Estimated Discharge and ChemicalConstituent Loading from the Upper Floridan 

Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

By R.M. Spechler

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 94-4132

Prepared in cooperation with the

U.S. ARMY CORPS OF ENGINEERS

Tallahassee, Florida

1995
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

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

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

For additional information Copies of this report can be

write to: purchased from:

District Chief U.S. Geological Survey

U.S. Geological Survey, WRD Earth Science Information Center

Suite 3015 Open-File Reports Section

227 North Bronough Street Box 25286, MS 517

Tallahassee, FL 32301 Denver Federal Center

Denver, CO 80225
Contents III

CONTENTS

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

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

Purpose and Scope....................................................................................................................................................... 2

Previous Investigations................................................................................................................................................ 2

Lower St. Johns River Drainage Basin.................................................................................................................................. 2

Description of the Lower St. Johns River.................................................................................................................... 4

Geologic Framework ................................................................................................................................................... 4

Hydrogeology .............................................................................................................................................................. 5

Potentiometric Surface of the Upper Floridan Aquifer ............................................................................................... 6

Water Quality.........................................................................................................................................................................12

Lower St. Johns River .................................................................................................................................................12

Upper Floridan Aquifer ...............................................................................................................................................13

Dissolved Solids ................................................................................................................................................13

Chloride .............................................................................................................................................................13

Sulfate................................................................................................................................................................13

Springs .........................................................................................................................................................................17

Estimated Ground-Water Discharge from the Upper Floridan Aquifer ................................................................................17

Estimated Chemical-Constituent Loads in Ground-Water Discharge ...................................................................................21

Summary................................................................................................................................................................................24

Selected References...............................................................................................................................................................30

FIGURES

1. Map showing location of study area ...................................................................................................................... 3

2. Diagram showing generalized geology and hydrogeology of northeastern Florida .............................................. 5

3-18. Maps showing:

3. Generalized thickness of the intermediate confining unit .............................................................................. 7

4. Altitude of the top of the Floridan aquifer system ......................................................................................... 8

5. Approximate extent of natural recharge and discharge areas in the Upper Floridan aquifer......................... 9

6. Potentiometric surface of the Upper Floridan aquifer, September 1990........................................................10

7. Potentiometric surface of the Upper Floridan aquifer, September 1991........................................................11

8. Generalized distribution of dissolved-solids concentrations in water from the Upper Floridan aquifer .......14

9. Generalized distribution of chloride concentrations in water from the Upper Floridan aquifer....................15

10. Generalized distribution of sulfate concentrations in water from the Upper Floridan aquifer ......................16

11. Study area model boundary and grid..............................................................................................................19

12. Estimated leakance coefficients of the intermediate confining unit...............................................................20

13. Estimated discharge from the Upper Floridan aquifer through the intermediate confining unit, 

September 1990..............................................................................................................................................22

14. Relation of total discharge and chemical-constituent load to change in Upper Floridan aquifer 

potentiometric surface....................................................................................................................................23

15. Estimated dissolved-solids flux, September 1990..........................................................................................25

16. Estimated chloride flux, September 1990 ......................................................................................................26

17. Estimated sulfate flux, September 1990.........................................................................................................27

18. Location of zones used to compute cumulative constituent load and flow of Upper Floridan 

aquifer water discharging into the surficial aquifer system ...........................................................................28

l9. Graph showing cumulative constituent load and flow of Upper Floridan aquifer water discharging 

into the surficial aquifer system, September 1990.........................................................................................29
IV Contents

TABLE

l. Average discharge, loads, and concentrations of selected chemical constituents of springs from the Upper Floridan 

aquifer in and near the study area ...................................................................................................................................

CONVERSION FACTORS VERTICAL DATUM, AND ADDITIONAL ABBREVIATIONS

Equations for temperature conversion between degrees Celsius (×°C) and degrees Fahrenheit (°×F):

×C = 5/9 x (×°F - 32)

×F = (9/5 °C) + 32

Sea level: In this report “sea level” refers to the National Geodetic Vertical Datum of 1929--a geodetic datum

derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called

Sea Level Datum of 1929.

Additional abbreviations

μS/cm microsiemens per centimeter at

 25 degrees Celsius (25 ×°C)

mg/L milligrams per liter

Acronyms

RASA Regional Aquifer-System Analysis

USGS U.S. Geological Survey

Multiply By To obtain

Length

inch (in.) 25.4 millimeter

feet (ft) 0.3048 meters

miles (mi) 1.609 kilometers

Area

square miles (mi2) 2.590 square kilometers

Flow

cubic feet per second (ft3/s) 0.02832 cubic meters per second

inch per year (in/yr) 25.4 millimeters per year

Mass

tons per year per square miles [(ton/yr)/mi2] .3503 megagrams per year per square kilometers

ton per year (ton/yr) .9072 megagrams per year

Leakance

feet per day per feet [(ft/d)/ft] 1.000 meters per day per meter
Introduction 1

Estimated Discharge and Chemical-Constituent Loading 

from the Upper Floridan Aquifer to the Lower St. Johns 

River, Northeastern Florida, 1990-91 

Rick M. Spechler

Abstract

The lower St. Johns River, a 101-mile long 

segment of the St. Johns River, begins at the confluence of the Ocklawaha River and ends where 

the river discharges into the Atlantic Ocean at 

Mayport. The St. Johns River is affected by tides 

as far upstream as Lake George, 106 miles from 

the mouth. Saltwater from the ocean advances 

inland during each incoming tide and recedes 

during each outgoing tide. The chemical quality 

of the lower St. Johns River is highly variable 

primarily because of the inflow of saltwater from 

the ocean, and in some areas, from the discharge 

of mineralized ground water.

Three hydrogeologic units are present in 

the study area: the surficial aquifer system, the 

intermediate confining unit, and the Floridan 

aquifer system. The surficial aquifer system overlies the intermediate confining unit and consists 

of deposits containing sand, clay, shell, and some 

limestone and dolomite. The intermediate confining unit underlies all of the study area and retards 

the vertical movement of water between the surficial aquifer system and the Floridan aquifer system. The intermediate confining unit consists of 

beds of relatively low permeability sediments that 

vary in thickness and areal extent and can be 

breached by sinkholes, fractures, and other openings. The Floridan aquifer system primarily consists of limestone and dolomite.

The quality of water in the Upper Floridan 

aquifer varies throughout the study area. Dissolved solids in water range from about 100 to 

more than 5,000 milligrams per liter. Chloride 

and sulfate concentrations in water from the 

Upper Floridan aquifer range from about 4 to 

3,700 milligrams per liter and from 1 to 1,300 

milligrams per liter, respectively.

The rate of leakage through the intermediate confining unit is controlled by the leakance 

coefficient of the intermediate confining unit and 

by the head difference between the Upper Floridan aquifer and the surficial aquifer system. The 

total ground-water discharge from the Upper 

Floridan aquifer to the St. Johns River within the 

lower St. Johns River drainage basin, based on the 

potentiometric surface of the Upper Floridan 

aquifer in September 1990, was estimated to be 

86 cubic feet per second. Total estimated groundwater discharge to the lower St. Johns River in 

September 1991, when heads in the Upper Floridan aquifer averaged about 4 feet higher than in 

1990, was 133 cubic feet per second.

The load of dissolved-solids that discharged 

from the Upper Floridan aquifer into the lower 

St. Johns River on the basis of September 1990 

heads is estimated to be 47,000 tons per year. 

Estimated chloride and sulfate loads are 18,000 

and 9,500 tons per year, respectively. Dissolvedsolids, chloride, and sulfate loads discharging into 

the lower St. Johns River are estimated to be 

81,000, 39,000, and 15,000 tons per year, respectively, on the basis of September 1991 heads.

INTRODUCTION

Few data are available on the quantity of water 

from the Upper Floridan aquifer that discharges to the 

lower St. Johns River. This lack of data has caused 

difficulty for various State regulatory agencies in 

implementing management plans to control point and 

nonpoint sources of effluent discharged to the lower 

St. Johns River. For example, chloride and sulfate 

usually are present in effluent and ground water entering the river from the Upper Floridan aquifer. It is 

probable that in some areas of the lower St. Johns 
2 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

River, ground-water discharge from the Upper Floridan aquifer could be more mineralized than the riverwater. Therefore, it is possible that some constituent 

loads to the river contributed by ground-water discharge from the Upper Floridan aquifer are greater 

than those contributed by manmade sources.

The U.S. Geological Survey (USGS), in cooperation with the U.S. Army Corps of Engineers, began a 

study in 1992 to estimate the quantity and quality of 

Upper Floridan aquifer discharge to the lower St. 

Johns River. Information about the quantity and quality of ground-water discharge can be used by agencies 

concerned with evaluating point and nonpoint discharge to the St. Johns River. This information also 

can be used to assess the potential effects of urbanization on the quality of the river water.

Purpose and Scope

This report describes the quantity and quality of 

ground-water discharge to the lower St. Johns River 

from the Upper Floridan aquifer. A short overview of 

the geology and hydrology of the study area includes 

maps constructed from geophysical logs, and geologic 

and driller logs.

Potentiometric-surface maps of the Upper Floridan aquifer published in recent reports depict heads in 

the aquifer for September 1990 and September 1991. 

The information contained in these maps is the basis 

for simulating and evaluating ground-water discharge 

from the Upper Floridan aquifer to the surficial aquifer 

and to the St. Johns River using existing subregional 

RASA ground-water flow models.

Water-quality data from recent reports were 

used to map the distribution of dissolved solids, 

chloride, and sulfate in water in the Upper Floridan 

aquifer. The quality of ground-water discharge is 

reported in terms of loads of these constituents using 

the simulated discharge values. Total constituent loads 

in water discharging from the Upper Floridan aquifer 

are estimated in zones along the lower St. Johns River 

drainage basin.

