Evaluation of the Feasibility of Freshwater 

Injection Wells in Mitigating Ground-Water 

Quality Degradation at Selected Well Fields in 

Duval County, Florida

By Nicasio Sepúlveda and Rick M. Spechler

U.S. Geological Survey

Water-Resources Investigations Report 03–4273

Prepared in cooperation with the

Jacksonville Electric Authority

St. Johns River Water Management District

Tallahassee, Florida

2004
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes 

only and does not constitute endorsement by the U.S. Geological Survey.

For additional information 

write to: 

U.S. Geological Survey

2010 Levy Avenue

Tallahassee, FL 32310

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

Additional information about water resources in Florida is available on the internet 

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

CONTENTS

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

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

Purpose and Scope ....................................................................................................................................................... 5

Description of Study Area ........................................................................................................................................... 5

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

Acknowledgments........................................................................................................................................................ 6

Hydrogeologic Framework .................................................................................................................................................... 7

Surficial Aquifer System.............................................................................................................................................. 9

Intermediate Confining Unit ........................................................................................................................................ 9

Floridan Aquifer System.............................................................................................................................................. 9

Upper Floridan Aquifer...................................................................................................................................... 9

Middle Semiconfining Unit ............................................................................................................................... 10

Lower Floridan Aquifer and Fernandina Permeable Zone ................................................................................ 10

Potential for Upward Flow of Poor-Quality Water ...................................................................................................... 11

Simulation of Ground-Water Flow......................................................................................................................................... 11

Conceptual Model........................................................................................................................................................ 12

Steady-State Flow Approximation............................................................................................................................... 13

Average Hydrologic Conditions for 2000.................................................................................................................... 14

Calibration of Ground-Water Flow Model................................................................................................................... 19

Hydraulic Properties from Calibrated Model .............................................................................................................. 23

Simulated Potentiometric Surfaces .............................................................................................................................. 30

Ground-Water Flow Budget......................................................................................................................................... 33

Sensitivity Analyses..................................................................................................................................................... 36

Application of Ground-Water Flow Model............................................................................................................................ 36

Application of Steady-State Ground-Water Flow Model............................................................................................. 39

Transient Ground-Water Flow Model.......................................................................................................................... 44

Effects of Parameter Uncertainty on Simulated Upward Flow from the Fernandina Permeable Zone................................. 52

Vertical Leakance of Semiconfining Unit.................................................................................................................... 52

Transmissivity of the Fernandina Permeable Zone...................................................................................................... 52

Specified-Head Cells Along the Lateral Boundaries of the Fernandina Permeable Zone........................................... 55

Model Limitations.................................................................................................................................................................. 56

Summary and Conclusions .................................................................................................................................................... 56

References.............................................................................................................................................................................. 58

FIGURES

1. Map showing location of wells tapping the Upper Floridan aquifer, the upper zone of the Lower Floridan 

aquifer, and the Fernandina permeable zone in and near the area with well fields of interest ................................... 3

2. Map showing location of hydrogeologic sections and average chloride concentrations measured during 

2002 at selected Jacksonville Electric Authority well fields....................................................................................... 4

3. Diagram showing stratigraphic units, general lithology, and hydrogeologic units in Duval County, Florida ............ 7

4. Graphs showing generalized hydrogeologic sections A-A' and B-B' ......................................................................... 8

5. Diagram showing geologic units and corresponding layering scheme in the model.................................................. 12

6-9. Maps showing:

6. Estimated altitude of the water table of the surficial aquifer system, average 2000 conditions ........................ 14

7. Average 2000 heads in the Upper Floridan aquifer, upper zone of the Lower Floridan aquifer, and 

Fernandina permeable zone................................................................................................................................ 15
IV Contents

8. Linearly regressed potentiometric surface of the Upper Floridan aquifer for the average 2000 

conditions obtained from average 1993-94 conditions...................................................................................... 17

9. Specified heads along the lateral boundary cells of the upper zone of the Lower Floridan aquifer, 

average 2000 conditions .................................................................................................................................... 18

10. Graph showing comparison of simulated to measured heads in all hydrogeologic units for the 

calibrated model ......................................................................................................................................................... 20

11-22. Maps showing:

11. Average 2000 ground-water withdrawal rates from the Upper Floridan aquifer............................................... 21

12. Average 2000 ground-water withdrawal rates from the upper zone of the Lower Floridan aquifer ................. 22

13. Transmissivity of the Upper Floridan aquifer from the calibrated model ......................................................... 23

14. Transmissivity of the upper zone of the Lower Floridan aquifer from the calibrated model ............................ 24

15. Leakance of the intermediate confining unit from the calibrated model .......................................................... 25

16. Simulated vertical leakage rates to and from the Upper Floridan aquifer through the 

intermediate confining unit, average 2000 conditions....................................................................................... 26

17. Leakance of the middle semiconfining unit from the calibrated model ............................................................ 27

18. Simulated vertical leakage rates from the upper zone of the Lower Floridan aquifer through the middle

semiconfining unit, average 2000 conditions .................................................................................................... 28

19. Simulated vertical leakage rates from the Fernandina permeable zone through the semiconfining unit, 

average 2000 conditions .................................................................................................................................... 29 

20. Simulated potentiometric surface of the Upper Floridan aquifer, average 2000 conditions ............................. 30

21. Simulated potentiometric surface of the upper zone of the Lower Floridan aquifer, average 

2000 conditions.................................................................................................................................................. 31

22. Simulated potentiometric surface of the Fernandina permeable zone, average 2000 conditions...................... 32

23. Diagram showing simulated steady-state volumetric flow budget for the model area, average 2000 conditions...... 33

24. Map showing simulated steady-state lateral flow to and from the upper zone of the Lower Floridan aquifer 

across model boundaries, average 2000 conditions.................................................................................................... 34

25. Map showing simulated steady-state lateral flow to and from the Fernandina permeable zone across model 

boundaries, average 2000 conditions.......................................................................................................................... 35

26. Diagram showing simulated steady-state volumetric flow budget for the well field subareas of Brierwood, 

Deerwood 3, Main Street, and Community Hall, time-averaged 2000 conditions..................................................... 37

27. Graph showing model sensitivity to changes in selected model parameters.............................................................. 38

28. Map showing steady-state head buildups in the upper zone of the Lower Floridan aquifer after 

6 million gallons per day (Mgal/d) are injected into Brierwood and 12 Mgal/d are injected into 

Deerwood 3 wells ....................................................................................................................................................... 40

29-31. Diagrams showing:

29. Simulated steady-state net volumetric flow budget differences between flows before and 

after withdrawing 18 million gallons per day (Mgal/d) from the Main Street well field and injecting 

the water into the upper zone of the Lower Floridan aquifer in Brierwood (6 Mgal/d) and 

Deerwood 3 (12 Mgal/d) wells .......................................................................................................................... 41

30. Simulated steady-state net volumetric flow budget differences for the well field subareas 

between flows before and after withdrawing 18 million gallons per day (Mgal/d) from the Main 

Street well field and injecting the water into the upper zone of the Lower Floridan aquifer in 

Brierwood (6 Mgal/d) and Deerwood 3 (12 Mgal/d) wells ............................................................................... 42

31. Simulated steady-state net volumetric flow budget differences for the well field subareas 

between flows before and after withdrawing 18 million gallons per day (Mgal/d) from the Main 

Street well field and injecting the water into the Upper Floridan aquifer in Brierwood (6 Mgal/d) and 

Deerwood 3 (12 Mgal/d) wells .......................................................................................................................... 43

32-38. Graphs showing:

32. Simulated monthly upward flows from the Fernandina permeable zone for 2000, for scenarios 1 

through 6, with injection rates of 6 million gallons per day (Mgal/d) in Brierwood and 12 Mgal/d in 

Deerwood 3 well fields ...................................................................................................................................... 47

33. Simulated monthly upward flows from the Fernandina permeable zone for 2000, for scenarios 7 

through 12, with injection rates of 6 million gallons per day (Mgal/d) in Brierwood and 12 Mgal/d in 

Deerwood 3 well fields ...................................................................................................................................... 48
Contents V

34. Simulated monthly vertical flows between the Fernandina permeable zone and the upper zone of 

the Lower Floridan aquifer for 2000 under scenario 8 conditions, for various injection rates in 

Brierwood and Deerwood well fields................................................................................................................. 50

35. Simulated monthly upward flows from the Fernandina permeable zone for 2000, under scenario 8 

conditions, for various withdrawal rates at Brierwood and Deerwood 3 well fields, and where water is 

injected, at the rate of 18 million gallons per day, in Deerwood 3 well field only ............................................ 51

36. Simulated monthly vertical flows between the Fernandina permeable zone and the upper zone of the 

Lower Floridan aquifer for 2000, under scenario 8 conditions, for various vertical leakances of the 

semiconfining unit overlying the Fernandina permeable zone, and where water is injected, at the rate 

of 18 million gallons per day, in Deerwood 3 well field only............................................................................ 53

37. Simulated monthly vertical flows between the Fernandina permeable zone and the upper zone of the 

Lower Floridan aquifer for 2000, under scenario 8 conditions, for various transmissivities of the 

Fernandina permeable zone, and where water is injected, at the rate of 18 million gallons per day, 

in Deerwood 3 well field only............................................................................................................................ 54

38. Simulated monthly upward flows from the Fernandina permeable zone for 2000, under scenario 8 

conditions, for various differences in specified heads, between the Fernandina permeable zone 

and the upper zone of the Lower Floridan aquifer, along lateral boundaries of the model, and where 

water is injected, at the rate of 18 million gallons per day, in Deerwood 3 well field only............................... 55

TABLES

1. Maximum chloride concentrations measured in ground water at selected wells in Duval County, Florida, 

1999 to 2002 ............................................................................................................................................................ 2 

2. Site identification numbers of wells used in this study and corresponding local well numbers ............................. 6

3. Geographical information system coordinates of the corners of the ground-water flow model grid...................... 11 

4. List of parameter values used to compute the average 2000 freshwater and environmental-water heads 

at Fernandina permeable zone wells........................................................................................................................ 18

5. Water-level residual statistics for the calibrated steady-state model ....................................................................... 20

6. Water-level residual statistics for the calibrated transient model ............................................................................ 45

7. Monthly withdrawals during 2000 at Brierwood, Deerwood 3, and Main Street well fields ................................. 46

8. Simulated scenarios of injection, withdrawal, and intervening rest months at Brierwood and 

Deerwood 3 well fields............................................................................................................................................ 46
VI Contents

CONVERSION FACTORS, DATUMS, ACRONYMS, AND ABBREVIATIONS 

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times foot of aquifer thickness [(ft

3

/d)/ft2]ft. 

In this report, the mathematically reduced form, foot squared per day (ft2

/d), is used for convenience.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F-32)/1.8

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29).

Horizontal coordinate information (latitude-longitude) is referenced to the North American Datum of 1927 (NAD27).

Acronyms and abbreviations used in report:

CY calendar year

FPZ Fernandina permeable zone

FAS Floridan aquifer system

g/mL grams per milliliter

ICU intermediate confining unit

JEA Jacksonville Electric Authority

LFA Lower Floridan aquifer

MSCU middle semiconfining unit

mg/L milligrams per liter

MLR multiple linear regression

MODFLOW U.S. Geological Survey Modular Three-Dimensional Ground-Water Flow Model

NWIS National Water Information System

RMS root-mean-square

SCU semiconfining unit

SLR simple linear regression

SJRWMD St. Johns River Water Management District

S storage coefficient

SAS surficial aquifer system

UFA Upper Floridan aquifer 

uzLFA upper zone of Lower Floridan aquifer

USGS U.S. Geological Survey

Multiply By To obtain

Length

inch (in.) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

acre 0.4047 hectare

square mile (mi2) 2.590 square kilometer 

Flow Rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second 

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

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

Hydraulic Conductivity

foot per day (ft/d) 0.3048 meter per day

*Transmissivity

foot squared per day (ft2/d) 0.09290 meter squared per day 

Leakance

foot per day per foot [(ft/d)/ft] 1.0 meter per day per meter
Abstract 1

Evaluation of the Feasibility of Freshwater Injection 

Wells in Mitigating Ground-Water Quality Degradation at 

Selected Well Fields in Duval County, Florida

By Nicasio Sepúlveda and Rick M. Spechler

ABSTRACT

The Fernandina permeable zone contains 

brackish water in parts of Duval County, Florida. 

Upward flow from the Fernandina permeable zone 

to the upper zone of the Lower Floridan aquifer 

increases chloride concentrations in ground water 

in parts of Duval County. Numerical models of the 

ground-water flow system in parts of Duval, St. 

Johns, and Clay Counties, Florida, were used to 

(1) estimate the vertical flows between the lowquality water of the Fernandina permeable zone 

and the high-quality water of the upper zone of the 

Lower Floridan aquifer in the vicinity of Deerwood 3 and Brierwood well fields, based on 2000 

ground-water withdrawal rates; (2) determine how 

such vertical flows change as several scenarios of 

injection, withdrawal, and intervening rest periods 

are simulated in the two well fields; and (3) evaluate the effects of changes in less certain hydraulic 

parameters on the vertical flows between the 

Fernandina permeable zone and the upper zone of 

the Lower Floridan aquifer. The ground-water 

flow system was simulated with a four-layer 

model using MODFLOW-2000, which was developed by the U.S. Geological Survey. The first 

layer consists of specified-head cells simulating 

the surficial aquifer system with prescribed water 

levels. The second layer simulates the Upper 

Floridan aquifer. The third and fourth layers simulate the upper zone of the Lower Floridan aquifer 

and the Fernandina permeable zone, respectively. 

Average flow conditions in 2000 were approximated with a steady-state simulation. The changes 

in upward flow from the Fernandina permeable 

zone due to periods of injections and withdrawals 

were analyzed with transient simulations. The 

grid used for the ground-water flow model was 

uniform and composed of square 250-foot cells, 

with 400 columns and 400 rows.

The active model area encompasses about 

360 square miles in parts of Duval, St. Johns, and 

Clay Counties, Florida. Ground-water flow simulation was limited vertically to the bottom of the 

Fernandina permeable zone. The steady-state 

ground-water flow model was calibrated using 

time-averaged 2000 heads at 20 control points. 

Environmental-water heads in the Fernandina permeable zone were calculated for wells with variable water density. Transmissivity of the Upper 

Floridan aquifer, the upper zone of the Lower 

Floridan aquifer, and the Fernandina permeable 

zone, and the leakance of the intermediate confining unit, the middle semiconfining unit, and the 

semiconfining unit were obtained from regional 

ground-water flow models and adjusted until a 

reasonable fit between simulated and computed 

heads was obtained.

Root-mean-square residuals, calculated from 

simulated and time-averaged heads for the steadystate model, in the Upper Floridan aquifer, the 

upper zone of the Lower Floridan aquifer, and the 

Fernandina permeable zone were 1.75, 1.99, and 

1.14 feet, respectively. Based on the 20 control 

points from all units, the overall residual for the 

steady-state model was 1.75 feet. Monthly measured heads at 20 sites during May and September 

2000 and at 16 sites for the remaining months of 
2 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well 

Fields in Duval County, Florida

2000 were used to compute residuals for the 12 

one-month-duration stress periods. These residuals 

were used to calibrate storage coefficient. Rootmean-square residuals for the transient model, 

calculated from simulated heads at the end of the 

12 stress periods and time-averaged heads, in the 

Upper Floridan aquifer, the upper zone of the 

Lower Floridan aquifer, and the Fernandina permeable zone, were 1.52, 1.79, and 1.52 feet, respectively, with 1.78 feet being the overall residual.

The calibrated hydraulic properties from the 

steady-state ground-water flow model, and the calibrated storage coefficient from the transient model, 

were used to simulate hypothetical transient scenarios of injection, withdrawal, and intervening 

rest periods to assess changes in flow between the 

Fernandina permeable zone and the upper zone of 

the Lower Floridan aquifer. Based on the simulated 

flows between the Fernandina permeable zone and 

the upper zone of the Lower Floridan aquifer and 

the 18 million gallons per day of water available 

for injection, the reversal of the prevailing upward 

flow from the Fernandina permeable zone was not 

achieved. However, steady-state and transient simulations indicate that the upward flow of water 

from the Fernandina permeable zone could be 

reduced by as much as 64 percent, from 0.11 to 

0.04 cubic foot per second, if only injection periods 

are simulated.

INTRODUCTION

The Floridan aquifer system (FAS) is the principal source of water supply in northeast Florida. As the 

population of this area increases, the demand for water 

also increases. In some areas of Florida, declining 

water levels and increasing mineralization of ground 

water have become problems for local and state watermanagement officials. Water samples from the upper 

zone of the Lower Floridan aquifer (LFA) in the Deerwood 3 well field in Duval County, Florida (fig. 1) 

have chloride concentrations as high as 290 milligrams 

per liter (mg/L) as listed in table 1. This chloride concentration is greater than the secondary drinking water 

standard for chloride, which was set to 250 mg/L by 

the Florida Administrative Code (p. 56, table 4, 2000), 

and by the U.S. Environmental Protection Agency 

(2000). Projected increases in ground-water withdrawals from the upper zone of the LFA could cause further 

upward migration of water with high chloride concentrations from the Fernandina permeable zone (FPZ) to 

the upper zone of the LFA, resulting in increased chloride concentrations.

In 2000, the U.S. Geological Survey (USGS), in 

cooperation with the Jacksonville Electric Authority 

(JEA) and the St. Johns River Water Management District (SJRWMD), initiated an investigation to assess the 

effects of freshwater injection wells in mitigating 

ground-water degradation in the vicinity of selected 

well fields in Duval County, Florida. The ground-water 

flow model developed for this study was used to assess 

the effects of a series of injection, withdrawal, and 

intervening rest scenarios on the upward flow from 

some areas in the FPZ with greater chloride concentrations to the high-quality water of the upper zone of the 

LFA in northeastern Florida. The steady-state groundwater flow model was calibrated by using time-averaged heads for calendar year (CY) 2000 at 20 control 

points from the Upper Floridan aquifer (UFA), the 

upper zone of the LFA, and the FPZ. The transient simulations were performed using the calibrated hydraulic 

properties from the steady-state model, and the calibrated storage coefficient from the transient model.

Table 1. Maximum chloride concentrations measured in 

ground water at selected wells in Duval County, Florida, 

1999 to 2002

[Source: Jacksonville Electric Authority, written communication, 2003; 

mg/L, milligrams per liter]

Well field Well

number

Total 

depth 

(feet)

Date 

of

measurement

Chloride

concentration 

(mg/L)

Brierwood 6001 1,100 10-09-02 14

Brierwood 6002 1,100 11-19-02 18

Brierwood 6003 1,100 10-09-02 14

Brierwood 6004 1,100 10-09-02 17

Brierwood 6005 1,100 10-11-02 26

Deerwood 3 5701 980 10-02-02 71

Deerwood 3 5702 1,198 10-02-02 140

Deerwood 3 5703 1,180 10-02-02 290

Deerwood 3 5704 1,000 10-02-02 71

Deerwood 3 5705 1,000 10-02-02 94

Deerwood 3 5706 970 1-25-99 20

Main Street 0103 1,282 10-19-02 15

Main Street 0104 1,302 11-01-02 17

Main Street 0108 1,248 10-19-02 16

Main Street 0119 1,284 10-19-02 14

Main Street 0120 1,282 10-19-02 13

Community Hall M501 624 10-14-02 10

Community Hall M502 900 11-18-02 10

Community Hall M503 1,225 3-19-99 10

Community Hall M504 1,225 10-09-02 9.2

Community Hall M505 1,100 3-19-99 9.7
Introduction 3

pPROPOSED INJECTION WELL

1 PROPOSED PRODUCTION WELL

PRODUCTION WELLS -- Locations and names of selected well fields in model area DEERWOOD 3 .

