ground-water flows 

through the Boulder Zone may be best described by 

conduit or fracture flow, which normally is faster than 

flow in porous media. No acceptable fracture flow equations are available to calculate such velocity.

EFFECTS OF RISING SEA LEVEL ON 

GROUND-WATER MOVEMENT

Sea level is generally accepted as the lower limit for 

hydraulic gradients in natural surface-water and groundwater flow systems. During periods of constant climate 

(no changes in rainfall and temperature) and rising sea 

level, freshwater in aquifer storage would gradually be 

displaced by seawater. During periods of constant climate and falling sea level, seawater in aquifer storage 

would gradually be displaced by freshwater.

TABLE 9. Estimated hydraulic gradients and related hydraulic data 

based on measured water levels and radiocarbon dating, Boulder 

Zone of the Lower Floridan aquifer

[Based on transmissivity of 2.46x 107 feet squared per day, thickness of 

650 feet, and porosity of 0.30. Site locations shown in fig. 2]

Measurement 

No.

1

2

3

Referenced 

segment

Site 9 to site 10 1

Site 9 to site 10 2

Subsea outcrop to

Distance 

(miles)

44.5

44.5

10.5

Head 

difference 

(feet)

0.2

.28

.021

Hydraulic 

gradient 

(feet per 

foot)

8.5 x 10"'

1.2 x 10" 6

3.8 x 10" 7

Average 

velocity 

(feet per 

year)

39.0

54.6

17.3

Transit 

time 

(years)

6,025

4,300

3,200

Subsea outcrop to 

site 10*.

7,500

Calculation based on fluid density, water-level measurements, and

Calculation based on radiocarbon age and assumed aquifer characteristic
G38 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

PLEISTOCENE

SANGAMON 

INTERGLACIATION

WISCONSIN 

GLACIATION

SEA 

LEVEL

-100

-200

-300

-400

-500

-600

-700

120 100 60 60 40 

YEARS BEFORE PRESENT X 1000

FIGURE 20. Sea-level fluctuations during the late Pleistocene and 

Holocene Epochs.

Sea-level fluctuations during the past 150,000 yr (Cronin, 1983) range from about 23 ft above present sea level 

at about 140,000 yr B.P. (before present) to about 330 ft 

below present sea level at about 18,000 yr B.P. (fig. 20). 

High stands of sea level represent interglacial stages, 

and low stands represent glacial stages. The last rise in 

sea level, called the Holocene transgression, began about 

18,000 yr ago and is continuing, although at a much 

slower rate- Predevelopment hydraulic gradients of the 

Floridan aquifer system in the late 1800's were, therefore, temporary and do not relate directly to antecedent 

flow conditions. However, flow rates based on radiocarbon dating (or the relative carbon-14 ages of water) 

reflect average antecedent hydraulic gradients. Therefore, estimates of hydraulic gradients, velocities, and 

transit times based on carbon-14 dating should consider 

the effects of changes in sea level, climate, topography, 

and perhaps permeability.

Data on paleoclimates, paleohydraulic gradients, paleotopography, and paleopermeability are generally lacking; however, data on changes in vegetative types during 

the past 8,500 yr in central Florida and southern Georgia 

(Watts, 1971) suggest that changes in forest types probably relate to rising sea level. Watts (1971, p. 676) 

reported that pollen studies of bottom sediments in 

upland lakes in central Florida and southern Georgia 

snowed that from about 8,500 to 5,000 yr B.P. (based on 

radiocarbon age) the vegetation was chiefly oak (Quercus) and that for the past 5,000 yr, the vegetation has 

chiefly been pine (Pinus). Prior to 8,500 yr B.P., a long 

hiatus occurred in sedimentation (about 27,000 yr) which 

probably resulted from greatly lowered water levels 

during the Wisconsin glaciation. Watts (1971, p. 686)

150

LAND 

SURFACE

100

50

SEA 

LEVEL

-50

-100

-150

-250

-300 -

HIATUS IN VEGETATION OAK

VEGETATIVE HISTORY BY WATTS, 1971

ESTIMATED HIGHEST WATER

LEVEL IN UPPER \ 

FLORIDAN AQUIFER ^x. / 

IN CENTRAL FLORIDA > 

(POLK CITY HIGH) /

INLAND TRANSIT TIME 

(YEARS BEFORE PRESENT X 1000)

PINE

/ / <_ £-

, , ESTIMATED SEA LEVEL 

//(Modified from unpublished data

from A.C. Neuman, 1982;

from Milliman and Emery, 1968;

and Cronin, 1983)

PLEISTOCENE 

WISCONSIN GLACIATION

I________I

HOLOCENE

25 20 15 10

YEARS BEFORE PRESENT X 1000 

(1950)

FIGURE 21. Comparisons of water level in central Florida, vegetation, and transit distance from subsea outcrop with sea level 

during the Holocene transgression. (Location of sites shown in 

fig. 2.)

estimated that during the hiatus, the water table was 

probably at least 40 ft below that of today, and he 

concluded that the changes in vegetation were chiefly 

caused by rising water tables and sea level rather than 

by increased rainfall.

The estimated rise in sea level during the Holocene 

transgression is shown in the conceptual cross section 

(fig. 21), along with the hypothetical rise of the water 

level in the Upper Floridan aquifer at the center of the 

Polk City high and the changes in forest vegetation 

according to Watts (1971). The hypothetical relationships 

suggest that at the beginning of the Holocene transgres-
GROUND-WATER MOVEMENT IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA

C' C

G39

C'

FEET

200 -i

SEA 

LEVEL

500 -

1,000

1.500

2,000

2.500

3,000

3,500

4,000

FEET

LOWER CONFINING UNIT 

(EVAPORITE DEPOSITS)

- 3.500

4,000

EXPLANATION 

 POTENTIOMETRIC SURFACE

FRESHWATER 

SALTWATER

FIGURE 22. Hypothetical hydrogeologic sections through the Floridan Plateau for (A) 18,000 years B.P. and (B) present.

sion, ground-water levels in central Florida were relatively high above sea level but low with respect to land 

surface. From 18,000 to 9,800 yr B.P., sea level was 

estimated to have risen about 0.03 ft/yr and water levels 

in central Florida were estimated to have risen about 

0.008 ft/yr; water levels during this period in central 

Florida were too far below land surface for appreciable 

forest growth. From 9,000 to 5,000 yr B.P., sea level 

probably rose about 0.01 ft/yr and ground-water levels in 

central Florida rose about 0.006 ft/yr; water levels 

during this period in central Florida were sufficiently 

near land surface (within 20 to 40 ft) to sustain growth of 

oak forests. From 5,000 yr B.P. to the present, both sea 

level and water level in central Florida probably rose at 

the same rate, about 0.004 ft/yr; water levels in central 

Florida were sufficiently near land surface (within 20 ft 

or less) to sustain growth of pine forests. Dissolution of 

limestone during the Holocene transgression probably 

resulted in a slight increase in the permeability of the 

Floridan aquifer system.

The retreat of the glaciers (and hence the rise in sea 

level) during the Holocene transgression probably 

lagged behind the change (global warming) in worldwide 

climate, and rainfall over the Floridan Plateau probably 

was subtropical. Drainage in central peninsular Florida 

was chiefly subsurface through the Floridan aquifer 

system, and the part beneath the exposed land mass 

probably was filled with freshwater (fig. 22A). As sea 

level rose (rapidly at first), seawater moved inland 

through the Floridan aquifer system and displaced the 

stored freshwater (fig. 225). Freshwater discharge to 

the Atlantic Ocean and the Gulf of Mexico by submarine 

springs decreased (principal discharge probably confined 

to the Straits of Florida). Continued subtropical rainfall 

maintained high water levels in central Florida so that 

the freshwater head was above land surface over most of 

the coastal areas. Circulation of freshwater in the Floridan aquifer system was restricted both laterally and 

vertically, and shallow circulation patterns developed in 

which karst features that earlier were sources of
G40 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

200

150

100

CARBON-14

20 15 10 5 0

YEARS BEFORE PRESENT X 1000

(1950)

FIGURE 23. Theoretical velocity distribution during the Holocene 

transgression, Boulder Zone of the Lower Floridan aquifer.

recharge to the system (during the low sea-level stand) 

became sources of discharge during the high sea-level 

stand. With high water tables, outflow (both surface 

water and ground water in the surficial aquifer) to the 

Atlantic Ocean and the Gulf of Mexico probably 

increased.

Results of radiocarbon dating of seawater from the 

Boulder Zone (in the Lower Floridan aquifer) suggest 

that the rise of movement inland may be directly related 

to the rise in sea level. Comparison of the estimated 

inland transit curve based on apparent carbon-14 ages of 

samples at sites 9 and 10 (see inset A in fig. 21) with the 

sea-level curve (fig. 21) suggests that seawater moved 

inland about 1 mi for each 1-ft rise in sea level. The 

velocity of seawater moving inland in south Florida 

probably was greatest at the beginning of the Holocene 

transgression and least at the end (present). The velocities, based on assumed aquifer characteristics (T=2.5 

xlO7 ft2/d, m=650 ft, and cj>=0.3) ranged from about 172 

to 5 ft/yr (fig. 23). The corresponding hydraulic gradients 

would, therefore, range from 3.7xlO~6 to l.lxl(T7 

(table 10), and the corresponding differences in head 

between sites 9 and 10 (44.5 mi apart) would be about 

0.87 and 0.026 ft. Again, these values are relatively small 

compared with transient effects of tides and changes in 

fluid density.

The distance traveled by a water particle moving 

inland through the Lower Floridan aquifer from the 

Straits of Florida during the Holocene transgression 

(330-ft rise x inland movement of 1 mi per 1 ft of rise =

TABLE 10. Estimated maximum and minimum velocities, hydraulic 

gradients, and head loss between sites 9 and 10 during the Holocene 

transgression, Lower Floridan aquifer

[Based on radiocarbon dating and theoretical relation between sea level 

rise and inland transit in fig. 23. Site locations shown in fig. 2]

Velocity

(feet per

year)

Hydraulic gradient

(feet per 

foot)

(feet per

mile)

Maxin 

Minin

3.7 

1.1

1.9 x 10 

5.8 x 10"

44.5

44.5

0.87

0.026

330 mi) would greatly exceed half the width of the 

Floridan Plateau (width (fig. 22A), 270 mi; half the 

width, 135 mi), suggesting that the movement of seawater inland must be accompanied by a circulation system, 

as proposed by Kohout (1965) (that is, movement upward 

through the middle confining unit and seaward through 

the Upper Floridan aquifer). The scenario of groundwater movement in southern Florida would, therefore, 

include (1) the continued rise in sea level and concurrent 

displacement of stored freshwater by inland-moving cool 

seawater chiefly through the Boulder Zone (the Lower 

Floridan aquifer), (2) heating of the seawater during 

inland transit (lowering of the density), (3) upward 

movement of the seawater through fractures and sinkholes that transect the middle confining unit, and (4) 

dilution and transport of the seawater back to the ocean 

by seaward-flowing freshwater in the Upper Floridan 

aquifer. However, the combined effects of heating and 

dilution alone probably would provide sufficient loss of 

head in the Boulder Zone to drive the circulation regardless of changes in sea level.

