U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report

Geochemistry and Origins of Mineralized

Waters in the Floridan Aquifer System,

Northeastern Florida

Prepared in cooperation with the

City of Jacksonville

Jacksonville Electric Authority

St. Johns River Water Management District

01-4112

By G.G. Phelps

Tallahassee, Florida

2001
For additional information write to: Copies of this report can be purchased

from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

888-ASK-USGS

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.

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

District Chief

U.S. Geological Survey

Suite 3015

227 North Bronough Street

Tallahassee, FL 32301

Additional information about water resources in Florida is available on the World Wide Web at

http://fl.water.usgs.gov
Contents III

CONTENTS

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

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

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

Previous Investigations................................................................................................................................................ 6

Well-Numbering System ............................................................................................................................................ 6

Acknowledgments ...................................................................................................................................................... 8

Methods of Sampling and Chemical Analysis ..................................................................................................................... 8

Water Samples ............................................................................................................................................................. 8

Quality Assurance Samples ......................................................................................................................................... 10

Analysis of Aquifer Materials .................................................................................................................................... 10

Hydrogeologic Framework ................................................................................................................................................... 11

Surficial Aquifer System and Intermediate Confining Unit........................................................................................ 11

Floridan Aquifer System ............................................................................................................................................. 11

Upper Floridan Aquifer .................................................................................................................................... 11

Middle Semi-Confining Unit and Lower Floridan Aquifer............................................................................... 15

Isotopic Data from Aquifers ....................................................................................................................................... 16

Chemical and Isotopic Composition of Ground Water ......................................................................................................... 18

Major Ions and Trace Constituents.............................................................................................................................. 18

Halogens ........................................................................................................................................................... 25

Sulfur ................................................................................................................................................................ 25

Ion Enrichment ................................................................................................................................................. 27

Saturation State with Respect to Minerals .................................................................................................................. 31

Isotopic Analyses ........................................................................................................................................................ 33

Oxygen and Deuterium ..................................................................................................................................... 33

Carbon .............................................................................................................................................................. 37

Sulfur ................................................................................................................................................................ 41

Strontium .......................................................................................................................................................... 42

Boron ................................................................................................................................................................ 46

Chlorine ............................................................................................................................................................ 46

Origins of Mineralized Waters .............................................................................................................................................. 51

Mass-Balance Models ................................................................................................................................................. 51

Ground-Water Ages .................................................................................................................................................... 53

Reconciling Hydrogeologic and Geochemical Evidence ........................................................................................... 58

Summary ............................................................................................................................................................................... 59

Selected References .............................................................................................................................................................. 60

FIGURES

1. Map showing selected wells in northeastern Florida .............................................................................................. 3

2. Graph showing chloride concentrations of water and water levels from well D-262 in Duval County,

Florida, 1950-2000 .................................................................................................................................................. 4

3. Map showing chloride concentrations in water from the Upper Floridan aquifer and the upper zone of

the Lower Floridan aquifer and locations of inferred faults and paleokarst features in Duval County, Florida..... 5

4. Chart showing generalized geology and hydrogeology of northeastern Florida..................................................... 12

5a. Map showing estimated pre-development potentiometric surface of the Upper Floridan aquifer.......................... 13

5b. Map showing potentiometric surface of the Upper Floridan aquifer in northeastern Florida, May 1996 ............. 14

6. Diagram showing comparison of strontium-87 to strontium-86 ratio for rock samples from four wells in

northeastern Florida................................................................................................................................................. 17
IV Contents

7. Trilinear diagrams for water from wells tapping various aquifer zones ................................................................. 19

8. Major ion diagrams for selected wells .................................................................................................................... 23

9. Box plots of calcium, chloride, sulfate, and strontium concentrations................................................................... 24

10-15. Graphs showing:

10. Relations between sodium and chloride concentration, and between the mass ratio of

chloride-to-bromide and chloride concentration ............................................................................................. 26

11. Relation between sulfate-chloride ratio and sulfate concentration ................................................................. 28

12. Enrichment of calcium compared to enrichment or depletion of sulfate, sodium, and magnesium

for mixing with seawater................................................................................................................................. 30

13. Relations between celestite and gypsum saturation indexes and sulfate concentration, and between

calcite saturation index and calcium concentration......................................................................................... 36

14. Relations between delta deuterium and delta oxygen-18 and between delta deuterium and chloride

concentration ................................................................................................................................................... 38

15. Relation between delta oxygen-18 and delta carbon-13 for rock and water samples ..................................... 39

16. Map showing distribution of delta carbon-13 in ground water............................................................................... 40

17-21. Graphs showing:

17. Relations between delta carbon-13 and (a) sulfate concentration, (b) delta sulfur-34,

and (c) sulfate enrichment ............................................................................................................................... 41

18. Relation between delta sulfur-34 and sulfate concentration, and calculation of enrichment factors

for delta sulfur-34 of sulfate............................................................................................................................ 43

19. Relation between strontium concentration and chloride concentration .......................................................... 44

20. Relation between inverse of strontium concentration and ratio of strontium-87 to strontium-86 .................. 45

21. Relations between delta boron-11 and chloride and boron concentrations..................................................... 47

22. Map showing ratio of chlorine-36 to total chlorine in water from wells in northeastern Florida........................... 48

23. Graph showing relation between ratio of chlorine-36 to total chlorine and chlorine-36 concentration ................. 50

24. Map showing carbon-14 ages of ground-water samples......................................................................................... 55

25. Graphs showing relations between chloride concentration and carbon-14 age, and between strontium

concentration and carbon-14 age ............................................................................................................................ 57

TABLES

1. Well information..................................................................................................................................................... 7

2. Lithology and isotopic content of aquifer samples................................................................................................. 16

3. Chemical and physical data for water samples....................................................................................................... 20

4. Percent excess of some constituents for selected wells.......................................................................................... 29

5. Calculated partial pressure of carbon dioxide and saturation indexes of water with respect to selected

minerals................................................................................................................................................................... 32

6. Isotopic data for water samples .............................................................................................................................. 34

7. NETPATH models for selected wells ..................................................................................................................... 52

8. Carbon-14 ages for ground-water samples............................................................................................................. 54
Contents V

CONVERSION FACTORS, VERTICAL DATUM, ABBREVIATIONS AND ACRONYMS

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8.

Sea level:  In this report, sea level refers to the National Geodetic Vertical Datum of 1929 (NGVD

of 1929)--a geodetic datum derived from a general adjustment of the first-order level nets of both

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

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD83).

Acronyms and additional abbreviations used in report:

AMS accelerator mass spectrometry

FPZ Fernandina permeable zone

GML Global Meteoric Line

JEA Jacksonville Electric Authority

LFA Lower Floridan aquifer

μg microgram

μL microliter

μm micrometer

mg/L milligrams per liter

mL milliliter

ng nanogram

NIST National Institute of Standards and Technology

NWIS National Water Data Information System

RASA Regional Aquifer-System Analysis

SJRWMD St. Johns River Water Management District

SI saturation index

SLAP Standard Light Antarctic Precipitation

SMOC Standard Mean Ocean Chloride

UFA Upper Floridan aquifer

UZLF Upper zone of the Lower Floridan aquifer

USGS U.S. Geological Survey

VPDB Vienna PeeDee Belemnite

VSMOW Vienna Standard Mean Ocean Water

Multiply By To obtain

Length

foot (ft) 0.3048 meter

inch (in.) 25.4 millimeter

Flow

foot per year (ft/yr) 0.3048 meter per year

gallon per minute (gal/min) 0.06309 liter per second

million gallons per day (Mgal/d) 0.04381 cubic meter per second

cubic feet per second (ft3

/s) 0.02832 cubic meter per second

Transmissivity

foot squared per day (ft2/d) 0.0929 meter squared per day
VI Contents
Abstract 1

Geochemistry and Origins of Mineralized Waters in the

Floridan Aquifer System, Northeastern Florida

By G. G. Phelps

ABSTRACT

Increases in chloride concentration have

been observed in water from numerous wells

tapping the Floridan aquifer system in northeastern Florida. Although most increases have been in

the eastern part of Duval County, Florida, no spatial pattern in elevated chloride concentrations is

discernible. Possible sources of the mineralized

water include modern seawater intrusion;

unflushed Miocene-to-Pleistocene-age seawater

or connate water in aquifer sediments; or mineralized water from deeper zones of the aquifersystem

or from formations beneath the Floridan aquifer

system. The purpose of thisstudy wasto document

the chemical and isotopic characteristics of

water samples from various aquifer zones, and

from geochemical and hydrogeologic data, to

infer the source of the increased mineralization.

Water samples were collected from 53 wells

in northeastern Florida during 1997-1999. Wells

tapped various zones of the aquifer including:  the

Fernandina permeable zone (FPZ), the upper

zone of the Lower Floridan aquifer (UZLF), the

Upper Floridan aquifer (UFA), and both the UFA

and the UZLF. Water samples were analyzed for

major ions and trace constituents and for isotopes

of carbon, oxygen, hydrogen, sulfur, strontium,

chlorine, and boron. Samples of rock from the

aquifer were analyzed for isotopes of oxygen,

carbon, and strontium.

In general, water from various aquifer zones

cannot be differentiated based on chemistry,

except for water from FPZ wells. Major-ion concentrations vary as much within the upper zone

of the Lower Floridan aquifer and the Upper

Floridan aquifer as between these two zones.

Simple models of mixing between fresh ground

water and either modern seawater or water from

the FPZ as a mineralized end member show that

many water samples from the UZLF aquifer and

the UFA are enriched in bicarbonate, calcium,

magnesium, sulfate, fluoride, and silica and are

depleted in sodium and potassium (as compared

to concentrations predicted by simple mixing).

Chemical mass-balance models of mixing and

reactions between a hypothetical initial seawater

and aquifer minerals cannot account for the

observed water chemistry in a few wells, implying a source other than seawater, either ancient or

modern, or the occurrence of other more complex

rock-water reactions.

Hydrogeologic and geochemical data from

water and aquifer samples indicate that the most

likely source of mineralized water in some wells

yielding water with increasing chloride concentrations is water from the FPZ. In other wells, the

flushing of Miocene-to-Pleistocene-age seawater can account for the observed chloride concentrations. The fact that most of the water

samples collected are a mixture of less than one

percent of mineralized water with more than

99 percent fresh or recharge water makes identifying the source of the mineralized water difficult. Differencesin carbon-14 and sulfur-34 values

probably reflect areal differences in aquifer mineralogy and distribution of organic carbon related to

paleokarst features. Geochemical mass-balance

models of seawater-rock interaction are unable to

account for the chemical and isotopic composition of mineralized water from the FPZ,
2 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

which implies another source of mineralized

water, such as a brine, or the occurrence of more

complex water-rock reactions.

INTRODUCTION

The Floridan aquifer system is the major source

of water supply in northeastern Florida. Ground-water

withdrawals in Duval County have increased from about

127 million gallons per day (Mgal/d) in 1965 to about

145 Mgal/d in 1995 (Marella, 1999, table 2). As a result,

the altitude of the potentiometric surface of the Floridan

aquifer system has gradually declined at a rate of about

0.3 to 0.75 foot per year (ft/yr) (Spechler, 1994).

Concomitant with declines in the potentiometric surface

have been increases in chloride concentrations in water

from numerous wells (figs. 1 and 2). Most of the

observed increases are in the eastern part of Duval

County, but a pattern in the locations of wells yielding

water with chloride concentrations higher than background values (greater than about 20-30 milligrams per

liter (mg/L)) is not discernible (fig. 3) and wells as far as

14 miles inland have been affected (Spechler, 1994). In

contrast, chloride concentrations in water from other

wells near the center of pumping have remained lessthan

20 mg/L (Phelps and Spechler, 1997, fig. 7.).

Background ground-water quality primarily

reflects the chemistry of local precipitation and aquifer

mineralogy. The mean chloride concentration of bulk

precipitation at 24 sitesthroughout Florida was 3.2 mg/L

with a standard deviation of 2.0 mg/L (Irwin and

Kirkland, 1980). Davis and others (2000) estimated that

the chloride concentration of precipitation probably

increases by a factor of 10 due to the combined effects of

evapotransipiration and dissolution of dry fallout before

the water is recharged into an aquifer. Thus, the assumption of a background chloride concentration of about

20 mg/L is reasonable. Leve (1966) reported that of the

26 samples collected in Duval County between the 1940s

and 1960s, only 2 had chloride concentrations greater

than 30 mg/L. Another constituent to consider is sulfate.

Dissolution of sulfur-bearing minerals is the primary

source of sulfate in water from the Floridan aquifer

system. A sulfate concentration of 5 mg/L was reported

by Spechler (1994) for a well in Clay County, Fla.,

and median sulfate concentrationslessthan 7 mg/L were

reported for about 40 wellstapping the surficial aquifer

system in central and north-central Florida (Adamski and

Knowles, 2001). A number of wellstapping the Floridan

aquifersystem in northeastern Florida have chloride concentrations less than 30 mg/L and sulfate concentrations

lessthan 100 mg/L (Spechler, 1994; Phelps and Spechler,

1997). The term mineralized water is used in this report

to describe water with chloride concentrations greater

than 30 mg/L and sulfate concentrations greater than

100 mg/L.

Previousinvestigations(Spechler, 1994; Phelps

and Spechler, 1997)indicate that themost likely source of

mineralized water to the upper zone of the Lower Floridan aquifer is the deepest zone of the aquifer system, the

Fernandina permeable zone (FPZ), which contains freshwater in the western part of the county butsaline water in

the eastern part. The ultimate source of mineralized water

in the FPZ, however, has not been determined. Possible

sources of mineralized water include intrusion of modern

seawater, entrapped connate water in the aquifer,

unflushed seawater from higher stands of seas level during Miocene to Pleistocene time, and upconing of highly

mineralized water (brine) from formations beneath the

aquifer. For example, a water sample from a depth of

about 5,000 ft below land surface in an oil test well drilled

near Fernandina Beach in 1970 had a chloride concentration of about 55,700 mg/L (R.E. Faye, U.S. Geological

Survey (USGS), written commun., 1997) compared to

about 19,000 mg/L forseawater. Another possible source

of brackish water is unflushed relict seawater in the

upper, freshwater-producing zones of the Floridan aquifer system. In some areas, zones bearing brackish water

are underlain by zones of freshwater, whereas in other

areas, freshwater is absent beneath the brackish water.

The pathways for saline water movement are

interconnecting vertical and horizontal fracture or solution zones probably developed along paleokarst features

(fig. 3; Phelps and Spechler, 1997). A single fracture or

solution feature wasthe source of brackish waterfound in

several wells in Duval County (Phelps and Spechler,

1997). These pathways for upward movement cannot be

delineated from land surface by using observational or

geophysicalmethods,makingmonitoring of existing well

fields and development of new well field sites difficult.

Geochemical analysis of water from saline parts of the

FPZ, from wells that yield water with increased chloride

concentrations, and from wellsin which chloride concentrations have not increased may help determine whether

the source of the mineralized water is the same throughout Duval County, or if more than one source exists.

Analysis of water samples for isotopes of hydrogen, oxygen, sulfur, strontium, and carbon is commonly used to help determine the origin of waters in

many hydrogeologic settings, to identify and quantify

dominant geochemical processes that occur along
Introduction 3

Figure 1.  Selected wells in northeastern Florida.
4 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

flow paths, and (for strontium and carbon) to determine the age of ground water. Boron and chlorine

isotopes are less widely used in ground-water

geochemical investigations because the analytical

methods used to determine the abundance of these

isotopes are performed by only a few researchers. The

hypothesis was made that saline water in the Floridan

aquifer system would have chemical and isotopic

characteristics that would be dependent upon the age

and origin of the water. For example, relict seawater

would have a chemical composition similar to seawater but its isotopic composition would indicate an age

older than modern seawater. Entrapped seawater from

previous higher stands of sea level also would show

the effects of some chemical interaction with the aquifer matrix, as well as having isotopic compositions

that would indicate an age that is not modern. Brines

would be expected to have undergone extensive chemical modification and have isotopic compositions that

indicate very old ages. To test this hypothesis, the

USGS, in cooperation with the City of Jacksonville,

the Jacksonville Electric Authority (JEA), and the St

Johns River Water Management District (SJRWMD),

began a 4-year study in 1996 to characterize the

geochemical and isotopic composition of water from

the Floridan aquifer system and to evaluate the possible sources of mineralized waters that have resulted in

the degradation of water quality in northeastern Florida, and particularly in Duval County.

Purpose and Scope

This report presents a summary and interpretation of the chemical analysis of water from 53 wellsin

northeastern Florida (fig. 1, table 1). The water was

analyzed for major and trace ions and for isotopes of

Figure 2.  Chloride concentrations of water and water levels from well D-262 in Duval County, Florida, 1950-2000

(modified from Phelps and Spechler, 1997, fig. 2).
Introduction 5

Figure 3.  Chloride concentrations in water from the Upper Floridan aquifer and the upper zone of the Lower Floridan

aquifer and locations of inferred faults and paleokarst features in Duval County, Florida (modified from Phelps and

Spechler, 1997, fig. 8).
6 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

hydrogen, oxygen,sulfur,strontium, carbon, chlorine,

and boron. The wells sampled were selected so that

most areas of Duval County and all zones of the Floridan aquifer system are represented. Wells that yield

water with increasing chloride concentrations and

wells that do not were sampled. A sample of modern

seawater was collected to provide a chemical endmember for seawater-freshwater mixing models.

Samples of rock from wells penetrating various

depths of the Floridan aquifer system were analyzed

for isotopes of oxygen, strontium, and carbon. A

geochemical modeling program, NETPATH (Plummer and others, 1994), was used to calculate carbon14 age dates and to provide insight into possible models of ground-water mixing and potential flow pathsin

the Duval County area. The hydrogeologic and

geochemical data were used to draw conclusions

about the source or sources of mineralized waters in

northeastern Florida.

The data and analyses presented in thisreport are

relevant to the goals of the USGS. Specifically, these

data help to better define the quality of drinking water

in a coastal area where saltwater intrusion may be

occurring. Also, this coastal area is one where existing

sources of water are being stressed by increasing withdrawals. Understanding the possible sources of mineralized waters can help local and state water managers

plan for the use of existing or alternative water-supply

sources. Similar conditions occur in the Floridan aquifer system in adjacent states, and the methods used in

this study could, therefore, be applicable to other areas.

Previous Investigations

The geology and hydrology of Duval County

and adjacent areas of northeastern Florida have been

discussed in numerous reports. The geology of northeastern Florida was described by Puri (1957), Puri and

Vernon (1964), and Miller (1986). The ground-water

resources of Duval County were described by Leve

(1966), Leve and Goolsby (1966), Fairchild (1972

and 1977), and Causey and Phelps (1978). Water

quality in Duval County was described by Thompson

(1982), and the intrusion of saline water into the Floridan aquifer system in northeastern Florida has been

documented by Fairchild and Bentley (1977), Leve

(1983), Toth (1990), and Spechler (1994). The relation between hydrogeology and water quality in the

Lower Floridan aquifer has been described by Phelps

and Spechler (1997). Data from test wells drilled into

the Lower Floridan aquifer in northeastern Florida

have been described by Brown (1980), Brown and

others (1984), Brown and others (1985), and Brown

and others (1986).

In addition to the numerous interpretive reports

describing the hydrogeology of the Duval County area,

a data-collection network is operated cooperatively by

the USGS, the SJRWMD, and the City of Jacksonville.

Water levels are collected in about 30 wells and water

samples from 24 wells are analyzed yearly for major

ions and quarterly for chloride concentration. Data

from the network are published annually by the USGS

(U.S. Geological Survey, 2000).

Well-Numbering System

Two well-numbering systems are used in this

report. The first is a 15-digit site identification number

(Site ID) based on latitude and longitude, used to identify wells in the USGS National Water Data Information System (NWIS). The first six digits denote the

degrees, minutes, and seconds of latitude; the next

seven digits denote degrees, minutes, and seconds of

longitude; and the last two digits denote a sequential

number for a site within a 1-second grid. For example,

well 302538081253101 is the first well inventoried at

latitude 30o25Ì38Ó N, longitude 081o25Ì31Ó W.