Previous Investigations

The study area has been described in numerous 

statewide and regional hydrologic and geologic investigations, and in several reports specifically about 

Duval, St. Johns, Clay, and Putnam Counties. 

Regional reports describing the geology, hydrology 

and geochemistry of the Floridan aquifer system 

include Stringfield (1966), and various RASA reports 

by Miller (1986), Bush and Johnston (1988), Johnston 

and Bush (1988), Krause and Randolph (1989), 

Sprinkle (1989), and Tibbals (1990).

The geology of the study area has been 

described in reports by Vernon (1951), Puri (1957), 

Puri and Vernon (1964), and Chen (1965). Groundwater resources were described by Bermes and others 

(1963), Clark and others (1964), Leve (1966), Bentley 

(1977b), Frazee and McClaugherty (1979), and 

Spechler and Hampson (1984).

The quality of water from the Floridan aquifer 

system in northeastern Florida was described in 

reports by Cooper (1942, 1944), Bermes and others 

(1963), Leve (1966), Fairchild (1972), Fairchild and 

Bentley (1977), Munch and others (1979), Brown 

(1984), Toth (1990), and Spechler (1994). Some of 

these reports discuss the intrusion of saltwater into the 

Floridan aquifer system.

Anderson and Goolsby (1973) describe in detail 

the physical and chemical characteristics of the water 

in the lower St. Johns River. Spechler and Stone 

(1983) evaluated the interconnection between the 

St. Johns River and the surficial aquifer system along a 

25-mi section of the channel east of Jacksonville, but 

the discussion did not include the evaluation of discharge from the Upper Floridan aquifer. The U.S. 

Army Corps of Engineers has prepared many internal 

reports on several aspects of the lower St. Johns River; 

however, many primarily pertain to navigational 

improvements along the channel.

LOWER ST. JOHNS RIVER DRAINAGE 

BASIN

The study area encompasses the drainage basin 

known as the lower St. Johns River basin (fig. 1). The 

lower St. Johns River drainage basin is approximately 

2,600 mi2

 in area and consists of the 101-mi long 

section of the St. Johns River (herein referred to as the 

lower St. Johns River) that begins at the confluence 

with the Ocklawaha River and ends where the 

St. Johns River discharges into the Atlantic Ocean at 

Mayport.

Basin land peripheral to the lower St. Johns 

River is drained by 12 major tributaries. The river 

also receives surface runoff and drainage from minor 

tributaries within its own hydrologic subbasin.

Landscape features within the lower St. Johns 

River basin are relatively low and flat. Three ridge 

systems border the drainage area. Land surface altitudes range from sea level to greater than 200 ft in the 

western part of the basin.
Lower St. Johns River Drainage Basin 3

ha 

Oc

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

15′

NASSAU COUNTY

DUVAL COUNTY

LAKE COUNTY

10

10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

MARION COUNTY

ALACHUA COUNTY BRADFORD COUNTY

PUTNAM COUNTY

CLAY COUNTY Orange

Park

GreenCove 

Springs

Picolata

Riverdale

Hastings

Palatka

Johns

St. 

River

15′

29° 00′

30° 30′

30° 00′

15′

45′

30′

0

0

Mandarin

St. Augustine

Welaka

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

A T L A N T I C O C E A N

ST. JOHNS COUNTY

VOLUSIA COUNTY

wa kla

Jacksonville

International

Airport

Drummond

Point

Buffalo

Bluff

Salt

Springs

Croaker Hole Spring

Little Lake George

Satsuma Spring

Mud Spring

Nashua Spring

Forest Spring

Welaka Spring

Beecher Spring

Federal

Point

Mill

Cove

Main Street Bridge

Wadesboro

Spring

RAINFALL GAGING STATION

EXPLANATION

Boundary of lower St. Johns River 

drainage basin

Mayport

Ponte Vedra

Beach

Study

Area

Map

Area

Fort

Pierce

Lake

Monroe

Figure 1. Location of study area.
4 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

Description of the Lower St. Johns River

The headwaters of the St. Johns River occurs 

near Ft. Pierce, more that 300 mi from its mouth. The 

river flows on a generally northward course to Jacksonville and then eastward to the ocean. In the study 

area, the St. Johns River passes through four counties 

that include most of the northeastern part of the state.

From the confluence with the Ocklawaha River 

to Palatka, the lower St. Johns River generally ranges 

in width from about 600 ft to one-half mile. In the 

reach from Palatka to Jacksonville, the river widens 

and ranges from about l to 3 mi. From Jacksonville to 

the mouth of the river at Mayport, the width of the 

river ranges from about 1,250 ft at the Main Street 

Bridge to more than 2 mi at Mill Cove.

Navigation is one of the primary uses of the 

St. Johns River. A navigational channel in the river, 

maintained by the U.S. Army Corps of Engineers, is 

about 12 ft deep and 100 ft wide from the Ocklawaha 

River to Palatka; about 13 ft deep and 200 ft wide 

from Palatka to Jacksonville; and about 40 ft deep and 

400 to 900 ft wide from Jacksonville to the ocean. 

Scouring by river currents near some bridge pilings 

has deepened the channel to more than 90 ft.

The St. Johns River is affected by tides as far 

upstream as Lake George, about 106 mi from the 

mouth. Occasionally, combined wind and tidal effects 

can influence river stages and flow as far as Lake 

Monroe, about 161 mi upstream (Anderson and 

Goolsby, 1973, p. 9). The tidal range averages about 

4.9 ft at the mouth of the river and varies unequally 

upstream because of channel geometry. Normal tidal 

range is about 1.2 ft at Jacksonville and about 0.7 ft at 

Orange Park; from Orange Park southward, the amplitude of the tidal wave increases gradually to about 

1.2 ft at Palatka; and from Palatka to Lake George, the 

amplitude again decreases and approaches zero near 

the outlet of Lake George.

Because of the tidal nature of the river, accurate 

measurements of streamflow are difficult to obtain and 

subject to error. Therefore, ground- water discharge to 

the lower St. Johns River cannot be quantified by the 

standard technique of making multiple streamflow 

measurements under low-flow conditions and computing ground-water discharge by determining differences in discharge between measurement sites. 

Discharge records for several sites along the river are, 

at best, poor. Net discharges above the mouth of the 

Ocklawaha River and at the Atlantic Ocean near Mayport are estimated to be 4,000 and 8,300 ft3

/s, respectively (Snell and Anderson, 1970, p. 35, 44). Average 

discharge at Palatka during 12 years of record is estimated at 6,000 ft3

/s (U.S. Geological Survey, 1983, 

p. 124). The average discharge of the St. Johns River 

at the Main St. Bridge in Jacksonville is estimated 

(from 22 years of record) to be 5,700 ft3

/s (U.S. Geological Survey, 1992, p. 132).

Geologic Framework

The study area is underlain by a thick sequence 

of sedimentary rocks that overlie a basement complex 

of metamorphic rocks. Major stratigraphic units and 

corresponding hydrogeologic units in northeastern 

Florida include sedimentary rocks ranging in age from 

late Paleocene to Holocene (fig. 2). Stratigraphic units 

of interest, in ascending order are: Cedar Keys Formation of late Paleocene age, 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 the 

undifferentiated deposits of late Miocene to Holocene 

age.

Subsurface structures (such as collapse features 

and related fractures, joints, and faults) can play a significant role in the quantity and quality of ground 

water discharging from the Upper Floridan aquifer to 

the lower St. Johns River. Previous investigators have 

inferred the presence of several faults within the study 

area. Leve (1978, 1983, p. 255) postulated two generally northward-trending faults located primarily in 

Duval County. The easternmost inferred fault extends 

from northeastern Duval County south to a few miles 

beyond the Duval-St. Johns County boundary. The 

westernmost inferred fault approximately follows the 

St. Johns River and extends from north-central Duval 

County south toward Green Cove Springs in Clay 

County. Fairchild (1977, p. 23) inferred the presence 

of two smaller faults just west of the St. Johns River in 

extreme northeastern Clay County. A northsouth fault 

near Lake George in southern Putnam County was 

inferred by Bermes and others (1963, fig. 8) and 

extends northward into north-central Putnam County.

Circular depressions are present on the surface 

of the Ocala Limestone (Spechler, 1994). Some of the 

depressions could be erosional features formed before 

the Hawthorn Formation was deposited. However, 

some were formed by sinkhole collapse caused by the 

gradual dissolution of the underlying carbonate materials. Recent marine seismic-reflection investigations 
Lower St. Johns River Drainage Basin 5

along the St. Johns River in Duval County (Spechler, 

l994) and in south-central Florida (Snyder and others, 

1989) reveal buried collapse features that originated in 

the rocks of the Floridan aquifer system. Marine seismic-reflection profiles collected off the coast of northeastern Florida also show the presence of these buried 

collapse features (Meisburger and Field, 1976; and 

Popenoe and others, 1984).

Hydrogeology

Two aquifer systems are present in the study 

area- - the surficial aquifer system and the Floridan 

aquifer system. The two systems are separated by the 

intermediate confining unit, which includes most of 

the Hawthorn Formation. The intermediate confining 

unit contains beds of lower permeability sediments 

that confine the water in the Floridan aquifer system. 