The time involved in the circulation is short by geologic 

standards but extremely long by man's standards. The 

circulation is best shown by flow lines in figure 24. 

Fractures and sinkholes, which transect the middle 

confining unit, hydraulically connect the Upper Floridan 

aquifer with the Boulder Zone (Lower Floridan aquifer) 

near the coastline. Relatively young, cold seawater flows 

into the Boulder Zone through the sinkholes and fractures, then is heated as it flows inland. Some saltwater 

moves upward into the Upper Floridan aquifer through 

sinkholes and fractures, where it is diluted and carried 

seaward by the flowing freshwater. The relative carbon14 activities and apparent ages of the ground waters in 

the Floridan aquifer system support the theory of this 

circulation, as does the uranium isotope data and the 

occurrence of anomalously cool saltwater in the Lower 

Floridan aquifer. Oxygen isotope data (table 4) show 

enrichment of oxygen-18 with respect to oxygen-16 

(8180) in the saltwater in the Lower Floridan aquifer and 

a possible relation between oxygen-18 concentration and 

radiocarbon age. However, the data are insufficient to 

draw conclusions on paleotemperatures of the ocean 

during the Holocene transgression. The role that rising
GROUND-WATER MOVEMENT IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G41

A*

GULF

OF 

MEXICO <n

FEET

BIG CYPRESS 

SWAMP

SITE 10

(ground-water divide 

is near this site) EVERGLADES

o

z>

3 SITE 9

STFtAITS

OF 

FLORIDA

FEET

MIAMi" !F!$if^tibl^^ SAND?

rnimSMmm HAWTHORN FORMATION m%&

^^^^Si^^^^^in^vKm^^^^^^f^^^^^^^i

V

94 (OCEAN WATER) 

Number is percent

V. , ""

2,500

3,500 

4,000

 3.500

4,000

10 20

10 20 30 KILOMETERS 

SCALE APPROXIMATE

30 MILES

I

-80-

EXPLANATION

SURFICIAL AQUIFER 

CONFINING UNITS 

TOP OF SEAWATER-LIKE SALINITY 

LINE OF EQUAL GROUND-WATER TEMPERATUREInterval 10 degrees Fahrenheit

^ SALTWATER FLOW LINE 

 ^- FRESHWATER FLOW LINE

(4.5 SAMPLED ZONE-Number is percent of modern 

carbon

FIGURE 24. Idealized cross section A-A' through southern Florida showing ground-water circulation and carbon-14 activities, Floridan aquifer system.

(Line of section shown in fig. 2.)

sea level plays in circulation is not well documented, and 

more research is needed to fully evaluate such an effect.

UPWELLING GROUND WATER AS EVIDENCE OF 

CIRCULATION

The existence of upwelling warm, salty ground water 

in the Upper Floridan aquifer in southern Florida is 

further evidence that vertical circulation does occur. At 

least five temperature anomalies in southern Florida 

have been identified and discussed by various investigators (fig. 25). Most of the reported anomalies are on the

southwestern coast of Florida, where the Upper Floridan aquifer is an important source of supply (and consequently there is a higher density of deep artesian wells) 

and where temperatures in the underlying Lower Floridan aquifer are highest. On the southeastern coast, 

temperature anomalies in Martin County were first 

reported by Lichtler (1960, p. 62) and expanded on by 

Sproul (1977, p. 84); data on temperature and salinity 

anomalies in St. Lucie County were later reported by 

Reece and others (1980).

The temperature anomalies in the Upper Floridan 

aquifer in the Martin County-St. Lucie County area (fig. 

26) are indicated by composite temperatures of ground
G42 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

87

28

 100 LINE OF EQUAL WATER

TEMPERATURE OF THE LOWER 

FLORIDAN AQUIFER-Interval 10 

degrees Fahrenheit

TEMPERATURE ANOMALY IN THE 

UPPER FLORIDAN AQUIFERNumber Indicates location and source:

1. Mudhole Spring (Kohout and others. 

1977, p. 25)

2. Warm Mineral Spring (Kohout and 

others, 1977. p. 22)

3. McGregor Isles (Sproul, 1977. p

4. Peace River Estuary (Sproul. 1977, p. 86)

5. Martin County-St. Lucie County area 

(this report)

SUBMARINE TOPOGRAPHY (Uchupi. 1966)-Shows 

depth below sea level. Contour Interval. In feet. 

Is variable

FLORIDA AND STUDY AREA

FIGURE 25. Location of some temperature anomalies in the Upper Floridan aquifer and isotherms in the Lower Floridan aquifer, south Floridan Plateau.

water from wells ranging in depth from about 800 to 

1,200 ft, exceeding the background temperature of about 

80 °F by as much as 10 °F. Temperatures that are above 

normal occur in wells aligned generally in a 

northwesterly-southeasterly trend, extending from the 

northwestern corner of St. Lucie County to the southeastern corner of Martin County. Above-normal temperatures also occur in the vicinity of south-central Martin 

County. The temperatures of the anomalies compare 

closely with the temperatures of saltwater in the underlying Boulder Zone of the Lower Floridan aquifer. Also, 

the salinity (dissolved solids) of the ground water in the 

Upper Floridan aquifer near the temperature anomalies 

is locally higher (fig. 27), thereby providing additional

evidence of upwelling from the Boulder Zone. The 

northwesterly-southeasterly trend of the salinity and 

temperature anomalies suggests that the source likely is 

water upwelling through sinkholes or vertical solution 

pipes, which are associated with major linears (fractures 

or perhaps faults) in the Tertiary limestone. During low 

stands of sea level, the same features probably facilitated 

recharge to the underlying Boulder Zone. The presence 

of brackish ground water with anomalously high carbon14 activity in the Upper Floridan aquifer in southern 

Florida also is evidence that upwelling is widespread. 

The theory of inland circulation of seawater and subsequent mixing with seaward-flowing freshwater is preferred to the theory that saltwater in the Floridan
SUBSURFACE STORAGE IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G43

INDIAN RIVER CO 

LUCIE CO EXPLANATION

80 LINE OF EQUAL TEMPERATURE FOR 

GROUND WATER IN THE BOULDER 

ZONE OF THE LOWER FLORIDAN 

AQUIFER-Dashed where approximately 

located. Interval 10 degrees Fahrenheit

75-- LINE OF EQUAL TEMPERATURE FOR 

GROUND WATER IN THE UPPER 

FLORIDAN AQUIFER-Dashed where approximately located. Interval 5 degrees 

Fahrenheit

DIRECTION OF GROUND-WATER 

MOVEMENT, UPPER FLORIDAN 

AQUIFER

WELL-Number is temperature in degrees 

Fahrenheit

HIGHLANDS COJ 

GLADES CO

ST LUCIE CO 

MARTIN CO

MARTIN CO

FALMBEA H co

UMAHATCHEE 

GLADES CO 

HENDHY CO

30 KILOMETERS

FIGURE 26. Temperature of ground water in the Upper and Lower Floridan aquifers, Martin County-St. Lucie County area.

aquifer system in southern Florida is connate or residual 

seawater from high stands of sea level.

SUBSURFACE STORAGE IN THE FLORIDAN 

AQUIFER SYSTEM IN SOUTHERN FLORIDA

Subsurface storage is the practice of emplacing fluids 

in permeable underground rocks (aquifers) by gravity 

flow or pressure-induced injection through wells. The 

receiving rocks must have sufficient confinement, porosity, and permeability to accept the fluids without endangering underground sources of drinking water. In most 

cases, the fluids are nontoxic liquid wastes that cannot

easily be disposed of at the surface. In some cases, 

however, the fluids are valuable and are temporarily 

emplaced underground for later recovery. The subsurface storage practice is commonly referred to as "underground injection," "deep disposal," and "deep-well injection."

The practice of injecting nontoxic liquid wastes into 

saline parts of the Floridan aquifer system began in 1943 

at an oil field in Collier County (fig. 28, site 1); oil field 

brine was injected into the cavernous, saltwater-filled 

Boulder Zone of the Lower Floridan aquifer (Vernon, 

1970). The injection of treated municipal wastewater into 

brackish zones of the Upper Floridan aquifer began in 

1959 at a wastewater-treatment plant (fig. 28, site 14) in
G44 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

EXPLANATION

 500-- LINE OF EQUAL TOTAL DISSOLVED SOLIDS FOR GROUND 

WATER IN THE UPPER 

FLORIDAN AQUIFER-Dashed 

where approximately located. Interval 500 milligrams per liter

4j|j WELL-Upper number is total dis- 

 solved solids, in milligrams per 

liter. Lower number is depth, in 

feet. Queried where unknown. 

E, estimated

30 KILOMETERS

FIGURE 27. Dissolved solids in ground water from the Upper Floridan aquifer, Martin County-St. Lucie County area.

Broward County (McKenzie and Irwin, 1984). Injection 

of treated municipal waste water into the saltwater-filled 

Boulder Zone began in 1971 at a wastewater-treatment 

plant (fig. 28, site 16) in Dade County (Meyer, 1974). 

Injection of industrial liquid wastes (chiefly acetic acid) 

into brackish zones of the Upper Floridan aquifer began 

in 1966 at a furfural plant (fig. 28, site 15) in Palm Beach 

County (Kaufman and McKenzie, 1975).

Prior to 1970, regulation of injection wells was a 

principal function of the Florida State Board of Health 

(chapter 170C-3, Florida Administrative Code), and permits were issued as though the injection well were a 

drainage well. The criterion for issuing the permit was 

that the receiving rocks contain water that was nonpotable and salty (water having a chloride concentration of 

1,500 mg/L or more). Subsequent assignment of the 

permitting function to the Florida Department of Pollution Control in about 1970 led to more stringent regulation, and permits were issued only after thorough review 

by the Florida Department of Natural Resources, the 

State Board of Health, and the local Water Management 

District in consultation with the U.S. Geological Survey. 

As injection wells proliferated in the early 1970's, the 

Federal Government became increasingly concerned 

about the impact of deep-well disposal practices on 

drinking-water supplies. In 1974, Congress passed the 

Safe Drinking Water Act (Public Law 93-523, as 

amended by Public Law 95-190), which required the
SUBSUEFACE STOEAGE IN THE FLOEIDAN AQUIFEE SYSTEM IN SOUTHEEN FLOEIDA G45

I INDIAN RIVER

MANATEE HARDEE

| - j- - -j HIGHLANDS

LAKE OKEECHOBEE

EXPLANATION

14 SITE NUMBER IN TEXT 

OILFIELD

(§) MUNICIPAL WASTEWATER DEEP-WELL INJECTION 

SYSTEM

INDUSTRIAL DEEP-WELL INJECTION SYSTEM 

FRESHWATER INJECTION PILOT PROJECT

25° 

FIGURE 28. Location of oil fields, municipal and industrial deep-well injection systems, and freshwater injection pilot projects, southern

Florida.
G46 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

U.S. Environmental Protection Agency (EPA) to 

develop and publish regulatory and minimum requirements to control underground injection. The regulations, 

called UIC (underground injection control) regulations, 

were published in the Congressional Federal Register on 

June 24, 1980 (chapter 40, parts 122 and 146). Responsibility for development and enforcement of the UIC 

regulations along the lines established by the EPA, for 

all but Class II injection wells, was delegated to the 

Florida Department of Environmental Regulation in 

1983. The regulation of injection wells associated with oil 

and gas production, Class II injection wells, is administered by the Florida Bureau of Geology, Florida Department of Natural Resources (chapter 377, Florida Statutes and Rules 16C-2 and 16C-26 through 16C-30, 

Florida Administrative Code), and by the EPA.