The second well-numbering system consists of

shortened, local identifiers used in place of the longer,

15-digit site identification numbers. Local numbers

were assigned by the USGS to wells in each county in

northeastern Florida asthe wells were inventoried. The

prefixes D, N, SJ, and C denote wellsin Duval, Nassau,

St. Johns and Clay Counties, respectively. The

sequence number denotes the order in which the well

was inventoried. For example, well D-164 is the 164th

well inventoried in Duval County. Other government

agencies also have local well-numbering systems. For

example, the City of Jacksonville Department of Regulatory and Environmental Services assigns local well

numbers beginning with a Ï J,Ó and the JEA Water Production Division assigns numbers to its water-supply

wells that denote the pumping station number and

sequence number. For example, well 0502 designates

the number 2 well at the number 5 well field. The

SJRWMD also useslocal well numbersthat begin with

the letter Ï DÓ for wells in Duval County. The local

numbers used by the SJRWMD may not correspond

with the local numbers used by the USGS. In this

report, USGS local well numbers are used. A few wells
Introduction 7

Table 1.  Well information

[Aquifer:  UFA, Upper Floridan aquifer; B, both Upper Floridan and upper zone of Lower Floridan aquifers; UZLF, Upper

zone of Lower Floridan aquifer; FPZ, Fernandina permeable zone; --, no information; na, not applicable]

Site

identification

number

Local

number Aquifer Depth(feet)

Casing

(feet)

Sample

date Remarks

300019081363301 SJ-3 UFA 500 07/15/97

300347081363701 SJ-168 UFA 630 590 07/15/97

301304081222701 SJ-103 UFA 857 385 03/19/98

301339081531201 D-326 UFA 887 400 01/28/99

301547081271901 SE-1 UFA 870 349 01/28/99

301714081233301 D-72 UFA 850 350 01/21/98

301830081375501 D-1551 UFA 660 540 01/27/99

302013081353801 D-673 UFA 814 578 01/21/98

302058081244101 D-1107 UFA 1,011 395 01/27/99

302113081322301 D-655 UFA 750 -- 10/29/98

302140081235501 D-1121 UFA 518 -- 10/28/98

302239081434401 NK-1 UFA 600 500 10/29/98 Well and casing depth estimated

302557081253101 D-913 UFA 556 435 07/15/97

302608081354903 D-264 UFA 700 -- 10/27/98

302609081260601 D-912 UFA 484 431 10/27/98

300856081382401 D-3847 B 1,220 470 03/18/98

301405081325601 D-3846 B 1,198 578 01/20/98

301407081324101 Deer3-03 B 1,180 578 01/20/98

301508081364601 BW-3 B 1,102 510 02/23/99

301704081233401 D-484 B 1,220 357 02/04/97

301725081305002 D-650 B 1,276 424 01/20/98

301740081361001 D-275 B 1,230 515 01/28/97

301743081303501 D-3825 B 1,093 440 01/20/98

301743081362301 D-225 B 1,277 547 01/28/97 High chloride

301743081362301 D-225 B 1,277 547 07/14/97 Low chloride

301748081383401 D-334 B 1,500 535 01/21/98

301758081303901 D-665 B 1,185 417 01/31/97

301817081374901 D-425T B 1,895 752 03/18/98

301846081240201 D-246 B 1,210 388 01/22/98

301934081292301 R-3 B 920 425 01/28/99

301937081471401 D-2223 B 1,315 553 01/28/99

301957081342301 D-313 B 1,150 576 01/22/98

302007081353201 D-479 B 1,350 606 03/18/98

302008081242101 D-307 B 1,300 407 01/27/99

302015081384501 D-335 B 1,290 531 03/18/98

302045081323101 D-1323 B 1,170 580 04/06/98

302342081320601 D-1902 B 1,209 515 03/17/98

302503081332001 D-1149 B 1,104 520 03/17/98

302510081260201 D-3841 B 1,311 771 04/08/98

302538081392501 D-329 B 1,210 545 01/21/98

303940081281801 N-22 B 1,000 -- 05/18/99

301335081355001 D-536-1 UZLF 1,140 372 02/04/97 Sample depth 1,042 ft

301335081355001 D-536-2 UZLF 1,140 372 02/04/97 Sample depth 1,059 ft

301335081355001 D-536-3 UZLF 1,140 372 02/04/97 Sample depth 1,078 ft

301522081331301 D-291 UZLF 1,237 537 01/31/97 Sample depth 1,213 ft

301537081441901 D-75 UZLF 1,290 970 01/29/97

301604081361501 D-450 UZLF 1,300 1,100 01/29/97

301639081330802 D-1155 UZLF 1,170 1,080 01/28/97

301936081292401 MW-2 UZLF 1,121 1,010 03/19/98

302022081393501 D-176 UZLF 1,270 800 01/29/97

302227081435001 D-592 UZLF 1,330 1,150 01/29/97

302608081354901 D-262 UZLF 1,237 1,163 01/29/97

302608081354902 D-263 UZLF 1,020 850 01/22/98

301132081225801 SJ-150 FPZ 2,035 1,980 01/30/97

301405081325601 D-425B FPZ 2,486 2,055 01/28/97

301725081305002 D-3060 FPZ 2,112 2,050 01/30/97

301743081303501 D-2386 FPZ 2,026 1,892 01/27/97

302737080571801 Seawater na na na 04/07/98
8 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

inventoried during thisstudy were given local numbers

based on nearby features, such as NK-1 and R-3.

Acknowledgments

The author would like to thank the staff of the

JEA Water Production Division for their assistance and

cooperation in providing access to wells for collection

of watersamples. Personnel of the City ofJacksonville,

Department of Regulatory and Environmental Services

also provided help in locating wells to sample. Thanks

also to United Water of Florida, Inc., for access to

selected wells and to private property owners for

permission to sample their wells. Ratios of 36Cl/Clwere determined by P. Sharma, Purdue University, and

strontium and boron isotopes of water samples and

strontium isotopes of aquifer samples were determined

by Thomas D. Bullen, USGS, Menlo Park, Calif.,

Thomas D. Bullen and L. Neil Plummer (USGS,

Reston, Va.) and Stanley N. Davis (University of

Arizona) provided helpful advice and discussions.

The author also is grateful to technical reviewers

Laura Sacks (USGS, Tampa, Fla.) and Peter McMahon

(USGS, Denver, Colo.).

METHODS OF SAMPLING AND CHEMICAL

ANALYSIS

Water samples were collected from 53 wells in

northeastern Florida. Many of the wells sampled for

this study have large open-hole intervals, although a

few are open to a relatively short aquifer interval

(table 1). The large open-hole intervals sampled in

many of the wells complicated the interpretation of

water-chemistry data. For example, many of the wells

are open to both the Upper Floridan aquifer (UFA) and

the upper zone of the Lower Floridan aquifer (UZLF).

This makes comparison of water chemistry from the

two aquifers difficult. In two wells(D-291 and D-536),

samples were collected by using a down-hole thief

sampler, so although the wells are open to both aquifers, the samples analyzed represent only the UZLF.

One well (D-225) produces water with varying water

chemistry. The chloride concentration fluctuates from

about 60 to 200 mg/L, depending on whether the well

is allowed to flow naturally or is pumped (Phelps and

Spechler, 1997). For this study, well D-225 was sampled twice so water of both high and low chloride

compositions could be analyzed.

Samples of aquifer material from four wells in

Duval County were analyzed for isotopes of oxygen,

carbon, and strontium. The rocks analyzed include

samples of limestone and dolomite that represent rocks

of the UFA (Ocala Limestone), the UZLF, and the FPZ

(Avon Park Formation). Also analyzed were two samples of organic carbon (peat) probably from cavity-fill

in the Avon Park Formation.

For most isotope determinations, standard δ

(delta) notation (Gonfiantini, 1981) is used in units of

parts per thousand (per mil), and is expressed by the

equation:

, (1)

More information about standards and reporting units

for various environmental isotopes is given in a later

section of this report.

Water Samples

Both public-supply and monitor wells were sampled for this study. Many of the wells sampled are public-supply wells that are in nearly continuous use.

Some of the wells flow naturally, but in most publicsupply wells, the flow rates are augmented by deepwell turbine pumps. All public-supply wells were sampled after at least three casing volumes had been

pumped. Other wells were pumped by using a submersible electric pump. Those wells were sampled after at

least three casing volumes had been pumped and after

field measurements of water temperature, pH, and specific conductance had stabilized.

Samples were collected following a standard

USGS protocol described by Wood (1976). Water

temperature, pH, and specific conductance were determined in the field. For most samples, alkalinity was

determined in the field by titration with sulfuric acid.

Sulfide was measured in the field for many samples

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

limit for sulfide is 0.01 mg/L. Many of the samples

contained sulfide concentrationsin excess of 0.6 mg/L,

in which case the samples were diluted with deionized

where

R is the ratio of heavy to light isotope.

δsample

Rsample

Rs dard tan

= ------------------- 1 Ò 1 000 × ,
Methods of Sampling and Chemical Analysis 9

water in a volumetric flask, which may have oxidized

some of the sulfide in the sample, thereby resulting in a

lower apparent sulfide concentration (C.J. Patton,

USGS, oral commun., 1998). As a comparison, sulfide

was precipitated in the field for some samples using

sodium hydroxide and zinc acetate and the resulting

precipitate was analyzed at the USGS laboratory in

Ocala, Fla.

Watersamplesfor anion and cation analysis were

filtered using 0.45 micrometer (μm) Gelman capsule

filters. Cation, trace metal, and 87Sr/86Sr samples were

collected in acid-washed bottles and acidified with

70 percent nitric acid. Major ion and trace constituent

samples were analyzed at the USGS laboratory in

Ocala, Fla.

The minimum reporting level for bromide determined by ion chromatography is 0.01 mg/L. For this

study, more accurate determinations were desired, so

low-level bromide determinations were made at the

USGS laboratory in Denver, Colorado. The method

involved Ï spikingÓ the samples with 10-microliter

(μL), 30-μL, 50-μL, or 100-μL volumes of a bromide standard solution (0.01 mg Br/milliliter (mL))

depending on the unspiked concentrations. Then addition of aliquots of the spike solution (10 μL, 30 μL,

50 μL, or 100 μL) to 10-mL volumes of sample

resulted in increased bromide concentration in the

samples by 10 μg/L, 30 μg/L, 50 μg/L, and 100 μg/L,

respectively. Dilution of the sample by the spike solution was 1 percent or less (worst case, 100 μL spike

in 10 mL of sample = 1 percent). A colormetric

method was then used to determine bromide concentrations, after correction for iodide concentrations. Relatively high and variable concentrations of chloride

and sulfate in the samples could interfere with the bromide recovery. During the first round of sampling, this

custom procedure had bromide recovery rates ranging

from 15 to 188 percent (the extremes representing two

samples with very low bromide concentrations). In

later sampling, the recovery rates ranged from 44 to

111 percent. The median recovery rate for all samples

was 92 percent.

Ratios of 87Sr/86Sr (on filtered, acidified samples) and 11B/10B (on filtered, unacidified samples)

were determined at the USGS research laboratory in

Menlo Park, Calif., using a Finnigan MAT 261 solid

source mass spectrometer. Strontium isotopes were

analyzed by the positive ion thermal massspectrometry

technique. For each analysis, sufficient fluid sample

containing approximately 1 microgram (μg) of strontium was loaded onto a pre-cleaned cation exchange

column, and strontium was separated from other cations and anions by using 2N hydrochloric acid as the

eluent. The strontium fraction was evaporated to dryness with 40 μL of 0.5N phosphoric acid and converted

to a nitrate. The sample was then loaded onto a tantalum ribbon filament and placed in the mass spectrometer. Samples were analyzed for isotopic composition

using a double collector dynamic data acquisition

approach. Measured 87Sr/86Sr ratios were corrected for

all stable isotope fractionation by simultaneously measuring 88Sr/86Sr, correcting this ratio to a value of

8.37521, and then correcting the 87Sr/86Sr ratio by

using an exponential mass-dependent fractionation

relation. NIST 987, a widely-used standard reference

material, routinely gives a value of 0.71024 on the

Finnigan MAT 261 instrument. Reported 87Sr/86Sr

ratios are precise to 0.00002 at the 95-percent

confidence level.

Boron isotopes were analyzed by the negative

ion thermal mass spectrometry technique. For each

analysis, sufficient sample containing approximately

2 nanograms (ng) of boron was evaporated in a Teflon

beaker and treated with 100 μL H2O2 to eradicate

organics. The dried sample was loaded onto a rhenium

filamentribbon and placed into the high vacuum source

of the mass spectrometer. Filaments were heated to

approximately 950 o

C, and data were collected when

ion beamsstabilized in their growth characteristics. Ion

beams were measured simultaneously at mass

42 (

10BO2) and 43 (11BO2) for approximately 30 minutes. The reported δ11B is the per mil difference of the

ratio of the measured 11BO2 to 10BO2 beams, relative

to the reference standard NIST 951 boric acid. The correction for oxygen isotope composition is negligible.

On this mass spectrometer, NIST 951 gives a 11B/10B

value of 4.004; seawater gives a value of 4.161, which

corresponds to a δ11B of +39.2 relative to NIST 951.

Precision is + 0.5 per mil.

For analysis of 36Cl and ratios of 35Cl/37Cl,

filtered, unacidified samples were collected in plastic

bottles secured with polyseal caps. Targets were prepared by using ion exchange techniques at the Purdue

Rare Isotope Measurement Laboratory, Purdue

University; the ratios of 36Cl-

/Cl- were determined by

using accelerator mass spectrometry (AMS). The

ratios of 35Cl/37Cl were determined at the Environmental Isotope Laboratory, University of Waterloo Ontario,

Canada. The chloride ion was precipitated as silver

chloride by addition of silver nitrate to an acidified
10 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

solution. Methyl iodide was added to the dried silver

chloride and reacted for approximately 48 hours at

85 o

C in a vacuum to produce methyl chloride. The

methyl chloride was then isolated and purified by gas

chromatography and cryogenics, and then analyzed on

a mass spectrometer. Ratios of 35Cl/37Cl were calculated relative to SMOC (Standard Mean Ocean Chloride). Precision is + 0.15 per mil.

Unfiltered samples were collected in glass bottles secured with polyseal caps for deuterium (δD) and

oxygen-18 (δ18O) analysis of the water and carbon-14

(

14C) and carbon-13 (δ 13C) analysis of total inorganic

carbon. Concentration ratios of deuterium and hydrogen isotopes (D/H) and oxygen-18 and oxygen-16 isotopes (

18O/16O) were determined by the USGS Isotope

Fractionation Laboratory in Reston, Va., using techniques described by Coplen (1988 and 1994). Analytical determinations of 14C and 13C/12C were made by

contract laboratories using AMS. Oxygen and hydrogen isotopic results are reported in per mil relative to

VSMOW (Vienna Standard Mean Ocean Water), and

are normalized on scales such that the oxygen and

hydrogen isotopic values of SLAP (Standard Light

Antarctic Precipitation) are -55.5 per mil and -428 per

mil, respectively. Carbon isotopic results are reported

in per mil relative to VPDB (Vienna PeeDee belemnite). The uncertainties of both carbon and oxygen isotopic results are + 0.2 per mil; the uncertainty for

hydrogen results is 2 per mil.

Samples also were collected for analysis of

sulfur-34 (

34S) of sulfate. Previous studies in Duval

County have shown that the sulfate concentrations in

nearly all ground water are much greater than 20 mg/L

and nearly all water containssubstantial concentrations

of sulfide. Samples for sulfate analysis were collected

in 1-L plastic bottles and then immediately acidified to

a pH between 3 and 4 by using 1N hydrochloric acid.

The samples were stripped of H2S by bubbling N2 gas

through the sample for at least 30 minutes, until there

was no discernible H2S odor. Samples were then

shipped to the USGS Isotope Fractionation Laboratory

in Reston, VA, where BaSO4 was precipitated for isotopic analysis (Carmody and others, 1998). The uncertainty associated with these data is + 0.3 per mil.

Quality Assurance Samples

For quality assurance purposes, various duplicate samples were collected from some wells and three

equipment blanks were collected by using laboratoryprepared inorganic blank water. Because nearly all of

the wells had been sampled previously for major ions

and trace metals (and some are sampled for major ions

on a regular basis), duplicate samples for those constituents were not collected. Ionic charge balances for all

analyses were within 10 percent, with the majority

within 5 percent. The three equipment blanks all had

concentrations of major ions and trace metals near or

below detection limits, indicating that negligible contamination was introduced by the sampling or

analytical processes.

A duplicate pair of samples for chlorine isotopes

indicated a 36Cl/Cl ratio of 59 with a 7.8 percent error

and 55.9 with a 9.1 percent error for water with a relatively low chloride concentration of 12 mg/L. The

ratios of 35Cl/37Cl for that pair of duplicates were 0.04

and 0.08, respectively, with a precision of + 0.4 per

mil. In general, errors in the determination of the

36Cl/Cl ratio tended to increase as chloride concentration increased, but there was considerable scatter in the

trend. Duplicate samples were analyzed for 14C at two

wells. For one well, the percent modern carbon values

were 1.37 + 0.06 percent and 1.33 + 0.05 percent. For

the other well, the values were 2.9 percent + 0.09 percent and 1.04 percent + 0.05 percent. The effects of

analytical errors on the calculated 14C ages of water

samples is discussed in a later section of this report.

For a duplicate pair ofsamples analyzed for low-level

bromide concentrations, the results were 61 μg/L and

45 μg/L, a 30 percent difference. The mean of the

two values (53), is estimated to be the Ï actualÓ value,

+ 15 percent.

Analysis of Aquifer Materials

Samples of cuttings from four wells in Duval

County were analyzed for δ 13C, δ 18O, and the ratio of

87Sr/86Sr to provide information about the isotopic

composition of aquifer materials that contain and interact with the ground water in northeastern Florida.

Washed cuttings were selected from depth intervals

corresponding to the UFA and the UZLF from well

MW-2 (drilled in 1995) and the UZLF from well D-3846

(drilled in 1994). Samples of cores from wells D-349

and D-425 that correspond to the FPZ also were analyzed. Bulk samplesidentified aslimestone or dolomite

in geologistsÌ logs were analyzed. In addition to samples of limestone and dolomite, two samples of peat

and several tests of the foraminifera Lepidocyclina

ocalana and Dictyoconus americanus collected from
Hydrogeologic Framework 11

well D-3846 were analyzed. The δ13C and δ18O samples were analyzed at the USGS Isotope Fractionation

Laboratory at Reston, Va. Analyses for 87Sr/86Sr were

done at the USGS research laboratory, Menlo Park,

Calif.

HYDROGEOLOGIC FRAMEWORK

Northeastern Florida is underlain by a thick

sequence of marine sedimentary rocks that overlie a

basement complex of metamorphic strata. Geologic

formations and hydrogeologic units penetrated by

wellsin the Jacksonville area are described in figure 4.

Rocks of the Cedar Keys Formation of late Paleocene

age underlie all of northeastern Florida. They are overlain, in ascending order, by the Oldsmar Formation of

early Eocene age, the Avon Park Formation of middle

Eocene age, the Ocala Limestone of late Eocene age,

the Hawthorn Formation of Miocene age, and undifferentiated deposits of late Miocene to Holocene age.

Surficial Aquifer System and Intermediate

Confining Unit

The sediments of late Miocene to Holocene age

generally consist of interbedded sand, shell, and clay

with some dolomitic limestone. These sediments

together make up the surficial aquifer system, which in

most areas of Duval County has two water-producing

zones separated by beds of lower permeability. The

surficial aquifer system usually is unconfined, but may

be semiconfined where overlying beds of lower permeability are sufficiently thick and continuous. The

thickness of the surficial aquifer system is variable,

ranging from about 20 to 120 feet (ft) in Duval County

and vicinity.

The Hawthorn Formation of Miocene age underlies the surficial sediments and consists of interbedded

clay, silt, sand, dolomite, and limestone that contain

abundant phosphate pebbles and granules and phosphatic sand. Throughout most of northeastern Florida

the clays and silts of the Hawthorn Formation serve as

an effective confining unit. Thisintermediate confining

unit retards the movement of water between the overlying surficial aquifer system and the underlying Floridan aquifer system.

Floridan Aquifer System

The Floridan aquifer system underlies all of the

Florida peninsula, as well as parts of Alabama, Georgia,

and South Carolina. It is the principal source of watersupply for the study area. The aquifer system ranges in

thickness from about 1,600 to 1,900 ft in the study area

and includes the Ocala Limestone, the Avon Park

Formation, the Oldsmar Formation, and locally, the

upper part of the Cedar Keys Formation. The beds

range in age from Paleocene (Cedar Keys Formation)

to Eocene (Oldsmar and Avon Park Formations and

Ocala Limestone). In northeastern Florida, sediments

of Oligocene age are absent (Miller, 1986; Stringfield,

1966). Miller (1986) definitively described the various

water-bearing zones of the Floridan aquifer system as

part of the Regional Aquifer-System Analysis (RASA)

study. The Floridan aquifer system is divided into two

aquifers of relatively high permeability, referred to as

the Upper Floridan and the Lower Floridan aquifers.

The Lower Floridan aquifer (LFA) can be subdivided

into two principal water-bearing zones, the UZLF and

the FPZ, separated by a less permeable unit. The

aquifer layers are delineated on the basis of rock

permeability, not on the basis of lithology, formation,

ortime-stratigraphic boundaries(Miller, 1986, p. B54).

In general, the upper zones of the aquifer system

produce freshwater, but mineralization increases with

depth. For example, some wells completed into the

UFA yield water with chloride concentrations less than

10 mg/L. In contrast, Leve (1966, p. 91) reported a

chloride concentration of about 33,000 mg/L in a

water sample from a depth of about 2,220 ft below

land surface (FPZ) in an oil test well in northwestern

Nassau County.

Upper Floridan Aquifer

The UFA generally corresponds to the Ocala

Limestone, and in some areasincludesthe upper part of

the Avon Park Formation. The UFA is about 300 to

500 ft thick in Duval County. The Ocala Limestone is

fossiliferous and characterized by high permeability

and high effective porosity. Permeability has been

enhanced by dissolution of the rock along bedding

planes, joints, and fractures. The surface of the Ocala

Limestone is a paleokarst plain that exhibits erosional

and collapse features that developed before the deposition of the overlying Hawthorn Formation. Leve (1983,

p. 251) attributed some of the abrupt variations in the

altitude of the surface of the Ocala Limestone to faults.
12 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Using additional data collected since 1983, Spechler

(1994, pl. 1) compiled a more detailed map of the top

of the Ocala Limestone, which indicates the presence

of numerous closed-contour depressions in the surface

of the unit. The observed configuration of the top of the

Ocala Limestone could be the result solely of paleokarst processes, rather than movement along faults.

The transmissivity of the UFA varies considerably throughout the study area. Transmissivity values

for six wells that penetrated 50 to 500 ft of the Floridan aquifer system, corresponding to the UFA and the

middle semi-confining unit, ranged from 20,000 to

50,000 feet squared per day (ft2

/d) (Franks and Phelps,

1979, p. 7). At Ft. George Island, in eastern Duval

County, transmissivity values of 31,000 and 49,000 ft2

/d

were determined (Environmental Science and Engineering, Inc., 1985, p. 3-36). Brown (1984, p. 27)

reported transmissivity values ranging from about

20,000 to 50,000 ft2/d for the UFA in Nassau County

and adjacent Camden Country, Ga. Transmissivity

values in Duval County from modeling simulations

range from 50,000 to 250,000 ft2/d (Bush and

Johnston, 1988, pl. 2). Bush and Johnston (1988)

noted that a contrast between field and model-derived

transmissivity data was evident in the Jacksonville

area, perhaps because model-derived values were

averaged for large grid blocks and, therefore, did not

reflect local variation.

Flow in the UFA is generally from west to east

(figs. 5a and 5b). The major recharge area in northeastern Florida is in the vicinity of Keystone Heights

(fig. 1), southwest of the study area. A deep cone of

depression has formed in Nassau County as a result of

heavy industrial pumping. In Duval County, a broad

cone of depression probably is due to a combination of

municipal and industrial pumping and natural discharge by diffuse upward leakage or through springs in

the St. Johns River (Spechler, 1995). Green Cove

Spring, at the southern end of the study area (fig. 5a),

discharges about 3 cubic feet per second (ft3/s) (about

2 million gallons per day) (Rosenau and others, 1977).

Figure 4. Generalized geology and hydrogeology of northeastern Florida (from Phelps and Spechler, 1997).
Hydrogeologic Framework 13

Figure 5a.  Estimated pre-development potentiometric surface of the Upper Floridan aquifer (from Johnston and others,

1980).
14 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Figure 5b.  Potentiometric surface of the Upper Floridan aquifer in northeastern Florida, May 1996 (from Phelps and others,

1996).
Hydrogeologic Framework 15

Middle Semi-Confining Unit and Lower Floridan

Aquifer

The middle semi-confining unit separates the

Upper and Lower Floridan aquifers and usually is composed of beds of hard, less permeable limestone and

dolomite. In Duval County, this unit generally occursin

the upper part of the Avon Park Formation and ranges

in thickness from about 100 to 150 ft (Phelps and

Spechler, 1997). Miller (1986) noted that the contrast

in permeability between rocks of the middle semi-confining unit in northeastern Florida relative to the rocks

above and below this unit is less than that of any other

confining unit mapped during the RASA study. Also,

the lithology of this unit does not differ much from that

of the permeable zones above and below it. This interval usually is observed to contribute little or no flow to

wells during flow logging.