D-1344 . FERNANDINA PERMEABLE ZONE WELL -- Location and identification

number of well tapping the Fernandina permeable zone

. UPPER ZONE OF LOWER FLORIDAN AQUIFER WELL -- Location and identification

number of well tapping the upper zone of the Lower Floridan aquifer

D-0094

. UPPER FLORIDAN AQUIFER WELL -- Location and identification number

of well tapping the Upper Floridan aquifer

D-3824

EXPLANATION

MODEL

AREA

Pa bl o C r e e k

B l a c k

C r e

e k

D o c t o r s L a k e

Julington

Cree

k

Ortega River

St. Johns R vi

er

Thoma

e

s Cr

ek

Ft. George

Island

ST. JOHNS COUNTY

DUVAL COUNTY

CLAY

COUNTY

NASSAU

COUNTY

. ....

p.p.

.

.

....

.

.. . .

. . ..

.

1

1p

p

p

MAIN STREET

DEERWOOD 3

BRIERWOOD

COMMUNITY

HALL

.

.

.

.

.

D-425B

D-3060

SJ-150

D-1344

D-2386

D-0075

.

. . .

.

.

.

D-0176

C-0094

D-1155

D-0450

D-4610

D-0263

.D-0592

D-0145

.

. .

. . .

.

. .

.

.

.

.

.

. . .

. .

.

. .

.

.

D-0420

D-3840 D-0264

D-0270

D-0395

D-122A

D-0018

D-0115

D-0129

D-1292

C-0005

C-0007

SJ-015

SJ-163

D-0169

SJ-005

SJ-063

SJ-060

D-0160

D-1313

D-2847

D-3824

D-3544

0 5 KILOMETERS

0 1 2 3 4 5 MILES

12 34

       	

	

	







Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

. ...

.

. ..

. .

.

.

. .

.

..

..

...

...

.

SOUTHEAST

RIDENOUR

OAKRIDGE

NORWOOD

FAIRFAX

RIVEROAKS

HENDRICKS

STUDY AREA

Jacksonville

Fernandina Beach

Crescent

Beach

Figure 1. Location of wells tapping the Upper Floridan aquifer, the upper zone of the Lower Floridan aquifer, and 

the Fernandina permeable zone in and near the area with well fields of interest.
4 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well 

Fields in Duval County, Florida

The injection and withdrawal scenarios simulated in this study are based on the assumption that 

18 million gallons per day (Mgal/d) will be withdrawn 

from the Main Street well field (fig. 1), in addition to 

the current withdrawals from this well field. This 

water will be pumped into the upper zone of the LFA 

injection wells in the Brierwood and Deerwood 3 well 

fields (fig. 2). Upward flows from the FPZ at the Brierwood and Deerwood 3 well fields are shown in this 

report with the purpose of assessing potential degradation of potable water in the upper zone of the Lower 

Floridan aquifer due to upward movement of brackish 

water from the FPZ in the vicinity of these two well 

fields. Steady-state simulations take into account

.

.

1 PROPOSED PRODUCTION WELL

p PROPOSED INJECTION WELL

EXPLANATION

EXISTING PRODUCTION WELL

WELL USED IN HYDROGEOLOGIC SECTION

AVERAGE CHLORIDE CONCENTRATION

AND AVERAGE TOTAL DEPTH – Top number is the

average chloride concentration measured among all

wells in the well field during fourth quarter of 2002.

Bottom number is average altitude of aquifer depth

penetrated by wells, in feet. Datum is NGVD 29

10

-1,090

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

Pa b l o C r e e k

D o c t o r s L a k e

St . J

o h n s R i v e r

O r t e g a

R i v

e r

J u l i n g t o n

C r e

e

k

  



	



0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

J J JJJ

pJpJ

J

J

JJJJ

J

BRIERWOOD

COMMUNITY

HALL

MAIN STREET JJ J J J J

JJ

J

1

1 p

p

p

DEERWOOD 3

.

. D-52A

D-425B

A A

B

B

D-1344

.

SOUTHEAST

OAKRIDGE

RIDENOUR

J JJJ

J J J

J J

J

J

J J

J

JJ

JJ

JJJ

JJJ

J

NORWOOD

FAIRFAX

HENDRICKS

RIVEROAKS

14

-1,253

13

-1,305

15

-1,266

13

-1,275

13

-1,210

18

-864

109

-1,130

14

-860

114

-1,015

17

-1,080

10

-1,090

Figure 2. Location of hydrogeologic sections and average chloride concentrations measured 

during 2002 at selected Jacksonville Electric Authority well fields.
Introduction 5

ground-water withdrawals from Brierwood and Deerwood 3 well fields, while transient simulations consider ground-water withdrawals from these two well 

fields only during stress periods of withdrawals. 

Ground-water withdrawals from all other well fields 

are considered in steady-state and transient simulations. Temporal variations in the upward flow from the 

FPZ resulting from several periods of injections 

followed by periods of withdrawals were simulated 

with the transient ground-water flow model.

Purpose and Scope

This report presents the results of a study to 

evaluate the feasibility of using injection and withdrawal scenarios to mitigate ground-water quality degradation of the potable water supply resulting from the 

upward flow of poor-quality water from the FPZ in 

well fields in Duval County, Florida. The injection and 

withdrawal scenarios were simulated using the calibrated transient model. The model was used to (1) estimate the vertical flows from the FPZ to the upper zone 

of the LFA, in particular in the vicinity of Deerwood 3 

and Brierwood well fields, based on 2000 groundwater withdrawal rates; (2) evaluate changes in the 

vertical flows under several hypothetical scenarios of 

injection, withdrawal, and intervening rest periods; 

and (3) evaluate the effects of selected parameter 

uncertainty on the simulated vertical flows between 

the FPZ and the upper zone of the LFA. A conceptual 

model of the flow system and applications of a finitedifference flow model based on this conceptualization 

are presented. The simulated scenarios are designed to 

represent realistic injection and withdrawal conditions 

considered by JEA. This report discusses the imposition of boundary conditions, regressions used to 

derive the specified heads along the lateral boundaries 

of the model, calibration strategies of steady-state 

simulations, sensitivity analyses, volumetric flow 

estimates among hydrogeologic units, and the 

transient simulation of injection, withdrawal, and 

intervening rest scenarios.

The initial distribution of hydraulic properties of 

the study area were obtained from Durden (1997) and 

Sepúlveda (2002a). The geologic structure of the study 

area was analyzed from geophysical logs and interpretive reports by Phelps and Spechler (1997); Spechler 

(1994, 1996); and Spechler and Wilson (1997).

Geographical information system data bases 

(Environmental Systems Research Institute, Inc., 1997) 

were developed to manage spatially distributed information that covered the model area. Digital coverages 

were projected into a uniform coordinate system to 

achieve consistency of coordinate systems among data 

bases. All data bases were projected to the Albers 

equal-area conic projection with standard parallel 

29o

30', 45o30', and central meridian -83o00' (Snyder, 

1983). The 1927 North American Datum was used for 

all data bases generated in this study; the unit length 

was feet.

Description of Study Area

Public-water supply wells in Duval County, 

Florida, are classified by location relative to the St. 

Johns River. Well fields southeast of the St. Johns 

River are referred to as south grid wells, whereas well 

fields northwest of the St. Johns River are referred to 

as north grid wells. Bierwood and Deerwood 3 well 

fields are in the south grid; Main Street well field is in 

the north grid (fig. 1).

The study area encompasses most of the well 

fields in the south grid of Duval County (fig. 2). Those 

wells south or east of the St. Johns River are referred 

to as the south grid. Some north-grid well fields are 

within the study area. The extent of the study area 

(fig. 2) is about 19 miles (mi) north to south from central Duval County to northern St. Johns County and 

about 19 mi west to east in Duval County. The landsurface altitude ranges from sea level to about 60 feet 

(ft). Most of the study area is characterized as a 

ground-water discharge area except in the north-central part, where the water-table altitude is higher than 

the potentiometric surface of the underlaying FAS. 

The climate is classified as subtropical and is characterized by warm, normally wet summers and mild, 

dry winters.

Within Duval County, Florida, the FPZ is the 

deepest productive unit of the FAS. The FPZ is characterized by increasing chloride concentrations in areas 

roughly east of longitude -81o

35' (fig. 2). The freshwater-saltwater interface in the FPZ is estimated to be 

east of Brierwood but west of the Deerwood 3 well 

field, based on chloride concentrations measured at 

well fields tapping the upper zone of the LFA. The 

upward flow of ground water from the FPZ causes 

increased chloride concentrations in wells tapping the 

upper zone of the LFA. The increased chloride concentrations are not observed in well fields that tap only 

the UFA. The water to be injected into the Deerwood 3 
6 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well 

Fields in Duval County, Florida

and Brierwood wells is proposed to be withdrawn 

from a north-grid well field, Main Street, located in the 

northwestern part of the study area. Chloride concentrations measured in the Main Street well field are less 

than 20 mg/L.

Ground-water withdrawals for CY 2000 within 

the study area totaled about 111 Mgal/d, or nearly 

172 cubic feet per second (ft3/s), distributed as 

23 and 88 Mgal/d, or about 36 and 136 ft3/s, from 

the Upper and Lower Floridan aquifers, respectively 

(Thomas Lund, Jacksonville Electric Authority, 

written commun., 2001). This includes 90 Mgal/d for 

public-water supply (including estimated pumping 

from self-supplied domestic wells), 19 Mgal/d for 

commercial or industrial (including thermoelectricpower generation and recreational uses), and 2 Mgal/d 

for irrigation purposes. All ground-water withdrawals 

were compiled from consumptive user permit data 

bases and water-use data files from the SJRWMD and 

biannual operating reports by JEA based on meter 

readings. The locations of self-supplied domestic 

wells were obtained from a data base supplied by the 

City of Jacksonville (Jason C. Sheasley, written 

commun., 2002). The estimated water-use rate from 

self-supplied domestic wells in Duval County was 

assumed to be 167 gallons per person per day 

(Beth Wilder, St. Johns River Water Management 

District, written commun., 2002).

Well-Numbering System

Two well-numbering systems are used in this 

report. The first is a 15-digit number based on latitude 

and longitude, used to identify wells in the USGS 

National Water 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. The second numbering system is based on 

local well numbers. Local numbers have been assigned 

to wells in each county in northeastern Florida as the 

wells were inventoried. The prefixes D, SJ, and C 

denote wells in Duval, St. Johns, and Clay Counties, 

respectively. All local numbers were assigned by the 

USGS, except for well number D-1344, which was 

assigned by the SJRWMD (table 2).

Acknowledgments

The authors would like to thank Jacksonville 

Electric Authority and the St. Johns River Water Management District for providing the 2000 water-use data 

for the study area.

Table 2. Site identification numbers of wells 

used in this study and corresponding local 

well numbers

[UFA, Upper Floridan aquifer; uzLFA, upper zone of the 

Lower Floridan aquifer; FPZ, Fernandina permeable zone; 

USGS, U.S. Geological Survey]

USGS site 

identification

Local well

number

Aquifer

300507081272701 SJ-163 UFA

300649081485901 C-0005 UFA

300717081381001 SJ-015 UFA

300758081230501 SJ-005 UFA

300824081305401 D-0169 UFA

300834081421301 C-0007 UFA

300926081343002 D-1313 UFA

301157081465201 D-1292 UFA

301212081252401 SJ-063 UFA

301333081324101 D-2847 UFA

301408081253101 SJ-060 UFA

301551081415701 D-0129 UFA

301617081421601 D-0115 UFA

301710081323603 D-3824 UFA

301844081403801 D-0018 UFA

301846081350901 D-3544 UFA

301852081234201 D-0160 UFA

302304081383202 D-122A UFA

302330081463001 D-0420 UFA

302550081331501 D-3840 UFA

302608081354903 D-0264 UFA

302724081244801 D-0395 UFA

302801081375101 D-0145 UFA

300656081463401 C-0094 uzLFA

301522081331303 D-4610 uzLFA

301537081441901 D-0075 uzLFA

301604081361501 D-0450 uzLFA

301639081330802 D-1155 uzLFA

302022081393501 D-0176 uzLFA

302127081411002 D-52A uzLFA

302227081435001 D-0592 uzLFA

302608081354902 D-0263 uzLFA

301132081225801 SJ-150 FPZ

301345081421701 D-13441

1

Local number assigned by St. Johns River Water 

Management District.

FPZ

301817081374902 D-425B FPZ

302052081323201 D-3060 FPZ

302159081235601 D-2386 FPZ
Hydrogeologic Framework 7

HYDROGEOLOGIC FRAMEWORK

The study area is underlain by a thick sequence 

of sedimentary rocks that overlie deeper volcanic, 

metamorphic, and sedimentary rocks. The primary 

water-bearing sediments are composed of limestone, 

dolomite, shell, and sand that range in age from late 

Paleocene to Holocene. Stratigraphic units and corresponding hydrogeologic units penetrated by wells in 

the study area are described in figure 3. Stratigraphic 

units, in ascending order, are: the 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 Group of Miocene age, and the undifferentiated surficial deposits of late Miocene to 

Holocene age.

The principal water-bearing units in the study 

area are the surficial aquifer system (SAS) and the 

FAS. The two aquifer systems are separated by the 

intermediate confining unit (ICU), which contains 

beds of lower permeability sediments that confine the 

water in the FAS. The three major water-bearing zones 

of the FAS (SAS, UFA, and LFA) are separated by 

less-permeable semiconfining units. Underlying the 

FAS are low permeability limestone and dolomite that 

contain considerable gypsum and anhydrites, which 

mark the base of the FAS. Generalized hydrogeologic 

sections based on geophysical and geologists’ logs 

were generated to show the thicknesses of the 

hydrogeologic units (fig. 4).

Holocene

to Late

Miocene

Miocene

Eocene

Paleocene Cedar Keys

Formation

Oldsmar

Formation

Avon Park

Formation

Ocala

Limestone

Hawthorn

Group

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

Sub-Floridan

confining unit

Fernandina

permeable zone

(FPZ)

Semiconfining

unit

Upper

zone

Middle semiconfining

unit

(MSCU)

Upper Floridan

aquifer

(UFA)

Intermediate

confining unit

(ICU)

Surficial aquifer

system

(SAS)

Sand, shell, limestone, and

coquina deposits provide

local water supplies.

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.

Public-water supply source.

Water from some wells shows

increasing salinity.

Low permeability limestone

and dolomite.

Public-water supply source.

High permeability. Water from

some wells shows increasing

salinity.

Low permeability limestone

and dolomite.

Salinity increases with depth.

Low permeability; contains

highly saline water.

Lower Floridan aquifer

(LFA)

Floridan aquifer system

(FAS)

Figure 3. Stratigraphic units, general lithology, and hydrogeologic units in Duval County, Florida (modified from Spechler, 

1994).
8 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well 

Fields in Duval County, Florida

.....................................................................................................

.....................................................................................................

.....................................................................................................

.....................................................................................................

.....................................................................................................

.....................................................................................................

................................................................

................................................................

................................................................

................................................................

................................................................

................................................................ SURFICIAL AQUIFER SYSTEM

INTERMEDIATE

CONFINING UNIT

UPPER FLORIDAN

AQUIFER

MIDDLE

SEMICONFINING UNIT

SEMICONFINING UNIT

FERNANDINA PERMEABLE ZONE

SURFICIAL AQUIFER SYSTEM (SAS)

INTERMEDIATE

CONFINING UNIT

(ICU)

UPPER FLORIDAN AQUIFER

(UFA)

MIDDLE

SEMICONFINING UNIT

(MSCU)

FERNANDINA PERMEABLE ZONE (FPZ)

SEMICONFINING UNIT

UPPER ZONE OF

LOWER FLORIDAN AQUIFER

UPPER ZONE OF

LOWER FLORIDAN AQUIFER (uzLFA)

SUB-FLORIDAN CONFINING UNIT

0

0 5 KILOMETERS

5 MILES

100

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

100

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

FEET

A WEST A EAST

FEET

B NORTH B SOUTH

FEET

FEET

100

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

100

NGVD 29

200

400

600

800

1,000

1,200

1,400

1,600

1,800

2,000

VERTICAL SCALE GREATLY EXAGGERATED

VERTICAL SCALE GREATLY EXAGGERATED

2,200 2,200

NGVD 29

NGVD 29 NGVD 29 Total depth 2,020 ft Total depth 2,486 ft 440 ft 400 ft

?

D-1344

St. Johns River

Brierwood

well field

Deerwood 3

well field

St. Johns River

D-52A

D-425B

Brierwood

well field

Community Hall

well field

?

? ?

? ?

? ?

? ?

EXPLANATION

DASHED WHERE

INFERRED

Figure 4. Generalized hydrogeologic sections A-A' and B-B' (section lines shown in figure 2).
Hydrogeologic Framework 9

Surficial Aquifer System

The SAS is the uppermost water-bearing unit in 

the study area. The SAS sediments are of late Miocene 

to Holocene age, and generally consist of interbedded 

quartz sand, shell, and clay with some beds of dolomitic limestone. The deposits generally are discontinuous; the lithology and texture of the deposits can vary 

considerably over short distances both vertically and 

laterally. In much of the area, the SAS has two waterproducing zones separated by beds of lower permeability. The aquifer generally is unconfined, but may 

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

thickness of the SAS is variable, ranging from about 

20 to 120 ft in the study area.

Intermediate Confining Unit

The ICU underlies the SAS and consists primarily of the Hawthorn Group of late-to-middle Miocene 

age. The unit consists of interbedded clay, silt, sand, 

limestone and dolomite containing abundant amounts 

of phosphatic sand, granules, and pebbles. Throughout 

the study area, the ICU serves as a confining layer that 

restricts the vertical movement of water between the 

SAS and the UFA. The thickness of the ICU varies 

from more than 500 ft north of Deerwood 3 well field 

to less than 250 ft in the extreme northern part of 

St. Johns County. The thickness of the ICU ranges 

from about 420 ft at the Community Hall and Deerwood well fields, to about 440 ft at the Brierwood well 

field (fig. 4).

Floridan Aquifer System

The FAS, the principal source of ground water 

in northeastern Florida, underlies all of Florida, and 

parts of Alabama, Georgia, and South Carolina. Miller 

(1986, p. B45) defined the FAS as a vertically continuous sequence of carbonate rocks of generally high permeability that is hydraulically connected in varying 

degrees and whose permeability is, in general, one to 

several orders of magnitude greater than those rocks 

that bound the system. In the study area, the aquifer is 

composed of a sequence of highly permeable carbonate rocks of Eocene and Late Paleocene age that averages about 1,650 ft in thickness and includes the 

following stratigraphic units in descending order: the 

Ocala Limestone, the Avon Park Formation, the Oldsmar Formation, and the upper part of the Cedar Keys 

Formation (fig. 3).

The FAS is divided into two aquifers of relatively high permeability, referred to as the Upper Floridan and the Lower Floridan aquifers. The waterbearing zones within the FAS consist of soft, porous 

limestone and porous highly fractured dolomite beds. 

These aquifers are separated by a less permeable unit 

called the middle semiconfining unit (MSCU), which 

restricts the vertical movement of water within the 

aquifer. The LFA can be subdivided into two principal 

water-bearing zones, the upper zone of the LFA and 

the FPZ, separated by a less permeable unit. The UFA 

produces freshwater, but mineralization increases with 

depth. Monitor well D-2386 in eastern Duval County 

(fig. 1), drilled to a depth of 2,026 ft, showed chloride 

concentrations increasing from 6.4 mg/L in the UFA to 

3,300 mg/L in the FPZ (Brown and others, 1984).

Upper Floridan Aquifer 

The UFA generally corresponds to the Ocala 

Limestone, and in some areas also includes the uppermost part of the Avon Park Formation. 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 top of the UFA is about 450 to 550 ft below 

NGVD 29 in the study area (fig. 4). However, in specific locations such as the southwestern part of the 

study area near Jacksonville Naval Air Station, the 

top of the UFA could be as shallow as 250 ft below 

NGVD 29 (Spechler, 1994). The top of the UFA averages about 450 ft below NGVD 29 at the Community 

Hall well field and about 500 ft below NGVD 29 at the 

Brierwood and Deerwood 3 well fields.