All injection wells other than those associated with oil 

and gas production are regulated by the Florida Department of Environmental Regulation (chapter 17-28, Florida Administrative Code). The purposes of the UIC 

regulations are to protect the quality of the State's 

underground sources of drinking water and to prevent 

degradation of the quality of other aquifers adjacent to 

the injection zone. They regulate the location, construction, operation, and monitoring of injection wells so that 

the injection does not interfere with any designated use 

of ground water or cause violations of water-quality 

standards for underground sources of drinking water. An 

underground source of drinking water is defined by the 

State as an aquifer or its portion that supplies drinking 

water for human consumption, or is classified by rule 

17-3.403, Florida Administrative Code, as Class G-I or 

G-II water, and is not an exempted aquifer. In general, 

ground water having a dissolved solids concentration of 

10,000 mg/L or less is protected by the UIC regulations.

This section of the report is concerned with Class I 

injection wells, those that are used to inject municipal 

and industrial wastewater, Class II injection wells, those 

that are used to inject oil field brine, and Class V, Group 

2, injection wells, those that are used to inject freshwater for storage.

OIL FIELD BRINE

Since the discovery of oil in southern Florida in 1943 at 

a field in Collier County, 12 other oil fields have been 

discovered that have produced commercial amounts of 

crude oil (fig. 28). Oil is chiefly produced from the Lower 

Cretaceous limestone, called the Sunniland Zone by 

drillers, that underlies the region at depths ranging from

 OIL PRODUCTION 

o- - -o BRINE PRODUCTION 

(1 BARREL = 42 U.S. GALLONS)

CUMULATIVE OIL PRODUCTION

1943-83 77.3X106 BBL

(3.25X109 GAL)

CUMULATIVE BRINE PRODUCTION

1943-83 193X106 BBL

(8.11X109 GAL)

600

500

400

200

FIGURE 29. Brine and oil production for southern Florida oil fields,

1943-83.

11,000 to 12,000 ft. Along with the crude oil produced 

are large quantities of saltwater, called brine. The brines 

are several times saltier than seawater, and small 

amounts spilled on the surface can render a potable 

water supply useless for many years. Analyses of 

selected oil field brine are shown in table 11. Chloride 

concentrations range from 108,000 to 164,570 mg/L, 

compared with about 19,200 mg/L for seawater.

Total oil production for the 13 fields during 1943-83 

was 77.3 million barrels (bbl) (3.2 billion gallons (Ggal)), 

and brine production was 193.2 million bbl (8.1 Ggal). 

The largest producer of oil and brine (fig. 28, site 4) 

yielded 35 million bbl of oil and 73 million bbl of brine 

during 1966-83. The next largest producer (fig. 28, site 1) 

yielded 18 million bbl of oil and 52 million bbl of brine 

during 1943-83. During 1943-66, the ratio of brine to oil 

was relatively stable, as production was mostly from the 

field shown as site 1 in figure 28. Subsequent oil production at site 4 resulted in significantly greater amounts of 

brine, and in 1971 oil production leveled off while brine 

production continued to increase exponentially (fig. 29). 

Ultimately, oil production began to decline in 1977, and 

brine production continued to rise. The brine-to-oil ratio 

in 1983 was 6.4 to 1.0 compared with a 1 to 1 ratio in 1964.
SUBSURFACE STORAGE IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G47

TABLE 11. Selected analyses of oil field brine, southern Florida

[Concentrations in milligrams per liter. Remarks: BM, U.S. Bureau of 

Mines; GS, U.S. Geological Survey; PL, private laboratory; SDS, 

saltwater disposal system. Site locations shown in fig. 28. Dashes 

indicate no data]

Site No. Date Calclun Magnesium Sodium Potassium Chloride

1 12/43 25.204 3.110 58.491 4.700 143.601 

12/77 31.700 4.070 65.600 -- 164.570 

6/55 23.800 3,400 48.300 3,150 129.000

2 H/7/55 6.910 3,010 53.500 2.030 108,000 

6/19/59 27.730 4.080 50.980 350 140,000 

8/1/65 27.700 4.770 56.900 3,950 152,000

3 11/6', 21.100 2.880 55.600 2,850 131.000 

11/13/64 21.600 2.970 51,500 2.920 129.000

4 2/1/78 23.165 3.699 65.154 -- 152.000 

2/1/78 23.165 3.946 62.730 -- 149.000

7 17/29/77 28.448 4.439 60,292 -- 156,000 

12/29/77 27,635 5.425 57.445 -- 153.000

9 12/29/77 26,010 4,192 62,896 -- 155,000

DlsSlte Date Sul- solved Remarks 

No. f.ite solids

I 12/'.3 275 230.827 Specific gravity 1.162 at 60.1'F. Drill stem

ysls by BN. References: Gunter (1945, 

p. 18); Babcock (1962, p. 20). 

12/77 215 -- Composite injection into SDS No. 1, well 2. 

Permit 102. Analysis by PL. Source: Exxon 

Co. 

6/55 139 >207,000 Density 1.16 g/cm' (grams per cubic centimeter) at 68.0'F. Drill stem test for 

nonproduclng wildcat. Permit 222. Analysis 

by GS, No. 8655.

2 11/7/55 1.380 >175.000 Density 1.134 g/cm' at 68.0'F. Pumped sample. 

Permit 167. Analysis by GS. No. 8016. 

6/19/59 408 246,000 Drill stem test. Pernlt 278. Analysis by GS. 

No. 17682. 

8/1/65 665 >2S4.000 Density 1.204 g/cm' at 68.0'F. Drill stem 

test. Permit 331. Analysis by GS. No. 

NSF-546.

3 11/64 1,030 271.000 Drill stem test. Permit 314. Analysis by CS. 

No. NSF-170. 

11/13/64 415 >209.000 Pumped sample. Pernlt 315. Analysis by GS. 

No. OKE-19.

4 2/1/78 140 >244.000 Specific gravity 1.171 at 68.0'F. Composite 

injected into SDS No. 1, well 1. Permit 

491. Analysis by PL. Source: Exxon Co. 

2/1/78 140 >239.000 Specific gravity 1.170 at 68.0'F. Composite 

injected into SDS No. 2. veil 1. Permit 

748. Analysis by PL. Source: Exxon Co.

7 12/29/77 130 >249.600 Specific gravity 1.177 at 73.0'F. Composite 

injected into SDS No. 2. well 1. Pernlt 

856. Analysis by PL. Source: Exxon Co. 

12/29/77 140 >244.000 Specific gravity 1.176 at 73.0'F. Composite 

Injected into SDS No. 1, well 1. Pernlt 

761. Analysis by PL. Source: Exxon Co.

9 12/29/77 l/iO >248,500 Specific gravity 1.176 at 73.0'F. Composite 

injected Into SDS No. 1. well 1. Pernlt 

812. Analysis by PL. Source: Exxon Co.

TABLE 12. Summary of brine production, and disposal for oilfields 

in southern Florida, 1943 83

[Volume of brine in barrels. Operator: B. Burns; C, Commonwealth; E, 

Exxon; G, Gulf; K, Kanaba; NRM, Natural Resources Management; 

S, Sun; W, Weiner. No. of injection wells: B, Boulder Zone; P, 

Paleocene or older rocks; S, Sunniland Zone in Lower Cretaceous 

limestone. Dashes indicate no data. Site locations shown in fig. 28]

No. of 

Site Oper- Brine iniec- Brine disposal

No. ator Period production tion Boulder Sunniland 

wells Zone Zone

1 E 1943-73 51,879,210 3B 51,879,210 0 

2 C/G 1954-55 98,700 -- 98,700 0 

3 S 1964-83 29,963,400 1B.9S 13,862,112 16,101,288 

4 S/E 1966-83 72,722,755 3B.1S 69,428,463 3,294,292 

5 K 1 1969-83 0 -- 0 0 

2 6 E 1970-75 1,118,625 -- 1,118,625 0 

7 E 1972-83 18,369,565 2B.8S 14,215,772 4,153,793 

8 W/K 1973-78 289,106 -- 289,106 0 

9 E 1974-83 17,658,580 2B 17,658,580 0 

10 E 1977-78 19,458 -- 19,485 0 

11 B 1977-83 1,000,222 IB 1,000,222 0 

12 E 1977-83 13,880 IB 13,880 0 

13 NRM 1982-83 60,396 IP 0 3 60,396

Total 4 193,193,924 169,584,155 23,609,769 

(8,114) (7,122) (992)

'Formerly owned by Mobil. 

2This site was included in site 4 in 1975.

'injection occurs below the Boulder Zone in the open hole between 3,835 

and 11,074 feet.

4 Total in parentheses is shown in million gallons.

Some of the produced brine was used to repressure the 

oil-producing zone during 1966-83 to enhance oil recovery. This process is termed "water flooding" or "secondary recovery" and generally involves injection of the 

brine back into an abandoned oil well. About 23.6 million 

bbl (991 million gallons; Mgal) of brine were reinjected 

into the producing zone for water flooding. 

A summary of brine production, by oil field, is presented in table 12. During 1943-83, about 193.2 million 

bbl (8.1 Ggal) of brine were produced, of which 169.6 

million bbl (7. 1 Ggal) were injected into the Boulder Zone 

(Lower Floridan aquifer) and 23.6 million bbl (1.0 Ggal) 

were injected back into the oil-producing zone. 

Figure 30 shows the hydrogeology and construction 

details of typical oil field brine disposal wells at two oil 

fields (sites 12 and 1). The injection well at site 12 was 

constructed since establishment of the UIC regulations 

and incorporates current design criteria. The injection 

well at site 1 is a converted oil-production well with a 

cement plug in the lower confining unit of the Floridan 

aquifer system. Both wells, however, inject brine 

through perforations into the Boulder Zone (Lower 

Floridan aquifer). The main difference between the 

injection wells is that the injection well at site 12 has two 

strings of casing that extend from land surface to the 

middle confining unit of the Floridan aquifer system, 

whereas the well at site 1 has only one string of casing to
G48 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

4,000 

LAND SURFACE 1 |°

 

UNDIFFERENTIATED 

HOLOCENE AND PLIOCENE 

DEPOSITS

UNDIFFERENTIATED 

MIOCENE DEPOSITS

BRINE DISPOSAL WELL 

PERMIT NO. 1121 

SITE 12

INCHES 

86^202468 

II II

ii|

SUWANNEE 

LIMESTONE

OCALA LIMESTONE

AVON PARK 

FORMATION

I UPPER DOLOSTONE

\ MIDDLE DOLOSTONE

~ ^-OLDSMAR ~~ 

/ FORMATION

t

/ PERFORATIONS 1

/ LOWER // 

/ DOLOSTONE jf \

f

CEDAR KEYS 

FORMATION

LIMESTONE

'

!

