Geochemical and Isotopic Composition of

Ground Water with Emphasis on Sources of 

Sulfate in the Upper Floridan Aquifer in Parts of 

Marion, Sumter, and Citrus Counties, Florida

By Laura A. Sacks

U.S. Geological Survey

Water-Resources Investigations Report 95–4251

Prepared in cooperation with the

SOUTHWEST FLORIDA WATER MANAGEMENT DISTRICT

Tallahassee, Florida

1996
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

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

and does not imply endorsement by the U.S. Geological Survey.

For additional information Copies of this report can be 

write to: purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey Open-File Reports Section

Suite 3015 Box 25286, MS 517

227 N. Bronough Street Denver Federal Center

Tallahassee, FL 32301 Denver, CO 80225
Contents III

CONTENTS

Abstract.................................................................................................................................................................................... 1

Introduction ............................................................................................................................................................................. 2

Purpose and Scope......................................................................................................................................................... 4

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

Geologic and Hydrologic Setting ............................................................................................................................................ 4

Stratigraphy.................................................................................................................................................................... 4

Structure......................................................................................................................................................................... 6

Physiography and Surface Water................................................................................................................................... 6

Hydrogeology ................................................................................................................................................................ 7

Mineralogical Data .......................................................................................................................................................... 9

Carbonate Minerals................................................................................................................................................ 9

Gypsum................................................................................................................................................................ 10

Geochemical and Isotopic Composition of Ground Water ..................................................................................................... 11

Sampling Methods ......................................................................................................................................................... 11

Determining Flow Zones for Wells with Large Open Hole Intervals ................................................................. 12

Sample Collection................................................................................................................................................ 13

Quality Assurance Samples ................................................................................................................................. 14

Chemical Composition of Ground Water ...................................................................................................................... 14

Saturation State of Ground Water........................................................................................................................ 18

Chemical Characterization of High Sulfate Ground Water ................................................................................. 19

Chemical Characterization of Low Sulfate Ground Water.................................................................................. 22

Isotopic Composition of Ground Water ........................................................................................................................ 24

Deuterium and Oxygen-18................................................................................................................................... 24

Carbon-13 ............................................................................................................................................................ 25

Sulfur-34 .............................................................................................................................................................. 26

Sulfate ........................................................................................................................................................ 26

Sulfide........................................................................................................................................................ 27

Total Sulfur ................................................................................................................................................ 28

Geochemical Modeling ........................................................................................................................................................... 29

Reactions Controlling the Composition of Deep, High Sulfate Ground Water ............................................................ 30

Evaluating Upwelling as Source of Sulfate ......................................................................................................... 32

Reactions Controlling the Composition of Deep, Low Sulfate Ground Water ............................................................. 34

Aerobic, Shallow Ground Water ......................................................................................................................... 35

Anaerobic, Shallow Ground Water...................................................................................................................... 35

Uncertainty in the Nature of Interaction between Shallow and Deep Flow System ..................................................... 36

Future Study Needs.............................................................................................................................................. 37

Summary and Conclusions ...................................................................................................................................................... 37

References ............................................................................................................................................................................... 39

Appendix—Chemical and isotopic data from ground-water samples and calculated saturation indexes and

partial pressure of carbon dioxide ........................................................................................................................................... 43
IV Contents

Water Resources of Duval County, Florida FIGURES

1. Map showing study area of northwest central Florida and inland locations where sulfate concentrations 

have been reported as greater than 100 milligrams per liter in water from the Upper Floridan aquifer.................... 2

2. Graph showing profiles of sulfate concentrations in ground water from the Upper Floridan aquifer, as part of the 

Southwest Florida Water Management District’s Regional Observation and Monitoring Program (ROMP) .......... 3

3-4. Maps showing:

3. Large-scale structural features in the study area, and locations of major surface-water bodies and 

inland springs ...................................................................................................................................................... 6

4. Physiographic features in the study area ............................................................................................................. 7

5. Sketch showing idealized flow patterns in the Upper Floridan aquifer between north-central Florida and

the Gulf Coast .............................................................................................................................................................. 8

6. Map showing potentiometric surface of the Upper Floridan aquifer, May 1993 ......................................................... 9

7. Graph showing water levels in wells finished at different depths in the Upper Floridan aquifer at ROMP LP-4, 

near Lake Panasoffkee.................................................................................................................................................. 9

8. Map showing ground-water and rock sampling locations .......................................................................................... 10

9. Graph showing profiles of upward flow using the heat-pulse flow meter in boreholes at ROMP 119 and 

ROMP 120, which are open exclusively to the Avon Park Formation ..................................................................... 13

10. Trilinear diagram showing chemical composition of water from shallow and deep wells from the Upper 

Floridan aquifer ......................................................................................................................................................... 15

11-12. Maps showing:

11. Sulfate concentrations in water from shallow and deep wells from the Upper Floridan aquifer ........................ 16

12. Sulfide concentrations in water from shallow and deep wells from the Upper Floridan aquifer ........................ 16

13-17. Graphs showing:

13. Relation between concentrations of sulfate and calcium, magnesium, strontium, aluminum, chloride, and

fluoride in water from the Upper Floridan aquifer ............................................................................. ................. 17

14. Relation between calcium-to-bicarbonate molar ratio and calcium-to-sulfate molar ratio in water from the 

Upper Floridan aquifer ........................................................................................................................................ 17

15. Relation between calcite and dolomite saturation index and sulfate concentration in water from the

Upper Floridan aquifer ........................................................................................................................................ 18

16. Relation between gypsum and celestite saturation index and sulfate concentration in water from the 

Upper Floridan aquifer ........................................................................................................................................ 19 

17. Relation between partial pressure of carbon dioxide and dissolved oxygen concentration for low sulfate 

and high sulfate ground water from the Upper Floridan aquifer ......................................................................... 19

18. Map showing partial pressure of carbon dioxide in water from shallow wells from the Upper Floridan aquifer ..... 19

19-26. Graphs showing:

19. Relation between calculated and observed aluminum and strontium concentrations for high sulfate

ground water from the Upper Floridan aquifer ................................................................................................... 20

20. Relation between concentrations of chloride and bromide, sodium, and potassium for high sulfate

ground water from the Upper Floridan aquifer ................................................................................................... 21

21. Relation between concentrations of dissolved oxygen and dissolved organic carbon, nitrate, and sulfide

for low sulfate ground water from the Upper Floridan aquifer ........................................................................... 22

22. Relation between concentrations of dissolved oxygen and iron, manganese, silica, calcium, and 

bicarbonate for low sulfate ground water from the Upper Floridan aquifer ....................................................... 23

23. Relation between delta deuterium and delta oxygen-18 in water from the Upper Floridan aquifer ................... 24

24. Relation between delta carbon-13 and partial pressure of carbon dioxide for low sulfate ground water 

from the Upper Floridan aquifer ......................................................................................................................... 24

25. Relation between delta sulfur-34 of sulfate and sulfate concentration and delta sulfur-34 of sulfide and 

sulfate concentration in water from the Upper Floridan aquifer ......................................................................... 26

26. Relation between delta sulfur-34 of total sulfur and sulfate concentration in water 

from the Upper Floridan aquifer, plotted with mixing relations between dilute ground water and water

in equilibrium with gypsum ................................................................................................................................ 28
Contents V

27. Map showing locations of flow paths between shallow and deep ground water modeled using NETPATH. ........... 30

28. Graph showing depth range of open hole intervals of shallow and deep wells for flow paths modeled 

with NETPATH ......................................................................................................................................................... 31

29. Sketch showing scenarios of major reactions modeled with NETPATH between shallow wells and

deep wells with high sulfate concentrations, considering (1) only reactions between open intervals of wells or 

(2) reactions plus upwelling of sulfate-rich water ..................................................................................................... 33

TABLES

1. Relation between stratigraphic and hydrogeologic units .................................................................................................. 5

2. Concentration of selected constituents in gypsum ............................................................................................................ 11

3. Information about sampled wells ...................................................................................................................................... 12

4. NETPATH models between shallow ground water and deep, high sulfate ground water ................................................ 31

5. NETPATH models between shallow ground water and deep, high sulfate ground water, including upwelling 

of sulfate-rich water .......................................................................................................................................................... 34

6. NETPATH models between shallow ground water and deep, low sulfate ground water .................................................. 35
VI Contents

ABBREVIATIONS, ACRONYMS, and VERTICAL DATUM 

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 the United

States and Canada, formerly called Sea Level Datum of 1929.

ft = feet mi = mile

ft2 = square feet PCO2 = partial pressure of carbon dioxide

ft3 = cubic feet δ = delta

in/yr = inches per year 34S = sulfur-34

min = minutes 13C = carbon-13

ppm = parts per million D = deuterium

L = liter 18O = oxygen-18

mL = milliliter SI = saturation index

mmol/L = millimoles per liter DO = dissolved oxygen

mg/L = miiligrams per liter DOC = dissolved organic carbon

μg/L = microgram per liter SWFWMD = Southwest Florida Water Management District

μm = micrometer USGS = United States Geological Survey
Abstract 1

Geochemical and Isotopic Composition of Ground 

Water with Emphasis on Sources of Sulfate in the 

Upper Floridan Aquifer in Parts of Marion, Sumter, 

and Citrus Counties, Florida

By Laura A. Sacks

Abstract

In inland areas of northwest central Florida, 

sulfate concentrations in the Upper Floridan aquifer are extremely variable and sometimes exceed 

drinking water standards (250 milligrams per 

liter). This is unusual because the aquifer is unconfined and near the surface, allowing for active 

recharge. The sources of sulfate and geochemical 

processes controlling ground-water composition 

were evaluated in this area. Water was sampled 

from thirty-three wells in parts of Marion, Sumter, 

and Citrus Counties, within the Southwest Florida 

Water Management District; these included at 

least a shallow and a deep well at fifteen separate 

locations. Ground water was analyzed for major 

ions, selected trace constituents, dissolved organic 

carbon, and stable isotopes (sulfur-34 of sulfate 

and sulfide, carbon-13 of inorganic carbon, 

deuterium, and oxygen-18).

Sulfate concentrations ranged from less than 

0.2 to 1,400 milligrams per liter, with higher sulfate 

concentrations usually in water from deeper wells. 

The samples can be categorized into a low sulfate 

group (less than 30 milligrams per liter) and a high 

sulfate group (greater than 30 milligrams per liter). 

For the high sulfate waters, concentrations of 

calcium and magnesium increased concurrently 

with sulfate. Chemical and isotopic data and massbalance modeling indicate that the composition of 

high sulfate waters is controlled by dedolomitization reactions (dolomite dissolution and calcite 

precipitation, driven by dissolution of gypsum). 

Gypsum occurs deeper in the aquifer than open 

intervals of sampled wells. Upward flow has been 

documented in deeper parts of the aquifer in the 

study area, which may be driven by localized 

discharge areas or rapid flow in shallow parts of the 

aquifer. Mixing between shallow ground water and 

sulfate-rich water that dissolved gypsum at the base 

of the aquifer is probably responsible for the range 

of concentrations observed in the study area. Other 

solutes that increased with sulfate apparently originate from the gypsum itself, from other mineral 

assemblages found deeper in the aquifer in association with gypsum, and from residual seawater 

from less-flushed, deeper parts of the aquifer. 

These ions are subsequently transported with 

sulfate to shallower parts of the aquifer where 

gypsum is not present.

The composition of low sulfate ground water 

is controlled by differences in the extent of microbially mediated reactions, which produce carbon 

dioxide. This, in turn, influences the extent of 

calcite dissolution. Ground waters which underwent limited microbial reactions contained 

dissolved oxygen and were usually in ridge areas 

where recharge typically is rapid. Anaerobic 

waters were in lower lying areas of Sumter 

County, where soils are poorly drained and aquifer 

recharge is slow. Anaerobic waters had higher 

concentrations of calcium, bicarbonate, sulfide, 

dissolved organic carbon, iron, manganese, and 

silica, and had lower concentrations of nitrate than 

aerobic ground waters. For low sulfate waters, 
2 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

sulfate generally originates from meteoric sources 

(atmospheric precipitation), with variable amounts 

of oxidation of reduced sulfur and sulfate reduction. Sulfide is sometimes removed from solution, 

probably by precipitation of a sulfide minerals 

such as pyrite. In areas where deep ground water 

has low sulfate concentrations, the shallow flow 

system is apparently deeper than where high 

sulfate concentrations occur, and upwelling of 

sulfate-rich water is negligible. The range of 

sulfate concentrations observed in the study area 

and differences in sulfate concentrations with 

depth indicate a complex interaction between 

shallow and deep ground-water flow systems.

INTRODUCTION

The Upper Floridan aquifer is a major source of 

drinking water for the state of Florida. Water in the 

aquifer is often nonpotable in coastal areas because 

sulfate concentrations exceed 250 mg/L (milligrams 

per liter). Sulfate concentrations are usually low in 

inland areas, particularly where the aquifer is unconfined. High sulfate concentrations, however, do occur 

in isolated locations of inland northwest central Florida 

(fig. 1). High concentrations in these areas are unusual 

because the aquifer is unconfined and near the surface 

and because this is primarily an aquifer recharge area. 

A better understanding of sulfate sources in inland 

areas is important because increased ground-water 

development could induce movement of this high 

sulfate water to fresher parts of the aquifer.

Several sources of sulfate exist in the aquifer. The 

most apparent source is dissolution of evaporite minerals (gypsum and anhydrite), which are found at the base 

of the Upper Floridan aquifer or in the underlying middle confining unit and Lower Floridan aquifer. This 

source requires an upward movement of water because 

the occurrence of evaporites are considerably deeper 

than zones in which drinking water wells are finished. 

Trace evaporites have not been observed in well cuttings or cores in shallow parts of the aquifer. Saltwater 

mixing is not a likely source of sulfate in inland areas 

because chloride concentrations are low (less than 50 

mg/L). Other possible sources include oxidation of 

sulfide minerals, such as pyrite, or diffusion of sulfate 

from clays in overlying beds. Surficial sources of 

sulfate include meteoric rainwater that contains sulfate, 

oxidation of organic sulfur, and anthropogenic sources 

such as fertilizers applied in the form of sulfate salts. 

Sulfate in rainwater originates primarily from marine 

aerosols and oxidation of reduced sulfur from natural 

and anthropogenic sources.

Sulfur isotope data collected in a regional study of 

the Upper Floridan aquifer were used successfully to 

delineate sulfate sources on a regional scale (Rightmire 

and others, 1974; Rye and others, 1981). For the most 

part, low sulfate concentrations (less than 100 mg/L) 

and isotopically light sulfate were found in the aquifer 

recharge area and were attributed to atmospheric precipitation. Downgradient and in confined parts of the 

aquifer, isotopically heavier sulfate was attributed to 

gypsum dissolution and marine sulfate. This regional 

study did not examine the localized occurrences of high 

sulfate ground water in the recharge area or the vertical 

variability of sulfur sources in the aquifer.

Figure 1. Study area of northwest central Florida and inland 

locations where sulfate concentrations have been reported 

as greater than 100 milligrams per liter in water from the 

Upper Floridan aquifer. (Modified from Sprinkle, 1989; 

Southwest Florida Water Management District, 1991; and 

Katz, 1992.)

82o45’

29o15’

29o00’

45’

28o 30’

82o30’ 15’ 00’

CITRUS COUNTY

HERNANDO

COUNTY

LEVY

COUNTY

SUMTER COUNTY

MARION COUNTY

Mexico

of

Gulf

EXPLANATION

ROMP 110

ROMP 119

ROMP 110

ROMP 120

C

C

B

SWFWMD BOUNDARY

A

C

Sulfate concentration reported as greater than 100 

milligrams per liter. Letter corresponds to source: A = 

Sprinkle (1989); B = Southwest Florida Water 

Management District

Location and name of site with profile of sulfate 

concentration shown in figure 2

Base from Southwest Florida Water Management District digital 

data, 1992. Universal Transverse Mercator projection, Zone 17

Scale 1:500,000

ROMP 112

Study

Area

0 10 MILES

0 10 KILOMETERS
Introduction 3

Water from the Upper Floridan aquifer is chemically stratified in central and northwest peninsular 

Florida, with higher sulfate concentrations found deeper 

in the aquifer (Faulkner, 1973; Sprinkle, 1989; Southwest Florida Water Management District, 1991; Katz, 

1992). Faulkner (1973) attributed higher sulfate concentrations in deeper ground water to an increase in 

gypsum and anhydrite in rocks found deeper in the 

aquifer. This deeper ground water is part of a slower, 

regional flow system with a longer aquifer residence 

time than the rapid, shallow flow system that discharges 

to springs. Southwest Florida Water Management 

District (1991) hypothesized that this high sulfate water 

moves upward in areas of low aquifer recharge, assisted 

by preferential flow through fractures and faults.

Although the chemical stratification of sulfate has 

been recognized, very little data exist on detailed chemical profiles in the aquifer in northwest central Florida. 

Water quality data collected during coring and drilling 

of several sites by Southwest Florida Water Management District (written commun., 1993) demonstrate 

how variable the increase in sulfate with depth is in this 

region (fig. 2). Sometimes sulfate concentrations 

increase rapidly with depth (for example, ROMP 110), 

but other times sulfate concentrations remain relatively 

low in the deeper part of the aquifer (for example, 

ROMP 120; fig. 2). This variability suggests that sulfate 

concentrations and sources vary both vertically and 

laterally in the aquifer. 

Figure 2. Profiles of sulfate concentrations in ground water from the Upper Floridan aquifer, as part of the Southwest

Florida Water Management District’s Regional Observation and Monitoring Program (ROMP). (Locations of sites shown

in figure 1.)

0 500 1,000 1,500 2,000 2,500

-600

-400

-200

SEA

ALTITUDE, IN FEET

0 100 200 300 400 500

SULFATE CONCENTRATION, IN MILLIGRAMS PER LITER

ROMP 110

ROMP 112

ROMP 119

ROMP 120

LEVEL

Abundant

Trace gypsum

Trace

gypsum

gypsum

(all samples collected by reverse air

during drilling)
4 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

The depth of freshwater resources in the unconfined aquifer is often assumed to be the thickness of the 

Upper Floridan aquifer (Causey and Levy, 1976; 

Ryder, 1985). However, the occurrence of high sulfate 

concentrations at relatively shallow depths results in a 

significantly shallower zone of potable water (Jones 

and Upchurch, 1994, pg. 78). Population growth in 

Marion and Citrus Counties will undoubtedly result in 

increased development of ground-water resources. 

In addition, fresh water resources in these counties are 

being considered as supplemental water supplies for 

water-stressed west-central Florida. A better understanding of the sources of sulfate will help water managers more judiciously develop water resources.

In 1991, the U.S. Geological Survey (USGS) 

began a cooperative study with the Southwest Florida 

Water Management District (SWFWMD) to evaluate 

sulfate sources in the Upper Floridan aquifer in two 

separate areas of the water management district. One 

area is in the northern part of the District, in parts of 

Marion, Sumter, and Citrus counties where isolated 

high sulfate concentrations have been observed (fig. 1). 

The other study area is south of Tampa Bay in the confined part of the aquifer. This report focuses on the 

northern study area.

Purpose and Scope

The purpose of this report is to provide additional 

information regarding areal and vertical distribution of 

sulfate in an unconfined part of the Upper Floridan 

aquifer, and to evaluate sources of sulfate and 

geochemical processes controlling ground-water composition in the aquifer. During 1992 and 1993, water 

was sampled from thirty-three wells in parts of Marion, 

Sumter, and Citrus Counties, within the SWFWMD 

boundaries. These included at least a shallow and deep 

well at fifteen separate locations. The sampling locations bracket an area where sulfate concentrations have 

been reported to be variable (Southwest Florida Water 

Management District, 1991). Ground water was sampled for major ions, selected trace elements, dissolved 

organic carbon, field parameters (temperature, pH, 

specific conductance, dissolved oxygen, alkalinity, 

sulfide), and stable isotopes (sulfur-34, deuterium, 

oxygen-18, carbon-13). The sources of sulfate and 

controls on chemical and isotopic composition of water 

were evaluated; hypotheses about reactions were 

examined using geochemical mass-balance modeling.

Acknowledgments

Well selection and sampling efforts depended upon 

the generous assistance of many individuals at the 

SWFWMD, including personnel from the Ambient 

Ground-Water Quality Monitoring Program, Regional 

Observation and Monitoring Program (ROMP), 

Resource Data Department, and Well Permitting 

Department. The author also would like to extend 

thanks to well owners, local water utility departments 

(Cities of Bushnell and Coleman), and local well drillers for allowing us to sample and for providing us with 

specific information about the wells. Rock core analyzed for this study was provided by the Florida Geological Survey. Shelley Kauffman, University of 

Virginia, prepared vials and provided equipment to 

determine presence of sulfate reducing bacteria.

GEOLOGIC AND HYDROLOGIC SETTING

The geology and hydrology of the study area are 

intricately associated because of the proximity of limestone to land surface and high aquifer recharge rates. 

Limestone in the shallow part of the Upper Floridan 

aquifer is highly karstified and the amount of surfacewater drainage in the study area is limited. The study 

area falls primarily within the Withlacoochee River 

Basin. The hydrology and shallow geology of the study 

area have been studied in detail because the proposed, 

but now-defunct, Cross Florida Barge Canal intersected 

the study area (Faulkner, 1973).

Stratigraphy

The study area is underlain by approximately 4,000 ft 

of limestones, dolomites and evaporites (gypsum and 

anhydrite), ranging in age from Cretaceous to Eocene. 

These rocks overlie deeper volcanic, metamorphic, and 

Mesozoic sedimentary rocks (Vernon, 1951; Applin 

and Applin, 1965; Smith, 1982). Of interest in this 

study are the upper 2,000 ft of Tertiary age rocks, 

which constitute the Floridan aquifer system (table 1). 

The base of the Cedar Keys Formation of Paleocene 

age occurs at about 2,000 ft below sea level. Rocks of 

the Cedar Keys consist of dolomite with variable 

amounts of gypsum and anhydrite. The base of the 

Floridan aquifer system occurs in areally extensive, 

thick anhydrite beds in the lower two-thirds of this 

formation (Miller, 1986). Overlying the Cedar Keys 

Formation is the Oldsmar Formation of early Eocene 

age, which is composed of limestones, dolomites, and 

thin beds of evaporites and chert.
Geologic and Hydrologic Setting 5

In the study area, water wells are drilled into the 

upper part of the Avon Park Formation of middle Eocene 

age and the Ocala Limestone of late Eocene age, which 

make up the Upper Floridan aquifer (table 1). The lower 

part of the Avon Park Formation (formerly the Lake City 

Limestone; Miller, 1986) consists primarily of dolomite, 

with intergranular gypsum and beds of anhydrite, peat, 

and chert occurring in some locations. The upper part of 

the Avon Park also is usually dolomitized, although 

sometimes it consists of limestone; evaporites presumably are not present. 

The Avon Park Formation was deposited in a 

peritidal warm-water carbonate bank. Evaporites were 

probably formed in a tidal flat or sabkha environment 

in an arid climate analogous to the modern Persian Gulf 

(Miller, 1986; Randazzo and others 1990; Cander, 

1991). Sometimes evaporites are bedded, but in other 

places they occur as nodules or secondary infilling of 

preexisting pore spaces in the rock. Evaporite deposits 

are not continuous and probably were formed in isolated evaporative basins on a carbonate bank separated 

from the shallow sea. Thin evaporite beds probably 

fragmented when carbonate deposition resumed, and 

these fragments were then incorporated into the host 

rock as nodules. Some of these nodules have an outer 

core of gypsum and a central core of anhydrite (Hickey, 

1990). Secondary gypsum results in significantly 

reduced permeability. It is generally thought that 

dolomitization occurred not long after deposition in the 

middle Eocene by a platform-wide influx of normal to 

hypersaline seawater (Faulkner, 1973; Cander, 1991). 

Multiple dolomitization episodes, however, probably 

occurred. Some of the dolomite probably formed in 

freshwater/saltwater mixing zones at later times 

(Hanshaw and Back, 1972; Randazzo and Hickey, 

1978; Cander, 1991).

The contact between the Avon Park and the overlying Ocala Limestone is an erosional unconformity 

that generally occurs at an elevation between 80 ft 

below sea level to several feet above sea level 

(Faulkner, 1973; Miller, 1986; Campbell, 1989). In 

southwest Marion County and bordering Levy County, 

the Ocala Limestone has been completely removed by 

erosion, and the Avon Park Formation is the uppermost 

carbonate rock unit. The Ocala Limestone generally is 

composed of soft, fossiliferous to micritic limestone 

(Faulkner, 1973; Miller, 1986). Lower parts of the 

Ocala may be locally dolomitized. Thin, irregular 

zones of chert of limited areal extent are sometimes 

present. The Ocala Limestone was formed in a shallow, 

open to marginal marine environment (Miller, 1986; 

Randazzo and others, 1990). Evaporite minerals are not 

present in the Ocala Limestone because of better openwater circulation and possibly a more humid climate 

than when the Avon Park was deposited. Upper parts of 

the Ocala have been eroded in much of the study area, 

and the unit is much thinner than in other parts of Florida. The Ocala Limestone is very permeable with well 

developed secondary porosity and a highly irregular 

karstified surface.

The Ocala Limestone is overlain by clastics of 

Miocene to Holocene age, which range in thickness 

from less than 10 ft to greater than 100 ft on the ridges. 

Most of these deposits are undifferentiated sands; however, the Hawthorn Group and Alachua Formation are 

present in isolated parts of the study area. The Hawthorn Group of early Miocene age, which is present in 

much of peninsular Florida (Scott, 1988), is absent in 

the study area except for a few erosional remnants 

associated with ridges, usually in Marion County 

(Faulkner, 1973; Brooks, 1981). These deposits 

generally consist of phosphatic sands, clayey sands, 

Table 1. Relation between stratagraphic and 

hydrogeologic units. Quaternary

Series

Holocene

Pleistocene

Pliocene

Miocene

Eocene

Paleocene

Oldsmar

Cedar Keys

Formation

Avon Park

Formation

Ocala

Limestone

Hawthorn

Group

1

Alachua

Formation

Alluvium

and terrace

Lower

Floridan

aquifer

confining

unit

Upper

Floridan

aquifer

Surficial

aquifer

system

Floridan aquifer system

Stratigraphic

Unit

Hydrogeologic

Unit

Formation

1

Based on nomenclature of Scott (1988)

and

deposits

General

Lithology

Sands and clays

Phosphatic sands

and clays

Phosphatic sands,

Upper part,

limestone and

dolostone

Lower part,

dolomite with

intergranular

gypsum; some

bedded gypsum,

Tertiary

System

and clays

Limestone,

fossiliferous to

micritic

peat and chert

Limestone and dolomite; some evaporites and chert

Dolomite with

evaporites

Middle

(Of limited

areal extent

in study area)

Typical open intervals of shallow (S) and deep (D) wells

S

D
6 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

and clays. The Alachua Formation of Miocene to 

Pliocene age is found beneath the Brooksville Ridge in 

Citrus County. This unit is composed of irregularly 

interbedded clays, sands, sandy clays, and phosphate 

(Vernon, 1951; Olson, 1972; Faulkner, 1973). The 

origin of the Alachua is unresolved, but it probably 

represents a complex depositional environment, 

incorporating eroded and reworked Hawthorn Group 

sediments (Scott, 1988).

Structure

The most significant structural feature in the study 

area is the Ocala Platform (also called the Ocala Uplift 

and the Ocala High), which is a broad structural high in 

rocks of middle Eocene and younger ages (fig. 3). This 

feature was produced by sedimentational processes, 

rather than tectonic processes, but the mechanisms 

responsible for producing this high are not understood 

(Winston, 1976; Miller, 1986). Because this area has 

been a structural high since the middle Eocene, it has 

undergone increased erosional activity over other parts 

of the Florida Peninsula. The Avon Park Formation is 

closer to land surface than in any other location, and the 

Ocala Limestone has undergone much erosion and is 

completely eroded in parts of southwest Marion 

County and neighboring Levy County. If Oligocene 

and Miocene age rocks were originally deposited over 

the study area, they have been almost completely 

eroded. Karst landscapes are well developed in the area 

of the Ocala Platform because of the presence of carbonate rocks near land surface and a poorly consolidated overburden (Schmidt and Scott, 1984). 

Numerous faults and fractures have been mapped in 

the region of the Ocala Platform that are probably associated with this structural high (Vernon, 1951; Faulkner, 

1973). These faults and fractures have trends that are 

parallel (northwest to southeast) and perpendicular 

(northeast to southwest) to the main axis of the Ocala 

Platform. In Marion County, cavern systems have 

developed in the Ocala Limestone that are oriented in 

these same directions, indicating preferential limestone 

dissolution along these fracture systems (Faulkner, 

1973; Phelps, 1994). The locations of Lake Panasoffkee 

and the Withlacoochee River along the Citrus/Sumter 

County line also follow these same trends (fig. 3), and 

their locations may similarly be related to preferential 

dissolution of carbonates at fault or fracture planes 

(Southwest Florida Water Management District, 1991).

To the east of the Ocala Platform is the Peninsular 

arch, a northwest trending feature that is much older 

than the Ocala Platform. This feature has been intermittently positive from the Mesozoic time through the 

Cenozoic, and is apparently an upwarp produced by 

compressional tectonics (Miller, 1986). The study area 

is along the western flank of the Peninsular arch 

(fig. 3).

Physiography and Surface Water

Changing sea levels, differential erosion, and karst 

processes control current landform features in the study 

area. Topographically high areas generally correspond 

to erosional remnants and ancient ridges from higher 

sea level stands (White, 1970; Brooks, 1981). Resistant 

clays may be partly responsible for the preservation of 

these relict highlands. In the study area, upland areas 

consist of the Brooksville Ridge, the Sumter Upland, 

and, of lesser extent, the Cotton Plant Ridge, Martel 

Hill, and Ocala Hill in Marion County (fig. 4). The 

soils in these upland areas are excessively to well 

drained, resulting in minimal surface water other than 

isolated ponds (Soil Conservation Service, 1979; 

1988a).

Figure 3. Large-scale structural features in the study area, 

and locations of major surface-water bodies and inland 

springs. (Modified from Faulkner, 1973; and Miller and 

others, 1981.)

SUMTER

COUNTY

82o45’

29o15’

29o00’

45’

28o30’

82o30’ 15’ 00’

CITRUS COUNTY

HERNANDO

COUNTY

LEVY

COUNTY

Mexico

of

Gulf

SWFWMD BOUNDARY

Base from Southwest Florida Water Management District digital 

data, 1992. Universal Transverse Mercator projection, Zone 17

Scale 1:500,000

0 10 MILES

0 10 KILOMETERS

Silver

Springs

Rainbow

Springs

Rainbow

Springs

Run River

Tsala Apopka Lake

River

Panasoffkee

Lake

PENINSULAR

ARCH

PLATFORM

OCALA

Lake

Rousseau

Withlacoochee

Oklawaha

EXPLANATION

Inland spring with discharge 

greater than 1 cubic foot per 

second.

MARION

COUNTY
Geologic and Hydrologic Setting 7

Lower lying areas include the Tsala Apopka Plain 

and the Western Valley, which cover eastern Citrus 

County and most of the study area in Sumter County 

(fig. 4). Poorly drained soils result in numerous ponds 

and lakes (Soil Conservation Service, 1988a; 1988b). 

The low elevation of the Western Valley is attributed to 

mature karst erosional cycles and a lack of resistant 

clays (White, 1970). The very flat expanse and lower 

elevation of the Tsala Apopka Plain probably formed 

from a large lake that once covered the area (White, 

1958; 1970). Lake Panasoffkee, the Tsala Apopka 

chain of lakes, and the Withlacoochee River are in low 

lying areas of this plain. 

The Withlacoochee River is the dominant surface 

drainage in the study area (fig. 3). This river originates 

in the Green Swamp to the southeast of the study area, 

in an area with low aquifer transmissivity and minimal 

aquifer recharge (Ryder, 1985; Swancar and Hutchinson, 1995). The Dunnellon Gap, which cuts through the 

Brooksville Ridge, is the outlet of the Withlacoochee 

River to the Gulf of Mexico (fig. 4). White (1958) concluded that this gap is relatively recent, and at one time 

the Withlacoochee River flowed southward toward 

Tampa Bay. The present course of the river probably 

follows solution channels in the limestone that may 

represent a fracture or fault trace (Faulkner, 1973). The 

Withlacoochee River is dammed to the west of the gap 

and forms Lake Rousseau, which was constructed to be 

part of the Cross Florida Barge Canal.

The area has several inland springs that discharge 

more than 1 ft3

/s (Rosenau and others, 1977; Miller and 

others, 1981; fig. 3). Rainbow Springs, in southwest 

Marion County, is the largest spring and discharges 

over 500 ft3

/s. Dissolved solids and tritium data from 

Rainbow Springs indicate that the springs discharge 

relatively recent ground water from shallow parts of the 

aquifer (Faulkner, 1973). Increased specific conductance along Rainbow Springs Run has been attributed 

to additional spring flow, which may have a contribution from deeper zones in the aquifer than discharge to 

Rainbow Springs (Jones and others, in press). Most of 

the other springs in the study area drain into the Withlacoochee River or Lake Panasoffkee and have not 

been studied in detail.

Hydrogeology

The water table usually occurs within the surficial 

deposits, and these saturated clastics constitute the 

surficial aquifer system. Clay beds generally are not 

thick enough to establish a confining unit, and water 

percolates from the surficial aquifer system into the 

underlying Upper Floridan aquifer. Clays of the 

Alachua Formation beneath the Brooksville Ridge may 

semiconfine the Floridan aquifer system (Miller, 

1986), although variability in lithologic logs does not 

support a continuous confining unit.

The Floridan aquifer system is defined as a vertically continuous sequence of carbonate rocks of high 

permeability and regional extent (Miller, 1986). In the 

study area it is separated by a middle confining unit into 

the Upper and Lower Floridan aquifers (table 1). The 

middle confining unit corresponds to low permeability 

gypsiferous dolomite and dolomitic limestone. This unit 

usually is within the middle to lower part of the Avon 

Park Formation at an elevation between 500 and 800 ft 

below sea level (Miller, 1986). The hydraulic conductivity of the middle confining unit (0.01 to 0.1 ft/d; Hickey, 

1990) is many orders of magnitude lower than in the 

overlying Upper Floridan aquifer. However, the degree 

of confinement is not well understood. Miller (1986) 

describes the middle confining unit as a “nonleaky confining bed,” whereas Hickey (1990) concludes that its 

confining properties may be more analogous to a finegrained sandstone than a compact clay bed. Beneath the 

middle confining unit, the Lower Floridan aquifer is 

rarely drilled into because of its poor quality water and is 

not utilized in the study area.

Figure 4. Physiographic features in the study area. 

(Modified from White, 1970; and Miller and others, 1981.)

Brooksville Ridge

Tsala

Apopka Plain

Western Valley

Western Valley

Central

Valley

Gulf Coastal

Lowlands

Sumter Upland

Brooksville Ridg

Dunnellon Gap

e

Cotton Plant

Martel

Hill

Fairfield

Hills

Ocala

Hill

82o 45’

29o15’

29o 00’

45’

28o30’

82o 30’ 15’ 00’

CITRUS

COUNTY

HERNANDO

COUNTY

LEVY

COUNTY

SUMTER

COUNTY

MARION COUNTY

Mexico

of

Gulf

Base from Southwest Florida Water Management District digital 

data, 1992. Universal Transverse Mercator projection, Zone 17

Scale 1:500,000

0 10 MILES

0 10 KILOMETERS

EXPLANATION

Ridge Upland Valley or

Plain

Lake

Upland
8 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

The top of the Upper Floridan aquifer usually 

corresponds to the surface of the Ocala Limestone 

(table 1), although in places the aquifer surface is very 

irregular and the first carbonate rock unit encountered 

is the deeper Avon Park Formation. The Ocala Limestone is more permeable than the Avon Park Formation 

because of more extensive development of secondary 

porosity (Faulkner, 1973). The Upper Floridan aquifer 

is the principle water supply for most of the study area, 

and large quantities of good quality water usually can 

be produced at relatively shallow depths.

Shallow and deep ground-water flow systems 

occur in the Upper Floridan aquifer in the study area 

(Faulkner, 1973; Ryder, 1985; Bush and Johnston, 

1988). The shallow flow system discharges to local 

springs and rivers. Much of this flow takes place in 

conduits through well-developed secondary porosity in 

the limestone, with a short aquifer residence time. 

Deeper ground water is part of a more sluggish regional 

flow system that bypasses local discharge areas and 

discharges near or offshore of the coast (fig. 5).

Maps of recharge and discharge areas and of the 

potentiometric surface of the Upper Floridan aquifer do 

not differentiate between these shallow and deep flow 

systems. Most of the study region is an aquifer recharge 

area (Anderson and Laughlin, 1982). Upland parts of 

Citrus and Marion Counties have somewhat higher aquifer recharge rates (10 to 20 in/yr) than lower lying parts 

of Sumter County (1 to 10 in/yr) (Ryder, 1985; Aucott, 

1988). Discharge areas are focused in narrow areas 

around the Withlacoochee River and Lake Panasoffkee. 

Localized discharge also occurs at springs.

The direction of ground-water flow is controlled by 

a high in the potentiometric surface in the Green Swamp 

in southeast Sumter County. Ground water flows westnorthwest from southern Sumter County toward the 

Withlacoochee River, and then flows westward in Citrus 

County toward the coast (fig. 6). The high in the potentiometric surface and relatively steep hydraulic gradient 

in Sumter County are due to relatively low aquifer transmissivity in this area (50,000 to 500,000 ft2

/d), compared 

to parts of Citrus and Marion Counties, where aquifer 

transmissivity is greater than 1,000,000 ft2

/d (Ryder, 

1985). In Marion County, the potentiometric surface is 

very flat, and recharge dissipates rapidly because of the 

high aquifer transmissivity. A saddle occurs in the potentiometric surface in central Marion County, where 

regional ground water converges from highs in the 

potentiometric surface to the north and south. Flow from 

there is either west or southwest toward the Withlacoochee River and Rainbow Springs or eastward toward 

Silver Springs.

Very little information exists about head distribution with depth in this part of the Upper Floridan aquifer. It is often assumed that vertical hydraulic gradients 

in the aquifer are minimal (Faulkner, 1973). Head distribution data are available for three wells finished at 

separate depths in the aquifer near Lake Panasoffkee 

(ROMP LP-4; Southwest Florida Water Management 

District, written commun., 1994; fig. 7). At this site, an 

upward head gradient occurs between deeper and 

shallower zones in the Avon Park Formation. The head 

in the well finished in the Ocala Limestone, however, 

is usually higher than the well completed in the upper 

part of the Avon Park Formation, indicating recharge or 

downward flow.

Figure 5. Idealized flow patterns in the Upper Floridan aquifer between north-central Florida and the Gulf Coast. 

(Modified from Bush and Johnston, 1988.)

Gulf of Mexico Major Stream

Floridan aquifer system

Freshwater-saltwater interface

Major Spring Land Surface

WEST EAST

EXPLANATION

Shallow flow system

Deep flow system
Geologic and Hydrologic Setting 9

Figure 7. Water levels in wells finished at different depths 

in the Upper Floridan at ROMP LP-4, near Lake 

Panasoffkee. (Data from Southwest Florida Water 

Management District; well locations shown in figure 8; 

index numbers referenced in table 3.)

J FM AM J J A S O ND J F M AM J J A S O ND

38

39

40

41

42

43

1992 1993 

WATER LEVEL, IN FEET ABOVE SEA LEVEL

AVON PARK

(12O) WELL

AVON PARK

(240) WELL

OCALA

WELL

Ocala

Avon Park (120)

Avon Park (240)

15 to 30 feet

100 to 120 feet

200 to 240 feet

Well Open Interval

Mineralogical Data

Mineralogy and the chemical and isotopic composition of major minerals in the aquifer were evaluated 

to assist in interpreting ground-water chemical and 

isotopic data. The mineralogy of the aquifer has been 

well defined in previous studies, with a thorough summary by Sprinkle (1989). Calcite and dolomite are, by 

far, the dominant minerals in the aquifer. Most of the 

calcite ranges from pure CaCO3 to low magnesian 

calcite (less than 2 mole percent magnesium; Hanshaw 

and others, 1971). Dolomites range in composition 

from stoichiometric dolomite (CaMg(CO3)2) to 

Ca1.12Mg0.88(CO3)2 (Hanshaw and others, 1971; 

Randazzo and Hickey, 1978). This variability in 

dolomite composition probably influences its solubility 

in ground water (Sprinkle, 1989). Minor minerals in the 

aquifer include gypsum (CaSO4·2H2O) and anhydrite 

(CaSO4) in deeper zones of the aquifer, quartz and 

chert, apatite, clay minerals (kaolinite, glauconite, and 

possibly montmorillonite), potassium feldspar, and 

metallic oxides and sulfides (for example, goethite and 

pyrite); localized thin beds of peat also occur within the 

aquifer (Fischer, 1953; Hanshaw and others, 1971; 

Sprinkle, 1989; Katz and others, 1995b).

Carbonate Minerals

For mass-transfer modeling, it is necessary to 

define the stable carbon isotopic composition (13C) of 

the carbonate minerals that interact with the ground 

water. Isotopic data are reported as ratios of the heavy 

to the light isotope (for example, 13C/12C), relative to a 

standard, in delta (δ) notation:

δsample = 1000 [(Rsample/Rstd) –1], (1)

where Rsample and Rstd are the ratio of the heavy to the 

light isotope in the sample and in the standard, respectively. Standards used in this report are Pee Dee belemnite (PDB) for carbon (δ13C), Cañon Diablo troilite 

(CDT) for sulfur (δ34S), and Standard Mean Ocean 

Water (SMOW) for hydrogen (δD) and oxygen (δ18O). 

Figure 6. Potentiometric surface of the Upper Floridan Units are in parts per thousand, denoted as “per mil.”

aquifer, May 1993. (Modified from Halford and others, 1993; 

and Mularoni, 1994.)

82o 45’

29o15’

29o00’

45’

28o30’

82o 30’ 15’ 00’

CITRUS

COUNTY

HERNANDO

COUNTY

LEVY

COUNTY

SUMTER

COUNTY

MARION

COUNTY

Mexico

of

Gulf

SWFWMD BOUNDARY

Base from Southwest Florida Water Management District digital 

data, 1992. Universal Transverse Mercator projection, Zone 17

Scale 1:500,000

LAKE COUNTY

Withlacoochee

30

40

1020

50

50

60

5

80

40

10

90

100

70

60

Silver

Springs

Rainbow

Springs

50

20

40

30

EXPLANATION

10 POTENTIOMETRIC CONTOUR -- Shows altitude of 

potentiometric surface of Upper Floridan aquifer, May 1993. 

Datum is sea level. Contour interval is 5 and 10 feet.

River

Green

Swamp

70

0 10 MILES

0 10 KILOMETERS

50

20

30
10 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

The δ13C composition of calcite and dolomite from 

the aquifer has been determined in previous studies to 

evaluate water-rock interactions in the aquifer and to 

better understand dolomitization and diagenetic processes (Hanshaw and Back, 1972; Cander, 1991). 

Values near 0 per mil are typical for marine carbonates. 

Most reported δ13C values for calcite are near 0 per mil, 

and range between -0.4 and 3.1 per mil in the aquifer 

(Hanshaw and Back 1972; Sprinkle, 1989; Cander, 

1991). The range of reported δ13C for dolomite is considerably greater than for calcite, between -7.5 and 

1.0 per mil (Hanshaw and Back 1972; Sprinkle, 1989; 

Cander, 1991). Hanshaw and Back (1972) noted two 

groupings of dolomite based on isotopic composition, 

one which was near seawater composition and the other 

which was isotopically lighter than seawater (between 

-7.5 and -2.8 per mil). Lighter dolomites were hypothesized as forming in the saltwater mixing zone.

Two calcite and two dolomite samples were 

analyzed for this study in order to obtain specific information from the study area. The calcite samples were 

from the Ocala Limestone from ROMP 110 (59 ft) and 

ROMP 119 (70 to 75 ft; fig. 1). The δ13C values for 

calcite were within ranges of previous work: 1.2 per 

mil for ROMP 110 and -0.4 per mil for ROMP 119. 

Dolomite samples from the Avon Park Formation at 

these same sites also were analyzed; x-ray diffraction 

analysis verified that dolomite was the only carbonate 

mineral present in the sample (John M. Neil, U.S. Geological Survey, written commun., 1994). The isotopic 

composition of carbon from the dolomite samples were 

very similar to each other: 0.3 per mil for ROMP 110 

(469 ft) and 0.4 per mil for ROMP 119 (490 to 495 ft; 

within the open interval of the sampled well). These 

values are similar to seawater composition but differ 

from the isotopically light carbon observed by 

Hanshaw and Back (1972). 

Gypsum

Gypsum and anhydrite have not been reported 

from the upper part of the aquifer (within the Ocala 

Limestone and upper part of the Avon Park Formation). 

However, gypsum becomes more abundant at the base 

of the aquifer and within the middle confining unit, 

which corresponds to the middle to lower part of the 

Avon Park Formation. Because gypsum is a probable 

source for sulfate in the ground water, the isotopic and 

trace element composition of gypsum from the middle 

confining unit was evaluated. 

Limited data are available for the sulfur isotope 

composition of gypsum from the Floridan aquifer 

system (Rye and others, 1981; Sprinkle, 1989), and 

they range from 18.9 to 23.2 per mil. From within the 

study area, two additional gypsum samples from the 

Avon Park Formation (within the middle confining 

unit) were analyzed. These samples were both from 

eastern Citrus County (ROMP 110 from 486 ft and 

W-7534 from 795 ft) (fig. 8). The δ34S values for these 

samples were very similar (24.5 per mil for ROMP 110 

and 24.0 per mil for W-7534; analytical uncertainty is 

assumed to be 0.5 per mil). This is slightly heavier than 

previously reported δ34Sgypsum values from the aquifer; 

however, these differences may be due to changes in 

extraction procedures and calibration standards since 

the early 1980’s (W.C. Shanks, U.S. Geological Survey, written commun., 1995; Rees and others, 1978). 

The δ34S of gypsum also is isotopically heavier than 

expected for evaporites deposited from Eocene seawater 

(about 20 per mil; Claypool and others, 1980). Gypsum 

that is isotopically heavier than seawater probably is the 

result of localized sulfate reduction in brines from which 

the gypsum precipitated. Sulfur is fractionated during 

Figure 8. Ground-water and rock sampling locations.

CITRUS

COUNTY

HERNANDO

COUNTY

82o45’

29o15’

29o 00’

45’

28o 30’

82o30’ 15’ 00’

LEVY

COUNTY

SUMTER COUNTY

MARION COUNTY

Mexico

of

Gulf

Base from Southwest Florida Water Management District digital 

data, 1992. Universal Transverse Mercator projection, Zone 17

Scale 1:500,000

0 10 MILES

0 10 KILOMETERS

ROMP 110

ROMP 119

W-7534

1,2 3,4

5,6 7,8,9

10,11

12,13 14,15 16,17

18,19 20,21

32,33

26,27,28

29,30,31

22,25

SWFWMD

BOUNDARY

23,24

EXPLANATION

ROMP 110

15 Ground-water sampling location and index 

number from table 3 and appendix A.

Rock sampling location and name.
Geochemical and Isotopic Composition of Ground Water 11

sulfate reduction. The reduced sulfur is enriched in the 

lighter isotope (sulfur-32), and isotopically heavier 

sulfate remains in solution. In the ROMP 110 core, zones 

of pyrite, organics, and plant remains are reported in the 

same and nearby depth intervals as the gypsum sample 

(Southwest Florida Water Management District, written 

commun., 1992). This indicates that anaerobic conditions were present, although it is not known if these sediments were anaerobic at the time gypsum precipitated. 

Attempts to analyze the isotopic composition of sulfur in 

the pyrite and peat were unsuccessful because sample 

sizes were too small (W.C. Shanks, U.S. Geological Survey, written commun., 1994).

Gypsum samples from the Floridan aquifer system 

in southwest Florida were analyzed for δ34S, as part of 

a related study. Values of δ34Sgypsum in the Avon Park 

Formation (within the middle confining unit) ranged 

from 20.0 to 24.8 per mil. Deeper samples from the 

underlying Oldsmar and Cedar Keys Formations 

(Lower Floridan aquifer and lower confining unit) also 

were analyzed and were slightly lighter, ranging from 

19.4 to 21.2 per mil.

Gypsum often contains trace amounts of other ions 

that can substitute for calcium in normal lattice sites or 

reside in interstitial positions (Kushnir, 1980). Data 

were not available on trace element concentrations in 

gypsum and anhydrite from the Floridan aquifer system (Sprinkle, 1989). Thus, in order to estimate concentrations of other elements present in gypsum, the 

ROMP 110 gypsum sample from the middle confining 

unit was analyzed for selected trace elements. A visually pure sample of gypsum was crushed with a mortal 

and pestle, dried in a desiccator, and precisely weighed. 

Then, the gypsum was dissolved with a known volume 

of deionized water, acidified with 1 mL of 70 percent 

hydrochloric acid, and analyzed for dissolved cations 

and trace metals. An unacidified sample was analyzed 

for sulfate concentration to compare with the theoretical weight percent expected for gypsum and anhydrite. 

This can be used as an indicator of sample purity. The 

concentrations of dissolved constituents were related to 

ppm based on the original sample weight. Strontium 

was the most abundant trace element, with a concentration of 850 ppm (table 2). Other minor (less than 500 

ppm) trace elements detected in the gypsum included 

magnesium, sodium, barium, aluminum, lithium, and 

iron. Data from six additional gypsum samples from 

the Floridan aquifer system from southwest Florida 

(part of a related study) also are summarized in table 2.

GEOCHEMICAL AND ISOTOPIC 

COMPOSITION OF GROUND WATER

Water samples were collected from shallow and 

deep wells to assess the areal and vertical distribution 

of sulfate in the Upper Floridan aquifer. Chemical and 

isotopic data were used to evaluate geochemical and 

isotopic controls on ground-water composition and to 

assess sulfate sources in the aquifer. Well sampling 

locations bracketed the area where high sulfate concentrations were reported by Southwest Florida Water 

Management District (1991).

Sampling Methods

Well selection was based on spatial and vertical 

coverage within the aquifer. Although stratigraphic 

data were not available for most wells, shallow wells 

were usually finished near the top of the aquifer and are 

assumed to be in the Ocala Limestone; deep wells were 

assumed to be finished in the Avon Park Formation, 

based on the nearest stratigraphic control point (Southwest Florida Water Management District, written commun., 1993). Deep and shallow wells were sampled in 

Table 2. Concentrations of selected constituents in gypsum

[ppm, parts per million; %, percent; concentrations determined by 

dissolving visually pure sample of known weight into known volume of 

deionized water and analyzing water sample at laboratory using 

standard methods of Fishman and Friedman (1989); potassium, 

silica, and manganese all less than detection limit of 90, 9, and 1 ppm, 

respectively]

1 Six gypsum samples from Floridan aquifer system (unpublished data).

2 For two sets of replicate analyses from southwest Florida (unpublished 

data).

3 Theoretical weight percent is 23 for gypsum (CaSO4•H2O) and 29 for 

anhydrite (CaSO4).

4 Theoretical weight percent is 56 for gypsum and 71 for anhydrite.

Constituent

Reporting

unit

ROMP

110

(486 ft)

Range of 

samples

from

southwest

Florida1

Detection

limit

Mean 

difference 

between 

replicate

analyses2 Minimum

Maximum

Magnesium ppm 480 120 1,400 90 28

Sodium ppm 140 90 ,380 90 21

Barium ppm 82 7 , 58 2 32

Iron ppm 25 5 , 32 4 6

Strontium ppm 850 480 2,000 1 310

Aluminum ppm 59 43 ,200 18 51

Lithium ppm 27 5 , 40 4 18

Calcium3 weight % 23 9 , 31 0.01 2.0

Sulfate4 weight % 55 56 , 75 0.01 1.6
12 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

the same general location. Because of the abundance of 

good quality water at shallow depths, most water wells 

in the study area are relatively shallow. Thus, sampling 

locations were limited by availability of deep wells. 

“Deep” wells had highly variable depths, depending 

upon local well availability. Consequently, it was more 

useful to compare deep wells to adjacent shallow wells 

than to other deep wells. Very few deep monitoring 

wells are in the study area; most sampled wells were 

domestic or public supply wells. Information on well 

and casing depths were required, particularly for deep 

wells. Much of this information was obtained from 

driller’s reports. Fourteen wells were sampled in 

Marion County at six separate locations; thirteen wells 

were sampled in Sumter County at six separate locations; and six wells were sampled in Citrus County at 

three separate locations (fig. 8; table 3).

Purging methods varied depending upon the type 

of well. Water supply wells had high yielding pumps 

that were used routinely. For these wells, sampling 

commenced after field parameters (temperature, pH, 

and specific conductance) stabilized. Samples were 

collected from as close to the well head as possible, 

prior to the holding tank. For monitoring wells (wells 

7, 8, 9, and 20), at least three casing volumes of water 

were pumped prior to sampling, while monitoring field 

parameters. A submersible pump was used for sampling most wells. Exceptions are noted in the appendix.

Determining Flow Zones for Wells with Large 

Open Hole Intervals

Two monitoring wells had large open hole intervals 

(ROMP 119 and ROMP 120, with open hole intervals 

of 396 ft and 293 ft, respectively). Prior to sampling 

these wells, a vertical flow survey was done under 

static conditions to determine if 

distinct flow zones could be identified in the borehole. If so, a sample could be collected from a 

more discrete depth interval than 

the entire borehole. The open 

hole intervals of both wells were 

completely within the Avon Park 

Formation.

The heat-pulse flowmeter can 

detect low (0.1 ft/min) upward or 

downward flow within a borehole 

(Hess, 1990; Hess and Paillet, 

1990; Crowder and others, 1994). 

The flow rate is determined by 

measuring the time for a pulse of 

heated borehole water to move to 

an upper or lower thermistor. 

Velocity values are determined 

from calibration data for tubes of 

similar diameter as the borehole 

under laboratory conditions 

(Hess and Paillet, 1990). A 

diverter was used with the meter 

to concentrate the flow in the 

borehole through the meter. 

Measurements were most accurate where the borehole was narrowest, allowing all of the flow to 

pass through the diverter to the 

meter. 

Table 3. Information about sampled wells

[Well locations shown in figure 8; ft, feet; --, data not available]

1

 Site identification number is composed of the latitue, longitude, and sequence number.

Well 

numberName

Site 

identification 

number1

Casing

depth

(ft)

Well

depth

(ft)

Elevation (ft 

above 

sea 

level)

County

1 Kellogg shallow well 283924081581501 50 60 93 Sumter

2 Kellogg deep well 283939081580701 210 250 92 Sumter

3 City of Bushnell #2 well 284002082064201 598 693 76 Sumter

4 St. Lawrence Church rectory well 284003082063201 83 105 78 Sumter

5 White well 284422082180601 252 268 65 Citrus

6 Owens well 284422082181001 62 70 60 Citrus

7 ROMP LP-4 Avon Park (240) 284628082073801 200 240 50 Sumter

8 ROMP LP-4 Avon Park (120) 284628082073802 100 120 50 Sumter

9 ROMP LP-4 Ocala 284628082073803 15 30 50 Sumter

10 City of Coleman well 284736082042301 90 200 67 Sumter

11 Rolling well 284743082041601 51 61 66 Sumter

12 Hawkins well 285055082122601 42 50 51 Sumter

13 Campers’ World well 285107082124101 130 160 52 Sumter

14 Pilot Oil well 285225082054101 84 126 56 Sumter

15 Union Oil well 285228082054301 189 253 54 Sumter

16 Lorenz well 285338082261701 475 570 125 Citrus

17 Wooten well 285340082252301 148 150 99 Citrus

18 Budd well 290053082313601 387 420 120 Citrus

19 Iacino well 290053082315101 168 170 100 Citrus

20 USGS observation well CE-78 290132082133001 61 82 89 Marion

21 ROMP 119 290133082140901 106 502 72 Marion

22 Quaglio well 290438082201501 165 167 87 Marion

23 Silver well 290438082272701 100 120 75 Marion

24 Brookshier well 290441082273301 270 360 80 Marion

25 Barton well 290506082202801 76 90 85 Marion

26 Bonnie Builders well 290736082123201 66 84 73 Marion

27 Saddle Oak deep well 290737082124601 225 282 85 Marion

28 Saddle Oak shallow well 290737082124602 86 160 85 Marion

29 ROMP 120 291059082190801 110 403 76 Marion

30 Rowland deep well 291104082191701 136 147 102 Marion

31 Rowland shallow well 291104082191702 -- 69 102 Marion

32 Jones well 291157082130301 442 460 97 Marion

33 Brosky well 291201082124401 150 165 93 Marion
Geochemical and Isotopic Composition of Ground Water 13

Results from the flow survey are presented in 

figure 9. Measurable upward flow was present in both 

boreholes, on the order of 0.1 to 0.3 ft/min. Although 

this is unexpected in an aquifer recharge area, the 

interaction between the rapid, shallow flow system in 

the overlying Ocala Limestone and the deeper regional 

flow system in the less permeable Avon Park Formation is poorly understood.

To obtain as discrete a sample as possible, samples 

were collected from the deepest flow zone (near the 

base of the open borehole) for both wells using a drop 

pipe pumped with a 2-inch submersible pump. Samples 

also were collected for sulfate and specific conductance at several shallower intervals in the borehole 

using a thief sampler. Little difference was seen in 

water quality in the open interval of the borehole, indicating that the water in the borehole was well mixed.

Sample Collection

Standard USGS protocols were used for waterquality sampling (Wood, 1976).Temperature, pH, 

specific conductance, and dissolved oxygen (DO) were 

monitored using a flow-through chamber so that the 

sample was isolated from the atmosphere. Water samples were collected after field parameters stabilized and 

sufficient volume was cleared from the well (for monitoring wells). Alkalinity was determined in the field by 

titration with sulfuric acid. Sulfide was measured in the 

field using the methylene blue method and a field spectrophotometer (Hach, 1989). The detection limit for 

sulfide was typically 0.01 mg/L, although it was higher 

for several turbid waters. Duplicate analyses for sulfide 

were within 35 percent, except for one sample 

(well 32) that had unstable readings that varied by 

Figure 9. Profiles of upward flow using the heat-pulse flow meter in boreholes at ROMP 119 and ROMP 120, which are open 

exclusively to the Avon Park Formation. (Locations of wells shown on figure 1.)

0 0.1 0.2 0.3 0.4 0.5 0 0.1 0.2 0.3 0.4 0.5

-500

-400

-300

-200

-100

SEA

100

ALTITUDE, IN FEET

AVON PARK FORMATION OL

AVON PARK FORMATION OL

OPEN HOLE INTERVAL

OPEN HOLE INTERVAL

Undifferentiated

sands and clays

OL Ocala Limestone

ROMP 119 ROMP 120

BELOW INSTRUMENT SENSITIVITY

LEVEL

UPWARD BOREHOLE FLOW, IN FEET PER MINUTE
14 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

85 percent; the median value was used for this sample. 

Sulfate concentration also was estimated in the field 

with the spectrophotometer (using the barium sulfate 

method) in order to determine the method and amount 

of sample necessary for sulfur isotope collection of 

sulfate. 

Samples were filtered through a 0.45 μm filter for 

anion and cation analysis. Cation and trace metal samples were collected in acid-washed bottles and acidified with 1 mL of 70 percent nitric acid. Nitrate 

samples were preserved with mercuric chloride. Dissolved organic carbon (DOC) samples were filtered 

through a 0.45 μm silver filter using a stainless steel filter unit. Unfiltered samples were collected in glass bottles with polyseal caps for deuterium (δD) and oxygen18 (δ18O) analysis of the water, and carbon-13 (δ13C) 

analysis of inorganic carbon. For δ13C, SrCO3 was precipitated by adding approximately 50 mL of ammoniacal strontium chloride solution to a 500-mL unfiltered 

sample, minimizing exposure to the atmosphere.

Samples were collected for analysis of sulfur-34 

(δ34S) of sulfate and sulfide (if present). (In this report, 

total reduced sulfur species are referred to as sulfide; 

H2S° and HS-

 are the predominant species in the pH 

range of the samples). The method and amount of sample necessary for δ34Ssulfate depended upon the sulfate 

concentration. If the sulfate concentration was greater 

than 20 mg/L, at least 250 mL of sample (depending 

upon sulfate concentration) was acidified to a pH of 

about 4 with 1N HCl, and immediately BaCl2 was added 

to precipitate the sulfate from solution as BaSO4. This 

precipitate was filtered and dried before sending to the 

laboratory. If the sulfate concentration was less than 20 

mg/L, between 20 and 50 L of acidified sample were 

pumped through an anion exchange column that previously was flushed with KCl and deionized water, following methods described by Carmody and others (in press). 

In the laboratory, the sulfate was eluted from the resin by 

rinsing with KCl; BaCl2 then was added to the sample to 

precipitate BaSO4, as described for the high sulfate samples. When sulfate concentrations were less than 1 mg/L, 

insufficient sulfate was obtained for δ34S analysis; insufficient sample was also obtained from well 13, which 

had a sulfate concentration of 4.2 mg/L.

For δ34Ssulfide, the sample was collected in a series 

of three 50-L carboys that were acidified with 100 mL 

of 6N sulfuric acid so that H2S° was the predominant 

reduced sulfur species. Gaseous H2S was stripped from 

the sample with ultra-high purity nitrogen gas (less 

than 10 ppm O2) in a system closed to the atmosphere. 

After leaving the carboy, the H2S entered a solution 

of AgNO3 and precipitated as Ag2S. This Ag2S was 

filtered and dried before sending to the laboratory. 

These methods closely follow those described by 

Carmody and others (in press). Usually if sulfide 

concentrations were less than 0.05 mg/L, insufficient 

sulfur was obtained for δ34S analysis. However, sufficient sample was obtained for well 14, which had a 

sulfide concentration of 0.03 mg/L. 

Quality Assurance Samples

For quality assurance purposes, duplicate samples 

were collected from two wells (wells 12 and 30; see 

appendix), and two deionized water equipment blanks 

were collected. For duplicate samples, there was generally less than seven percent difference between major 

ion concentrations and less than 40 percent difference 

between trace element concentrations (except for several 

analyses that had very low concentrations near the detection limit). Charge balances for all analyses were within 

four percent. For the isotopes, precision according to 

duplicate analyses was 0.1 per mil for δ18O, 1.1 per mil 

for δD, 0.4 per mil for δ13C, 0.2 per mil for δ34Ssulfate, and 

0.5 per mil δ34Ssulfide (see appendix).

Deionized water equipment blanks were collected 

through field equipment in contact with sample water 

(peristaltic or submersible pump, filter unit, and tubing). The deionized water was also analyzed directly 

from its source in the laboratory. Most constituents 

were below detection limits for both types of samples. 

There was negligible difference between water from 

the laboratory and water passed through field equipment, indicating that no contamination was introduced 

by the field equipment.

Chemical Composition of Ground Water

The chemical composition of most of the ground 

water was dominated by calcium and bicarbonate. Major 

cation and anion equivalent concentrations are illustrated 

on a trilinear diagram in figure 10. All shallow ground 

waters were dominated by calcium and bicarbonate, 

whereas deep waters had a wider range in composition. 

For some deep ground waters, the dominant anion was 

sulfate, and the relative proportion of magnesium 

increased (fig. 10). 

Analytical data are presented in the appendix. Most 

constituents were less than primary and secondary drinking water standards, with the exception of sulfate, iron, 

and dissolved solids in some waters (Florida Department 
Geochemical and Isotopic Composition of Ground Water 15

of State, 1993). Sulfate concentrations were greater than 

the secondary drinking water standard of 250 mg/L in 

water from four deep wells (fig. 11). Iron concentrations 

were greater than the secondary drinking water standard 

of 300 μg/L in water from 11 wells. Samples with the 

highest iron concentrations (greater than 1,000 μg/L) 

were from shallow wells in Sumter County; samples 

with iron concentrations between 300 and 1,000 μg/L 

were from deep wells in all three counties. Calculated 

dissolved solids concentrations ranged from 69 mg/L to 

2,092 mg/L. Calculated dissolved solids concentrations 

were greater than the secondary drinking water standard 

of 500 mg/L for the same wells that had sulfate concentrations greater than 250 mg/L.

A wide range of sulfate concentrations (less than 

0.2 to 1,400 mg/L) were observed in the ground water. 

The ground waters were grouped into two categories 

based on sulfate concentration: low sulfate (less than 

30 mg/L) and high sulfate (greater than 30 mg/L). This 

was done because chemical characteristics differ 

between low and high sulfate waters. All low sulfate 

waters had concentrations less than 5 mg/L, except for 

water from three wells that had slightly higher sulfate 

concentrations, ranging between 20 and 25 mg/L. 

Water from most of the shallow wells had low sulfate 

concentrations, and only two shallow ground-water 

samples had sulfate concentrations greater than 30 

mg/L (fig. 11). Many of the deep waters had concentrations greater than 30 mg/L; however, six deep samples 

had sulfate concentrations less than 5 mg/L (fig. 11). 

Sulfate increased between the shallow and deep ground 

water at nine out of the 15 sampling sites.

The deeper ground water had higher sulfide concentrations than the shallow water (fig. 12). None of 

the shallow ground water from Marion and Citrus 

Counties contained sulfide; all of these waters contained dissolved oxygen (DO). In Sumter County water 

from the shallow wells often contained sulfide. All 

deep ground water had detectable sulfide and DO 

below the detection limit, except for water from two 

wells at the same site in Marion County (wells 27 and 

28), which also had relatively high DO concentrations 

(greater than 3 mg/L).

A number of solutes increased with sulfate at high 

concentrations (usually greater than 30 mg/L), but not 

at low sulfate concentrations (fig. 13). This indicates 

different controls on sulfate at low and high concentrations. At low sulfate concentrations, sulfate does not 

appear to be directly related to other solute concentrations. For the high sulfate waters, good relations exist 

between sulfate and calcium, magnesium, strontium, 

aluminum, and fluoride (correlation coefficient greater 

than 0.7). This can indicate similar sources for sulfate 

and these ions (for example, mineral dissolution) or 

similar processes responsible for the elevated concentrations (for example, upwelling of deeper ground 

water where other mineral assemblages may control 

the sources of other solutes).

Bicarbonate concentrations and pH were controlled by equilibrium with calcite. Bicarbonate concentrations ranged from 61 to 344 mg/L, and pH values 

ranged from 6.91 to 8.24. An inverse relation exists 

between bicarbonate and pH, which is expected for 

waters in equilibrium with calcite under a wide range 

of carbon dioxide partial pressures (PCO2

) (Stumm and 

Morgan, 1981). This is because water with a higher 

PCO2 

can dissolve more calcite than water with a 

lower PCO2

.

Figure 10. Chemical composition of water from shallow 

and deep wells from the Upper Floridan aquifer.

CATIONS ANIONS

80

60

40

20

20

80

40

60

60

40

80

20

SO4

+ Cl

Mg

CO3

+ HCO3

20

80

80

40

Na + K

Ca + Mg

SO4

60

60

60

40

40

80

20

20

20

40

60

80

80

60

40

20

Ca Cl

PERCENT OF TOTAL

MILLIEQUIVALENTS PER LITER

EXPLANATION

Shallow well

Deep well
16 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

Figure 11. Sulfate concentrations in water from shallow and deep wells from the Upper Floridan 

aquifer. (Index numbers and specific information about wells shown in figure 8 and table 3.)

82o45’

29o 15’

29o00’

45’

28o30’

82o30’ 15’ 00’

HERNANDO

COUNTY

LEVY

COUNTY

SUMTER COUNTY

Mexico

of

Gulf

Base from Southwest Florida Water Management District digital 

data, 1992. Universal Transverse Mercator projection, Zone 17

Scale 1:500,000

0 10 MILES

0 10 KILOMETERS

CITRUS

COUNTY CITRUSCOUNTY

82o45’

29o 15’

29o00’

45’

28o30’

82o30’ 15’ 00’

HERNANDO

COUNTY

LEVY

COUNTY

SUMTER COUNTY

Mexico

of

Gulf

Base from Southwest Florida Water Management District digital 

data, 1992. Universal Transverse Mercator projection, Zone 17

Scale 1:500,000

0 10 MILES

0 10 KILOMETERS

EXPLANATION

SULFATE CONCENTRATION--Circle shows location of well. Circle diameter indicates sulfate concentration, 

in milligrams per liter.

less than 5 5 to 29 30 to 250 greater than 250

SWFWMD

BOUNDARY SWFWMD SHALLOW BOUNDARY

WELLS

DEEP 

WELLS

MARION

COUNTY

MARION

COUNTY

Figure 12. Sulfide concentrations in water from shallow and deep wells from the Upper Floridan 

aquifer. (Index numbers and specific information about wells shown in figure 8 and table 3.)

82o45’

29o15’

29o00’

45’

28o30’

82o30’ 15’ 00’

CITRUS COUNTY

HERNANDO

COUNTY

LEVY

COUNTY

SUMTER COUNTY

MARION COUNTY

Mexico

of

Gulf

Base from Southwest Florida Water Management District digital 

data, 1992. Universal Transverse Mercator projection, Zone 17

Scale 1:500,000

0 10 MILES

0 10 KILOMETERS

82o45’

29o15’

29o00’

45’

28o30’

82o30’ 15’ 00’

CITRUS COUNTY

HERNANDO

COUNTY

LEVY

COUNTY

SUMTER COUNTY

MARION COUNTY

Mexico

of

Gulf

Base from Southwest Florida Water Management District digital 

data, 1992. Universal Transverse Mercator projection, Zone 17

Scale 1:500,000

0 10 MILES

0 10 KILOMETERS

SHALLOW 

WELLS

EXPLANATION

SULFIDE CONCENTRATION--Circle shows location of well. Circle diameter indicates sulfide concentration, 

in milligrams per liter as S.

Less than 0.03 due 

to interferences less than 0.01 0.01 to 0.09 0.1 to 0.9 1.0 to 2.0

SWFWMD

BOUNDARY

SWFWMD

BOUNDARY DEEP 

WELLS
Geochemical and Isotopic Composition of Ground Water 17

Calcium and bicarbonate concentrations increased 

for most waters at the 1:2 molar ratio expected for 

dissolution of calcite by carbonic acid:

. (2)

Outliers are waters from deep wells that had the highest 

calcium and sulfate concentrations (sulfate greater than 

150 mg/L). Plotting the molar ratios of Ca/HCO3

against Ca/SO4 illustrates how calcium concentrations 

CO CO

2

2+−

CaCO3 H2CO3 Ca2 + 2HCO3

– + → +

are controlled by both calcite and gypsum (fig. 14). 

Most waters plot along the calcite dissolution line 

(Ca/HCO3 molar ratio of 0.5), with a large range in the 

Ca/SO4 ratio. However, as calcium concentrations 

increased over bicarbonate, sulfate concentrations also 

increase. This other subset of data plots near to slightly 

below the line expected for gypsum dissolution 

(Ca/SO4 ratio of 1):

. (3)

This indicates that gypsum dissolution controls the 

elevated calcium and sulfate concentrations.

CaSO4 2H2O Ca2 + SO4

2 – ⋅ → + + 2H2O

Chloride concentrations were low for all ground 

water (less than 30 mg/L), which is three orders of 

magnitude lower than seawater (19,000 mg/L). For the 

high sulfate waters, chloride concentrations increased 

from 5 to 20 mg/L as sulfate concentrations increased 

to 150 mg/L; above this, chloride concentrations did 

not increase (fig. 13). For the low sulfate waters, 

chloride concentrations were more widely scattered, 

ranging from 2.8 to 29 mg/L. Bromide and sodium 

concentrations increased with chloride for most waters, 

indicating similar sources for these ions.

Figure 13. Relation between concentrations of sulfate 

and calcium, magnesium, strontium, aluminum, chloride, 

and fluoride in water from the Upper Floridan aquifer.

1,000

100

10

1

0.3

10,000

30,000

1,000

100

10

10

1

0.1

0.1 1 10 100 1,000 10,000

0.03

100

SULFATE CONCENTRATION,

IN MILLIGRAMS PER LITER

(Less than values plotted at detection limit.) detection limit

Fluoride detection limit

Aluminum detection limit

Low Sulfate High Sulfate

CALCIUM

MAGNESIUM

STRONTIUM

ALUMINUM

CHLORIDE

FLUORIDE

CALCIUM OR MAGNESIUM

CONCENTRATION, IN

MILLIGRAMS PER LITER

CONCENTRATION, IN

MILLIGRAMS PER LITER

STRONTIUM OR ALUMINUM CHLORIDE OR FLOURIDE

CONCENTRATION, IN

MICROGRAMS PER LITER

Figure 14. Relation between calcium-to-bicarbonate 

molar ratio and calcium-to-sulfate molar ratio in water from 

the Upper Floridan aquifer.

calcite dissolution (1:2)

gypsum dissolution (1:1)

6

5

3

0

0.1 1 10 1,000 10,000 100

1

2

4

CALCIUM-TO-BICARBONATE

MOLAR RATIO

CALCIUM-TO-SULFATE MOLAR RATIO
18 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

Saturation State of Ground Water

The saturation state of ground water with respect to 

mineral phases and the PCO2

 were computed with the 

aqueous speciation model WATEQ4F (Ball and Nordstrom, 1991). Information on mineral saturation states 

is useful for interpreting mineral controls on ion concentrations and for hypothesizing probable reactions 

for mass-balance modeling. The saturation index (SI) is 

a measure of the departure from equilibrium of the 

water with respect to mineral phases.

SI = log (IAP/KT) , (4)

where IAP is the ion activity product of the components of the mineral phase, and KT is the solid phase 

solubility equilibrium product at the specified temperature. An SI value of zero, with an associated range of 

uncertainty, indicates the water is in equilibrium or saturated with respect to the mineral phase; a value less 

than zero indicates undersaturation (mineral dissolution is possible); and a value greater than zero indicates 

supersaturation (mineral precipitation is possible). 

These calculations assume that the dissolved species in 

the ground water are at chemical equilibrium. Distributions of species that are sensitive to redox potential 

were computed using the Eh value determined from 

sulfate and sulfide concentrations when sulfide was 

present in the water; otherwise dissolved oxygen was 

used in the calculations.

Most of the waters were saturated to supersaturated 

with respect to calcite (equilibrium is assumed when SI 

values fall between -0.15 and 0.15, given uncertainties 

in analytical values and pH). Water from one shallow 

well (well 33) was undersaturated with calcite. This 

water had a low concentration of dissolved solids 

(74 mg/L) and probably represents recent recharge 

with a very short residence time in the aquifer. Over 

half of the deep ground waters were supersaturated 

with calcite. In addition, waters with highest sulfate 

concentrations (greater than 250 mg/L) were supersaturated with respect to calcite (fig 15).

Most waters were undersaturated with respect to 

dolomite (equilibrium is assumed for SI values between 

-0.3 and +0.3). The exact value for the equilibrium reaction constant (Kr

) of dolomite in the aquifer is uncertain 

(Hsu, 1963; Hanshaw and others, 1971; Plummer, 

1977); thus, SI values are reported in the appendix for 

both disordered (or poorly crystalline) dolomite (log Kr

of -16.54) and crystalline dolomite (log Kr

 of -17.09; 

Nordstrom and others, 1990). Most waters were undersaturated with respect to disordered dolomite; however, 

three deep waters with the highest sulfate concentrations 

were in equilibrium with disordered dolomite (fig. 15). 

All shallow ground waters were undersaturated with 

respect to crystalline dolomite, but over half of the deep 

waters were in equilibrium or supersaturated with 

crystalline dolomite (see appendix).

All of the waters were undersaturated with respect to 

gypsum (equilibrium is assumed for SI values between 

-0.03 and +0.03). The water with the highest sulfate concentration approached, but did not reach, gypsum equilibrium (-0.16 for well 7; fig. 16). However, ground 

water collected by SWFWMD above and within depth 

intervals containing gypsum at ROMP 110 (located in 

eastern Citrus County; fig. 1) was at equilibrium with 

respect to gypsum (Southwest Florida Water Management District, written commun., 1993) (see appendix). 

All of the waters were undersaturated with celestite 

Figure 15. Relation between calcite and dolomite 

saturation index and sulfate concentration in water from 

the Upper Floridan aquifer.

0.6

0.4

0.2

0

-0.2

-0.4

0.5

-0.5

-1.0

-1.5

-2.0

-2.5

0.1 10 100 10,000 1 1,000

0

EQUILIBRIUM

EQUILIBRIUM

CALCITE SATURATION INDEX DISORDERED DOLOMITE SATURATION INDEX

SULFATE CONCENTRATION,

IN MILLIGRAMS PER LITER

(Less than values plotted at detection limit)

detection

limit
Geochemical and Isotopic Composition of Ground Water 19

(SrSO4), with values approaching, but not reaching, 

equilibrium for waters with high sulfate concentrations 

(fig. 16). All waters containing sulfide were supersaturated with respect to pyrite; this mineral is unstable and 

can be oxidized when DO is present in the water.

The partial pressure of carbon dioxide (PCO2

) also 

was calculated with WATEQ4F. The low sulfate waters 

had a greater range of PCO2

 (from 10-1.35 to 10-3.46 atm) 

than the high sulfate waters (between 10-1.57 to 10-2.69

atm). Low sulfate waters containing DO generally had 

lower PCO2

 values than anaerobic, low sulfate waters 

(fig. 17). Shallow ground water from Marion and Citrus 

Counties usually had low PCO2

 values (less than 10-2.4

atm; fig. 18), whereas shallow ground water from 

Sumter County generally had higher PCO2

 values (greater 

than 10-2.4 atm; fig. 18). Deeper ground water also usually had higher PCO2

 values. Swancar and Hutchinson 

(1995) found that in unconfined parts of the Upper Floridan aquifer in west-central Florida, water with high PCO2

values were near swamps and river systems where 

recharge is reduced and soils are finer grained.

Figure 16. Relation between gypsum and celestite 

saturation index and sulfate concentration in water from the 

Upper Floridan aquifer.

EQUILIBRIUM

SATURATION INDEX

SULFATE CONCENTRATION, IN MILLIGRAMS PER LITER

0

-1

-2

-3

-4

-5

-6

0.1 1 10 100 1,000 10,000

GYPSUM

CELESTITE

Figure 17. Relation between partial pressure of carbon 

dioxide and dissolved oxygen concentration for low 

sulfate (less than 30 milligrams per liter) and high sulfate 

ground water from the Upper Floridan aquifer.

0 2468

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

IN MILLIGRAMS PER LITER

DISSOLVED OXYGEN CONCENTRATION,

LOG PARTIAL PRESSURE OF CARBON

DIOXIDE, IN ATMOSPHERES

Low sulfate

ground water

High sulfate

ground water

(Less than values plotted at detection limit) detection limit

Figure 18. Partial pressure of carbon dioxide in water from 

shallow wells from the Upper Floridan aquifer. (Index 

numbers and specific information about wells shown in figure 

8 and table 3.)

CITRUS

COUNTY

SUMTER COUNTY

82o 45’

29o 15’

29o 00’

45’

28o30’

82o 30’ 15’ 00’

HERNANDO

COUNTY

LEVY

COUNTY

Mexico

of

Gulf

Base from Southwest Florida Water Management District digital 

data, 1992. Universal Transverse Mercator projection, Zone 17

Scale 1:500,000

0 10 MILES

0 10 KILOMETERS

MARION COUNTY

EXPLANATION

PARTIAL PRESSURE OF CARBON DIOXIDE--Circle shows 

location of well. Circle diameter indicates partial pressure of 

carbon dioxide, in atmospheres.

Greater than 10

-2.4

Greater than 10

-2.4

and

sulfate concentration greater 

than 30 milligrams per liter

Less than 10

-2.4

SWFWMD

BOUNDARY
20 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

Chemical Characterization of High Sulfate 

Ground Water

High sulfate ground water (typically greater than 

30 mg/L) was chemically distinct from low sulfate 

ground water. A number of ions increased in concentration along with sulfate for the high sulfate water. 

For example, calcium and magnesium concentrations 

increased with sulfate (fig. 13). Aquifer minerals that 

control the concentrations of these ions are calcite, 

dolomite, and gypsum. Calcium was slightly depleted 

relative to sulfate (in mmol/L) for the highest sulfate 

ground waters (Ca/SO4 molar ratio less than one), compared to a Ca/SO4 ratio of one expected if only gypsum 

dissolution were occurring (fig. 14). This indicates a 

sink for some of the calcium, such as calcite precipitation, which is possible because these ground waters 

were supersaturated with respect to calcite. 

Dedolomitization reactions (dissolution of gypsum 

and dolomite and precipitation of calcite) are important 

in controlling the composition of ground water along 

regional flow paths in the confined part of the Upper 

Floridan aquifer in southwest Florida (Plummer, 1977; 

Plummer and others, 1983; Sacks and others, 1995). In 

the present study area, dedolomitization reactions also 

may be occurring. These reactions probably occur deep 

in the aquifer, where gypsum (which drives these reactions) is present and dolomite is more abundant. This 

deeper water is part of a regional, slow moving flow 

system with long aquifer residence times.

Strontium and aluminum concentrations also 

increased with increasing sulfate concentrations 

(fig. 13). Strontium and aluminum were observed in 

trace concentrations in gypsum from the Floridan aquifer system (table 2). The possibility that elevated strontium and aluminum concentrations in ground water 

originated from trace concentrations in gypsum was 

further evaluated by assuming that sulfate in excess of 

30 mg/L comes from gypsum. The fraction of sulfate 

originating from gypsum (fgyp) was computed as:

f

gyp = (SO4gw

 – 30) /1800 , (5)

where SO4gw is the sulfate concentration of the high sulfate ground water, and it was assumed that ground water 

in equilibrium with gypsum has a sulfate concentration 

of 1,800 mg/L (see appendix). Using this fraction and 

the ratio of strontium-to-sulfate or aluminum-to-sulfate 

(Rgyp) in gypsum from ROMP 110 (table 2), an expected 

strontium or aluminum concentration (iexp) was computed for each of the high sulfate waters:

i

exp = fgypRgyp (6)

A good relation exists (r2

 of 0.90) between calculated and observed aluminum concentrations, which 

implies that gypsum is the source of elevated aluminum 

in the ground water (fig. 19). Additional work is needed 

to quantify the association of aluminum with gypsum, as 

a source for aluminum in evaporative brines is not apparent. Possibly, aluminum could be in the form of aluminum hydroxides, rather than within the gypsum lattice or 

interstitial positions in an ionic form.

For strontium, the calculated concentrations were 

lower than observed concentrations (fig. 19), and 

significantly more strontium (on the order of about 

3,000 ppm) would need to be present in the gypsum to 

explain the measured ground-water concentrations. 

The strontium concentration in gypsum is dependent 

on temperature and salinity (Kushnir, 1980). The 

concentration of strontium in gypsum from ROMP 110 

(850 ppm; table 2) is lower than for primary gypsum 

Figure 19. Relation between calculated and observed 

aluminum and strontium concentrations for high sulfate 

ground water (greater than 30 milligrams per liter) from the 

Upper Floridan aquifer.

1:1 LINE

1:1 LINE

Calculated values below

detection limit of 20

200

150

100

50

0

0 50 100 150 200

CALCULATED

ALUMINUM CONCENTRATION,

IN MICROGRAMS PER LITER

CALCULATED

STRONTIUM CONCENTRATION,

IN MICROGRAMS PER LITER

OBSERVED ALUMINUM CONCENTRATION,

IN MICROGRAMS PER LITER

10,000

5,000

2,000

1,000

500

200

100

50

20

20 50 100 200 500 1,000 2,000 5,000 10,000

OBSERVED STRONTIUM CONCENTRATION,

IN MICROGRAMS PER LITER
Geochemical and Isotopic Composition of Ground Water 21

formed from evaporating seawater (1,000 and 2,000 

ppm; Ichikuni and Musha, 1978). Strontium concentrations in gypsum collected from the Avon Park Formation in southwest Florida had strontium concentrations 

ranging up to 2,000 ppm (table 2). Similarly high concentrations may occur within the study area. 

An additional strontium source, such as the mineral 

celestite, may be present within the study area. Celestite has been observed in association with gypsum from 

the Avon Park Formation at a site about 30 mi southwest of the study area, in southwest Hernando County 

(Cook and others, 1985). It has also been observed in 

shallow intervals of the confined Upper Floridan aquifer in southwest Florida (McCartan and others, 1992).

Other ions (chloride, bromide, sodium, and potassium) also increased slightly with increasing sulfate concentrations for the high sulfate ground water (fig. 20). 

These increased concentrations may be related to water 

flowing through deeper, more sluggish zones in the aquifer that had less efficient flushing of seawater than the 

shallower more rapid-flowing ground water. Bromide 

concentrations increased with chloride at the same ratio 

as conservative saltwater mixing, which supports a 

residual seawater source for these ions (fig. 20). Sodium 

concentrations were slightly enriched relative to conservative seawater mixing, and potassium concentrations 

were considerably more enriched than conservative seawater mixing (fig. 20). (In contrast, strontium and aluminum concentrations were several orders of magnitude 

higher than conservative seawater mixing.) 

Cation exchange is a possible source for sodium 

and potassium. Minor clay minerals present in the aquifer are possible exchange sites. At a time when saltwater was being flushed from the aquifer, calcium may 

have replaced sodium and potassium on exchange sites, 

elevating their concentrations in solution.

Alternatively, high sulfate waters could have 

recharged during more arid conditions than the present, 

resulting in more evaporative concentration of rainwater. Another possible source for potassium is weathering of silicate minerals such as potassium feldspar, 

which has been reported in trace quantities in the aquifer (Sprinkle, 1989; Katz and others, 1995b). Sodium 

and potassium do not occur in sufficient trace quantities in gypsum or carbonates to measurably influence 

ground-water concentrations.

Fluoride concentrations also increased with sulfate 

(fig. 13) and was many orders of magnitude higher than 

conservative saltwater mixing based on chloride. Thus, 

residual saltwater cannot explain the elevated concentrations. The generally accepted source of fluoride in 

the aquifer is calcium fluoroapatite from the overlying 

Hawthorn Group or the Alachua Formation (Lawrence 

and Upchurch, 1982; Sprinkle, 1989; Katz, 1992), 

neither of which are continuous over the study area. 

Thus, a source of fluoride from overlying rock units is 

not likely. In addition, fluoride concentrations usually 

were higher in deeper ground water than in shallow 

ground water, which is not consistent with a shallow 

fluoride source. Regional ground-water flow paths 

Figure 20. Relation between concentrations of chloride 

and bromide, sodium, and potassium for high sulfate 

ground water (greater than 30 milligrams per liter) from the 

Upper Floridan aquifer.

0.10

0.08

0.06

0.04

0.02

0

25

20

15

10

5

0

5

4

3

2

1

0

0 10 20 30 5 15 25

PO

TASSIUM

CHLORIDE CONCENTRATION, IN

MILLIGRAMS PER LITER

SODIUM

CONCENTRATION, IN

MILLIGRAMS PER LITER

CONCENTRATION, IN

MILLIGRAMS PER LITER

CONCENTRATION, IN

MILLIGRAMS PER LITER

BROMIDE

SALTWATER DILUTION LINE

SALTWATER DILUTION LINE

SALTWATER

DILUTION LINE
22 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

could have originated far upgradient at the potentiometric high to the southeast, where the Hawthorn 

Group is present. The shallow flow system, on the 

other hand, recharges locally in the unconfined part of 

the aquifer where fluoride minerals are not present. 

An alternative source of fluoride is trace amounts 

of fluoride in a mineral phase from deeper zones in 

the Avon Park Formation. Cook and others (1985) 

observed the mineral fluorite (CaF2) in association with 

gypsum from the Avon Park Formation in a core from 

southwest Hernando County. This association of fluorite with gypsum may be fairly widespread in the Avon 

Park Formation, which could explain the relation 

between sulfate and fluoride.

The large range in sulfate concentrations observed 

in the high sulfate ground water (between 48 to 1,400 

mg/L) indicates a range in the extent of gypsum dissolution or mixing between high and low sulfate waters. 

Gypsum dissolution, which is the major reaction influencing the high sulfate ground water, occurs deep in the 

aquifer. Other solutes that increase with sulfate apparently originate from the gypsum itself, from other 

mineral assemblages from deeper in the aquifer in association with gypsum, and from residual seawater from 

less-flushed, deeper parts of the aquifer. These ions are 

subsequently transported with the sulfate to shallower 

parts of the aquifer where gypsum is not present.

Chemical Characterization of Low Sulfate 

Ground Water

The chemical composition of low sulfate water is 

strongly controlled by the redox condition of the water 

(aerobic or anaerobic). This is consistent with differences in the extent of microbially mediated reactions 

(Chapelle, 1993). Microbial respiration initially consumes O2 and organic matter (simplified here as CH2O) 

and produces carbon dioxide (CO2):

. (7)

In Sumter County, where soils are organic-rich and 

poorly drained, shallow ground water was often anaerobic. In contrast, shallow ground water was aerobic in 

upland areas of Citrus and Marion Counties, where 

soils are well drained and sandy. The organic matter 

driving microbial respiration may originate in the soil 

horizon, resulting in anaerobic waters in shallow parts 

of the Upper Floridan aquifer. Dissolved organic 

carbon (DOC) concentrations were usually higher in 

O2 + CH2O CO → 2 + H2O

the anaerobic waters (fig. 21). This is probably related 

to the greater amount of organic matter in the soils and 

the presence of intermediate fermentation reactions 

during the oxidation of particulate organic carbon 

(Chapelle, 1993). This dissolved carbon may be available for anaerobic bacteria that use mineral electron 

acceptors such as sulfate.

Figure 21. Relation between concentrations of dissolved 

oxygen and dissolved organic carbon, nitrate, and sulfide 

for low sulfate ground water (less than 30 milligrams per 

liter) from the Upper Floridan aquifer.

0 2 468

DISSOLVED OXYGEN CONCENTRATION,

0.001

1

0.01

0.1

0.01

0.1

1

6

5

4

3

2

1

0

IN MILLIGRAMS PER LITER

NITRATE SULFIDE

(sulfide <0.03)

(Less than values plotted at detection limit.)

CONCENTRATION, IN

MILLIGRAMS PER LITER

CONCENTRATION, IN

MILLIGRAMS PER LITER

CONCENTRATION, IN

MILLIGRAMS PER LITER

DISSOLVED ORGANIC CARBON

7 points here

detection limit
Geochemical and Isotopic Composition of Ground Water 23

After dissolved O2 is by consumed (aerobic respiration), microbial populations typically use nitrate 

(denitrification), ferric iron (Fe (III) reduction), and 

sulfate (dissimilatory sulfate reduction) as electron 

acceptors (anaerobic respiration) (Chapelle, 1993). The 

source of iron is probably iron oxyhydroxides, either 

from overlying surficial deposits or from trace amounts 

in the carbonate rocks. These reactions can be represented as:

, (8)

, (9)

. (10)

4NO3

– 5H + 5CH2O 5CO2 2N2 + + → + + 7H2O

4Fe OH ( )3 CH2O 8H + CO2 4Fe2 + + + → + + 11H2O

SO4

2 – 2CH2O H + 2CO2 HS + + → + + – H2O

Anaerobic waters usually had higher PCO2 values 

(greater than 10-2.4) than aerobic waters (fig. 17). PCO2

is, thus, an indicator of the extent of microbial reactions. Compared to aerobic ground waters, anaerobic 

waters had distinctly lower nitrate concentrations (less 

than 0.01 mg/L as nitrogen), higher iron concentrations 

(greater than 50 μg/L), and usually contained sulfide 

(figs. 21 and 22). This is consistent with microbially 

mediated denitrification, iron reduction, and sulfate 

reduction. These reactions also increase alkalinity 

(or acid neutralizing capacity) because hydrogen ion is 

consumed (Stumm and Morgan, 1981; p. 193-194).

High iron concentrations in shallow ground water 

in Sumter County is problematic because it is often 

above drinking water standards. These shallow, anaerobic waters with high iron concentrations (greater than 

1,000 μg/L) had relatively low sulfide concentrations 

(less than 0.05 mg/L). This is consistent with iron 

reducing bacteria inhibiting sulfate reducing bacteria 

by outcompeting them for available electron donors 

(H2 or acetate; Chapelle and Lovely, 1992). Deeper low 

sulfate waters had lower iron concentrations and often 

had higher sulfide concentrations. As sulfide concentrations increase in the deeper anaerobic waters, iron 

may be removed by precipitation of insoluble iron sulfide minerals. A good relation exists between manganese and iron concentrations (r2

 of 0.72), indicating that 

manganese reducers also may be active, with similar 

efficiencies for using electron donors as iron reducers.

Silica concentrations were higher in the anaerobic 

waters than in the aerobic waters (fig. 22). Several 

studies have shown that organic acids may enhance the 

dissolution of quartz and potassium feldspars (Bennett, 

1991; McMahon and others, 1995). The higher DOC 

concentrations in anaerobic waters from this study 

may also increase the dissolution of silicate minerals. 

Potassium and silica concentrations usually increase 

together, suggesting that potassium feldspar may be the 

source of this additional silica.

The CO2 produced by microbial oxidation of 

organic matter allows more calcite to dissolve 

(equation 2). Thus, although all of the low sulfate 

Figure 22. Relation between concentrations of dissolved 

oxygen and iron, manganese, silica, calcium, and 

bicarbonate for low sulfate ground water (less than 30 

milligrams per liter) from the Upper Floridan aquifer.

14

12

10

8

6

4

400

300

200

100

0

024 6 8

DISSOLVED OXYGEN CONCENTRATION,

IN MILLIGRAMS PER LITER

(Less than values plotted at detection limit.)

CONCENTRATION, IN

MILLIGRAMS PER LITER

CONCENTRATION, IN

MILLIGRAMS PER LITER

SILICA CALCIUM OR BICARBONATE

CONCENTRATION, IN

MICROGRAMS PER LITER

IRON OR MANGANESE

Calcium

Bicarbonate

Iron

Manganese

detection limit

10,000

1,000

100

10

1

Analysis with higher

detection limit
24 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

waters were near calcite equilibrium, higher PCO2, 

anaerobic waters sometimes contained more than twice 

as much calcium and bicarbonate as the lower PCO2

, aerobic waters (fig. 22). The greater amount of calcite that 

the anaerobic waters dissolve, compared to the aerobic 

waters, may be important in the geomorphic evolution 

of the area. For example, the parts of Citrus and Marion 

Counties that were the focus of this study are topographically higher than most of the study area in 

Sumter County. Less calcite dissolution in these parts 

of Citrus and Marion Counties (where ground water is 

aerobic) could perpetuate the existence of topographic 

highs. In lower lying Sumter County (where ground 

water is anaerobic), CO2 charged ground water is 

chemically more aggressive, resulting in more calcite 

dissolution and possibly additional karstification in 

these areas.

Deeper ground waters with low sulfate concentrations tend to be chemically similar to shallow anaerobic 

ground waters. These shallow waters probably have a 

longer residence time in the aquifer than their aerobic 

counterparts. Similarly, deeper waters have longer 

aquifer residence times, resulting in a greater extent of 

microbially mediated reactions.

Specific microbial populations were not quantified 

for this study. However, the presence of sulfate reducing 

bacteria was determined for selected aerobic and anaerobic waters (wells 3, 5, 6, 10, 11, 12, 13, 22, and 25). 

Methods for preparing autoclaved vials, which contained a selective media favorable for the growth of sulfate reducing bacteria, were modified from Postgate 

(1979) and are described in detail by Kauffman (1994). 

In the field, a 3-mL sample of unfiltered ground water 

was injected into the vial using a sterile syringe and 

needle. A black precipitate appeared after several days in 

all samples, indicating that sulfate reducing bacteria 

were present in all water samples. The presence of 

bacteria on the media is not evidence that they are active 

in the subsurface. However, sulfate reducing bacteria 

were viable when conditions were favorable for their 

growth. (Sampling methods precluded the collection of 

water directly from the aquifer, and so the bacteria, alternatively, could have originated from the well or pump.)

Isotopic Composition of Ground Water

Isotopes can be useful in understanding sources of 

water and reactions affecting the chemical composition 

of ground water (Fritz and Fontes, 1980; Toran, 1982; 

Mazon, 1991). Ground-water samples were analyzed 

for stable isotopes of hydrogen (deuterium or 2

H), 

oxygen (oxygen-18 or 18O), inorganic carbon (carbon13 or 13C), and sulfur (sulfur-34 or 34S). Isotopes are 

presented in delta (δ) notation as the ratio of the heavy 

to the light isotope, normalized to a standard (equation 

1). A larger δ value is, thus, considered “heavier” or 

enriched in the heavier isotope, compared to a smaller 

value, which is referred to as “lighter” or more 

depleted. Results and interpretation of the isotopic data 

are discussed in the following section.

Deuterium and Oxygen-18

Deuterium and oxygen-18 are influenced by 

processes affecting the water, rather than the solutes, at 

the low temperature and pressure of shallow aquifers. 

These isotopes are useful in identifying waters that 

underwent evaporation, recharge under different 

climatic conditions than the present, and mixing of 

waters from different sources. Modern meteoric water 

usually falls along a global meteoric line, defined by 

the relation between δD and δ18O (δD = 8 δ18O +10; 

Craig, 1961). Rainwater within Florida generally plots 

along the global meteoric water line (Swancar and 

Hutchinson, 1995; Meyers and others; 1993; Katz and 

others, 1995a). A good relation exists between δD and 

δ18O in ground water from this study (r2

 of 0.94; 

fig. 23). Isotopically light waters plot near the global 

meteoric water line (Craig, 1961), but isotopically 

heavier waters are offset to the right of the line 

(enriched in δ18O). This same relation was found by 

Swancar and Hutchinson (1995) for water from the 

Upper Floridan aquifer in west-central Florida.

Figure 23. Relation between delta deuterium and delta 

oxygen-18 in water from the Upper Floridan aquifer.

DELTA OXYGEN-18, IN PER MIL

DELTA DEUTERIUM, IN PER MIL

Aerobic low sulfate*

ground water

Anaerobic low sulfate

ground water GLOBAL METEORIC WATER LINE

(Craig, 1961)

REGRESSION EQUATION (this study)

DELTA DEUTERIUM 

= 4.9 (DELTA OXYGEN-18) + 0.17

EVAPORATION

LINE

(Katz and others,

1995a)

* Less than 30 milligrams per liter.

0

-10

-20

-5

-5 -4 -3 -2 -1 0

-15

-25

Aerobic high sulfate 

ground water

Anaerobic high sulfate

ground water
Geochemical and Isotopic Composition of Ground Water 25

The δD and δ18O composition of the low sulfate 

waters appears to be related to ground-water age and 

evaporation prior to recharge. Isotopically light waters 

contained DO and are probably recent recharge that is 

part of the rapid, shallow flow system described by 

Faulkner (1973). Swancar and Hutchinson (1995) 

reported a relation between tritium and δD and δ18O in 

waters from the shallow part of the Upper Floridan 

aquifer in west-central Florida, and concluded that isotopically light waters were younger than heavy waters. 

The isotopically heavier, low sulfate waters from this 

study were anaerobic and occur either at shallow 

depths in Sumter County, where aquifer recharge and 

transmissivity are lower (Ryder, 1985), or in the deeper 

ground water. These waters are probably older than the 

oxygenated waters from shallow wells in Citrus and 

Marion County.

Some of the isotopically enriched waters appear to 

have undergone evaporation prior to recharging the 

aquifer. The heaviest δD and δ18O values were from 

two wells with low sulfate concentrations from eastern 

Sumter County (wells 1 and 2). These waters had 

elevated chloride concentrations (19 and 29 mg/L, 

respectively), which is consistent with evaporative 

concentration. This site is surrounded by numerous 

ponds and lakes. Katz and others (1995a) found that 

ground water downgradient from a seepage lake in 

north-central Florida was enriched in δD and δ18O 

because of recharge from the lake. In that study, δD and 

δ18O from the ground water plotted along a mixing line 

between the lake, which reflected evaporation, and the 

meteoric water line. The enriched δD and δ18O values 

for this study plot near Katz and others’ (1995a) evaporation line (fig. 23), indicating that evaporation may 

have caused the isotopic enrichment and offset from 

the global meteoric water line.

The high sulfate waters usually were more 

enriched in δD and δ18O than low sulfate waters that 

contain DO (fig. 23). However, high sulfate waters 

were not isotopically distinct from anaerobic, low 

sulfate waters. The range of δD and δ18O for the high 

sulfate waters may indicate mixing between recent 

recharge and older waters. However, a straightforward 

mixing relation is not apparent. The most isotopically 

enriched high sulfate water (from well 18) was analyzed for tritium during a previous sampling. It contained no tritium (A. Swancar, U.S. Geological Survey, 

written commun., 1992), indicating that this water is 

not recent recharge. Climatic differences during the 

time of recharge, including more evaporation, may 

explain the isotopic composition of the high sulfate 

waters, which are presumably older than the aerobic 

low sulfate waters.

Carbon-13

Processes affecting the stable isotope composition 

of inorganic carbon (δ13C) include carbonate mineral 

dissolution and precipitation, microbially mediated 

processes that oxidize organic carbon and generate 

CO2, and mixing of waters. δ13C values ranged from 

-12.3 to -6.6 per mil. For low sulfate waters, δ13C values usually were lighter for waters with higher PCO2

 values (fig. 24). This is consistent with microbial 

oxidation of organic matter, which typically has an isotopically light δ13C value around -25 per mil (Rightmire and Hanshaw, 1973; Buchardt and Fritz, 1980). 

As light organic carbon is oxidized to CO2 (equations 7 

through 10), the amount of dissolved CO2 and the corresponding PCO2

 increase, provided that the system is 

closed to the atmosphere. 

Several low sulfate waters were enriched in δ13C 

(well 1 and to a lesser degree well 2) (fig. 24). These 

waters also had enriched δD and δ18O values, indicative 

of water recharged from an evaporating surface-water 

body. Katz and others (1995b) attributed isotopically 

heavy inorganic carbon in ground water beneath a northcentral Florida lake to methanogenesis produced by the 

reduction of CO2. The anomalously heavy δ13C value 

Figure 24. Relation between delta carbon-13 and partial 

pressure of carbon dioxide for low sulfate ground water (less 

than 30 milligrams per liter) from the Upper Floridan aquifer.

LOW SULFATE

GROUND WATER

well 1

well 2

Aerobic

ground water

Anaerobic

DELTA

ground water

CARBON-13, IN PER MIL

LOG PARTIAL PRESSURE OF CARBON DIOXIDE,

IN ATMOSPHERES

-6

-7

-8

-9

-10

-11

-12

-13

-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0
26 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

Figure 25. Relation between delta sulfur-34 of sulfate 

and sulfate concentration and delta sulfur-34 of sulfide 

and sulfate concentration in water from the Upper Floridan 

aquifer.

Range of delta sulfur-34 of gypsum

from middle confining unit

Aerobic

ground water

Anaerobic

Range of delta sulfur-34 of sulfate in 

ground water

rainfall from north-central Florida,

from Katz and others (1995b)

35

30

25

20

15

10

5

0

20

10

0

-10

-20

-30

-40

-50

0.1 1 10 100 1,000 10,000

DELTA SULFUR-34 OF SULFATE,

IN PER MIL

DELTA SULFUR-34 OF SULFIDE,

IN PER MIL

SULFATE CONCENTRATION, 

IN MILLIGRAMS PER LITER

observed in ground water from well 1 similarly may be 

the result of methanogenesis, although methane was not 

analyzed. During CO2 reduction, methanogenic bacteria 

preferentially oxidize isotopically light CO2 to CH4

(methane), resulting in heavy inorganic carbon in the 

water. Water from well 1 did not contain DO or sulfide, 

and had a very low sulfate concentration (less than 0.2 

mg/L). Thus, activity of sulfate reducing bacteria was 

probably minimal. After O2 was consumed and Fe(III) 

was reduced, methanogenic bacteria probably became 

active and caused 13C enrichment of the remaining inorganic carbon in solution. The deeper ground water at this 

site (well 2) is probably a mixture between the shallow 

ground water and water more typical of that from the 

Upper Floridan aquifer.

High sulfate waters also had a large range in δ13C 

but had no obvious relation with PCO2

. However, as sulfate concentrations increased to more than 150 mg/L, 

the range of δ13C narrowed to relatively heavy values 

between about -10 to -8 per mil. Similar reactions 

involving inorganic carbon probably control δ13C 

values at high sulfate concentrations. Higher sulfate 

waters apparently have undergone a greater extent of 

dedolomitization reactions (gypsum and dolomite 

dissolution and calcite precipitation) than low sulfate 

waters. Dolomite dissolution adds isotopically heavy 

carbon to the water. However, calcite precipitation 

removes some of this heavier carbon, resulting in δ13C 

values that still are isotopically lighter than the 

dolomite. 

Most deep ground waters also had heavier δ13C values than their shallow counterparts. This is probably 

related to the greater abundance of dolomite deeper in 

the aquifer. Dolomite dissolution in deeper ground 

water is also evidenced by increased magnesium 

concentrations and SI of dolomite in deeper water.

Sulfur-34

Sources of sulfate and sulfide in the ground water 

can be evaluated by examining the sulfur isotope composition of both sulfur species (sulfate and sulfide; 

δ34Ssulfate and δ34Ssulfide). Sulfate sources such as atmospheric precipitation, marine sulfur (from gypsum dissolution or seawater), and pyrite oxidation all have 

distinctly different sulfur isotope signatures. The extent 

of sulfate reduction and precipitation of sulfide minerals also can be evaluated using sulfur isotopes. Microbially mediated sulfate reduction transforms sulfate to 

reduced sulfur (sulfide). The bacteria responsible for 

these reactions fractionate the sulfur by preferentially 

using isotopically light sulfur (sulfur-32), causing the 

remaining sulfate to become isotopically enriched in 

sulfur-34.

Sulfate

Values of δ34Ssulfate in the ground water ranged 

from 1.7 to 33.1 per mil. The low sulfate ground water 

had a much greater range in δ34Ssulfate than the high sulfate ground water (fig. 25). Most of the low sulfate 

waters are within the range of δ34Ssulfatevalues reported 

by Rightmire and others (1974) for waters from the 

recharge area of the Upper Floridan aquifer (8.1 to 23.2 

per mil). Rainwater collected during 1991 and 1992 

from an inland site in western Putnam County (about 

40 mi to the northeast of the study area) had threemonth composite δ34Ssulfate values ranging between 3.4 

and 5.9 per mil (Katz and others, 1995b). This is similar 

to ranges reported by Östlund (1959) and Jensen and 

Nakai (1961) for δ34Ssulfate in rainwater in unindustrialized regions (between 3.2 and 8.2 per mil). 
Geochemical and Isotopic Composition of Ground Water 27

The greater range of δ34Ssulfate for the low sulfate 

waters compared to rainwater probably reflects differences in the extent of reactions influencing sulfate and 

spatial variability in δ34Ssulfate in recharge waters. 

Biological uptake and microbial sulfate reduction fractionate sulfur isotopes, resulting in isotopically heavier 

sulfate. Adsorption of sulfate onto soils and clays also 

may fractionate some of the sulfur (Krouse, 1980). In 

addition, very small amounts of mixing with sulfaterich water that dissolved gypsum can result in heavier 

δ34Ssulfate values (discussed further below). Marine 

aerosols in coastal areas are probably another source 

for heavier sulfate in recharge waters. Oxidation of 

reduced sulfur, in the form of organic sulfur or a sulfide 

mineral such as pyrite, may explain δ34Ssulfate values 

lighter than rainwater. Alternatively, localized anthropogenic sources (for example, industrial emissions and 

fertilizers) may result in isotopically light sulfate.

As sulfate concentrations increase, δ34Ssulfate

approaches a more constant value between 21 and 25 

per mil (fig. 25). This is within the range of δ34Sgypsum

from the middle confining unit, and further indicates 

that gypsum dissolution is the primary source of sulfate 

for high sulfate waters. The only outlier is water from 

well 3, which had a heavier δ34Ssulfate value of 27.9 per 

mil. This water also had one of the highest sulfide concentrations (1.2 mg/L), which supports that fractionation during sulfate reduction caused isotopic 

enrichment of sulfate.

Sulfide

Values of δ34Ssulfide ranged from -42.1 to 14.6 per 

mil for the 11 samples that had sufficient sulfide to 

analyze (fig. 25). Low sulfate waters all had positive 

δ34Ssulfide values, whereas high sulfate waters all had 

negative δ34Ssulfide values. The positive values are 

indicative of a greater extent of sulfate reduction from 

a limited pool of sulfate, resulting in lower sulfate concentrations and heavier δ34Ssulfide values. For the high 

sulfate waters, the abundant supply of sulfate with a 

relatively constant δ34Ssulfate value resulted in fairly 

uniform δ34Ssulfide values (usually between -30 and -45 

per mil). Rye and others (1981) found positive 

δ34Ssulfide values in water from the Upper Floridan 

aquifer near the recharge area and negative values 

further downgradient, where sulfate concentrations and 

aquifer residence time increased.

For low sulfate waters, the range of δ34Ssulfide (7.7 

to 14.6 per mil) is very similar to the range of δ34Ssulfate

in waters not containing sulfide. This suggests that 

most of the original sulfate was reduced to sulfide. 

However, sulfate concentrations for sulfide-bearing 

waters were higher than their corresponding sulfide 

concentrations (in mmol/L), which indicates a removal 

mechanism for sulfide. Bicarbonate concentrations 

were also elevated for these waters (greater than 240 

mg/L) and were in excess of that expected from calcite 

dissolution based on calcium concentrations. This is 

consistent with microbially mediated sulfate reduction, 

which increases alkalinity. Only one low sulfate water 

was analyzed for sulfur isotopes of both sulfate and 

sulfide (well 14). This water had an unusually heavy 

δ34Ssulfate value (33.1 per mil), which is further 

evidence of sulfate reduction. Because the sulfide 

concentration was relatively low for this water (0.03 

mg/L), significant amounts of sulfide must have been 

removed from solution, probably as a sulfide mineral 

such as pyrite. Based on isotope mass balance calculations and assuming that the total amount of sulfur 

reduced (in mmol/L) equals the excess bicarbonate 

concentration over that expected for calcite dissolution 

(equation 2), between 50 and 85 percent of the original 

sulfur may have been removed from solution by pyrite 

precipitation.

The difference between the δ34S values of sulfate 

and sulfide (∆34S) ranged from 25.4 to 66.3 per mil for 

the nine waters with δ34S analyses of both sulfate and 

sulfide (see appendix). Most of the ∆34S values were 

around 60 per mil, which is near the value expected for 

fractionation at isotopic equilibrium between sulfate 

and sulfide (Rye and others, 1981). This indicates slow 

sulfate reduction in waters with long residence times in 

the aquifer. These waters were all from deeper wells, 

supporting that they are part of a slower regional flow 

system. Lesser ∆34S values may indicate that sulfate 

reduction took place at a considerably faster rate (Rye 

and others, 1981). Sulfate reduction from laboratory 

experiments, which are run at considerably faster rates 

than sulfate reduction in ground water with long residence times, typically have lower ∆34S values between 

20 and 30 per mil (Pearson and Rightmire, 1980; Rye 

and others, 1981). The shallow ground water from well 

14 had the lightest ∆34S value of 25.4 per mil. This may 

indicate faster sulfate reduction, perhaps during 

recharge through organic-rich soils, where carbon is 

not limited.
28 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

Total Sulfur

When sulfur occurs as both sulfate and sulfide, 

evaluating the sulfur isotope composition of total sulfur 

in solution (δ34Stotal) can allow a more accurate assessment of sulfur sources. The computed δ34Stotal value is 

based on the concentration and isotopic composition of 

sulfate and sulfide:

where msulfate and msulfide are the sulfate and sulfide 

concentrations in mmol/L. A δ34Stotal value was not 

computed for waters that had insufficient sulfate or sulfide for δ34S analysis; when sulfide was not present it 

was assumed that δ34Stotal equals δ34Ssulfate . Water from 

well 3 had the largest difference between δ34Ssulfate

(27.9 per mil) and δ34Stotal (24.3 per mil). This δ34Stotal

value is within the range of the sulfur isotope composition of gypsum from the middle confining unit. For the 

remainder of ground waters, the calculated δ34Stotal

value differed by less than 2 per mil from the δ34Ssulfate

value (see appendix).

Although a common source of gypsum is evident 

for the high sulfate waters, two separate groups are 

apparent based on δ34Stotal values. One group has values 

between 23.6 to 24.3 per mil, and the other group has 

δ34Stotal values between 21.0 and 22.2 per mil. Some of 

the waters in the isotopically heavier group had the 

highest sulfate concentrations or were from the deepest 

wells. None of these waters contained DO, and all are 

located in Sumter or Citrus Counties. The isotopically 

lighter group of waters could represents a mixture of 

more recent recharge with deeper ground water. Some 

of these lighter waters also contained DO, which 

supports mixing with recent recharge.

Isotope mass balance calculations were used to test 

the possibility that mixing is responsible for the 

observed range of δ34Stotal for the high sulfate waters. 

Mixing also was evaluated for the two waters with 

slightly elevated sulfate concentrations (between 20 and 

25 mg/L) that had δ34Ssulfate values in the range of marine 

sulfate (wells 19 and 26). The calculations considered 

mixing between a typical recharge water (sulfate concentration of 2 mg/L and δ34Stotal value of 5 per mil) and 

a deep ground water in equilibrium with gypsum (sulfate 

concentration of 1,800 mg/L). Initially the mixing line 

was calculated with a δ34S value of 24.0 per mil for the 

water in equilibrium with gypsum (similar to measured 

δ34S of gypsum from the study area of 24.0 and 24.5 per 

mil). Calculated δ34S values were much heavier than 

observed values for the isotopically lighter group of 

waters, when assuming the δ34S value for gypsum was 

24.0 per mil (fig. 26). Thus, the mixing line was recomputed with lighter δ34Sgypsum values. The isotopically 

lighter waters plot between mixing lines with δ34Sgypsum

of 21.5 and 22.5 per mil (fig. 26). These mass balance 

calculations illustrate how the computed δ34S composition for the water is equivalent to the assumed isotopic 

composition of the gypsum when sulfate concentrations 

are greater than about 100 mg/L. Thus, gypsum of a variable isotopic composition (between 21 and 24 per mil) 

apparently influences sulfate in the ground water. Waters 

with lower sulfate concentrations plot on mixing lines 

between the recharge water and the isotopically lighter 

gypsum. This includes the two samples with low, but 

slightly elevated, sulfate concentrations between 20 and 

25 mg/L (fig. 26).

The sulfur isotope composition of gypsum may 

vary spatially in the study area. All high sulfate waters 

from Marion County were isotopically lighter (δ34Stotal

values between 21.2 and 22.0 per mil) than high sulfate 

waters from Citrus County (24.0 per mil, similar to δ34S 

values of gypsum from Citrus County). When evaporites were deposited during the middle Eocene, evaporative basins were isolated, and gypsum and anhydrite 

were not formed simultaneously or uniformly over the 

study area. Reducing conditions were probably present 

Figure 26. Relation between delta sulfur-34 of total sulfur 

(sulfate plus sulfide) and sulfate concentration in water 

from the Upper Floridan aquifer, plotted with mixing 

relations between dilute ground water and water in 

equilibrium with gypsum.

well 3

24.0 per mil

22.5 per mil

21.5 per mil

Ground water from:

Citrus County Sumter County

Marion County

20

20

50 100 200 500 1,000 2,00

SULFATE CONCENTRATION, IN MILLIGRAMS PER LITER

[Lines calculated assuming mixing between a low sulfate wate (2 milligrams per liter) with a delta sulfur-34 value of 5 per mi

and a sulfate-rich ground water (1,800 milligrams per liter) wit

the delta sulfur-34 value specified on the line.]

25

24

23

22

21

DELTA SULFUR-34 OF TOTAL SULFUR, 

IN PER MIL

δ34Ssulfatemsulfate + δ34Ssulfidemsulfide δ34Stotal = (11)

msulfate + msulfide
Geochemical Modeling 29

in some of the basins, resulting in gypsum that was 

isotopically heavier than the seawater. This is consistent with the association of pyrite with gypsum from 

the ROMP 110 core (Southwest Florida Water Management District, written commun., 1993). 

Isotopically lighter gypsum indicates that more 

oxidizing conditions existed when evaporites were 

deposited, perhaps due to better circulation with open 

marine water. In eastern Marion County, which is outside of the study area, continuous evaporites do not 

occur in the Avon Park Formation (Miller, 1986). This 

could indicate that there was a connection to the open 

sea to the east during the middle Eocene, precluding the 

development of evaporative basins in that area. 

The part of Marion County within the study area would 

have been closer to this area of open circulation, 

whereas Citrus County would have been further away, 

perhaps making it more prone to reducing conditions. 

Further evaluation of the sulfur isotope composition of 

gypsum in the study area may help resolve spatial variability in evaporites and allow a better understanding of 

the depositional environment.

Other explanations for the differences in sulfur 

isotope composition of high sulfate ground waters are 

precipitation and oxidation of sulfide minerals (such as 

pyrite). Isotopically lighter waters may have oxidized 

isotopically light pyrite. Several of these waters 

contained DO, and so pyrite oxidation is possible. 

However, about half of the high sulfate waters with 

δ34Stotal values between 20 and 22 per mil did not contain DO, which was probably consumed by microbial 

populations. Given the similar isotopic composition for 

the aerobic and anaerobic ground waters, pyrite oxidation is probably not a significant source of sulfate.

Conversely, the isotopically heavier waters could 

be influenced by removal of reduced sulfur from solution, probably by precipitation of pyrite. Water from 

well 3 had a δ34Stotal value that was considerably 

heavier than predicted from strict mixing between 

dilute recharge and water in equilibrium with gypsum 

(fig. 26). This indicates that some of the sulfide was 

removed from solution, presumably as a sulfide mineral. Likewise, the isotopically heavier waters from 

Citrus County also could be influenced by sulfate 

reduction and pyrite precipitation. These waters are 

downgradient from the isotopically lighter waters in 

Marion County, presumably along deep slow flow 

paths from Marion County (fig. 6). Therefore, these 

waters probably had a longer residence time in the 

aquifer than waters from Marion County. 

GEOCHEMICAL MODELING

Geochemical mass-balance modeling was used to 

evaluate chemical and isotopic differences between 

shallow and deep ground water. The interaction 

between shallow and deep ground water in the study 

area is poorly understood. Shallow ground water is 

usually characterized by a rapid flow system and relatively low concentrations of dissolved solids, whereas 

deeper ground water is usually characterized by a 

slower regional flow system with higher sulfate concentrations. High sulfate concentrations, however, do 

occur in the shallow ground water in isolated locations 

(fig. 11). In these areas, the source of sulfate appears to 

be similar to the deeper ground water. Sulfate also does 

not increase with depth at all locations, which may 

mean that the shallow flow system penetrates deeper in 

these area. An evaluation of chemical changes between 

shallow and deep ground water may help define the 

relation between the flow systems. The wide range of 

sulfate concentrations in the high sulfate waters and 

concurrent increases in other solutes indicate that differences in the extent of geochemical reactions or a 

range of mixing control sulfate concentrations.

The model NETPATH (Plummer and others, 1991) 

computes a set of reactions (including mineral dissolution or precipitation, ion exchange, and gas exchange) 

based on the net mass transfer of elements between initial and final waters, given a set of constraints (elemental, isotopic, electron balance). The model also can 

compute an isotopic composition for the final water, 

given the isotopic composition of the initial water, 

isotopic composition of dissolving phases, and fractionation factors of precipitating phases. Although the 

resulting models are not unique and cannot always be 

validated, certain models can be rejected based on violations of thermodynamics (for example, dissolution of 

a mineral from a supersaturated water) or large discrepancies between observed and computed δ13C and δ34S 

values of the final water. Descriptions of the mass-balance modeling approach, including relevant equations, 

are presented in detail elsewhere (Plummer, 1977; 

Plummer and Back, 1980; Plummer and others, 1983; 

Plummer and others, 1990; Busby and others, 1991).

Modeling in this report focuses on vertical changes 

in chemical composition between shallow and deep 

ground water. To be consistent with modeling terminology these are referred to as “flow paths.” Because the 

study area is dominated by aquifer recharge, the shallow ground water is probably a good approximation of 

water that recharged the aquifer before moving to 
30 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

deeper parts of the aquifer. However, the modeling scenarios are limited by lack of head measurements in the 

deeper part of the aquifer, and a direct hydraulic connection between the two waters usually has not been 

established. Nonetheless, geochemical modeling can 

elucidate dominant reactions in the evolution of the 

deeper ground water and can give insights into how the 

shallow and deep flow systems are related.

Constraints considered in all models were sulfur, 

calcium, magnesium, carbon, iron, and electron balance. Electron balance was necessary for modeling 

redox reactions such as oxidation of organic matter. 

Reaction phases considered were gypsum, dolomite, 

calcite, organic matter (represented as CH2O), CO2 gas, 

pyrite, and goethite. Pyrite precipitation was included 

as a sink for sulfur, which also allowed for the fractionation of δ34S during sulfate reduction. Goethite was 

included in models to balance iron, which was necessary when pyrite precipitation was calculated. When 

shallow waters contained dissolved oxygen and nitrate, 

nitrate was added as a constraint and O2 and N2 gas 

were added as phases (representing aerobic respiration 

and nitrate reduction; equations 7 and 8). Mixing with 

sulfate-rich water also was used to assess the feasibility 

of upwelling in explaining high sulfate waters. Nine 

flow paths were modeled where sulfate concentrations 

are high (greater than 30 mg/L) in the deep ground 

water, and six flow paths were modeled where sulfate 

remains low in the deep ground water. The locations of 

the flow paths are shown in figure 27; the depth of open 

hole intervals of wells for modeled flow paths are 

shown in figure 28.

Reactions Controlling the Composition of 

Deep, High Sulfate Ground Water

Gypsum and dolomite dissolution were calculated 

for all high sulfate models (table 4). These reactions are 

consistent with the saturation state of the waters. 

Calcite precipitation was computed for all models 

expect for flow path I, which computed calcite dissolution. Water from the shallow well for this flow path 

(well 33) was undersaturated with respect to calcite, 

making additional calcite dissolution possible. Deep 

ground waters were all saturated to supersaturated with 

calcite, making calcite precipitation a plausible reaction. Although similar reactions were modeled, the 

magnitude of dominant reactions between flow paths 

varied by over an order of magnitude. This is because 

of the wide range of differences between the chemical 

composition of shallow and deep ground waters.

Several models were computed for most flow 

paths. These models varied by differences in minor 

amounts of mass transfer of pyrite, goethite, CO2 or O2

exchange, and CH2O; major reactions described above 

were very similar for most models for a specific flow 

path. Differences in minor reactions influence the calculated δ13C and δ34S values of the final water. Models 

including pyrite precipitation had δ34S values that were 

heavier than observed values, whereas models not 

including pyrite precipitation had computed δ34S 

values closer to observed values. Therefore, models 

without pyrite precipitation or with the least amount of 

pyrite precipitation are included in table 4.

For flow path G, models with pyrite and goethite 

precipitation were virtually identical. Both shallow and 

deep waters contained DO. Thus, pyrite precipitation is 

Figure 27. Locations of flow paths between shallow and 

deep ground water modeled using NETPATH. (Index 

numbers and specific information about wells shown in figure 

8 and table 3.)

82o45’

29o 15’

29o00’

45’

28o30’

82o30’ 15’ 00’

CITRUS

COUNTY

HERNANDO

COUNTY

LEVY

COUNTY

SUMTER COUNTY

MARION COUNTY

Mexico

of

Gulf

Base from Southwest Florida Water Management District digita

data, 1992. Universal Transverse Mercator projection, Zone 17

Scale 1:500,000

0 10 MILES

0 10 KILOMETERS

SWFWMD

BOUNDARY

A M

J B

O

N C D

E

F

I

G

H

L K

EXPLANATION

A Location of flow path modeled with NETPATH between 

shallow ground water and deep, high sulfate ground water (greater than 30 milligrams per liter). Letter refers to flow 

path in tables 4 through 6.

Location of flow path modeled with NETPATH between 

shallow ground water and deep, low sulfate ground water 

(less than 30 milligrams per liter).
Geochemical Modeling 31

Table 4. NETPATH models between shallow ground water and deep, high sulfate ground water

[Units in millimoles per liter (mmol/L) unless otherwise noted; positive mass transfer indicates dissolution or ingassing; negative mass transfer 

indicates precipitation or outgassing; gyp, gypsum; dol, dolomite; cal, calcite; δ34S, delta sulfur-34; ‰, per mil; calc, calculated; obs, observed; 

δ13C, delta carbon-13; --, not modeled; isotope data used in models unless otherwise specified: δ13C of CH2O = -25.0 per mil; δ13C of calcite = 

0 per mil when dissolving; for precipitating phases, δ13C or δ34S computed by model based on Rayleigh calculations and fractionation factors 

defined at the midpoint (X=0.5) between initial and final water compositions; δ13C (observed) based on DOC of -25.0 per mil; δ34S (observed) 

based on measured δ34S of sulfate and sulfide; all models also include goethite dissolution or precipitation (less than 0.06 mmol/L); all models 

include nitrogen outgassing (less than 0.04 mmol/L), except flow paths A, C, and G]

1

 Using δ34S of gypsum equal to 22 per mil.

2

 Using variable δ34S of gypsum shown in previous column.

3

 Using δ13C of dolomite equal to 0 per mil.

4

 Using isotopically lighter δ13C of dolomite shown in previous column.

5

 Model has pyrite precipitation (0.04 mmol/L).

Flow

path

Initial

well

Final

well Gyp Dol Cal CH2O O2 CO2

δ13C (total C), ‰

calc1 gyp calc2 obs

δ13C (total C), ‰

calc3 dol calc4 obs

A 4 3 0.56 0.32 -0.52 0.03 -- -0.19 21.4 24.5 23.7 24.5 -9.6 -- -- -9.6

B 9 7 14.11 4.25 -8.13 0.23 -- -1.05 22.0 23.5 23.4 23.6 -2.0 -8 -9.6 -9.5

C5 14 15 0.86 0.41 -1.77 -- -- -1.79 26.1 20.0 24.1 22.2 -9.4 -- -- -9.9

D 17 16 7.93 3.04 -6.74 0.98 0.77 -- 22.0 24.0 24.0 24.1 -4.9 -6 -9.9 -9.6

E 19 18 9.19 2.32 -4.12 1.37 0.97 -- 22.0 24.0 23.9 24.0 -7.4 -3 -9.5 -9.4

F 20 21 6.05 1.53 -3.43 0.82 0.34 -- 22.0 21.5 21.5 21.5 -8.0 -2 -9.2 -9.4

G 26 27 2.24 0.64 -1.30 0.29 0.20 -- 21.9 -- -- 22.0 -8.3 -5 -10.3 -10.2

H 31 30 0.11 0.12 -0.21 0.10 -- -0.64 21.1 -- -- 21.7 -11.1 -- -- -10.2

I 33 32 1.56 0.81 0.14 2.09 1.90 -- 21.6 -- -- 21.3 -13.5 -- -- -12.5

Figure 28. Depth range of open hole intervals of shallow and deep wells for flow paths modeled with NETPATH.

-600

-400

-200

SEA

LEVEL

ALTITUDE, IN FEET

AB CD E F GH I J K LMN O

deep, high sulfate ground waters deep, low sulfate ground waters

sample

approximate extent of Avon Park Formation

approximate extent

of Ocala Limestone

depth range of

middle confining unit

continues below

scale of graph

open hole

interval of

sampled well

FLOW PATH

4

3

9

7

14

15

17

16

19

18

20

21

26

27

31

30 33

32

6

5

25

22

23

24

1

2

12

13

11

10

well

number

top of open interval

not known ?
32 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

not possible, and the model containing goethite precipitation (which is supersaturated in both waters) is 

included in table 4. A model with pyrite oxidation also 

was considered because of the aerobic ground water. 

This model is virtually identical to the one presented in 

table 4, except that 1 percent of the sulfur (0.02 

mmol/L) comes from pyrite oxidation, resulting in 

calculated δ34S being 1 per mil lighter.

The sulfur isotope composition of gypsum probably varies within the study area, as previously noted. 

Changing the δ34S value of gypsum within the reported 

range for the Floridan aquifer system (20.0 to 24.5 per 

mil) resulted in a better agreement between modeled 

and observed values for most flow paths (table 4). Like 

many modeling endeavors, however, solutions are not 

unique. For example, changing pyrite fractionation factors to be more representative of kinetic fractionation 

seen in the shallow ground water (for example, ∆34S 

value of 25.4 per mil for well 14) also significantly 

affects calculated δ34S values.

Calculated and observed δ34S values did not agree 

for two flow paths (A and C). This could indicate that 

too much or too little pyrite precipitation was modeled, 

or that the shallow water is not representative of water 

that reacted to form the deeper water. The latter is probably the case for flow path C, where the shallow water 

had already undergone a significant amount of sulfate 

reduction, reflected by an unusually heavy δ34Ssulfate

value (33.1 per mil). For flow path A, the observed δ34S 

value for the deep water was heavier than the modeled 

value, indicating that some pyrite precipitation probably occurred that was not computed by the model. This 

deep water (well 3) had a heavy δ34Stotal value, indicating pyrite precipitation (fig. 26).

The difference between observed and modeled 

δ13C values for some of the flow paths can be explained 

by analytical uncertainties and assumed values of δ13C 

of organic matter (CH2O). However, five flow paths (B, 

D, E, F, and G) had calculated δ13C values that were 

considerably heavier than observed values when 

assuming 0 per mil for δ13C of dolomite (near values 

measured in the study area). For these flow paths, an 

isotopically lighter dolomite (between -2 and -8 per 

mil) is needed for the calculated δ13C to approximately 

equal the observed value (table 4). Isotopically light 

dolomites (-2.8 to -7.5 per mil) have been observed in 

the Floridan aquifer system (Hanshaw and Back, 

1972). Based on mass balance modeling, Plummer 

(1977) concluded that isotopically light dolomite 

(between -1.5 and -3.9 per mil) was dissolving along a 

regional flow path in the Upper Floridan aquifer in 

southwest Florida. Isotopically light dolomites may be 

more soluble in ground water than heavy dolomites, 

perhaps due to differences in their origin, stoichiometry, or degree of crystallinity.

Most models had some oxidation of organic matter 

and CO2 or O2 exchange. In Citrus and Marion Counties, where aquifer recharge is high, shallow waters 

contained DO and nitrate, whereas deep waters typically were anaerobic (except for flow path G). Two different types of models were computed for flow paths in 

these counties. One had significant amounts of CO2

ingassing; the other did not contain CO2, but instead 

had O2 ingassing (or reduction) and usually had more 

than twice as much organic matter oxidation as the corresponding CO2 ingassing model. Isotopic data for 

these two types of models were identical. The O2

reduction models simulate aerobic respiration in the 

shallow ground water. These models are probably more 

realistic because an oxygen source is readily apparent. 

Thus, the aerobic respiration models are included in 

table 4. The CO2 ingassing models apparently are a net 

result of the CO2 produced by the consumption of 

organic matter in aerobic respiration (equation 7). The 

amount of modeled O2 reduction, however, was higher 

than the amount of dissolved oxygen in the shallow 

ground water. Additional dissolved O2, as well as soil 

CO2, could enter the shallower part of the aquifer as 

rapid recharge, perhaps through preferential flow 

through sinkholes.

Flow paths with CO2 outgassing models typically 

are in Sumter County, where recharge is slow and shallow ground water is probably older. Mechanisms for 

outgassing of CO2 are not apparent. Calculated outgassing may be related to charge imbalance in the analytical data. Alternatively, the shallow ground water for 

these flow paths may not be representative of water that 

evolved to the deeper water.

Evaluating Upwelling as Source of Sulfate

This modeling exercise illustrates that dedolomitization reactions are significant in controlling the composition of deep, high sulfate ground water. These 

reactions are driven by gypsum dissolution. However, 

gypsum occurs deeper in the aquifer than open intervals of most deep wells (fig. 28; table 1). Although it is 

possible that trace gypsum occurs in upper intervals of 

the Avon Park Formation, it has not been observed in 

core samples or well cuttings. Alternatively, the deeper 

ground water could have a mild upward flow component from near the base of the aquifer, where evaporites 

occur. Upward flow was measured within the open 

borehole of several wells finished in the Avon Park 

Formation in Marion County (fig. 9), and an upward 

head gradient exists in ground water from the Avon 

Park Formation near Lake Panasoffkee (fig. 7).
Geochemical Modeling 33

Figure 29. Scenarios of major reactions modeled with 

NETPATH between shallow wells and deep wells with 

high sulfate concentrations, considering (1) only 

reactions between open interval of wells or (2) reactions 

plus upwelling of sulfate-rich water.

shallow

deep

well

well

(1) (2)

Scenario Scenario

reactions

(gyp, dol, -cal)

reactions

(dol, -cal)

upwelling

sulfate-rich

water

of

EXPLANATION

Open hole interval of well

Surficial sands and clays

Limestone

Dolomite

Gypsum

dol

gyp

-cal

Dolomite dissolution

Gypsum dissolution

Calcite precipitation

Modeled reactions:

(larger font size indicates more

mineral reacted)

In southwest Florida, high sulfate concentrations in 

shallow parts of the confined Upper Floridan aquifer 

are attributed to upwelling of sulfate-rich water that 

dissolved evaporites at the base of the aquifer (Jones 

and others, 1993; Sacks and Bullen, 1993; Johnson, 

1994; Sacks and others, 1995). This corresponds to an 

area where dedolomitization reactions had previously 

been modeled (Plummer, 1977; Plummer and others, 

1983). In the present study area, dedolomitization reactions similarly could be occurring deeper in the aquifer, 

where gypsum is present, rather than between the sampled shallow and deep wells.

To test whether upwelling could explain the high 

sulfate concentrations observed in the deeper ground 

water, upwelling was modeled with NETPATH 

(fig. 29). The shallow ground water was mixed with a 

sulfate-rich ground water at equilibrium with respect 

to gypsum. Water from ROMP 110 (collected by 

SWFWMD during drilling) was used as the composition of the upwelling water because it had a very high 

sulfate concentration (1,800 mg/L) and was at equilibrium with gypsum (see appendix).

Upwelling models are presented in table 5. For five 

of the flow paths, gypsum dissolution was not computed, and mixing accounted for all of the sulfate in the 

deep ground water. Models for the other flow paths had 

significantly less gypsum dissolution than models 

without mixing. The amount of mass transfer of carbonate minerals also was usually much smaller than for 

models not considering upwelling (fig. 29). Therefore, 

in addition to gypsum dissolution, the chemical composition of the deep, upwelling water already reflects significant amounts of dolomite dissolution and calcite 

precipitation. 

Isotopic data were not available for the high sulfate 

water from ROMP 110. Thus, it is difficult to evaluate 

the reasonableness of reactions based on calculated 

isotopic data. δ13C and δ34S values were assumed for 

the upwelling water to compare the calculated and 

observed isotope compositions (table 5). The δ34S 

composition of the upwelling waters apparently varies 

within the aquifer (reflecting a variable δ34Sgypsum). 

Computed and observed δ34S values were similar when 

δ34S of the upwelling water was within the range of 

gypsum from the aquifer. However, exceptions noted in 

the preceding section still held (flow paths A and C). 

Computed δ13C values were always lighter than 

observed values, when an assumed δ13C value for the 

upwelling water was similar to that of high sulfate 

ground water from the study area (about -8.5 per mil). 

Thus, δ13C for the upwelling water is probably heavier 
34 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

than measured values from the study area (about -4 per 

mil). However, for several flow paths (D, E, F, and G), 

calculated δ13C values were still too light when δ13C of 

the upwelling water was as heavy as 0 per mil (similar 

to dolomite measured from the study area). The heavier 

observed δ13C values may reflect isotopically heavier 

CH2O; for example, using a δ13C value of -15 per mil 

(rather than -25 per mil) usually resulted in a closer 

match between computed and observed values (table 5). 

Alternatively, isotopic exchange (or recrystallization) 

of calcite could result in enrichment of δ13C in the final 

water (Plummer and others, 1991; Katz and others, 

1995b). Analytical uncertainties in the chemical data 

for ROMP 110 also probably are reflected in the 

computed carbon mass transfer.

These models indicate that mixing between shallow ground water and sulfate-rich upwelling water can 

explain the high sulfate concentrations in the deep 

ground water. Differences between the shallow and 

deep ground water do not have to be explained solely 

by mineral dissolution and precipitation reactions 

between the open intervals of the wells (fig. 29). 

Mixing from a deeper source is plausible because gypsum is not found in observable quantities in intervals 

where wells are finished.

Reactions Controlling the Composition of 

Deep, Low Sulfate Ground Water

Sulfate concentrations were low in both the shallow and the deep ground water at six sites (fig. 27 and 

28). Reactions occurring between the shallow and the 

deep ground water for these flow paths were further 

evaluated with NETPATH. One of these flow paths 

(between wells 11 and 10) had significantly higher 

sulfate in the shallow ground water (21 mg/L) than in 

the deep ground water (3.0 mg/L). Models for this flow 

path compute unrealistically high pyrite precipitation 

to explain the lower sulfate concentration in the deep 

well, which results in unrealistically heavy δ34S values. 

Table 5. NETPATH models between shallow ground water and deep, high sulfate ground water, including upwelling 

of sulfate-rich water

[Units in millimoles per liter (mmol/L) unless otherwise noted; positive mass transfer indicates dissolution or ingassing; negative mass transfer 

indicates precipitation or outgassing; gyp, gypsum; dol, dolomite; cal, calcite; δ34S, delta sulfur-34; ‰, per mil; calc, calculated; obs, observed; 

δ13C, delta carbon-13; --, not modeled; isotope data used in models unless otherwise specified: δ13C of CH2O = -25.0 per mil; δ13C of calcite 

(when dissolving) and dolomite = 0 per mil; δ34S of gypsum = 22 per mil; for precipitating phases, δ13C or δ34S computed by model based on 

Rayleigh calculations and fractionation factors defined at the midpoint (X=0.5) between initial and final water compositions; δ13C (observed) 

based on DOC of -25.0 per mil; δ34S (observed) based on measured δ34S of sulfate and sulfide; all models also include goethite dissolution or 

precipitation (less than 0.06 mmol/L); all models include nitrogen outgassing (less than 0.04 mmol/L), except flow paths A, C, and G]

1

 Upwelling computed as amount of mixing between shallow ground water and water from ROMP 110 at equilibrium with gypsum collected by 

Southwest Florida Water Management District (written commun., 1993). Analytical results from ROMP 110 (248.5 feet) in the appendix; dissolved 

organic carbon concentration assumed to equal 1.5 milligrams per liter; iron concentration assumed to equal 10 micrograms per liter; nitrate concentration 

assumed to equal 0 milligrams per liter; pH assumed to equal 7.0; temperature assumed to equal 25 degrees C.

2

 Assumed δ34S value of upwelling water.

3

 Using assumed δ13C value of upwelling water equal to -4 per mil and δ13C of CH2O equal to -25 per mil.

4

 Using assumed δ13C value of upwelling water equal to -4 per mil and δ13C of CH2O equal to -15 per mil, except where noted.

5

 Model has pyrite precipitation (0.04 mmol/L).

6

 Same as previous column except δ13C of CH2O equal to -20 per mil.

Flow

path

Initial

well

Final

well

Fraction

upwelling1 Gyp Dol Cal CH2O O2 CO2

δ34S (total S), ‰

Upwelling2 calc obs

δ13C (total C), ‰

calc3 calc4 obs

A 4 3 0.03 -- 0.14 -0.22 0.02 -- -0.13 24.5 23.7 24.5 -10.5 -- -9.6

B 9 7 0.72 0.94 -- -1.14 0.34 0.28 -- 23.5 23.4 23.6 -8.6 -- -9.5

C5 14 15 0.05 -- 0.14 -1.26 -- -- -1.62 22.0 27.8 22.2 -10.3 -- -9.9

D 17 16 0.42 -- 0.47 -2.68 1.38 1.29 -- 24.5 24.5 24.1 -13.6 -9.7 -9.6

E 19 18 0.39 1.96 -- -0.48 1.77 1.45 -- 24.5 23.9 24.0 -15.6 -11.2 -9.4

F 20 21 0.25 1.38 -- -0.94 1.18 0.80 -- 22.0 22.0 21.5 -15.3 -11.8 -9.4

G 26 27 0.10 0.30 -- -0.28 0.41 0.36 -- 22.0 21.9 22.0 -12.5 -10.9 -10.2

H 31 30 0.01 -- 0.09 -0.15 0.10 -- -0.61 24.5 21.4 21.7 -11.2 -- -10.2

I 33 32 0.08 -- 0.30 0.91 2.17 1.98 -- 22.0 21.7 21.3 -13.8 6-11.7 -12.5
Geochemical Modeling 35

Water from the shallow well for this flow path had an 

anomalously light δ34S value (1.7 per mil), which 

probably is from a localized surficial source and is not 

representative of the shallow ground water in the area. 

Based on its more typical δ34Ssulfate value and low 

sulfate concentration, the deep ground water does not 

reflect this same source of sulfate. Thus, models for 

this flow path are considered to be unrealistic and are 

not presented in table 6.

Aerobic, Shallow Ground Water

Similar reactions were modeled for three flow 

paths (J, K, and L) that had aerobic, shallow ground 

water. Models with δ13C values closest to observed 

values are presented in table 6. Major reactions were O2

reduction, calcite and dolomite dissolution, and organic 

matter oxidation. Minor amounts of goethite dissolution, pyrite precipitation, and N2 outgassing (nitrate 

reduction) also were usually computed. No mass transfer of gypsum was modeled. Dolomite dissolution is 

consistent with undersaturation in the ground water and 

increasing amounts of dolomite deeper in the aquifer. 

Calcite is within the range of saturation in the shallow 

ground water (-0.104 to 0.069; see appendix) and is saturated to supersaturated in the deep ground water 

(0.060 to 0.332; see appendix). However, microbial 

oxidation of organic matter produces CO2, which 

should drive additional calcite dissolution.

As were the high sulfate models with aerobic, shallow ground water, models with CO2 ingassing (rather 

than O2 reduction) also were computed for these low 

sulfate models. The CO2 ingassing models had about 

half as much CH2O oxidation as the O2 reduction models. The calculated CO2 is probably the net result of aerobic respiration (equation 7), and so only O2 ingassing 

models are presented in table 6. Mass transfer of minerals and computed isotope compositions are identical 

for CO2 and O2 ingassing models for a given flow path. 

The amount of calculated O2 reduction is similar to that 

observed in the shallow water for flow path K, but is 

higher than measured DO in the shallow waters for the 

other two flow paths. Dissolved O2 (and probably some 

soil-zone CO2) may be added to the water by rapid 

recharge in the karstified shallow part of the aquifer.

Models for these three flow paths have considerably less mass transfer than most of the high sulfate 

models. For these modeling scenarios, the lesser 

amounts of mass transfer and minimal reactions 

involving sulfur indicate that the deep wells are within 

the shallow flow system.

Anaerobic, Shallow Ground Water

Similar reactions were modeled for the other two 

low sulfate flow paths (flow paths M and N, in Sumter 

County), both of which had anaerobic, shallow ground 

water. Reactions include calcite precipitation, dolomite 

dissolution, CO2 outgassing, and lesser amounts of 

gypsum dissolution, CH2O oxidation, and pyrite precipitation (table 6). Deep waters were supersaturated 

with respect to calcite, and so precipitation is possible. 

As dolomite dissolves between the shallow and deep 

ground water, the addition of calcium and inorganic 

carbon probably drives this calcite precipitation.

Table 6. NETPATH models between shallow ground water and deep, low sulfate ground water

[Units in millimoles per liter (mmol/L) unless otherwise noted; positive mass transfer indicates dissolution or ingassing; negative mass transfer 

indicates precipitation or outgassing; gyp, gypsum; dol, dolomite; cal, calcite; δ34S, delta sulfur-34; ‰, per mil; calc, calculated; obs, observed; 

δ13C, delta carbon-13; --, not modeled; n/d; insufficient data; <, less than; isotope data used in models unless otherwise specified: δ34S of 

gypsum = 22 per mil; δ13C of calcite (when dissolving) and dolomite = 0 per mil; for precipitating phases, δ13C or δ34S computed by model based 

on Rayleigh calculations and fractionation factors defined at the midpoint (X=0.5) between initial and final water compositions; δ13C (observed) 

based on DOC of -25.0 per mil; δ34S (observed) based on measured δ34S of sulfate and sulfide; all models except flow path K include pyrite 

precipitation (less than 0.02 mmol/L for flow paths J and L, 0.69 mmol/L for flow path M, and 0.08 mmol/L for flow path N)]

1 Using δ13C of CH2O = -25.0 per mil.

2

 Using δ13C of CH2O = -20.0 per mil.

3

 Model has nitrogen outgassing (less than 0.03 mmol/L) and goethite dissolution (less than 0.05 mmol/L).

Flow

path

Initial

well

Final

well Gyp Dol Cal CH2O O2 CO2

δ34S (total S), ‰

calc obs

δ13C (total C), ‰

calc1 calc2 obs

J3 6 5 -- 0.03 0.45 0.71 0.28 -- n/d n/d -13.1 -12.2 -11.6

K3 25 22 <0.01 0.07 0.12 0.28 0.09 -- 9.5 10.7 -12.0 -11.5 -11.0

L3 23 24 -- 0.49 0.19 1.44 1.12 -- n/d n/d -12.8 -10.8 -10.0

M 1 2 0.15 0.51 -0.83 0.12 -- -1.67 n/d n/d -5.8 -- -9.2

N 12 13 0.23 0.48 -1.01 0.26 -- -0.84 n/d n/d -11.1 -- -12.7
36 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

In contrast to the aerobic ground waters for the other 

flow paths, these shallow ground waters already had 

been influenced by microbially mediated reactions that 

generate CO2. This is illustrated by the lack of DO and 

nitrate; sulfide is also present in the shallow water for 

flow path N. Microbially produced CO2 results in high 

PCO2

 values in the shallow ground water, allowing more 

calcite to initially dissolve compared to the aerobic shallow waters. The greater extent of microbially mediated 

reactions is probably related to longer aquifer residence 

times due to a slower, shallow flow system and less 

recharge compared to the aerobic ground waters.

Sulfate is very low in both shallow and deep 

ground waters (less than 5 mg/L). Insufficient sulfur 

isotope data make it difficult to evaluate the reasonableness of reactions involving sulfur. The low amount 

of mass transfer of gypsum indicates that these “deep” 

waters are mostly isolated from the deeper flow system. However, a very small amount of upwelling may 

influence the composition of the deep water for flow 

path N (well 13). For example, gypsum dissolution is 

not computed when the shallow water is mixed with 

about one percent sulfate-rich water from ROMP 110, 

representing upwelling. (This model still contained 

CO2 outgassing.) Upwelling is plausible because the 

site is near the Withlacoochee River, where localized 

discharging conditions exist in the aquifer. Nonetheless, significant amounts of discharge do not occur 

from the deep flow system because the sulfate concentration is still low in the deep well.

Deep wells for the low sulfate flow paths are somewhat shallower (average depth of 234 ft) than many of 

the deep wells for the high sulfate flow paths (average 

depth of 396 ft) (fig. 28). The low sulfate, deep ground 

water is apparently part of the same flow system as the 

shallow ground water, with a longer residence time in 

the aquifer than the shallow water (based on the greater 

extent of dolomite dissolution and microbially mediated reactions). The amount of mass transfer for these 

flow paths is considerably less than for most of the high 

sulfate flow paths. This indicates either less water-rock 

interactions or, more likely, insignificant amounts of 

upwelling of sulfate-rich water. A well finished deeper 

in the aquifer than open intervals of these “deep wells” 

is expected to encounter more mineralized high sulfate 

water. This is supported by profiles of sulfate during 

drilling (fig. 2).

Uncertainty in the Nature of Interaction 

between Shallow and Deep Flow System

The variability in chemical and isotopic composition of deep and shallow ground water indicates that 

differences exist in the amount of interaction between 

the shallow and deep ground-water flow systems 

within the study area. Deeper ground water in some 

areas had significantly more sulfate than in other parts 

of the study area. Similarly, shallow ground water had 

elevated sulfate concentrations in some areas, but had 

low concentrations in most of the study area. The 

hydraulic connection between shallow and deep flow 

systems is poorly defined. Upward flow has been noted 

within the less permeable Avon Park Formation (figs. 7 

and 9). Upwelling can be driven by aquifer discharge 

near the Withlacoochee River and Lake Panasoffkee. 

However, upwelling also occurs in recharge areas of 

Marion County. This upward flow may be induced by 

rapid shallow flow within the more permeable Ocala 

Limestone. Pumpage from shallow zones in the aquifer 

also may cause localized upward flow. In addition, 

upwelling could be driven by a zone of stagnation in 

the aquifer at the drainage divide between Rainbow 

Springs in western Marion County and Silver Springs 

in eastern Marion County (Faulkner, 1973). In this 

area, regional flow paths with long aquifer residence 

times converge from potentiometric highs to the north 

and south (fig. 6).

Alternatively, elevated sulfate concentrations at 

shallow depths in Marion County may be the result of 

reduced recharge where clays cap ridges (Lamonds, 

1976). Jones and others (in press) concluded that 

recharge was slow in the Fairfield Hills area of northwest Marion County (north of the present study area, 

where sulfate concentrations are low). They attributed 

this to relict clays overlying the aquifer. In other parts 

of western Marion County, however, they concluded 

that recharge was rapid. Factors supporting relatively 

rapid recharge in the study area in Marion County 

include the presence of DO and tritium in shallow 

ground water (Faulkner, 19723; Swancar and Hutchinson, 1995), the lack of surface drainage, and a very flat 

potentiometric surface combined with high aquifer 

transmissivities (Ryder, 1985).

Numerous faults and fractures in the study area are 

associated with the structural high of the Ocala Platform (Vernon, 1951; Faulkner, 1973). Upwelling is 

probably accelerated by preferential vertical flow 

through fractures and faults that connect deeper and 

shallower parts of the aquifer. This study did not find an 
Summary and Conclusions 37

apparent relation between areas of high sulfate concentrations and mapped fault and fracture traces (Faulkner, 

1973). However, a recent detailed study of groundwater quality in western Marion County found that 

high sulfate concentrations fell along a linear trend, 

similar to that of a fracture trace in the area (Jones and 

others, in press).

High sulfate concentrations in the deeper part of 

the aquifer occur at shallower intervals than zones 

where gypsum occurs. For example, at ROMP 110 

sulfate concentrations are very high (over 1,000 mg/L) 

about 200 ft above the first mention of gypsum in the 

core (fig. 2). Besides upwelling, diffusion also probably transports some of the sulfate-rich ground water 

that dissolved gypsum upward within the more stagnant, deeper part of the aquifer. The occurrence of gypsum is highly variable in the deeper part of the Avon 

Park Formation (fig. 2). This variability undoubtedly 

controls how high sulfate concentrations are deep in the 

aquifer. It is also possible that gypsum occurs very 

sparsely in shallower intervals in the aquifer or that it 

already completely dissolved, with sulfate remaining in 

the aquifer because of the slow rate of flushing.

The “deep” ground waters that had low sulfate 

concentrations were probably too shallow to intersect 

the high sulfate water that occurs deeper in the aquifer. 

These lower sulfate concentrations indicate areas of 

insignificant upwelling and relatively deep circulation 

of the shallow flow system. Several of these wells are 

near Rainbow Springs and the Withlacoochee River, 

which are significant focuses of discharge from the 

shallow flow system.

Future Study Needs

Additional data are necessary to better define the 

complex interaction between the shallow and deep 

ground-water flow systems. A better network of monitoring wells finished at discrete depth intervals in the 

aquifer will help distinguish areas of upward flow. A 

potentiometric map that focuses on the deeper flow system can be used to define deep flow paths in the aquifer. 

Hydrologic data from this detailed network of shallow 

and deep wells can then be linked with measured sulfate 

concentrations to identify areas that are vulnerable to 

upwelling. Additional coring and water quality sampling 

throughout the Upper Floridan aquifer to gypsiferous 

zones of the middle confining unit are necessary to help 

evaluate mineralogical controls on chemical stratification in the aquifer. Vertical hydraulic conductivity measurements could help establish the connection between 

shallow and deep parts of the aquifer. Detailed groundwater flow modeling, coupled with new hydrologic data, 

also can be useful in evaluating physical processes 

responsible for the movement of high sulfate water and 

in assessing whether increased ground-water development can induce upwelling of sulfate-rich water. These 

additional hydrologic, geologic, and chemical data can 

provide a better understanding of the variable nature and 

occurrence of high sulfate ground water, which can help 

optimize ground-water resources and protect them from 

future degradation.

SUMMARY AND CONCLUSIONS

In inland areas of northwest central Florida, sulfate 

concentrations in the Upper Floridan aquifer are 

extremely variable and sometimes exceed drinking 

water standards (250 mg/L). This is unusual because 

the aquifer is unconfined and near the surface, allowing 

for active recharge. The most apparent source of sulfate 

is gypsum found at the base of the aquifer. However, 

gypsum has not been observed in shallower intervals of 

the aquifer. Shallow and deep flow systems occur in the 

Upper Floridan aquifer in the study area. Flow within 

the shallow system is relatively fast and discharges 

locally to springs and rivers; flow within the deep 

system is more stagnant and bypasses local discharge 

areas to discharge near or offshore of the coast. The 

sources of sulfate and geochemical processes controlling ground-water composition were evaluated in an 

unconfined part of the aquifer where sulfate is variable. 

A better understanding of sulfate sources is important 

because increased ground-water development could 

induce movement of high sulfate water to fresher parts 

of the aquifer. Water was sampled from thirty-three 

wells in parts of Marion, Sumter, and Citrus Counties, 

within the boundaries of the SWFWMD; these 

included at least a shallow and deep well at fifteen 

separate locations. Ground water was analyzed for 

major ions, selected trace constituents, DOC, and 

stable isotopes (δD, δ18O, δ13C of inorganic carbon, and 

δ34S of sulfate and sulfide).

Sulfate concentrations ranged from less than 0.2 to 

1,400 mg/L, with higher concentrations usually in 

water from deeper wells. The waters were separated 

into a low sulfate group (less than 30 mg/L) and a high 

sulfate group (greater than 30 mg/L). Different processes control the chemical composition of these 

waters. High sulfate waters had concurrent increases 

between sulfate and other ions (for example, calcium, 

magnesium, strontium, aluminum, fluoride, and chlo-
38 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

ride). Gypsum dissolution is the major reaction controlling the composition of high sulfate ground water. 

Other solutes that increase with sulfate apparently originate from the gypsum itself, from other mineral 

assemblages which occur deeper in the aquifer in association with gypsum, and from residual seawater from 

less-flushed, deeper parts of the aquifer. These ions are 

subsequently transported with sulfate to shallower 

parts of the aquifer where gypsum is not present. The 

large range in sulfate concentrations indicates a range 

in the extent of gypsum dissolution or mixing between 

high and low sulfate waters.

The chemical composition of low sulfate waters is 

controlled by differences in the extent of microbially 

mediated reactions, which produce CO2. This CO2, in 

turn, influences the extent of calcite dissolution. Ground 

waters which underwent limited microbial reactions still 

contained DO and were usually in ridge areas where 

recharge is rapid. Anaerobic waters were in lower lying 

areas of Sumter County, where soils are poorly drained 

and aquifer recharge is slow, or were from deep wells. 

Anaerobic waters usually had higher concentrations of 

calcium, bicarbonate, sulfide, DOC, iron, manganese, 

and silica, and lower concentrations of nitrate compared 

to aerobic waters.

The δD and δ18O composition of the ground water 

appears to be related to ground-water age and extent of 

evaporation prior to recharge. Aerobic, low sulfate 

waters had isotopically light δD and δ18O values, 

whereas anaerobic, low sulfate waters usually were 

isotopically heavier. Some of the isotopically enriched 

waters apparently underwent evaporation prior to 

recharging the aquifer. High sulfate waters were usually 

more enriched in δD and δ18O than low sulfate waters 

that contained DO. Mixing between recent recharge and 

older waters probably controls this range of δD and δ18O.

For most of the low sulfate waters, δ13C of inorganic carbon reflects the extent of microbially mediated reactions. Waters with higher PCO2

 values usually 

had isotopically lighter δ13C values, which is consistent 

with oxidation of isotopically light organic matter. For 

high sulfate waters no obvious relation was apparent 

with PCO2

. At the highest sulfate concentrations, however, the range of δ13C narrows, indicating that similar 

processes control inorganic carbon sources and sinks at 

high sulfate concentrations. This is probably related to 

dolomite dissolution adding isotopically heavy carbon 

and calcite precipitation removing isotopically heavy 

carbon from the water.

The range of δ34Ssulfate in the ground water was 

between 1.7 and 33.1 per mil. Low sulfate waters had a 

much greater range in δ34Ssulfate than high sulfate 

waters. This greater range of δ34Ssulfate is probably controlled by processes influencing sulfate during 

recharge, including sulfate reduction and oxidation of 

reduced sulfur. As sulfate concentrations increase, the 

δ34Ssulfate values approach a more constant value, generally between 21 and 24 per mil. This is consistent 

with gypsum dissolution. Several waters with isotopically heavy δ34Ssulfate values appear to have lost sulfide 

from solution, probably by precipitation of a sulfide 

mineral such as pyrite. Values of δ34Ssulfide ranged from 

-42.1 to 14.6 per mil. Low sulfate waters had positive 

δ34Ssulfide values, whereas high sulfate waters had negative δ34Ssulfide values. Sulfate reduction is slow in the 

deeper part of the aquifer, which is part of a sluggish, 

regional flow system. Isotope mass balance calculations indicate that mixing is responsible for most of the 

observed range of δ34Stotal for the high sulfate ground 

water. However, the δ34S of gypsum apparently is variable and often isotopically lighter than gypsum analyzed from the study area.

Geochemical mass-balance modeling was used to 

evaluate reactions occurring between shallow and deep 

ground water at the same location. When sulfate is high 

in the deep ground water, dedolomitization reactions 

(gypsum and dolomite dissolution and calcite precipitation) control the composition of the deep water. 

These reactions are driven by gypsum, which occurs 

deeper in the aquifer than open intervals of sampled 

wells. Computed and measured δ34S values generally 

matched when the δ34S of gypsum ranged between 21.5 

and 24.5 per mil, which is in the range of measured 

values for gypsum in the middle confining unit. 

Upward flow has been documented in deeper parts of 

the aquifer in the study area. To test whether upwelling 

could explain the high sulfate concentrations observed 

in the deeper ground water in the study area, upwelling 

was modeled with NETPATH. These models indicate 

that mixing between shallow ground water and sulfaterich upwelling water can explain most of the high 

sulfate concentrations in the deep ground water. Therefore, differences between the shallow and deep ground 

water do not have to be explained solely by reactions 

occurring between open intervals of the wells.

When sulfate is low in the deep ground water, 

modeled reactions depend upon whether the shallow 

ground water contained DO. For aerobic, shallow 

ground water, major reactions were calcite and 
References 39

dolomite dissolution, organic matter oxidation, and O2

reduction. For anaerobic, shallow ground water, major 

reactions included calcite precipitation, dolomite dissolution, and CO2 outgassing. Differences between 

these models are related to the extent of microbially 

mediated reactions occurring shallow in the aquifer, 

which, in turn, is probably related to aquifer residence 

time and recharge rate. The amount of mass transfer for 

low sulfate flow paths is considerably less than for 

most of the high sulfate flow paths. This indicates 

either less water-rock interactions or insignificant 

amounts of upwelling of sulfate-rich ground water. 

The shallow flow system is apparently deeper in areas 

where low sulfate concentrations are found in the deep 

ground water.

The range of sulfate concentrations observed in the 

study area and differences in sulfate concentrations 

with depth indicate a complex interaction between the 

shallow and deep ground-water flow systems. The 

hydraulic connection between shallow and deep flow 

systems is poorly defined. Upwelling is probably 

responsible for high sulfate concentrations in intervals 

of the aquifer where gypsum is not found. This 

upwelling can be driven by localized aquifer discharge 

areas and perhaps by rapid flow within the shallow, 

more permeable, part of the aquifer. In addition, diffusion probably transports sulfate-rich water that dissolved gypsum upward within the more stagnant 

deeper part of the aquifer. The occurrence of gypsum is 

highly variable in deeper parts of the aquifer. It is also 

possible that gypsum occurs very sparsely in shallower 

intervals of the aquifer, or that it already completely 

dissolved, with sulfate remaining because of slow rates 

of flushing. Additional hydrologic, geologic, and 

chemical data, particularly from the deeper part of the 

aquifer, are necessary to better define the interaction 

between shallow and deep flow systems. These data 

can provide a better understanding of the variable 

nature and occurrence of high sulfate ground water, 

which can help optimize ground-water resources in the 

study area.

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Appendix 43

APPENDIX
44 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

Appendix—Chemical and isotopic data from ground-water samples and calculated saturation indexes and partial pressure of 

carbon dioxide

[deg C, degrees Celsius; μS/cm, microsiemens per centimeter; mg/L, milligrams per liter; μg/L, micrograms per liter; atm, atmospheres; 

-, no data; <, less than; δ13C, delta carbon-13; δD, delta deuterium; δ18O, delta oxygen-18; δ34S, delta sulfur-34; ∆34S, difference between 

the δ34S of sulfate and sulfide; sampling pump type: S = submersible; P = peristaltic; C = centrifugal; A = airlifted from corehole during drilling]

1

 Number refers to well locations shown in figure 8.

2

 Sample collected by Southwest Florida Water Management District (written commun., 1993), from 248.5 ft depth in corehole during drilling. Analysis used 

in NETPATH modeling of upwelling water. 3 Duplicate analysis was -32.5 per mil.

4 Duplicate analysis was 27.7 per mil.

Well

No.1 Well name

Sampling

date

(Year/

Month/

Day)

Temperature

water

(deg C)

Specific

conductance,

field

(μS/cm)

Oxygen,

dissolved

(mg/L)

Field

pH

(standard

units)

Nitrogen,

NO2+NO3

dissolved

(mg/L

as N)

Organic

carbon,

dissolved

(mg/L

as C)

1 Kellogg shallow well 19921215 21.7 495 <0.5 7.05 0.006 3.5 

2 Kellogg deep well 19921215 22.5 460 <.5 7.50 <.002 2.1 

3 City of Bushnell #2 well 19930204 24.6 416 <.5 7.71 .007 <.1

4 St. Lawrence Church rectory well 19930616 23.8 322 <.5 7.43 <.002 .4 

5 White well 19930224 23.8 368 <.5 7.35 <.002 4.0 

6 Owens well 19930224 23.4 267 1.73 7.50 .630 1.0 

7 ROMP LP-4 Avon Park (240) 19921208 24.8 2,260 <.8 7.37 <.002 1.5 

8 ROMP LP-4 Avon Park (120) 19921209 24.2 333 1.4 7.71 1.800 .1 

9 ROMP LP-4 Ocala 19921208 24.5 383 3.08 7.49 1.500 1.6 

10 City of Coleman well 19930324 27.4 457 <.5 7.31 .009 1.4 

11 Rolling well 19930324 22.8 366 1.27 7.54 .076 1.2 

12 Hawkins well 19930203 22.0 529 - 6.91 <.002 5.2 

12-d Hawkins well duplicate 19930203 - - - - .002 4.9 

13 Campers’ World well 19930203 22.5 531 <.5 7.17 .002 2.6 

14 Pilot Oil well 19921117 24.8 500 <.5 6.98 .003 3.4 

15 Union Oil well 19921117 25.1 461 <.5 7.39 .003 1.2 

16 Lorenz well 19930113 24.6 1,451 <.5 7.66 .003 .3 

17 Wooten well 19930113 23.6 179 3.08 7.92 .808 .3 

18 Budd well 19930106 24.6 1,625 <.5 7.31 <.002 2.7 

19 Iacino well 19930106 22.7 228 2.09 7.96 .406 .4 

20 USGS observation well CE-78 19930112 22.7 226 7.45 7.64 .242 <.1

21 ROMP 119 19930301 24.0 1,238 <.5 7.37 <.002 2.1 

22 Quaglio well 19930325 24.1 275 <.5 7.85 <.002 1.3 

23 Silver well 19930107 22.9 123 4.91 8.24 .633 <.1

24 Brookshier well 19930107 24.1 311 <.5 7.61 <.002 .7 

25 Barton well 19930325 23.2 228 3.89 7.75 .280 .9 

26 Bonnie Builders well 19930112 24.9 259 5.84 7.74 .549 <.1

27 Saddle Oak deep well 19921216 25.7 689 3.25 7.46 1.000 .1 

28 Saddle Oak shallow well 19921216 25.1 607 3.65 7.55 1.300 .1 

29 ROMP 120 19930302 24.6 602 <.5 7.31 <.002 1.4 

30 Rowland deep well 19921119 26.7 645 <.5 7.04 .003 .5 

30-d Rowland deep well duplicate 19921119 - - - - .003 -

31 Rowland shallow well 19930615 25.1 653 2.17 7.03 .081 .3 

32 Jones well 19921118 25.0 739 <.5 7.20 .006 2.6 

33 Brosky well 19921118 24.0 133 5.43 8.00 .870 3.8 

R1102 ROMP 110, 248.5 ft 19890912 22.5 2,600 - - - -
Appendix 45

Appendix—Continued

Well

No.1

Calcium,

dissolved

(mg/L 

as Ca)

Magnesium,

dissolved

(mg/L

as Mg)

Sodium,

dissolved

(mg/L

as Na)

Potassium,

dissolved

(mg/L

as K)

Chloride,

dissolved

(mg/L

as Cl)

Sulfate,

dissolved

(mg/L

as SO4)

Fluoride,

dissolved

(mg/L

as F)

Silica,

dissolved

(mg/L

as SiO2)

Barium,

dissolved

(μg/L

as Ba)

1 94 1.8 12 0.68 19 <0.2 <0.1 11 <5

2 75 6.9 9.1 1.3 29 .6 .2 13 8

3 69 10 5.8 .95 8.7 55 .4 14 46

4 55 2.3 4.1 .15 7.4 4.9 .1 8.2 11

5 66 2.9 5.3 .23 11 <.2 <.1 7.1 8

6 47 2.2 7.0 .17 8.6 3.6 <.1 8.5 6

7 470 110 10 2.5 13 1,400 1.0 12 32

8 53 3.5 3.5 .85 6.3 57 <.1 5.0 8

9 61 6.9 3.1 4.6 5.2 48 .2 4.4 12

10 77 1.9 14 .45 28 3.0 .1 8.1 24

11 58 1.9 13 3.2 21 21 <.1 7.2 9

12 106 1.3 5.2 <.10 7.8 <.2 <.1 9.7 13

12-d 106 1.5 5.2 .10 7.8 .2 <.1 9.9 10

13 94 13 4.6 .60 7.3 4.2 .2 10 14

14 92 3.0 9.2 .80 17 3.2 <.1 9.8 9

15 72 13 6.0 1.3 9.6 75 .5 13 16

16 200 76 22 2.7 18 760 .9 14 4

17 31 2.2 3.0 .11 4.9 1.3 <.1 7.6 2

18 330 62 7.3 1.5 11 900 1.0 14 34

19 34 5.6 3.4 .16 5.0 23.0 <.1 6.6 <5

20 44 0.8 1.8 .15 2.8 1.7 <.1 6.7 3

21 210 38 11 2.2 15 580 1.0 15 29

22 50 3.3 2.7 .12 4.5 2.8 .1 8.0 8

23 18 2.2 2.6 .13 4.3 2.3 <.1 6.4 <5

24 45 14 3.3 .32 4.8 <.2 .1 9.4 <5

25 42 1.6 2.5 <.10 4.3 2.3 <.1 5.6 7

26 47 2.5 3.3 .17 5.7 25.0 <.1 7.0 5

27 110 18 8.4 1.1 14 240 .2 9.7 8

28 95 14 8.0 .97 14 180 .2 9.7 <5

29 93 17 8.4 1.3 12 76 .7 18 7

30 100 19 10 1.5 15 100 .6 16 9

30-d 100 19 10 1.5 14 99 .6 17 14

31 99 16 9.1 1.3 13 90 .6 17 11

32 120 21 12 1.5 20 150 .5 17 12

33 20 1.4 3.3 .40 5.4 2.8 <.1 6.4 5

R1102 635 148 18 3.0 19 1,800 .9 - -
46 Geochemical and Isotopic Composition of Ground Water with Emphasis on Sources of Sultate in the Upper Floridan 

Aquifer in Parts of Marion, Sumter, and Citrus Counties, Florida

Appendix—Continued

Well

No.1

Boron,

dissolved

(μg/L

as B)

Iron,

dissolved

(μg/L

as Fe)

Manganese,

dissolved

(μg/L

as Mn)

Strontium,

dissolved

(μg/L

as Sr)

Aluminum,

dissolved

(μg/L

as Al)

Lithium,

dissolved

(μg/L

as Li)

Bromide,

dissolved

(mg/L

as Br)

Bicarbonate,

field

(mg/L

as HCO3)

Sulfide,

field

total

(mg/L

as S)

Dissolved

solids,

calculated

(mg/L)

1 29 4,000 17 72 32 <5 0.07 310 <0.01 -

2 <20 160 <5 97 33 <5 .09 241 .13 254

3 <20 12 3 810 20 4 .03 199 1.2 263

4 <20 2,200 26 49 20 5 .03 190 .02 178

5 <20 990 33 130 20 6 .01 217 .05 -

6 20 <3 <1 67 20 7 .03 167 <.01 162

7 32 1,000 15 8,200 140 5 .04 131 .02 2,092

8 <20 <5 <5 300 <20 <5 <.01 108 <.01 191

9 24 <5 <5 200 26 <5 .02 157 <.03 218

10 <20 870 15 140 40 <4 .02 226 .02 245

11 <20 39 9 55 20 <4 .03 155 <.01 202

12 <20 4,700 33 94 40 8 <.01 331 .03 -

12-d <20 4,400 38 92 30 4 <.01 330 .02 -

13 <20 32 7 940 20 <4 <.01 344 .61 304

14 22 5,000 50 130 31 <5 .09 293 .03 284

15 <20 25 <5 1,300 26 <5 .03 186 1.0 283

16 100 280 8 5,000 50 12 .05 121 .61 1,159

17 <20 11 <1 62 <20 4 .02 102 <.01 104

18 33 950 17 2,400 100 12 .05 198 1.7 1,428

19 20 19 <5 67 40 5 .03 107 <.03 132

20 <20 20 1 33 <20 <4 .02 147 <.01 131

21 22 52 2 7,100 60 13 .03 158 .80 957

22 <20 100 6 33 20 <4 <.01 162 .02 151

23 <20 23 <5 31 30 <5 .01 62 <.01 69

24 24 400 7 21 40 5 .02 207 .01 -

25 <20 5 <1 24 <20 <4 .01 131 <.01 -

26 <20 17 1 320 <20 4 .02 126 <.01 155

27 <20 10 <5 2,200 43 <5 .07 137 <.01 476

28 <20 <5 <5 1,700 38 <5 .05 127 <.01 392

29 <20 23 3 2,000 30 6 .04 286 .12 369

30 23 520 <5 3,000 37 5 .05 280 .14 403

30-d 24 400 5 2,800 24 <5 .05 280 .10 402

31 25 <3 2 2,000 30 <4 .05 306 <.01 399

32 21 720 5 2,600 44 <5 .07 271 .74 479

33 <20 6 <5 54 <20 <5 .02 61 <.01 74

R1102 - - - - - - - 168 odor 2,773
Appendix 47

Appendix—Continued

Well

No.1

Stable isotope ratio, in per mil

∆34S

(per 

δ mil) 13C δD δ18O 

δ34S

of

sulfide

δ34S

of

sulfate

δ34S

of

total

sulfur

1 -6.6 -5.8 -1.21 - - - -

2 -8.5 -6.8 -1.69 14.6 - - -

3 -9.6 -10.3 -2.18 3-32.0 427.9 24.3 59.9

4 -11.6 -17.8 -3.39 - 15.7 15.5 -

5 -10.5 -11.5 -2.21 - - - -

6 -12.1 -17.6 -3.28 - 10.4 10.3 -

7 -8.6 -14.0 -2.87 - 23.6 23.6 -

8 -10.7 -17.4 -3.70 - 21.9 21.9 -

9 -6.6 -15.3 -3.64 - 21.8 21.8 -

10 -11.2 -18.5 -3.70 - 15.0 14.7 -

11 -11.5 -21.3 -4.06 - 1.7 1.7 -

12 -11.7 -13.8 -2.78 - - - -

12-d -12.1 -13.5 -2.79 -32.5 - - -

13 -12.3 -10.1 -1.83 7.9 - - -

14 -12.2 -15.4 -2.86 7.7 33.1 32.5 25.4

15 -9.4 -12.5 -2.48 -34.4 24.3 22.0 58.7

16 -9.4 -16.5 -3.60 -42.1 24.2 24.0 66.3

17 -11.4 -16.6 -3.44 - 20.9 20.4 -

18 -8.4 -8.7 -1.83 -30.1 24.3 24.0 54.4

19 -10.7 -19.7 -3.86 - 20.2 20.1 -

20 -11.4 -18.3 -3.88 - 13.2 13.0 -

21 -8.4 -10.8 -2.55 -40.7 21.7 21.4 62.4

22 -10.4 -18.6 -3.67 - 11.4 11.2 -

23 -10.1 -19.7 -3.95 - 3.7 3.7 -

24 -9.8 -11.4 -2.46 - - - -

25 -11.4 -17.1 -3.61 - 6.5 6.4 -

26 -10.8 -18.8 -3.84 - 21.3 21.3 -

27 -10.1 -15.8 -3.56 - 22.0 22.0 -

28 -9.5 -16.2 -3.51 - 22.2 22.2 -

29 -7.7 -17.0 -3.65 -40.2 21.4 21.1 61.6

30 -10.1 -17.4 -3.58 -15.2 21.8 21.7 37.0

30-d -9.7 -16.3 -3.64 -15.0 21.8 21.7 36.8

31 -11.9 -18.2 -3.66 - 21.0 21.0 -

32 -12.0 -15.6 -3.11 -32.9 22.0 21.2 54.9

33 -9.4 -18.7 -3.84 - 1.8 1.8 -

R1102 - - - - - - -

Log 

partial

pressure

of CO2

(atm)

Saturation Index

Pump

Cal- type

cite

Dolomite

(disordered)

Dolomite

(crystalline)

GypsumCelestite

-1.60 0.008 -1.957 -1.394 - -S

-2.15 .271 -0.735 -0.175 -3.745 -4.927 S

-2.44 .368 -0.312 0.239 -1.837 -2.056 S

-2.17 .013 -1.573 -1.018 -2.906 -4.248 S

-2.03 .060 -1.455 -0.900 - -S

-2.29 -0.035 -1.627 -1.070 -3.085 -4.224 S

-2.33 .334 -0.184 0.367 -0.161 -0.204 P

-2.69 .019 -1.358 -0.805 -1.873 -2.410 S

-2.31 .014 -1.130 -0.578 -1.915 -2.689 S

-1.95 .138 -1.494 -0.954 -3.029 -4.054 S

-2.37 .032 -1.657 -1.098 -2.264 -3.581 C

-1.43 -0.053 -2.267 -1.705 - -S

-1.43 -0.054 -2.205 -1.643 - -S

-1.67 .169 -0.762 -0.202 -2.856 -3.152 S

-1.53 -0.055 -1.799 -1.248 -2.957 -4.099 S

-2.14 .036 -0.874 -0.324 -1.703 -1.735 S

-2.64 .320 -0.004 0.548 -0.608 -0.496 S

-2.92 .021 -1.330 -0.774 -3.650 -4.640 S

-2.08 .370 -0.208 0.343 -0.382 -0.807 S

-2.95 .076 -0.870 -0.310 -2.403 -3.400 S

-2.49 .021 -1.937 -1.378 -3.416 -4.834 S

-2.22 .205 -0.563 -0.009 -0.644 -0.403 S

-2.66 .332 -0.731 -0.177 -3.175 -4.645 S

-3.46 -0.104 -1.355 -0.797 -3.588 -4.643 S

-2.31 .141 -0.440 0.114 - -S

-2.65 .069 -1.511 -0.954 -3.301 -4.837 S

-2.65 .093 -1.291 -0.741 -2.250 -2.706 S

-2.35 .090 -0.806 -0.259 -1.128 -1.113 S

-2.47 .104 -0.831 -0.281 -1.271 -1.306 S

-1.88 .224 -0.498 0.053 -1.638 -1.596 S

-1.61 -0.005 -0.909 -0.366 -1.513 -1.322 S

-1.61 -0.004 -0.908 -0.365 -1.516 -1.356 S

-1.57 .000 -0.994 -0.444 -1.551 -1.536 S

-1.80 .168 -0.626 -0.076 -1.296 -1.249 S

-3.22 -0.286 -1.944 -1.39 -3.459 -4.318 S

 - - - - -0.008 -A