The LFA underlies the middle semi-confining

unit. Most wells in the Jacksonville area deeper than

900 ft penetrate the Lower Floridan aquifer. The LFA

contains two main water-bearing zones, the UZLF

and the FPZ, which are separated by a less-permeable semi-confining unit (Brown, 1984). In some

parts of Duval County the UZLF may have two subzones. The top of the UZLF usually can be identified

on flow logs as an interval contributing a noticeable increase in flow to the well.

The UZLF consists of approximately the lower

two-thirds of the Avon Park Formation and is composed of alternating beds of limestone and dolomite.

The top of the unit is about 850 to 950 ft below sea

level in Duval County (Phelps and Spechler, 1997,

p. 9). In some parts of Duval County, the UZLF has a

single flow zone and a maximum thickness of about

300 ft. In other areas, less permeable strata separate

two distinct flow zones. At test well D-3060, flowmeter traverses indicate that the altitude of the top of

the UZLF is about 870 ft below sea level. A second

highly productive zone was penetrated about 1,200 ft

below sea level (Brown and others, 1985, fig. 15).

A flow zone at 950 ft below land surface and others at

about 1,150 and 1,250 ft below land surface were

observed at test well D-2386 (Brown and others 1984,

figs. 15-16). Test well D-425 also has two flow zones,

one about 880 ft below sea level and another, more productive zone at about 1,080 to 1,280 ft below sea level.

At other sites in Duval County, the UZLF has only one

flow zone (Phelps and Spechler, 1997, p. 9).

The LFA, like the UFA, also exhibits evidence of

paleokarst development. Miller (1986) described a

high-permeability cavernous zone in the lower part of

the rocks of middle Eocene age. The zone is a fairly

thick horizon of large-scale solution features representing a period when paleowater tables were at a level that

permitted karstification of the carbonate rocks at or

near land surface (Miller, 1986, p. B69). Borehole televiewer surveys of wells penetrating a similar zone in

southern Florida show that the paleokarst levels, separated by intervals of undissolved rock, commonly are

connected by vertical fractures, which can be enlarged

by solution into vertical Ï pipesÓ that connect the horizontal cavernous levels (Miller, 1986, p. B66).

The permeability, and thus the transmissivity,

of the UZLF is strongly related to secondary porosity

developed along bedding planes, joints, and fractures

(Phelps and Spechler, 1997, p. 9). The transmissivity

of the zone has not been determined by aquifer tests

of wells open only to that zone. Transmissivity values

derived from digital model simulations of the Jacksonville area range from 17,000 to 320,000 ft2/d

(R.E. Krause, USGS, written commun., 1991). Transmissivity estimates based on specific capacity tests

range from 2,000 to about 194,000 ft2/d, with a median

of 24,000 ft2/d for 50 tests analyzed by Phelps and

Spechler (1997, table 2).

Hydraulic heads generally increase with depth in

the Floridan aquifer system in northeastern Florida. In

some areas, however, where the potentiometric surface

of the UZLF is lowered by pumping, the head may be

higher in the overlying UFA than in the LFA (Phelps

and Spechler, 1997, p. 12). About half of the water

pumped by large municipal and industrial wells in the

Jacksonville area is withdrawn from the UZLF

(Spechler, 1994, p. 16). Many wells are open to both

the UFA and the UZLF, resulting in local equalization

of heads in the two aquifer zones.

The FPZ is a high-permeability unit that lies at

the base of the Floridan aquifer system in parts of

southeastern Georgia and northeastern Florida (Miller,

1986, p. B70). The FPZ was first tapped by a 2,130-ft

deep test well at Fernandina Beach (fig. 1) in 1945

(Brown, 1984, p. 39). Little is known about the extent

or thickness of the FPZ because of the sparsity of data.

Only four wellsin Duval County and one test well each

in Fernandina Beach, Nassau County, and Ponte Vedra,

St. Johns County (fig. 1) penetrate the FPZ. The thickness of the zone is estimated to range from about 100 ft

in the Jacksonville area to about 500 ft in southeastern

Georgia (Krause and Randolph, 1989, p. D23). No

aquifer test data are available to estimate the transmissivity of the FPZ.
16 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

The base of the Floridan aquifer system (the top

of the sub-Floridan confining unit) represents a composite surface that crosses formation and time boundaries (Miller, 1986, p. B73). Below the top of this

surface there are no high-permeability carbonate rocks.

The top of the sub-Floridan confining unit generally

corresponds with the top of the Cedar Keys Formation

in northeastern Florida.

Isotopic Data from Aquifers

Information about the chemical and isotopic

composition of the Floridan aquifer system sediments

can aid in understanding the chemical and isotopic

composition of the ground water. The aquifer system is

composed primarily of limestone and dolomite, as indicated by the lithologic logs of deep test wells in northeastern Florida (Brown, 1980; Brown and others, 1984;

Brown and others, 1986). Most of the limestone ranges

from pure calcite (CaCO3) to low-magnesium calcite

(less than 2 percent magnesium on a molar basis; Hanshaw, Back and Deike, 1971). The dolomites of the

Avon Park Formation probably formed relatively soon

after deposition; however, multiple episodes of dolomitization probably occurred (Cander, 1994). Some of

the dolomites may have formed at later times in saltwater mixing zones (Hanshaw and Back, 1972; Randazzo

and Hickey, 1978). Gypsum and anhydrite occur in the

Avon Park Formation and deeper deposits in much of

the Florida peninsula (Miller, 1986), although neither

was noted in the three deep test wells in northeastern

Florida. Localized carbonaceous materials also occur

in the aquifer system as thin beds of peat, particularly

in the Avon Park Formation, and as cavity fill. Appreciable amounts of quartz sand (SiO2) also are found in

the aquifer, but because quartz has a low solubility, the

δ18O of oxygen in the quartz would have little effect on

ground-water chemistry and was, therefore, not analyzed. The isotopic data for 14 samples of aquifer

materials are given in table 2.

Table 2.  Lithology and isotopic content of aquifer samples

[Aquifer:  UFA, Upper Floridan aquifer; UZLF, upper zone of Lower Floridan aquifer; FPZ, Fernandina permeable zone. δ 13C, delta carbon-13;

δ 18O, delta oxygen-18; LS, limestone; na, not analyzed. Well locations shown in figure 1]

Site

identification

number

Local

number Lithology Depth(feet) Aquifer δ 13C

(per mil)a

a

Isotope standard for analyses is Pee Dee belemnite (PDB).

δ 18O

(per mil)b

bIsotope standard for analyses is Vienna Standard Mean Ocean Water (VSMOW).

87Sr/ 86S r

ratioc

c

All data have been normalized to 88Sr/86Sr = 0.1194. National Institute of Standards and Technology (NIST) 987 is measured as

0.71024.

301405081325601 D-3846 Peat 1,110-1,120 UZLF -22.74 na 0.70796

301405081325601 D-3846 Very hard gray LS 1,110-1,120 UZLF 1.15 29.62 .70788

301405081325601 D-3846 White LS 1,180-1,190 UZLF 1.40 29.62 .70784

301405081325601 D-3846 Lepidocyclina ocalana 1,180-1,190 UZLF na na .70781

301405081325601 D-3846 Dictyoconus americanus 1,180-1,190 UZLF na na .70783

301405081325601 D-3846 Peat 1,180-1,190 UZLF -22.24 na .70800

301817081374901 D-425 Gray dolomite 2,075-2,080 FPZ 2.02 33.42 .70790

301817081374901 D-425 Light gray dolomite 2,105 FPZ 2.40 33.39 .70809

301936081292401 MW-2 White fossiliferous LS 670-680 UFA 0.39 30.41 .70782

301936081292401 MW-2 White fossiliferous LS 1,102-1,112 UZLF .89 29.90 .70777

301936081292401 MW-2 White LS 1,112-1,121 UZLF 1.23 29.90 .70781

301936081292401 MW-2 Tan to gray dolomite 1,112-1,121 UZLF 1.69 32.53 .70789

302416081522601 D-349 Very hard gray dolomite 1,893-1,894 FPZ -1.66 31.28 .70778

302416081522601 D-349 Soft white LS 1,991-1,992 FPZ 1.75 33.91 .70794
Hydrogeologic Framework 17

The values of δ13C for the limestone and dolomite samples range from -1.66 to 2.40 per mil. Values

for the peat samples (which are interpreted to be cavity

fill and, therefore, younger than the limestone) are

much lighter (about -22 per mil). Values near 0 per mil

indicate little change from seawater composition

(Sacks and Tihansky, 1996). The limestone samples

analyzed in this study have values similar to those

reported for calcite by Hanshaw and Back (1972) and

Cander (1994), whereas the dolomite samples have a

narrower range (-1.66 to 2.40) than those reported in

the previous studies (-7.5 to 1.0). The dolomites analyzed in this study were nearly contemporaneousin origin, which could explain the narrower range of values.

The δ18O values for the limestone and dolomite

samplesranged from 29.62 to 33.91 per mil. Oxygen is

present in CaCO3 or CaMg(CO3)2, which presumably

precipitated from ancient seawater at the time of deposition. In general, the δ18O values for limestones were

isotopically lighter than the dolomites. This variation

may reflect the fact that dolomites formed in a more

saline (compared to the environment of limestone deposition)sabka environment, or could be due to diagenic

changes after deposition. Values of δ18O were not analyzed for the peat and foraminifera samples.

Samples of aquifer materials also were analyzed

for the ratio of 87Sr/86Sr, which has varied in seawater

over geologic time (DePaolo and Ingram, 1985; Elderfield, 1986; Hess, Bender, and Schilling, 1986).

Because of later diagenesis, the 87Sr/86Sr ratio of rocks

may differ from the ratio of the seawater in which the

rocks were deposited (Sacks and Tihansky, 1996).

Most of the limestone samples from the study area

(from the Avon Park Formation and the lower part of

the Ocala Limestone) had ratios indicating an Eocene

age, concordant with the time of deposition (fig. 6).

One sample from well D-349, however, had a ratio

indicating an Oligocene age. This sample was taken

from a core (not cuttings) from a depth of 1,991 ft

below land surface and could have been affected by

recrystallization in Oligocene seawater, thus incorporating the Oligocene-age strontium isotope ratio.

Figure 6.  Comparison of strontium-87 to strontium-86 ratio for rock samples from four wells in northeastern Florida.
18 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Strontium isotope ratios for dolomite samples

were more diverse. A dolomite sample from a depth of

2,075 ft below land surface in well D-425 (Oldsmar

Formation) had the same ratio as a sample from

1,112 ft below land surface in well MW-2 (Avon Park

Formation). Another dolomite sample from a depth of

2,105 ft in well D-425 had an even younger ratio, indicating an apparent middle-Oligocene age. The diverse

isotope ratios for dolomites tend to confirm the hypothesis that at least some episodes of dolomitization

occurred later than during Eocene time (Sacks and

Tihansky, 1996).

The two peat samples, both from the Avon Park

Formation, had slightly different 87Sr/86Sr ratios,

which are, nevertheless, significant based on the analytical precision of the method. This variation for the

peat samples could reflect the fact that peat easily

adsorbs ions from circulating water and the two samples could represent the effects of two episodes of dolomitization. Neither the peat sample from a depth of

1,180 ft nor the sample from 1,110 ft had the same ratio

as the carbonate rocks from the same depth intervals.

CHEMICAL AND ISOTOPIC COMPOSITION

OF GROUND WATER

During 1997-99, water samples were collected

from 53 wells in Duval County and vicinity, and a sample of seawater was collected about 25 miles off the

coast of Duval County. Four of the wells sampled tap

only the Fernandina permeable zone (referred to as

FPZ wells), 10 tap only the upper zone of the Lower

Floridan aquifer (UZLF wells), 24 tap both the UFA

and the UZLF (Ï bothÓ wells), and 15 tap only the

Upper Floridan aquifer(UFA wells). One well(D-225),

which has a wide fluctuation in chloride concentration,

was sampled under both free-flowing, low chloride

concentration (35 mg/L) and pumped, high chloride

concentration (200 mg/L) conditions. In another well

(D-536), three samples were collected from different

depths by using a down-hole thief sampler.

Major Ions and Trace Constituents

Watersamples were analyzed for major ions and

trace constituents of iron, barium, boron, manganese,

strontium, lithium, iodide, and bromide. During the

first round of sampling, aluminum also was analyzed

but results were inconsistent, possibly because high

strontium concentrations interfered with the method of

analysis for aluminum, so those data are not reported.

Water chemistry varied within the aquifer zones as well

as among the various zones (table 3). Of the major

ions, chloride and sulfate are important because the

U.S. Environmental Protection Agency (U.S. Environmental Protection Agency, 2000) has established secondary recommended limits of 250 mg/L for both

constituentsin water used for public supply. Increasing

chloride concentrations have received the most attention from water managers, but in some areas, sulfate

concentrations are more of a concern than chloride

because they exceed the recommended limits.

Three of the four FPZ wells yield sodium-chloride type water (fig. 7). The water from well SJ-150 is

chemically similar to modern seawater except for

having higher concentrations of calcium and strontium and a slightly higher sulfate concentration. Well

D-425B yields calcium-sulfate type water that is much

lower in chloride concentration than the other FPZ

wells(100 mg/L compared to greater than 6,000 mg/L

in the other three wells tapping the FPZ). On the basis

of these data, the lateral interface between fresh and

saline water in the FPZ probably occurs within the

approximately 6-mile distance between wells D-425B

and D-3060 (fig. 1).

Some questions exist about the integrity of the

well completion in well D-425, a multizone monitor

well. The bottom zone, D-425B, was designed to tap

the FPZ and the upper zone, D-425T, taps both the

UZLF and the UFA. When the wellhead was being

reworked during the summer of 2000, there was some

indication that the separation between the two zones in

the well may have been breached and the well might be

open to both the FPZ and the UZLF. The major ion data

do not indicate conclusively whether or not the well is

open to both zones, but saturation index and isotopic

data, discussed in later sections of this report, may be

more useful.

Twelve water samples were collected from the

UZLF. Of these, three were collected from different

zones of one well, D-536. The sample collected by

using a thief sampler just above a fracture at a depth of

1,042 ft in well D-536 had the highest chloride concentration (230 mg/L) of all wells tapping only the UZLF.

A sample from a depth of 1,059 ft had water with

chemical characteristics very similar to the firstsample

(table 3). A third sample from a depth of 1,078 ft had a

slightly lower chloride concentration (180 mg/L) but

slightly higher sulfate concentration (340 mg/L).
Chemical and Isotopic Composition of Ground Water 19

Figure 7.  Trilinear diagrams for water from wells tapping various aquifer zones.
20 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Table 3.  Chemical and physical data for water samples

[°C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °C; mg/L, milligrams per liter; μg/L, micrograms per liter; --, no data. Aquifer:  UFA, Upper

Floridan aquifer; B, both Upper Floridan and upper zone of Lower Floridan aquifers; UZLF, upper zone of Lower Floridan aquifer; FPZ, Fernandina permeable zone. All constituents are dissolved]

Site

identification

number

Local

number

Aquifer Date

Water

temperature

(°C)

Specific

conductance

(μS/cm)

pH

(standard

units)

field

Calcium  

(mg/L as

Ca)

Magnesium  

(mg/Las

Mg)

Sodium  

(mg/L

as Na)

Potassium  

(mg/L as

K)

Chloride  

(mg/L

as Cl)

Sulfate  

(mg/Las

SO4)

Fluoride  

(mg/L as

F)

Silica

(mg/L as

Si)

300019081363301 SJ-3 UFA 07/15/97 29.1 1,330 7.44 180 68 11 3.1 17 660 0.9 17

300347081363701 SJ-168 UFA 07/15/97 29.6 1,350 7.52 180 63 18 3.2 45 600 .7 17

301304081222701 SJ-103 UFA 03/19/98 23.0 557 7.18 62 29 18 2.2 26 140 .8 27

301339081531201 D-326 UFA 01/28/99 22.8 241 7.87 26 11 5.5 1.5 5.6 20 .34 15

301547081271901 SE-1 UFA 01/28/99 23.0 650 7.60 66 33 13 2.3 14 190 .8 22

301714081233301 D-72 UFA 01/21/98 21.2 676 7.33 64 36 21 2.3 32 160 .76 26

301830081375501 D-1551 UFA 01/27/99 23.4 569 7.51 65 24 12 1.9 14 140 .69 24

302013081353801 D-673 UFA 01/21/98 27.2 1,040 7.10 100 39 45 2.2 160 140 .55 27

302058081244101 D-1107 UFA 01/27/99 24.1 584 7.56 62 28 12 1.7 16 140 .7 25

302113081322301 D-655 UFA 10/29/98 24.0 600 7.62 65 24 17 1.7 34 110 .65 26

302140081235501 D-1121 UFA 10/28/98 23.0 587 7.44 66 24 13 2.1 16 130 .68 25

302239081434401 NK-1 UFA 10/29/98 23.0 646 7.60 55 29 33 2.7 14 160 .75 20

302557081253101 D-913 UFA 07/15/97 26.1 1,830 7.24 110 57 160 3.6 380 200 .5 30

302608081354903 D-264 UFA 10/27/98 24.0 500 7.45 53 22 15 1.5 19 72 .56 30

302609081260601 D-912 UFA 10/27/98 23.0 506 7.53 48 22 19 2.1 30 58 .63 28

300856081382401 D-3847 B 03/18/98 23.3 367 7.52 39 21 8.4 2.4 9.1 86 .71 17

301405081325601 D-3846 B 01/20/98 26.4 797 7.37 81 39 25 2.8 63 180 .81 24

301508081364601 BW-3 B 02/23/99 26.5 650 7.70 69 32 12 2.4 15 180 .75 23

301704081233401 D-484 B 02/04/97 28.0 1,000 7.24 95 40 66 2.6 180 140 .3 30

301725081305002 D-650 B 01/20/98 25.3 711 7.14 78 31 24 2.1 64 140 .69 25

301740081361001 D-275 B 01/28/97 28.1 1,150 7.00 100 39 70 2.3 200 140 .6 26

301743081303501 D-3825 B 01/20/98 26.0 1,200 7.44 96 40 70 2.4 180 160 .6 26

301743081362301 D-225 B 01/28/97 28.0 1,160 7.27 100 39 71 2.3 200 150 .6 26

301743081362301 D-225 B 07/14/97 28.8 650 7.47 69 28 16 2.1 35 160 .7 23

301748081383401 D-334 B 01/21/98 25.3 600 7.42 73 29 12 2.3 12 180 .61 22

301758081303901 D-665 B 01/31/97 26.8 3,140 7.85 180 78 320 4.1 830 250 .6 26

301817081374901 D-425T B 03/18/98 25.0 535 7.29 68 25 12 1.9 15 140 .68 23

301846081240201 D-246 B 01/22/98 26.0 599 7.30 63 31 16 1.9 24 140 .73 26

301934081292301 R-3 B 01/28/99 23.5 495 7.78 68 26 13 1.8 17 140 .69 25

301937081471401 D-2223 B 01/28/99 27.0 550 7.43 64 24 11 1.9 12 140 .6 21

301957081342301 D-313 B 01/22/98 23.1 902 7.18 83 32 39 2 110 120 .58 27

302007081353201 D-479 B 03/18/98 26.3 750 7.18 82 30 34 1.9 110 120 .59 26

302008081242101 D-307 B 01/27/99 26.0 930 7.40 73 35 56 2.2 130 140 .63 27

302015081384501 D-335 B 03/18/98 26.0 490 7.22 63 22 13 1.8 15 110 .71 25

302045081323101 D-1323 B 04/06/98 26.6 1,200 7.21 110 42 100 2.4 270 150 .55 27

302342081320601 D-1902 B 03/17/98 25.0 474 7.32 59 24 15 1.7 23 91 .64 26

302503081332001 D-1149 B 03/17/98 21.0 464 7.22 55 23 15 1.5 22 79 .58 29

302510081260201 D-3841 B 04/08/98 22.3 435 7.55 56 22 11 1.5 16 94 .63 26

302538081392501 D-329 B 01/21/98 23.3 530 7.32 58 24 15 1.6 20 74 .56 31

303940081281801 N-22 B 05/18/99 24.0 650 6.50 64 33 19 1.8 28 150 .6 34

301335081355001 D-536-1a

a

Sampling depth 1,042 ft below land surface.

UZLF 02/04/97 30.0 1,400 7.76 150 57 69 3.1 230 330 .8 24

301335081355001 D-536-2b

b

Sampling depth 1,059 ft below land surface.

UZLF 02/04/97 30.0 1,430 7.60 150 57 68 3.1 220 320 .8 24

301335081355001 D-536-3c

c

Sampling depth 1,078 ft below land surface.

UZLF 02/04/97 30.0 1,200 7.50 150 52 50 3.1 180 340 .4 24

301522081331301 D-291d

d

Sampling depth 1,210 ft below land surface.

UZLF 01/31/97 -- 884 7.80 86 33 34 2.2 110 160 .7 25

301537081441901 D-75 UZLF 01/29/97 24.5 349 7.90 41 16 6.5 1.8 7.6 65 .4 17

301604081361501 D-450 UZLF 01/29/97 24.1 879 7.78 92 37 30 2.3 100 170 .7 24

301639081330802 D-1155 UZLF 01/28/97 -- 1,030 7.08 110 41 39 2.4 140 180 .7 25

301936081292401 MW-2 UZLF 03/19/98 22.6 560 7.53 72 29 15 1.8 15 160 .69 23

302022081393501 D-176 UZLF 01/29/97 24.1 591 7.66 73 27 11 1.9 13 160 .7 22

302227081435001 D-592 UZLF 01/29/97 23.5 585 7.70 75 26 9.8 1.6 12 160 .6 23

302608081354901 D-262 UZLF 01/29/97 26.1 620 7.60 68 28 20 1.6 46 98 .6 31

302608081354902 D-263 UZLF 01/22/98 22.6 500 7.43 56 24 16 1.6 19 73 .6 31

301132081225801 SJ-150 FPZ 01/30/97 29.7 47,100 7.40 760 940 9,700 290 18,000 2,600 .4 19

301405081325601 D-425B FPZ 01/28/97 27.7 1,860 7.65 290 60 63 4.4 100 850 .9 24

301725081305002 D-3060 FPZ 01/30/97 28.7 27,200 7.40 800 540 4,900 70 9,700 1,500 .3 23

301743081303501 D-2386 FPZ 01/27/97 27.0 20,400 7.50 540 390 3,400 62 6,800 1,100 .4 25

302737080571801 Seawater 04/07/98 -- 41,800 7.76 370 1,200 10,400 360 19,000 2,500 .49 3.4
Chemical and Isotopic Composition of Ground Water 21

Table 3.  Chemical and physical data for water samples--Continued

[°C, degrees Celsius; μS/cm, microsiemens per centimeter at 25 °C; mg/L, milligrams per liter; μg/L, micrograms per liter; --, no data. Aquifer:  UFA, Upper

Floridan aquifer; B, both Upper Floridan and upper zone of Lower Floridan aquifers; UZLF, upper zone of Lower Floridan aquifer; FPZ, Fernandina permeable zone. All constituents are dissolved]

Site

identification

number

Local

number

Aquifer

Barium

(μg/L as

Ba)

Boron

(μg/Las

B)

Iron

(μg/Las

Fe)

Manganese

(μg/L as

Mn)

Strontium

(μg/L as

Sr)

Lithium

(μg/Las

Li)

Iodide

(mg/Las

I)

Bromide

(mg/L as

Br)

Field

alkalinity as

CaCO3

Lab

alkalinity as

CaCO3

Field

sulfide

(mg/L)

Lab

sulfide

(mg/L)

300019081363301 SJ-3 UFA 20 24 6 0.4 7,800 6 0.015 0.066 96 93 -- --

300347081363701 SJ-168 UFA 22 22 3 .5 8,200 8 .017 .163 116 105 -- --

301304081222701 SJ-103 UFA 25 37 3 .3 2,400 4 .021 .085 160 150 1.7 --

301339081531201 D-326 UFA 14 28 16 1.1 1,200 4 .009 .045 118 99 .5 1.6

301547081271901 SE-1 UFA 25 40 15 .5 2,700 6 .019 .062 125 128 1.6 4.6

301714081233301 D-72 UFA 24 26 20 2.8 2,100 6 .017 .109 142 133 1.5 --

301830081375501 D-1551 UFA 21 35 4.6 .3 3,000 6 .017 .062 140 138 1.3 4.6

302013081353801 D-673 UFA 31 19 110 1.3 3,000 7 .024 .378 137 141 1.5 --

302058081244101 D-1107 UFA 25 34 2.6 .4 2,100 5 .018 .036 -- 140 1.9 5.8

302113081322301 D-655 UFA 19 35 82 10 2,200 6 .022 .122 -- 133 1.3 2.6

302140081235501 D-1121 UFA 28 36 11 1.4 2,300 7 .02 .059 -- 144 2.4 3

302239081434401 NK-1 UFA 16 43 4.2 3.9 1,500 9 .015 .04 -- 159 1.8 2.4

302557081253101 D-913 UFA 60 49 310 2.9 1,800 10 .031 1.32 153 145 -- --

302608081354903 D-264 UFA 26 34 53 4.7 500 5 .021 .037 162 162 1.1 1.9

302609081260601 D-912 UFA 24 33 37 9.7 560 6 .023 .12 -- 154 1.2 2.6

300856081382401 D-3847 B 18 29 50 .7 3,600 4 .01 .012 122 106 .7 --

301405081325601 D-3846 B 30 16 20 .6 4,300 7 .018 .213 149 130 1.5 --

301508081364601 BW-3 B 24 33 19 .8 4,200 5 .021 .061 122 131 1.5 1.8

301704081233401 D-484 B 38 52 6 .9 2,500 10 .022 .538 -- 149 -- --

301725081305002 D-650 B 26 16 7 .5 3,200 7 .02 .217 148 138 1.7 --

301740081361001 D-275 B 33 36 40 1.2 3,700 10 .022 .689 -- 138 -- --

301743081303501 D-3825 B 34 20 40 1.5 3,700 7 .023 .405 150 140 1.2 --

301743081362301 D-225 B 33 37 50 1.3 3,700 20 .022 .682 -- 139 -- --

301743081362301 D-225 B 23 32 40 2.5 3,300 5 .019 .119 121 138 -- --

301748081383401 D-334 B 20 12 30 7 4,000 6 .018 .033 130 127 1.2 --

301758081303901 D-665 B 77 93 150 5.8 6,300 20 .028 2.879 -- 137 -- --

301817081374901 D-425T B 19 29 7 .7 3,300 4 .018 .049 198 139 1.1 --

301846081240201 D-246 B 27 24 5 .8 2,100 5 .017 .06 135 143 1.8 --

301934081292301 R-3 B 22 36 13 .7 2,800 6 .021 .073 144 143 1.9 3.7

301934081471801 D-2223 B 18 36 14 .6 3,300 6 .013 .02 -- 130 1.2 4.2

301957081342301 D-313 B 26 22 20 .7 2,400 6 .019 .328 138 146 1.5 --

302007081353201 D-479 B 25 34 20 1.1 2,500 4 .021 .357 196 145 1.3 --

302008081242101 D-307 B 36 43 16 .9 2,000 7 .023 .405 -- 149 2.06 5.8

302015081384501 D-335 B 19 32 3 .4 2,500 4 .018 .051 185 147 .9 --

302045081323101 D-1323 B 40 42 40 5.6 2,800 5 .03 .996 144 147 1.5 2.2

302342081320601 D-1902 B 24 33 60 5.4 1,800 4 .022 .087 154 157 1.7 --

302503081332001 D-1149 B 24 39 9 .5 880 4 .022 .066 160 157 1.9 --

302510081260201 D-3841 B 28 37 100 10 1,300 4 .019 .061 138 149 1.4 1.8

302538081392501 D-329 B 27 28 50 1.5 560 5 .019 .056 161 160 1.5 --

303940081281801 N-22 B 28 40 5.1 .3 590 8.1 .084 -- 156 -- 3.8

301335081355001 D-536-1 UZLF 35 50 10 1 8,400 10 .024 .738 -- 127 -- --

301335081355001 D-536-2 UZLF 35 51 20 .9 8,300 10 .023 .747 -- 126 -- --

301335081355001 D-536-3 UZLF 29 53 20 .9 7,800 10 .023 .555 -- 125 -- --

301522081331301 D-291 UZLF 26 46 20 3.5 3,800 10 .021 .369 -- 91 -- --

301537081441901 D-75 UZLF 22 39 160 1.6 3,300 9 .01 .028 -- 108 -- --

301604081361501 D-450 UZLF 33 29 660 13 4,300 10 .021 .365 -- 141 -- --

301639081330802 D-1155 UZLF 29 28 220 5 4,800 8 .019 .465 -- 137 -- --

301936081292401 MW-2 UZLF 28 32 730 15 2,900 4 .02 .06 133 145 .3 --

302022081393501 D-176 UZLF 41 28 530 7.1 3,900 8 .017 .051 -- 134 -- --

302227081435001 D-592 UZLF 42 41 270 3.1 2,800 8 .012 .03 -- 132 -- --

302608081354901 D-262 UZLF 35 30 20 3 630 10 .025 .146 -- 158 -- --

302608081354902 D-263 UZLF 27 27 30 3.1 730 5 .022 .058 160 164 1 --

301132081225801 SJ-150 FPZ 88 3,600 1,200 38 14,100 200 .121 81 -- 120 -- --

301405081325601 D-425B FPZ 19 76 130 5.2 8,900 10 .015 .34 -- 118 -- --

301725081305002 D-3060 FPZ 170 1,300 1,500 59 21,800 100 .082 58 -- 130 -- --

301743081303501 D-2386 FPZ 160 910 50 6.3 11,400 100 .07 27 -- 136 -- --

302737080571801 Seawater 25 4,300 20 25 650 120 .105 62 -- 113 -- --
22 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Well D-536 had the warmest water temperature

(32.5 o

C) measured during geophysical logging of

numerous wellsin Duval County (Phelps and Spechler,

1997). The sample collected during the present study

had a lower temperature because the water cooled as

the sampler was being raised to the land surface. These

samples and the sample from well D-291 (also a thief

sample from near the bottom of the well) were calciummagnesium sulfate-chloride type water.

Most of the other samples from UZLF wells

were identified as calcium-magnesium sulfate-bicarbonate type water (fig. 7). Wells with the lower chloride concentrations are generally west (or north) of the

St. Johns River (fig. 1), except for MW-2. Chloride

concentrations do not show an increasing trend with

depth; wells D-450 and D-592 both are about 1,300 ft

deep, but the chloride concentration is 100 mg/L in

D-450 and only 12 mg/L in D-592. These wells are

located about 10 miles apart in central Duval County

(fig. 1).

Twenty-five water samples were collected from

wells tapping both the UZLF and the UFA. Many public supply wells in northeastern Florida have been

drilled to penetrate both zones to take advantage of the

increased yield of water from the UZLF. The chemistry

of water from wells open to both zones varies greatly,

from calcium-magnesium-bicarbonate type water

(wells D-329 and D-334) with relatively low specific

conductance (about 360-500 μS/cm) to calciumsodium-magnesium chloride-sulfate type water (wells

D-665 and D-225) with much higher specific conductance (about 1,000-3,000 μS/cm) (figs. 7 and 8).

Water from well D-665 is chemically more similar to the high chloride water from the FPZ wells than

to water from wells tapping the UZLF (fig. 7). Most of

the water produced from well D-665 apparently comes

from a single fracture at a depth of about 1,080 ft below

land surface (1,040 ft below sea level) (Phelps and

Spechler, 1997). High-resolution seismic reflection

surveys near this well (Odum and others, 1997) indicate the presence of a paleokarst feature that possibly

breaches the semi-confining unit separating the UZLF

from the FPZ. Thisfeature could provide a pathway for

mineralized water to move from the FPZ to the UZLF.

When the well is not allowed to flow, water from the

UZLF moves up the wellbore and flows into the UFA,

thusintroducing high chloride water into the UFA. This

type of situation probably accounts for some of the

variability in water quality observed in wells tapping

both zones and also in the UFA.

Well D-225 also is finished in both aquifers, and

the water chemistry reflectsthis construction. This well

is sampled regularly and chloride concentrations

fluctuate between about 35 and 200 mg/L (Phelps and

Spechler, 1997). When the well is flowing naturally

(flow rate about 1,000 gallons per minute (gal/min)),

the water at the wellhead and from a zone of fractures

orsolution features at a depth of about 1,225 to 1,245 ft

below land surface are nearly identical (Phelps and

Spechler, 1997, table 3), indicating that most of the

flow comes from that zone. Non-pumped conditions

represent Ï low chlorideÓ conditions. A sample collected when the well was being pumped at about

2,000 gal/min had a different chemistry, however,

including a higher chloride concentration (table 3,

fig. 8). When the well was pumped, concentrations of

chloride, calcium, magnesium, and sodium were much

higher than under non-pumped conditions, but the sulfate concentration was about the same (150-160 mg/L).

Other water samples collected monthly from this well

had chloride concentrations intermediate between the

values sampled during this study. Although the well

has been geophysically logged, the source of the higher

chloride water is unclear, but may be related to whether

or not other nearby wells are being pumped.

Water chemistry of the 15 samples collected

from the UFA wells varied considerably (figs. 7 and 8).

Well D-326 yielded calcium-magnesium bicarbonate

type water with a low specific conductance (241 μS/cm)

and low chloride (5.6 mg/L) and sulfate (20 mg/L) concentrations. Well SJ-3 also had a relatively low chloride

concentration (17 mg/L) but was much higher in sulfate (660 mg/L) (a calcium-magnesium sulfate type

water). Well D-913 had the highest chloride concentration of the UFA wells sampled (380 mg/L). The majorion diagram of water from this well more closely

resembles that of water from well D-665 than water

from other wells tapping the UFA. Both wells are

located near paleokarst collapse features (fig. 3).

Except for water from the FPZ, water from the

various aquifer zones cannot be differentiated based on

chemistry; major-ion concentrations vary as much

within a particular zone as among zones (fig. 9). The

UZLF wells showed the least variability. Wellstapping

both the UFA and the UZLF had a wider range in chloride concentration than wells tapping just the UFA or

just the UZLF. Water from the UFA had the widest

range of sulfate concentrations (58-660 mg/L).

A Kruskal-Wallis test of the means of chemical constituents also confirms the similarity of water chemistry

among the various aquifer zones. Only the trace constituents barium, iron, strontium, and lithium were significantly different (at a probability level of 0.05) in

samplesfrom the UFA, the UZLF, and wells completed

in both aquifer zones. Means for FPZ wells were not

included because there were so few data available.
Chemical and Isotopic Composition of Ground Water 23

Figure 8. Major ion diagrams for selected wells.
24 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Figure 9.  Box plots of calcium, chloride, sulfate, and strontium concentrations.
Chemical and Isotopic Composition of Ground Water 25

In general, water samples collected in northeastern Florida show a trend similar to the dilution of seawater with freshwater (fig. 10) for the relation between

chloride and sodium. The data, however, cannot indicate the age of a possible seawater end member. Water

from UFA well D-326 could represent the freshwater

recharge end member, and water from FPZ well SJ-150

could represent a saline-water end member. Samples

with chloride concentrations ranging between about

20-1,000 mg/L have excess chloride, possibly indicating loss of sodium by ion exchange. Water from well

NK-1 (UFA well) departsslightly from the mixing line,

indicating the presence of excess sodium. This well

may be open to the overlying surficial aquifer system

and would, thus, have different water chemistry from

UFA wells. Although figure 10 implies slightly different trends for lower and higher chloride samples, the

relation between chloride and sodium cannot definitely

indicate the source of the mineralized water.

Halogens

More insight into the possible sources of mineralized ground water that mixes with fresher water can

be gained by comparing the ratio of chloride to bromide (Cl-

/Br-) in the water. Seawater has a mass ratio

of chloride to bromide ranging between 288 and 292

(Davis and others, 1998a). In most ground-water samples in the United States reported by Davis and others

(1998a), water with chloride concentrations less than

200 mg/L had Cl-

/Br- ratios less than 200, and the

Cl-

/Br- ratios increased as chloride concentrations

increased within a given area. Those trends generally

could be attributed to a source of mineralized water

such as an oil-field brine or treated wastewater. In

coastal areas, however, Davis and others (1998a) suggested that Cl-

/Brratios would likely be affected by the

flushing of small amounts of residual Cenozoic seawater and by sea salt in the atmosphere, which affects the

Cl-

/Br- ratio in precipitation. The ratio generally is less

than 200 in inland areas.

Water samples collected in northeastern Florida

(fig. 10) generally support the conclusion of dilution of

seawater, although the ground-water samples with the

highest chloride concentrations tended to have excess

bromide. Among the low chloride samples (Cl- less

than 100 mg/L), well D-3847 had excess chloride,

whereas well D-326 had excess bromide. Excess bromide in ground water has been attributed to the diagenesis of organic material (such as peat) in the aquifer

material (Davis and others, 1998a). Peat commonly is

observed in the Floridan aquifer system.

Possible errors in the determination of low-level

concentrations of bromide could affect the calculated

ratios of bromide to chloride. Based on a sample split,

an estimated error for the low-level bromide determinations is about 15 percent. A few examples of how

this error affects the calculated chloride-to-bromide

ratio are given in the following table:

For most wells, although the ratio may change slightly

because of errors in bromide determinations, the range

of Cl-

/Br- still is indicative of whether or not the ratio

is consistent with seawater dilution (the range for seawater dilution is 247 to 334).

Thus, for some wells, such as D-225, the Cl-

/Brratio implies dilution of seawater but for wells such as

D-3847 and D-326, this is clearly not the case. For

other wells, however, possible errors in bromide determinations can obscure any conclusions based on the

Cl-

/Br- ratio. The dilution trend (fig. 10) showing the

gradual flushing ofseawater out of the aquifer probably

accountsfor background chloride concentrationsin the

Floridan aquifer system, but probably does not explain

increasing chloride concentrations. If unflushed pockets of relict seawater from higher stands of sea level

were the source of the increased chloride concentrations,such pockets would be expected to occur in strata

of low permeability; however, geophysical logging has

shown that fracture flow zones are the sources of

high chloride water in numerous wells (Phelps and

Spechler, 1997).

Sulfur

Sources of sulfur in the ground water include

atmospheric precipitation, dissolution of gypsum or

sulfide minerals from deeper zones of the aquifer, and

mixing with saline waters (Sacks and Tihansky, 1996).

Well Cl Br Cl-

/Brratio Br error

Ratio

range

with error

D-326 5.6 0.045 124 0.0068 108-147

D-3847 9.1 .012 758 .0018 650-910

D-225 (low) 35 .119 294 .0178 255-346

D-225 (high) 200 .682 293 .102 255-344
26 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Figure 10.  Relations between sodium and chloride concentration, and between the mass ratio of chloride-to-bromide

and chloride concentration.
Chemical and Isotopic Composition of Ground Water 27

Two common forms of sulfur are the oxidized sulfate

ion and the reduced sulfide (S-2) ion (Hem,

1985). High sulfate concentrations in ground water

contribute to high dissolved solids and the gaseous sulfide, hydrogen sulfide, which produces the unpleasant

Ï rotten eggÓ smell in water. In anoxic ground-water

systems, bacteria may metabolize sulfate, reducing it to

sulfide (Long and others, 1988). A representative equation for the process from Berner (1971) is:

. (2)

Sulfate concentrations in water from FPZ wells

ranged from 850 mg/L at well D-425B to 2,600 mg/L

at well SJ-150 (table 3), of which the latter wasslightly

higher than the sulfate concentration in the seawater

sample (2,500 mg/L). For wells tapping all other aquifer zones, the highest concentrations were from UFA

wells SJ-3 (660 mg/L) and SJ-168 (600 mg/L). In well

D-665 (completed in both the UFA and the UZLF),

which had a chloride concentration of 830 mg/L, the

sulfate concentration was only 250 mg/L. All three

samples from well D-536 (UZLF) had relatively high

sulfate concentrations of about 330 mg/L. The lowest

sulfate concentration measured was 20 mg/L in well

D-326, which taps the UFA.

Nearly all of the watersamples collected in northeastern Florida have the characteristic smell of hydrogen sulfide. Sulfide concentrations measured in the field

ranged from 0.3 mg/L at well MW-2 to 2.4 mg/L at

well D-1121(table 3). As discussed previously, the field

technique for determining sulfide concentrations probably aerates the sample, resulting in some loss of hydrogen sulfide gas, so for a number of samples, both field

and laboratory determinations were made. Field and

laboratory determinations were similar for well BW-3

(1.5 and 1.8 mg/L, respectively). For wells with higher

concentrations, however, the field procedure apparently

allowed significant loss. At well D-1107, the field value

was 1.9 mg/L, whereas the lab value was 5.8 mg/L; and

at well D-307 the field value was about 2.1 mg/L,

whereas the lab value also was 5.8 mg/L.

Sources ofsulfur in the ground water include dissolution of sulfur-bearing minerals in the aquifer and

mixing with water having a chemical composition similar to that ofseawater. Dissolution of gypsum and dilution of seawater have distinct trends on a plot of the

relation of the mass ratio of sulfate to chloride and the

sulfate concentration of water samples (Rightmire and

others, 1974). Water samples from the high chloride

FPZ wells follow a trend similar to that for seawater

mixing with water from well D-326 (fig. 11). Other

samples follow a trend similar to that for gypsum dissolution. In many of the samples, sulfate reduction

probably has affected sulfate concentrations. The dominant chemical processes involving sulfur in northeastern Florida ground water apparently are varying

degrees of freshwater-seawater mixing, gypsum dissolution, and sulfate reduction.

Ion Enrichment

Additional insight into possible geochemical

reactions and the source of mineralized water that

mixes with fresh ground water in northeastern Florida

can be gained by comparing observed ground-water

chemistry with hypothetical mixing using various

saline waters. Water from UFA well D-326 was

selected to represent the chemistry of Ï ambientÓ fresh

ground water. A spreadsheet was made to tabulate mixing of D-326 water with water from well SJ-150 and

with modern seawater (based on chloride concentration

as the mixing variable). Compared to simple mixing

with water either from well SJ-150 or modern seawater,

the samples generally are enriched in bicarbonate, calcium, magnesium, sulfate, fluoride and silica, and

depleted in sodium and potassium (table 4). The fraction of saline water (x) was computed using the

following equation:

, (3)

The ion concentrations observed in each sample were

then compared to the expected concentrations based on

the relation:

(4)

SO4

Ò2 ( )

SO4

Ò2 + 2( ) CH2O H2S 2HCO3Ò = +

where

Cl is the chloride concentration, in milligrams per liter, and

w, fw, and sw refer to the sample from a well, freshwater end member (D-326) or saline water

end member, respectively.

where

ie ision enrichment if positive and ion depletion

if negative,

iobs is the observed ion concentration in the well,

and

isw and ifw are the ion concentrations in the saline water

and freshwater end members, respectively.

x Clw Clfw ( ) Ò Clsw Clfw = ⁄ ( ) Ò

ie iobs isw ( ) x ifw = Ò [ ] + ( ) ( ) 1 Ò x
28 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

   Sulfate is enriched in all samples compared to

conservative mixing. The low chloride waters were

most enriched and the high chloride waters least

enriched, which agrees with observations in west-central Florida by Sacks and Tihansky (1996). In northeastern Florida, however, sulfate enrichment was

related more to chloride concentrations than to aquifer

zone sampled; the high chloride samplesfrom the UFA

had sulfate enrichment similar to that for high chloride

waters from the LFA and the FPZ. For low chloride

waters, the sulfate enrichment is indicative of gypsum

dissolution (Sacks and Tihansky, 1996).

Calcium is enriched in all samples compared to

conservative mixing, and is more enriched in high

chloride waters. If conservative mixing with modern

seawater is assumed, calcium enrichment for some of

the high chloride waters is related to sulfate enrichment, indicating that dissolution of gypsum is the

source of both calcium and sulfate (fig. 12). For other

samples, such as that from D-665, there must be

another source of calcium. Cation exchange (of calcium for sodium) could be a mechanism for calcium

enrichment. As sodium-rich seawater moves through

an aquifer, two sodium ions can replace one calcium

ion at exchange sites(probably in the minor amounts of

clay minerals that occur in the aquifer) (Sacks and

Tihansky, 1996). This cation exchange results in

ground water that is enriched in calcium and depleted

in sodium. In northeastern Florida, water samples from

the FPZ were slightly depleted in sodium, whereas the

other high chloride samples were more depleted. For

the high chloride waters, calcium enrichment islinearly

related to sodium depletion, supporting cation

exchange as a mechanism for reducing the sodium and

increasing the calcium concentrations in the ground

water. The low chloride samples were slightly enriched

in sodium compared to conservative mixing.