S 

I

1

S

; : ;

£

10 12 10 

1 1 1

STEEL CASING 

 CEMENT

 - 95/fe-INCH 

STEEL CASING

7-INCH 

STEEL CASING

31/fc-INCH 

FIBERGLASS 

TUBING

±^" PACKER

J 5-INCH 

__ STEEL LINER

27/fe-INCH 

""FIBERGLASS TUBING

\

BOULDER 

ZONE

^ PACKER

ANHYDRITE

-

20-INCH ti. 

STEEL CASING

MIOCENE 

DEPOSITS

133/fe-INCH-~~~"~~~^ 

STEEL CASING 

CEMENT--

STEEL CASING

BRINE DISPOSAL WELL 

PERMIT NO. 15 

SITE 1

INCHES 

6642024 68 10 12 FEET 

II | III || LAND SURFACE 

e27/fe-INCH 

TUBING

UPPER DOLOSTONE 

PACKER -_

PERFORATIONS 1

LOWER / I 

DOLOSTONE L L

s-=

i

<

 »

A

-^==

11

BEE

r^r..

-.' .

' ' ' '.' 

k>xx 

%$&

%$$ v«s li %M

' ;' 

a

.".-.

 r 

?

' "JliJ SURFCIAL 

'; '. AQUIFER

' UPPER 

.' ' CONFINING

: UNIT

SUWANNEE 

. ' LIMESTONE

OCALA 

LIMESTONE

AVON PARK 

FORMATION

>j

, OLDSMAR 

/ FORMATION

BOULDER 

J ZONE

CEDAR KEYS 

FORMATION

 CEMENT 

PLUG

S

I

DC

t

1

*

LOWER FLORIDAN AQUIFER

LOWER 

CONFINING UNIT

- 4-INCH TUBING

- BENTONITE CLAY 

AND ROCK 

CUTTINGS TO 

11,506 FEET

 2,000

 3,000

FIGURE 30. Hydrogeology and typical construction of oil field brine disposal wells.

protect brackish ground water in the Upper Floridan 

aquifer. Also, the injection well at site 12 has associated 

with it a separate monitoring well (not shown in fig. 30) 

used to provide an early warning for leaks and upward 

migration of saltwater.

MUNICIPAL AND INDUSTRIAL LIQUID WASTES

Injection of municipal and industrial liquid wastes 

through wells into the Floridan aquifer system is prevalent in the southeastern part of the Florida peninsula 

(fig. 28). The start of this practice was mentioned previously in the report. Liquid wastes from both sources 

were injected into brackish water-bearing zones of the

Upper Floridan aquifer because the criteria at that time 

required only that the receiving rocks contain water 

having a chloride concentration of at least 1,500 mg/L. 

Problems ultimately developed with the operation of 

both systems. In the municipal wastewater injection 

system (site 14), the low transmissivity of the aquifer and 

the high suspended solids in the injectant caused frequent plugging of the wellbore and excessive injection 

pressure (McKenzie and Irwin, 1984). In the furfural 

plant system (site 15), the hot acid waste migrated 

upward from the lower part of the Floridan aquifer to 

appear in a monitored zone near the top (Kaufman and 

McKenzie, 1975; McKenzie, 1976; Vecchioli and others, 

1979).
SUBSURFACE STORAGE IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G49

The practice of deep-well injection of liquid wastes 

became increasingly attractive in 1969 when a test 

injection well drilled at a wastewater-treatment plant 

(fig. 28, site 16) tapped the highly transmissive 

saltwater-filled Boulder Zone of the Lower Floridan 

aquifer. An evaluation of the natural water-level fluctuations in the well by Meyer (1974) suggested that the 

transmissivity of the Boulder Zone was about 3.2xlO6 

ft2/d; however, a later pumping test at a wastewatertreatment plant (fig. 28, site 22) suggested that the 

transmissivity was about 2.5xl07 ft2/d (Singh and others, 1983). The success of the injection well at site 16 

soon led to rapid exploitation of the Boulder Zone as a 

receptacle for nonhazardous municipal and industrial 

liquid wastes.

The characteristics of the Boulder Zone meet the 

current criteria for Class I injection wells as required by 

the Florida Department of Environmental Regulation 

(that is, the receiving zone is deep, confined, thick, 

porous, and highly transmissive and contains ground 

water whose dissolved solids concentration exceeds 

10,000 mg/L).

During 1959-70, the volume of liquid wastes injected 

into the Floridan aquifer system increased gradually 

from 98 to 465.6 Mgal/yr (fig. 31, table 13). In 1971, the 

volume of liquid wastes injected began to increase exponentially, and in 1983 it reached 26.8 billion gallons per 

year (Ggal/yr). The total amount injected for the 25-yr 

period (1959-83) was 112 Ggal. Of that, 4 Ggal were 

industrial liquid wastes (sites 15 and 20) and 108 Ggal 

were treated municipal wastewater.

FIGURE 31. Annual and cumulative volumes of municipal and industrial liquid wastes injected, 1959--83.

The injected industrial liquid waste at site 15 is chiefly 

acetic acid, a byproduct of the production of furfural. 

Neutralization of the acid waste takes place in the 

receiving zone by dissolution of the carbonate rocks and 

release of carbon dioxide. High concentrations of biogenie hydrogen sulfide and methane also result from 

reaction in the receiving zone. Characteristics of the

TABLE 13. Summary of municipal and industrial injection of liquid wastes in southern Florida, 1959 83

[In million gallons. Site locations shown in fig. 28]

Site 14' Site 16 Site 19 Site 20 Site 23

Yearly 

total

1959 

1960 

1961 

1962 

1963 

1964 

1965 

1966 

1967 

1968 

1969 

1970 

1971 

1972

1973

1974

1975

1976

1977

1978

1979

1980

1981

1982

1983

Total

98 

182 

182 

182 

182 

219 

?! » 

219 2 

182 45 

219 104 

219 121 

265 200 

223 213 

248 246

293 307

259 284

10 311

317

157

187

272

375

358

201

161

3.401 3.868

0 

4 

6 

8 

6 

4 

5

5

3

1

6

6

8

1

6

4

3

1

7

577.0 

1,046.0

1.275.0

1.341.0

1.537.0

.732.0

.715.5

,734.8

.957.7

,723.5

.754.8

1.856.0

139.3

18,389.6

179.5

483.9

582.3

531.4

646.8

902.1

1.134.5

1.006.5

1,277.9

1.022.0

82.3

7,849.2

570.6

1.299.4

1.284.0

1,415.1

1.671.3

1.816.6

1,756.7

1,850.6

1.993.6

2.001.3

15.659.2

175.7

4.253.3

5.673.7

8,531.5

8,910.2

10.639.3

11.125.1

49,308.8

0.2

20.4

44 . ?

66.8

55.3

186.9

1 

1

2

2

3

3

4

8

10

13

14

81.2 15

742.5 12.376.9 144.6 26

823.7 12.376.9 144.6 112

98 

182 

182 

182 

182 

219 

219 

221 

227.4 

323.6 

340.8 

465.6 

.013.4 

.540.5

.055

,938

.739

,865

,110

.749

.854

.414

.196

.860

.828

.008

0

8

8

0

7

3

8

2

1

2

4

6

98 

280 

462 

644 

826 

1.045 

1.264 

1.485 

1.712.4 

2,036.0 

2.376.8 

2,842.4 

3.855.8 

5.396.3

7.451.3

10.390.1

14.129.9

17.994.9

22.105.6

30.854.9

41.709.7

55.123.9

69,320.0

85.180.2

112.008.6

'Estimated except for 1969 through 1974.
G50 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

TABLE 14. Selected water-quality characteristics of injectant of 

industrial liquid wastes, local water supply, and native ground 

water in the Boulder Zone at site 15, Palm Beach County

[Concentrations in milligrams per liter, except where noted; analyses 

by U.S. Geological Survey. Location of site 15 shown in fig. 28. 

Dashes indicate no data]

Ground water

Boulder Zone*

Ma I or Inorganics and related physical characteristics

Acidity, as H* 208 0 0 

Bicarbonate (HCO,) 0 ISO 200 

Calcium (Ca) 140 44 430 

Chloride (Cl) 160 99 19.000 

Dissolved solids 9.720 >380 36.100 

(residue at 180'C). 

Magnesium (Mg) 63 21 1.300 

pH (units) 2.9 8.6 7.9 

Potassium (K) 310 5 610 

Sodium (Na) 110 60 12,000 

Specific conductance 2,400 700 51. 500 

(nlcroslenens per 

centimeter). 

Sulfnte (SO.) 290 66 2.400 

Suspended solids 1,500

Temperature (*C) 4 75.0 26.5

Carbon, total organic 7,500 20

Nitrogen, total as N 138 1.6 

Phosphorus, total as P 47 .02

'Sample of plant effluent was collected on July 8, 1974.

pump station on April 18, 1974. 

'Sample was collected from Injection well 3 At 3,130 feet on June 29, 1976. 

4 Tenperature of effluent Is reduced to about SO.O'C prior to Injection.

TABLE 15. Selected water-quality characteristics of injectant of 

secondary-treated wastewater, local water supply, and native 

ground water in the Boulder Zone at site 1 9, Palm Beach County

[Concentrations in milligrams per liter, except where noted; analyses 

by U.S. Geological Survey. Location of site 19 shown in fig. 28. 

Dashes indicate no data]

Ground water 

Characteristic Injectant 1 Water supply2 in the 

Boulder Zone 3

Hal or Inorganics and related physical characteristics

Bicarbonate (HCO,) 120 150 180 

Calcium (Ca) 50 40 390 

Chloride (Cl) 240 78 21,000 

Dissolved solids 1,060 330 37,400 

(residue at 180°C). 

Magnesium (Mg) 16 8.9 1,300 

pH (units) 6.7 8.3 7.6 

Potassium (K) 15 3.1 450 

Sodium (Na) 160 45 12,000 

Specific conductance 1,220 550 >50,000 

(microsiemens per 

centimeter) . 

Sulfate (SO,) 110 33 2,800 

Suspended solids 12 -- 43 

(residue at 110°C) . 

Temperature (°F) 78.8 79.7 <17.5

Selected nutrients and related characteristics

Carbon, total organic 15 -- 8.2 

Nitrogen, ammonia as N .03 -- <.01 

Nitrogen, total as N 1.6 >.02 .00 

Phosphorus, total as P .08 -- .02

'Sample of effluent from site 19 collected on April 18, 1978.

2 Sample of raw surface-water supply (Clear Lake) for site 19 collected on 

May 9, 1979.

3 Sample of native ground water collected from injection well 2 at site 19 

on May 30, 1972. Density was 1.022 g/mL (grams per milliliter) at 68°F and 

hydrogen sulfide was 2.4 mg/L (milligrams per liter).

TABLE 16. Selected water-quality characteristics of injectant of 

secondary-treated wastewater, local water supply, and native 

ground water in the Boulder Zone at site 22, Dade County

[Concentrations in milligrams per liter, except where noted. Location 

of site 22 shown in fig. 28. Dashes indicate no data]

Ground water

Boulder Zone 3

Bicarbonate (HCO,) -- 260 146 

Calcium (Ca) -- 92 430 

Chloride (Cl) 65 25 19,000 

Dissolved solids 360 322 37,900 

(residue at 180°C). 