In the northern part of the study area, the Floridan 

Series

Holocene 

to Upper

Miocene

Miocene

Eocene

Lower

Floridan aquifer system

Upper Floridan 

aquifer

Upper

zone

Fernandina

permeable zone

Lower Floridan 

aquifer

Middle Upper

Paleocene

Stratigraphic

unit

Hawthorn

Formation

Ocala

Limestone

Avon Park

Formation

Oldsmar

Formation

Cedar Keys

Formation

Undifferentiated

surficial deposits

Discontinuous sand, 

clay, shell beds, and

limestone

Interbedded

phosphatic sand,

clay, limestone, and

dolomite

Massive fossiliferous

chalky to granular

marine limestone

Alternating beds of

massive granular and 

chalky limestone, and

dense dolomite

Uppermost appearance

of evaporites; dense

limestones

General

lithology Hydrogeologic

unit

Intermediate

confining unit

Sub-Floridan

confining unit

Middle semiconfining 

unit

Semiconfining 

unit

Surficial aquifer

system

Hydrogeologic properties

Sand, shell, limestone, and

coquina deposits provide

local water supplies.

Principal source of

ground water. High permeability

overall. 

Low permeability limestone

and dolomite.

Principal source of ground

water. 

Low permeability limestone

and dolomite.

High permeability; salinity

increases with depth.

Contains highly saline water;

low permeability.

Sand, shell, and 

carbonate deposits provide 

limited local water supplies.

Low permeability clays serve as the 

principle confining beds for the

Floridan aquifer system below.

Figure 2. Generalized geology and hydrogeology of northeastern Florida (modified from Spechler, 1994).

aquifer system has three major water-bearing zones 

(the Upper Floridan aquifer, the upper zone of the 

Lower Floridan aquifer, and the Fernandina permeable 

zone) separated by less permeable semiconfining 

units. The major hydrogeologic units underlying the 

study area, their stratigraphic equivalent, and hydrogeologic properties are shown in figure 2.

The uppermost water-bearing unit is the surficial aquifer system. It primarily is composed of deposits of sand, clay, shell, and some limestone and 

dolomite of Holocene to upper Miocene age (fig. 2). 

The thickness of the surficial aquifer system is highly 

variable, ranging from about 20 to 160 ft throughout 

the study area.

The depth to the water table in the surficial aquifer system generally is less than 5 ft. In areas of higher 

relief, such as in the western part of the study area, the 

water table can be more than 15 ft below land surface. 

In areas of poor drainage and low topographic relief, 
6 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

the water table can be less than 1 ft below land surface. 

The water table fluctuates seasonally and varies within 

about a 5-ft range. Maximum depth to water occurs 

during the spring and early summer months. Water 

levels generally recover during the wet summer 

months and reach their annual high in September or 

October.

The surficial aquifer system yields small to 

moderate amounts of water to wells and is an important source of water supply in those areas where the 

Floridan aquifer system water is too highly mineralized for use. In the areas where the Floridan aquifer 

system is less mineralized, the surficial aquifer system 

is used only to supply small amounts of water for 

domestic and irrigation use.

The intermediate confining unit, which underlies the surficial aquifer system, primarily consists of 

the Hawthorn Formation of Miocene age. However, 

in the extreme southern part of the area where the 

Hawthorn Formation is thin or absent, other deposits 

of Miocene age or younger can act as a confining unit. 

The intermediate confining unit consists of interbedded clay, silt, sand, limestone, and dolomite. The 

thickness of the intermediate confining unit varies 

considerably: it ranges from 0 in southern Flagler and 

northern Volusia Counties where it is absent, to more 

than 500 ft in the central part of Duval County (fig. 3). 

The intermediate confining unit also can be locally 

breached by sinkholes where it is relatively thin, or by 

other openings that serve to connect the Floridan aquifer system directly with the river.

The Floridan aquifer system is a vertically continuous sequence of Tertiary-age carbonate rocks of 

generally high permeability that are hydraulically connected to each other in varying degrees (Miller, 1986, 

B45). The aquifer system consists of rock units which 

vary in age from late Paleocene to Eocene. The Floridan aquifer system ranges in thickness from 1,500 to 

2,000 ft and underlies all of the study area. It is the 

principal source of municipal, industrial and agricultural water supply for most of northeastern Florida. 

The top of the Floridan aquifer system ranges from 

less than 50 ft below sea level in the extreme southern 

part of the study area to more than 550 ft below sea 

level in parts of central Duval County (fig. 4).

The Floridan aquifer system in northeastern 

Florida is divided into the Upper Floridan aquifer and 

the Lower Floridan aquifer (fig. 2) which are separated 

by the a zone of lower permeability (Miller, 1986, 

p. B45). Two major water-bearing zones exist within 

the Lower Floridan: the upper zone of the Lower 

Floridan aquifer and the Fernandina permeable zone. 

These zones are separated by another less permeable 

semiconfining unit. The Fernandina permeable zone is 

thought to be absent in the southern part of the study 

area (Miller, 1986, p. B72). In this study, however, 

only hydrogeologic and chemical data obtained from 

the Upper Floridan aquifer are presented.

The principal recharge area of the Upper Floridan aquifer is in the western part of the study area. In 

this area, water enters the Floridan aquifer system by 

several means: by downward leakage where the water 

table is above the potentiometric surface of the Upper 

Floridan aquifer; through sinkholes and other features 

having enhanced permeability; and by lateral inflow 

from adjacent areas. Areas of discharge are present 

along the coast and throughout the lower St. Johns 

River basin (fig. 5). Discharge from the Floridan aquifer system is by diffuse upward leakage in areas where 

the potentiometric surface of the Floridan aquifer system is above the water table, by pumping and flowing 

wells, by springs, and by lateral outflow.

Potentiometric Surface of the Upper Floridan 

Aquifer

The potentiometric surfaces of the Upper 

Floridan aquifer for September 1990 and 1991 were 

selected for the estimate of discharge from the Upper 

Floridan aquifer to the lower St. Johns River. Groundwater levels in much of northeastern Florida were at or 

near record lows in September 1990 (fig. 6). Rainfall 

during this period was about 21 in. below normal at 

the Jacksonville International Airport and about 12 in. 

below normal at Federal Point near Palatka (National 

Oceanic and Atmospheric Administration, 1990) 

(fig. 1). Water levels in September 1991 (fig. 7) were 

about 2 to 6 ft higher than in September 1990 because 

of near-record rainfall in northeastern Florida. Rainfall 

was about 22 in. above normal at Jacksonville International Airport and about 17 in. above normal at 

Federal Point (National Oceanic and Atmospheric 

Administration, 1991).

The altitude of the potentiometric surface of the 

Upper Floridan aquifer ranged from about 75 ft above 

sea level in the western parts of Clay and Putnam 

Counties to less than 10 ft above sea level in the southern part of Flagler County in September 1990 (fig. 6), 
Lower St. Johns River Drainage Basin 7

100

100

0

50

50

150

150

300

250

200

300

250

200

350

500

350

400

450

INTERMEDIATE CONFINING UNIT--

Interval 50 feet

LINE OF EQUAL THICKNESS OF THE

EXPLANATION

100

ha wa

Ockla

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

15′

VOLUSIA COUNTY

10

10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

ST. JOHNS COUNTY

MARION COUNTY

ALACHUA COUNTY BRADFORD COUNTY

PUTNAM COUNTY

CLAY COUNTY Orange

Park

Picolata

Palatka

Johns

St. 

River

15′

29° 00′

30° 30′

30° 00′

45′

30′

NASSAU COUNTY

DUVAL COUNTY

VOLUSIA COUNTY

LAKE COUNTY

0

0

FLAGLER COUNTY

ST. JOHNS COUNTY

MARION COUNTY

PUTNAM COUNTY

CLAY COUNTY

Mandarin Orange

Park

Green

Cove

Springs

Picolata St. Augustine

Palatka

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

River

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

400

Figure 3. Generalized thickness of the intermediate confining unit (data from Scott and others, 1991, and 

files of the U.S. Geological Survey and St. Johns River Water Management District).
8 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

Green

Cove

Springs

ha wa

Ockla

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

15′

VOLUSIA COUNTY

LAKE COUNTY

0 10

10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

MARION COUNTY

ALACHUA COUNTY BRADFORD COUNTY

Palatka

15′

29° 00′

30° 30′

30° 00′

15′

45′

30′

NASSAU COUNTY

DUVAL COUNTY

0 10

10

20 MILES

0 20 KILOMETERS

ST. JOHNS COUNTY

PUTNAM COUNTY

CLAY COUNTY

Mandarin Orange

Park

Picolata St. Augustine

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

River

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

-50

-450

-50

-400 -500

-500

-150

-150

-250

-250

-450

-450

-350

-350

-550

-100

-100

-200

-200

-300

-300

-400

-400

-350

-300

of the top of the Floridan aquifer system. 

Datum is sea level. Hachures indicate 

depression. Contour interval is 50 feet

STRUCTURE CONTOUR--Shows altitude

EXPLANATION

-50

-350

-450

Figure 4. Altitude of the top of the Floridan aquifer system (modified from Scott and others, 1991; and 

Spechler, 1994).
Lower St. Johns River Drainage Basin 9

Boundary of lower St. Johns River 

drainage basin

EXPLANATION 

Recharge area

Discharge area

ha wa

Ockla

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

15′

VOLUSIA COUNTY

LAKE COUNTY

0

0

10

10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

ST. JOHNS COUNTY

MARION COUNTY

ALACHUA COUNTY BRADFORD COUNTY

PUTNAM COUNTY

CLAY COUNTY Orange

Park

Green

Cove

Springs

Palatka

Johns

St. 

River

15′

29° 00′

30° 30′

30° 00′

15′

45′

30′

NASSAU COUNTY

DUVAL COUNTY

MARION COUNTY

Mandarin

Picolata St. Augustine

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

Figure 5. Approximate extent of natural recharge and discharge areas in the Upper Floridan aquifer (from 

Phelps, 1984).
10 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

Green

Cove

Springs

50

30

30

30

35

35

40

40

45

45

ha wa

Ockla

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

15′

VOLUSIA COUNTY

LAKE COUNTY

0 10

10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

MARION COUNTY

ALACHUA COUNTY BRADFORD COUNTY

Palatka

15′

29° 00′

30° 30′

30° 00′

15′

45′

30′

NASSAU COUNTY

DUVAL COUNTY

0 10

10

20 MILES

0 20 KILOMETERS

ST. JOHNS COUNTY

PUTNAM COUNTY

CLAY COUNTY

Mandarin Orange

Park

Picolata St. Augustine

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

River

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

0

0

5

-5

-5

10

25

25

10

15

20

25

10

10

30

30

35

35

40

35

30

25

25

20

20

25

20

20

5

5

5

10

10

10

15

15

15

15

15

15

25

25

20

25

30

35

25

25

20

75

75

50

50

55

55

60

60

65

65

70

70

EXPLANATION 

POTENTIOMETRIC CONTOUR--Shows 

altitude at which water level would

have stood in tightly cased wells. 

Hachures indicate depressions.

Contour interval 5 feet. Datum is 

sea level

10

40

35

80

20

30

Figure 6. Potentiometric surface of the Upper Floridan aquifer, September 1990 (modified from Spechler 

and others, 1991).
Lower St. Johns River Drainage Basin 11

ha R. wa

Ockla

MARION COUNTY

Lake

George

VOLUSIA COUNTY

Picolata

Johns

PUTNAM COUNTY

BRADFORD COUNTY

EXPLANATION 

POTENTIOMETRIC CONTOUR--Shows 

altitude at which water level would

have stood in tightly cased wells. 

Hachures indicate depressions.

Contour interval 5 feet.Datum is

sea level.

0

0

5

10

20

20

40

20

20

15

0 10

10

20 MILES

0 20 KILOMETERS

5

10

30

30

30

30

35

25

25

25

25

25

60

35

35

65

70

25

15

15

15

15

15

10

20

20

5

-5

10

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

15′

LAKE COUNTY

BAKER COUNTY

FLAGLER COUNTY

ALACHUA COUNTY

Palatka

15′

29° 00′

30° 30′

30° 00′

15′

45′

30′

NASSAU COUNTY

DUVAL COUNTY

ST. JOHNS COUNTY

CLAY COUNTY

Mandarin Orange

Park

St. Augustine

Crescent

Beach

Jacksonville

GEORGIA

St. 

River

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

Green

Cove

Springs

75

60

65

70

80

75

15

30

35

40

40

30

50

45

35

55

30

30

35

35

30

504540

35

55

50

55

Figure 7. Potentiometric surface of the Upper Floridan aquifer, September 1991 (modified from Sumner and 

others, 1992).
12 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

and from about 80 to less than 15 ft above sea level in 

these respective areas in September 1991 (fig. 7). The 

large depression in the potentiometric surface south 

of Jacksonville is probably caused, in part, by withdrawals from industrial and public-supply wells, and 

possibly by diffuse upward leakage or undetected 

spring discharge into the St. Johns River. The depression in the potentiometric surface of the Upper Floridan aquifer in the Green Cove Springs area is probably 

caused by a combination of factors including diffuse 

upward leakage, spring flow, and withdrawals for public, agricultural, and domestic use. Spring discharge 

and diffuse upward leakage are the probable cause of 

the depression in the potentiometric surface near the 

confluence of the St. Johns and the Ocklawaha Rivers, 

near Welaka. The intermediate confining bed in this 

area is thin, making conditions favorable for spring 

formation. Additionally, it is possible that channel 

dredging or excavation in the St. Johns River has, in 

places, breached or partially breached the semiconfining unit overlying the Upper Floridan aquifer and has 

allowed water from the Upper Floridan aquifer to 

discharge into the river (Tibbals, 1990, E28).

WATER QUALITY

The quality of surface water, ground water, and 

springs in the study area are briefly discussed in the 

following sections. Most of the water-quality data 

used in this investigation are from previous reports. 

These data were obtained primarily during the 1970’s 

and 80’s as part of areal hydrologic studies by the 

USGS and St. Johns River Water Management 

District.

Lower St. Johns River

The quality of water in the lower St. Johns River 

has been documented in many publications. Although 

data collection and descriptive reports for the St. Johns 

River from the Ocklawaha River to Orange Park are 

limited, considerable data have been collected for tributaries and for the St. Johns River near Jacksonville. 

Generally, water quality in the river is characterized as 

good in the sparsely populated area of the lower 

St. Johns River or poor in the urban area of Jacksonville (Campbell and others, 1989). Water-quality problems include low dissolved oxygen concentrations, 

and elevated nutrient concentrations and bacterial populations which result from point and nonpoint sources 

such as industrial discharges, agricultural runoff, 

municipal water treatment plants, septic tanks, and 

dairy farms. A detailed description of the nutrient and 

bacterial concentrations and the concentrations of 

other pollutants discharged into the water in the lower 

St. Johns River is beyond the scope of this report. 

However, certain chemical constituents (dissolved 

solids, chloride, and sulfate) in discharging ground 

water that could affect the quality of water in the lower 

St. Johns River will be discussed in a later section of 

this report.

Water quality in the Iower St. Johns River varies 

significantly with changing flow conditions. The flow, 

hence the water quality of the river is affected by 

ocean tides, rainfall, surface-water runoff, evapotranspiration, and wind. The extent to which these factors 

can affect the flow is primarily controlled by channel 

geometry and the available storage capacity of the 

river. Of these factors, ocean tides probably have the 

greatest effect on the quality of water in the river. Saltwater from the ocean advances up the St. Johns River 

during each incoming tide and recedes during each 

outgoing tide. A transition zone forms where the ocean 

water mixes with freshwater in the river. The length of 

the transition zone varies with each tidal cycle and 

with the amount of freshwater entering the river 

upstream. During periods of large freshwater inflow to 

the river, downstream flow can increase, reducing the 

length of the transition zone and shifting the zone 

toward the ocean. Reduced freshwater inflow can 

cause the transition zone to move a considerable distance upstream during periods of drought or reduced 

rainfall.

The chemical quality of water in the river also 

can change significantly over a short period of time as 

a result of the constant movement of the transition 

zone. At the Main Street Bridge in Jacksonville, for 

example, chloride concentrations increased more than 

fourfold in a few hours and more than tenfold in 

several days (Anderson and Goolsby, 1973, p. 54). 

Anderson and Goolsby (1973, p. 37) also documented 

changes in specific conductance of as much as 

12,000 μS/cm during a single tide cycle.

Between the headwaters of the St. Johns River 

and the point at which it discharges into the Atlantic 

Ocean, an undetermined amount of mineralized 

ground water from the Upper Floridan aquifer discharges through springs and by upward leakage 

through the intermediate confining unit. Ground-water 

discharge of mineralized water is a potential source of 
Water Quality 13

highly concentrated solute species such as chloride 

and sulfate. In some areas along the lower St. Johns 

River, water from the Upper Floridan aquifer could be 

more mineralized than the riverwater. Under certain 

conditions, such as during a drought when low-flow 

conditions exist, ground water could cause increased 

mineralization of the riverwater. A small amount of 

water flows into the St. Johns River from surfacewater sources during these periods and the mineralized 

ground water discharging into the river can significantly affect water quality. However, in the areas 

where saltwater from the ocean affects the quality of 

water in the lower St. Johns River, water that enters 

the river from the Upper Floridan aquifer probably is 

of better quality than the riverwater.

Upper Floridan Aquifer

Available data for ground-water quality in the 

study area were used to evaluate the potential effect 

of natural ground-water discharge from the Upper 

Floridan aquifer into the lower St. Johns River basin. 

Maps indicating the distribution of selected constituents in the study area are included in this section. 

These data were obtained from the files of the U.S. 

Geological Survey and the St. Johns River Water Management District, and from published reports (Bermes 

and others, 1963; Fairchild, 1977; Munch and others, 

1979; Ross and Munch, 1980; Rutledge, 1982; 

Spechler and Hampson, 1984; Navoy and Bradner, 

1987; and Spechler, 1994).

Dissolved Solids

The concentration of dissolved solids in water is 

a common indicator of mineralization. The areal distribution of dissolved solids in water collected from the 

Upper Floridan aquifer in parts of Duval, Clay, St. 

Johns, Putnam, Flagler, and Volusia Counties is shown 

in figure 8. Dissolved-solids concentrations range 

from about 100 to 5,000 mg/L (unpublished data in 

files of the U.S. Geological Survey) and generally 

increase toward the east and the south. Lowest dissolved-solids concentrations are in much of Clay and 

western Putnam Counties, where concentrations of 

less than 150 mg/L are common. Highest dissolvedsolid concentrations are in the area near Hastings in 

southwestern St. Johns County. Relatively high 

dissolved-solids concentrations also are present in 

west-central Flagler County, and in several areas 

adjacent to the St. Johns River near Welaka, east of 

Palatka, and north of Riverdale.

Specific conductance data were used to estimate 

dissolved-solids concentrations in some areas where 

dissolved-solids concentrations were not available. 

Although specific conductance values cannot be used 

to precisely determine dissolved-solids concentrations 

in natural waters, they can be used to estimate dissolved-solids concentrations. Based on the specific 

conductance in the study area, multiplication of the 

specific conductance values by 0.66 gives a reasonable 

approximation of the dissolved-solids concentration.

Chloride

Chloride is a major constituent of dissolved 

solids in parts of St. Johns, Putnam, and Flagler 

Counties. The primary mechanisms by which chloride 

increases in ground water include the dissolution of 

minerals in rocks of the aquifer, relict or connate water 

mixing with freshwater, and the intrusion of seawater. 

The distribution of chloride in water from the Upper 

Floridan aquifer in the study area is shown in figure 9. 

Concentrations range from 4 to 3,700 mg/L and generally increase toward the south (Navoy and Bradner, 

1987; unpublished data in files of the U.S. Geological 

Survey). The lowest chloride concentrations are 

present in Clay, southwestern Duval, and western 

Putnam Counties where concentrations of less than 

10 mg/L are common. Highest chloride concentrations 

were measured in southwestern Flagler County. Relatively high chloride concentrations (concentrations 

exceeding 500 mg/L) also are present in southern 

St. Johns and northern Flagler Counties and in areas 

adjacent to the St. Johns River near Palatka and 

Welaka (fig. 9). The area north of Riverdale also could 

be an area where chloride concentrations in ground 

water exceed 500 mg/L.

Sulfate

Sulfate concentrations in water from the Upper 

Floridan aquifer range from about 1 to 1,300 mg/L 

(Spechler, 1994; unpublished data in files of the U.S. 

Geological Survey) and generally increase toward the 

east and south (fig. 10). Lowest concentrations typically are in Clay and Putnam Counties, where concentrations of sulfate are less than 10 mg/L. Highest 

sulfate concentrations are present north of Riverdale. 

Sulfate concentrations greater than 500 mg/L also are 

present in southern St. Johns County and in two small 

areas east of Green Cove Springs.
14 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

EXPLANATION 

Less than 250

250 to 500

500 to 1,000

Greater than 1,000

DISSOLVED SOLIDS CONCENTRATIONS IN

WATER FROM THE UPPER FLORIDAN

AQUIFER, IN MILLIGRAMS PER LITER

Boundary of lower St. Johns River 

drainage basin

ha wa

Ockla

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

15′

VOLUSIA COUNTY

LAKE COUNTY

0

0

10

10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

ST. JOHNS COUNTY

MARION COUNTY

ALACHUA COUNTY BRADFORD COUNTY

PUTNAM COUNTY

CLAY COUNTY Orange

Park

Green

Cove

Springs

Palatka

Johns

St. 

River

15′

29° 00′

30° 30′

30° 00′

15′

45′

30′

NASSAU COUNTY

DUVAL COUNTY

MARION COUNTY

Mandarin

Picolata St. Augustine

Riverdale

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

Welaka

Hastings

Figure 8. Generalized distribution of dissolved-solids concentrations in water from the Upper Floridan aquifer 

(data from Munch and others, 1979; Ross and Munch, 1980; Rutledge, 1982; Spechler and Hampson, 1984; 

Navoy and Bradner, 1987; Spechler, 1994; and the files of the U.S. Geological Survey and St. Johns River 

Water Management District).
Water Quality 15

EXPLANATION 

Less than 30

30 to 249

250 to 500

Greater than 500

CHLORIDE CONCENTRATIONS IN

WATER FROM THE UPPER FLORIDAN

AQUIFER, IN MILLIGRAMS PER LITER

Boundary of lower St. Johns River 

drainage basin

ha wa

Ockla

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

15′

VOLUSIA COUNTY

LAKE COUNTY

0

0

10

10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

ST. JOHNS COUNTY

MARION COUNTY

ALACHUA COUNTY BRADFORD COUNTY

PUTNAM COUNTY

CLAY COUNTY Orange

Park

Green

Cove

Springs

Palatka

Welatka

Johns

St. 

River

15′

29° 00′

30° 30′

30° 00′

15′

45′

30′

NASSAU COUNTY

DUVAL COUNTY

MARION COUNTY

Mandarin

Picolata St. Augustine

Riverdale

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

Figure 9. Generalized distribution of chloride concentrations in water from the Upper Floridan aquifer (data 

from Bermes and others, 1963; Fairchild, 1977; Munch and others, 1979; Ross and Munch, 1980; Rutledge, 

1982; Spechler and Hampson, 1984; Navoy and Bradner, 1987; Spechler, 1994; and unpublished data from 

the files of the U.S. Geological Survey and St. Johns River Water Management District).
16 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

Figure 10. Generalized distribution of sulfate concentrations in water from the Upper Floridan aquifer (data 

from Bermes and others, 1963; Ross and Munch, 1980; Spechler and Hampson, 1984; Spechler, 1994; and 

the files of the U.S. Geological Survey and St. Johns River Water Management District).

EXPLANATION 

Less than 100

100 to 249

250 to 500

Greater than 500

SULFATE CONCENTRATIONS IN

WATER FROM THE UPPER FLORIDAN

AQUIFER, IN MILLIGRAMS PER LITER

Boundary of lower St. Johns River 

drainage basin

ha wa

Ockla

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

30° 00′

15′

NASSAU COUNTY

DUVAL COUNTY

VOLUSIA COUNTY

LAKE COUNTY

0

0

10

10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

ST. JOHNS COUNTY

MARION COUNTY

ALACHUA COUNTY BRADFORD COUNTY

PUTNAM COUNTY

CLAY COUNTY Orange

Park

Green

Cove

Springs

Palatka

Johns

St. 

River

15′

29° 00′

30° 30′

15′

45′

30′

MARION COUNTY

Mandarin

Picolata St. Augustine

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

Riverdale

Welaka
Estimated Ground-Water Discharge from the Upper Floridan Aquifer 17

High sulfate concentrations in southern 

St. Johns and parts of northeastern Putnam and northern Flagler Counties are often associated with high 

chloride concentrations. This could indicate that the 

high concentrations of sulfate are, in part, related to 

the presence of ancient seawater that has not been 

flushed from the aquifer by freshwater. However, in 

some areas of northwestern and west-central St. Johns 

County, the high concentrations of sulfate are not associated with high concentrations of chloride. High 

sulfate concentrations are probably the result of the 

dissolution of sulfate-bearing minerals.

Springs

Nine gaged springs that discharge water into the 

St. Johns River are located in or near the study area 

(fig. l). Only five springs are known to discharge water 

into the St. Johns River between the Ocklawaha River 

and Mayport. The remaining four springs, Beecher, 

Croaker Hole, Mud, and Forest Springs, are located a 

few miles to the south of the study area.

The quality of water discharging from these 

springs is variable. Limited water-quality data indicate 

that average dissolved-solids concentrations range 

from about 133 to 3,564 mg/L and sulfate concentrations from 10 to 260 mg/L (table l). However, sulfate 

concentrations in most of the springs generally are less 

than 100 mg/L.

Table 1. Average discharge, loads, and concentrations of selected chemical constituents of springs from the Upper Floridan 

aquifer in and near the study area (from Rosenau and Faulkner, 1975; Rosenau and others, 1977; and the files of the USGS)

[ft3/s, cubic feet per second; mg/L, milligrams per liter; ton/yr, tons per year; --, no data]

Spring (see 

figure 1 for 

location)

Average 

discharge 

(ft3/s)

Dissolved 

solids 

(mg/L)

Dissolved 

solids 

loads 

(ton/yr)

Chloride 

(mg/L)

Chloride 

load 

(ton/yr)

Sulfate 

(mg/L)

Sulfate 

load 

(ton/yr)

Beechera ........................................ 10 252 2,483 78 769 13 128

Croaker Holea ............................... 84 1,834b,d 151,822 720d 59,603 160d 13,245

Green Cove ................................... 3.5 181 624 5.8 20 54 186

Foresta ........................................... .30 1,518b 449 600d 177 87d 26

Muda ............................................. 2.3 688d 1,559 290d 657 43d 98

Nashua .......................................... .27 -- -- -- -- -- --

Satsuma......................................... 1.8 3,564b 6,322 1,600 2,838 260 460

Wadesboroc ................................... 1.2 133 157 8.5 10 10 12

Welaka........................................... 2.2 849 1,841 370 802 57 124

Average chloride concentrations range from 

5.8 to 1,600 mg/L. Chloride concentrations in water 

from Green Cove and Wadesboro Springs in the northern part of the study area are less than 10 mg/L.

 Chloride concentrations in water from springs near 

the confluence of the Ocklawaha and St. Johns Rivers 

are higher and more variable. The average chloride 

concentration in water discharging from Satsuma 

Spring is 1,600 mg/L but at Beecher Springs, several 

miles to the south, chloride concentrations average 

only 78 mg/L. A water sample collected at the mouth 

of Croaker Hole Spring, located in Little Lake George 

(St. Johns River), had a chloride concentration of 

720 mg/L; however, inside the spring, several vents 

observed discharging water had chloride concentrations ranging from 740 to 880 mg/L. Chloride concentrations as high as 1,600 mg/L also have been 

determined in water discharging from Salt Springs, 

about 8 mi south of the study area (Tibbals, 1990, 

p. E94).

ESTIMATED GROUND-WATER 

DISCHARGE FROM THE UPPER 

FLORIDAN AQUIFER

One purpose of this study was to estimate total 

ground-water discharge from the Upper Floridan aquifer to the St. Johns River within the lower St. Johns 

River drainage basin. To expedite the process, hydrogeologic data from two subregional RASA models 

that overlapped the study area were used. The subregional model by Krause and Randolph (1989) extends 

south to central St. Johns County in northern Florida. 

a

Spring located outside of study area. bDissolved-solids concentrations estimated from specific conductance. c

Possibly not an Upper Floridan aquifer spring. dSingle sample value.
18 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

The subregional model by Tibbals (1990) includes the 

remainder of the study area (fig. 11).

Features of and assumptions for the RASA 

models are described in Krause and Randolph (1989) 

and Tibbals (1981, 1990). The reader is referred to 

these reports for discussions of the steady-state model 

calibration, boundary conditions, methods used to 

simulate spring flow, and derivation of confining-unit 

leakage coefficients.

Data input within the study area involved 

assigning hydrogeologic data to nodes within a grid. 

The grid consists of 14 rows and 27 columns. Each of 

the 378 nodes measures 4 mi on each side and 16 mi2 

in area. Because of the configuration of the study area 

boundaries, only 174 of the nodes were considered 

“active.” The position of the grid on the study area is 

shown in figure 11. Orientation of the grid coincides 

with the position of the grid used in the two subregional RASA models.

Ground-water discharge from the Upper 

Floridan aquifer to the lower St. Johns River drainage 

basin occurs as diffuse upward leakage or spring flow. 

Water from the Upper Floridan aquifer moves upward 

through the intermediate confining unit to the surficial 

aquifer system in the discharge areas of the lower 

St. Johns River drainage basin. The water then moves 

laterally through the surficial aquifer system or into 

streams and creeks where it eventually discharges into 

the St. Johns River.

The rate of leakage through the intermediate 

confining unit is controlled by the leakance coefficient 

of the intermediate confining unit and the head difference between the Upper Floridan aquifer and the surficial aquifer system. The equation representing this 

relationship can be expressed by Darcy’s Law as

Q = K’/b’(hr-hf)A C, (1)

where

Q is ground-water discharge or leakage,

in cubic feet per second;

K’ is vertical hydraulic conductivity of the intermediate confining unit, in feet per day;

b’ is thickness of the intermediate confining unit, 

in feet;

K’/b’ is leakance coefficient, in feet per day per 

foot; 

hr is altitude of water level of the surficial aquifer 

system, in feet above sea level; 

hf is altitude of water level of the Upper Floridan 

aquifer system, in feet above sea level; 

A is surface area, in square feet; and 

C is conversion factor from days to seconds.

Leakance coefficients of the intermediate confining unit are highly variable. Leakance coefficients 

for the intermediate confining unit required to deliver 

RASA-simulated rates of vertical flow between the 

Upper Floridan aquifer and the surficial aquifer system reported for the study area range from less than 

1×10-8 (ft/d)/ft to 1×10-4 (ft/d)/ft (fig. 12), (C.H. Tibbals and R.E. Krause, U.S. Geological Survey, written 

commun., 1993). The lowest leakance coefficients 

generally are in areas where diffuse recharge and discharge rates are low—areas where the confining units 

are

relatively thick or have low permeability. The highest 

leakance coefficients generally are in areas where 

diffuse recharge and discharge rates are highest-areas 

where the confining units are relatively thin or permeable.

Total diffuse ground-water discharge to the 

lower St. Johns River basin was calculated by determining leakance coefficients and average heads for the 

surficial aquifer system and Upper Floridan aquifer at 

each node within the lower St. Johns River drainage 

basin. Leakance coefficients, as previously mentioned, 

were obtained from the subregional RASA studies. 

Average heads in the Upper Floridan aquifer at each 

node were determined by interpolating directly from 

the September 1990 potentiometric-surface map 

(fig. 6), which represented a time when water levels in 

much of northeastern Florida were at or near record 

lows. Water-level altitudes in the surficial aquifer system were estimated from USGS 1:100,000 scale topographic maps.

Discharge rates for springs in and near the study 

area were determined by standard streamflow measurements (table l). In Krause and Randolph (1989), 

spring discharge was not explicitly simulated by a 

separate water-budget program. The effects of known 

springs on the potentiometric surface of the Upper 

Floridan aquifer are implicitly accounted for by the 

diffuse upward leakage rates determined during model 

calibration. Tibbals (1990), however, did a separate 

water-budget program for springs. Therefore groundwater discharge from Satsuma and Welaka Springs 

was added to the diffuse upward leakage rate to get 

total ground-water discharge from the Upper Floridan 

aquifer to the surficial aquifer system.
Estimated Ground-Water Discharge from the Upper Floridan Aquifer 19

27

13

12

25

11

10

9

8

6

5

4

3

2 1 21

7

17

16

15

14

COLUMN

ROW

26

24

23

22

20

19

18

1

2

3 4

5

6

7

8 9 10 11 12 13 14

MODEL BOUNDARY OF STUDY AREA

EXPLANATION 

Regional Aquifer Systems

Analysis (RASA) southeast

Georgia and adjacent parts

of Florida and South

Carolina model grid

Regional Aquifer Systems

Analysis (RASA) east-central

Florida model grid

ha 

Oc

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

VOLUSIA COUNTY

LAKE COUNTY

BAKER COUNTY

FLAGLER COUNTY

ST. JOHNS COUNTY

MARION COUNTY

ALACHUA COUNTY

BRADFORD COUNTY

PUTNAM COUNTY

CLAY COUNTY Orange

Park

Green

Cove

Springs

Palatka

Johns

St. 

River

0 10

0 10

20 MILES

20 KILOMETERS

15′

29° 00′

30° 30′

45′

30′

NASSAU COUNTY

DUVAL COUNTY

MARION COUNTY

Mandarin

Picolata

Crescent Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

30° 00′

15′

wa kla

St. Augustine

Figure 11. Study area model boundary and grid.
20 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

EXPLANATION 

LEAKANCE COEFFICIENT,

IN FEET PER DAY PER FOOT

Less than 10

10 to 10

10 to 10

10 to 10

10 to 10

-8 

-7

-6

-5

-4

-8 

-7

-6

-5

ha wa

Ockla

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

15′

NASSAU COUNTY

DUVAL COUNTY

VOLUSIA COUNTY

LAKE COUNTY

0

0

10

10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

ST. JOHNS COUNTY

ALACHUA COUNTY

MARION COUNTY

BRADFORD COUNTY

PUTNAM COUNTY

CLAY COUNTY Orange

Park

Green

Cove

Springs

Palatka

Johns

St. 

River

15′

29° 00′

30° 30′

30° 00′

15′

45′

30′

MARION COUNTY

Mandarin

Picolata St. Augustine

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

Figure 12. Estimated leakage coefficients of the intermediate confining unit (from Krause and Randolph, 

1989; Tibbals, 1990; and C.H. Tibbals and R.E. Krause, U.S. Geological Survey, written commun., 1993).
Estimated Chemical-Constituent Loads in Ground-Water Discharge 21

Total ground-water discharge to the lower 

St. Johns River basin, determined by adding the calculated flow from all nodes with an upward hydraulic 

gradient in the study area and spring flow at Welaka 

and Satsuma Springs, was estimated at 86 ft3

/s based 

on September 1990 Upper Floridan heads. Estimated 

leakage rates for each node within the lower St. Johns 

River drainage basin ranged from about 0 to 9 in/yr 

(fig. 13).

Total ground-water discharge to the lower 

St. Johns River also was computed for September 

1991. Variables used to compute ground-water discharge for September 1990 were again used for the 

September 1991 estimate, except for potentiometricsurface altitudes, which increased in the drainage 

basin by an average of about 4 ft from the September 

1990 levels. Potentiometric-surface altitudes rose 

between 2 and 6 ft as a result of near-record rainfall in 

much of the study area. The calculated discharge rate 

from the Upper Floridan aquifer to the river was about 

133 ft3

/s, an increase of about 55 percent from the 

September 1990 calculated discharge. The increase in 

ground-water discharge was due to a greater upward 

hydraulic gradient in discharge areas, as well as to a 

reversal in the hydraulic gradient from recharge to discharge in areas where the two heads were nearly equal.

Potentiometric-surface altitudes in the Upper 

Floridan aquifer in most of the study area were near or 

at historical lows during September l990. Accordingly, 

ground-water discharge to the lower St. Johns River 

(assuming that the surficial aquifer system heads have 

remained relatively constant since predevelopment 

times) also could have been at a historical low. 

The average potentiometric surface of the Upper Floridan aquifer was higher during predevelopment times 

than at present (Johnston and others, 1980; Krause and 

Randolph, 1989; Tibbals, 1990). Estimated declines 

from predevelopment heads to heads measured in 

September 1990 range from less than 10 to more 

than 30 ft in the areas of higher pumpage (near 

Jacksonville).

Although changes in the potentiometric surface 

do not occur equally over the entire area, ground-water 

discharge to the river during predevelopment times 

was considerably higher than during September 199O. 

The potential effects of a higher potentiometric surface 

on total groundwater discharge to the river are shown 

in figure 14. For example, if in the past, the potentiometric surface of the Upper Floridan aquifer averaged 

12 ft higher than the September 1990 levels, total 

ground-water discharge to the lower St. Johns River 

would have been about 300 ft3/s, an increase of 

approximately 249 percent from the September 1990 

calculated discharge. Conversely, a continued average 

decline of another 4 ft from the September 1990 levels 

could decrease the amount of ground-water discharge 

to the river to about 50 ft 3

/s, a decline of about 

42 percent.

ESTIMATED CHEMICAL-CONSTITUENT 

LOADS IN GROUND-WATER DISCHARGE

The contribution of constituent loads into the 

lower St. Johns River from the Upper Floridan aquifer 

were estimated for this report. Total loads were determined by adding the calculated loads at each node 

with an upward hydraulic gradient within the lower 

St. Johns River drainage basin. It is assumed that all 

the water from the Upper Floridan aquifer that moves 

upward through the intermediate confining unit to the 

surficial aquifer system within the drainage basin 

eventually drains into the lower St. Johns River. 

Constituent loads from the Upper Floridan aquifer 

for each node were calculated using the following 

equation:

L = Q × C × 0.9855, (2)

where

L is load, in tons per year;

Q is ground-water discharge, in cubic feet per 

second; 

C is concentration of chemical constituents in 

water from the Upper Floridan aquifer, in 

milligrams per liter; and

 0.9855 is a conversion factor.

Because load is a function of ground-water 

discharge and constituent concentration, changes in 

either variable can affect the calculation of load. An 

average constituent concentration in water from the 

Upper Floridan aquifer for each node was determined 

by interpolating concentrations in water from selected 

wells. However, some uncertainty concerning the 

accuracy of constituent concentrations exists because 

of long-term and seasonal changes in the chemical 

quality of water within the aquifer. In much of the 

area, water analyzed from the Upper Floridan aquifer 

was collected during the 1980’s however, in some 

areas, the most recent water samples were collected 
22 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

EXPLANATION 

0 to 1.0

1.0 to 2.0

Greater than 2.0

Recharge area

LEAKAGE, IN INCHES PER YEAR

ha wa

Ockla

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

15′

NASSAU COUNTY

DUVAL COUNTY

VOLUSIA COUNTY

LAKE COUNTY

0

0

10

10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

ST. JOHNS COUNTY

ALACHUA COUNTY

MARION COUNTY

BRADFORD COUNTY

PUTNAM COUNTY

CLAY COUNTY Orange

Park

Green

Cove

Springs

Palatka

Johns

St. 

River

15′

29° 00′

30° 30′

30° 00′

15′

45′

30′

MARION COUNTY

Mandarin

Picolata St. Augustine

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

Figure 12. Estimated discharge from the Upper Floridan aquifer through the intermediate confining unit, 

September 1990.
Estimated Chemical-Constituent Loads in Ground-Water Discharge 23

during the 1970’s. Increases in constituent concentration might have occurred with time in selected wells. 

Seasonal changes in water levels due to pumpage also 

can affect the constituent concentration in water from 

the Upper Floridan aquifer. Munch and others (1979) 

documented the relation between chloride concentrations and the potentiometric surface in southwestern 

St. Johns County and near Palatka. Chloride concentrations in many wells increased during the spring, 

generally a period of greater irrigation-water use and 

lower potentiometric surface.

Total estimated constituent loads also can vary 

depending on the chemical and physical changes in the 

Upper Floridan aquifer water as it moves through the 

intermediate confining unit and surficial aquifer system. The degree of mineralization and the chemicalconstituent loading of water are largely determined by 

0 0 

20,000 

40,000 

60,000 

80,000 

200,000 

Dissolved-solids load

September 1990

September 1991

Total Upper Floridan

aquifer discharge

Chloride load

Sulfate load

180,000 

160,000 

140,000 

120,000 

100,000 

220,000 

-10 -8 -6 -4 -2 0 2 

AVERAGE AREAL CHANGE IN ALTITUDE OF POTENTIOMETRIC SURFACE 

OF UPPER FLORIDAN AQUIFER FROM SEPTEMBER 1990, IN FEET 

4 6 8 10 12 14 

50 

100 

150 TOTAL DISCHARGE, IN CUBIC FEET PER SECOND

CONSTITUENT LOAD, IN TONS PER YEAR

200 

300 

250 

350 

Figure 13. Relation of total discharge and chemical-constituent load to change in Upper Floridan aquifer 

potentiometric surface.

the initial chemical composition of water entering the 

overlying hydrogeologic units, the composition and 

solubility of rocks with which it comes in contact, and 

the length of time the water remains in contact with 

these rocks. Therefore, estimated constituent loads 

could vary depending on the geochemical reactions 

that occur. For the purpose of this report, the assumption was made that little chemical change occurs as the 

water moves through the intermediate confining unit 

and the surficial aquifer.

Dissolved-solids loads discharging into the 

lower St. Johns River were estimated at 47,000 ton/yr 

based on September 1990 heads. Estimates of chloride 

and sulfate loads based on September 1990 heads were 

18,000 and 9,500 ton/yr, respectively. The average 4-ft 

increase in the altitude of the potentiometric surface of 

the Upper Floridan aquifer throughout the entire lower 
24 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

St. Johns River drainage basin during September 1991 

increased dissolved-solids loads discharging into the 

lower St. Johns River to about 81,000 ton/yr, a 76 percent increase from September 1990. Chloride loads 

increased to 39,000 ton/yr, a 117 percent increase; and 

sulfate loads increased to about 15,000 ton/yr, a 

58 percent increase. Constituent fluxes for dissolved 

solids, chloride, and sulfate, in (ton/yr)/mi2 based on 

September 1990 heads, also were calculated to compare loading rates for the various nodes within the 

study area and are shown in figures 15-17.

Estimated ground-water discharge and loads in 

four zones of the lower St. Johns River (fig. 18) were 

calculated based on September 1990 heads. The drainage basin was subdivided based on ground-water flow 

lines within the surficial aquifer system. The contribution of ground-water discharge from the Upper 

Floridan aquifer to four zones in the lower St. Johns 

River drainage basin is shown in figure 19. The largest 

contribution of ground water to the river is from zone 

A. In this zone, approximately 35 ft3/s of ground water 

discharges to the St. Johns River. Conversely, only 

2 ft3

/s were discharged in zone D. Nearly all of zone D 

is underlain by the thickest part of the intermediate 

confining unit that has very low leakance coefficients.

Approximately 16,000 of the 18,000 ton/yr of 

the chloride load potentially could enter the St. Johns 

River through the surficial aquifer system in zone A 

(fig. 19). Little chloride load is contributed through 

zones B, C, and D. The largest contribution of sulfate 

load is through zone B, of which approximately 6,000 

ton/yr could be discharged into the river. The total dissolved-solids constituent load that could enter the 

St. Johns River is estimated at 47,000 ton/yr. Approximately 41,000 ton/yr of dissolved solids are discharged into the river from zones A and B.

SUMMARY

The lower St. Johns River, a 101-mile long 

segment of the St. Johns River, begins at the confluence with the Ocklawaha River and ends where the 

river discharges into the Atlantic Ocean at Mayport. 

The river ranges in width from about 600 feet to 3 

miles. The St. Johns River is affected by tides as far 

upstream as Lake George, 106 miles from the mouth 

and, under certain conditions, as far as Lake Monroe, 

approximately 161 miles upstream. Because of the 

tidal nature of the river, the chemical quality of the 

riverwater is highly variable. Saltwater from the ocean 

advances inland during incoming tides and recedes 

during outgoing tides. The advancing ocean water 

mixes with the existing fresh riverwater, thereby 

increasing the salinity of the river.

Three hydrogeologic units are present in the 

study area: the surficial aquifer system, the intermediate confining unit, and the Floridan aquifer system. 

The surficial aquifer system overlies the intermediate 

confining unit and contains deposits of sand, clay, 

shell, limestone, and dolomite. The intermediate 

confining unit, which consists primarily of the Hawthorn Formation, underlies the entire area. The clay 

and silts of the intermediate confining unit retard the 

vertical movement of water between the surficial aquifer system and the Floridan aquifer system. The intermediate confining unit sediments, generally of low 

permeability, vary in thickness and areal extent. The 

intermediate confining unit also could be locally 

breached by sinkholes, fractures, and other openings, 

providing connections between the Floridan aquifer 

system and the surficial aquifer system. Leakage 

coefficients are estimated to range from 1×10-8 to 

more than 1×10-4 foot per day per foot. The highest 

values generally are where aquifer recharge or discharge rates are greatest, coinciding with areas where 

the confining units are relatively thin or permeable.

The Floridan aquifer system, the major source 

of ground water in northeastern Florida, consists of 

limestone and dolomite. The potentiometric surface of 

the Upper Floridan aquifer ranged from about 75 to 

less than 10 feet above sea level in September 1990. 

The Upper Floridan aquifer discharges ground water 

along the entire length of the lower St. Johns River. 

Large depressions in the potentiometric surface are 

believed to be caused by one or more of the following: 

(1) water withdrawals for industry, public-supply, and 

agriculture use; (2) diffuse upward leakage; and 

(3) spring discharge.

The quality of water in the Upper Floridan aquifer varies throughout the study area. Dissolved solids 

in water from the Upper Floridan aquifer range from 

about 100 to 5,000 milligrams per liter. Chloride and 

sulfate concentrations are estimated to range from 4 to 

3,700 milligrams per liter and l to 1,300 milligrams 

per liter, respectively. Saltwater from the ocean affects 

much of the lower St. Johns River; therefore, water 

discharging from the Upper Floridan aquifer in most 

areas probably is less mineralized than riverwater. 

However, in some areas of the lower St. Johns River, 

water discharging from the Upper Floridan aquifer 

could be more mineralized than the riverwater.
Summary 25

ha wa

Ockla

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

15′

NASSAU COUNTY

DUVAL COUNTY

VOLUSIA COUNTY

LAKE COUNTY

0

0

10

10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

ST. JOHNS COUNTY

MARION COUNTY

ALACHUA COUNTY

BRADFORD COUNTY

PUTNAM COUNTY

CLAY COUNTY Orange

Park

Green

Cove

Springs

Palatka

Johns

St. 

River

15′

29° 00′

30° 30′

30° 00′

15′

45′

30′

MARION COUNTY

Mandarin

Picolata St. Augustine

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

EXPLANATION 

Less than 10

10 to 35

Greater than 35

Recharge area

DISSOLVED SOLIDS FLUX, IN

TONS PER YEAR PER SQUARE MILE

Figure 14. Estimated dissolved-solids flux, September 1990.
26 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

EXPLANATION 

Less than 1

1 to 10

Greater than 10

Recharge area

CHLORIDE FLUX, IN TONS

PER YEAR PER SQUARE MILE

ha wa

Ockla

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

15′

NASSAU COUNTY

DUVAL COUNTY

VOLUSIA COUNTY

LAKE COUNTY

0

0

10

10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

ST. JOHNS COUNTY

ALACHUA COUNTY

MARION COUNTY

BRADFORD COUNTY

PUTNAM COUNTY

CLAY COUNTY Orange

Park

Green

Cove

Springs

Palatka

Johns

St. 

River

15′

29° 00′

30° 30′

30° 00′

15′

45′

30′

MARION COUNTY

Mandarin

Picolata St. Augustine

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

Figure 15. Estimated chloride flux, September 1990.
Summary 27

EXPLANATION 

Less than 1

1 to 10

Greater than 10

Recharge area

SULFATE FLUX, IN TONS

PER YEAR PER SQUARE MILE

ha wa

Ockla

R.

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

15′

NASSAU COUNTY

DUVAL COUNTY

VOLUSIA COUNTY

LAKE COUNTY

0

0

10

10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

ST. JOHNS COUNTY

MARION COUNTY

ALACHUA COUNTY

BRADFORD COUNTY

PUTNAM COUNTY

CLAY COUNTY Orange

Park

Green

Cove

Springs

Palatka

Johns

St. 

River

15′

29° 00′

30° 30′

30° 00′

15′

45′

30′

MARION COUNTY

Mandarin

Picolata St. Augustine

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

Figure 16. Estimated sulfate flux, September 1990.
28 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

ZONE D

ZONE C

ZONE B

ZONE A

EXPLANATION 

STUDY AREA

82° 15′ 82° 00′ 45′ 30′ 15′ 81° 00′

ha wa

Oc

R.

30° 00′

15′

45′

VOLUSIA COUNTY

LAKE COUNTY

0 10

0 10

20 MILES

20 KILOMETERS

BAKER COUNTY

FLAGLER COUNTY

ST. JOHNS

COUNTY

MARION COUNTY

ALACHUA COUNTY BRADFORD COUNTY

PUTNAM COUNTY

CLAY COUNTY Orange

Park

Green

Cove

Springs

Palatka

River

15′

29° 00′

30° 30′

30° 00′

15′

45′

30′

NASSAU COUNTY

DUVAL COUNTY

MARION COUNTY

Mandarin

Picolata St. Augustine

Crescent

Beach

Jacksonville

Lake

George

GEORGIA

Johns

St. 

A T L A N T I C O C E A N

Mayport

Ponte Vedra

Beach

kla

Figure 17. Location of zones used to compute cumulative constituent load and flow of Upper Floridan 

aquifer water discharging into the surficial aquifer system.
Summary 29

Water from the Upper Floridan aquifer moves 

upward through the intermediate confining unit to the 

surficial aquifer system in the discharge areas of the 

lower St. Johns River drainage basin. The water then 

moves laterally through the surficial aquifer system or 

into streams and creeks where it eventually discharges 

into the St. Johns River. Ground-water discharge to the 

river is controlled by the leakance coefficient of the 

intermediate confining unit and the head difference 

between the Upper Floridan aquifer and the surficial 

aquifer system.

Ground-water discharge and constituent loads 

from the Upper Floridan aquifer to the lower St. Johns 

River were estimated based on the potentiometric surfaces within the drainage basin for September 1990 

and 1991. Estimated total discharge by diffuse upward 

50,000 125

100

75

50

25

40,000

OCKLAWAHA RIVER

BOUNDARY OF ZONES A-B

GREEN COVE SPRINGS

BOUNDARY OF ZONES B-C

BOUNDARY OF ZONES C-D

JACKSONVILLE

MAYPORT

30,000

20,000

10,000

0

0 20 40 60

RIVER MILES

Total groundwater

discharge from the

Upper Floridan aquifer

to the surficial 

aquifer system

Dissolved-solids load

Chloride load

Sulfate load

CUMULATIVE CONSTITUENT LOAD, IN TONS PER YEAR

CUMULATIVE DISCHARGE, IN CUBIC FEET PER SECOND

80 100

0

Figure 18. Cumulative constituent load and flow of Upper Floridan aquifer water 

discharging into the surficial aquifer system, September 1990.

leakage and spring flow to the lower St. Johns River 

was 86 cubic feet per second, based on September 

1990 heads. Total estimated dissolved-solids load to 

the river from the Upper Floridan aquifer was 47,000 

tons per year. Total estimated chloride and sulfate 

loads were 18,000 and 9,500 tons per year, respectively. Water levels in the Upper Floridan aquifer in 

September 1991 averaged about 4 feet higher than in 

September 1990 as a result of near record rainfall 

throughout most of the study area. Total estimated 

Floridan aquifer system discharge to the lower 

St. Johns River was about 133 cubic feet per second. 

Estimated dissolved-solids, chloride, and sulfate loads 

discharging into the river also increased to 81,000, 

39,000, and 15,000 tons per year, respectively.
30 Estimated Discharge and Chemical-Constituent Loading from the Upper Floridan Aquifer to the Lower St. Johns River, 

Northeastern Florida, 1990-91

SELECTED REFERENCES

Applin, P.L., 1951, Preliminary report on buried preMesozoic rocks in Florida and adjacent States: U.S. 

Geological Survey Circular 91, 28 p.

Applin, P.L., and Applin, E.R., 1944, Regional subsurface 

stratigraphy and structure of Florida and southern 

Georgia: American Association of Petroleum Geologists Bulletin, v. 28, no. 12, p. 1673-1753.

Anderson, Warren, and Goolsby, D.A., 1973, Flow and 

chemical characteristics of the St. Johns River at Jacksonville, Florida: Florida Bureau of Geology Information Circular no. 82, 57 p.

Bentley, C.B., 1977a Aquifer test analyses for the Floridan 

aquifer in Flagler, Putnam, and St. Johns Counties, 

Florida: U.S. Geological Survey Water-Resources 

Investigations Report 77-36, 50 p.

———1977b, Surface-water and ground-water features, 

Clay County, Florida: U.S. Geological Survey WaterResources Investigations Report 77-87, 59 p.

Bermes, B.J., Leve, G.W., and Tarver, G.R., 1963, Geology 

and ground-water resources of Flagler, Putnam, and 

St. Johns Counties, Florida: Florida Geological Survey 

Report of Investigations no. 32, 97 p.

Boniol, Don, Munch, D.A., and Williams, Marvin, 1990, 

Recharge areas of the Floridan aquifer in the Crescent 

City Ridge of southeast Putnam County, Florida—a 

pilot study: St. Johns River Water Management District Technical Publication SJ 90-9, 72 p.

Brown, D.P., 1984, Impact of development on availability 

and quality of ground water in eastern Nassau County, 

Florida, and southeastern Camden County, Georgia: 

U.S. Geological Survey Water-Resources Investigations Report 83-4190, 113 p.

Bush, P.W., and Johnston, R.H., 1988, Ground-water 

hydraulics, regional flow, and ground-water development of the Floridan aquifer system in Florida and in 

parts of Georgia, South Carolina, and Alabama: U.S. 

Geological Survey Professional Paper 1403-C, 80 p., 

17 pl.

Campbell, Dean, Bergman, Martinus, Curtis, Donna, 

Koenig, Tracy, Morris, Fred, Neubauer, Clifford, 

Schell, John, Watkins, Bill, 1989, Swim plan for the 

Lower St. Johns River Basin: St. Johns River Water 

Management District, 134 p.

Campbell, Dean, Munch, D.A., Johnson, Richard, Parker, 

M.P., Parker, Bruce, Rao, D.V., Marella, Richard, and 

Albanesi, Edward, 1984, Section III, Water administration and regional management, Chapter 13, St. Johns 

River Water Management District, in Fernald, E.A., 

and Patton, D.J., eds., Water resources atlas of Florida: 

Tallahassee, Florida State University, p. 158-177.

Causey, L.V., and Phelps, G.G., 1978, Availability and quality of water from shallow aquifers in Duval County, 

Florida: U.S. Geological Survey Water-Resources 

Investigations 78-92, 36 p.

Chen, C.S., 1965, The regional lithostratigraphic analysis of 

Paleocene and Eocene rocks of Florida: Florida Geological Survey Bulletin 45, 105 p.

Clark, W.E., Musgrove, R.H., Menke, C.G., and Cagle, 

J.W., Jr., 1964, Water resources of Alachua, Bradford, 

Clay, and Union Counties, Florida: Florida Geological 

Survey Report of Investigations No. 35, 170 p.

Conover, C.S., and Leach, S.D., 1975, River basin and 

Hydrologic Unit map of Florida: Florida Bureau of 

Geology Map Series 72.

Cooke, C.W., 1945, Geology of Florida: Florida Geological 

Survey Bulletin 29, 339 p.

Cooper, H.H., Jr., 1942, The possibility of saltwater intrusion in northeast Florida: U.S. Geological Survey 

Open-File Report FL-42004.

———1944, Ground-water investigations in Florida (with 

special reference to Duval and Nassau Counties): 

American Water Works Association Journal v 36, 

no. 2, p. 169-185.

Fairchild, R.W., 1972, The shallow-aquifer system in Duval 

County, Florida: Florida Bureau of Geology Report of 

Investigations no. 59, 50 p.

———1977, Availability of water in the Floridan aquifer in 

southern Duval and northern Clay and St. Johns Counties, Florida: U.S. Geological Survey Water-Resources 

Investigations 76-98, 53 p.

Fairchild, R.W., and Bentley, C.B., 1977, Saline-water 

intrusion in the Floridan aquifer in the Fernandina 

Beach area, Nassau County, Florida: U.S. Geological 

Survey Water-Resources Investigations 77-32, 27 p.

Franks, B.J., and Phelps, G.G., 1979, Estimated drawdowns 

in the Floridan aquifer due to increased withdrawals, 

Duval County, Florida: U.S. Geological Survey WaterResources Investigations 79-84, 22 p.

Frazee, J.M., Jr., and McClaugherty, D.R., 1979, Investigation of ground- water resources and saltwater intrusion 

in the coastal areas of northeast Florida: Palatka, Fla., 

St. Johns River Water Management District Technical 

Report No. 3, 136 p.

Hayes, E.C., 1981, The surficial aquifer in east-central St. 

Johns County, Florida: U.S. Geological Survey WaterResources Investigations 81-14, 19 p.

Johnston, R.H., and Bush, P.W., 1988, Summary of the 

hydrology of the Floridan aquifer system in Florida 

and in parts of Georgia, South Carolina, and Alabama: 

U.S. Geological Survey Professional Paper 1403-A, 

24 p., 4 pls.
Selected References 31

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