Other ions, such as magnesium, potassium, and

silica, also show enrichment or depletion (table 4). Magnesium generally was enriched compared to conservative mixing with seawater for all samples except those

from the FPZ, which were depleted. The sample from

well D-665 was about 20 percent enriched, compared to

about 45 to 50 percent for the other high chloride samples. These data indicate that the source of mineralized

water in well D-665 could be the FPZ. The low chloride

samples generally were more enriched in magnesium

(50 to 60 percent) than the higher chloride samples.

Figure 11. Relation between sulfate-chloride ratio and sulfate concentration (Maritime rainfall data from Rightmire

and others, 1974).
C

hemicalandIsotopicCompositionofGroundWater29

Table 4. Percent excess of some constituents for selected wells[Aquifer: UFA, Upper Floridan aquifer; B, both Upper Floridan and upper zone of Lower Floridan aquifers; UZLF, upper zone of Lower Floridan aquifer; FPZ, Fernandmg/L, milligrams per liter. First number is percent excess over conservative mixing of water from well D-326 with water from well SJ-150. Second number is mixing of modern seawater sample. Positive numbers indicate enrichment, negative numbers indicate depletion]

Local

No. Aquifer

Chloride

concentration,

mg/L

Percent

saline

water

Bicarbonate Calcium Magnesium Sodium Potassium Sulfate Fluoride 

D-673 UFA 160 0.8 0.9 30 30 71 68 47 51 -100 -97 -101 -81 71 70 38 38 D-913 UFA 380 2.0 2.1 32 31 70 62 40 47 -32 -30 -138 -108 66 63 31 32 D-225 (high) B 200 1.0 1.1 29 29 70 66 41 46 -57 55 -125 -101 70 68 43 43 D-665 B 830 4.3 4.6 27 27 77 70 20 31 -43 -41 -316 -259 49 45 42 43 D-536-1 UZLF 230 1.2 1.2 22 22 80 77 56 60 -86 -83 -85 -64 85 84 57 57 D-1155 UZLF 140 0.7 0.7 28 28 74 71 53 56 -103 -100 68 -52 79 78 51 51 D-3060 FPZ 9,700 51.0 54.0 18 15 75 47 -14 5 -8 -7 -164 -124 14 6 -39 -24 D-2386 FPZ 6,800 35.8 37.8 24 21 72 44 -12 7 -10 -8 -109 -78 18 10 2 9 SE-1 UFA 14 <0.1 <0.1 23 23 60 60 65 65 23 22 29 28 89 89 58 58 D-1551 UFA 14 <0.1 <0.1 28 28 60 60 52 52 16 16 14 13 85 85 51 51 NK-1 UFA 14 <0.1 <0.1 38 38 52 52 61 60 70 69 40 39 87 87 55 55 D-3847 B 9 <0.1 <0.1 7 7 33 33 47 47 12 12 35 35 76 76 52 52 BW-3 B 15 0.1 <0.1 24 24 62 62 64 64 12 11 31 30 88 88 55 55 D-334 B 12 <0.1 <0.1 22 22 64 64 61 61 25 25 30 30 88 88 44 44 D-2223 B 12 <0.1 <0.1 24 24 59 59 53 52 19 18 16 15 85 85 43 43 D-335 B 15 0.1 <0.1 33 33 58 58 48 47 19 18 8 7 81 81 52 52 D-75 UZLF 7.6 <0.1 <0.1 8 8 36 36 31 30 -1 -2 15 15 69 69 15 15 MW-2 UZLF 15 0.1 <0.1 32 32 63 64 60 60 30 29 8 7 87 87 51 51 D-176 UZLF 13 <0.1 <0.1 26 26 64 64 58 58 14 14 15 14 87 87 51 51 D-592 UZLF 12 <0.1 <0.1 25 25 65 65 56 56 9 8 0 -1 87 87 43 43 
30 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Figure 12. Enrichment of calcium compared to enrichment or depletion of sulfate, sodium, and

magnesium for mixing with seawater.
Chemical and Isotopic Composition of Ground Water 31

These data agree with observations in west-central

Florida (Sacks and Tihansky, 1996), and are consistent

with dedolomitization reactions(dolomite and gypsum

dissolution and calcite precipitation) in the freshwaterbearing zones of the aquifer and either calcium/magnesium exchange or dolomitization of calcite in the

saline water-bearing zones. Potassium was depleted

for all the high chloride samples and enriched for all

the low chloride samples. Silica also is enriched in all

samples, with those from the FPZ wells showing the

greatest enrichment.

Comparisons of ion enrichment or depletion

based on simple mixing models indicate that water

from the FPZ could be the source of the mineralized

water in well D-665 (fig. 12). For water from the FPZ

wells and from well D-665, however, the data are not

consistent with simple mixing of seawater, either

ancient or modern. This could imply that mixing with

water of a different (but as yet unknown) chemical

type, or more complex reactions with aquifer materials,

may have been involved in the increased mineralization of water in northeastern Florida. For the low chloride samples, common reactions with aquifer materials

and simple mixing (dilution) during the process of

flushing after the last retreat of Pleistocene seas could

account for the observed chemical composition of the

water samples.

Saturation State with Respect to Minerals

To better understand the chemical changes that

can occur in water as it moves through the ground, the

chemistry of the water and aquifer materials and the

interactions between them can be examined. One

aspect of such geochemical interactions is the dissolution or precipitation of minerals along flow paths in the

ground-water system. The saturation index (SI) is a

measure of the departure from equilibrium of the water

with respect to various mineral phases. An SI value of

zero indicates that the water is at equilibrium (saturated) with respect to the mineral phase. A negative

value indicates undersaturation (dissolution of the mineral is possible), and a positive value indicates supersaturation (precipitation is possible). SIs for the water

samples were calculated using the aqueousspeciation

model WATEQ4F (Ball and Nordstrom, 1991):

, (5)

where

The determination of whether or not carbonate

minerals are at equilibrium is dependent upon the accuracy of the chemical analysis and pH and water temperature measurements (Sacks and Tihansky, 1996), so

equilibrium is defined to occur within a range of SI

values and not at a single specific value. For calcite,

equilibrium is assumed for SI values between -0.15 and

0.15 and for dolomite the range is -0.30 to 0.30 (Sacks

and Tihansky, 1996). For dolomite, the value is uncertain because the exact value of the equilibrium reaction

constant is uncertain (Hsu, 1963); therefore, SI values

are given (table 5) for both Ï crystallineÓ and Ï disorderedÓ dolomite (Nordstrom and others, 1990). Comparing saturation indexes among various wells can

show which geochemical reactions can or cannot occur

as water moves along a flow path between a pair of

wells, assuming the reactions are controlled by thermodynamic equilibrium. For example, if a possible model

indicates that calcite is dissolving at a particular well,

but the saturation index calculated for water from that

well shows calcite supersaturation, then the model

must be rejected.

All water samples from the FPZ were supersaturated with respect to calcite (CaCO3), and both crystalline and disordered dolomite (CaMg(CO3)2) (table 5).

This is consistent with long residence times in the aquifer. The samples were undersaturated with respect to

gypsum   and celestite (SrSO4),

although water from the FPZ was closer to saturation

for those minerals than samples from other zones.

Mineral saturation data also may be useful for

evaluating the integrity of well D-425, a multi-zone

monitor well. The SI data for well D-425B are more

similar to those from the other FPZ wells than to the

UZLF wells. The water from well D-425B is supersaturated with respect to calcite and to both crystalline and

disordered dolomite, as are the other samples from the

FPZ. The only other sample that was supersaturated

with respect to all three minerals was the sample from

well D-665, which has a much higher chloride concentration. Further field investigation of well D-425 is

needed to evaluate whether or not the sampling zones

are adequately separated. SI IAP KT = log( ) ⁄

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.

CaSO4 ( ) • 2H2O
32 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Table 5.  Calculated partial pressure of carbon dioxide and saturation indexes of water with respect to selected minerals

[Aquifer:  UFA, Upper Floridan aquifer; B, both Upper Floridan and upper zone of Lower Floridan aquifers; UZLF, upper zoneupper zone of Lower

Floridan aquifer; FPZ, Fernandina permeable zone; CO2, carbon dioxide; atm, atmospheres; for calcite, equilibrium is assumed in the saturation index

range between -0.15 and 0.15; for dolomite, equilibrium is assumed in the saturation index range between -0.30 and 0.30]

Site

identification

number

Local

number Aquifer

Log partial

pressure of

CO2 (atm)

Calcitea

a

Values in black are at equilibrium with respect to the mineral. Values in red are undersaturated and those in blue are supersaturated.

Crystalline

dolomitea

Disordered

dolomitea Gypsuma Celestitea

300019081363301 SJ-3 UFA -2.35 0.09 0.13 -0.42 -0.69 -0.33

300347081363701 SJ-168 UFA -2.37 .28 .49 -.05 -.72 -.34

301304081222701 SJ-103 UFA -1.94 -.27 -.54 -1.08 -1.55 -1.26

301339081531201 D-326 UFA -2.82 -.08 -.22 -.76 -2.59 -2.22

301547081271901 SE-1 UFA -2.46 .03 .07 -.47 -1.42 -1.10

301714081233301 D-72 UFA -2.11 -.24 -.44 -.98 -1.50 -1.28

301830081375501 D-1551 UFA -2.33 -.00 -.11 -.65 -1.52 -1.15

302013081353801 D-673 UFA -1.8 -.30 -.64 -1.18 -1.43 -1.24

302058081244101 D-1107 UFA -2.38 .04 .06 -.48 -1.55 -1.31

302113081322301 D-655 UFA -2.45 .11 .13 -.41 -1.62 -1.38

302140081235501 D-1121 UFA -2.24 -.04 -.21 -.75 -1.54 -1.29

302239081434401 NK-1 UFA -2.36 .06 .17 -.37 -1.55 -1.41

302557081253101 D-913 UFA -1.97 -.08 -.09 -.63 -1.33 -1.40

302608081354903 D-264 UFA -2.18 -.00 -.11 -.65 -1.85 -2.17

302609081260601 D-912 UFA -2.29 -.02 -.06 -.61 -1.98 -2.20

300856081382401 D-3847 B -2.35 -.23 -.40 -.94 -1.88 -1.21

301405081325601 D-3846 B -2.16 .03 .11 -.43 -1.39 -.95

301508081364601 BW-3 B -2.52 .22 .46 -.08 -1.43 -.93

301704081233401 D-484 B -1.95 -.08 -.15 -.69 -1.46 -1.32

301725081305002 D-650 B -1.87 -.28 -.61 -1.15 -1.49 -1.16

301740081361001 D-275 B -1.79 -.29 -.61 -1.15 -1.44 -1.16

301743081303501 D-3825 B -2.17 .08 .14 -.40 -1.40 -1.10

301743081362301 D-225 (high) B -2.09 .01 -.02 -.56 -1.42 -1.13

301743081362301 D-225 (low) B -2.33 .04 .06 -.48 -1.47 -1.07

301748081383401 D-334 B -2.22 -.07 -.20 -.74 -1.40 -.94

301758081303901 D-665 B -2.74 .71 1.43 .89 -1.15 -.88

301817081374901 D-425T B -1.94 -.02 -.13 -.67 -1.52 -1.12

301846081240201 D-246 B -2.1 -.19 -.34 -.88 -1.55 -1.32

301934081292301 R-3 B -2.59 .30 .50 -.04 -1.51 -1.19

301937081471401 D-2223 B -2.25 -.06 -.19 -.73 -1.53 -1.10

301957081342301 D-313 B -2.02 -.21 -.52 -1.06 -1.53 -1.36

302007081353201 D-479 B -1.84 -.02 -.12 -.66 -1.54 -1.35

302008081242101 D-307 B -2.18 -.03 -.02 -.56 -1.54 -1.38

302015081384501 D-335 B -1.86 -.13 -.36 -.90 -1.63 -1.32

302045081323101 D-1323 B -1.99 -.08 -.21 -.74 -1.40 -1.28

302342081320601 D-1902 B -2.05 -.15 -.35 -.89 -1.73 -1.53

302503081332001 D-1149 B -1.65 -.31 -.71 -1.25 -1.80 -1.89

302510081260201 D-3841 B -2.41 .04 -.00 -.54 -1.72 -1.65

302538081392501 D-329 B -2.04 -.16 -.37 -.91 -1.81 -2.12

303940081281801 N-22 B -1.25 -.95 -1.86 -2.40 -1.52 -1.85

301335081355001 D-536-1 UZLF -2.65 .56 1.07 .53 -1.01 -.54

301335081355001 D-536-2 UZLF -2.45 .36 .66 .12 -1.02 -.55

301335081355001 D-536-3 UZLF -2.35 .26 .42 -.12 -.98 -.55

301522081331301 D-291 UZLF -2.79 .24 .41 -.13 -1.41 -1.05

301537081441901 D-75 UZLF -2.8 .16 .26 -.28 -1.96 -1.34

301604081361501 D-450 UZLF -2.6 .43 .79 .25 -1.37 -.99

301639081330802 D-1155 UZLF -1.91 -.20 -.48 -1.02 -1.30 -.95

301936081292401 MW-2 UZLF -2.33 .01 -.07 -.61 -1.44 -1.13

302022081393501 D-176 UZLF -2.5 .24 .37 -.17 -1.44 -1.00

302227081435001 D-592 UZLF -2.53 .23 .33 -.21 -1.42 -1.14

302608081354901 D-262 UZLF -2.33 .22 .42 -.12 -1.67 -1.98

302608081354902 D-263 UZLF -2.14 -.08 -.21 -.75 -1.83 -2.01

301132081225801 SJ-150 FPZ -2.45 .34 1.20 .66 -.38 -.38

301405081325601 D-425B FPZ -2.59 .61 .88 .34 -.46 -.25

301725081305002 D-3060 FPZ -2.38 .48 1.19 .65 -.42 -.25

301743081303501 D-2386 FPZ -2.44 .47 1.20 .66 -.59 -.54
Chemical and Isotopic Composition of Ground Water 33

With respect to calcite, most water samples

from the UZLF were supersaturated. The exceptions

were water samples from wells D-1155, which was

slightly undersaturated, and MW-2 and D-263, which

were at equilibrium. Water from most of the wells

tapping this zone was at equilibrium with respect to

disordered dolomite, except for the first sample from

D-536 (supersaturated) and samplesfrom wells D-1155,

MW-2, and D-263 (undersaturated). With respect to

crystalline dolomite, the water was supersaturated

except at wells D-1155 (undersaturated) and D-75,

MW-2, and D-263 (equilibrium). All the samples from

the UZLF were undersaturated with respect to gypsum

and celestite. The three samples from D-536, along

with the samples from the FPZ, were closer to celestite

saturation than any other samples. Those three samples

were from fracture flow zones and the chemical similarity between those samples and water from the FPZ

tends to confirm the FPZ as the source of mineralized

water in at least some of the UZLF wells.

Samples from wells tapping both the UZLF and

the UFA had more diverse saturation indexes than

wells tapping only the UZLF or only the FPZ. For calcite, the majority of the water samples were at equilibrium and nearly all of the rest were undersaturated;

however, water from wells D-665, BW-3 and R-3 was

supersaturated. The same distribution was calculated

for crystalline dolomite, with D-665, BW-3 and R-3

again the exceptions (supersaturated). For disordered

dolomite, allsamples were undersaturated, except D-665

(supersaturated), and BW-3 and R-3 (equilibrium). All

samples were undersaturated with respect to gypsum

and celestite.

Nearly all of the water samples from wells tapping only the UFA were at equilibrium with respect to

calcite, except SJ-168 (supersaturated); and SJ-103,

D-72, and D-673 (undersaturated). All samples also

were at equilibrium with respect to crystalline dolomite, with the same exceptions: SJ-168 (supersaturated); SJ-103, D-72, and D-673 (undersaturated). All

the samples from the UFA also were undersaturated

with respect to gypsum and celestite, although the samples from wells SJ-3 and SJ-168 were closest to saturation. These two wells, although yielding water with

relatively low chloride concentrations, have high sulfate concentrations and are more similar with respect to

celestite saturation to wells tapping the FPZ than to

other UFA wells (fig. 13), possibly suggesting a differentsource of water to these wells. The water from these

wells also had the highest temperature (29o

C) (table 3)

of all the UFA wells, possibly indicating a deeper

source of water. There is a strong relation between gypsum saturation index and sulfate concentration, implying that gypsum dissolution is the primary source of

sulfate in most wells(fig. 13). The relation, however, is

not as clear for the more highly mineralized FPZ samples (D-3060, D-2386, and SJ-150).

Isotopic Analyses

Another technique to better understand the

sources of mineralized water in northeastern Florida is

the use of environmental isotopes in water. Stable isotopes of oxygen-18, deuterium, carbon-13, sulfur-34,

and boron-11, as well asradiogenic isotopes carbon-14,

chlorine-36, and the ratio of strontium-87 to strontium86 were analyzed in most of the water samples from

northeastern Florida (table 6). Isotopic similarities or

differences among waters from the various aquifer

zones can help differentiate the source or sources of

mineralized waters.

Oxygen and Deuterium

The oxygen and hydrogen that make up water

molecules contain a mixture of isotopes of both elements, including the stable isotopes oxygen-18 and

deuterium. These isotopes can be fractionated by

hydrologic processes such as evaporation. The abundance of these isotopes can help researchers to understand the movement or evolution of ground water,

including processes such as recharge and mixing.

Deuterium and oxygen-18 do not evaporate asreadily

as their lighter isotopes, so they are concentrated in

seawater relative to atmospheric water vapor. Conversely, when it rains, deuterium and oxygen-18 are

more easily condensed, so precipitation becomes isotopically lighter and lighter as it continuously condenses from clouds (Coplen, 1993). Thus, rain that

falls inland will be isotopically lighter than rain that

falls along the coast. Both oxygen-18 and deuterium

are reported in delta (δ) units of parts per thousand

(per mil) relative to an arbitrary standard, VSMOW.

The Global Meteoric Line (GML) (Craig, 1961) is an

average of δD and δ18O values for precipitation and

freshwater worldwide.
34 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida Table 6. Isotopic data for water samples

[δ D, delta deuterium; δ 18O, delta oxygen-18; δ 34S, delta sulfur-34; δ 37Cl, delta chlorine-37; 36Cl, chlorine-36; 14C, carbon-14; δ 13C, delta carbon-13; 87Sr, strontdelta boron-11; na, not analyzed; --, no data. Aquifer: UFA, Upper Floridan aquifer; B, both Upper Floridan and upper zone of

Lower Floridan aquifers; UZLF, uppe

FPZ, Fernandina permeable zone]

Site

identification

number

Local

number

Aquifer δ D (per mil)

δ 18

O

(per mil)

δ 34

S of

sulfate

(per mil)

δ 37Cl

(per mil)

δ 37Cl,repeat

(per mil)

36Cl/Cl

ratio

(x 10-15)

Error,

+ (x 10-15)

14C,

percent

modern

14

C error

(percent)

300019081363301 SJ-3 UFA -8.9 -2.42 20.9 -0.52 -0.35 37.1 5.7 3.37 0.11 300347081363701 SJ-168 UFA -10.9 -2.42 20.9 .24 .19 17.4 5.8 3.5 .12 301304081222701 SJ-103 UFA -9.9 -2.51 21.7 .23 -- 48 7 0.56 .05 301339081531201 D-326 UFA -10 -2.35 27.6 -.14 -.17 38.1 5 2.9 .09 301547081271901 SE-1 UFA -8.8 -2.47 22.2 na -- na na 1.47 .06 301714081233301 D-72 UFA -9.5 -2.51 24.3 .15 -- 23 13 2.12 .05 301830081375501 D-1551 UFA -8.3 -2.31 23.4 .29 .18 52.4 4.7 1.23 .05 302013081353801 D-673 UFA -10 -2.43 25.2 .08 -- 8 8 1.3 .06 302058081244101 D-1107 UFA -9.9 -2.48 24.1 na -- na na 1.79 .07 302113081322301 D-655 UFA -8.8 -2.4 24.2 na -- na na 1.86 .07 302140081235501 D-1121 UFA -6.9 -2.25 23.5 .22 .15 94 9 1.48 .06 302239081434401 NK-1 UFA -7.7 -2.48 26 na -- na na 1.24 .05 302557081253101 D-913 UFA -11.9 -2.58 24.9 -.07 -.39 9.6 5.9 1.34 .08 302608081354903 D-264 UFA -10.2 -2.49 28.2 na -- na na 2.35 .08 302609081260601 D-912 UFA -8.5 -2.38 16.9 .06 -- 50 6 2 .07 300856081382401 D-3847 B -9.4 -2.34 22.8 -.02 -- 66.5 5.5 4.59 .10 301405081325601 D-3846 B -10.4 -2.37 22.2 .41 .13 10.2 4.8 .68 .05 301508081364601 BW-3 B -8.2 -2.4 22.5 na -- na na 1.56 .09 301704081233401 D-484 B -10.9 -2.5 25.3 .28 -- 10.4 2.5 2.28 .09 301725081305002 D-650 B -9.1 -2.44 22.9 .09 -- 16.9 2.8 .83 .06 301740081361001 D-275 B -9.5 -2.39 23.7 .61 -- 7 2.1 1.95 .08 301743081303501 D-3825 B -8.2 -2.36 23.8 .35 -- 5 4.7 2.46 .07 301743081362301 D-225 B -7.3 -2.4 23.5 -.07 -.31 9.9 2.3 2.19 .08 301743081362301 D-225 B -8.8 -2.4 23 .65 .33 30 11 1.81 .09 301748081383401 D-334 B -9.9 -2.27 21.6 .04 .08 59 4.6 .97 .06 301758081303901 D-665 B -10.4 -2.31 22.6 .34 -- 3.6 2.1 3.64 .13 301817081374901 D-425T B -10.3 -2.48 22.9 .07 -- 55.8 4 .88 .06 301846081240201 D-246 B -9.8 -2.54 21.7 -.38 -.33 60 80 2.96 .08 301934081292301 R-3 B -9.5 -2.34 23 na -- na na 1.54 .06 301937081471401 D-2223 B -8.1 -2.36 23.4 na -- na na 1.41 .06 
Chemical and Isotopic Composition of Ground Water 35

301957081342301 D-313 B -10.3 -2.49 25.9 -.06 -- 7.5 4.8 1.99 .08 -1302007081353201 D-479 B -9.1 -2.35 25.3 .49 -- 11.7 2.7 1.12 .08 -302008081242101 D-307 B -9.3 -2.53 24.5 .14 -- 3 12 1.33 .05 -302015081384501 D-335 B -10.4 -2.38 23.1 -.21 -- 56.6 4.1 .98 .06 -302045081323101 D-1323 B -9.2 -2.47 23.8 .37 -- 370 240 .91 .05 -1302342081320601 D-1902 B -10.5 -2.45 26.9 .66 -- 47 9 2.02 .07 -302503081332001 D-1149 B -9.6 -2.4 29.5 -.24 -- 55.5 4.1 .62 .05 -1302510081260201 D-3841 B -10.1 -2.32 22.7 .05 -- 75.3 4.2 .53 .05 -1302538081392501 D-329 B -9.1 -2.49 27.1 -.12 -.26 51.5 4.1 1.37 .07 -1303940081281801 N-22 B -12.6 -2.68 27.5 na na na na 9.96 .19 -301335081355001 D-536-1 UZLF -9.7 -2.34 21.7 .34 .45 2 13 2.2 .08 -301335081355001 D-536-2 UZLF -8.9 -2.38 21.8 .11 -.12 7.5 1.9 2.32 .09 -301335081355001 D-536-3 UZLF -9.4 -2.36 21.5 .3 .15 3.6 5.5 4.07 .11 -301522081331301 D-291 UZLF -8.8 -2.27 21.9 .54 -- 13.5 1.4 3.05 .10 -301537081441901 D-75 UZLF -9 -2.35 23.4 .24 -- .5 3.3 3.09 .10 -301604081361501 D-450 UZLF -8.1 -2.4 23.2 .29 -- 10 12 1.54 .07 -301639081330802 D-1155 UZLF -7.2 -2.35 22.6 .73 -- 14.6 2.9 1.95 .08 -301936081292401 MW-2 UZLF -8.8 -2.44 23 .33 .33 76.2 4.3 .96 .05 -302022081393501 D-176 UZLF -9.5 -2.39 22.7 .02 -- 57.1 2.9 3.07 .10 -302227081435001 D-592 UZLF -9.4 -2.3 22.4 .16 -- 51.2 2.5 2.47 .09 -302608081354901 D-262 UZLF -8.6 -2.57 27.2 -.03 -- 29.3 3.3 2.48 .09 -1302608081354902 D-263 UZLF -10.8 -2.49 25.8 .02 -- 67 33 .88 .06 -1301132081225801 SJ-150 FPZ 2.6 .12 20.9 -.03 -- .8 2.3 1.31 .07 -301405081325601 D-425B FPZ -10.6 -2.38 21.5 -.44 -.42 20.7 2.4 1.27 .07 -301725081305002 D-3060 FPZ -4 -1.18 21 .25 -- 4 2.4 1.63 .07 -301743081303501 D-2386 FPZ -4.7 -1.66 21.3 .07 -- 11.5 1.7 1.68 .08 -302737080571801 Seawater 6.2 .66 20.6 .52 .52 1.1 3.7 109.96 .55 -

Table 6. Isotopic data for water samples (Continued)

[δ D, delta deuterium; δ 18O, delta oxygen-18; δ 34S, delta sulfur-34; δ 37Cl, delta chlorine-37; 36Cl, chlorine-36; 14C, carbon-14; δ 13C, delta carbon-13; 87Sr, strontium

delta boron-11; na, not analyzed; --, no data. Aquifer: UFA, Upper Floridan aquifer; B, both Upper Floridan and upper zone of Lower Floridan aquifers; UZLF, upper zo

FPZ, Fernandina permeable zone]

Site

identification

number

Local

number

Aquifer δ D (per mil) δ 18O (per mil) δ 34S of sulfate (per mil) δ 37Cl (per mil) δ 37Cl, repeat (per mil) 36Cl/Cl ratio (x 10-15) Error, + (x 10-15) 14C, percent modern 14C error (percent) δ 

(pe
36 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Figure 13.  Relations between celestite and gypsum saturation indexes and sulfate concentration, and between

calcite saturation index and calcium concentration.
Chemical and Isotopic Composition of Ground Water 37

Most of the water samples from the study area

plot near the GML (fig. 14). Except for the high chloride samples from the FPZ, no trend by aquifer zone is

apparent. This indicates that climatic conditions probably did not vary substantially during the time period

that the water was recharged to the aquifer. The regression line for the water samples (fig. 14) is similar to

that for water from the UFA in west-central Florida

(Swancar and Hutchinson, 1992). The samples that

depart from the GML (that is, are enriched in oxygen18) have higher chloride concentrations and are indicative of freshwater-salt water mixing. The deuterium and

oxygen-18 values for the water from well SJ-150 are

similar to those of seawater. These data, however, cannot indicate whether the seawater is ancient or modern.

Carbon

Two isotopes of carbon in dissolved inorganic

carbon were analyzed in the samples from the Duval

County area:  the stable isotope carbon-13 (

13C) and

the radiogenic isotope carbon-14 (

14C). Values of delta

carbon-13 (δ13C) for dissolved inorganic carbon also

are needed to correct the calculations of water age

using carbon-14. Processes affecting the stable carbon

isotope composition (δ13C) of inorganic carbon in

ground water include carbonate mineral dissolution

and precipitation, microbially mediated processes that

oxidize organic carbon and generate carbon dioxide

(CO2), and mixing of waters (Sacks and Tihansky,

1996). A general equation for carbonate dissolution

driven by CO2 is:

. (6)

Values of δ13C depend on the source of the

carbon. For example, most vegetation and atmospheric

CO2 have δ13C values averaging about -25 per mil

(Deines and others, 1974), so organically derived carbon tends to add lighter 13C to the system. On the other

hand, dissolution of calcium carbonate adds heavier

13C. According to equation 6, calcite having a δ13C

value of 1 per mil (table 2) reacting with CO2 having a

δ13C value -22 per mil (table 2) would produce

dissolved inorganic carbon with a δ13C value of -11.5

per mil. δ13C values ranged from -3.48 to -11.96 per

mil in the study area (table 6). The generally heavier

δ13C values measured in the study area indicate that a

larger proportion of the dissolved inorganic carbon in

those samples was derived from carbonate minerals.

Delta 13C values do not seem to be related to

aquifer zones (figs. 15 and 16). In the UFA, δ13C was

heaviest (-3.48 and -3.82) in the high-sulfate waters

from wells SJ-3 and SJ-168, respectively. The lightest

value was at well SE-1 (-11.02). In wells tapping both

the UFA and the UZLF, the heaviest value (-4.87) was

at well D-3847 in southern Duval County (a low

chloride water). The lightest values were north of the

St. Johns River at wells D-329 (-11.57) and D-1149

(-11.22). In samples from the UZLF, the heaviest

values (-5.35 to -6.38) were from well D-536 in southern Duval County and the lightest from D-262 (-11.96).

The samples from the FPZ ranged from -6.3 at well

SJ-150 to -9.83 at well D-2386. These data show that

the water from well SJ-150, although chemically

almost identical to modern seawater, is isotopically

different with respect to δ13C (fig. 15, table 6), as are

the other high chloride water samples from the FPZ.

The δ13C for the dolomite sample from a depth of

1,893 ft in well D-349 was similar to that for modern

seawater, whereasthe othersamples of aquifer material

had heavier values.

In general, in all aquifer zones, water with the

heaviest δ13C values was found in the southern part of

the study area and water with the lightest values was in

the northern part(fig. 16). Thisissomewhat inconsistent

with the fact that all of northeastern Florida is a discharge area and the principal recharge area for the study

area is to the southwest. Data from west-central Florida

showed that δ13C in the UFA waslightest in the recharge

area (less than -8 per mil) and heaviest in the discharge

areas (greater than -5 per mil) and generally increased

with depth in the UFA (Sacks and Tihansky, 1996). In

the Keystone Heights recharge area, Plummer (1997)

reported a value of -12 per mil. In northeastern Florida,

however, each aquifer zone except the FPZ had at least

one well with a δ13C value of -11 per mil or lighter.

Light values could be related to the presence of buried

organic materials associated with paleokarst features.

The mean value of δ13C for all wells sampled

ranged from -8.09 per mil for wells tapping only the

UFA to -8.79 for wells tapping both the UFA and the

UZLF. This is heavier than the average value (-13 per

mil) reported by Deines and others (1974) for a

closed ground-water system in contact with carbonates in Pennsylvania. The heavier values could result

from dissolution of isotopically heavier marine carbonates in northeastern Florida (Long and others,

1988). Sacks and Tihansky (1996) reported an average δ13C value for marine dolomites in southwestern

Florida of 1.7 per mil. In northeastern Florida, the

values ranged from -1.66 to 2.4 per mil (table 2).

CaCO3 + + Ca CO2 H2O 2 + 2HCO3 +

Ò

→
38 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Figure 14.  Relations between delta deuterium and delta oxygen-18 and between delta deuterium and chloride concentration.
Chemical and Isotopic Composition of Ground Water 39

Heavier δ13C values also could result from recrystallization of unstable carbonate phases such as aragonite

or high-magnesium calcite, which also would enrich

the dissolved inorganic carbon in 13C without increasing the dissolved inorganic carbon concentration.

In contrast, the light values of δ13C can result

from sulfate reduction, which produces isotopically

light CO2 (Long and others, 1988) from the oxidation

of organic carbon, such as the peat found in the aquifer

(table 2). If values of δ13C were strongly related to sulfate reduction, then δ13C values should be lighter in

waters with lower sulfate concentrations. There is a

general trend for lighter δ13C values where sulfate concentrations are lower (fig. 17), although the data show

considerable scatter. Water samples from the FPZ do

not show such a trend. Also, the delta sulfur-34 (δ34S)

values of dissolved sulfide become more positive as

sulfide reduction progresses. Therefore, δ13C values

should become more negative as δ34S values become

more positive, if sulfate reduction is occurring. That

trend in carbon and sulfur isotopes was observed in the

aquifer system (fig. 17). The trend of increased sulfate

enrichment and heavier δ13C values (fig. 17) is indicative of the occurrence of dedolomitization reactions

(gypsum and dolomite dissolution and calcite precipitation). Apparently, in northeastern Florida the competing processes of sulfate reduction and dissolution of

isotopically heavier marine carbonates both influence

the 13C isotopic content of the ground water, as does

the areal distribution of gypsum in the aquifer.

The percent modern carbon (

14C) for dissolved

inorganic carbon also was determined for the samples.

Percent modern carbon valuesranged from 0.53 in well

D-3841 to 9.96 in well N-22 (table 6). Radiogenic 14C

can be used to age-date waters, although in a carbonate

aquifer dissolution of carbon from the aquifer matrix

(which contains zero percent modern carbon) can complicate the calculations(Wigley, 1975), as can dissolved

inorganic carbon derived from the oxidation of buried

peat. Another difficulty in calculating radiocarbon ages

is defining the initial carbon activity of the sample. A

geochemical modeling program, NETPATH (Plummer

and others, 1994), was used to correct the measured

14C activity values for water samples from the study

area. A complete discussion of this process is given in

a later section of this report.

Figure 15. Relation between delta oxygen-18 and delta carbon-13 for rock and water samples.
40 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Figure 16.  Distribution of delta carbon-13 in ground water.
Chemical and Isotopic Composition of Ground Water 41

Sulfur

The stable isotope of sulfur, 34S, can be used to

evaluate the sources of sulfur in ground water and to

test geochemical models by calculating the δ34S balance of chemical reactions along flow paths (Coplen,

1993). Sulfur sources such as atmospheric precipitation, gypsum, and seawater all can have differentsulfur

isotope signatures. Atmospheric sulfur and most fresh

waters have sulfur isotopic compositions ranging from

0 to about +15 per mil (Long and others, 1988). This

agrees with data from rain water in north-central Florida that ranged from 3.4 to 5.9 per mil (Katz and others,

1995). A water sample from the surficial aquifer system in central Florida had a sulfur isotopic composition

of 8.3 per mil (Sacks and Tihansky, 1996).

If the source of sulfur in ground water is from

dissolution of sulfate minerals in the aquifer, the isotopic composition would reflect the composition of the

various minerals. In southwestern Florida, evaporites

from the Avon Park Formation (the middle semi-confining unit of the Floridan aquifer system) had an average δ34S value of 23.4 per mil. In the same area,

gypsum from the Lower Floridan aquifer and the lower

confining unit (Oldsmar and Cedar Keys Formations)

had lighter values, averaging 20.1 per mil (Sacks and

Tihansky, 1996).

The modern seawater sample collected during

this study had a δ34S value of 20.6 per mil (table 6),

almost identical to that of the sample from well SJ-150

(20.9 per mil). The sulfur isotopic composition cannot

be used to differentiate ancient from modern seawater.

In fact, the δ34S of seawater probably has not changed

substantially since the middle Miocene, about

15 million years ago (Claypool and others, 1980).

Figure 17. Relations between delta carbon-13 and (a) sulfate concentration, (b) delta sulfur-34, and (c) sulfate

enrichment.
42 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

The sulfur isotopic composition of ground water

can be altered by biological processes. Bacterial reduction of sulfate to sulfide fractionates sulfur by preferentially using isotopically lighter sulfur-32, causing the

remaining sulfate to be isotopically enriched in sulfur34. As mentioned previously, the sulfate reduction process also results in ground water with a lighter δ13C

composition and a lower sulfate concentration. Some

water samples from the study area show a sulfate

reduction trend (figs. 17 and 18).

The δ34S values for the samples collected

varied somewhat from one aquifer zone to another. For

wells tapping the FPZ, δ34S values ranged from 20.9

to 21.5 per mil. This corresponds to the values from

Sacks and Tihansky (1996) for evaporites from the

Oldsmar and Cedar Keys Formations and to post-midMiocene seawater. Samplesfrom the UZLF had values

ranging from 21.5 per mil to 27.2 per mil, with a

median of about 22.5. Wells tapping both the UZLF

and the UFA had δ34S valuesranging from 21.6 per mil

to 29.5 per mil, with a median value of about 23.2 per

mil. The median for samples from the UFA was 23.5

per mil, with values ranging from 16.9 to 28.2 per mil.

If the concentration of sulfate in ground water

were altered by bacterial reduction of sulfate to sulfide,

then samples with lower sulfate concentrations should

have heavier values of δ34S. For some wells, this trend

was observed (fig. 18). For example, water from well

D-1149 had the heaviest δ34S value, a light δ13C value,

and a relatively low sulfate concentration (79 mg/L).

The sulfide concentration from thatsample (1.9 mg/L),

as measured in the field, was not among the highest values, which could have been because sulfide wasreleased

during the field analysis or because sulfide precipitation is occurring in the aquifer. Other samples that fit

this general trend are those from wells D-329 and

D-262. The low sulfate concentration in water from

well D-912, accompanied by the lightest δ34S value

and a relatively light δ13C value, is more difficult to

explain. That sample may show the effects of more

recent recharge water. The UFA wells with the highest

sulfate concentrations (SJ-3 and SJ-168) have light

δ34S values, similar to Oldsmar and Cedar Keys

evaporites, and heavy δ13C values, possibly indicating

the influence of water from another source, or the

effects of more extensive dedolomitization reactions.

The extent to which sulfate reduction increases

the abundance of 34S in a ground-water system can be

quantified, assuming that no other processes such as

gypsum dissolution or pyrite oxidation are occurring,

by calculating the enrichment factor. This can be done

by plotting the relation between δ34S of sulfate and the

natural logarithm of the ratio of measured sulfate concentration to an assumed initial (unreacted) sulfate concentration (fig. 18). The slope ofthe best-fit (regression)

line is the enrichment factor. For each group of wells,

the highest measured sulfate value was used as the initial value; for UFA wells this was the sulfate concentration from well SJ-3 (660 mg/L), for UZLF wells the

value from D-536-3 (340 mg/L) was used, and for

wells tapping the UFA and the UZLF, the value from

D-665 (250 mg/L) was used. The enrichment factors

were -2.1 per mil for UFA wells, -2.3 per mil for UZLF

wells, and -4.0 per mil for wells tapping both the UFA

and the UZLF. For all three groups plotted together

(excluding FPZ wells and seawater) the factor is-3.1 per

mil. The plot of all wells, adding FPZ wells and seawater (fig. 18)showstwo trends:  a trend for possible dilution of seawater that includes the FPZ wells and UFA

wells SJ-3 and SJ-168; and an apparent fractionation

trend for most of the other wells, along which δ34S

increases assulfate decreases. The trend for decreasing

sulfate concentration and increasing δ34S content could

represent changes down a flow path. Long and others

(1988) reached a different conclusion, however, for

aquifers they studied in Michigan. Long and others

(1988) concluded that sulfate reduction had been

occurring for different lengths of time in different parts

of the aquifers. This also could be the case in northeastern Florida. Other processes such as gypsum dissolution or pyrite oxidation also are likely occurring. Thus,

the isotopic content of watersamplesfrom northeastern

Florida indicate that two chemical processes probably

are occurring, but the sulfur isotope data cannot be used

to help determine the origin of mineralized water

affecting wells such as D-665.

Strontium

The aqueous chemistry of strontium is similar

to that of calcium because of a similar ionic size and

valence. The minerals strontium carbonate (strontianite) and strontium sulfate (celestite) are common

in sediments (Hem, 1985). Fossils in limestone tend

to be enriched in strontium, relative to an overall

ratio of strontium-to-calcium in massive limestone

(Kulp and others, 1952). The ratio of radiogenic

strontium-87 (

87Sr) to stable strontium-86 (86Sr) in

marine carbonates is presumed to have been the

same as that in the oceans at the time of deposition.
Chemical and Isotopic Composition of Ground Water 43

Figure 18.  Relation between delta sulfur-34 and sulfate concentration, and calculation of enrichment factors for delta

sulfur-34 of sulfate.
44 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

That ratio persists in calcite and gypsum, but in dolomite may be changed by diagenetic processes during

dolomitization (Sacks and Tihansky, 1996). The variations in the ratio of 87Sr to 86Sr in seawater over geologic time have been well-documented (DePaolo and

Ingram, 1985; Elderfield, 1986; Hess and others,

1986), so the ratio measured in rock or water samples

may help in interpretation of the relative ages of various samples.

Strontium concentrations in ground-water samples ranged from 500 μg/L at well D-264 (UFA) to

21,800 μg/L in well D-3060 (FPZ) (table 3). The modern seawater sample had a strontium concentration of

650 μg/L, much lower than the high chloride samples

from the FPZ (range 11,400 to 21,800 μg/L) and lower

than the 8 mg/L (8,000 μg/L) reported by Hem (1985).

If mixing between seawater (using the strontium concentration from Hem) and water from well D-326 is

assumed, most of the samples are enriched in strontium, compared to the mixing trend (fig. 19). This

probably is the result of dissolution of strontium from

the aquifer matrix. Areal variations in strontium

enrichment may be related to the distribution of strontium-rich mineralsin the aquifer. Another possibility is

that strontium concentrations are related to aquifer residence time (and, thus, ground-water Ï ageÓ ); this is

discussed in a later section of this report. Another

trend, with water from well SJ-150 as the mineralized

end member, includes wells D-665, D-536, SJ-168,

and SJ-3. A third group of samples (wells north of the

St. Johns River) have low strontium and chloride concentrations, and plot in a group below the trend line.

The strontium data, therefore, seem to indicate that

dilution of at least two different mineralized end members probably is occurring.

Ratios of 87Sr to 86Sr for most of the samples are

in the range of Eocene-age (approximately 37-58 million years before present) seawater, as a result of dissolution of the Eocene-age rocks of the aquifer matrix

(fig. 20). The exceptions include the samples from the

FPZ with high strontium concentrations and high chloride concentrations (SJ-150, D-2386, and D-3060) and

samples from several wells north of the St.Johns River

with relatively low strontium concentrations. The

Figure 19. Relation between strontium concentration and chloride concentration.
Chemical and Isotopic Composition of Ground Water 45

strontium-depleted samples have the strontium isotope

ratio of Oligocene-age (approximately 24-37 million

years before present) seawater. There are no

Oligocene-age rocks present in northeastern Florida,

so non-Eocene strontium isotope ratios must be the

result either of diagenetic changes during dolomitization or waters of different ages mixing, or both. Possibly, contact with dolomites could explain the

Oligocene ages for water samples from the UZLF, but

is unlikely for wells tapping the UFA because of the

absence of dolomite. If chemical changes during dolomitization were the cause of non-Eocene ratios, then

considerable scatter might be expected in the plot of

strontium isotope ratios as a function of inverse strontium concentration (fig. 20). Instead, the plot in

figure 20 clearly shows a mixing trend. For the wells

north of the St. Johns River that have low strontium

concentrations and strontium isotope ratios of

Oligocene age, it is possible that water from this group

of wells was recharged from a different, closer

recharge area than that for the wells south of the

St. Johns River. The source of increasing chloride

concentrations in some of these wells (for example,

D-262; fig. 2) could be seawater of an age different

from the water from well SJ-150.

Water from well SJ-150 has a strontium isotope

ratio between that of Oligocene and Miocene age

(approximately 5-24 million years before present);

water from wells D-3060 and D-2386 (the other high

chloride waters from the FPZ) have ratios consistent

with Oligocene-age waters. The dolomite samples

analyzed in this study had strontium isotope ratios

ranging from Eocene-to-Oligocene-age (fig. 6), so it is

unlikely that dissolution of dolomite from the aquifer

matrix is the major factor controlling the strontium

isotope ratio in the sample from well SJ-150. Instead,

the water from well SJ-150 could contain a

mixture of strontium from younger seawater (with a

higher strontium isotope ratio) and strontium

dissolved from Eocene-age rocks. Strontium isotope

data point to mixing of waters of different ages, but do

not conclusively indicate the age of mineralized waters

involved in the mixing process.

Figure 20.  Relation between inverse of strontium concentration and ratio of strontium-87 to strontium-86.
46 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Boron

Boron is chemically and physically similar to

silicon, although it is less abundant. Boron is not found

free in nature, occurring only in oxy-compounds. The

most common forms are boric acid (H3BO3) and borax

(hydrated sodium borate, ), an

evaporite deposit. Boric acid in seawater is ingested by

foraminifera and incorporated into their tests. Comparing the delta boron-11 (δ11B) content of foraminifera

from different formations has been used to assess the

pH of the ocean, and thus the atmospheric carbon dioxide concentration through geologic time (Ramesh and

Krishnamurthy, 1998; Pearson and Palmer, 1999).

Also, the δ11B content of fertilizers and detergents, the

source of which is evaporite deposits, generally has a

different boron-11 (

11B) content from that metabolized

by marine foraminifera. Thus, the influence of relatively recent recharge water affected by anthroprogenic

activities can be observed. The worldwide δ11B content

for modern seawater is about 40 per mil (Barth, 1998).

Values of δ11B were measured for 34 of the samples collected during this study; however, samples collected during the first round of data collection,

including those from the FPZ and the UZLF, were not

analyzed for δ11B. Water samples with higher chloride

concentrations tend to have heavier values of δ11B,

whereas samples with lower chloride concentrations

show more scatter in the boron-chloride relation

(fig. 21). The relation between δ11B and chloride concentration does not help to clarify the source of the high

chloride water. The lack of a relation between δ11B and

boron concentration, however, indicates that dilution

of modern seawater is not the source of boron in most

of the samples. Additional sampling of wells from

the Lower Floridan aquifer and from the FPZ could

confirm this conclusion. The low δ11B content of

water from some UFA wells (NK-1 and D-912) could

be the result of anthroprogenic influences.

Chlorine

The long half-life of chlorine-36 (

36Cl) (about

3 x 105 years) and its low chemical activity make 36Cl

suitable for establishing model dates for some old

ground-water systems (Bentley and others, 1986a).

However, radioactive 36Cl is a relatively scarce isotope

and, therefore, difficult to analyze. The scarcity of 36Cl

has limited use of this isotope in ground-water investigations until recently, when more laboratories began to

offer analysis for 36C by AMS. Chlorine-36 is formed

naturally in the atmosphere by cosmic ray spallation of

argon-40 (

40Ar) (meteoric 36Cl); in the shallow zones

of the EarthÌs crust by the interaction of cosmic rays

and chlorine-35 (

35Cl) in near-surface rocks (epigene 36Cl); and in deeper rocks by the activation of 35Cl by

neutrons resulting from alpha particle emission during

the decay of uranium and thorium (hypogene 36Cl)

(Bentley and others, 1986a; Fontes and Andrews,

1994). Anthroprogenic sources of 36Cl include atmospheric testing of nuclear weapons(Bentley and others,

1986b) and disposal of waste from nuclear facilities

(Cecil and others, 1992). For example, Davis and others (1998b) attributed a ratio of 36Cl to total chlorine of

377 x 10-15 in water from a well in Tallahassee, Fla., to

the presence of bomb-produced 36Cl. At the Savannah

River Site nuclear fuel reprocessing facility in South

Carolina, Beasley and others (1992) reported ratios in

shallow ground water as high as 4,400 x 10 -15.

Ideally, the ratio of 36Cl to total chlorine in

ground water can be determined by the initial meteoric

36Cl of recharge water and a Ï backgroundÓ value (secular equilibrium) for the aquifer based on location and

lithology. When recharge water enters the aquifer, the

radiometric Ï clockÓ is set; the decay of 36Cl gradually

decreases the ratio unless chlorine is added to the

system by dissolution of evaporites or mixing with

water from other aquifers. Thus, the time since

recharge can be calculated. This method works best for

ground water isolated from the atmosphere for

50,000 years or longer (Bentley and others, 1986a).

The analysis made during the presentstudy islikely the

first attempt to use the 36Cl method to study water from

the Floridan aquifer system.

Water from 49 of the wells sampled during this

study was analyzed for the ratio of 36Cl to total chlorine. Ratios ranged from 0.5 x 10-15 for well D-75 to

370 x 10-15 for well D-1323 (table 6). The anomalously

high value for well D-1323 probably results from contamination of the sample by water from the surficial

aquifer system; geophysical logs of the well indicate

that the casing isleaking at a depth of 120 ft below land

surface where the casing reducesfrom 12 to 6 inchesin

diameter. This ratio (370 x 10 -15) is, therefore, not

considered to be representative of the Floridan aquifer

system and is not included in the data analysis. Ratios

of 36Cl to total chlorine vary both by aquifer and by

location (fig. 22). Ratios for the FPZ ranged from

0.8 x 10-15 to 20.7 x 10-15. For wells tapping only the

UZLF, the range was 0.5 x -15 to 76.2 x 10-15, with a

median of about 15 x 10-15. Wells tapping both the

UZLF and the UFA had ratiosranging from 3 x 10-15 to

75.3 x 10-15, with a median of about 20 x 10-15. For

wells tapping only the UFA, the range was from

8 x 10-15 to 94 x 10-15, with a median of 37 x 10-15.

Na2B4O7 • 10H2O
Chemical and Isotopic Composition of Ground Water 47

Figure 21.  Relations between delta boron-11 and chloride and boron concentrations.
48 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Figure 22. Ratio of chlorine-36 to total chlorine in water from wells in northeastern Florida.
Chemical and Isotopic Composition of Ground Water 49

The natural (pre-bomb) ratio of 36Cl to total

chlorine in Florida ground water has been estimated to

be 20 x 10-15 or less (Bentley and others, 1986a). However, Davis and others (1998b) concluded that during

the time interval 12,000-40,000 years before present,

the average production rate of meteoric 36Cl was higher

than the present-day rate by more than 33 percent.

Based on this, an initial value of at least 27 x 10-15 is

estimated. Even for water samples with low chloride

concentrations(20 mg/L or less), many of the ratios are

larger than this estimated initial ratio. Therefore, there

must be anothersource of 36Cl to the ground water, and

as a result, the decay of 36Cl from an initial value cannot be used to date the water. The 36Cl data may, however, still be useful in understanding flow systems

in northeastern Florida.

The chemical evolution of ground water in a

region sometimes can be evaluated by looking at the

relation between the ratio of 36Cl to total chlorine and

the number of 36Cl atomsin the watersamples(Bentley

and others, 1986b). The ratio of 36Cl to total chlorine

for water samples that have evolved from a common

initial composition by evaporation or ion filtration will

plot on a horizontal line. Samples that have evolved by

addition of Ï deadÓ chlorine (having little or no 36Cl,

such as from evaporite deposits) will plot on a vertical

line. Low chloride waters that have evolved from an

initial composition by aging alone will plot on a

straight line between the initial composition and the

origin of the plot.

The plot of the relation between the ratio of 36Cl

to total chlorine and 36Cl concentration for watersfrom

the Duval County area shows several trends (fig. 23).

The low chloride samples (chloride concentration less

than 100 mg/L) plot on a steeply sloping line possibly

indicating a 36Cl decay trend. Along such a trend, both

the number of 36Cl atoms and the ratio of 36Cl to total

chlorine should decrease along flow lines as water

flowsfrom recharge to discharge areas. In northeastern

Florida, however, this is not always the case. The sample with the lowest ratio of 36Cl to total chlorine, UZLF

well D-75, is closer to the present-day recharge area

than the sample from the same aquifer zone with the

highest ratio (well MW-2). The same trend is apparent

for low chloride samples from wells tapping the

UFA; well D-326, closer to the present-day recharge

area, has a lower ratio than well D-1121, which is

located downgradient. This could indicate that water in

the Floridan aquifersystem in the Jacksonville area did

not recharge the aquifer at modern-day recharge areas,

or that the wellssampled do not lie along the same flow

paths. Another possible conclusion is that these data

indicate a higher initial 36Cl concentration for water

recharged during the last approximately 10,000 to

30,000 years than previously estimated. For all of the

samples(except well D-75, which could be anomalous)

with chloride concentrations less than 15 mg/L, the

36Cl to total chlorine ratio was in the range 50 to

66 x 10-15.

Samples with higher chloride concentrations

follow trends that are different from the trend for low

chloride waters. The high chloride samples from the

FPZ (wells SJ-150, D-3060, and D-2386) form a nearly

horizontal trend (fig. 23, inset). The watersamples with

chloride concentrations between 100 and 150 mg/L

form a trend, as do most of the samples with chloride

concentrations greater than 150 mg/L. These trends

could represent the superposition of the effects of

mixing with high chloride concentration water from

the FPZ (such as from well SJ-150) on a simple

decay trend.

To illustrate the possible effects of mixing, values of the ratio of 36Cl to total chlorine and the number

of atoms of 36Cl were calculated for the mixing of various percentages of water from well D-650 (chloride

concentration 64 mg/L) with water from well SJ-150.

The plotted points (fig. 23) show how mixing could

cause trendsthat depart from a simple decay trend. This

hypothetical mixing, however, does not explain the values for samples from well D-665 (chloride concentration 830 mg/L) and well D-913 (chloride concentration

380 mg/L). According to the calculations, a mixture of

about 5 percent water from well SJ-150 with 95 percent

water from well D-650 should plot along one of the

trend lines shown in figure 23. However, water from

well D-665, which according to mass-balance mixing

contains about 5 percent water similar to SJ-150 water,

does not plot along any of the other trends. Likewise,

the sample from well D-913 does not plot on a trend.

Thus the source of chlorine to water from wells D-665

and D-913 could be different from the source to the

other wells sampled.

The fact that, in general, the ratio of 36Cl to total

chlorine decreases as chlorine concentration increases

probably indicates that old chloride from a source deep

in the aquifer (or from formations beneath the aquifer)

is mixing with fresher water in the upper zones of the

aquifer (S. Davis, University of Arizona, written commun., 2000). Analysis of additional water samples in

northeastern Florida would be useful for understanding

this complex problem.
50 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Figure 23. Relation between ratio of chlorine-36 to total chlorine and chlorine-36 concentration.
Origins of Mineralized Waters 51

ORIGINS OF MINERALIZED WATERS

The chemical and isotopic data discussed in

previous sections have shown that there are several

possible sources of mineralized water, including

unflushed Miocene-to-Pleistocene-age seawater, modern seawater, or brine. However, differentiating among

the possible sources is difficult. To aid in this task, the

geochemical modeling program NETPATH (Plummer

and others, 1994) was used to investigate possible mixing scenariosthat could yield the chemical and isotopic

composition observed in the waters at some of the

wells sampled. NETPATH can calculate the mixing

proportions of two or more initial waters and the net

geochemical mass-balance reactions that can account

for the observed composition of a final water. The

program also was used to evaluate possible flow paths

in the aquifer system and to correct initial 14C values

for age-dating the water samples. Finally, hydrogeologic and geochemical evidence can be evaluated

together to provide conclusions about the source or

sources of mineralized water.

Mass-Balance Models

A series of NETPATH runs was made to investigate possible mixing of an initial water that represents

ambient ground water with the highly mineralized

waters from the FPZ and with the modern seawater

sample. Water from D-326, a UFA well yielding water

with a chloride concentration of 5.6 mg/L and a sulfate

concentration of 20 mg/L, was selected as the ambient

(freshwater) end member for wells tapping the UFA

and both the UFA and the UZLF. The ambient initial

water was mixed with mineralized initial water from

well SJ-150 and with modern seawater to simulate

water samples from wells with elevated chloride concentrations such as D-665, D-673, D-913, D-275,

D-479, D-307, D-3846, and D-225 (both low and high

chloride conditions), and wells N-22 and SE-1 which

have a low chloride concentration. Mixing models

also were created for UZLF wells using the chemical

composition of water from well D-75 (chloride concentration 7.6 mg/L and sulfate concentration 65 mg/L;

table 3) as the ambient initial water and with water

from well SJ-150 and modern seawater as the mineralized initial water. Mixing scenarios were simulated

for wells D-450, D-291, D-1155, and for the three

samples from D-536 (table 7). Simulations also were

made for wells MW-2, D-262, and D-263, which have

chloride concentrations of 15 mg/L, 46 mg/L and

19 mg/L, respectively.

The geochemical modeling program NETPATH

(Plummer and others, 1994) computes sets of possible

reactions (including mineral dissolution or precipitation, ion exchange, and gas exchange) based on the net

masstransfer of elements, given a set of constraints and

plausible mineral phases. Possible constraints include

available elements and concentrations, isotopic data,

and electron balance. The model also can compute an

isotopic composition for the final water based on the

isotopic compositions of the initial waters, calculated

mass transfer of elements, and fractionation factors for

precipitating mineral phases. The models are not

unique but do indicate possible reactions that could

result in an observed final water.

The models used in this study were constrained

by carbon, calcium, chloride, sodium, sulfur, magnesium, iron, and electron balance (redox state). Chloride

was used as the constraint for mixing of fresh and mineralized waters. Electron balance was needed for modeling redox reactions such as oxidation of organic

matter. Reaction phases included in the models were

calcite, dolomite, gypsum, CO2 gas, organic material

(represented as CH2O and constrained to dissolve

only), pyrite (FeS2, constrained to precipitate only),

geothite (FeOOH, constrained to dissolve only), and

both calcium/sodiumand magnesium/sodiumexchange.

Trace amounts of clay minerals in the aquifer could

serve as ion-exchange sites (Sacks and Tihansky,

1996). Although the aquifer system is expected to be

closed to CO2 gas mass transfer, chemical reactions

(such as oxidation of organic matter accompanying sulfate reduction, methanogenesis, or carbonate mineral

dissolution or precipitation) can produce or consume

CO2 within the aquifer (Plummer and others, 1990).

The observed final-water chemistry could be

simulated with either of the mineralized initial

waters. Furthermore, the isotopic compositions of

the final waters also could be simulated using either

FPZ water or modern seawater. This is because the

fraction of mineralized water needed to produce the

final water in most of the samples was so small that

the major-ion composition and isotopic signature of

the ambient (freshwater) end member dominated the

modeled chemical and isotopic composition of the

final water. For the sample from well D-665 (chloride concentration 830 mg/L), the proportions were

about 95.5 percent ambient water and 4.5 percent

mineralized water (table 4). For samples from the

other UFA and from both the UFA and UZLF, the

proportions were 97 to 98 percent ambient water.
52 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Table 7. NETPATH models for selected wells[Units in millimoles per liter (mmol/L) unless otherwise noted; positive mass transfer indicates dissolution or ingassing; negative mass transfer indicates precipitation Floridan aquifer; B, both Upper Floridan and upper zone of Lower Floridan aquifers; UZLF, upper zone of Lower Floridan aquifer. δ34S, delta sulfur-34; δ13C, delta ca--, not modeled; na, not analyzed. Isotopic data used in models unless otherwise specified: δ13

C of CH2O and CO2 = -25.0 per mil. For dissolving phases: δ34S of gy

per mil; δ13

C of dolomite = 2 per mil. For precipitating phases, δ13

C or δ34S computed by model based on Rayleigh calculations and additive fractionation factors def

of pyrite = -30.0 per mil; δ34S (observed) based on measured δ34S of sulfate. Remarks give more information about phases: X, calcium-sodium exchange modeled; Meled; P, pyrite not modeled; G, goethite not modeled; C, all models included CO2 outgassing]

Final well Initial well 1 Initial well 2 Aquifer Gypsum Dolomite Calcite CH2O CO2

δ34

S (total S),

per mil

δ13

C (total C),

per mil

Carbon-14,

percent mode

calc obs calc obs calc obs 

SE-1 D-326 SJ-150 UFA 1.94 -- -- 0.36 0.22 23.9 19.2 -12.8 -11.0 32.7 1.4SE-1 D-326 Seawater UFA 1.85 -- 0.09 .19 .30 22.1 19.2 -11.9 -11.0 32.9 1.4D-673 D-326 SJ-150 UFA 1.02 0.82 -1.10 -- .70 22.1 23.9 -8.2 -9.4 17.4 1.3D-673 D-326 Seawater UFA 1.04 .75 -.99 -- .73 22.1 23.9 -8.6 -9.4 18.1 1.3D-913 D-326 SJ-150 UFA 1.32 1.10 -1.73 -- .93 22.2 24.2 -8.1 -10.3 13.5 1.3D-913 D-326 Seawater UFA 1.37 .93 -1.46 -- 1.01 22.2 24.2 -9.0 -10.3 14.8 1.3D-3846 D-326 SJ-150

B 1.58 1.03 -1.54 -- .66 22.2 21.1 -7.1 -6.8 14.9 .6

D-3846 D-326 Seawater

B 1.59 1.00 -1.51 -- .67 22.2 21.1 -7.2 -6.8 15.1 .6

D-275 D-326 SJ-150

B 1.81 .74 -1.77 1.58 -- 22.0 23.7 -12.4 -9.1 12.5 1.9

D-275 D-326 Seawater

B 1.88 .65 -1.68 1.67 -- 22.0 23.7 -13.2 -9.1 12.9 1.9

D-225 (high) D-326 SJ-150

B 1.73 .74 -1.67 1.24 .58 22.0 23.5 -11.0 -9.3 13.7 2.1

D-225 (high) D-326 Seawater

B 1.80 .65 -1.58 1.33 -- 22.0 23.5 -11.8 -9.3 14.1 2.1

D-225 (low) D-326 SJ-150

B 1.91 .64 -1.62 .92 -- 22.1 23.0 -10.4 -8.2 15.7 1.8

D-225 (low) D-326 Seawater

B 1.92 .62 -1.60 .93 -- 22.1 23.0 -10.5 -8.2 15.8 1.8

D-665 D-326 SJ-150

B 1.18 1.01 -2.00 .02 .65 21.5 21.6 -7.2 -9.0 13.6 3.6

D-665 D-326 Seawater

B 1.29 .63 -1.43 .03 .82 21.5 21.6 -9.7 -9.0 17.2 3.6

D-479 D-326 SJ-150

B .89 .56 -.76 -- 2.01 22.5 23.6 -15.6 -9.9 16.7 1.1

D-479 D-326 Seawater

B .90 .51 -.69 -- 2.04 22.5 23.6 -13.9 -9.9 17.1 1.1

D-307 D-326 SJ-150

B 1.20 .72 -1.23 .26 .59 22.0 19.7 -9.3 -9.0 17.1 1.3

D-307 D-326 Seawater

B 1.21 .67 -1.14 .26 .62 22.0 19.7 -9.7 -9.0 17.7 1.3

N-22 D-326 SJ-150

B 1.39 .89 -1.35 .14 2.51 22.2 24.4 -14.1 -8.2 12.8 9.9

N-22 D-326 Seawater

B 1.39 .88 -1.32 .14 2.52 22.2 24.4 -14.1 -8.2 12.9 9.9

D-536-1 D-75 SJ-150 UZLF 2.82 1.22 -2.78 .72 -- 22.1 21.7 -6.8 -6.0 10.1 2.2D-536-1 D-75 Seawater UZLF 2.91 1.12 -2.68 .83 -- 22.1 21.7 -7.6 -6.0 10.5 2.2D-536-2 D-75 SJ-150 UZLF 2.53 1.23 -2.41 .35 .16 22.1 21.8 -4.9 -6.4 11.74 2.2D-536-2 D-75 Seawater UZLF 2.61 1.14 -2.31 .44 -- 22.1 21.8 -5.7 -6.4 12.2 2.3D-536-3 D-75 SJ-150 UZLF 2.62 1.12 -2.26 .01 .45 22.2 21.5 -5.9 -5.4 13.0 4.0D-536-3 D-75 Seawater UZLF 2.64 1.04 -2.14 .01 .48 22.2 21.5 -6.3 -5.4 13.7 4.0D-291 D-75 SJ-150 UZLF .90 -- -- .12 -.46 22.4 21.9 -7.8 -8.6 36.4 3.0D-291 D-75 Seawater UZLF .90 -- -- .08 -.42 22.4 21.9 -7.6 -8.6 37.0 3.0D-450 D-75 SJ-150 UZLF 1.36 .67 -1.42 .76 -- 22.4 23.2 -9.0 -8.0 16.6 1.5D-450 D-75 Seawater UZLF 1.40 .63 -1.38 .81 -- 22.3 23.2 -9.4 -8.0 16.9 1.5D-1155 D-75 SJ-150 UZLF 1.54 .75 -1.54 1.00 -- 22.2 22.6 -9.9 -8.1 14.9 1.9D-1155 D-75 Seawater UZLF 1.59 .69 -1.48 1.06 -- 22.2 22.6 -10.4 -8.1 15.2 1.9MW-2 D-75 SJ-150 UZLF .99 .20 -- .02 .20 22.5 22.7 -8.3 -8.3 2.76 .9MW-2 D-75 Seawater UZLF 1.22 .62 -1.06 .45 -- 28.2 22.7 -7.7 -8.3 19.6 .9D-262 D-75 SJ-150 UZLF .85 .41 -.78 1.06 -- 22.4 27.2 -11.3 -12.0 19.2 2.4D-262 D-75 Seawater UZLF .87 .40 -.76 1.07 -- 22.4 27.2 -11.5 -12.0 19.3 2.4D-263 D-75 SJ-150 UZLF .13 .31 -- .13 .50 25.5 23.6 -9.7 -11.2 25.4 .8D-263 D-75 Seawater UZLF .10 .32 -- .07 .53 23.5 23.6 -9.4 -11.2 25.4 .8
Origins of Mineralized Waters 53

For wells tapping only the UZLF, the mixtures were

98.8 to 99.9 percent ambient water. The only apparent

exception to this generalization is well MW-2, where

the water from SJ-150 more accurately simulated the

observed 14C value than did modern seawater.

The dominant reaction simulated is dedolomitization (dolomite dissolution and calcite precipitation)

driven by gypsum dissolution. This generally is consistent with the chemical reactionsthat would be expected

to occur as ground water flows through the Floridan

aquifer system. For some wells, however, this reaction

may not be realistic. The samples from wells D-673

(UFA), D-275 and N-22 (both UFA and UZLF), and

D-1155 (UZLF) are undersaturated with respect to calcite, precluding the precipitation of calcite. This could

be because water from well D-326 does not represent

the initial freshwater end member for water from wells

D-673, D-275, and N-22 (and,similarly, that well D-75

is not the initial freshwater end member for water from

well D-1155). Alternatively, there could be another

Ï initial waterÓ involved in the evolution of the observed

final water but not simulated in these models. This possibility is especially likely for the wells with large

open-hole intervals.

Some other discrepancies exist between the simulated and observed water chemistry. For example,

simulated values for 14C for wells SE-1 and D-291 are

poor regardless of which mineralized end member is

used. This also is true for water from wells D-262 and

D-263. Finally, many of the models include CO2 ingassing. Models with CO2 ingassing are considered to be

unrealistic in an aquifer discharge area (Sacks and

Tihansky, 1996). Small amounts of CO2 (a few tenths

of a millimole per liter) could be explained by uncertainties in the analytical data; however, larger values

can be attributed to uncertainties in the sulfur isotopic

composition of gypsum in the aquifer (Plummer and

others, 1990), which could, in turn, affect the models

for sulfur isotopic composition.

Ground-Water Ages

Ground-water ages were estimated based on the

carbon-14 composition of total inorganic carbon in

the water samples. For most samples, the ages were

estimated with NETPATH using the Fontes and

Garnier (1979) model, which assumes a simple mixing of the three carbon phases (gaseous CO2 and solid

and aqueous carbonate) in the ground-water system.

The Fontes and Garnier model is applicable to

present-day recharge in the karst of the Floridan

aquifer system; it, therefore, is assumed that the

model is applicable to past conditions. In the models,

the following δ13C values were assumed:  soil gas

CO2 of -23 per mil; carbonate minerals in the aquifer

of 1.0 per mil; CO2 gas at all wells of -23 per mil; and

water at each well was given the measured value.

The calculated 14C ages, using the Fontes and

Garnier model, ranged from about 9,500 years at well

N-22 to about 35,700 years at well D-3841 (table 8,

fig. 24). Both wells tap both the UFA and the UZLF.

For wells tapping the UFA only, ages ranged from

about 10,500 at well SJ-3 to 32,000 at well SJ-103.

Wells tapping the UZLF only had ages ranging from

about 13,200 years for the sample from a depth of

1,078 ft in well D-536 to 32,400 years for the sample

from well D-263.

Errors in the determination of 14C values will

affect the ages calculated. In addition to possible sample variability, two processes are subject to error:  the

laboratory preparation of targets and the AMS analysis. Of the two, the target preparation probably has

more potential for the introduction of error. The AMS

determinations of percent modern carbon had errors

ranging from 0.05 to 0.19 percent. The effects of

AMS errors on age determinations were negligible.

To determine the effect of target preparation errors,

two duplicate samples were analyzed for 14C. The

duplicates were collected at wells D-326 (chloride

concentration 5.6 mg/L and lab alkalinity 99 mg/L)

and D-307 (chloride concentration 130 mg/L and lab

alkalinity 149 mg/L). For well D-307 the percent

modern carbon values were 1.37 and 1.33, resulting in

calculated ages of 26,700 and 27,100 years, respectively (a difference of about 400 years). The differences

were greater for well D-326:  1.04 and 2.90 percent

modern carbon, with calculated ages of 29,200 and

20,900 years, respectively (a difference of about

8,300 years). To be conservative, 14C ages could be

considered to be accurate to + 4,000 years.

Trends in the areal distribution of 14C ages are

difficult to discern. Relatively young waters were

found in wells SJ-3, SJ-168, and D-3847, in the

southern part of the study area, suggesting the presence of a different flow path than that supplying

most of the water to wells in Duval County (figs. 1,

5b, and 24). The youngest age calculated was for

water from well N-22, located in an area of heavy

industrial pumping just to the north of the study area.
54 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Table 8.  Carbon-14 ages for ground-water samples

[

14C, carbon-14; δ 13C, delta carbon-13; dup, duplicate; na, not analyzed. Aquifer:  UFA, Upper Floridan aquifer; B, both

Upper Floridan and upper zone of Lower Floridan aquifers; UZLF, upper zone of Lower Floridan aquifer; FPZ, Fernandina

permeable zone. FG, Fontes and Garnier (1979) model; T, Tamers (1975) model; uncertainty in 14C ages is + 4,000 years]

Site identification

number

Local

number Aquifer

14C

(percent

modern)

14C error

(percent)

δ 13C

(per mil)

14C FG age

(years)

14C T age

(years)

300019081363301 SJ-3 UFA 3.37 0.11 -3.48 10,500 22,800

300347081363701 SJ-168 UFA 3.5 .12 -3.82 11,100 22,400

301304081222701 SJ-103 UFA 0.56 .05 -6.98 32,000 38,000

301339081531201 D-326 UFA 2.90 .09 -9.22 20,900 23,800

301339081531201 D-326dup UFA 1.04 .05 -9.02 29,200 32,200

301547081271901 SE-1 UFA 1.47 .06 -11.02 28,100 29,600

301714081233301 D-72 UFA 2.12 .05 -7.26 21,400 26,900

301830081375501 D-1551 UFA 1.23 .05 -7.9 26,600 31,100

302013081353801 D-673 UFA 1.30 .06 -9.4 27,600 31,400

302058081244101 D-1107 UFA 1.79 .07 -7.97 23,600 28,000

302113081322301 D-655 UFA 1.86 .07 -8.96 24,300 27,600

302140081235501 D-1121 UFA 1.48 .06 -9.24 26,500 29,700

302239081434401 NK-1 UFA 1.24 .05 -7.82 26,500 31,000

302557081253101 D-913 UFA 1.34 .08 -10.3 28,200 31,000

302608081354903 D-264 UFA 2.35 .08 -7.3 20,600 25,800

302609081260601 D-912 UFA 2.0 .07 -10.66 25,200 27,100

300856081382401 D-3847 B 4.59 .10 -4.87 11,400 20,200

301405081325601 D-3846 B .68 .05 -6.79 30,100 36,100

301508081364601 BW-3 B 1.56 .09 -6.04 22,200 29,000

301704081233401 D-484 B 2.28 .09 -9.37 23,000 26,400

301725081305002 D-650 B .83 .06 -8.48 30,400 34,900

301740081361001 D-275 B 1.95 .08 -9.09 24,000 28,000

301743081303501 D-3825 B 2.46 .07 -8.37 21,400 25,500

301743081362301 D-225 B 2.19 .08 -9.29 23,200 26,500

301743081362301 D-225 B 1.81 .09 -8.22 23,700 27,900

301748081383401 D-334 B .97 .06 -7.95 28,600 33,200

301758081303901 D-665 B 3.64 .13 -9.01 18,800 21,800

301817081374901 D-425T B .88 .06 -8.12 29,600 34,100

301846081240201 D-246 B 2.96 .08 -7.67 19,100 24,100

301934081292301 R-3 B 1.54 .06 -8.64 25,600 29,000

301937081471401 D-2223 B 1.41 .06 -8.62 26,200 30,000

301957081342301 D-313 B 1.99 .08 -10.12 24,800 27,500

302007081353201 D-479 B 1.12 .08 -9.88 29,300 32,300

302008081242101 D-307 B 1.33 .05 -9.02 27,100 30,600

302008081242101 D-307dup B 1.37 .06 -8.80 26,700 30,300

302015081384501 D-335 B .98 .06 -9.08 29,700 33,400

302045081323101 D-1323 B .91 .05 -10.11 31,200 34,000

302342081320601 D-1902 B 2.02 .07 -9.56 24,200 27,200

302503081332001 D-1149 B .62 .05 -11.22 35,300 37,200

302510081260201 D-3841 B .53 .05 -10.05 35,700 38,000

302538081392501 D-329 B 1.37 .07 -11.57 29,000 30,500

303940081281801 N-22 B 9.96 .19 -8.23 9,500 16,100

301335081355001 D-536-1a

a

Sampling depth 1,042 ft below land surface.

UZLF 2.2 .08 -5.98 19,300 26,000

301335081355001 D-536-2b

b

Sampling depth 1,059 ft below land surface.

UZLF 2.32 .09 -6.38 19,400 25,700

301335081355001 D-536-3c

c

Sampling depth 1,078 ft below land surface.

UZLF 4.07 .11 -5.35 13,200 21,200

301522081331301 D-291d

d

Sampling depth 1,210 ft below land surface.

UZLF 3.05 .10 -8.6 19,900 23,400

301537081441901 D-75 UZLF 3.09 .10 -8.53 19,700 23,200

301604081361501 D-450 UZLF 1.54 .07 -7.99 24,900 29,000

301639081330802 D-1155 UZLF 1.95 .08 -8.11 23,000 27,900

301936081292401 MW-2 UZLF .96 .05 -8.28 29,100 32,200

302022081393501 D-176 UZLF 3.07 .10 -7.79 19,000 23,400

302227081435001 D-592 UZLF 2.47 .09 -8.99 22,000 25,200

302608081354901 D-262 UZLF 2.48 .09 -11.96 24,400 25,200

302608081354902 D-263 UZLF .88 .06 -11.22 32,400 34,000

301132081225801 SJ-150 FPZ 1.31 .07 -6.3 na na

301405081325601 D-425B FPZ 1.27 .07 -7.19 25,500 30,600

301725081305002 D-3060 FPZ 1.63 .07 -9.28 na na

301743081303501 D-2386 FPZ 1.68 .08 -9.83 na na

302737080571801 Seawater 109.96 .55 -1.75 na na
Origins of Mineralized Waters 55

Figure 24. Carbon-14 ages of ground-water samples.
56 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Much of the water discharged from well N-22 possibly

follows flow paths from recharge areas in southeastern

Georgia rather than from the Keystone Heights

recharge area. Carbon-14 ages also are apparently not

related to chloride concentration (fig. 25). In general,

water samples from wells in the southern part of the

study area have younger ages, heavier δ13C values, and

have undergone more extensive dedolomitization reactions; however, this could be related to the distribution

of gypsum in the aquifer, rather than to residence time

in the aquifer.

The simulated agesfor mostsamplesranged from

about 20,000 to about 30,000 years, regardless of

chloride concentration. The samples having the oldest

simulated ages, from wells D-3841 and D-1149, were

north of the St. Johns River and could represent a flow

divide between pumping centers in Nassau and Duval

Counties, along which older water has not been intercepted by pumping. Because the ages calculated are

approaching the upper limits of the 14C age dating technique, the water could in fact be older than estimated.

At Ft. George Island and in the area of well D-665,

waters with the lowest chloride concentrations have the

oldest calculated 14C ages. For example, at

Ft. George, water from wells D-3841 (chloride 16 mg/L)

and D-1149 (chloride 22 mg/L), a few miles to the

west, both have ages of about 35,000 years. Water from

well D-913 (chloride 380 mg/L) has an apparent age of

about 28,000 years. Similarly, water from well D-665

(chloride concentration 830 mg/L) has an apparent age

of about 19,000 years, but waterfrom nearby well D-650

(chloride 64 mg/L) has an apparent age of about

30,000 years. In other areas, water with a higher chloride concentration has an older age. Water from well

D-1323 (chloride 270 mg/L), about 3.5 miles north of

well D-665, has an apparent age of about 30,000 years;

whereas, water from the nearby UFA well D-655

(chloride 34 mg/L) has an age of about 24,000 years

(although given the error range, this difference may not

be significant).

The relation between 14C age and strontium

concentration also was considered (fig. 25).

Apparently, residence time in the aquifer (Ï ageÓ ) is not

related to strontium concentration; the wells with water

having the youngest apparent ages (N-22, SJ-3, and

SJ-168) had a variation of more than an order of magnitude in strontium concentration. Also, other wells

with relatively low strontium concentrations (D-264,

D-262, D-912, D-329, D-263, and D-1149) had ages

ranging from about 20,000 to more than 35,000 years.

Thus, the strontium concentrations probably relate to

aquifer mineralogy.

Carbon-14 ages for the mineralized samples

from the FPZ are more difficult to calculate because the

initial 14C activity at the time the water entered the

aquifer is unknown. If it is assumed that the FPZ water

began as seawater left from a higher stand of sea level,

then NETPATH could be used to simulate reactions

between seawater and aquifer materials that could

result in the observed water in the FPZ. Several models

were found but none was entirely satisfactory in simulating the water from well SJ-150, particularly with

regard to carbon isotope values(L.N. Plummer, USGS,

written commun., 2000). Reasons for the discrepancy

could include errors in the chemical analysis or

the possibility that the sample has mixed with saline

water that differsin chemical and isotopic composition

from seawater. Attempts to simulate mineralized water

in the LFA in southwestern Florida also have been

problematic (L.A. Sacks, USGS, written commun.,

2001). This suggests that saline water underlying both

northeastern and southwestern Florida may have had

a similar chemical history; however, a solution to this

problem is beyond the scope of the present study.

Based on the low values of percent modern carbon (less than 2 percent) and the light values of δ13C

(-6 to -9 per mil), the FPZ waters are old. It islikely that

the mineralized waters from the FPZ are too old to be

dated using 14C. Sea level has not been higher than at

present during approximately the last 125,000 years

(Chappell and Shackleton, 1986). According to their

data, during the period 25,000-30,000 years ago, sea

level was about 150 feet lower than at present.

In summary, 14C data indicate that most ground

water in northeastern Florida was recharged to the

aquifer about 25,000-30,000 years ago, although some

samples indicate recharge about 10,000 years ago.

Mineralized water from the FPZ is old (probably

greater than 25,000 years). Water samples from wells

D-665 and D-913, which had the highest chloride concentrations of all non-FPZ wells, had slightly younger

ages than samples from nearby wells having lower

chloride concentrations. In well D-536, however, the

sample with the highest chloride water had the youngest age. These data could indicate at least two different

sources of mineralized water in northeastern Florida.
Origins of Mineralized Waters 57

Figure 25.  Relations between chloride concentration and carbon-14 age, and between strontium concentration

and carbon-14 age.
58 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

ReconcilingHydrogeologicandGeochemical

Evidence

Previous investigations (Spechler, 1994; Phelps

and Spechler, 1997) have documented the distribution

of increasing chloride concentrations in ground water

in northeastern Florida. Several sources of the mineralized water and mechanisms of movement have been

postulated:  lateral movement of the freshwater-saltwater interface from the Atlantic Ocean (modern seawater intrusion); unflushed connate water (from the

time of sediment deposition) or relict seawater from

high sea level stands in the geologic past that was

trapped in the upper, freshwater-bearing zones of the

aquifer; or upconing of mineralized water from deeper

zones of the aquifer or from underlying formations. In

northeastern Florida, upward movement of mineralized water from deeper aquifer zones probably occurs

through preferential structural pathways such as

joints, fractures, faults, or paleokarst features rather

than on a regional scale in response to lowering of the

potentiometric surface.

Modern seawater, if intruding into the aquifer

from the Atlantic Ocean, would be found in most or all

wells near the coast and in few or no wells farther

inland. This is not the case in northeastern Florida.

Unflushed connate or relict seawater in upper, freshwater-bearing zones would be expected to be found in

zones of low transmissivity. In northeastern Florida,

mineralized water was more likely to be found in high

transmissivity or fracture flow zones than in low transmissivity zones (Spechler, 1994; Phelps and Spechler,

1997). Connate water would be expected to have major

ion concentrations similar to or higher than seawater,

strontium isotope data indicating an Eocene age, and

14C activity near zero. The most mineralized watersampled during the study, however, had major ion concentrations that could not be accounted for by model

reactions of seawater with aquifer rock, strontium isotope ratios generally in the Oligocene-age range, and

measurable 14C activity. It is possible that upconing of

mineralized water through thin or permeable confining

layersin response to increased pumping is the source of

the observed chloride increases in some wells in Duval

County; information is sparse about the thickness and

vertical hydraulic conductivity of the semi-confining

layersseparating the upper, freshwater-bearing zones of

the Floridan aquifer system from deeper zones. Many

wells in the Jacksonville area, however, have been

pumped for as long as 60 years with no increase in

chloride concentration (Phelps and Spechler, 1997,

p. 41-42), whereas others have increasing chloride concentrations after only a few years.

The geochemical and isotopic data indicate that

the likely source of increased mineralization in some

water samples from the upper zones of the Floridan

aquifer system is mineralized water from the FPZ. The

assumption is that mineralized water moves upward in

response to decreased heads through interconnecting

vertical and horizontal fracture or solution zones probably developed along deep, subsurface paleokarst features. Geophysical logging and surface geophysical

investigations have described the occurrence of fracture

flow zones in wells and subsurface paleokarst features

near several wells that have increasing chloride concentrations. This mineralized water could be relictseawater

that was not flushed from the aquifer during lower

stands of sea level, perhaps because the thick sequence

of overlying carbonates and confining units prevented

flushing during the peak of Pleistocene glaciation, when

the sea level was much lower than present day. However, the fact that the chemical and isotopic composition

of the FPZ water cannot be simulated by simple reactions ofseawater with aquifer rocksindicatesthat either

some other, more complex, rock-water reactions are

occurring, or that the water could be affected by a brine,

probably from formations beneath the FPZ.

Waterfrom well SJ-150 probably represents, both

chemically and isotopically, the mineralized water end

member affecting water from some wells with increasing chloride concentrations (such as well D-665). Both

the δ13C and strontium data (figs. 17 and 20) indicate

that although water from well SJ-150 is chemically

similar to modern seawater, it is isotopically different,

precluding the possibility that the source of mineralized

water is modern seawater intrusion. The 14C data also

point to old, rather than modern, seawater as the source

of mineralized water. Waters from the other two high

chloride FPZ wells (D-3060 and D-2386) also are isotopically different from modern seawater. All of the samplesfromthe FPZ are saturated with respect to calcite and

both crystalline and disordered dolomite, indicative of

long residence time in the aquifer. For example, the

chemistry of water samples with low chloride and strontium concentrations, such as those from wells D-329,

D-264, and D-1149, could result from the residual

effects of gradual flushing of seawater from the aquifer

system by freshwater (figs. 19 and 20). Thus, it islikely

that there are at least two sources of mineralization for

ground-water samples in northeastern Florida.
Summary 59

The water sampled from well D-326, in western

Duval County, probably most nearly represents the

chemistry of ambient ground water in the Floridan

aquifer system. The water has very low concentrations

of dissolved constituents and is at equilibrium with

respect to calcite and crystalline dolomite, probably

indicating a shorter residence time in the aquifer

(because the area is closer to the recharge area), but

perhaps also indicating a difference in aquifer mineralogy in that area.

The geochemical modeling program NETPATH

was used to calculate proportions of the respective end

members that would produce water of the composition

found in the water samples analyzed. Water from most

of the wells sampled is a mixture of about 99 percent

recharge water and less than 1 percent water with the

chemical composition of water from well SJ-150. The

sample with the highest chloride concentration (from

well D-665) is about 95.5 percent recharge water and

about 4.5 percent water chemically similar to that from

well SJ-150. The small percentage of mineralized

water present in most samples makes it difficult to find

a unique solution to the problem of estimating the

chemical and isotopic composition, and thus, the

source of the mineralized water.

Based on the potentiometric surface (figs. 5a and

5b) and on the chemical and isotopic data, there may be

at least two different flow systems in northeastern

Florida. For example, wells north of the St.Johns River

have relatively low sulfate and strontium concentrations, heavy δ34S content, and light values of δ13C. The

distribution of 14C ages for that area (fig. 24) apparently indicates that this may be due to areal differences

in aquifer mineralogy, rather than differences in aquifer residence time. In these wells north of the St.

Johns River, the data do not clearly indicate whether or

not the mineralized water from the FPZ is the source of

increased chloride concentrations; however, strontium

data (fig. 20) indicate that waters in that area could be

mixing with higher chloride water from a source geochemically different from the water from well SJ-150.

The source of higher chloride water to these wells

could be pockets of unflushed seawater from higher

stands of sea level during Miocene to Pleistocene time.

SUMMARY

Increasesin the chloride concentration have been

observed in the water from numerous wells in northeastern Florida. Most of the increases have been in the

eastern part of Duval County, but a pattern in the locations of wells with chloride increases is not readily

discernible. Possible sources of the mineralized water

include lateral intrusion of modern seawater; unflushed

connate water in aquifer sediments or relict seawater

from higher stands of sea level in the past; or upconing of mineralized water from formations beneath the

Floridan aquifer system. Chemical and isotopic data

were used to differentiate water samples from

various wells and infer the source of the increased

mineralization observed.

Water samples were collected from 53 wells in

northeastern Florida during 1997-99. Wells tapped

various zones of the aquifer including:  the Fernandina

permeable zone (FPZ), the upper zone of the Lower

Floridan aquifer (UZLF), the Upper Floridan aquifer

(UFA), and both the UFA and the UZLF. Some wells

had large open-hole intervals, which could have

complicated the analysis of the chemical data. Water

samples were analyzed for major ions and trace constituents and for isotopes of carbon, oxygen, hydrogen,

sulfur, strontium, chlorine, and boron. Samples of rock

from the aquifer also were analyzed for isotopes of

oxygen, carbon, and strontium.

In general, water from various aquifer zones

cannot be differentiated based on chemistry; major-ion

concentrations vary as much within a particular zone as

among zones. Three of the four wells tapping the FPZ

had sodium-chloride-type water. Waterfrom well SJ-150

was chemically similar to seawater except for higher

concentrations of calcium, strontium, and sulfate,

which probably are the result of dissolution of aquifer

materials. Most of the samples from the UZLF were

calcium-magnesium sulfate-bicarbonate-type water.

Chloride concentrations did not show an increasing

trend with depth. The water with the highest chloride

concentrations in the UZLF came from fracture flow

zones and was collected by using a thiefsampler at one

well. Water samples from wells tapping both the

UZLF and the UFA had widely varying chemistries.

Some wells yielded calcium-magnesium-bicarbonatetype water with low specific conductance, whereas

others were sodium-chloride type and calciumsodium-magnesium chloride-sulfate-type waters. The

water from well D-665 was chemically more similar to

the high chloride water of the FPZ than to water from

wells tapping the UZLF. The water chemistry from the

UFA also varied widely. The sample from D-326 had

very low concentrations of dissolved constituents and

probably closely resembles recharge water. Other

wells had low chloride concentrations but very high

sulfate concentrations.
60 Geochemistry and Origins of Mineralized Waters in the Floridan Aquifer System, Northeastern Florida

Saturation indexes calculated for various minerals based on water chemistry can give an indication

of the residence time of water in the aquifer. All of the

water samples from the FPZ were supersaturated with

respect to calcite, and both crystalline and disordered

dolomite, which is consistent with a long residence

time. Most of the samples from the UZLF were supersaturated with respect to calcite and crystalline dolomite, and at equilibrium with respect to disordered

dolomite. Wells tapping both the UZLF and the UFA

and those tapping only the UFA had more diverse saturation indexes. Most were at equilibrium with respect

to calcite, but dolomite saturation varied.

Hydrogeologic and geochemical data from water

and aquifersamplesindicate that the most likely source

of mineralized water in some wells yielding water with

increasing chloride concentrations is water from the

FPZ. In other wells, flushing of Miocene-to-Pleistoceneage seawater can account for the observed chloride

concentrations.The fact that most of the water samples

collected are a mixture of less than 1 percent of the

mineralized type water with more than 99 percent fresh

or recharge water makes identifying the source of the

mineralized water difficult. Differences in carbon-13

and sulfur-34 values probably reflect areal differences

in aquifer mineralogy and distribution of organic carbon related to paleokarst features. Geochemical massbalance models ofseawater-rock interaction are unable

to account for the chemical and isotopic composition of

mineralized water from the FPZ, which implies another

source of mineralized water, such as a brine, or the

occurrence of more complex water-rock reactions.

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