Magnesium (Mg) -- 3.2 1,200 

pH (units) 6.0 7.5 7.1 

Potassium (K) -- 1.7 200 

Sodium (Na) -- 16 1,100 

Specific conductance 700 540 52,900 

(microsiemens per 

centimeter) . 

Sulfate (SO,) -- 28 2,600 

Temperature4 (°F) 87.8 82.4 <19.0

Selected nutrients and related characteristics

Carbon, total organic 7.65 2.0 3.9 

Nitrogen, ammonia as N 17.5 .01 .12

Phosphorus, total as P 1.56 <.01 <.01

'Sample of treated effluent collected on August 14, 1984. Analyses by the 

Miami -Dade Water and Sewer Authority.

2 Sample of raw water from the Biscayne aquifer collected on June 6, 1975, 

at site 22. Analyses by the U.S. Geological Survey.

3 Sample of native ground water collected from monitor well BZ-1 between 

depth of 2,689 and 2,960 feet on October 22, 1981. Sample contained high 

metal concentrations due to pipe erosion. Analyses by the U.S. Geological 

Survey.

injected industrial liquid wastes (site 15) are compared 

with those of the local water supply and the native 

ground water in the Boulder Zone in table 14. 

At site 20, the industrial liquid waste is caustic (chiefly 

aluminum hydroxide and sodium chloride), a byproduct 

of the production of pectin. Analyses of the injectant 

from site 20 were unavailable. 

The injected municipal liquid waste is secondarytreated wastewater (that is, wastewater that has had at 

least 90 percent of the suspended solids and biochemical

oxygen demand removed by treatment). The characteristics of the treated wastewater vary from plant to plant, 

but the wastewaters are distinguished from local water 

supply by high concentrations of nutrients. The characteristics of the injected wastewater at two waste watertreatment plants (sites 19 and 22) are compared with 

those of the local water supply and native ground water 

in the Boulder Zone in tables 15 and 16. 

Injection into the brackish water-bearing zones of the 

Upper Floridan aquifer occurred only at sites 14 and 15.

The combined amount (municipal and industrial) for both 

sites during 1959-75 was 5 Ggal. 

Injection into the middle confining unit and perhaps

the upper unit of the Lower Floridan aquifer occurred 

only at site 15, where about 656.7 Mgal were injected 

during 1972-75. 

Injection into the Boulder Zone of the Lower Floridan 

aquifer occurred at the eight remaining sites during
SUBSURFACE STORAGE IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G51

1971-83 and at site 15 during 1977-83. The total amount 

injected into the Boulder Zone during 1971-83 was 106.4 

Ggal.

Injection rates have increased exponentially since 

1971, when the injection well at site 16 became operational and injection was directed to the Boulder Zone. 

The rate in 1983 was 73.5 Mgal/d, and the estimated rate 

in 1984 was 112 Mgal/d (table 17).

In 1983, two injection wells (fig. 28, sites 16 and 17) 

were removed from service because of small leaks in 

uncemented (conductor) inner casings, and the effluent 

from the plant was directed to other treatment facilities 

of the Miami-Bade Water and Sewer Authority. Also in 

1983, a small leak was detected in the uncemented inner 

casing of a third injection well (fig. 28, site 18), and 

construction of a replacement well was required by the 

Florida Department of Environmental Regulation before 

remedial work could be performed on the leaking well. 

Despite these minor problems, which have been resolved 

by enforcement of the UIC regulations, the outlook for 

deep-well injection in southern Florida is for continued 

expansion. The outlook, however, should include caution 

because the injected liquid waste will ultimately conform 

to the regional ground-water circulation system. The 

injected waste, thus, will move with the hypothesized 

inland and upward flow of seawater from the Florida 

Straits.

Typical construction characteristics of nontoxic municipal and industrial liquid wastes disposal wells are shown 

in figures 32 and 33 along with the local hydrogeology. 

The construction of the municipal liquid wastes disposal 

well (fig. 32) is based on that of well 3 at the city of Fort 

Lauderdale's Port Everglades wastewater-treatment

plant

TABLE

(fig. 28, site 28). The well is constructed with

17. Average rate of municipal and 

injection, 1959-84

industrial liquid wastes

[In million gallons per day; e, estimated]

Year

1959

1960

1961

1962

1963

1964

1965

1966

1967

1968

1969

1970

1971

Rate

0.268

.499

.499

.499

.499

.600

.600

.605

.623

.887

.934

1.276

2.776

Year Rate

1972 4.221

1973 5.630

1974 8.052

1975 10.520

1976 10.589

1977 11.262

1978 23.971

1979 29.739

1980 36.751

1981 38.B94

1982 43.453

1983 73.502

1984 112a

TYPICAL MUNICIPAL WASTEWATER DISPOSAL WELL

FORT LAUDERDALE WELL NO. 3 

FEET INCHES FEET 

LAND SURFACE 32 , " , . * , ° ,

24 32

JNOIFFEREHTIMED HOLOCB4E1 

AND PLEISTOCENE DEPOSITS V

TAM1AMI 

FORMATION

HAWTHORN FORMATION

TAMPA LIMESTONE

SUWANNEE LIMESTONE

AVON PARK I.'. 

FORMATION VUPPER DOLOSTONE Y

f ('

^ MIDDLE DOLOSTONE J 

5 V

 54-1 

STEEL

42-1 

STEEL

~^CE

34-1 

STEEL

~~^24-l 

STEEL

i :><> LOWER DOLOSTONE » j

ysAr

 4CH 

SASING

4CH 

CASING

MENT

JCH - 

CASING

MCH 

CASING

^ -_

 !/ SURFK 

 '.'.'.I AQUIFl IAL

UPPER 

- ' '1 CONFINING 

' '/I UNIT

I

1

' . MIDDLE £ 

' ] CONFINING 5 

. ' \ UNIT

\

T BOULDER ZONE

f~- MINIMUM 22-WCH 

___-==> OPEN HOLE

CEDAR KEYS £§^§^- - 17-INCH 

FORMATION £5^58$ EXPLORATORY HOLE 

ANHYDRITE AND GYPSUM R5&8888 LOWER CONFINING UNIT 

_ _ _ / _____ __ $£x>ooo<i \ 

LOWER FLORIDAN AQUIFER 

FLORIDAN AQUIFER

FIGURE 32. Hydrogeology and typical construction 

characteristics of a municipal wastewater disposal well.

telescoping steel casings to protect drinking water 

resources in the surficial aquifer and the Upper Floridan 

aquifer. The casings are cemented in place from top to 

bottom with special sulfate-resistant cement. The steel

TYPICAL INDUSTRIAL DISPOSAL WELL 

SITE 15 WELL NO. 3

1,000 

3,000-

4nnn 

LAND SURFACE 32 , 2*

UNWTEnENTlAtED PtftSTO- 1: '. 

CENE AND PUOCENE DEPOSTISJ'. 

UNDIFFERENTIATED 

MIOCENE DEPOSITS

CICALA LIMESTONE

AVON PARK

OLDSMAR 

FORMATION

CEDAR KEYS 

FORMATION

16

J: 

:

j

a

':

li

L

6

i

J

16 2* 32

'. . ' " ; . ! SURFICIAL AQUIFER- 

. .' ' ^4 42-INCH STEEL CASING

'. - *

 n- - 32-INCH 

 .V. STEEL CASING

' ' ^_ 24-INCH 

' T. STEEL CASING

 ~~ CLASS H CEMENT

^ CLASSH 

__^ DENSIFIED CEMENT

- 16-INCH STEEL CASING 

(LOWER 90 FEET IS STAINLESS) 

1 . CLASS H THIXOTFOPIC CEMENT 

 103/4-INCH E-EBRITE 26-1 

j SCHED 20 CASING 

'- CLASS H DENSIFIED CEMENT 

- DENSIFIED CEMENT 

f STAINLESS STEEL PACKER

Jr

FD 3,163 FEET BOULDER ZONE

 P) 

 .

UPPER CONFINING UNIT

,

£ 

yl^i =

! !

f i

o

-1,000

 3,000

FIGURE 33. Hydrogeology and typical construction characteristics 

of an industrial liquid wastes disposal well.
G52 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

inner (conductor) casing is 24 inches in diameter and V2 

inch thick. The well has a minimum injection capacity of 

15 Mgal/d.

The construction of the industrial liquid wastes disposal well is based on that of well 3 at the furfural plant 

(fig. 28, site 15). The well is also constructed with several 

steel casings that are cemented in place with special 

cement to resist heat and corrosion to protect drinking 

water sources. The inner (conductor) casing is made of a 

special alloy that is acid and heat resistant. Not shown in 

figures 32 and 33 are monitor wells, which are located 

nearby to detect leaks and upward-migrating wastes.

FRESHWATER STORAGE

Subsurface storage of freshwater as an alternative to 

surface storage has become increasingly attractive to 

water managers in southern Florida as urbanization and 

population growth have placed increasing demands on 

the water supply. The advantages of the subsurface 

storage concept are that subsurface space is free, water 

loss by evapotranspiration is nonexistent, and the site 

may be located at the point of greatest need (provided 

hydrogeologic conditions are favorable). The concept is 

particularly desirable in southern Florida, where real 

estate has become very expensive, the availability of 

water is seasonal, and underlying artesian aquifers in the 

intermediate (Miocene) aquifer system and Floridan 

aquifer system contain nonpotable saline ground water.

The source of freshwater for injection might be surplus 

within the surface-water storage system or the surficial 

aquifer system during the annual wet season. On an 

annual basis, the surplus freshwater would be injected 

through Class V wells into suitable artesian aquifers 

during the wet season, stored for a short period (perhaps 

3 to 6 months (mo)), and then withdrawn as needed 

during the dry season hence the term cyclic injectionstorage-recovery. The measure of success of a cycle is 

recovery efficiency, which is defined as the volume of 

freshwater recovered before it fails to meet an established (or a prescribed) chemical standard, expressed as 

a percentage of the volume of freshwater that was 

injected. Pilot studies to date in southern Florida have 

assumed the standard established by EPA for chloride 

concentration (250 mg/L) in public water supply. Other 

criteria may be used depending on the particular use of 

the recovered water. For example, a higher chloride 

standard could be used if the recovered water were 

mixed with surface water to yield a blend that would 

meet drinking water standards.

Theoretical and pilot-operational studies to date indicate that recovery efficiency usually improves with successive cycles, provided that recovery ceases when the 

recovered water reaches the standard and that the

TABLE 18. Results of Florida Department of Natural Resources and 

Florida Department of Environmental Regulation injection, storage, and recovery tests of freshwater at site 24 in Palm Beach 

County, 1975-76

[Quantities in million gallons and rates in gallons per minute. From J. J. 

Plappert, Florida Department of Environmental Regulation, written 

commun., 1977. Location of site 24 shown in fig. 28]

Cycle 1 Cycle 2 Cycle 3 Cycle 4

Quantity Injected

Storage period (days)

Quantity of potable water

recovered 1

Injection rate

Withdrawal rate

20.5

15

0

2.000

1.000

100

30

4.7

2,000

1,000

Unknown

306

30

55.5

2.000

1.000

102

120

36.1

2,000

1,000

Injected water chloride concentration: 65 rag/L (milligrams per liter)

Resident water chloride concentration: 1,980 rag/L

Op*n hole: 990 to 1,280 feet (Ocala Limestone and Avon Park Formation)

'Recovery was terminated when the chloride concentration of the recovered 

water reached 250 mg/L.

storage period is sufficiently short to prevent significant 

migration of the injectant away from the point of recovery.