The surface of the UFA is irregular and paleokarstic, and includes sinkhole-like depressions. Some 

of the depressions could be erosional features formed 

before the Hawthorn Group was deposited. However, 

most were formed by sinkhole collapse caused by the 

gradual dissolution of the underlying carbonate material. Marine seismic reflection profiles show that the 

continental shelf off the coast of northeastern Florida 

is underlain by solution-deformed limestone of Late 

Cretaceous to Eocene age (Meisburger and Field, 

1976; Popenoe and others, 1984; Kindinger and others, 2000). Dissolution and collapse features are scattered throughout the area. Seismic reflection 
10 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

investigations along the St. Johns River in northeastern 

Florida by Snyder and others (1989) and Spechler 

(1994, 1996) also revealed the presence of buried collapse and other karstic features that originated in the 

rocks of the FAS. Using land-based seismic reflection, 

such features also were observed in Duval and St. 

Johns County (Odum and others, 1997). At Fort 

George Island, located northeast of the study area in 

eastern Duval County (fig. 1), land-based seismic 

reflection surveys show a large solution feature estimated to measure about 650 ft by 1,550 ft (Odum and 

others, 1997).

Seismic profiles also show that the karst solution feature likely breached the MSCU within the FAS 

and possibly the semiconfining unit (SCU) that separates the upper zone of the LFA from the FPZ. Two 

collapse features that are visible in the seafloor off the 

coast of St. Johns County were studied by Spechler 

and Wilson (1997) and Swarzenski and others (2001). 

The largest of the two, Red Snapper Sink, is located 

26 miles east of Crescent Beach, Florida, and is 

approximately 400 ft in diameter and 482 ft deep 

(Spechler and Wilson, 1997). Collapse features can 

create conduits of relatively high vertical hydraulic 

conductivity, providing a hydraulic connection 

between freshwater zones and deeper, more saline 

zones within the aquifer system.

Middle Semiconfining Unit

The MSCU separates the UFA and LFA and is 

composed of beds of relatively less permeable limestone and dolostone of variable thickness. In the study 

area, the MSCU generally is present in the upper part 

of the Avon Park Formation, but also can include the 

lower part of the Ocala Limestone, where hard dolostone or limestone is present. Flow logs indicate that 

the MSCU is considerably less transmissive than 

either the UFA or LFA, and the unit restricts vertical 

ground-water flow in the aquifer system.

The top of the MSCU, determined primarily by 

using flow logs, is variable throughout the area and 

ranges from about 700 to 800 ft below NGVD 29. The 

top of the unit generally is recognized by a decrease in 

flow as observed on flowmeter logs. Thickness of the 

unit ranges from about 100 to 250 ft over the study 

area and ranges from about 165 ft at the Community 

Hall well field to about 200 ft at the Deerwood 3 and 

Brierwood well fields.

Lower Floridan Aquifer and Fernandina Permeable 

Zone

The LFA underlies the MSCU and includes the 

lower part of the Avon Park Formation, all of the Oldsmar Formation, and the upper part of the Cedar Keys 

Formation. The aquifer is highly productive and is 

composed of alternating beds of limestone and dolomite. The LFA contains two main water-bearing 

zones, the upper zone of the LFA and the FPZ, separated by a less-permeable semi-confining unit. The top 

of the upper zone of the LFA usually can be identified 

on flow logs as an interval contributing a noticeable 

increase in flow to the well. Permeability within this 

zone is related mostly to secondary porosity developed 

along bedding planes, joints, and fractures, developed 

by repeated episodes of active dissolution of the rock 

matrix (Phelps and Spechler, 1997).

Flowmeter logs show that the upper zone of the 

LFA commonly contains a single flow zone, whereas 

in other areas, less permeable strata separate two 

distinct flow zones (Leve, 1966). The top of the upper 

zone of the LFA is variable throughout the study area 

and generally ranges from about 800 to 950 ft below 

NGVD 29. At the Community Hall well field, flowmeter traverses indicate that the altitude of the top of the 

upper zone of the LFA is about 875 ft below NGVD 29 

(fig. 4). At the Brierwood and Deerwood 3 well fields, 

the top of the aquifer is estimated at about 900 and 

950 ft below NGVD 29, respectively. At Brierwood, 

Deerwood 3, and Community Hall well fields, the total 

thickness of the LFA, including the SCU and the FPZ, 

ranges from about 1,150 to 1,250 ft.

The FPZ is a high-permeability unit that lies at 

the base of the FAS in parts of southeastern Georgia 

and northeastern Florida (Miller, 1986, p. B70). In the 

areas of Fernandina Beach and Jacksonville (fig. 1), 

the unit is present in the lower Oldsmar and upper 

Cedar Keys Formations (Krause and Randolph, 1989, 

p. D23). The upper part of the zone consists of limestone that is commonly dolomitized and locally cavernous. Little is known about the extent or thickness of 

the FPZ because of the sparsity of data. In the few 

wells that have penetrated the zone in northeastern 

Florida, data indicate that the zone extends over the 

northern half of St. Johns and all of Duval and Nassau 

Counties. The top of the FPZ is estimated at 1,900 ft 

below NGVD 29 within the study area. The thickness 

of the zone is estimated to range from about 100 ft in 

the Jacksonville area to more than 500 ft in southeastern Georgia (Krause and Randolph, 1989, p. D23).
Simulation of Ground-Water Flow 11

The sub-Floridan confining unit underlies the 

LFA. The unit typically is characterized by low 

permeability and serves as the hydraulic base of the 

FAS. The sub-Floridan confining unit consists of 

dolomite and limestone deposits that may contain 

abundant evaporite minerals. The top of the subFloridan confining unit generally corresponds 

with the top of the Cedar Keys Formation in the 

study area.

Potential for Upward Flow of Poor-Quality 

Water

Chloride concentrations of ground water from 

the FPZ southeast of the St. Johns River generally are 

greater than those of the upper zone of the LFA 

throughout Duval County. Water from wells tapping 

the FPZ in the eastern part of Duval County generally 

has greater chloride concentration than water from 

wells farther inland (Sepúlveda, 2002a). The vertical 

leakance of the SCU and the vertical hydraulic gradient between the upper zone of the LFA and the FPZ 

determine the resulting upward flux of water. Most 

public-water supply wells in the study area deeper than 

900 ft penetrate the UFA and parts of the upper zone 

of the LFA; therefore the potential exists for upward 

migration of water of poor quality in areas where 

water in the FPZ has elevated chloride concentrations 

and where the vertical leakance of the SCU is large. 

Even in areas where the vertical leakance of the SCU 

might be relatively small, the presence of discrete 

fractures or deeply buried karst features provide pathways for upward migration of poor-quality water in 

areas of elevated chloride concentrations in the FPZ 

(Spechler, 1994). Ground-water development has 

resulted in increased upward flow from the FPZ 

through the fractures or karst features. A single fracture or solution feature was the source of brackish 

water in several wells in Duval County (Phelps and 

Spechler, 1997).

SIMULATION OF GROUND-WATER FLOW

MODFLOW-2000 (Harbaugh and others, 2000) 

was used to simulate ground-water flow in the FAS in 

Duval County. The regional ground-water flow 

system was simulated as a quasi three-dimensional 

ground-water flow model with four layers, 

representing the SAS, the UFA, the upper zone of 

the LFA, and the FPZ. A steady-state ground-water 

flow model in the FAS was constructed and 

calibrated to time-averaged data for CY 2000. 

Simulated hydraulic properties obtained from Durden 

(1997) and Sepúlveda (2002a) were integrated with 

the hydrogeologic data discussed in previous sections 

to generate the initial distribution of model 

parameters. The model parameters were further 

refined with calibration to time-averaged heads in 

2000. Monthly measured heads were used to calibrate 

the storage coefficient of the transient ground-water 

flow model. The calibrated transient model was used 

to assess the rate of upward flow from the FPZ to the 

upper zone of the LFA by simulating scenarios of 

injection, withdrawal, and intervening rest months at 

the Deerwood 3 and Brierwood well fields (fig. 2) 

while maintaining withdrawal rates for CY 2000 at 

all other wells. The potential range of values in 

vertical flow between the FPZ and the upper zone of 

the LFA due to parameter uncertainty was assessed 

with the calibrated model.

A uniformly spaced grid of square 250-ft cells 

was used to discretize the ground-water flow system 

horizontally. The coordinates of the grid corners given 

in table 3 facilitate reproduction of the grid. The grid 

consisted of 400 rows and 400 columns and was oriented along a north-south axis for simplicity because 

boundary conditions were not aligned along any particular axis. The solution of the ground-water flow 

equation allows for areal variations in transmissivity to 

simulate regional heterogeneities. Because no estimates of anisotropy were available, an isotropic transmissivity distribution was assumed.

Table 3. Geographical information system coordinates of 

the corners of the ground-water flow model grid

[Albers X and Y coordinates refer to Albers equal-area conic projection 

with standard parallels 29o

30' and 45o30' and central meridian -83o00'; 

UTM X and Y coordinates refer to Universal Transverse Mercator projection, zone 17 (Snyder, 1983)]

Grid 

corner

Albers X 

coordinate 

(meters)

Albers Y 

coordinate 

(meters)

UTM X 

coordinate 

(feet)

UTM Y 

coordinate 

(feet)

 Upper left 120208.16 813936.00 1405000 11030000

 Upper right 151255.75 813936.00 1505000 11030000

 Lower right 151255.75 782792.38 1505000 10930000

 Lower left 120208.16 782792.38 1405000 10930000
12 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

Conceptual Model

The SAS, UFA, the upper zone of the LFA, and 

the FPZ were designated layers 1 through 4, respectively (fig. 5). The SAS (layer 1) provides a specifiedhead boundary that, in part, determines the movement 

of water to and from the UFA. The areal distribution of 

the water-table altitude, estimated from the algorithm 

presented by Sepúlveda (2002b) by using river-stage 

data and water-level measurements from SAS wells 

for CY 2000, was used to specify heads in layer 1. 

Confining layers (ICU, MSCU, and SCU) were simulated by using vertical leakance arrays. A quasi threedimensional flow model was developed by simulating 

lateral flow within the aquifers and vertical flow 

within the confining units.

Average hydrologic conditions for 2000 (discussed below) were used for calibration of steady-state 

simulations in the model area. Hydrologic conditions 

change in time due to changes in ground-water withdrawal patterns and changes in discharge or recharge 

patterns. Specified heads for each month of CY 2000 

were derived by performing linear regressions between 

time-averaged heads for CY 2000 and monthly measured heads.

The vertical ground-water flow is determined by 

the vertical leakance of the confining units and the differences among the altitude of the water table of the 

SAS, and the heads in the UFA, the upper zone of the 

LFA, and the FPZ. The altitude of the water table of 

the SAS is influenced by recharge from rainfall and 

irrigation infiltration, and by diffuse upward leakage 

from the UFA in areas where the water table is below 

the potentiometric surface of the underlying UFA. The 

heads representing average hydraulic conditions for 

CY 2000 in the UFA, the upper zone of the LFA, and 

2,000

1,800

1,600

1,400

1,200

1,000

800

600

400

200

NGVD 29

100

FEET

LAND SURFACE

SURFICIAL AQUIFER SYSTEM (SAS)

INTERMEDIATE

CONFINING UNIT

(ICU)

UPPER FLORIDAN AQUIFER

(UFA)

MIDDLE

SEMICONFINING UNIT

(MSCU)

UPPER ZONE OF

LOWER FLORIDAN AQUIFER

(uzLFA)

SEMICONFINING UNIT

FERNANDINA PERMEABLE ZONE (FPZ)

STRATIGRAPHIC

UNIT

HAWTHORN

GROUP

UNDIFFERENTIATED

SURFICIAL

DEPOSITS

OCALA

LIMESTONE

OLDSMAR

FORMATION

AVON PARK

FORMATION

CEDAR KEYS

FORMATION

SURFICIAL AQUIFER

SYSTEM

(LAYER 1)

UPPER FLORIDAN

AQUIFER

(LAYER 2)

FERNANDINA

PERMEABLE ZONE

(LAYER 4)

UPPER ZONE OF

LOWER FLORIDAN AQUIFER

(LAYER 3)

MIDDLE

SEMICONFINING UNIT

SEMICONFINING UNIT

INTERMEDIATE

CONFINING UNIT

HYDROGEOLOGIC UNITS

PRINCIPAL

HYDROGEOLOGIC UNITS

AND

EQUIVALENT LAYERS

IN MODEL

SUB-FLORIDAN

CONFINING UNIT

(LOW PERMEABILITY)

SIMULATED

IN MODEL AS

SPECIFIED

HEAD

BOUNDARY

VERTICAL

LEAKANCE

ARRAY

ACTIVE

MODEL

LAYER

VERTICAL

LEAKANCE

ARRAY

ACTIVE

MODEL

LAYER

VERTICAL

LEAKANCE

ARRAY

ACTIVE

MODEL

LAYER

NO-FLOW

BOUNDARY

Figure 5. Geologic units and corresponding layering scheme in the model.
Simulation of Ground-Water Flow 13

the FPZ were calculated at 20 control points. These 

time-averaged heads were used to perform linear 

regressions with 1993-94 measured heads for which a 

more detailed areal distribution of heads was known. 

These linear regressions made it possible to assess the 

hydraulic gradients between each set of two hydrogeologic units separated by a common confining unit. 

Details of these average heads and the linear regressions are presented later in the section Average Hydrologic Conditions for 2000.

In most of the study area, there is upward flow 

from the UFA to the SAS. The UFA discharges to the 

SAS throughout the St. Johns River. The flow from the 

UFA to the St. Johns River in the study area is difficult 

to measure; however, in 1990, Spechler (1995) estimated this upward flow, for a substantially larger reach 

of the St. Johns River than the reach within the study 

area, by using the results of a regional ground-water 

flow model. Ground-water discharge from the UFA to 

the SAS along the St. Johns River reach within the 

study area was estimated to be about 11 ft3

/s by 

Spechler (1995), a rough estimate obtained from a 

more areally extensive river reach.

Model boundaries were assigned to approximate 

the ground-water flow system as accurately as possible. The simulation of the SAS as a layer of specified 

heads allows the UFA to discharge to or receive leakage from the SAS at rates dictated by the relative head 

difference between the water table and the UFA and 

the vertical leakance of the ICU. The altitude of the 

water table was used to define these specified heads. 

A no-flow condition was applied along the lateral 

boundaries of layer 2 (UFA) based on the estimated 

average potentiometric surface for CY 2000, which is 

shown later. Flow entering or leaving cells in the UFA 

is assumed to occur either as horizontal flow to neighboring cells or vertical flow to either the SAS (layer 1) 

or the upper zone of the LFA (layer 3), through the 

ICU or MSCU.

Heads were specified at cells comprising the lateral boundaries of the upper zone of the LFA and the 

FPZ. Details of the computations of these heads are 

presented later in the section Average Hydrologic 

Conditions for 2000. The lateral inland movement of 

water with greater chloride concentrations in the FPZ 

made it impractical to impose a no-flow boundary 

along the lateral boundaries of the model. Although 

the FPZ in the eastern half of the study area is saline, 

small horizontal hydraulic gradients could be generated. This model is restricted to simulating the movement of freshwater within the aquifers.

Steady-State Flow Approximation

The assessment of the error introduced in the 

model by a steady-state flow approximation required 

analyzing the differences in heads at the beginning and 

end of the year. Head differences between the beginning and the end of the year were computed for the 

period 1995-2000 in the UFA and the upper zone of 

the LFA. Wells at which these differences were computed included two wells with continuous water-level 

recorders tapping the UFA (D-3824 and D-3840 in 

fig. 1), and wells for which monthly head measurements were available in and near the study area (UFA 

wells C-0007, D-0018, D-0129, D-0145, D-0160, D-0264, 

D-122A, D-3544, SJ-005, SJ-015, and the upper zone 

of the LFA and FPZ wells in fig. 1). The year that 

resulted in the smallest difference in heads at these 

wells was 2000. Head data for CY 2000 were used to 

evaluate the magnitude of the error introduced in the 

model by a steady-state approximation.

A steady-state flow approximation over a time 

interval, , implies that the magnitude of the product 

of the storage coefficient, , and the time rate of 

changes in head over the same time interval, / , 

is small compared to other aquifer stresses such as 

ground-water withdrawals, aquifer-river flux 

exchanges, or, in general, to the non-storage terms of 

the ground-water flow equation (Harbaugh and others, 

2000). The storage coefficient, , of a confined limestone aquifer in Duval County could vary from 0.001 

to 0.02 (Domenico, 1972). For calculation purposes, 

the specific storage of the UFA and the upper zone of 

the LFA are assumed to be 2.5 x 10-5 ft-1. The specific 

storage is defined as the storage coefficient divided by 

the thickness of the aquifer. Assuming the average 

thickness of the UFA is 250 ft and that of the upper 

zone of the LFA is 420 ft, the storage coefficients used 

in this model for these two aquifers become 0.0063 

and 0.0105, respectively.

The largest head difference between January 2000 

and December 2000 was 3.26 ft, measured at D-1155, 

a well tapping the upper zone of the LFA. This value 

was obtained by analyzing data from continuous 

water-level recorders tapping the UFA and from wells 

for which monthly head measurements were available. 

This head difference implies that the largest value 

 can assume for 2000 is 0.000094 foot per 

day (ft/d), or approximately 0.41 inches per year 

(in/yr). This suggests that the error introduced to the 

model is within the limits of measuring aquifer 

recharge to the aquifer from rainfall infiltration.

∆t

S

∆h ∆t

S

S ∆h/∆t
14 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

Average Hydrologic Conditions for 2000

The development of a ground-water flow model 

based on the local hydrogeologic framework described 

above required the computation of average 2000 heads 

at the control points for the layers of the model. The 

water-table altitude was approximated by using a 

statistical approach that produced reliable results in 

Florida (Sepúlveda, 2002b).

The altitude of the water table for 2000 was 

approximated by using a multiple linear regression 

among the measured levels in SAS wells, the interpolated minimum water-table altitude, and the difference 

between land-surface altitude and the minimum watertable altitude (Sepúlveda, 2002b). The minimum 

water-table altitude is defined as the surface interpolated strictly from the measured altitude at drains in 

the SAS such as streams and lakes. The altitude of the 

water table was strongly correlated with the minimum 

water-table altitude and the difference between landsurface altitude and minimum water-table altitude. 

Water-level measurements at SAS wells were compiled 

from SJRWMD and USGS data bases. A digital landsurface elevation model was generated from digitized 

hypsography obtained from the SJRWMD and USGS.

The minimum water-table altitude was generated from the stages interpolated for all points forming 

the river meanderings. The shoreline, assigned a 

water-table altitude of 0 ft, was used in the generation 

of the minimum water table in the northeastern corner 

of the model area (fig. 6). The minimum water table 

was bounded above by land-surface altitude and below 

by the altitude of the top of the ICU. Water-table altitudes, which were computed at the center of the grid 

cells, generally decrease coastward. The estimated dis-

  



	



0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985 Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

Pa b l o C r e e k

D o c t o r s L a k e

St . J

o h n s R i v e r

Or g t e

a

R i v e r

J u l i n g t o n

C r e

e

k

. . . ..

. .

.

.

..

.

.

.

.. . . . .

. .

.

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

p p

1

1 p

p

p

EXPLANATION

WATER-TABLE ALTITUDE, IN FEET ABOVE NGVD 29

0.00 to 5.00 5.01 to 10.00 10.01 to 20.00 20.01 to 40.00 40.01 to 62.11

1 PROPOSED p

PRODUCTION WELL

PROPOSED

INJECTION WELL

. EXISTING

PRODUCTION WELL

Figure 6. Estimated altitude of the water table of the surficial aquifer system, average 2000 conditions. 
Simulation of Ground-Water Flow 15

tribution of the water table shows an extensive area 

where the altitude of the water table is at least 20 ft 

(fig. 6). The water-table altitude generally is less than 

5 ft in the vicinity of the St. Johns River.

Time-averaged heads for CY 2000 in the UFA 

were calculated for 12 wells with monthly head measurements in Duval County and parts of St. Johns and 

Clay Counties, and regressed for another 12 wells for 

which only biannual head measurements were available. These head measurements were compiled from 

USGS data bases (U.S. Geological Survey, 2001a, 

2001b, 2002a, 2002b). Annual average heads calculated for wells with monthly head measurements were 

regressed with the heads measured during May 2000 

and September 2000 at the same wells by using the 

multiple linear regression (MLR):

, (1)

where

is the calculated 2000 average head at a

UFA well, in feet;

is the measured May 2000 head at the

 same UFA well, in feet;

is the measured September 2000 head at

the same UFA well, in feet;

are the dimensionless MLR coefficients;

and

is the intercept of the MLR, in feet.

Time-averaged 2000 heads in the model area 

were calculated at 11 sites in the UFA (fig. 7), seven of 

which were calculated by averaging monthly head measurements and four were computed by using equation 1. 

EXPLANATION

FERNANDINA PERMEABLE

ZONE WELL

UPPER FLORIDAN

AQUIFER WELL

UPPER ZONE OF LOWER

FLORIDAN AQUIFER WELL

J J J 30.04 33.05 34.90

Number indicates altitude of head in feet above NGVD 29

1 p PROPOSED

PRODUCTION WELL

PROPOSED

INJECTION WELL

J EXISTING

PRODUCTION WELL

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

Pa b l o C r e e k

D o c t o r s L a k e

St . J

o h n s R i v e r

Or g t e

a

R i v e r

J u l i n g t o n

C r e

e

k

  



	



0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985 Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

COMMUNITY HALL

WELL FIELD

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

MAIN STREET

WELL FIELD

J J JJJ

pJpJ

J

J

JJJJ

J

JJ J J J J

JJ

J

1

1 p

p

p

J

J

J

34.90

32.15

(measured

in 2002)

32.71

J

J J

J

J

J

29.97

31.32

33.05

34.04

28.45

35.22

J

J J

J

J

J

J

J

J J

J

32.63

25.35

26.13

21.82

22.14

33.64

30.04

21.80

23.69

22.50

24.85

Figure 7. Average 2000 heads in the Upper Floridan aquifer, upper zone of the Lower Floridan aquifer, 

and Fernandina permeable zone.

hAve βM hMay βS hSep βI = + +

hAve

hMay

hSep

βS and βM

βI
16 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

The time-averaged 2000 heads calculated from monthly 

measurements at 12 UFA sites and the average heads 

for the UFA regressed from equation 1 using May 2000 

and September 2000 measurements at another 12 sites 

were used to represent the average 2000 hydrologic 

conditions or “average hydrologic conditions.” 

Regression coefficients and represent 

the influence of the May 2000 ( ) and September 

2000 ( ) measurements on regressed annual averages . A total of 12 points was available to perform the MLR in equation 1, which had a correlation 

coefficient of 0.99, a root-mean-square (RMS) residual of 0.15 ft, and residuals ranging from -0.20 to 

0.21 ft. The regression coefficients, , 

, and , were used to estimate 

2000 average heads at UFA wells for which only 

May 2000 and September 2000 measurements were 

available. Equation 1 was used only for UFA wells 

because there were no wells tapping the upper zone of 

the LFA or the FPZ with biannual head measurements. 

The areal distribution of the time-averaged 2000 

heads in the UFA was obtained by regressing the heads 

in the UFA within the area shown in figure 1 with the 

1993-94 time-averaged heads in the UFA at the same 

sites and by using the regression coefficients to interpolate heads in the UFA from the estimated potentiometric-surface map for 1993-94 (Sepúlveda, 2002b, 

fig. 18). The simple linear regression (SLR) used to 

generate average 2000 heads in the UFA was:

, (2)

where

is the calculated 2000 average head at a UFA 

well, in feet;

is the calculated 1993-94 average head at the 

same UFA well, in feet;

is the dimensionless SLR coefficient; and

is the intercept of the SLR, in feet.

A total of 24 points was available to perform the 

SLR, which had a correlation coefficient of 0.96, an 

RMS residual of 2.25 ft, and residuals ranging from 

-6.67 to 3.21 ft. Of these 24 points, 11 were inside the 

model area. The regression coefficients, 

and , were used to estimate CY 2000 average heads in the UFA at the center of the grid cells. 

These heads were used to obtain a computer-generated 

potentiometric surface for the UFA, which suggests 

that lateral flow entering or leaving the model area is 

negligible (fig. 8).

Time-averaged 2000 heads were calculated from 

monthly head measurements at eight wells in the 

upper zone of the LFA, six of which were in the model 

area (fig. 1). The time-averaged heads computed for 

the upper zone of the LFA were used to generate specified-head conditions along the lateral boundaries of 

the model by performing another SLR to generate the 

areal distribution of time-averaged 2000 heads. Heads 

in the UFA at these sites were interpolated from the 

potentiometric surface for the UFA derived from equation 2. The following SLR was used to regress calculated time-averaged 2000 heads in the upper zone of 

the LFA with interpolated UFA heads:

, (3)

where

is the calculated 2000 average head at an upper 

zone of the LFA well, in feet;

is the interpolated 2000 average UFA head at 

the same site, in feet;

is the dimensionless SLR coefficient; and

is the intercept of the SLR, in feet.

Eight points were available to perform the SLR, 

which had a correlation coefficient of 0.96, an RMS 

residual of 0.63 ft, and residuals ranging from -1.10 to 

0.83 ft. The SLR represented by equation 3 resulted in 

regression coefficients and , 

which were used to regress 2000 average heads in the 

upper zone of the LFA at the center of the grid cells 

from interpolated 2000 average heads in the UFA. 

Results of the SLR were used to regress the specified 

heads at the center of the grid cells comprising the lateral boundaries of the upper zone of the LFA (fig. 9).

Time-averaged 2000 heads were calculated from 

monthly head measurements at five sites in the FPZ, 

three of which were in the model area. Environmentalwater and freshwater heads were computed for wells 

tapping the FPZ and having variable ground-water 

density (Lusczynski, 1961). Environmental-water head 

at a given point in ground water of variable density is 

the freshwater head reduced by the difference of salt 

mass in freshwater and the salt mass in the environmental water between the given point and the top of the 

saturated zone (Lusczynski, 1961). Wells D-425B, 

D-1344, D-3060, D-2386, and SJ-150 (fig. 1) tap 

only the FPZ. The open interval of well D-425B 

extends from an altitude of -2,035 to -2,466 ft. The 

water from well D-425B is more characteristic of the 

FPZ wells than of the upper zone of the LFA wells 

(Phelps, 2001). Measured chloride concentrations at 

D-425B and D-1344 (Phelps, 2001; St. Johns River 

Water Management District, 2002) indicated freshwater in the FPZ, thus, the point-water, environmental-water, and freshwater heads were the same. 

Measured chloride concentrations at D-3060, D-2386, 

and SJ-150 implied the need to compute environmental-water and freshwater heads to account for variable 

ground-water density.

βS βM

hMay

hSep

hAve

βI = 0.24

βM = 0.34 βS = 0.66

h2000 α1h1993-94 β1 = +

h2000

h1993-94

α1

β1

β1 = –9.12

α1 = 1.14

hUZLFA α2hUFA β = + 2

hUZLFA

hUFA

α2

β2

β2 = –0.71 α2 = 0.91
Simulation of Ground-Water Flow 17

 The environmental-water heads were computed from 

(Lusczynski, 1961):

, (4)

where

is the environmental-water head, in feet;

is the point-water head, in feet;

is the altitude of the top of the open interval, 

referred to as point , in feet;

is the altitude of a reference point from 

which average density is computed, in feet;

is the density of freshwater, equal to 1.000 

gram per milliliter (g/mL);

is the density of water at the top of the open 

interval or point , in g/mL; and

is the average density of water between points 

 and , in g/mL.

The elevation could be taken to be the land-surface 

altitude. The freshwater head was computed from the 

equation (Lusczynski, 1961):

. (5)

The maximum difference between freshwater and 

environmental-water heads among wells tapping the 

FPZ within the study area was about 2 ft (table 4).

EXPLANATION

REGRESSED POTENTIOMETRIC CONTOUR IN THE UPPER FLORIDAN AQUIFER --

Contour interval is 1 foot. Hachures indicate depression. Datum is NGVD 29

Derived from simple linear regression established by equation 2.

30

PROPOSED

PRODUCTION WELL

PROPOSED

INJECTION WELL

EXISTING

PRODUCTION WELL

UPPER FLORIDAN AQUIFER WELL -- Number indicates difference, in feet,

between regressed and measured head in 2000

+0.29

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

Pa b l o C r e e k

Or g t e

a

R i v e r

D o c t o r s L a k e

St . J

o h n s R i v e r

J u l i n g t o n

C r e

e

k

  



	



0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985 Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

27

30

29

28

27

26

23

31

30

29

31

30

25

26

28

21

32

24

22

23

32

29

28

27

26

25

24

-1.93

+0.02

+0.31

+0.77

+0.77

-3.26

-0.93

+0.02

+0.29

-0.55

+1.00

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

Figure 8. Linearly regressed potentiometric surface of the Upper Floridan aquifer for average 

2000 conditions obtained from average 1993-94 conditions.

ρf Hew ρi Hp Zi ρi ρa ( ) – Zr ρa ρf = – – ( ) –

Hew

Hp

Zi

i

Zr r

ρf

ρi

i

ρa

i r

Zr

ρf Hfw ρi Hp Zi ρi ρf = – ( ) –
18 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

EXPLANATION

SPECIFIED HEAD, IN FEET ABOVE NGVD 29

24.00 - 26.00 26.01 - 28.00 28.01 - 30.00 30.01 - 32.00 32.01 - 34.00

PROPOSED

PRODUCTION WELL

PROPOSED

INJECTION WELL

EXISTING

PRODUCTION WELL

P

a

b l o C r e e k

D o c t o r s L a k e

St . J o h

n s R i v

e r

O r g t e a

R i v e r

J u l i n g t o n

C r e

e

k

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

  



	



J J JJJ

J J

J

J

JJJJ

J

JJ J J J J

JJ

J

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

p p

1

1 p

p

p

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985 Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

Figure 9. Specified heads along the lateral boundary cells of the upper zone of the Lower Floridan 

aquifer, average 2000 conditions.

Table 4. List of parameter values used to compute the average 2000 freshwater and environmental-water 

heads at Fernandina permeable zone wells

[Datum is NGVD 29; g/mL, grams per milliliter. For the calculation of freshwater and environmental-water heads, refer to equations 4 

and 5. Ground-water density values were estimated from a simple linear regression between specific conductance and density values (Phelps 

and Spechler, 1997)]

Well 

name

Average 

point-water 

head,

(feet)

Altitude of 

top of 

open interval 

 (feet)

Altitude of 

reference 

point 

(feet)

Density 

of water 

at point

(g/mL)

Average 

ground-water 

density 

between 

points

and

(g/mL)

Average 

freshwater 

head,

(feet)

Average 

environmental-water 

head,

(feet)

D-1344 32.71 -1,858 7 1.000 1.000 32.71 32.71

D-425B 32.15 -2,032 18 1.000 1.000 32.15 32.15

D-3060 18.54 -2,027 23 1.009 1.001 36.95 34.90

D-2386 37.88 -1,874 17 1.002 1.001 41.73 39.81

SJ-150 -4.56 -1,975 5 1.023 1.002 40.76 36.80

H

p Zi

Zr

ρi

Zi

a

ρ

Zi

Zr

Hfw Hew
Simulation of Ground-Water Flow 19

Heads specified along the lateral boundaries of 

the FPZ (layer 4) were 1.5 ft higher than the specified 

heads at the upper zone of the LFA (fig. 9). The differences between the environmental-water heads in the 

FPZ and the heads in the upper zone of the LFA, 

which ranged from 1.3 to 2.2 ft, averaged about 1.5 ft. 

Heads in the upper zone of the LFA at the sites of the 

FPZ wells (fig. 1) were interpolated using the results 

of equation 3. The average difference between the 

environmental-water heads in the FPZ and the heads in 

the upper zone of the LFA was used to determine the 

heads in the FPZ to be specified along the lateral 

boundaries of the model.

Calibration of Ground-Water Flow Model

Computed time-averaged heads in the UFA, the 

upper zone of the LFA, and the FPZ for CY 2000 were 

the control points used to calibrate the steady-state 

ground-water flow model. The number of control 

points for the UFA, the upper zone of the LFA, and the 

FPZ, respectively, were 11, 6, and 3. Hydraulic parameters were systematically adjusted to reduce the difference between simulated and computed average heads 

until an acceptable model calibration was achieved.

The steady-state ground-water flow model was 

calibrated by adjusting input hydraulic parameters 

within reasonable ranges from the initial distributions 

of values, which were taken from Sepúlveda (2002a), 

for all hydrogeologic units except the FPZ, and from 

Durden (1997) for the FPZ. The initial distributions of 

aquifer properties were adjusted until simulated heads 

closely approximated average 2000 heads (fig. 7). 

Simulated heads and flows from a calibrated, deterministic ground-water flow model commonly depart 

from measured heads and flows, even after a diligent 

calibration effort. The difference between model 

results and what actually occurs in the aquifers, 

referred to as model error, is the cumulative result of 

simplification of the conceptual model, grid scale, 

measurement errors, and the difficulty in obtaining 

sufficient measurements to account for all spatial variations in hydraulic properties and stresses throughout 

the model area.

Hydraulic parameters that were adjusted during 

calibration of the steady-state ground-water flow 

model included: the transmissivity of the UFA, the 

upper zone of the LFA, and the FPZ; and the vertical 

leakance of the ICU, the MSCU, and the SCU. The 

calibration process was iterative and consisted of 

(1) assigning initial hydraulic parameters from models 

developed by Durden (1997) and Sepúlveda (2002a); 

(2) comparing simulated and average heads for the 

steady-state period of 2000 at the 20 control points 

(fig. 7); and (3) adjusting and generalizing hydraulic 

parameters to minimize the difference between simulated and average heads for 2000. The guiding principle of calibration was that the model parameter with 

the highest sensitivity for a given area or aquifer was 

adjusted first; other less sensitive parameters were 

adjusted only if reasonable residuals were not 

achieved. In cases where two parameters were about 

equally sensitive, each was adjusted separately.

The initial distributions of hydraulic parameters 

were used to define contiguous zones of equal values. 

The areal extents of the transmissivity zones for the 

UFA, upper zone of the LFA, and FPZ were independent from each other. The areal extents of the vertical 

leakance zones for the ICU, MSCU, and the SCU also 

were independent from each other. The areal extents of 

these zones were modified during calibration. Due to 

the limited number of control points in the FPZ, the 

calibration of transmissivity for the FPZ and of the 

vertical leakance of the SCU was limited to selecting 

one uniform value for the study area for each of these 

two parameters. The hydraulic properties of the FPZ 

were calibrated based on the average environmentalwater head in the FPZ. This was different from the 

freshwater-equivalent head only at D-3060 (table 4). 

The environmental-water head was computed because 

this head defines the hydraulic gradients along the vertical component of flow. The assumption that the vertical component of flow in the FPZ exerts a more 

dominant effect than the lateral component of flow in 

the FAS resulted in computing the environmentalwater head at D-3060 (eq. 4).

Simulated heads, at the center of the cells where 

the control points were located, were compared to the 

measured heads at the control points, as no spatial 

interpolation was deemed necessary given the small 

size of the uniform grid cells. Vertical interpolation 

was not necessary because of the discontinuity and 

associated refraction of potential fields across zones of 

different transmissivities.

Simulated and measured heads in the UFA, 

upper zone of the LFA, and FPZ agreed reasonably 

well throughout the study area (fig. 10). The RMS 

residuals between simulated and measured heads for 

the UFA, upper zone of the LFA, and FPZ were 1.75, 

1.99, and 1.14 ft, respectively, with all residuals less 
20 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

than 3.8 ft (table 5). The histogram of residuals for all 

hydrogeologic units appears to be normally distributed 

(fig. 10), indicating that the error in the model also is 

normally distributed.

The process of modifying the initial transmissivity distribution of the UFA, the upper zone of the LFA, 

and the FPZ during calibration included: (1) finding 

values that decreased the absolute value of residuals at 

control points; (2) generalizing transmissivity values 

to two significant figures; (3) making transmissivity 

changes to areas where residuals were large and sensitive to changes in transmissivity; (4) making changes 

to areas where new ground-water withdrawals 

occurred; and (5) distributing previously simulated 

transmissivity values for the entire thickness of the 

LFA (Sepúlveda, 2002a) into transmissivity values for 

the upper zone of the LFA and for the FPZ.

The process of modifying the initial vertical leakance distribution of the ICU, the MSCU, and the 

SCU during calibration included: (1) finding values 

that decreased the absolute value of residuals at control points; (2) generalizing vertical leakances to one 

significant figure; (3) making vertical leakance 

changes to areas where residuals were large and sensitive to changes in leakance; (4) limiting recharge to the 

UFA to less than 1 in/yr in areas where the estimated 

altitude of the water table was higher than the simulated heads in the UFA; and (5) limiting the discharge 

from the UFA to the SAS along the St. Johns River 

reaches to about 11 ft3/s, as a ground-water flow 

model-based estimate by Spechler (1995) suggested.

Average ground-water withdrawals for 2000 in 

the UFA (23 Mgal/d) and in the upper zone of the LFA 

(88 Mgal/d) were compiled from JEA data bases and 

assigned to corresponding grid cells. Self-supplied 

water from domestic wells was estimated to be, at 

most, 0.01 Mgal/d from each well; these were simulated as withdrawals from the UFA (fig. 11). Assigned 

ground-water withdrawals to each aquifer for wells 

with open intervals tapping more than one aquifer 

were set equal to the total withdrawal rate multiplied 

by the ratio of the simulated transmissivity of the interval open to the aquifer and the simulated transmissivity of the entire open interval of the well. Larger 

withdrawals from the upper zone of the LFA than 

those from the UFA reflect that the upper zone of the 

LFA is much more transmissive than the UFA (fig. 12).

Table 5. Water-level residual statistics for the calibrated 

steady-state model

[Min., minimum; Max., maximum; RMS, root-mean-square]

Aquifer

Number 

of 

measurements

Residual 

(feet) 

Min. Max. Mean RMS

Upper Floridan 

aquifer

11 -2.71 3.75 0.14 1.75

Upper zone of the 

Lower Floridan aquifer

6 -3.44 2.15 -.98 1.99

Fernandina permeable zone3 -1.29 1.39 .21 1.14

Entire model 20 -3.44 3.75 -.19 1.75

Upper Floridan aquifer

EXPLANATION

Upper zone of Lower Floridan aquifer

Fernandina permeable zone

CONTROL POINTS – Symbol indicates

water-level measurement made for the:

Root-mean-square residual = 1.75 ft

MEASURED HEAD, IN FEET ABOVE NGVD 29

SIMULATED HEAD, IN FEET ABOVE NGVD 29

20

25

35

40

30

20 25 30 35 40

0

1

2

3

4

5

HEAD RESIDUAL, IN FEET

-5 0 5

NUMBER OF

MEASUREMENTS

Figure 10. Comparison of simulated to measured heads 

in all hydrogeologic units for the calibrated model.
Simulation of Ground-Water Flow 21

EXPLANATION

GROUND-WATER WITHDRAWALS,

IN MILLION GALLONS PER DAY

0.001 to 0.010 0.011 to 0.100 0.101 to 0.917

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

Pa b l o C r e e k

D o c t o r s L a k e

St . J

o h n s R i v e r

J u l i n g t o n

C r e

e

k

Or g t e

a

R i v e r

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



Figure 11. Average 2000 ground-water withdrawal rates from the Upper Floridan aquifer.
22 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

0.001 to 0.010 0.101 to 1.000 0.011 to 0.100

EXPLANATION

GROUND-WATER WITHDRAWALS,

IN MILLION GALLONS PER DAY

1.001 to 4.687

Pa b l o C r e e k

D o c t o r s L a k e

St . J

o h n s R i v e r

Or g t e

a

R i v e r

J u l i n g t o n

C r e

e

k

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

  



	



0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

Figure 12. Average 2000 ground-water withdrawal rates from the upper zone of the Lower Floridan 

aquifer.
Simulation of Ground-Water Flow 23

Hydraulic Properties from Calibrated 

Model

Transmissivity in the UFA from the calibrated 

model ranged from 16,000 feet squared per day (ft2/d) 

in the northeastern and north-central parts of the 

model area to 24,000 ft2/d in south-central parts of the 

model area (fig. 13). The distribution of transmissivity 

for the UFA in the model area is similar to that simulated by Sepúlveda (2002a). The main difference 

between the calibrated distribution of the UFA in this 

study and that simulated in regional models (Durden, 

1997; Sepúlveda, 2002a) occurred in the northwestern 

part of the model area, where the simulated transmissivity (18,000 ft2/d, fig. 13) is one-fourth of that simulated by regional models. This difference could be 

explained by the fact that the locations of self-supplied 

domestic wells were not available in the regional models. The rearrangement of aquifer stresses necessitated 

the redistribution of transmissivity to account for 

changes in aquifer stresses. Only seven zones of transmissivity were generated as a result of calibration.

EXPLANATION

UPPER FLORIDAN AQUIFER

TRANSMISSIVITY,

IN FEET SQUARED PER DAY

24,000

1

p

PROPOSED PRODUCTION WELL

PROPOSED INJECTION WELL

. EXISTING PRODUCTION WELL

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

Pa b l o C r e e k

J u l i n g C k t o n r e e

D o c t o r s L a k e

St . J

o h n s R i v e r

Or g t e

a

R i v e r

J u l i n g t o n

C r e

e

k

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



. . . ..

. .

.

.

..

.

.

.

.. . . . .

. .

.

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

p p

1

1 p

p

p

MAIN STREET

WELL FIELD

18,000

16,000

24,000

20,000

22,000

18,000

18,000

Figure 13. Transmissivity of the Upper Floridan aquifer from the calibrated model.
24 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

Transmissivity of the upper zone of the LFA 

ranged from 480,000 to 700,000 ft2

/d, with only three 

distinct transmissivity zones in the model area 

(fig. 14). The only difference between the transmissivity distribution shown in figure 14 and that presented 

in regional models is in the northwestern corner of the 

model area, where the upper zone of the LFA has a 

value that is 80,000 ft2/d less transmissive than the 

simulated value in both regional models (Durden, 

1997; Sepúlveda, 2002a). The model area had only 

one transmissivity zone for the FPZ, with a calibration 

value of 60,000 ft2

/d. The limited number of control 

points in the FPZ precluded the discretization of the 

model area into more transmissivity zones.

EXPLANATION

UPPER ZONE OF LOWER FLORIDAN

AQUIFER TRANSMISSIVITY, IN FEET

SQUARED PER DAY

480,000

PROPOSED PRODUCTION WELL

PROPOSED INJECTION WELL

EXISTING PRODUCTION WELL

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

Pa b l o C r e e k

D o c t o r s L a k e

St . J

o h n s R i v e r

Or g t e

a

R i v e r

J u l i n g t o n

C r e

e

k

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



JJ J J J J

JJ

J

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

480,000

600,000

700,000

Figure 14. Transmissivity of the upper zone of the Lower Floridan aquifer from the calibrated model.
Simulation of Ground-Water Flow 25

The distribution of the simulated vertical leakance of the ICU (fig. 15) is similar to that presented 

in previous regional models. The ICU is simulated to 

have the largest vertical leakance value in parts of the 

St. Johns River. The cumulative discharge from the 

UFA to the SAS from cells in reaches of the St. Johns 

River in the model area was simulated to be 10.79 ft3

/s. 

This flow is within the discharge range that was estimated from Spechler (1995). A large area of the model 

where the water-table altitude is higher than simulated 

heads of the UFA was assigned the lowest leakance 

value to match measured heads in the UFA within this 

area of low recharge (fig. 16). The simulated vertical 

leakage between the SAS and the UFA ranged from 

5.88 in/yr of upward leakage from the UFA to 0.67 in/yr 

of recharge from the SAS to the UFA.

LEAKANCE,

IN FEET PER DAY PER FOOT

EXPLANATION

1 x 10-6

1

p

PROPOSED PRODUCTION WELL

PROPOSED INJECTION WELL

J EXISTING PRODUCTION WELL

5 x 10

-5

4 x 10

-5

8 x 10

-6

2 x 10

-6

2 x 10

-6

1 x 10

-6

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

Pa b l o C r e e k

D o c t o r s L a k e

St . J

o h n s R i v e r

Or g t e

a

R i v e r

J u l i n g t o n

C r e

e

k

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



J J JJJ

J J

J

J

JJJJ

J

JJ J J J J

JJ

J

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

p p

1

1 p

p

p

Figure 15. Leakance of the intermediate confining unit from the calibrated model.
26 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

EXPLANATION

VERTICAL LEAKAGE RATE, IN INCHES PER YEAR --

Negative leakage rates indicate upward leakage and

positive leakage rates indicate downward leakage

-5.88 to -1.00 -0.99 to 0.00 0.01 to 0.67

PROPOSED PRODUCTION WELL

PROPOSED INJECTION WELL

EXISTING PRODUCTION WELL

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



Pa b l o C r e e k

D o c t o r s L a k e

St . J

o h n s R i v e r

Or g t e

a

R i v e r

J u l i n g t o n

C r e

e

k

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

Figure 16. Simulated vertical leakage rates to and from the Upper Floridan aquifer through the 

intermediate confining unit, average 2000 conditions.
Simulation of Ground-Water Flow 27

The lack of control points in the upper zone of 

the LFA limited the number of zones of uniform vertical leakance assigned to the MSCU (fig. 17). Except 

for zones of uniform vertical leakance of the MSCU in 

the northwestern and south-central parts of the model 

area, a uniform value of 1 x 10-4 foot per day per foot 

(ft/d)/ft was assigned as vertical leakance of the 

MSCU. The model area was simulated to be a discharge area from the upper zone of the LFA (fig. 18). 

The highest leakage rate from the upper zone of the 

LFA to the UFA, simulated in the northwestern part of 

the model area, was 8.45 in/yr.

1

p

PROPOSED PRODUCTION WELL

PROPOSED INJECTION WELL

J EXISTING PRODUCTION WELL

LEAKANCE, IN FEET

PER DAY PER FOOT

EXPLANATION

1 x 10-4

4 x 10-4

3 x 10-4

1 x 10-4

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

Pa b l o C r e e k

D o c t o r s L a k e

St . J

o h n s R i v e r

Or g t e

a

R i v e r

J u l i n g t o n

C r e

e

k

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



J J JJJ

J J

J

J

JJJJ

J

JJ J J J J

JJ

J

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

p p

1

1 p

p

p

Figure 17. Leakance of the middle semiconfining unit from the calibrated model.
28 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

EXPLANATION

VERTICAL LEAKAGE RATE, IN INCHES PER YEAR --

All rates are negative, indicating upward leakage

-8.45 to -5.00 -0.99 to - -4.99 to 0.04 -2.00 -1.99 to -1.00

PROPOSED PRODUCTION WELL

PROPOSED INJECTION WELL

EXISTING PRODUCTION WELL

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

Pa b l o C r e e k

D o c t o r s L a k e

St . J

o h n s R i v e r

Or g t e

a

R i v e r

J u l i n g t o n

C r e

e

k

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



Figure 18. Simulated vertical leakage rates from the upper zone of the Lower Floridan aquifer 

through the middle semiconfining unit, average 2000 conditions.
Simulation of Ground-Water Flow 29

The vertical leakance of the SCU was simulated 

with a uniform value of 2.0 x 10-5 (ft/d)/ft. Upward 

leakage from the FPZ was simulated throughout the 

model area (fig. 19). Upward vertical leakage rates 

from the FPZ ranged from 0.08 to 0.37 in/yr. However, 

because chloride concentrations of ground water from 

the FPZ generally are greater southeast of the St. Johns 

River than to the northwest, it is more likely greater 

chloride concentrations would be observed in the upper 

zone of the LFA at Deerwood 3 and Brierwood wells 

than at the Main Street well field due to the upward 

migration of water with greater chlorides from the 

FPZ. The simulated rates of upward flow from the FPZ 

are about the same at the locations of the Deerwood 3 

and Brierwood wells. The fact that greater chloride 

concentrations were measured in samples from Deerwood 3 than from Brierwood wells in 2002 may be 

explained by either the additional upward flow induced 

at Deerwood 3 due to increased pumping during 2002 

compared to 2000 or by the possibility that ground 

water from the FPZ in the vicinity of Deerwood 3 may 

be more mineralized than that in the Brierwood area. 

The magnitude of the upward leakage from the FPZ 

(fig. 19) is substantially smaller than the upward leakage from the upper zone of the LFA (fig. 18), mainly 

due to the fact that the MSCU is leakier than the SCU.

EXPLANATION

VERTICAL LEAKAGE RATE, IN INCHES PER YEAR --

All rates are negative, indicating upward leakage

-0.37 to -0.25 -0.24 to -0.15 -0.14 to -0.08

PROPOSED

PRODUCTION WELL

PROPOSED

INJECTION WELL

EXISTING

PRODUCTION WELL

Pa b l o C r e e k

D o c t o r s L a k e

St . J

o h n s R i v e r

Or g t e

a

R i v e r

J u l i n g t o n

C r e

e

k

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985 Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



Figure 19. Simulated vertical leakage rates from the Fernandina permeable zone through the 

semiconfining unit, average 2000 conditions.
30 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

Simulated Potentiometric Surfaces

The simulated potentiometric surface of the 

UFA includes cones of depression at the Deerwood 3 

well field, at the Brierwood well field extending into 

the St. Johns River, and at the Oakridge well field 

(fig. 20). These cones of depression coincide with 

areas of substantial pumping in the UFA (fig. 11). 

Simulated ground-water flow in the UFA is from the 

north and southeast towards the location of Brierwood, 

Deerwood 3, and Oakridge well fields. Simulated 

ground-water flow in the UFA also is from the Community Hall wells to the southwest corner of the model 

area along the St. Johns River.

EXPLANATION

SIMULATED POTENTIOMETRIC CONTOUR IN THE

UPPER FLORIDAN AQUIFER --

Contour interval is 1 foot.

Hachures indicate depression. Datum is NGVD 29

Derived from

model simulation.

30

UPPER FLORIDAN AQUIFER WELL -- Number indicates

difference between simulated and measured head

+0.29

PROPOSED PRODUCTION WELL

PROPOSED INJECTION WELL

EXISTING PRODUCTION WELL

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

Pa b l o C r e e k

Or g t e

a

R i v e r

D o c t o r s L a k e

St . J

o h n s R i v e r

J u l i n g t o n

C r e

e

k

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



27

30

29

28

27

26

23

24

29

31

30

30

29

31

20

23

21

25

26

24

28

27

26

24

25

23

22

24

21

32

24

28

22

22

25

22

23

25

32

23

22

26

-2.71

+1.66 +0.38

+3.75

+0.71

-2.11

-1.12

-0.50

+0.29

-0.47

+1.62

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

SOUTHEAST

WELL FIELD

OAKRIDGE

WELL FIELD

RIDENOUR

WELL FIELD

FAIRFAX

WELL FIELD

HENDRICKS

WELL FIELD

RIVEROAKS

WELL FIELD

Figure 20. Simulated potentiometric surface of the Upper Floridan aquifer (model layer 2), 

average 2000 conditions.
Simulation of Ground-Water Flow 31

The simulation of flow in the upper zone of the 

LFA indicates that ground water moves from the 

northwest and southeast areas towards the center of the 

model area (fig. 21). Although some of the water discharges through the eastern boundary of the model 

area, another flow pattern is indicated by the decreasing heads towards the southwestern boundary of the 

model area. A depression in the potentiometric surface 

of the upper zone of the LFA surrounding the Deerwood 3 well field is indicative of the approximately 

11 Mgal/d pumped from this aquifer at this well field 

in 2000. The simulation of flow in the FPZ indicated 

flow patterns similar to those that occur in the upper 

zone of the LFA (fig. 22).

EXPLANATION

SIMULATED POTENTIOMETRIC CONTOUR IN THE

UPPER ZONE OF THE LOWER FLORIDAN

AQUIFER -- Contour

interval is 1 foot. Hachures indicate depression.

Datum is NGVD 29

Derived from model simulation.

30

UPPER ZONE OF LOWER FLORIDAN AQUIFER

WELL -- Number indicates difference between

simulated and measured head

-0.48

PROPOSED PRODUCTION WELL

PROPOSED INJECTION WELL

EXISTING PRODUCTION WELL

Pa b l o C r e e k

Or g t e

a

R i v e r

D o c t o r s L a k e

St . J

o h n s R i v e r

J u l i n g t o n

C r e

e

k

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

-0.48

-2.20

-3.44 -0.56

+2.15

-1.40

33

32

31

30

29

29

30

31

32

33

25

26

27

28

29

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



Figure 21. Simulated potentiometric surface of the upper zone of the Lower Floridan aquifer 

(model layer 3), average 2000 conditions.
32 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

EXPLANATION

SIMULATED POTENTIOMETRIC CONTOUR IN THE

FERNANDINA PERMEABLE ZONE --

Contour interval is 1 foot. Datum is

NGVD 29

Derived from

model simulation.

30

FERNANDINA PERMEABLE ZONE WELL --

Number indicates difference between simulated

and measured head

-1.29

PROPOSED PRODUCTION WELL

PROPOSED INJECTION WELL

EXISTING PRODUCTION WELL

Pa b l o C r e e k

Or g t e

a

R i v e r

D o c t o r s L a k e

St . J

o h n s R i v e r

J u l i n g t o n

C r e

e

k

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

-1.29

+1.39

+0.53

34

35

33

32

31

29

28

27

31

32

33

34

35

30

30

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



Figure 22. Simulated potentiometric surface of the Fernandina permeable zone (model layer 4), 

average 2000 conditions.
Simulation of Ground-Water Flow 33

Ground-Water Flow Budget

Volumetric flow rates simulated in the UFA, the 

upper zone of the LFA, and the FPZ were computed to 

quantify contributions of each applicable component 

of the ground-water flow system (fig. 23). Groundwater withdrawals and flow through the lateral boundaries of the model in the upper zone of the LFA were 

the largest net fluxes in the model area (135.70 and 

187.43 ft3

/s, respectively). Vertical leakages through 

the confining units in the FAS reflect that the MSCU 

is more than 2 times leakier than the ICU and more 

than 10 times leakier than the SCU. The vertical 

hydraulic gradient between the SAS and the UFA is 

larger than that between the units within the FAS in 

many areas. Large vertical hydraulic gradients 

between the SAS and the UFA increase the vertical 

leakage through the ICU, whereas the presence of 

clay in the ICU decreases the vertical leakage.

EXPLANATION

FLOW ACROSS BOUNDARY

NET FLOW ACROSS BOUNDARY

WEN WELL WITHDRAWALS FROM LAYER N

SH FLOW ACROSS SPECIFIED-HEAD BOUNDARY

FINM NET FLOW ACROSS INTERFACE OF LAYERS N AND M

23.13 FLOW RATE, IN CUBIC FEET PER SECOND

SURFICIAL

AQUIFER

SYSTEM

(Layer 1)

UPPER

AQUIFER

(Layer 2)

FLORIDAN

UPPER ZONE

LOWER

FLORIDAN

AQUIFER

(Layer 3)

WE =135.70 3

FI =56.72 23

20.94

0.15

FI =20.79 12

WE =35.94 2

18.14

SH=4.99

23.13

56.72

0.00

FERNANDINA

PERMEABLE

ZONE

(Layer 4)

176.69

SH=187.43

FI =4.99 364.12 34

0.00

4.99

Figure 23. Simulated steady-state volumetric flow budget for the model area, average 

2000 conditions.
34 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

Flow through cells along the lateral boundary in 

the upper zone of the LFA ranged from leaving the 

model area at a rate of 2.51 ft3/s to entering the model 

area at a rate of 2.18 ft3

/s (fig. 24). The majority of the 

cells along this boundary, however, had flows less than 

0.50 ft3

/s. Simulations indicated that water in the upper 

zone of the LFA discharges through the eastern boundary of the model area and through parts of the southeastern boundary, whereas the aquifer is recharged through 

the western, northern, and southwestern boundaries.

EXPLANATION

LATERAL FLOW, IN CUBIC FEET PER SECOND -- Negative

flow indicates water is leaving the model area and positive

flow indicates water is entering the model area

-0.49 to 0.00 0.01 to 0.50 0.51 to 1.00 -2.51 to -1.00 -0.99 to -0.50 1.01 to 2.18

SUBAREAS OF FLOW ANALYSES -- Well field areas

where flow budget was computed

PROPOSED PRODUCTION WELL

PROPOSED INJECTION WELL

Pa b l o C r e e k

Or g t e

a

R i v e r

D o c t o r s L a k e

St . J

o h n s R i v e r

J u l i n g t o n

C r e

e

k

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



Figure 24. Simulated steady-state lateral flow to and from the upper zone of the Lower Floridan 

aquifer across model boundaries, average 2000 conditions.
Simulation of Ground-Water Flow 35

Flows through the lateral boundary cells in the 

FPZ were mostly less than 0.05 ft3

/s, with exceptions 

along the southwestern boundary and scattered cells 

near the corners of the model area (fig. 25). These 

flows across the lateral boundaries of the FPZ are 

substantially lower than those across the boundaries of 

the upper zone of the LFA because the calibrated 

transmissivity of the FPZ (60,000 ft2

/d) is at least eight 

times lower than that for the upper zone of the LFA 

(equal or greater than 480,000 ft2

/d).

EXPLANATION

LATERAL FLOW, IN CUBIC FEET PER SECOND -- Negative

flow indicates water is leaving the model area and positive

flow indicates water is entering the model area

0.00 to 0.05 0.06 to 1.04 -1.01 to -0.06 -0.05 to -0.01

SUBAREAS OF FLOW ANALYSES -- Well field areas

where flow budget was computed

PROPOSED PRODUCTION WELL

PROPOSED INJECTION WELL

Pa b l o C r e e k

Or g t e

a

R i v e r

D o c t o r s L a k e

St . J

o h n s R i v e r

J u l i n g t o n

C r e

e

k

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



Figure 25. Simulated steady-state lateral flow to and from the Fernandina permeable zone 

across model boundaries, average 2000 conditions.
36 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

A more detailed analysis of the ground-water 

flow in the Brierwood, Deerwood 3, Main Street, and 

Community Hall wells is facilitated by computing all 

flows within smaller areas surrounding the well fields 

(fig. 25). Simulations indicate that the lateral flow in 

the FPZ in the Community Hall subarea is greater than 

for any other well field subarea. The magnitude of the 

upward flow from the FPZ can be used to assess the 

risk of increased chloride concentrations in overlying 

aquifers due to the greater chloride concentrations in 

the FPZ. Ground water pumped from wells tapping the 

upper zone of the LFA generally have chloride concentrations that are the result of a mixing of the 

respective chloride concentrations. The consistently 

low chloride concentrations measured in ground water 

in the Community Hall and Main Street well fields 

(less than 18 mg/L, table 1), combined with the simulated upward flow from the FPZ, indicates the FPZ 

contains freshwater in these two well fields. The simulated upward flow from the FPZ in the Deerwood 3 

well field subarea (0.11 ft3/s, fig. 26) and the elevated 

chloride concentrations at Deerwood 3 wells (table 1) 

indicate the FPZ contains greater chloride concentrations than those found in Community Hall or Main 

Street well fields. The analysis of volumetric flows in 

the upper zone of the LFA indicates that as groundwater withdrawals increase from this aquifer, the lateral flows entering the subareas increase while lateral 

flows leaving the subareas decrease (fig. 26). The 

induced upward flow from the FPZ also tends to 

increase as ground-water withdrawals from the upper 

zone of the LFA at Deerwood 3 wells increase.

Sensitivity Analyses

Sensitivity analyses were conducted to assess 

the relative response of simulated heads to uniform 

changes in the simulated value of selected model 

parameters and to address model uncertainty. Model 

parameters considered for sensitivity analysis included 

the transmissivities of the UFA, the upper zone of the 

LFA, and the FPZ; the leakances of the ICU, the 

MSCU, and the SCU; and the specified heads along 

the lateral boundaries of the upper zone of the LFA 

and the FPZ. The RMS residuals of the differences 

between simulated and computed heads were the criteria used to assess the effects of changes made to 

parameter values used in the model.

The effect each parameter had on simulation 

results was assessed by varying independently from 

0.2 to 2.0 times the values of transmissivity; from -3.0 

to 3.0 ft the departure from specified heads along lateral boundaries of the upper zone of the LFA and FPZ; 

and from 0.1 to 10.0 times the leakance values 

(fig. 27). These ranges of values may not include all 

the uncertainties associated with some of the parameters, but provided a perspective on parameter sensitivity.

The sensitivity analyses indicated that simulated 

heads were more sensitive to changes in vertical leakances of the ICU and MSCU than to changes in vertical leakance of the SCU (fig. 27). Simulated heads 

were more sensitive to changes in transmissivity of the 

UFA and upper zone of the LFA than to changes in 

transmissivity of the FPZ; however, this may be the 

result of having only three control points in the FPZ. 

Changes in the calibrated vertical leakance of the SCU 

and in the transmissivity of the FPZ and the effect 

these have on the simulated upward leakage from the 

FPZ are presented later in the report.

The no-flow boundary condition imposed along 

the lateral boundaries of the UFA was replaced with a 

specified-head boundary condition, with heads 

obtained from equation 1, to analyze the sensitivity of 

model results to changes in this boundary condition. 

The largest change in net vertical flow between the 

UFA and the upper zone of the LFA was 0.02 ft3/s. 

This change was simulated in the Main Street well 

field subarea, where the upward flow from the upper 

zone of the LFA was 0.48, instead of 0.50 ft3

/s 

(fig. 26). There were no changes in the simulated 

upward flow from the FPZ when this change in boundary condition was performed in the UFA.

APPLICATION OF GROUND-WATER FLOW 

MODEL

The calibrated steady-state ground-water flow 

model described in the previous sections was used to 

predict the effects of hypothetical changes in withdrawals and injections on the FAS. Steady-state simulations were used to determine head buildups in the 

potentiometric surface of the upper zone of the LFA. 

Transient simulations, presented later in this section, 

were used to analyze the effects ground-water injection and withdrawal stress periods have on the upward 

flow from the FPZ at the Brierwood and Deerwood 3 

well fields. Reduced upward flows from the FPZ were 

used as the criterion to identify potential success of 

transient scenarios of injections and withdrawals. Storage coefficient values and the sensitivity of the simulated heads to changes in storage coefficient are 

presented in the transient simulations section.
Application of Ground-Water Flow Model 37 EXPLANATION FLOW ACROSS BOUNDARY NET FLOW ACROSS BOUNDARY WEN WELL WITHDRAWALS FROM LAYER N FL FLOW ACROSS BOUNDARIES OF WELL FIELD AREA FINM NET FLOW ACROSS INTERFACE OF LAYERS N AND M 8.73 FLOW RATE, IN CUBIC FEET PER SECOND

SURFICIAL

AQUIFER

SYSTEM

(Layer 1)

UPPER

AQUIFER

(Layer 2)

FLORIDAN

UPPER ZONE

OF THE LOWER

FLORIDAN

AQUIFER

(Layer 3)

FERNANDINA

PERMEABLE

ZONE

(Layer 4)

WE =8

3

WE =1

2

WE =12.53 3

WE =0.87 2

0.00

FL=13.00

0.31

0.38

FL=0.03

0.41

0.02 FI =0.02 12

FI =0.50 23

0.00

0.00

13.00

Main Street

0.70

FL=0.39

0.50

0.00

0.03

FI =0.03 34

WE =17.18 3

WE =4.12 2

0.23

FL=18.16

18.39

0.30

FL=0.11

0.41

0.00

FI =0.02 12

FI =1.09 23

0.02

1.09

0.00

Deerwood 3

0.00

FL=3.01

3.01

FI =0.11 34 0.11

0.00

0.00

FL=11.17

1.61

0.23

FL=0.03

0.26

0.01

FI =0.01 12

WE =1.50 2

WE =10.91 3

FI =0.29 23

0.00

0.29

0.00

FI =0.03 34

0.00

Brierwood

0.03

0.39

FL=1.22

11.17

Figure 26. Simulated steady-state volumetric flow budget for the well field subareas of Brierwood, Deerwood 3, Main Street, and 2000 conditions.
38 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida ROOT-MEAN-SQUARE RESIDUAL, IN FEET

1.75

1.80

0.0 0.5 1.0 1.5 2.0

1.70

-3.0 -2.0 -1.0 0.0 1.0 2.0 3.0

3.0

2.5

2.0

1.5

DIFFERENCE FROM SPECIFIED HEADS, IN FEET DIFFERENCE FROM SPECIFIED HEADS, IN FEET

-3.0 -2.0 -1.0 0.0 1.0 2.0 3.0

2.1

2.0

1.9

1.8

1.7

1.6

2.2

MULTIPLIER FROM CALIBRATED MODEL VALUE MULTIPLIER FROM CALIBRATED MODEL VALUE

0.10 1.00 10.00

1.85

1.80

1.75

1.70

6.0

5.5

5.0

4.5

4.0

3.5

3.0

1.5

2.0

2.5

0.10 1.00 10.00

0.10 1.00 10.00

11.5

14.0

9.0

6.5

4.0

1.5

1.0 1.5 2.0

0.5 1.0 1.5 2.0

1.6

1.7

1.8

1.9

2.0

2.1

0.0

0.0 0.5

1.6

1.7

1.8

1.9

2.0

2.1

MULTIPLIER FROM CALIBRATED MODEL VALUE MULTIPLIER FROM CALIBRATED MODEL VALUE

MULTIPLIER FROM CALIBRATED MODEL VALUE MULTIPLIER FROM CALIBRATED MODEL VALUE

SPECIFIED HEADS IN THE

FERNANDINA PERMEABLE

ZONE ALONG LATERAL

BOUNDARIES

SPECIFIED HEADS IN THE

UPPER ZONE OF THE LOWER

FLORIDAN AQUIFER ALONG

LATERAL BOUNDARIES

LEAKANCE OF THE

SEMICONFINING UNIT

OVERLYING THE

FERNANDINA

PERMEABLE ZONE

LEAKANCE OF THE MIDDLE

SEMICONFINING UNIT

LEAKANCE OF THE INTERMEDIATE

CONFINING UNIT

TRANSMISSIVITY OF

THE FERNANDINA

PERMEABLE ZONE

TRANSMISSIVITY OF

THE UPPER ZONE OF THE

LOWER FLORIDAN AQUIFER

TRANSMISSIVITY OF THE

UPPER FLORIDAN AQUIFER

E

C

G H

F

D

B A

Figure 27. Model sensitivity to changes in selected model parameters.
Application of Ground-Water Flow Model 39

The simulation of injecting the water into the 

FPZ at Deerwood 3 and Brierwood well fields would 

require a density-dependent ground-water flow model 

to study how chloride concentrations change as injection and withdrawal stresses are implemented. The 

ground-water flow model presented herein cannot simulate such changes in water density. Traditional aquifer storage and recovery operations inject freshwater 

into poor-quality water, not as a barrier, but for storage 

and later recovery. The buoyancy effect would retain 

the freshwater near the top of the FPZ and would not 

be lost by downward movement. In addition, the sluggish movement of water in the FPZ would prevent 

much lateral loss of the freshwater bubble to be created with the injection, although annual replenishment 

would probably be necessary to counter bubble reduction during pumping from the south grid.

Application of Steady-State Ground-Water 

Flow Model

The 18 Mgal/d of ground water withdrawn from 

the Main Street well field was simulated to be injected 

into the upper zone of the LFA to reduce upward flow 

from the FPZ through the SCU. A steady-state run 

simulated the injection of 6 Mgal/d through two Brierwood injection wells and 12 Mgal/d through three 

Deerwood 3 injection wells, into the upper zone of the 

LFA, to assess the head buildups that such injections 

could have on the potentiometric surface of the upper 

zone of the LFA. These injections resulted in a head 

buildup of less than 1 ft in the upper zone of the LFA 

near the Deerwood 3 well field and a drawdown of 

0.5 ft in the Main Street well field, where the 18 Mgal/d 

are withdrawn (fig. 28). No withdrawals were considered from Brierwood or Deerwood 3 well fields when 

injections were simulated. The ground-water flow 

budget analysis for the model area indicates that the 

elimination of ground-water withdrawals at Brierwood 

and Deerwood 3 and the injection of 18 Mgal/d distributed between Brierwood and Deerwood 3 wells 

results in a net outflow of almost equal amount across 

the specified-head boundary in the upper zone of the 

LFA (fig. 29). The ground-water flow budget analysis 

for the well field subareas shows that most of the 

injected water in Deerwood 3 moves laterally away 

from the well field with less than 1 percent moving 

downward towards the FPZ. The upward flow from the 

FPZ was simulated to decrease from 0.11 ft3

/s when 

pumping occurred at Deerwood 3 well field to 0.04 ft3

/s, 

a net change of 0.07 ft3

/s or a reduction of 64 percent, 

when injection occurred (figs. 26, 30). The upward 

flow from the FPZ was not reversed at any cell in the 

model area during the injections into the upper zone of 

the LFA at Brierwood and Deerwood 3.

The 18 Mgal/d withdrawn from Main Street 

wells also were simulated to be injected into the UFA 

at Brierwood and Deerwood 3, rather than into the 

upper zone of the LFA, to assess the contrast such 

injection would have on the upward flow from the FPZ. 

A comparison of the volumetric flows shown in figure 

31 with those shown in figure 30 was facilitated by 

making the injection rates into Brierwood and Deerwood 3 wells the same for both simulations. The only 

difference between the simulations shown in figures 30 

and 31 was the layer into which the water was injected 

in Brierwood (6 Mgal/d) and Deerwood 3 (12 Mgal/d).

If the ground water withdrawn from the Main 

Street well field was simulated to be injected into the 

UFA, then about 17 percent of the injected water at 

Deerwood 3 would move downward to the upper zone 

of the LFA and almost all of the remaining 83 percent 

would move laterally in the UFA within the Deerwood 

3 well field subarea (fig. 31). In the case of the Brierwood well field, the injected water moving downward 

is only 6 percent. The injection into the UFA reduced 

the upward flow from the FPZ by about 45 percent 

compared to the upward flow when only withdrawals 

and no injections occurred at the Deerwood 3 well 

field (from 0.11 to 0.06 ft3/s, figs. 26, 30). At no cell in 

the model area was the upward flow from the FPZ 

reversed as a result of simulated injections into the 

UFA at Brierwood and Deerwood 3 well fields. This 

type of injection was less effective in reducing the 

upward flow from the FPZ, a reduction in upward flow 

of 0.05 ft3

/s, compared to a reduction of 0.07 ft3/s if 

water was injected into the upper zone of the LFA. 

Thus, injecting water into the UFA would be less 

effective in reducing chloride concentrations in the 

upper zone of the LFA. Simulation results indicate, 

however, that whether the injection takes place in the 

UFA or in the upper zone of the LFA, most of the 

injected water moves laterally to areas outside the well 

field subareas (figs. 30, 31). The net changes in flow 

simulated at the Community Hall well field (figs. 30, 

31) due to the injections at Brierwood and Deerwood 3 

well fields, and the additional withdrawals at Main 

Street well field, indicated these stresses have a negligible effect on the heads in the vicinity of Community 

Hall well field. Therefore, the analysis of vertical 

flows between the upper zone of the LFA and the FPZ 

will focus on the Main Street, Brierwood, and Deerwood 3 well fields, wells where additional withdrawals or injections are simulated.
40 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

EXPLANATION

HEAD BUILDUP (OR DRAWDOWN) CONTOUR

IN THE UPPER ZONE OF THE LOWER FLORIDAN

AQUIFER DUE TO INJECTED (OR WITHDRAWN)

WATER --

Contour interval is 0.5 foot

Derived from model simulation.

Changes in head are relative to simulated heads

shown in figure 20. Negative values indicate

drawdown caused by withdrawal of 18 Mgal/d.

0.5

SUBAREAS OF FLOW ANALYSES --

Areas where flow budget was computed

PROPOSED PRODUCTION WELL

PROPOSED INJECTION WELL

Pa b l o C r e e k

Or g t e

a

R i v e r

D o c t o r s L a k e

St . J

o h n s R i v e r

J u l i n g t o n

C r e

e

k

ST. JOHNS COUNTY

CLAY

COUNTY

DUVAL COUNTY

0

0

5 KILOMETERS

5 MILES

Base modified from U.S. Geological Survey digital data; 1:100,000, 1985

Albers Equal-Area Conic projection

Standard parallels 29 30 and 45 30 , central meridian -83 00   

  



	



DEERWOOD 3

WELL FIELD

BRIERWOOD

WELL FIELD

COMMUNITY HALL

WELL FIELD

MAIN STREET

WELL FIELD

-0.5

0.0

0.5

Figure 28. Steady-state head buildups in the upper zone of the Lower Floridan aquifer after 

6 million gallons per day (Mgal/d) are injected into Brierwood and 12 Mgal/d are injected into 

Deerwood 3 wells (no withdrawals were simulated from Brierwood or Deerwood 3).
Application of Ground-Water Flow Model 41

EXPLANATION

NET FLOW DIFFERENCE ACROSS BOUNDARY

WEN DIFFERENCE IN WELL WITHDRAWALS FROM LAYER N

SH FLOW DIFFERENCE ACROSS SPECIFIED-HEAD BOUNDARY

FINM NET FLOW DIFFERENCE ACROSS INTERFACE OF LAYERS N AND M

0.71 DIFFERENCE IN FLOW RATE, IN CUBIC FEET PER SECOND











SH=32.52

SURFICIAL

AQUIFER

SYSTEM

(Layer 1)

UPPER

AQUIFER

(Layer 2)

FLORIDAN

UPPER ZONE

LOWER

FLORIDAN

AQUIFER

(Layer 3)


SH=0.71

FERNANDINA

PERMEABLE

ZONE

(Layer 4)


FI =0.71 34


WE =30.32 3


FI =2.91 23


WE =3.33 2


FI =0.42 12

Figure 29. Simulated steady-state net volumetric flow budget differences between flows before 

and after withdrawing 18 million gallons per day (Mgal/d) from the Main Street well field and 

injecting the water into the upper zone of the Lower Floridan aquifer in Brierwood (6 Mgal/d) 

and Deerwood 3 (12 Mgal/d) wells (no withdrawals were simulated from Brierwood or 

Deerwood 3, see figure 23 for net flows before the injections).
42 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

EXPLANATION

NET FLOW DIFFERENCE ACROSS BOUNDARY

WEN DIFFERENCE IN WELL WITHDRAWALS FROM LAYER N

FL FLOW DIFFERENCE ACROSS SPECIFIED-HEAD BOUNDARY

FINM NET FLOW DIFFERENCE ACROSS INTERFACE OF LAYERS N AND M

0.07 DIFFERENCE IN FLOW RATE, IN CUBIC FEET PER SECOND










SURFICIAL

AQUIFER

SYSTEM

(Layer 1)

UPPER

AQUIFER

(Layer 2)

FLORIDAN

UPPER ZONE

OF THE LOWER

FLORIDAN

AQUIFER

(Layer 3)

FERNANDINA

PERMEABLE

ZONE

(Layer 4)

WE =0.0

3 


WE =0.00 2 






WE =25.62 3

Main Street

FI =0.01 34





FL=2.05


FL=25.78


FL=0.01

FI =0.17 23




FI =0.00 12




WE =2.22 2

WE =35.74 3 


WE =4.10


 2

Deerwood 3

FI =0.01 12





FL=36.24


FL=0.07


FL=3.52

FI =0.07 34




FI =0.57 23




WE =20.19


 3

WE =1.46 2 


Brierwood


FI =0.00 12


FL=0.02


FL=20.23


FL=1.40

FI =0.06 23




FI =0.02 34




Figure 30. Simulated steady-state net volumetric flow budget differences for the well field subareas between flows before and after wper day (Mgal/d) from the Main Street well field and injecting the water into the upper zone of the Lower Floridan aquifer in Brierwood (12 Mgal/d) wells (no withdrawals were simulated from Brierwood or Deerwood 3).
Application of Ground-Water Flow Model 43 EXPLANATION NET FLOW DIFFERENCE ACROSS BOUNDARY WEN DIFFERENCE IN WELL WITHDRAWALS FROM LAYER N FL FLOW DIFFERENCE ACROSS SPECIFIED-HEAD BOUNDARY FINM NET FLOW DIFFERENCE ACROSS INTERFACE OF LAYERS N AND M 0.07 DIFFERENCE IN FLOW RATE, IN CUBIC FEET PER SECOND 




SURFICIAL

AQUIFER

SYSTEM

(Layer 1)

UPPER

AQUIFER

(Layer 2)

FLORIDAN

UPPER ZONE

OF THE LOWER

FLORIDAN

AQUIFER

(Layer 3)

FERNANDINA

PERMEABLE

ZONE

(Layer 4)


WE =0.00 3


WE =0.00 2


WE =25.62 3


WE =2.22 2

Main Street

FI =0.01 34 



FL=2.11


FL=25.72


FL=0.01

FI =0.11 23 


FI =0.00 12 



WE =17.18 3


WE =22.66 2

Deerwood 3

FI =0.04 12 



FL=20.97


FL=0.05


FL=18.78

FI =0.05 34 


FI =3.84 23 


WE =10.91 
 3


WE =10.74 2

Brierwood


FI =0.01 12


FL=0.01


FL=11.56


FL=10.07

FI =0.66 23 


FI =0.01 34 


Figure 31. Simulated steady-state net volumetric flow budget differences for the well field subareas between flows before and afte

per day (Mgal/d) from the Main Street well field and injecting the water into the Upper Floridan aquifer in Brierwood (6 Mgal/d) and (no withdrawals were simulated from Brierwood or Deerwood 3).
44 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

Transient Ground-Water Flow Model

The transmissivity and vertical leakance values 

derived from the calibrated steady-state ground-water 

flow model were not modified for the simulation of 

transient stresses. The storage coefficient was calibrated for the transient ground-water flow model, a 

parameter needed to simulate transient stresses.

The CY 2000 was divided into 12 stress periods 

of 1-month duration each and heads measured for each 

month were compared to simulated heads at the end of 

each period. The storage coefficients for the UFA, 

upper zone of the LFA, and FPZ were calibrated based 

on the residuals or differences between simulated and 

measured heads at control points for each stress 

period. The minimum, maximum, mean, and RMS 

residuals were used as criteria to calibrate the storage 

coefficient. The SAS was assigned a storage coefficient of 0.14. Storage coefficient values were assigned 

to the UFA, the upper zone of the LFA, and the FPZ by 

calibrating a uniform specific storage (storage coefficient divided by the thickness of the hydrogeologic 

unit), and using corresponding average thicknesses of 

250, 420, and 170 ft. Simulated heads did not change 

significantly for specific storage values between 

1.5 x 10-5 and 3.5 x 10-5 ft-1. Storage coefficient values outside this range resulted in overall increases to 

residuals. Residuals for each stress period in the transient ground-water flow model were similar to those 

obtained for the calibrated steady-state model 

(table 6). The calibrated specific storage for the transient ground-water flow model was 2.5 x 10-5 ft-1, 

which resulted in storage coefficients of 0.0063, 

0.0105, 0.0042 for the UFA, upper zone of the LFA, 

and FPZ, respectively. Changes in transmissivity and 

vertical leakance values to the calibrated steady-state 

model decreased residuals for some stress periods but 

increased them in others, indicating the calibrated 

transmissivity and vertical leakance values from the 

steady-state model were the optimal values to use for 

the transient model.

Heads specified along the lateral boundaries of 

the upper zone of the Lower Floridan aquifer and 

Fernandina permeable zone were derived for each 

stress period by linearly regressing the monthly measured heads during CY 2000 with the time-averaged 

heads for CY 2000. The specified heads along the lateral boundaries of the upper zone of the LFA for each 

stress period were computed by first performing the 

SLR:

, (6)

where

is the measured head at a control point for a 

given month, in feet;

is the time-averaged head for CY 2000 at the 

same site, in feet;

is the dimensionless SLR coefficient; and

is the intercept of the SLR, in feet.

With the exceptions of May and September, 16 

points were available to perform the SLR. The number 

of control points for May and September 2000 was 20. 

Correlation coefficients were either 0.98 or 0.99 for 

the 12 SLR performed. RMS residuals ranged from 

0.34 ft for January to 0.96 ft for April, with overall 

residuals ranging from -2.17 to 3.04 ft. Slope and 

intercept values derived for each stress period are 

shown in table 6. These regression coefficients were 

used to calculate the heads to be specified along the 

lateral boundaries of the upper zone of the LFA for 

each stress period by using the specified heads for the 

steady-state simulations as (eq. 6). The specified 

heads along the lateral boundaries of the FPZ were set 

to be 1.5 ft higher than the specified heads for the 

upper zone of the LFA. Residuals were computed for 

each stress period (table 6). In addition, residuals were 

calculated between the time-averaged heads for CY 

2000 and the simulated heads at the end of the transient simulations. The small differences between the 

residuals for the steady-state model (table 5) and those 

computed from the transient model when using timeaveraged heads for 2000 indicate the final distribution 

of heads in the steady-state and transient simulations 

were similar.

Ground-water withdrawals for CY 2000 varied 

from one month to another at most well fields. In particular, temporal variations in ground-water withdrawals occurred at Brierwood, Deerwood 3, and Main 

Street well fields (table 7), where the stress periods of 

injections, withdrawals, and intervening rest periods 

were simulated. The simulated transient scenarios were 

based on a withdrawal of 18 Mgal/d, or 27.85 ft3

/s, 

from the Main Street well field during stress periods of 

injections and intervening rest, in addition to withdrawals shown in table 7, and the injection, withdrawal, or 

intervening rest periods at Brierwood and Deerwood 3 

well fields. All other withdrawals from other well 

fields in the model area remained unchanged. A stress 

period of injection is when 18 Mgal/d of water are 

withdrawn from the Main Street well field and 

injected in Brierwood and Deerwood 3 injection wells.

hMon α3hAve β3 = +

hMon

hAve

α3

β3

hAve
Application of Ground-Water Flow Model 45

Table 6. Water-level residual statistics for the calibrated transient model[All residuals are in feet unless otherwise indicated; 11 control points were used for the UFA in May 2000 and September 2000, 7 in all other months for the UFA

the FPZ (Jan.-Dec.). Residual is the difference between simulated and measured head. Min, minimum residual; Max, maximum residual; Mean, mean residual; 

Upper Floridan aquifer; uzLFA, upper zone of the Lower Floridan aquifer; FPZ, Fernandina permeable zone. Intercept and dimensionless slope refer to regressio

heads from equation 6; na, not applicable]

Month

UFA uzLFA FPZ

Overall 

mean

Min Max Mean RMS Min Max Mean RMS Min Max Mean RMS 

January -3.44 0.57 -1.74 2.20 -3.99 1.71 -1.36 2.31 -2.06 -0.62 -1.16 1.33 -1.49 February -2.30 1.68 -.49 1.44 -3.68 .71 -1.57 2.09 -1.46 .58 -.60 1.05 -.91 March -2.94 4.32 .18 2.17 -3.52 1.22 -1.60 2.20 -2.61 1.23 -.72 1.73 -.66 April -3.62 4.74 1.23 2.74 -4.88 -.87 -2.99 3.27 -1.77 .80 -.84 1.44 -.74 May -4.12 4.88 2.27 3.68 -3.90 3.03 -1.17 2.81 -1.43 1.41 -.05 1.16 .89 June -2.84 4.75 2.49 3.66 -3.98 2.15 -1.05 2.14 -.61 2.26 .92 1.49 .87 July -2.54 4.29 1.94 3.06 -2.84 3.36 -.20 2.33 -1.36 2.32 .89 1.84 .94 August -2.17 3.87 1.55 2.54 -2.13 1.09 -.11 1.18 -.83 2.68 1.33 2.04 .89 September -3.37 1.19 -2.08 2.42 -4.03 3.31 -1.21 2.61 -2.18 2.27 .38 1.92 -1.45 October -3.01 2.20 -.20 1.68 -4.70 3.49 -1.04 2.70 -1.95 2.27 .48 1.85 -.39 November -3.05 3.03 -.83 2.15 -3.34 3.31 -.62 2.21 -1.76 2.03 .11 1.55 -.57 December -2.15 1.78 -.46 1.38 -2.47 .17 -.88 1.33 -1.39 2.39 .02 1.69 -.52 Year 2000 -4.12 4.88 .30 2.59 -4.88 3.49 -1.15 2.33 -2.61 2.68 .06 1.61 -.26 

Residuals computed from time-averaged heads for 2000 and heads at the end of transient simulation

-2.11 3.41 .51 1.52 -2.62 3.13 .04 1.79 -.32 2.07 1.11 1.52 .46 
46 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

A stress period of withdrawal is when pumping from 

Brierwood and Deerwood 3 occurs at the corresponding rate for that period during 2000 (table 7) and no 

additional ground-water withdrawal occurs at Main 

Street. A stress period of rest is when water is withdrawn from the Main Street well field and is put 

directly into the service line and no injection or 

withdrawal occurs at the Brierwood and Deerwood 3 

well fields.

The first nine transient scenarios simulated in 

this study included a minimum of 6 months of injections, with alternating withdrawal and rest months 

(table 8). The scenarios simulated were based on 

injecting 12 Mgal/d in three Deerwood 3 injection 

wells and 6 Mgal/d in two Brierwood injection wells. 

Scenarios 10 through 12 (table 8) were used as standards for comparison of upward flow from the FPZ. 

A ground-water flow analysis of the Deerwood 3 well 

field subarea indicates that the minimum annual average upward flow from the FPZ was 0.04 ft3

/s (figs. 32, 

33). Additional simulations showed that increasing the 

duration of the injection stresses in Deerwood 3 does 

not necessarily reduce upward flow. If no injection or 

withdrawal months are simulated in Deerwood 3 

(scenario 12 in table 8), then the simulated average 

annual upward flow from the FPZ becomes 0.08 ft3

/s 

(fig. 33). Pumping at Deerwood 3 wells continuously 

for all 12 months (scenario 11 in table 8) results in an 

upward flow from the FPZ of 0.11 ft3

/s, the highest 

simulated upward flow among all scenarios.

Table 7. Monthly withdrawals during 2000 at Brierwood, Deerwood 3, and Main Street well fields

[Source: Jacksonville Electric Authority, written communication, 2003; all measurements in cubic feet per second]

Well field Jan. Feb. Mar. Apr. May June July Aug. Sept. Oct. Nov. Dec.

Average 

for 

2000

Brierwood 9.16 9.80 12.85 12.28 16.16 14.37 10.72 13.94 8.43 12.61 14.57 13.57 12.37

Deerwood 3 13.54 14.47 13.62 20.85 21.94 15.80 17.96 29.74 23.88 16.44 38.70 28.43 21.28

Main Street 9.97 10.66 13.69 8.37 9.61 14.70 15.61 16.44 14.25 15.64 16.46 15.21 13.38

Table 8. Simulated scenarios of injection, withdrawal, and intervening rest months at Brierwood and Deerwood 3 

well fields

Scenario 

number Injection months Withdrawal months Rest months

1 Jan., Mar., May, July, Sept., Nov. Feb., Apr., June, Aug., Oct., Dec.

2 Jan. - Mar., May - July, Sept. - Nov. Apr., Aug., Dec.

3 Jan. - Mar., May - July, Sept. - Nov. Apr., Aug., Dec.

4 Jan., Mar., May, July, Sept., Nov. Feb., Apr., Aug., Oct., Dec. June

5 June-Sept. Jan. - May Oct. - Dec.

6 Jan. - Feb., Apr. - May, July - Aug., Oct. - Nov. Mar., Sept., Dec. June

7 Jan., Mar., Apr., July, Sept., Nov. Feb., June, Oct. Apr., Aug., Dec.

8 Jan. - Mar., May - July, Sept. - Nov. Apr., Dec. Aug.

9 Jan., Mar., May, July, Sept., Nov. Apr., Oct. Feb., June, Aug., Dec.

10 Jan. - Dec.

11 Jan. - Dec.

12 Jan. - Dec.
Application of Ground-Water Flow Model 47

BRIERWOOD

DEERWOOD 3

MAIN STREET

EXPLANATION

MONTHLY UPWARD FLOW -- All flows are negative and indicate upward flow from

the Fernandina permeable zone. Upward flows apply to delinated subareas in

figure 24

ANNUAL AVERAGE FLOW -- Number indicates simulated annual average upward

flow from the Fernandina permeable zone, in cubic feet per second

-0.02

-0.08

-0.04

-0.02 -0.06

-0.04

-0.02

-0.07

-0.04

-0.02

-0.05

-0.04

-0.02

-0.08

-0.04

-0.02

-0.06

-0.04

-0.02

VERTICAL FLOW, IN CUBIC FEET PER SECOND

J FMAMJ J A SOND J FMAMJ J A SOND

MONTH OF 2000

Scenario 1

Scenario 2

Scenario 3 Scenario 4

Scenario 5 Scenario 6

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

Figure 32. Simulated monthly upward flows from the Fernandina permeable zone for 2000, for scenarios 1 

through 6, with injection rates of 6 million gallons per day (Mgal/d) in Brierwood and 12 Mgal/d in Deerwood 3 

well fields (see table 8 for description of scenarios).
48 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

BRIERWOOD

DEERWOOD 3

MAIN STREET

EXPLANATION

MONTHLY UPWARD FLOW -- All flows are negative and indicate upward flow from

the Fernandina permeable zone. Upward flows apply to delinated subareas in

figure 24

ANNUAL AVERAGE FLOW -- Number indicates simulated annual average upward

flow from the Fernandina permeable zone, in cubic feet per second

-0.03

-0.07

-0.04

-0.02

-0.06

-0.04

-0.02

-0.04

-0.04

-0.02

-0.07

-0.04

-0.02

-0.11

-0.03

-0.03 -0.08

-0.04

-0.02

VERTICAL FLOW, IN CUBIC FEET PER SECOND

J FMAMJ J A SOND J FMAMJ J A SOND

MONTH OF 2000

Scenario 7 Scenario 8

Scenario 9

Scenario 10

Scenario 11 Scenario 12

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

Figure 33. Simulated monthly upward flows from the Fernandina permeable zone for 2000, for scenarios 7 

through 12, with injection rates of 6 million gallons per day (Mgal/d) in Brierwood and 12 Mgal/d in Deerwood 3 

well fields (see table 8 for description of scenarios).
Application of Ground-Water Flow Model 49

Simulated upward flows from the FPZ in scenarios 8 and 9 showed that as the time between withdrawals is increased and the intervening periods of rest 

are replaced by periods of injection, the annual average upward flow from the FPZ decreases. The upward 

flow from the FPZ to the upper zone of the LFA was 

never reversed in any cell in the model area when 

6 Mgal/d were injected at Brierwood and 12 Mgal/d in 

Deerwood 3. Upward flow from the FPZ was observed 

at all model cells and for all simulated stress periods 

and scenarios.

Among the first nine transient scenarios (table 8), 

scenario 8 had the most periods of injections, at most 

two periods of withdrawals, and the longest time interval between periods of withdrawals. This scenario uses 

all three types of stress periods and provides the optimal combination of stress periods that could minimize 

the upward flow from the FPZ. Scenario 8 was used to 

simulate how changes in the magnitude of the injection and withdrawal rates in the Deerwood 3 and Brierwood well fields would affect vertical fluxes 

between the FPZ and the upper zone of the LFA.

The sensitivity of transient ground-water flow 

to changes in storage coefficient values was analyzed 

by simulating two sets of values. Specific storage 

values of 1.5 x 10-5 and 3.5 x 10-5 ft-1 were used to 

determine how the corresponding storage coefficient 

values would change the results shown in figures 32 

and 33. A specific storage value of 1.5 x 10-5 ft-1

results in storage coefficient values of 0.0038, 0.0063, 

and 0.0026 assuming average thicknesses of 250, 420, 

and 170 ft for the UFA, upper zone of the LFA, and 

FPZ, respectively. A second set of simulated storage 

coefficient values was 0.0088, 0.0147, and 0.0060 for 

the UFA, upper zone of the LFA, and FPZ, respectively. This second set of storage coefficient values 

corresponds to a specific storage value of 3.5 x 10-5 ft-1. 

No significant changes in upward flow from the FPZ 

from those shown in figure 33, scenario 8, were 

simulated when either set of storage coefficient values 

was used.

The magnitude of upward flow from the FPZ 

decreased as the injection rate increased, but did not 

change during both months of withdrawals (fig. 34). 

Of the 18 Mgal/d withdrawn from Main Street wells 

and available for injection to Deerwood 3 and Brierwood wells, no possible combination of injection rates 

resulted in the reversal of the upward hydraulic gradient from the FPZ. The upward hydraulic gradient from 

the FPZ could not be reversed with a combined injection rate of 18 Mgal/d between Deerwood 3 and Brierwood well fields. The higher injection rate into the 

Deerwood 3 wells (18 Mgal/d) resulted in the lowest 

upward flow from the FPZ within the area delineated 

around the well field. Greater injection rates were simulated to determine the necessary injection rate (as an 

integer value in million gallons per day units) to Deerwood 3 for which a reversal of the upward flow from 

the FPZ would be achieved. An injection rate greater 

than 30 Mgal/d would be required to reverse the 

upward flow from the FPZ (fig. 34F). However, a 

period of withdrawal or rest at Deerwood 3 well field 

following a period of injection of this rate (30 Mgal/d) 

causes the return of upward flow from the FPZ.

Based on the upward flows from the FPZ 

(fig. 34), the injection of the 18 Mgal/d into Deerwood 

3 wells and no injection into Brierwood wells resulted 

in the lowest annual average upward flow from the 

FPZ at Deerwood 3 well field. Because greater chloride concentrations in ground water were measured in 

Deerwood 3 wells than at Brierwood wells (table 1), 

all simulations that followed considered the injection 

of 18 Mgal/d into Deerwood 3 wells to explore the 

conditions under which a reversal of the upward flow 

from the FPZ at these wells could occur. The withdrawal rates from Deerwood 3 and Brierwood wells 

for 2000 were varied to determine the effect these 

changes would have on the upward flow from the FPZ 

during April and December (withdrawal months). 

Simulation results showed that the upward flow from 

the FPZ increased as the withdrawals from the Deerwood 3 and Brierwood well fields increased (fig. 35). 

Upward flow from the FPZ increases when withdrawal 

rates at Deerwood 3 and Brierwood increase above 

2000 rates.

The impact of head drawdowns or buildups, 

resulting from the withdrawal of 18 Mgal/d from the 

Main Street well field or the injections into Brierwood 

and Deerwood 3 wells, on the specified heads along 

the lateral boundaries of the model was analyzed. A 

regional steady-state ground-water flow model 

(Sepúlveda, 2002a) was used to compute simulated 

head changes along the lateral boundaries of the model 

area (fig. 1) due to the withdrawal of 18 Mgal/d from 

Main Street and its injection into the Brierwood and 

Deerwood 3 wells. The initial distribution of heads for 

the regional model was the steady-state solution from 

the same regional model using average 1993-94 conditions. The head changes along the boundaries of the 

model (fig. 1) ranged from an increase of 0.05 ft to a 

decrease of 0.06 ft. This indicates the effect of the 

specified heads along the boundaries of the model during the withdrawal of water from the Main Street wells 

and its injection into the Brierwood and Deerwood 3 

wells was minimal.
50 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

BRIERWOOD

DEERWOOD 3

MAIN STREET

EXPLANATION

MONTHLY VERTICAL FLOW -- Negative flow indicates upward flow from the Fernandina

permeable zone. Positive flow indicates downward flow from the upper zone of the Lower

Floridan aquifer. Vertical flows apply to delineated subareas in figure 24

INJECTION RATES -- Applied to Brierwood and Deerwood 3 injection wells during

simulated months of injections, in million gallons per day

B = 15 Rate distributed uniformly between the two Brierwood injection wells

D3 = 3 Rate distributed uniformly among the three Deerwood 3 injection wells

ANNUAL AVERAGE VERTICAL FLOW -- Number indicates simulated annual

average vertical flow between the Fernandina permeable zone and the upper zone

of the Lower Floridan aquifer, in cubic feet per second

-0.02

D3 = 3

B = 15 D3 = 6

B = 12

D3 = 12

D3 = 9 B = 6

B = 9

D3 = 18

B = 0

D3 = 30

B = 0

-0.07

-0.04

-0.01

-0.06

-0.04

-0.02

-0.06

-0.04

-0.02

-0.06

-0.04

-0.02

-0.03

-0.04

-0.02

-0.05

-0.04

-0.02

VERTICAL FLOW, IN CUBIC FEET PER SECOND

J FMAMJ J A SOND J FMAMJ J A SOND

MONTH OF 2000

A B

C D

E F

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

Figure 34. Simulated monthly vertical flows between the Fernandina permeable zone and the upper zone of the 

Lower Floridan aquifer for 2000 under scenario 8 conditions, for various injection rates in Brierwood and Deerwood 

well fields (see table 8 for description of scenario 8).
Application of Ground-Water Flow Model 51

VERTICAL FLOW, IN CUBIC FEET PER SECOND

MONTH OF 2000

J FMAMJ J A SOND J FMAMJ J A SOND

A B

C D

E F

-0.20

-0.16

-0.12

-0.08

-0.04

0.00

-0.20

-0.16

-0.12

-0.08

-0.04

0.00

-0.20

-0.16

-0.12

-0.08

-0.04

0.00

-0.20

-0.16

-0.12

-0.08

-0.04

0.00

-0.20

-0.16

-0.12

-0.08

-0.04

0.00

-0.20

-0.16

-0.12

-0.08

-0.04

0.00

WITHDRAWAL RATES -- Simulated percentage of 2000 ground-water withdrawals

applied to Brierwood and Deerwood 3 production wells during months of withdrawals

W = 200

BRIERWOOD

DEERWOOD 3

MAIN STREET

EXPLANATION

MONTHLY UPWARD FLOW -- All flows are negative and indicate upward flow from

the Fernandina permeable zone. Upward flows apply to delinated subareas in

figure 24

ANNUAL AVERAGE FLOW -- Number indicates simulated annual average upward

flow from the Fernandina permeable zone, in cubic feet per second

-0.02

W = 50 W = 75

W = 100, W = 125

CALIBRATION VALUE

W = 150

W = 200

-0.05

-0.04

-0.02

-0.05

-0.04

-0.02

-0.06

-0.04

-0.02

-0.05

-0.04

-0.02

-0.06

-0.04

-0.02

-0.07

-0.04

-0.02

Figure 35. Simulated monthly upward flows from the Fernandina permeable zone for 2000, under scenario 8 

conditions, for various withdrawal rates at Brierwood and Deerwood 3 well fields, and where water is injected, at the 

rate of 18 million gallons per day, in Deerwood 3 well field only (see table 8 for description of scenario 8).
52 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

EFFECTS OF PARAMETER UNCERTAINTY 

ON SIMULATED UPWARD FLOW FROM 

THE FERNANDINA PERMEABLE ZONE

This section presents a discussion of the effects 

of parameter changes on the simulated flow between 

the upper zone of the LFA and the FPZ. Vertical leakance of the SCU, transmissivity of the FPZ, and specified heads in the FPZ along the lateral boundaries of 

the model are the hydraulic parameters that can have 

the most substantial effects on the flow exchange 

between these two hydrogeologic units. The uncertainty of these parameters implies the true vertical 

fluxes between the upper zone of the LFA and the FPZ 

may be quantified by simulating changes to parameter 

values within feasible limits. The effects of the uncertainty of some model parameters on the vertical fluxes 

between the upper zone of the LFA and the FPZ under 

the conditions of scenario 8 (table 8) were analyzed 

simulating the injection of all available 18 Mgal/d into 

the Deerwood 3 injection wells. Parameter values that 

resulted in substantial increases of the RMS residual 

of 1.75 ft derived from the steady-state calibration 

were not tested in these parameter uncertainty simulations. The rationale for the sensitivity analysis on the 

upward flow from the FPZ was to study uncertainty in 

model prediction that results from the inability to completely describe the physical system.

Vertical Leakance of Semiconfining Unit

Vertical leakance of the SCU was tested for values that ranged from slightly leakier than the ICU to as 

leaky as the MSCU. These values ranged from half the 

calibrated value, or 1.0 x 10-5 (ft/d)/ft, to about 

25 times the calibrated value, or 5 x 10-4 (ft/d)/ft. 

Increases in the simulated vertical leakance of the 

SCU resulted in increased upward flow from the FPZ 

during the simulated months of withdrawals, April and 

December. During the simulated month of rest, 

August, the Brierwood and Deerwood 3 well field 

subareas had increases in upward flow from the FPZ 

relative to the remaining 9 months of the year, months 

of injections. In contrast, upward flow from the FPZ 

decreased in the Main Street well field during April 

and December because the withdrawals at Brierwood 

and Deerwood 3 during these 2 months did not require 

the additional withdrawal of 18 Mgal/d at the Main 

Street well field (fig. 36).

During stress periods of rest or withdrawal, the 

upward flow from the FPZ in the Deerwood 3 well 

field subarea increased as the simulated vertical 

leakance of the SCU increased and decreased during 

stress periods of injection. The injection of water into 

the upper zone of the LFA in Deerwood 3 wells 

reverses the hydraulic gradient between the FPZ and 

the upper zone of the LFA when the vertical leakance 

of the SCU is simulated to be at least 5 times greater 

than the calibration value of 0.00002 (ft/d)/ft. For 

example, if the simulated vertical leakance of the SCU 

is 0.00050 (ft/d)/ft, then the flows from the FPZ in the 

Deerwood 3 well field subarea during the months of 

April and December were upward at the rate of about 

0.6 and 1.0 ft3

/s; flow during August, a month of rest, 

was downward at about 0.12 ft3/s; and flows during 

the 9 months of injection were downward at an 

average of about 0.3 ft3/s (fig. 36).

Transmissivity of the Fernandina 

Permeable Zone

Transmissivity of the FPZ was tested for values 

that ranged from about the average simulated transmissivity of the UFA to a fourth of the transmissivity 

of the upper zone of the LFA. These values ranged 

from 20,000 to 120,000 ft2

/d. Decreasing the simulated transmissivity of the FPZ decreases the upward 

flow from the FPZ to the upper zone of the LFA (fig. 37). 

This was observed for stress periods of injection, withdrawal, and intervening rest months. The results for the 

simulated transmissivities for the FPZ were obtained 

using the calibrated values for all other model parameters, and assuming that 18 Mgal/d were injected in the 

Deerwood 3 well field. A simulated transmissivity of 

20,000 ft2

/d in the FPZ resulted in the reversal of the 

upward hydraulic gradient from the FPZ (fig. 37).

Vertical flow rates between the FPZ and the 

upper zone of the LFA (fig. 37) indicate that if the 

transmissivity of the FPZ was 20,000 or 40,000 ft2

/d 

(less than the calibration value), then the injection of 

water into Deerwood 3 wells would result in the reduction of the upward flow from the FPZ in the subarea of 

this well field. The injection of water into Deerwood 3 

wells also would reduce the upward flow from the FPZ 

during the months of withdrawals if the transmissivity 

of the FPZ was less than the calibrated value. Transmissivity values of the FPZ greater than the calibration 

value of 60,000 ft2/d increased the upward flow from 

the FPZ. The scales used for the vertical axis in figures 

36 and 37 indicate the simulated changes to the vertical leakance of the SCU had a greater effect on the 

simulated upward flow from the FPZ than the simulated changes to the transmissivity of the FPZ.
Effects of Parameter Uncertainty on Simulated Upward Flow from the Fernandina Permeable Zone 53

VERTICAL LEAKANCE OF SEMICONFINING UNIT -- Simulated vertical leakance

of unit overlaying the Fernandina permeable zone, in units of foot per day per foot

L = 0.0005

BRIERWOOD

DEERWOOD 3

MAIN STREET

EXPLANATION

MONTHLY VERTICAL FLOW -- Negative flow indicates upward flow from the Fernandina

permeable zone. Positive flow indicates downward flow from the upper zone of the Lower

Floridan aquifer. Vertical flows apply to delineated subareas in figure 24

ANNUAL AVERAGE VERTICAL FLOW -- Number indicates simulated annual

average vertical flow between the Fernandina permeable zone and the upper zone

of the Lower Floridan aquifer, in cubic feet per second

-0.06

VERTICAL FLOW, IN CUBIC FEET PER SECOND

J FMAMJ J A SOND J FMAMJ J A SOND

MONTH OF 2000

A B

C D

E F

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

L = 0.00001

L = 0.00005

L = 0.0002

L = 0.00002,

CALIBRATION VALUE

L = 0.0001

L = 0.0005

-0.03

-0.02

-0.01

-0.05

-0.04

-0.02

-0.10

-0.16

-0.09 -0.05

-0.09

-0.04

-0.06

-0.27

-0.06

0.11

-0.48

-0.06

Figure 36. Simulated monthly vertical flows between the Fernandina permeable zone and the upper zone of the Lower 

Floridan aquifer for 2000, under scenario 8 conditions, for various vertical leakances of the semiconfining unit overlying 

the Fernandina permeable zone, and where water is injected, at the rate of 18 million gallons per day, in Deerwood 3 

well field only (see table 8 for description of scenario 8).
54 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

TRANSMISSIVITY OF FERNANDINA PERMEABLE ZONE -- Simulated

transmissivity of the Fernandina permeable zone, in feet squared per day

T = 120,000

BRIERWOOD

DEERWOOD 3

MAIN STREET

EXPLANATION

MONTHLY VERTICAL FLOW -- Negative flow indicates upward flow from the Fernandina

permeable zone. Positive flow indicates downward flow from the upper zone of the Lower

Floridan aquifer. Vertical flows apply to delineated subareas in figure 24

ANNUAL AVERAGE VERTICAL FLOW -- Number indicates simulated annual

average vertical flow between the Fernandina permeable zone and the upper zone

of the Lower Floridan aquifer, in cubic feet per second

-0.02

VERTICAL FLOW, IN CUBIC FEET PER SECOND

J FMAMJ J A SOND J FMAMJ J A SOND

MONTH OF 2000

A B

C D

E F

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

T = 40,000

T = 60,000,

CALIBRATION VALUE

T = 100,000

T = 80,000

T = 20,000

T = 120,000

-0.04

-0.04

-0.02

-0.05

-0.04

-0.02

-0.06

-0.04

-0.02

-0.05

-0.04

-0.02

-0.06

-0.04

-0.02

-0.06

-0.04

-0.02

Figure 37. Simulated monthly vertical flows between the Fernandina permeable zone and the upper zone of the Lower 

Floridan aquifer for 2000, under scenario 8 conditions, for various transmissivities of the Fernandina permeable zone, 

and where water is injected, at the rate of 18 million gallons per day, in Deerwood 3 well field only (see table 8 for 

description of scenario 8).
Effects of Parameter Uncertainty on Simulated Upward Flow from the Fernandina Permeable Zone 55

Specified-Head Cells Along the Lateral 

Boundaries of the Fernandina Permeable 

Zone

Several variations to the differences in heads 

along the lateral boundaries of the model were simulated for the purpose of assessing how these changes 

affect the upward flow from the FPZ. If the difference 

between the specified heads along the lateral boundaries of the model in the FPZ and the upper zone of the 

LFA is varied, then the rates of water exchanged 

between these two units change. Hydrologists believe 

that flow from the FPZ is always upward or lateral, but 

not downward within the model area.

The simulated upward flow from the FPZ 

decreases as the difference in specified heads, along 

the lateral boundaries of the model, between the FPZ 

and the upper zone of the LFA decreases (fig. 38). 

This was expected based on Darcy’s law because the 

vertical leakance was maintained constant, but the 

vertical hydraulic gradient decreased, thus reducing 

the exchange of water.

SPECIFIED HEADS ALONG LATERAL BOUNDARIES OF THE FERNANDINA

PERMEABLE ZONE -- Expressed as difference between the specified head at the

Fernandina permeable zone and that at the upper zone of the Lower Floridan aquifer

for cells lying along the lateral boundaries of the model, in feet

DIFF = 1.50

BRIERWOOD DEERWOOD 3 MAIN STREET

EXPLANATION

MONTHLY UPWARD FLOW -- All flows are negative and indicate upward flow from

the Fernandina permeable zone. Upward flows apply to delinated subareas in figure 24

ANNUAL AVERAGE FLOW -- Number indicates simulated annual average upward

flow from the Fernandina permeable zone, in cubic feet per second

-0.02

VERTICAL FLOW, IN CUBIC FEET PER SECOND

J FMAMJ J A SOND J FMAMJ J A SOND

MONTH OF 2000

A B

C D

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

-0.18

-0.15

-0.12

-0.09

-0.06

-0.03

0.00

0.03

DIFF = 1.0

DIFF = 0.75 DIFF = 0.50

-0.05

-0.04

-0.02

-0.04

-0.04

-0.02

-0.02

-0.03

-0.01

-0.03

-0.04

-0.02

DIFF = 1.50,

CALIBRATION

VALUE

Figure 38. Simulated monthly upward flows from the Fernandina permeable zone for 2000, under scenario 8 

conditions, for various differences in specified heads, between the Fernandina permeable zone and the upper zone 

of the Lower Floridan aquifer, along lateral boundaries of the model, and where water is injected, at the rate of 

18 million gallons per day, in Deerwood 3 well field only (see table 8 for description of scenario 8).
56 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

MODEL LIMITATIONS

Ground-water flow simulations generally are 

based on conceptual models that are simplified representations of complex heterogeneous ground-water 

flow systems. Assumptions such as isotropy, vertical 

homogeneity within each layer, and the absence of 

preferential flow zones are examples of simplified 

representations that can be sources of error in a groundwater flow model. The lack of sufficient measurements 

to account for the spatial variation of hydraulic properties throughout the model area necessitated these simplifications. Simplifying the model does not invalidate 

model results, although model results should be interpreted at scales larger than the representative grid cell.

The most important limitations of the groundwater flow models presented in this report are: simplifications in the conceptual model, inherent model 

assumptions, and lack of head and flow measurements 

in areas where spatial variability of hydraulic and 

hydrologic properties is poorly known. Model simulations were performed using calibrated hydraulic properties, specified heads in the upper zone of the LFA 

and in the FPZ along lateral boundaries of the model, 

time-averaged Floridan aquifer heads, and steady-state 

conditions in the UFA, upper zone of the LFA, and 

FPZ for CY 2000. An error in any of these can limit 

the accuracy of model simulations.

The ground-water flow equation solved by the 

ground-water flow model is the continuity equation for 

flow, derived from the principal of conservation of 

mass and the assumption that water is incompressible 

and of constant viscosity, incorporated with Darcy’s 

law (Todd, 1980). This equation is valid for groundwater flow conditions where the velocity of ground 

water is low and flow is laminar. In karstic aquifers, it 

is possible to have turbulent flow through caverns and 

solution channels. Thus, the ground-water flow equation may not be valid for the entire FAS. The assumptions of laminar flow and uniform effective 

transmissivity throughout each grid cell to conserve 

mass were made.

Inaccuracies inherent in the algorithm used to 

estimate the heads in the UFA, the upper zone of the 

LFA, and the FPZ (such as lack of data and residual 

errors) could produce errors in simulated recharge or 

discharge. These inaccuracies can lead to errors in 

simulated vertical leakance of the confining units and 

leakage rates to or from the UFA, the upper zone of the 

LFA, and the FPZ. The assumption of uniform heads 

throughout the vertical thickness of each grid cell is 

another possible source of error in the simulated 

heads.

The simulated transmissivities in the UFA, the 

upper zone of the LFA, the FPZ, and the vertical 

leakances of the ICU, MSCU, and SCU in this study 

had minor changes to those reported in previous 

ground-water flow models. The modifications to transmissivity and vertical leakance values from previous 

ground-water flow models were made to reduce differences between simulated and measured heads. Areas 

in the UFA or the upper zone of the LFA where 

ground-water withdrawals were minimal for 2000 may 

require changes to simulated transmissivity and vertical leakance because additional ground-water withdrawals may reflect unexpected responses in aquifer 

areas previously not stressed.

Lack of data for the upper zone of the LFA, the 

FPZ, the MSCU, and the SCU precludes a reliable 

estimation of transmissivity and vertical leakances; at 

most, only 20 control points were available for this 

study, and at times only 16. In addition, flow between 

the upper zone of the LFA and the FPZ can be better 

understood as the potentiometric surfaces of these 

units are refined with additional head measurements. 

Some heads were computed based on environmentalwater heads because the assumption of uniform density in the FPZ is not valid in areas where water is of 

greater density.

In spite of the limitations of the ground-water 

flow models, these models can indicate the general 

movement of ground water in the study area. The calibrated steady-state flow model can be used to assess 

long-term head buildups (or drawdowns) due to the 

injections or withdrawals of water. The transient 

model can be used to assess the effects of various 

injection and withdrawal scenarios on the magnitude 

of upward flow from the FPZ. The sensitivity analysis 

of the upward flow from the FPZ was performed by 

simulating changes in the calibrated hydraulic properties of the SCU and FPZ, which allowed the identification of parameter values under which the upward flow 

from the FPZ would be reversed.

SUMMARY AND CONCLUSIONS

The water-bearing zones within the Floridan 

aquifer system (FAS), the Upper Floridan aquifer 

(UFA) and the Lower Floridan aquifer (LFA), consist 

of soft, porous limestone and porous highly fractured 

dolomite beds. The LFA is subdivided into two 
Summary and Conclusions 57

principal water-bearing zones, the upper zone of the 

LFA and the Fernandina permeable zone (FPZ), 

separated by a less permeable unit, the semiconfining 

unit (SCU). The UFA and LFA are separated by the 

middle semiconfining unit (MSCU), a unit that 

restricts vertical movement of water from one aquifer 

to another based on vertical leakance.

A four-layer finite-difference steady-state 

ground-water flow model of the FAS in parts of Duval, 

St. Johns, and Clay Counties, Florida, was developed 

and calibrated. The initial distribution of hydraulic 

properties, extracted from larger-scale, regional 

ground-water flow models, was calibrated to average 

2000 hydrologic conditions based on 20 control 

points, representing wells tapping the UFA, the upper 

zone of the LFA, and the FPZ. A transient groundwater flow model was developed by using the calibrated hydraulic properties of the steady-state flow 

model and by calibrating the storage coefficient based 

on residuals between simulated heads and monthly 

measured data at the control points.

Two well fields within the study area, Deerwood 

3 and Brierwood, have had elevated chloride concentrations in ground-water samples. The FPZ, which 

contains ground water with substantially greater chloride levels in the eastern part of the study area, is the 

likely source of these elevated chlorides. Throughout 

the model area, ground water flows upward from the 

FPZ to the upper zone of the LFA, and from the upper 

zone of the LFA to the UFA. Twelve scenarios of 

injection, withdrawal, and intervening rest periods 

were simulated to explore conditions under which the 

upward flow from the FPZ could be decreased or 

reversed, thus minimizing the mixing of poorer quality 

water from the FPZ with freshwater from the upper 

zone of the LFA.

Ground-water withdrawal rates from 2000 were 

used in both steady-state and transient models. As 

withdrawal rates increased from one month of pumping to another, upward flow from the FPZ increased, 

even when pumping periods were preceded by several 

injection periods. Injection periods were characterized 

by an injection rate of 18 million gallons per day 

(Mgal/d) distributed among Deerwood 3 and Brierwood wells. The injections into the upper zone of the 

LFA were more efficient in reducing the upward flow 

from the FPZ than injections into the UFA. During 

intervening rest periods, the water withdrawn from the 

Main Street well field would be connected directly 

into the service line, thus there would be no pumping 

from or injection into Deerwood 3 or Brierwood wells.

An annual average upward flow from the FPZ of 

0.11 cubic feet per second (ft3/s) was simulated before 

the injection of water into the Deerwood 3 wells. As 

water was simulated as being injected into the Deerwood 3 and Brierwood wells, upward flow from the 

FPZ decreased; however, the direction of the vertical 

flow was not reversed until simulated injection rates in 

Deerwood 3 were at least 30 Mgal/d. This rate 

exceeded the proposed rate of 18 Mgal/d to be withdrawn from the Main Street wells. The injection rate 

of 18 Mgal/d into the upper zone of the LFA in Deerwood 3 wells reduced the upward flow from the FPZ 

in the vicinity of this well field to an annual average of 

0.04 ft3/s, less than the 0.11 ft3/s before any injection 

of water. However, a large percentage of the injected 

water into the upper zone of the LFA moves laterally 

within the same aquifer. As ground-water withdrawals 

at Deerwood 3 and Brierwood increase in the future, 

so will the induced upward flow from the FPZ.

If 18 Mgal/d of water withdrawn from the Main 

Street well field is injected into the UFA, instead of 

into the upper zone of the LFA, most of the water 

leaves the Deerwood 3 and Brierwood areas through 

lateral flow in the UFA and as a result, less water 

moves downward to the upper zone of the LFA. Such 

injection into the UFA would reduce the water moving 

downward into the FPZ, and thus, would not reduce 

the upward flow from the FPZ to the upper zone of the 

LFA.

Sensitivity analyses of parameters from the calibrated steady-state model showed that the model was 

least sensitive to the vertical leakance of the SCU, the 

transmissivity of the FPZ, and the specified hydraulic 

gradient between the upper zone of the LFA and the 

FPZ. As the simulated vertical leakance of the SCU 

was increased from 0.00002 (calibration value) to 

0.0001 (ft/d)/ft or higher, injection into the upper zone 

of the LFA at the Deerwood 3 well field reversed the 

hydraulic gradient, although the upward leakage from 

the FPZ increased for higher vertical leakances during 

stress periods of withdrawals at Deerwood 3. As the 

simulated transmissivity of the FPZ was decreased 

from 60,000 feet squared per day (calibration value), 

upward flow from the FPZ decreased during periods of 

withdrawals. As the simulated difference in specified 

heads between the upper zone of the LFA and the FPZ 

decreased, so did upward flow from the FPZ.
58 Evaluation of the Feasibility of Freshwater Injection Wells in Mitigating Ground-Water Quality Degradation at Selected Well

Fields in Duval County, Florida

The SCU was simulated to be leakier than the 

intermediate confining unit (ICU), but tighter than the 

MSCU. The thickness of the SCU was estimated to be 

about 500 feet, more than the thicknesses of the ICU 

or MSCU. The lack of any field measurements for the 

vertical leakance of the SCU resulted in relying on the 

calibration value of 0.00002 (ft/d)/ft. Simulation 

results from a wide range of injection, withdrawal, and 

intervening rest scenarios indicated that the upward 

flow from the FPZ can only be reversed if simulations 

are conducted using other than the calibrated values 

for the transmissivity of the FPZ, the vertical leakance 

of the SCU, and the difference in heads between the 

upper zone of the LFA and the FPZ.

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