Pilot studies have been conducted at four sites (fig. 28) 

in southern Florida, with varying degrees of success 

(Merritt and others, 1983; Wedderburn and Knapp,

1983). Also, data on the recovery of injected wastewater 

(freshwater) from Class I injection wells during repairs, 

testing, and abandonment have yielded valuable information on recovery efficiency (McKenzie and Irwin,

1984). Aspects of the existing pilot studies are summarized in tables 18 through 21. Of the four studies, three 

(sites 24, 25, and 27) involved injection into waterbearing zones of the Upper Floridan aquifer and one (site 

26) involved injection into water-bearing zones of the 

intermediate aquifer system. Plugging of the wellbore by 

suspended solids in the injectant was a significant problem in all four studies.

At site 24 in Palm Beach County (table 18), injection 

was chiefly into water-bearing zones of the Ocala Limestone and Avon Park Formation (units of the Upper 

Floridan aquifer). The study involved four injectionstorage-recovery cycles (J.J. Plappert, Florida Department of Environmental Regulation, written commun., 

1977). Recovery efficiencies ranged from 0 to 35.2 percent. The transmissivity of the injection zone(s) is probably on the order of 10,000 to 20,000 ft2/d, although data 

are lacking to support that assumption. The injection 

zones are apparently associated with zones of dissolution 

at or near unconformities that separate formations.

At site 25 in Bade County (table 19), injection of 

freshwater chiefly was into water-bearing zones of the 

Suwannee Limestone although the injection well tapped 

parts of the Tampa Limestone and Avon Park Formation 

(all units of the Upper Floridan aquifer). The study 

involved three injection-storage-recovery cycles. Recov-
SUBSURFACE STORAGE IN THE FLORIDAN AQUIFER SYSTEM IN SOUTHERN FLORIDA G53

TABLE 19. Results of U.S. Geological Survey injection, storage, and 

recovery tests of freshwater at site 25 in Bade County, 1975-80

[Quantities in million gallons and rates in gallons per minute. Location 

of site 25 shown in fig. 28]

Cycle 1 Cycle 2 Cycle 3

Quantity injected

Storage period (days)

Percent of recovery

Injection rate

Withdrawal rate 2

41.9

2

32.9

'440-780

330

85

54

47.8

854

494

208

181

38.5

800

450

Transmissivity3 : 10,950 ft2 /d (feet squared per day) 

Storage coefficient 3 : 8.4 x 10" 5

Injected water chloride concentration: 65 mg/L (milligrams per liter) 

Resident water chloride concentration4 : 1,200 mg/L

Open-hole: 955 to 1,105 feet (Tampa Limestone, Suwannee Limestone, and Avon 

Park Formation).

'Progressive decline due to wellbore plugging.

2 Natural artesian flow.

3 Estimated by computer simulation of well G-3062 pumping test.

 "Multilevel composite, range from 800 to 2,000 mg/L.

ery efficiencies ranged from 32.9 to 47.8 percent. A 

decline in efficiency was recorded for the third cycle 

which probably was related to migration of the injectant 

downgradient from the injection recovery well during 

the 181 days (d) of storage. The transmissivity of the 

injection zone(s) is 10,950 ft2/d. The results of the tests at 

this site were the basis for theoretical studies that used 

a mathematical model to evaluate the effects of varying 

aquifer characteristics, fluid density, regional flow, well 

arrays, and operating schedules on recovery efficiency 

(Merritt, 1985).

At site 26 in Lee County (table 20), injection of 

freshwater was into water-bearing zones in limestone of 

the Hawthorn Formation (unit of the intermediate aquifer system). The study involved three injection-storagerecovery cycles. Recovery efficiencies ranged from 9.7 to 

38.7 percent. The efficiency of the first cycle, which had 

the greatest efficiency value, is probably not representative of the true efficiency because of the small amount 

injected and the short storage period. The value for the 

third cycle (30.4 percent) probably represents the efficiency and the storage capability of the aquifer. The 

transmissivity of the injection zone(s) is about 750 ft2/d.

At site 27 in St. Lucie County (table 21), injection of 

freshwater chiefly was into water-bearing zones of the 

Ocala Limestone and the Avon Park Formation of the 

Upper Floridan aquifer. The water-bearing zones are 

associated with zones of dissolution near formation contacts. The study involved one injection-storage-recovery 

cycle, for which the recovery efficiency was only 3 

percent. The low efficiency was due to the high chloride 

concentration (200 mg/L) of the injectant. The 3-percent 

recovery efficiency represented a 79-percent blend of the 

injectant (chloride concentration of 200 mg/L) with 

native ground water (chloride concentration of 1,000

TABLE 20. Results of U.S. Geological Survey injection, storage, and 

recovery tests of freshwater at site 26 in Lee County, 1980 82

[Quantities in million gallons and rates in gallons per minute. From 

Fitzpatrick, 1985. Location of site 26 shown in fig. 28]

Cycle 1 Cycle 2 Cycle 3

Quantity Injected

Storage period (days)

Percent of recovery

Withdrawal rate 4

0.571

0

38.7

95-110

'6.831

47

S 9.7 

300

165-175

*29.026

99

8 819

30.4

ISO

Transmissivity: 700 to 800 ft'/d (feet squared per day)

Storage coefficient: About 1 x 10"*

Injected water chloride concentration: Cycle 1, 60 mg/L (milligrams per

liter); cycle 2. 150 to 350 mg/L*; cycle 3. 80 to 100 ng/L (finished water)

and 60 mg/L (raw water).

Resident water chloride concentration: 550 mg/L 

Open hole: 647 to 600 feet (limestone of the Hawthorn Fomatlon)

'Estimated after loss of water due to equipment failure.

'8.568 x 10* gallons of finished water, followed by 20.873 x 10* gallons of 

raw river water.

'Low due to relatively high chloride concentration of Injected water. 

Purpose was to test well after acidification.

'Natural artesian flow; Improvement In cycle 2 due to acidification of well.

'Abnormally high due to record low.flows In Caloosahatchee River source; 

decreased during Injection.

mg/L). A recovery efficiency of 33 percent would have 

been realized had the chloride concentration of the 

injectant been 50 mg/L (based on the indicated rate of 

mixing and the limit of 250 mg/L for chloride in drinking 

water). The transmissivity of the injection zone(s) is 

about 6,000 ft2/d.

During 1975-77, the U.S. Geological Survey, in cooperation with the Florida Department of Environmental 

Regulation, conducted a study of the quality of recovered 

secondary-treated wastewater from subsurface storage 

in the Upper Floridan aquifer at site 14 in Broward 

County (table 22). The injection system consisted of two 

wells that were in operation from 1959 to 1975. Injection 

ceased in January 1975 when the plant's function was 

transferred to the Broward County North Regional 

wastewater treatment plant. Recovery of the injected

TABLE 21. Results of South Florida Water Management District 

injection, storage, and recovery tests of freshwater at site 27 in St. 

Lucie County, 1982-83

[Quantities in million gallons and rates in gallons per minute. From 

Wedderburn and Knapp, 1983. Location of site 27 shown in fig. 28]

Quantity Injected

Storage period (days)

Quantity of potable water recovered

Percent of recovery 1

Injection rate

Withdrawal rate

Cycle

1.488 

37.5

.061 

2.76 

331 

140-190

Transmissivity: 6.000 ft*/d (feet squared per day) 

Storage coefficient: 1.6 x 10" 4

Injected water chloride concentration: 200 mg/L (milligrams per liter) 

Resident water chloride concentration: 1,000 mg/L

Open hole: 600 to 775 feet (limestone of the Hawthorn Formation, Ocala 

Limestone, and Avon Park Fomatlon).

'Percent of recovery would have been 33 percent If the chloride 

concentration of the Injected water were 50 mg/L.
G54 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

TABLE 22. Results of U.S. Geological Survey wastewater recovery 

tests of freshwater at site 14 in Broward County, 1975 77

[Quantities in million gallons and rates in gallons per minute. From 

McKenzie and Irwin, 1984. Location of site 14 shown in fig. 28]

Quantity injected

Storage period (years)

Quantity of potable water recovered

Percent of recovery

Injection rate

Withdrawal rate

Cycle

3,401 

16 

69.2

2

400 

4-132

Transmissivity and storage coefficient: Not determined

Injected water chloride concentration: 84 mg/L (milligrams per liter)

Resident water chloride concentration: 2,360 mg/L

Open hole: 995 to about 1,250 feet (Avon Park Formation)

treated wastewater began in April 1975 and ended in 

March 1977, when the chloride concentration reached 250 

mg/L. The recovery efficiency based on reaching a 

chloride concentration of 250 mg/L was only 2 percent, 

which was much less than expected for the great volume 

(3.4 Ggal) that was injected during the 16 yr of operation. 

The transmissivity of the injection zone was not determined but was probably greater than for previously 

discussed freshwater storage pilot studies. Records of 

the construction of one injection well suggest that injection occurred to a greater depth (perhaps as deep as 

1,600 ft) than previously reported. The low recovery 

efficiency is probably a result of higher aquifer transmissivity, higher chloride concentration (and, hence, higher 

density) of the resident water, and construction problems. As with the previous pilot studies, plugging of the 

wellbore by suspended solids was a significant problem.

Unpublished data collected by the Florida Department 

of Environmental Regulation on the amount of freshwater effluent from abandoned injection wells at sites 16 

and 17 in Bade County suggest that recovery efficiency 

for wells that tap the Boulder Zone of the Lower 

Floridan aquifer is virtually nonexistent. The injection 

well at site 16 was abandoned in 1983 after 13 yr of 

operation and after 18.4 Ggal of effluent were injected; 

the injection well at site 17, also abandoned in 1983, was 

operated for 11 yr, during which time 7.8 Ggal of effluent 

were injected. At both sites, the chloride concentration 

of the injectant was about 60 mg/L, and the chloride 

concentration of the resident water was about 19,200 

mg/L. For both sites, the amount of effluent recovered 

before the chloride concentration exceeded 250 mg/L did 

not exceed 1 Mgal. The recovery tests indicate that there 

is no potential for recovering freshwater stored in the 

highly transmissive Boulder Zone.

Dissolution zones at erosional unconformities between 

the Suwannee and Ocala Limestones and the Avon Park 

Formation probably offer the best opportunity for largescale storage of freshwater in the subsurface of southern 

Florida. Detailed maps of the dissolution zones are

unavailable, but maps showing the configuration of the 

top of the middle and upper Eocene rocks are shown in 

chapter B (Miller, 1986, pis. 6, 8) of this professional 

paper. The surface is irregular and shows the effects of 

large-scale erosion at the close of the Eocene Epoch. 

Erosion removed the Ocala Limestone from much of 

southeastern Florida and exposed the underlying (older) 

Avon Park Formation. Zones of dissolution are prominent near this erosion surface; therefore, the maps in 

Miller (1986) may be used to estimate the depth at which 

favorable injection zones may be present.

SUMMARY AND CONCLUSIONS

In southern Florida, the Floridan aquifer system is 

divided into three general hydrogeologic units: (1) the 

Upper Floridan aquifer, which contains brackish ground 

water, (2) the middle confining unit, which contains salty 

ground water, and (3) the Lower Floridan aquifer, which 

contains ground water whose chemical composition compares closely with that of seawater. The aquifer system 

is about 3,000 ft thick and is composed chiefly of carbonate rocks that range in age from early Miocene to 

Paleocene. Zones of high permeability are present in the 

Upper Floridan aquifer at the unconformable contacts of 

the Suwannee Limestone with the Ocala Limestone and 

the Ocala Limestone with the Avon Park Formation. 

Zones of high permeability in the Lower Floridan aquifer 

are present in three dolostones in the Oldsmar Formation; the lowermost, locally called the Boulder Zone, is 

perhaps one of the most permeable units in the world. 

The transmissivity of the Upper Floridan aquifer is 

estimated to range from 10,000 to 60,000 ft2/d, whereas 

that of the Lower Floridan aquifer (Boulder Zone unit) is 

as much as 2.5x 107 ft2/d. The porosity of both aquifers is 

estimated at 0.3. In southeastern Florida, the salinity of 

the ground water in the aquifer system generally 

increases with increasing depth, whereas water temperature decreases with increasing depth. Temperatures of 

salty ground water in the Lower Floridan aquifer (Boulder Zone) range from 50.0 °F at Fort Lauderdale on the 

southeastern coast to about 110.0 °F near Punta Gorda 

on the southwestern coast.

Seismic reflection profiles in the western Straits of 

Florida show submerged karst on the Miami Terrace at 

about a 1,000-ft depth and in deeper parts of the straits. 

Ground-water movement in the Upper Floridan aquifer 

is generally southward to the Gulf of Mexico and the 

Atlantic Ocean from recharge areas in central Florida. 

Hydraulic gradients in the Upper Floridan aquifer in 

southeastern Florida suggest that eastward-flowing, 

brackish ground water is actively discharging through 

unfilled sinkholes on the Miami Terrace as submarine
SUMMARY AND CONCLUSIONS G55

springs. The middle confining unit is relatively less 

permeable than the Upper and Lower Floridan aquifers, 

and it separates the two flow systems. However, hydraulic connection between the aquifers is inferred from the 

presence of sinkholes and fractures and from local temperature and salinity anomalies in the Upper Floridan 

aquifer.

Samples of ground water from the Upper and Lower 

Floridan aquifers were analyzed for natural carbon and 

uranium isotopes to determine rates of ground-water 

movement. Analyses of principal anions and cations were 

also performed. Tritium was determined in selected 

samples to check for contamination by modern water, 

and oxygen isotopes were determined in selected samples to assess their use as climate indicators.

Vertical variations in carbon-14 activity were indicated earlier by samples of ground water from selected 

depths in the Alligator Alley test well (site 10), and areal 

variations in carbon-14 activity were reported by Hanshaw and others (1965). Carbon-14 activity in the ground 

water in the Upper Floridan aquifer decreases southward and downgradient from the principal recharge area 

in central Florida to site 5 near Lake Okeechobee, where 

the activity is hardly detectable (apparent age, 50,000 

Libby yr), and increases slightly downgradient from the 

lake to the coastal area of Bade and Broward Counties. 

The apparent reversal in the carbon-14 activity gradient 

at site 5 near Lake Okeechobee is coincident with the 

area where the southward-flowing fresh ground water 

becomes brackish and is no longer potable. Southward 

and downgradient from the Lake Okeechobee area, the 

increase in salinity and in carbon-14 activity is probably 

related to upwelling of relatively young seawater (with 

relatively higher carbon-14 activity) from the Lower 

Floridan aquifer. Carbon-14 activity in ground water in 

the Lower Floridan aquifer is highest at Fort Lauderdale 

(which is closest to the source area in the Straits of 

Florida) and decreases radially inland. Carbon-14 activity in the ocean is estimated at 94 percent of modern 

carbon, the activity at Fort Lauderdale (well G-2333) 

was 62.9 percent, and the activity at well G-2296 (site 10) 

about 44.5 mi west of Fort Lauderdale was 37.1 percent. 

The apparent carbon-14 ages of the samples suggest that 

the past rate of inland flow was decreasing and may have 

been affected by the rise in sea level during the Holocene 

transgression. The average rate of movement between 

Fort Lauderdale (site 9) and well G-2296 (44.5 mi west of 

Fort Lauderdale) is estimated at 54.6 ft/yr, and the 

average rate between Fort Lauderdale and the subsea 

source (about 10.5 mi east of Fort Lauderdale) is estimated at 17.3 ft/yr.

The results of the uranium isotope analyses of ground 

water from the Upper Floridan aquifer show a relation 

that apparently parallels the carbon-14 data. The uranium concentration gradient generally decreases down 

the hydraulic gradient to about the latitude of Lake 

Okeechobee and then increases slightly along the southeastern coast, again implying upwelling of uranium-rich 

saltwater from the Lower Floridan aquifer and (or) 

lateral encroachment of seawater from the Straits of 

Florida. Concentrations of dissolved uranium averaged 

0.75 fxg/L in the outcrop area of west-central Florida, 

0.080 fxg/L in central Florida, 0.048 fxg/L in southern 

Florida, and 0.568 fxg/L in distal southeastern Florida. 

The alpha-activity ratios for the Upper Floridan aquifer 

generally showed an inverse relation to the dissolved 

uranium. Ratios generally increased downgradient from 

the outcrop area in west-central Florida but decreased in 

southern and distal southeastern Florida. Increasing 

ratios indicate enrichment of uranium-234 with respect to 

uranium-238, and decreasing ratios indicate depletion of 

uranium-238 with respect to uranium-234. Ratios averaged 0.98 in the outcrop area of west-central Florida, 

1.83 in central Florida, 1.24 in southern Florida, and 1.11 

in distal southeastern Florida.

In the Lower Floridan aquifer, the uranium concentration gradient parallels the carbon-14 gradient. The 

dissolved uranium concentration at Fort Lauderdale is 

almost identical to that for seawater, and the concentration decreases radially inland as did the carbon-14. The 

alpha-activity ratio decreased radially inland, suggesting 

enrichment of uranium-234 with respect to uranium-238 

during transit. Therefore, the concentration gradients 

for both carbon-14 and uranium indicate inland flow of 

seawater from the Straits of Florida; the relative carbon14 ages of the seawater in the Lower Floridan aquifer 

imply that velocity is greatest toward the center of the 

Floridan Plateau, where temperatures are highest.

Measurements of water levels in the Upper and Lower 

Floridan aquifers were used to estimate hydraulic gradients, which were then used to estimate rates of 

ground-water movement. The hydraulic rates were then 

compared with estimates of rates based on relative 

carbon-14 ages of the ground water. Rates and transit 

times were calculated for segments of the flow lines from 

Polk City to Key Largo (227 mi) and from Polk City to 

Fort Lauderdale (194 mi) based on estimated predevelopment hydraulic gradients and assumed aquifer characteristics. Although predevelopment gradients represent 

a relatively short transient event compared with longterm averages represented by the carbon-14 data, the 

estimated rates and transit times for predevelopment 

conditions are considered an indication of the long-term 

flow pattern. For the predevelopment gradient, a particle of water traveling 227 mi from Polk City to Key 

Largo would be in transit from 159,000 to 779,000 yr, and 

a particle of water traveling 152 mi from the southernmost sinkhole lakes in central Florida to Key Largo
G56 REGIONAL AQUIFER-SYSTEM ANALYSIS-FLORIDAN AQUIFER SYSTEM

would be in transit from 155,000 to 768,000 yr. Both 

estimates of transit time exceed the maximum dating 

capability of carbon-14. Only the transit time between 

site 9 (Fort Lauderdale) and site 10 (well G-2296, 44.5 mi 

west of Fort Lauderdale) is within the dating capability 

of carbon-14. Therefore, the occurrence of measurable 

carbon-14 activity in the brackish ground water of the 

Upper Floridan aquifer in southern Florida strongly 

suggests that infiltrating freshwater in central Florida is 

not the only source.

For the Lower Floridan aquifer, water-level measurements and estimates of fluid density at site 9 (well 

G-2334 at Fort Lauderdale) and site 10 (well G-2296, 

44.5 mi west of Fort Lauderdale) were used to calculate 

the difference in head and the apparent hydraulic gradient. The head at site 9 (in terms of equivalent density) is 

about 0.2 ft higher than the head at site 10, thereby 

indicating an inland hydraulic gradient of about 

8.5xlO"7. The transit time between wells is from 3,900 

to 7,900 yr based on the estimated hydraulic gradient and 

assumed aquifer characteristics. Comparison of the 

hydraulically derived rates and transit times with those 

based on relative carbon-14 ages suggests that the 

estimated hydraulic gradient based on the water-level 

measurements may be too high. The particle velocities, 

transit times, and hydraulic gradients based on relative 

carbon-14 ages of the seawater in the Straits of Florida 

and ground water at sites 9 and 10 probably represent 

average values for the period represented by the differences in apparent age of the water samples.

Changing sea level, and perhaps changing climate, had 

significant effects on hydraulic gradients in the Floridan 

aquifer system. During periods of rising sea level and 

constant climate, freshwater in aquifer storage would 

gradually be displaced by seawater. Estimates of sealevel fluctuation during the past 150,000 yr suggest that 

sea level ranged from about 23 ft above present sea level 

to about 330 ft below present sea level. The last rise in 

sea level, called the Holocene transgression, began about 

18,000 yr ago. Sea level rose about 330 ft during this 

period, and it is still rising, although at a slower rate. 

The rise of freshwater levels in the Florida peninsula 

during the past 9,000 yr was recorded by changes in 

vegetative types. Pollen studies of bottom sediments in 

lakes by Watts (1971) indicated a change from oak forest 

to pine forest as water tables in central Florida rose in 

response to rising sea level about 5,000 yr ago. A 

hypothetical relation between sea level and the water 

table in central Florida resulted from the pollen studies. 

The apparent carbon-14 ages of samples of saltwater 

from the Lower Floridan aquifer at sites 9 and 10 in 

southern Florida suggest that the inland transit of seawater from the Straits of Florida is also related to the 

rise in sea level during the Holocene transgression.

Estimates of the rates based on assumed aquifer characteristics and relations between sea-level rise and the 

apparent carbon-14 ages of the samples ranged from 

about 172 ft/yr at the beginning of the Holocene transgression to about 5 ft/yr at present. The estimated 

differences in the head between sites 9 and 10 (44.5 mi 

apart) associated with these rates are 0.87 and 0.026 ft, 

respectively.

The present-day scenario of circulation proposed for 

the Floridan aquifer system of southern Florida is, 

therefore, (1) continuing rise in sea level and concurrent 

displacement of stored fresh ground water by inlandmoving cold seawater chiefly through the Lower Floridan aquifer, (2) heating of the seawater in the Lower 

Floridan aquifer during inland transit, which results in 

lowered fluid density, (3) upwelling of seawater from the 

Lower Floridan aquifer through sinkholes and fractures 

that transect the middle confining unit, and (4) dilution of 

the seawater (again reducing the fluid density) and 

transport of the seawater back to the ocean by seawardflowing freshwater in the Upper Floridan aquifer. This 

circulation theory is generally similar to that proposed 

earlier by Kohout (1965). The inland circulation of seawater is an argument against the theory that the saltwater in the Floridan aquifer system in southern Florida 

is connate or residual from former high stands of sea 

level.

Evidence of the upwelling phenomenon is indicated by 

salinity and temperature anomalies (also by anomalous 

carbon-14 activity) in the ground water of the Upper 

Floridan aquifer. In the Martin County-St. Lucie County 

area of southern Florida, wells ranging from 800 to 1,200 

ft in depth produce brackish water that exceeds the 

background temperature of 80.0 °F by 10.0 °F. Associated with the local temperature anomalies are salinity 

anomalies. Beneath the temperature and salinity anomalies, at a depth of about 3,000 ft in the Lower Floridan 

aquifer, is saltwater having temperatures that compare 

with anomalous temperatures in the Upper Floridan 

aquifer. The temperature-salinity anomalies generally 

trend northwestward to southeastward and seem to 

originate from sinkholes or vertical solution pipes that 

are aligned with the major system of fractures or joints 

in the Tertiary limestone.

The Floridan aquifer system has been used as a 

receptacle for oil field brines since 1943. During 1943-83, 

8.1 Ggal of brine were produced with 3.2 Ggal of oil. Of 

the 8.1 Ggal of brine, about 7.1 Ggal were injected into 

the Florida aquifer system. During 1959-83, 112 Ggal of 

nontoxic liquid waste were injected into the Floridan by 

municipal wastewater-treatment systems and industry. 

The average rate of injection increased from 268,000 

gal/d in 1959 to 73.75 Mgal/d in 1983. In 1984, the 

estimated rate of injection was 112 Mgal/d. Injection of
SELECTED REFERENCES G57

nontoxic liquid wastes is chiefly into the Boulder Zone of 

the Lower Floridan aquifer, although small amounts 

have been injected into the Upper Floridan aquifer.

Pilot studies indicate that the Upper Floridan aquifer 

can be used for temporal storage of freshwater. However, storage of freshwater in the Lower Floridan aquifer is not feasible.

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Osmond, J.K., Rydell, H.S., and Kaufman, M.I., 1968, Uranium 

disequilibrium in ground water An isotopic dilution approach to 

hydrologic investigations: Science, v. 162, p. 997-999.

Parker, G.G., Ferguson, G.E., Love, S.K., and others, 1955, Water 

resources of southeastern Florida, with special reference to geology and ground water of the Miami area: U.S. Geological Survey 

Water-Supply Paper 1255, 965 p.

Pearson, F.J., and Bodden, Martha, 1975, U.S. Geological Survey, 

Water Resources Division, Radiocarbon measurements I: Radiocarbon, v. 17, no. 1, p. 135-148.

Pearson, F.J., and Hanshaw, B.B., 1970, Sources of dissolved carbonate species in groundwater and their effects on carbon-14 dating, 

in Isotope hydrology: Symposium of the International Atomic 

Energy Agency, Vienna, Proceedings, p. 271-286.

Plummer, L.N., 1977, Defining reactions and mass transfer in part of 

the Floridan aquifer: Water Resources Research, v. 13, no. 5, p. 

801^812.

Puri, H.S., and Vernon, R.O., 1964, Summary of the geology of Florida 

and a guidebook to the classic exposures: Florida Geological 

Survey Special Publication 5, 312 p.

Puri, H.S., and Winston, G.O., 1974, Geologic framework of the high 

transmissivity zones in south Florida: Florida Bureau of Geology 

Special Publication 20, 101 p.

Reece, D.E., Brown, M.P., and Hynes, S.D., 1980, Hydrogeologic data 

collected from the upper east coast planning area: South Florida 

Water Management District Technical Publication 80-5, 117 p.

Rydell, H.S., 1969, The implications of uranium isotope distributions 

associated with the Floridan aquifer in north Florida: Unpublished 

doctoral dissertation, Florida State University (Tallahassee), 119

PSchroeder, M.C., Klein, Howard, and Hoy, N.D., 1958, Biscayne

aquifer of Dade and Broward Counties Florida: Florida Geological

Survey Report of Investigations 17, 56 p. 

Schroeder, M.C., Milliken, D.L., and Love, S.K., 1954, Water

resources of Palm Beach County, Florida: Florida Geological

Survey Report of Investigations 13, 63 p. 

Sellards, E.H., and Gunter, Herman, 1913, The artesian water supply

of eastern and southern Florida: Florida Geological Survey Fifth

Annual Report, p. 113-280.
SELECTED REFERENCES G59

Singh, W.P., Eichler, G.E., Sproul, C.R., and Garcia-Bengochea, J.I., 

1983, Pump testing Boulder Zone aquifer, south Florida: Journal of 

Hydraulic Engineering, v. 9, no. 8, p. 1152-1160.

Smith, C.A., Lidz, Lauralee, and Meyer, F.W., 1982, Data on selected 

deep wells in south Florida: U.S. Geological Survey Open-File 

Report 82-348, 144 p.

Sprinkle, C.L., in press, Geochemistry of the Floridan aquifer system 

in Florida and in parts of Georgia, South Carolina, and Alabama: 

U.S. Geological Survey Professional Paper 1403-1.

Sproul, C.R., 1977, Spatial distribution of ground-water temperatures 

in south Florida, in Smith, D.L., and Griffin, G.M., eds., Geothermal nature of the Floridan Plateau: Florida Bureau of Geology 

Special Publication 21, p. 65-89.

Sproul, C.R., Boggess, D.H., and Woodard, H.J., 1972, Saline-water 

intrusion from deep artesian sources in the McGregor Isles area of 

Lee County, Florida: Florida Bureau of Geology Information 

Circular 75, 30 p.

Stringfield, V.T., 1936, Artesian water in the Florida peninsula: U.S. 

Geological Survey Water-Supply Paper 773-C, p. 115-195.

 1953, Artesian water in the Southeastern United States, in 

McGrain, Preston, ed., Proceedings of the Southeastern Mineral 

Symposium: Kentucky Geological Survey Special Publication 1, 

ser. 9, p. 24-39. 

-1966, Artesian water in Tertiary limestone in the Southeastern

States: U.S. Geological Survey Professional Paper 517, 226 p. 

Stuiver, Minze, 1980, Carbon-14 distribution in the Atlantic Ocean:

Journal of Geophysical Research, v. 85, no. C5, p. 2711-2718. 

Stuiver, Minze, and Ostlund, H.G., 1980, GEOSECS Atlantic radiocarbon: Radiocarbon, v. 22, no. 1, p. 1-24. 

Sutcliffe, Horace, Jr., 1975, Appraisal of the water resources of

Charlotte County, Florida: Florida Bureau of Geology Report of

Investigations 78, 53 p. 

 1979, Hydrologic data from a deep test well, city of Sarasota,

Florida: U.S. Geological Survey Open-File Report 79-1275, 23 p. 

Tamers, M.A., 1967, Radiocarbon ages of groundwater in an arid zone

unconfined aquifer: American Geophysical Union, Geophysical

Monograph 11, p. 143-192.

Tamers, M.A., Stipp, J.J., and Weiner, Robert, 1975, Radiocarbon 

ages as a basis for the determination of safe limits of aquifer 

exploitation: Environmental Research, v. 9, p. 250-264.

Thatcher, L.L., Janzer, V.J., and Edwards, K.W., 1977, Methods for 

determination of radioactive substances in water and fluvial sediments: U.S. Geological Survey Techniques of Water-Resources 

Investigations, Book 5, Chapter A5, 95 p.

Uchupi, Elazar, 1966, Map showing relation of land and submarine 

topography, De Soto Canyon to Grand Bahama Bank: U.S. Geological Survey Miscellaneous Investigations Series 1-475, scale 

1:1,000,000 [1967].

U.S. Navy, 1962, Handbook of oceanographic tables: U.S. Naval 

Oceanographic Office Special Publication 68.

Vecchioli, John, McKenzie, D.J., Pascale, C.A., and Wilson, W.E., 

1979, Active waste-injection systems in Florida, 1976: U.S. Geological Survey Open-File Report 79-1296, 33 p.

Vernon, R.O., 1970, The beneficial uses of zones of high transmissivities in the Florida subsurface for water storage and waste disposal: 

Florida Bureau of Geology Information Circular 70, 39 p.

 1973, Top of the Floridan artesian aquifer: Florida Bureau of 

Geology Map Series 56.

Watts, W.A., 1971, Postglacial and interglacial vegetation history of 

southern Georgia and central Florida: Ecology, v. 52, p. 676-689.

Wedderburn, L.A., andKnapp, M.S., 1983, Field investigation into the 

feasibility of storing freshwater in saline portions of the Floridan 

aquifer system, St. Lucie County, Florida: South Florida Water 

Management District Technical Publication 83-7, 135 p.

Wedderburn, L.A., Knapp, M.S., Waltz, D.P., and Burns, W.S., 1982, 

Hydrogeologic reconnaissance of Lee County, Florida: South Florida Water Management District Technical Publication 82-1, pt. 1 

(text), 192 p.; pt. 2 (atlas), 29 p.; pt. 3 (app.), 210 p.

Wigley, T.M.L., Plummer, L.N., and Pearson, F.J., Jr., 1978, Mass 

transfer and carbon isotope evolution in natural water systems: 

Geochimica et Cosmochimica Acta, v. 42, p. 1117-1139.

Wilson, W.E., 1977, Ground-water resources of De Soto and Hardee 

Counties, Florida: Florida Bureau of Geology Report of Investigations 83, 102 p.

SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports of 

wide and lasting interest and importance to professional scientists and engineers. Included are reports on die results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic.

Bulletins contain significant data and interpretations that are of lasting scientific interest but are generally more limited in scope or 

geographic coverage than Professional Papers. They include the results 

of resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrogeology, 

availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific 

information of wide popular interest in a format designed for distribution 

at no cost to the public. Information is usually of short-term interest.

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at 

depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material mat are made available for public consultation at 

depositories. They are a nonpermanent form of publication that may be 

cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in? 1/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric 

bases at various scales; they show results of surveys using geophysical 

techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. 

Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 71/2-minute quadrangle photogeologic maps on planimetric bases 

which show geology as interpreted from aerial photographs. Series also 

includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information 

for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; principal scale 

is 1:24,000 and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from the U.S. Geological Survey, Books 

and Open-File Reports Section, Federal Center, Box 25425, Denver, 

CO 80225. (See latest Price and Availability UsL)

"Publications of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982,1983,1984,1985,1986, and for subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by mail 

and over the counter in paperback booklet form only.

"Price and Availability List of US. Geological Survey Publications," issued annually, is available free of charge in paperback booklet form only.

Selected copies of a monthly catalog "New Publications of the U.S. 

Geological Survey" available free of charge by mail or may be obtained 

over the counter in paperback booklet form only. Those wishing a free 

subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 

National Center, Reston, VA 22092.

Note. Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications.