U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 9

Ground-Water Flow in the Surficial Aquifer 

System and Potential Movement of 

Contaminants from Selected WasteDisposal Sites at Cecil Field Naval Air 

Station, Jacksonville, Florida

Prepared in cooperation with the

Southern Division Naval Facilities Engineering

Command, U.S. Navy

7–4278

By Keith J. Halford

Tallahassee, Florida

1998
U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Thomas J. Casadevall, Acting Director

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

from:

U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

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.

District Chief

U.S. Geological Survey, WRD

Suite 3015

227 North Bronough Street

Tallahassee, FL 32301
Contents III

CONTENTS

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

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

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

Description of the Study Area .................................................................................................................................... 3

Acknowledgments ...................................................................................................................................................... 3

Geohydrology ....................................................................................................................................................................... 3

Water Budget and Recharge Estimates ................................................................................................................................. 7

Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System ................ 18

Description of Ground-Water Flow Model ................................................................................................................. 18

Hydraulic Properties ......................................................................................................................................... 23

Surface-Water Features .................................................................................................................................... 25

Boundary Conditions......................................................................................................................................... 26

Simulation Approach................................................................................................................................................... 28

Model Calibration........................................................................................................................................................ 32

Water-Level and Discharge Measurements ...................................................................................................... 34

Parameter Estimation......................................................................................................................................... 34

Sensitivity Analysis ..................................................................................................................................................... 51

Simulation of Ground-Water Flow.............................................................................................................................. 53

Simulation of Movement of Contaminants ................................................................................................................ 54

Model Limitations ...................................................................................................................................................... 64

Summary................................................................................................................................................................................ 66

References ............................................................................................................................................................................. 67

FIGURES

1. Map showing location of the study area, sites of contamination, and well D-4560 ................................................ 2

2. Generalized geologic and hydrogeologic units and aquifers beneath Cecil Field Naval Air Station...................... 4

3. Diagram showing gamma-ray log and geologist’s log of well D-4560, a typical hydrogeologic

column of the first 300 feet below Cecil Field Naval Air Station, and model-layer correlations for this study ..... 4

4-11. Maps showing:

4. Location of selected wells used to define the hydrogeologic framework and the potentiometric

surface of the Upper Floridan aquifer, May 1993, in the study area, Black Creek Basin, regional 

model areas, and the location of stream-gaging station Black Creek near Middleburg .................................. 5

5. Altitude of base of the surficial-sand aquifer in the study area ....................................................................... 6

6. Thickness of the surficial-sand aquifer............................................................................................................ 8

7. Thickness of the blue-marl confining unit....................................................................................................... 9

8. Thickness of the upper-rock aquifer ................................................................................................................ 10

9. Thickness of the gray-marl confining unit....................................................................................................... 11

10. Thickness of the lower-rock aquifer ................................................................................................................ 12

11. Thickness of the lower confining unit ............................................................................................................. 13

12. Generalized hydrogeologic sections A-A′ and B-B′ in the study area..................................................................... 14

13. Diagram showing the water budget and its components within the study area ....................................................... 15

14. Map showing locations of hydrograph and domestic supply wells, stream-gaging station Rowell Creek 

near Fiftone, the drainage areas of the Fiftone and Sal Taylor Basins, and the 75-foot contour

of land surface elevation .......................................................................................................................................... 16
IV Contents

15. Graph showing monthly rainfall and ground-water levels from selected wells at Cecil Field and daily 

discharge at stream-gaging stations Black Creek and Rowell Creek from November 1993 to November 1994 ... 17

16. Graph showing discharge at stream-gaging station on the Black Creek and selected base-flow

recession curves for 1992-94................................................................................................................................... 19

17. Map showing model grid and extent ....................................................................................................................... 20

18. Map showing location of aquifer test site at Cecil Field Naval Air Station, background observation wells, 

a cross-section showing well placement and model grid within 250 feet of the wells pumped for the aquifer 

tests, and nearby hydrologic features ...................................................................................................................... 24

19. Graph showing long-term hydrographs from wells DS-238, 226, and 227 located near well D-4560................... 26

20. Map showing grid, boundaries, and simulated volumetric budget of the regional model used to test 

the lateral boundary conditions for the study area................................................................................................... 27

21. Map showing simulated potentiometric surface of the surficial-sand, upper-rock, and lower-rock aquifers

in the regional model area ....................................................................................................................................... 29

22. Graph showing hypothetical cross section that shows the expected effective recharge distribution for an 

aquifer system similar to the surficial-sand aquifer system .................................................................................... 31

23. Graph showing hypothetical time series that shows the expected water-level changes and average

effective recharge rates for an aquifer system similar to the surficial-sand aquifer system.................................... 33

24-29. Maps showing:

24. Simulated potentiometric surface of the top third of the surficial-sand aquifer (layer 1) 

on August 22, 1994 ......................................................................................................................................... 36

25. Simulated potentiometric surface of the middle third of the surficial-sand aquifer (layer 2) 

on August 22, 1994 ......................................................................................................................................... 39

26. Simulated potentiometric surface of the bottom third of the surficial-sand aquifer (layer 3)

on August 22, 1994 ......................................................................................................................................... 42

27. Simulated potentiometric surface of the blue-marl confining unit (layer 4) on August 22, 1994 .................. 45

28. Simulated potentiometric surface of the upper-rock aquifer (layer 5) on August 22, 1994 ........................... 48

29. Simulated potentiometric surface of the lower-rock aquifer (layer 6) on August 22, 1994 ........................... 49

30. Graph showing comparison of simulated to measured water levels and histogram showing residual 

distribution for the calibrated model ....................................................................................................................... 50

31. Graph showing model sensitivity to independent changes in model calibration parameters.................................. 52

32. Graph showing simulated volumetric flow budget using a recharge rate of 6 inches per year............................... 54

33-39. Maps showing:

33. Locations of simulated rejected recharge in the study area ............................................................................ 55

34. Average simulated head difference between the bottom of the surficial-sand aquifer

(layer 3) and upper-rock aquifer (layer 5) potentiometric surfaces ................................................................ 56

35. Simulated deep infiltration to the Upper Floridan aquifer (layer 7) from the lower-rock aquifer

(layer 6) using a recharge rate of 6 inches per year ........................................................................................ 57

36. Simulated flow paths from selected sites to their respective discharge points using a recharge

rate of 6 inches per year .................................................................................................................................. 58

37. Simulated flow paths, particles, traveltimes, and potentiometric contours near site 3, using

a recharge rate of 6 inches per year................................................................................................................. 62

38. Simulated flow paths from the north fuel farm to their respective discharge points, using

a recharge rate of 6 inches per year................................................................................................................. 63

39. Simulated contributing areas for domestic supply wells screened in the upper-rock aquifer

(layer 5) and drawdowns associated with producing 750 gallons per day per well........................................ 65
Contents V

TABLES

1. Aquifer and confining unit properties determined from aquifer tests .................................................................... 25

2. Hydraulic properties of the regional model............................................................................................................ 28

3. Stress period durations, recharge rates, and evapotranspiration rates used in the two-dimensional,

cross-sectional model for testing applicability of effective recharge rates............................................................. 32

4. Initial and final values of parameters estimated to calibrate the model ................................................................ 35

5. Water-level error statistics from calibrated Cecil Field Naval Air Station model by layer and 

synoptic-survey period............................................................................................................................................ 35

6. Simulated and measured discharges from Rowell and Black Creeks during the four 

synoptic-survey periods ......................................................................................................................................... 51

7. Correlation coefficients between parameters from the calibrated model ............................................................... 51

8. Approximate traveltimes of particle movement from selected sites to their respective discharge points.............. 61

CONVERSION FACTORS, VERTICAL DATUM, ABBREVIATIONS AND ACRONYMS 

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

°C=(°F-32)/1.8

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

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

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times foot of aquifer thickness [(ft3/d)/ft2]ft. In

this report, the mathematically reduced form, foot squared per day (ft2/d), is used for convenience.

Acronyms and additional abbreviations used in report:

DEM Digital Elevation Model

DI Deep infiltration

ET Evapotranspiration

MODFLOW Modular Finite-Difference Model

NAS Naval Air Station

RMS Root-mean-square

SR State Road

T* Time base

Tc Critical times

USGS U.S. Geological Survey

Multiply By To obtain

Length

foot (ft) 0.3048 meter

foot per mile (ft/mi) 0.1894 meter per kilometer

Area

square foot (ft2

) 0.0929 square meter

square mile (mi2

) 2.590 square kilometer 

Flow

cubic foot per day (ft3/d) 0.02832 cubic meter per day

gallon per minute (gal/min) 0.06309 liter 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) 0.0929 meter squared per day 
VI Contents
Introduction 1

Ground-Water Flow in the Surficial Aquifer System 

and Potential Movement of Contaminants from 

Selected Waste-Disposal Sites at Cecil Field Naval 

Air Station, Jacksonville, Florida

By Keith J. Halford

Abstract

As part of the Installation Restoration 

Program, Cecil Field Naval Air Station, 

Jacksonville, Florida, is considering remedialaction alternatives to control the possible 

movement of contaminants from sites that may 

discharge to the surface. This requires a 

quantifiable understanding of ground-water flow 

through the surficial aquifer system and how the 

system will respond to any future stresses. 

The geologic units of interest in the study 

area consist of sediments of Holocene to Miocene 

age that extend from land surface to the base of 

the Hawthorn Group. The hydrogeology within 

the study area was determined from gamma-ray 

and geologists’ logs. 

Ground-water flow through the surficial 

aquifer system was simulated with a seven-layer, 

finite-difference model that extended vertically 

from the water table to the top of the Upper 

Floridan aquifer. Results from the calibrated 

model were based on a long-term recharge rate of 

6 inches per year, which fell in the range of 4 to 

10 inches per year, estimated using stream 

hydrograph separation methods. More than 

80 percent of ground-water flow circulates within 

the surficial-sand aquifer, which indicates that 

most contaminant movement also can be expected 

to move through the surficial-sand aquifer alone. 

The surficial-sand aquifer is the uppermost unit of 

the surficial aquifer system. 

Particle-tracking results showed that the 

distances of most flow paths were 1,500 feet or 

less from a given site to its discharge point. For an 

assumed effective porosity of 20 percent, typical 

traveltimes are 40 years or less. At all of the sites 

investigated, particles released 10 feet below the 

water table had shorter traveltimes than those 

released 40 feet below the water table. 

Traveltimes from contaminated sites to their point 

of discharge ranged from 2 to 300 years. The 

contributing areas of the domestic supply wells 

are not very extensive. The shortest traveltimes 

for particles to reach the domestic supply wells 

from their respective contributing areas ranged 

from 70 to 200 years. 

INTRODUCTION 

Inorganic and organic priority contaminants 

defined by the U.S. Environmental Protection Agency 

(USEPA, 1988) have been detected in surface water, 

sediment, and ground-water samples collected near 

abandoned waste-disposal sites at Cecil Field Naval 

Air Station (NAS), Jacksonville, Florida (fig. 1). 

Concern exists that such contaminants may move with 

the underlying ground water into nearby creeks where 

contaminants could be readily transported to other parts 

of the air station or downstream to neighboring properties. As part of the Installation Restoration Program, 

Cecil Field NAS is considering remedial-action alternatives to control the movement of contaminants from 

those sites that may otherwise discharge to the surface. 
2 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 1. Location of the study area, sites of contamination, and well D-4560.
Geohydrology 3

This requires a quantifiable understanding of how the 

ground-water flow system responds to current conditions and to any future stresses imposed on the system. 

Numerical simulation provides the most tractable way 

of achieving this level of understanding. 

Cecil Field NAS, located in west Duval County, 

Florida (fig. 1), serves the U.S. Navy as a master jet 

base. From the early 1950’s to the early 1980’s, a large 

variety of solid and liquid waste materials generated 

by Cecil Field NAS was disposed in unlined pits, landfills, or in open areas of the air station. Twelve sites 

(fig. 1) were identified in 1986 for further investigation and potential remediation (U.S. Department of the 

Navy, 1992a). In some cases, wastes were placed in 

direct contact with the ground water in the surficialsand aquifer at the time of disposal. Suspected wastes 

include fuels and oils, solvents, paints, pesticides, 

detonated ordnance, ash, and debris. Analyses of 

ground-water, sediment, and surface-water samples 

collected by previous investigators near many of these 

sites have yielded detectable concentrations of extractable organic compounds and heavy metals. 

The dissolved constituents of jet fuel, lubricants, 

and solvents (benzene, dichlorobenzene, toluene, 

acetone, napthalene, and trichloroethene) are the 

primary contaminants that may be transported by 

ground water at Cecil Field NAS. The movement of 

these dissolved constituents is similar to the advective 

flow of the ground water, as the solubility of these 

contaminants is usually low and the concentrations are 

not great enough to significantly alter the density of 

the ground water. These dissolved constituents sorb to 

the porous media of the aquifers and confining units, 

which retards the rate of travel, but does not affect the 

direction of travel. 

Purpose and Scope

This report presents the results of a study to 

analyze ground-water flow in the surficial aquifer 

system in a 36-mi2 area that includes most of the Cecil 

Field NAS (fig. 1). The report includes ground-waterlevel and surface-water-discharge data, a description 

of the hydrogeologic framework, and an estimate of 

the water budget and base flow in the study area. 

Simulation results are presented from a calibrated, 

three-dimensional finite-difference, ground-water 

flow model of the 36-mi2

 area. A regional and crosssectional model are presented to support the lateral 

boundaries of the study area and the approach used for 

model calibration, respectively. Estimates of groundwater movement from existing and potential sites of 

contamination to their discharge points by way of 

particle tracking (Pollock, 1989) are presented. 

Description of the Study Area

The study area is in the northeastern corner of the 

Black Creek Basin (fig. 1) and is drained by Yellow 

Water, Sal Taylor, and Rowell Creeks (fig. 1). A section 

of Rowell Creek was impounded to create Lake 

Fretwell. Most soils in the basin are comprised of fine 

sands. There is little topographic relief; slopes generally 

are 0 to 2 percent. Depth to the water table generally 

ranges from 0 to 5 ft below land surface (Stem and 

others, 1978). Most of the area is forested by pine trees. 

The climate of Duval County is humid, subtropical. Average precipitation over the study area is about 

54 in/yr with more than half falling from June to 

September (Owenby and Ezell, 1992). The long-term 

potential evaporation rate from the study area has been 

estimated to be 48 in/yr (Farnsworth and others, 1982, 

map 3). The average yearly temperature is 79 °F 

(Owenby and Ezell, 1992). 

The geology and hydrology of the study area and 

surroundings have been described in numerous reports 

as summarized in Spechler (1994), but most describe 

features at a county-wide scale and focus primarily on 

the Upper Floridan aquifer. Causey and Phelps (1978) 

described the extent and availability of water from the 

surficial-aquifer system in Duval County. 

Acknowledgments

The author extends his appreciation to Cliff 

Casey, Southern Division Naval Facilities Engineering 

Command, and Maria Pinenburg and Drew Lonergan, 

ABB Environmental Services, for assistance provided 

during this study. 

GEOHYDROLOGY

The geologic units of interest in the study area 

include sediments of Holocene to Miocene age that 

extend from land surface to the base of the Hawthorn 

Group (fig. 2). Previous investigators have defined this 

sequence as two regional hydrogeologic units: the surficial aquifer system and the intermediate confining unit 

(Spechler, 1994). In this report, this sequence has been 
4 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

further subdivided into six local hydrogeologic units, 

because this study is primarily concerned with groundwater movement near the surface. 

The surficial aquifer system was subdivided into 

three hydrogeologic units: the surficial-sand aquifer, 

the blue-marl confining unit, and the upper-rock 

aquifer (fig. 2). The surficial-sand aquifer consists of 

silty sand with interbedded clay lenses that are about 

1 ft thick. The first 10 to 20 ft of the blue-marl confining unit is usually a blue-green clay which grades to a 

mixture of sand, shell, and clay at the top of the upperrock aquifer. The upper-rock aquifer is a layered composite of limestone and sand. 

The intermediate confining unit defined by 

Spechler (1994) was subdivided into three hydrogeologic units: the gray-marl confining unit, the lower-rock 

aquifer, and the lower confining unit (fig. 2). The graymarl confining unit consists of a gray clay interspersed 

with phosphatic sand stringers. The composition of the 

lower-rock aquifer is similar to that of the upper-rock 

aquifer. The lower confining unit is about 200 ft thick in 

the vicinity of Cecil Field NAS and consists of marine 

clays and discontinuous limestone stringers. 

The hydrogeologic structure within the study 

area was defined by depth and thickness data from 

gamma-ray logs and geologists’ logs. Typical gamma 

ray and geologist’s logs are shown in figure 3 for 

Series Formation Lithology

Spechler

(1994) This report

Hydrogeologic unit Model 

layer

Holocene 

to Upper

Miocene

Undifferentiated

 surficial

 deposits

Silty-sand and clay lenses

Surficial 

aquifer 

system

Surficial-sand aquifer 1, 2, 3

Clay and shell beds Blue-marl confining 

unit

4

Limestone and sand Upper-rock aquifer 5

Miocene Hawthorn

Group

Clay and interbedded 

phosphatic sands Intermediate

confining 

unit

Gray-marl confining 

unit

--

Limestone and sand Lower-rock aquifer 6

Clay, interbedded 

phosphatic sands, 

limestone, and dolomite

Lower confining unit --

Eocene Ocala

Limestone Marine limestone

Upper 

Floridan 

aquifer

Upper Floridan aquifer 7

(Modified from Spechler, 1994)

Figure 2. Generalized geologic and hydrogeologic units and aquifers beneath Cecil Field Naval Air Station. 

0

300

50

100

150

200

250

DEPTH, IN FEET BELOW LAND SURFACE

GAMMA 

GRAY-MARL

LOWER

1

2

3

4

5

6

BASE-WIDE MODEL LAYERS

SURFICIAL

BLUE-MARL

UPPER-ROCK

LOWER-ROCK

CONFINING UNIT

CONFINING UNIT

CONFINING UNIT

GAMMA-RAY LOG GEOLOGIST’S LOG

SAND

Figure 3. Gamma-ray log and geologist’s log of well 

D-4560, a typical hydrogeologic column of the first 300 

feet below Cecil Field Naval Air Station, and model-layer 

correlations for this study.
Geohydrology 5

well D-4560. The gamma-ray log responds to the 

natural radioactivity of the formation which generally increases with increasing clay content. Phosphatic sands are an exception and are usually much 

more radioactive than clays. Within the surficial 

aquifer system, the hydrogeologic units are clearly 

defined by the gamma-ray log. This definition 

becomes less clear for the deeper units in the intermediate confining unit because the gamma-ray log is 

responding to phosphatic sands in addition to clayey 

materials.

Figure 4. Location of selected wells used to define the hydrogeologic 

framework and the potentiometric surface of the Upper Floridan aquifer, May 

1993, in the study area, Black Creek Basin, regional model areas, and the 

location of stream-gaging station Black Creek near Middleburg (02246000).

Additional logs from beyond the study area were 

used to define the base of the surficial-sand aquifer and 

the thicknesses of the other hydrogeologic units (fig. 4). 

These additional logs ensured that the hydrogeologic 

framework estimated for the study area would be consistent with the regional framework. The configuration 

of the base of the surficial-sand aquifer is irregular 

(fig. 5), but generally dips to the south and east in the 

study area. Regionally, the base of the surficial-sand 

aquifer dips to the east at a slope ranging from 5 to 

20 ft/mi (Leve, 1966). 
6 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 5. Altitude of base of the surficial-sand aquifer in the study area.
Water Budget and Recharge Estimates 7

The thickness of the surficial-sand aquifer was 

estimated by subtracting the elevation at the base of 

the surficial-sand aquifer from the elevation at land 

surface (fig. 6). Thicknesses of the five remaining 

hydrogeologic units (figs. 7-11) were estimated by 

interpolation between logs located both within the 

study area and from beyond the study area (fig. 4). The 

blue-marl confining unit (fig. 7) is thickest near Lake 

Fretwell and is thinnest northeast of the runways. The 

thickness of the upper-rock aquifer ranges from less 

than 15 to more than 25 ft (fig. 8) and averages about 

20 ft. The gray-marl confining unit is thickest north of 

State Road (SR) 228 (fig. 9) and generally thins 

toward the southeast. The thickness of the lower-rock 

aquifer ranges from about 30 ft north of SR 228 to 

more than 70 ft in the southern part of the study area 

(fig. 10). The lower confining unit ranges from about 

170 ft to more than 220 ft (fig. 11) and averages about 

200 ft over the study area. Total thickness was used 

because the ground-water pathlines are of interest. If 

the unit thicknesses represented on figures 5 

through 11 are composited, generalized hydrogeologic 

sections could be cut from the total volume. Two such 

sections are shown in figure 12 that illustrate the 

variations in thickness and altitude of the five uppermost hydrogeologic units. 

WATER BUDGET AND RECHARGE 

ESTIMATES

A long-term water budget of the surficial aquifer system can be described by the following equation:

 , (1)

where

 is precipitation, in inches per year;

 is evapotranspiration, in inches per year;

 is stream discharge, in inches per year, which 

is composed of surface runoff, , and base 

flow, ; 

 is deep infiltration to the Upper Floridan, in 

inches per year; and

 is change in storage, in inches per year, which 

is assumed to be negligible over the long 

term. 

P ET – – Q – DI = ∆S ≅ 0

P

ET

Q

QS

QB

DI

∆S

Where these various budget components enter 

and exit the system is depicted in figure 13. Precipitation (54 in/yr) and stream discharge at Black Creek near 

Middleburg (02246000) (fig. 4), 15 in/yr, were measured. Deep infiltration (1 in/yr) was estimated previously (Krause and Randolph, 1989). Evapotranspiration 

(38 in/yr) is precipitation minus stream discharge and 

deep infiltration. All terms represent averages over periods of 30 years or more. 

Pumpage was not considered in the water budget 

because Cecil Field NAS is supplied only by wells 

screened in the Upper Floridan aquifer (U.S. Department of the Navy, 1992a). Several domestic wells existing adjacent to Cecil Field NAS (fig. 14) are probably 

screened in the upper-rock aquifer (U.S. Department of 

the Navy, 1992a). If all of the 17 domestic wells shown 

in figure 14 continually withdraw 13,000 gal/d, then the 

total discharge amounts to 0.008 in/yr across the study 

area. 

Recharge, , is the subcomponent of the water 

budget that drives ground-water flow through the 

surficial aquifer system. Both the surficial-sand and 

upper-rock aquifers respond to recharge events which 

correspond with periods of intense rainfall. Part of the 

rainfall is rejected as recharge and contributes to stream 

discharge as surface runoff, (fig. 13). These storm 

events appear as abrupt increases in stream discharge 

(fig. 15). 

The long-term recharge rate can be defined as: 

 or . Direct 

estimates of are complicated by the uncertainties 

associated with evaluating surface runoff ( ), evapotranspiration (ET), and deep infiltration (DI). 

Periodic recharge to ground water can be 

estimated by hydrograph separation. Many methods 

have been documented and most assume that stream 

discharge has two components, surface runoff and base 

flow (Meyboom, 1961; Rutledge, 1993; Domenico and 

Schwartz, 1990). Base flow is the component of stream 

discharge contributed by ground-water discharge and 

can serve as an estimate of recharge, assuming that 

surface-water basin boundaries generally correspond 

with ground-water divides and that measured stream 

discharge is uncontrolled by upstream diversion or 

regulation. 

N

QS

N P ET – QS = – N QQS = – + DI

N

QS
8 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

Figure 6. Thickness of the surficial-sand aquifer.
Water Budget and Recharge Estimates 9

Figure 7. Thickness of the blue-marl confining unit.
10 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

Figure 8. Thickness of the upper-rock aquifer.
Water Budget and Recharge Estimates 11

Figure 9. Thickness of the gray-marl confining unit.
12 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 10. Thickness of the lower-rock aquifer.
Water Budget and Recharge Estimates 13

Figure 11. Thickness of the lower confining unit.
Sites at Cecil Field Naval Air Station, Jacksonville, Florida

14 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

100 SEA LEVEL 100 200 100 SEA LEVEL 100 200 100 SEA LEVEL 100 200 VERTICAL EXAGGER

Sal Taylor Lake Yellow Water Creek Creek A-A B-B Aquifer Test Site FEET LOWER-ROCK BLUE-MARL UPPER-ROCK GRAY-MARL LOWER-ROCK LOWER BLUE-MARL GRAY-MARL LOWER Sal Taylor Creek WATER TABLE LAND 

WATER TABLE LAND SURFACE

B

A

0 

0 

12356

4

12356

4

(See traces in figure 5.) 

B

SURFICIAL

SAND

SURFICIAL

SAND

 FEET

UPPER-ROCK

Fretwell

Figure 12. Generalized hydrogeologic sections A-A′ and B-B′ in the study area.
Water Budget and Recharge Estimates 15

P = 54 in/yr

ET = 38 in/yr

QS 

Q = 15 in/yr

DI = 1 in/yr

N

INTERMEDIATE CONFINING UNIT 

Figure 13. The water budget and its components within the study area.

SURFICIAL AQUIFER SYSTEM

EXPLANATION

P PRECIPITATION

ET EVAPOTRANSPIRATION

N RECHARGE

QS SURFACE RUNOFF

Q STREAM DISCHARGE

QB BASE

DI DEEP INFILTRATION TO UPPER 

FLORIDAN

QB
16 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 14. Locations of hydrograph and domestic supply wells, stream-gaging station Rowell Creek near Fiftone 

(02245918), the drainage areas of the Fiftone and Sal Taylor Basins, and the 75-foot contour of land surface elevation.
Water Budget and Recharge Estimates 17

N D JF M A M JJA S O N

1993 1994

0.1

10,000

1

10

100

1,000

DISCHARGE, IN CUBIC FEET PER SECOND

02245918

02246000

0

12

6

MONTHLY RAINFALL,

IN INCHES

65

80

70

75

WATER LEVEL, IN FEET ABOVE SEA LEVEL

BK-2S

BK-7S

3-7S

3-7DD

16-7S

SURFICIAL-SAND AQUIFER

UPPER-ROCK AQUIFER

SURFICIAL-SAND AQUIFER

CALIBRATION

PERIODS

AVERAGE OVER

SAMPLING PERIOD

Black Creek

Rowell Creek

Figure 15. Monthly rainfall and ground-water levels from selected wells at Cecil Field and daily discharge at 

stream-gaging stations Black Creek (02246000) and Rowell Creek (02245918) from November 1993 to 

November 1994.
18 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

The recession-curve-displacement method 

(Rorabaugh, 1964; Rutledge, 1993) was used to 

estimate the long-term recharge rate to the Black 

Creek Basin (fig. 4) using daily stream discharge 

collected at station 02246000 during 1932-94. 

Although this method has some theoretical basis, the 

method is strongly governed by three empirical 

values: the recession index, , critical time, , and 

time base, . The recession index and critical time 

are inversely related to the diffusivity, Kb/S, of the 

contributing aquifer system (Linsley and others, 1958; 

Rorabaugh, 1964) and are positively correlated to one 

another. Estimates of range from 25 d/logQ to 

210 d/logQ (fig. 16) at station 0224600 on the Black 

Creek. Large estimates of (210 d/logQ) assume that 

the short-term responses are due to surface-water 

impoundments and that base flow is only represented 

by the long-term recession. The computed recharge 

rates estimated with this range of are fairly insensitive to this parameter. 

The time base is the duration of time after a 

peak in streamflow when overland runoff is still a 

component of stream discharge. A frequently cited 

estimate of time base is the empirical equation 

 (Linsley and others, 1958), where is the 

time base in days and is the drainage area in square 

miles. For the Black Creek Basin, the time base is 

3 days according to this estimate. Low slopes and 

impoundments such as those present in the study area 

tend to prolong the duration of overland runoff events. 

Estimates of under these conditions tend to be 

greater than estimates based on drainage area. 

Recession indices of 25, 70, and 210 d/logQ and 

time bases of 3, 6, and 9 days were used to estimate 

recharge rates. Ranges of values of the two parameters 

were used to assess the uncertainty associated with 

these parameters. Critical times ( ) were not tested 

independently because is a function of 

(Rutledge, 1993). Potential recharge rates to the Black 

Creek Basin ranged from 4 to 10 in/yr (fig. 16). 

The water-budget analysis and recharge rate estimates provide a general idea of how much water passes 

through the surficial aquifer system, but cannot discern 

what fraction of flow passes through the surficial-sand 

or upper-rock aquifers or the quantity of deep infiltration. The direction and velocity of the movement of 

contaminants from specific sites are also not determined 

through a water-budget analysis. A ground-water flow 

model is needed to address these more specific questions. 

κ Tc

T*

κ

κ

κ

T* A0.2 = T*

A

T*

Tc

Tc κ

SIMULATION OF GROUND-WATER FLOW 

AND ADVECTIVE TRANSPORT OF 

CONTAMINANTS IN THE SURFICIAL 

AQUIFER SYSTEM

A three-dimensional numerical model was used 

to quantitatively analyze ground-water flow and the 

advective transport of contaminants through the surficial aquifer system. The McDonald and Harbaugh 

(1988) modular finite-difference model (MODFLOW) 

was used to simulate flow in the surficial aquifer system and solve the governing equation: 

, (2)

where 

∇ is del, the vector differential operator;

 is hydraulic conductivity, in feet per day; 

 is thickness, in feet; 

 is hydraulic head, in feet; 

 is a source or sink, in feet per day; 

 is precipitation minus surface runoff, in feet 

per day; 

 is evapotranspiration, in feet per day; 

 is storage coefficient in confined aquifers and 

the specific yield in unconfined aquifers, 

dimensionless; and

 is time, in days.

Description of Ground-Water Flow Model 

To implement a finite-difference model, the 

study area was discretized into a rectangular grid of 

cells by row and column. The model grid covered an 

area greater than 36 mi2 and was divided into 

117 rows of 91 columns (fig. 17). Smaller cells were 

used in more stressed areas such as along creeks or 

drains and near observation wells to avoid over-linearization in areas of interest. Variably spaced, small 

cells were used near sites 5, 16, and 17, the north fuel 

farm, and along reaches of Sal Taylor and Rowell 

Creeks, where water-level data are relatively numerous and discharge is considerable. Of the 74,529 

model cells, 72,534 were active cells that ranged in 

size from 2,500 to 13x106 ft2

. The largest cells were in 

peripheral areas of little stress or away from areas of 

interest. 

–∇( ) Kb∇h q PQS + + ( ) – – ET S

∂t

∂h =

K

b

h

q

P QS –

ET

S

t
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 19 10

10,000

100

1,000

0 200 400 600 800 

DISCHARGE, IN CUBIC FEET PER SECOND

TIME, IN DAYS

1992 1993 19

k = 210 d/logQ

k = 210 d/logQ

k = 25 d/logQ

Time base, 

in days

Recharge, inc

25 d/logQ 70 d/lo

3 10 8 6 76 9 55 

Figure 16. Discharge at stream-gaging station 02246000 on the Black Creek and selected base-flow recession curves for 1
20 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 17. Model grid and extent.
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 21

Figure 17a. Model grid and extent.
22 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 17b. Model grid and extent.
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 23

The grid was generally oriented parallel to the 

primary drains: Sal Taylor (east-west), Yellow 

Water, and Rowell Creeks (north-south) (fig. 1). No 

measurements of anisotropy were available and a lateral anisotropy ratio of 1:1 was used for simulation. 

Values of aquifer and confining-unit hydraulic properties were assigned to the center of each cell, 

defined as a node, by interpolation from observed 

point values. 

The model was vertically discretized into 

seven layers, three of which were used to simulate 

the surficial-sand aquifer (fig. 12). The surficial-sand 

aquifer was discretized into layers 1, 2, and 3 by 

dividing the wetted thickness into three equal parts. 

Layers 4, 5, and 6 represented the blue-marl confining unit, the upper-rock aquifer, and lower-rock aquifer, respectively. Simulation of the blue-marl 

confining unit as an active layer (4) is atypical and is 

not necessary to effectively simulate flow and transport in the surficial-sand aquifer. Because contaminant movement through the blue-marl confining unit 

is a regulatory concern, the confining unit was simulated as an active model layer rather than as a leakance unit. Simulation of the blue-marl confining 

unit as an active layer allows for lateral migration 

within the confining unit which cannot be accounted 

for if it is simulated as a leakance unit. Layer 7 represented the Upper Floridan aquifer during the calibration period and was a specified-head lower boundary 

for the model. 

Vertical impedance to flow within the surficial-sand aquifer and between the other aquifers was 

simulated by assigning leakance values at each cell 

between model layers. The leakance is the average 

vertical hydraulic conductivity of the aquifer or confining unit material between nodes divided by the 

vertical distance between corresponding nodes in 

adjacent model layers and is in units of feet per day 

per foot. Within the intermediate confining unit, the 

leakances represent the resistance to flow across the 

gray-marl and lower confining units (fig. 2). 

Hydraulic Properties

Three aquifer tests were conducted near well 

D-4560 (fig. 1) to provide initial estimates of the 

hydraulic properties of the aquifers and confining 

units in and immediately beneath the surficial aquifer 

system. The lateral hydraulic conductivities of the 

surficial-sand, upper-rock, and lower-rock aquifers 

were estimated from the aquifer tests. Vertical 

hydraulic conductivities also were estimated for the 

blue-marl and gray-marl confining units as well as the 

surficial-sand aquifer. 

An aquifer test consists of pumping water from 

the aquifer at a known rate and measuring the drawdowns in nearby observation wells. Hydraulic conductivities are estimated by fitting a model to the 

measured drawdowns. The appropriate flow model to 

use is dictated by the geology of the site and the configuration of observation wells. The drawdown 

response in a single, confined aquifer can be described 

quite well by the analytical Theis (1935) model 

because it meets the assumptions of this solution. 

More complicated systems, such as the one at Cecil 

Field NAS, require a model that can account for 

responses to stress in the shallowest aquifer, which is 

unconfined, and the hydraulic connection between 

lower aquifers, which are confined. 

Three production wells and eight observation 

wells were used for the aquifer tests. Individual production wells were screened in the surficial-sand, 

upper-rock, and lower-rock aquifers, respectively 

(fig. 18). Six of the observation wells were screened 

within 200 ft of the production wells, with four in the 

surficial-sand aquifer and one each in the upper-rock 

and lower-rock aquifers. The remaining two observation wells were screened at the water table, 1,500 ft 

away and in the upper-rock aquifer, 8,500 ft away 

from the site, respectively. These two wells monitor 

background water levels for detrending water-level 

responses measured in the surficial-sand and upperrock aquifers. 

The surficial-sand, upper-rock, and lower-rock 

aquifers were pumped at rates of 5, 54, and 

85 gal/min, respectively, and water levels were monitored continuously in wells screened in the aquifer 

being stressed and in vertically adjacent aquifers during each test. The aquifer tested was stressed for 

about 2 days and allowed to recover about 2 weeks 

before testing the next aquifer. Absolute drawdowns 

were estimated by measuring the water-level change 

in each observation well during the test. These drawdowns were then adjusted for regional trends using 

the water-level change measured in the background 

well screened in the same aquifer. A background well 

for the lower-rock aquifer did not exist and no trend 

was assumed to exist in that aquifer during any of the 

tests. 
24 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

Hydraulic properties were estimated from the 

aquifer tests by fitting results computed using a variably saturated, radially symmetric, numerical model, 

VS2DT (Lappala and others, 1987; Healy, 1990) to 

the measured drawdowns. The VS2DT aquifer-test 

model spanned the entire shallow-aquifer system from 

land surface to the base of the lower-rock aquifer and 

Figure 18. Location of aquifer test site at Cecil Field Naval Air Station, background observation wells, a cross-section 

showing well placement and model grid within 250 feet of the wells pumped for the aquifer tests, and nearby hydrologic 

features.

USGS-19

OBSERVATION WELL AND NUMBER

SCREENED INTERVAL

WT-200

USGS-17, 18

USGS-20

WT-200

USGS-24, 23, 22

USGS-16

3-33DD

30°14′

30°13′

81°54′

0 1,000 2,000 FEET

0 500 METERS

228

Rowell Creek

AQUIFER TEST SITE

WT-BK

Perimeter Rd.

Perimeter Rd.

0 100 200 FEET

0 50 METERS

0 50 100 150

RADIAL DISTANCE FROM PUMPED WELL, IN FEET

200 250

USGS-16

USGS-17

USGS-18

USGS-19

USGS-20

USGS-22

WT-200

USGS-23

USGS-24

LAND

50

100

150

200

250

DEPTH, IN FEET BELOW LAND SURFACE

SURFACE

N

Lake

Fre

wt ell

NO-FLOW BOUNDARY

228

EXPLANATION

was used to analyze all three tests. The entire vertical 

section was simulated for all test analyses to avoid 

prescribing boundary conditions within the section. 

The VS2DT analyses were based on assumptions that the lateral and vertical hydraulic conductivity and specific storage of each aquifer or confining 

unit could be described by a single value. Only a 
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 25

fraction of these parameters were estimated during any 

individual test. The results of several aquifer tests 

were used in succession to update unestimated parameters. Parameters were estimated iteratively until the 

properties used for all three aquifer tests were internally consistent (table 1).

Additional lateral hydraulic conductivity data 

within the surficial-sand aquifer were determined from 

slug tests (U.S. Department of the Navy, 1992b) at 

sites 1, 2, 3, 5, 16, 17, and the north and south fuel 

farms (fig. 17, A and B). Three wells were tested at 

each site except for site 3, where four wells were 

tested. These lateral hydraulic conductivity values 

represent averages over the first 10 to 15 ft of saturated, surficial-sand aquifer and range from 0.6 to 

5 ft/d. The variability in lateral hydraulic conductivity 

at each site, as defined by dividing the largest value by 

the smallest value, ranged from twofold at sites 5 and 

16 to fivefold at site 2 and the south fuel farm. 

The initial transmissivity arrays input to the 

study-area flow model were calculated by multiplying 

the lateral hydraulic conductivity for each unit from 

table 1 by the corresponding thickness for that layer 

(figs. 6-11). One value of lateral hydraulic conductivity was assigned to the surficial-sand aquifer because 

the variability at each site was not significantly less 

than the variability for all of the sites, as determined 

by the slug tests. 

The initial leakance arrays input to the model 

were calculated by dividing the vertical hydraulic conductivity from table 1 by the internode-distance 

between layers. 

Table 1. Aquifer and confining unit properties determined 

from aquifer tests

[Values of Kxy, Kz, and Ss estimated from aquifer test results unless otherwise noted; all thicknesses were measured; ft/d, feet per day; ft, feet; --, not 

applicable]

Aquifer or 

confining unit Kxy, ft/d Kz, ft/d Ss, 10-6/ft Thickness, ft

Model 

layer

Surficial-sand aquifer 5 0.4 40 54 1, 2, 

and 3

Blue-marl confining 

unit 

0.01a

 .01 9 36 4

Upper-rock aquifer 40 4a 2 20 5

Gray-marl confining 

unit

.002a .002 5a 95 --

Lower-rock aquifer 20 2a 2 32 6

a

 Assumed Kxy/Kz = 10 for aquifers and Kxy/Kz = 1 for confining 

units. Estimated Ss = 5 x 10-6/ft by extrapolation from shallower intervals. 

Surface-Water Features 

The distribution of surface-water features controls 

the direction and rate of flow in the surficial aquifer system. In addition to Yellow Water, Sal Taylor, and Rowell Creeks, many smaller tributaries exist, both natural 

and man-made (fig. 14). Most of the stream elevation 

data were taken from 1:24,000 scale, U.S. Geological 

Survey (USGS) quadrangle maps. This information was 

supplemented by spot elevations obtained near sites 1, 5, 

16, 17, and the north fuel farm. 

Interaction between the surficial-sand aquifer and 

the creeks, runway drains, and Lake Fretwell was simulated by river nodes. These surface-water features were 

simulated as a group because they all contribute to the 

Black Creek Basin discharge. The flow rate in or out of 

the aquifer at a river node is defined by: 

, (3)

where

 is the hydraulic conductivity of the riverbed, in 

feet per day; 

 is the length of river reach across a cell, in feet; 

 is the average width of a reach, in feet; 

 is the average thickness of the riverbed, in feet; 

 is the average stage of the river or lake, in feet; 

and 

 is the head in the aquifer beneath the river, 

in feet. 

This equation only applies if is 

greater than the assigned elevation of the river or lake 

bottom. 

Uniform , , and values of 0.05 ft/d, 

10 ft, and 1 ft, respectively, were assumed for all creeks 

throughout the study area. Initial values were selected 

so that riverbed conductance could control discharge 

from the model cells. All river reaches shown in 

figure 17 were represented in the model and the length, 

, of river reach equalled the amount of reach bounded 

by the cell. A total of 2,007 river nodes was assigned to 

layer 1. The river bottom elevation for all creeks was set 

equal to the river stage to ensure all simulated reaches 

would be either gaining or inactive. 

Different conductances and river bottom elevations were used to simulate Lake Fretwell, because the 

areal extent of the lake in contact with the surficial-sand 

aquifer was greater than the areal extent of the river bottoms. In addition, portions of the lake lose water to the 

surficial-sand aquifer. The simulated lake was defined by 

the model nodes that fell within the shaded outline shown 

QB

KRBLW

M ------------------ HRIVER HAQUIFER = ( ) –

KRB

L

W

M

HRIVER

HAQUIFER

HAQUIFER

KRB W M

L
26 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

in figure 17, inset A. For each lake cell, the area in contact with the surficial-sand aquifer, , was set to the 

surface area of the cell and the river bottom was set far 

below the lake stage so the lake could be gaining or losing. A uniform hydraulic conductivity value was used 

for all of the riverbed and lake bottom conductances and 

was estimated by model calibration. 

A variable hydraulic conductivity was also used 

to simulate the runway drains, which are pipes buried 

beneath the runways. Cracks in the drain walls and 

joints between pipe sections probably are the primary 

conduits of flow and controlled seepage into the drains. 

An independent hydraulic conductivity could not easily 

be estimated for the drain lines and the final estimates 

were based on model calibration. 

North of SR 228 and along the eastern edge of the 

study area, perennial swampy areas exist where the 

water table ranges from less than 1 ft below land surface 

to land surface. During intense rainfall, the water table 

approaches land surface over a larger area that is 

approximated by the 75-ft land surface elevation contour (fig. 14). The area where the land surface elevation 

was greater than 75 ft was simulated as drains with high 

conductances and outlet heads set at land surface. If the 

water table rises to land surface, the drains behave as 

specified heads, otherwise they have no effect. Conceptually, the drain nodes simulated the effects of relatively 

LW

40

80

45

50

55

60

65

70

75

WATER LEVEL, IN FEET ABOVE SEA LEVEL

1984 1986 1988 1990 1992

YEAR

UPPER-ROCK AQUIFER

LOWER-ROCK AQUIFER

UPPER FLORIDAN AQUIFER

Figure 19. Long-term hydrographs from wells DS-238, 226, and 227 located near well D-4560.

high evapotranspiration and ground-water discharge in 

these areas. 

Boundary Conditions

Proper representation of model boundary conditions is one of the most important aspects in the simulation of an aquifer system. Model boundaries are assigned 

to represent the actual hydrologic boundaries as accurately as possible. If model boundaries are necessarily 

highly generalized, they are placed far enough away from 

the influence of hydrologic stresses in the model area to 

minimize their effects on simulation results.

The upper boundary, layer 1, is the water table 

and is represented in MODFLOW as a free surface 

except where it rises to land surface. A spatially uniform recharge rate was applied to this boundary in all 

simulations. This did not result in a spatially uniform 

flux across the water table due to rejection of water in 

areas where the water table is near or at land surface. 

The lower model boundary, layer 7, is a sink for the 

lower-rock aquifer. Specified heads were used to define 

the lower boundary and represent typical Upper Floridan water levels throughout the study area (fig. 4). 

Seasonal changes were not considered because they are 

typically small (2 to 4 ft) relative to the difference in 

water levels between the upper-rock and lower-rock 

aquifers and the Upper Floridan aquifer (20 to 25 ft, 

fig. 19). 
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 27

Figure 20. Grid, boundaries, and simulated volumetric budget of the regional model used to test the 

lateral boundary conditions for the study area.
28 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

All the lateral model boundaries in each layer are 

assumed to be ground-water divides that coincide with 

surface-water features. This assumption prescribes that 

areas contributing recharge to the surficial-sand, upperrock, and lower-rock aquifers are defined predominantly by surface-water divides, not the Upper Floridan 

aquifer. The northern and eastern edges of the study 

area coincide with the boundaries of the Black Creek 

Basin (fig. 4). This divide constitutes a no-flow boundary because water flows downward and perpendicularly away from the boundary. The southern boundary 

and the western boundary north of Yellow Water Creek 

follow ridges from the edges of the basin to the Yellow 

Water Creek (fig. 17). These divides constitute no-flow 

boundaries because water flows parallel to but not 

across them. The southern segment of the western 

boundary lies along the Yellow Water Creek where 

water moves upward from deeper to shallower aquifers 

and is discharged to river nodes. 

A simplified, regional-scale model was 

constructed initially to investigate the validity of 

assigning no-flow boundary conditions to the lateral 

boundaries of the study area (fig. 17). Laterally, the 

regional model covered the area between Trail Ridge 

and the St. Johns River from west to east and between 

Trout Creek and Black Creek from north to south 

(fig. 20). The regional model was divided into uniform, 

square cells that were 2,000 ft on a side in 83 rows of 

65 columns and was vertically discretized into four layers. Layers 1, 2, and 3 represented the surficial-sand, 

upper-rock, and lower-rock aquifers, respectively. 

Layer 4 represented the Upper Floridan aquifer and 

was the specified-head lower boundary for the regional 

model. The base of the surficial-sand aquifer was set at 

10 ft below sea level; uniform hydraulic conductivity 

and thickness values were used, except for the thickness of the surficial-sand aquifer, which varied due to 

the water table (table 2). Lateral hydraulic conductivities assigned to the regional model layers were those 

determined by aquifer test, except for layer 1, which 

according to slug-test results may be biased high when 

representing regional conditions. Rivers, lakes, and 

swampy areas were simulated as previously described. 

The upper and lower boundaries of the regional model 

were defined in a manner similar to that used for the 

study area model. No-flow conditions were assigned to 

lateral boundaries for layers 1, 2, and 3. Only one 

steady-state solution using a recharge rate of 6 in/yr 

was considered. 

The lateral boundaries of the study area are 

entirely internal to the regional model. Accordingly, if 

the assignment of no-flow boundaries to the study-area 

model is valid, then the regional model should simulate 

no-flow between the study area and the immediately 

adjacent areas. A volumetric budget from the regional 

model indicates that only 6 percent of the water entering 

the study area leaves laterally. The largest lateral flow 

was ft3

/d (4 percent of the total flow) across 

the southern boundary in the upper-rock aquifer. Given 

the small magnitude of this error, the no-flow boundaries assigned to the study-area model are considered 

reasonable. Part of the apparent lateral flow is a byproduct of the finite-difference approximation of the study 

area where the flows across a boundary are similar to or 

greater than the net flow across a boundary. The simulated potentiometric surfaces of the surficial-sand, 

upper-rock, and lower-rock aquifers also suggest that 

little or no lateral flow would be expected across the 

study area boundaries (fig. 21). 

Simulation Approach

Within the study area, the long-term water levels in and ground-water discharge from the surficialsand aquifer change very little. Therefore, most of the 

questions currently raised by site assessments and 

remediation plans can be addressed with a steady-state 

ground-water flow model based on a modification of 

equation 2:

 , (4)

where 

 is the long-term recharge rate, in feet per day. 

Table 2. Hydraulic properties of the regional model

[All hydraulic characteristic values were based directly on aquifer-test 

results unless otherwise noted; ft/d, feet per day; ft, feet; 1/d, per day]

Aquifer (layer) Hydraulic conductivity, ft/d Thickness, ft Vertical 

leakance, 1/d

Surficial-sand (1) 3a Variable 3. x 10-4

Upper-rock (2) 40. 20b 2. x 10-5

Lower-rock (3) 20. 50b 5. x 10-6 c

a

 The average hydraulic conductivity is closer to 3 ft/d instead of 

5 ft/d when slug test results are considered. b Represents an average thickness over Cecil Field Naval Air Station.

c

 Value obtained from Krause and Randolph (1989). 

0.05 6×10

∇( ) K∇h + + q N = 0

N
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 29

Figure 21. Simulated potentiometric surface of the surficial-sand, upper-rock, and lower-rock aquifers in the regional 

model area.
30 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

The water-level and base-flow data used for 

model calibration were obtained from several 

synoptic surveys conducted over approximately a 

year-long period. Given the infrequent sampling over 

a short period, long-term trends in water levels are 

difficult to assess. However, available data indicate 

that during any given year, water levels will fluctuate 

seasonally about 3 ft, and the surficial-sand aquifer 

infrequently approaches, at best, a quasi-steady state 

condition. Model calibration strategies are complicated by attempting to fit discontinuous, synoptic 

data to the transient equation 2. Such efforts require 

estimating the specific yield and storage coefficients, 

estimating the temporal distribution of ground-water 

recharge and evapotranspiration during the calibration period, estimating the initial head distribution, 

and establishing the hydraulic characteristics to 

develop the model. 

Alternatively, the synoptic surveys can be 

treated as independent “snapshots” of the groundwater system taken at various times. The data from 

these surveys can be fitted to the simpler steady-state 

equation: 

, (5)

where 

 is the effective recharge rate during a given 

survey, in feet per day, or 

. 

The effective recharge rate estimated for each 

synoptic-survey period serves as a dummy variable 

and is not an estimate of the long-term recharge rate. 

Several assumptions are necessarily made to 

apply this approach.

• The change in head with respect to time, 

, at any given location can be treated 

as a constant during each synoptic 

survey. 

• Survey periods are short enough to 

assume that all measurements are made 

∇( ) K∇h + + q N' = 0

N'

N' P Qo ( ) – – ET Sy∂t

∂h = –

∂t

∂h

at the same instant. Most of the waterlevel surveys at Cecil Field were performed over 2-day periods. 

• The spatial distribution of effective 

recharge, , is unknown. This leads to 

assumptions of a spatially uniform field 

of , because the true distribution is 

usually not known. The problems associated with spatial variability are not 

unique to this approach, because the 

variables , , and 

required by the transient solution can 

also be spatially variable. 

• No water is contributed to the groundwater system from compressibilitybased storage, and, thus, . The 

reasonableness of this assumption is 

derived from the fact that specific yield, 

, is usually three orders of magnitude 

greater than compressibility-based storage. 

The restrictiveness of these assumptions was 

tested with a hypothetical, two-dimensional, crosssectional, MODFLOW model. The cross-sectional 

model was constructed to be similar to a transect that 

extends perpendicularly from Rowell Creek to a 

ground-water divide 3,000 ft away and generally 

parallel to ground-water flow. The cross-sectional 

model was constructed with two layers and used the 

hydraulic properties and dimensions shown in 

figure 22. A variably spaced grid covered the 3,000 ft 

in 40 columns, beginning with a 3-ft-wide column 

and multiplying successive columns by 1.13. Both 

lateral boundaries and the base of the confined aquifer at 30 ft below sea level were no-flow boundaries. 

The upper boundary was a specified recharge rate 

applied to a free surface. A 100-day period was simulated in six stress periods with the recharge and 

evapotranspiration rates listed in table 3. The creek 

was simulated as a specified-head boundary using the 

water levels shown in figure 23 over the 100-day 

simulation period. A steady-state solution with a 

long-term recharge rate, , of 6 in/yr and a creek 

elevation of 45 ft above sea level served as the initial 

condition. 

N'

N'

P Qo ( ) – ET Sy

S

∂t

∂h = 0

Sy

N
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 31

80

40

SEA 

40

LEVEL

Kxy = 3 ft/d

Kxy = 30 ft/d

Kz = 0.03 ft/d

DEPTH ABOVE OR BELOW SEA LEVEL, IN FEET

S-2300

S-150 Sy = 0.2

S = 0.001

t = 1 d

D-2300

t = 56 d

150

0

10

2

4

6

8

EFFECTIVE RECHARGE, IN INCHES PER YEAR

3,000 2,500 2,000 1,500 1,000 500 0

DISTANCE FROM CREEK, IN FEET

-100

200

0

100

EFFECTIVE RECHARGE,

IN INCHES PER YEAR

100 50 0

2

5

11

25

55

5

2

11

55

DISTANCE FROM CREEK, IN FEET

DAYS AFTER FIRST RECHARGE EVENT

P Qo – – ET Sydt

dh –

25

DAYS AFTER FIRST RECHARGE EVENT

Figure 22. Hypothetical cross section that shows the expected effective recharge distribution for an aquifer 

system similar to the surficial-sand aquifer system.
32 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

The results of the cross-sectional model demonstrate that the assumptions necessary to apply 

synoptic-survey data to model calibration will probably be met when using effective-recharge. The 

simulated change in head with respect to time, , 

in both aquifers approached a constant value a few 

days after each recharge event (fig. 23), and water 

levels at distances of 150 ft or more from the creek 

changed less than 0.1 ft during any 2-day period. 

The spatial distribution of effective recharge 

( ) is fairly uniform over 

about 80 percent of the recharge area (fig. 22) and 

departs most from the assumed uniform distribution 

either immediately after a recharge event or during 

the later periods of a ground-water recession. The 

net effective recharge rate does not depart greatly 

from the long-term average of 6 in/yr except during 

and immediately after large recharge events 

(fig. 23). Some water is contributed to the groundwater system from compressibility-based storage, 

but the amounts are small relative to the quantities 

released by specific yield. This is shown directly 

and by the small difference between and base 

flow, (fig. 23). 

Although the effective recharge rates, , 

estimated for each synoptic-survey period are not 

estimates of the long-term recharge rate, estimates 

obtained during extreme conditions apparently can 

serve to bracket the long-term recharge rate. 

Model Calibration

Calibration is the attempt to reduce the difference between model results and measured data by 

Table 3. Stress period durations, recharge rates, and 

evapotranspiration rates used in the two-dimensional, 

cross-sectional model for testing applicability of effective

recharge rates

Boundary 

condition

Stress period

1 2 3 456

End of period, day 1 56 57 63 64 100

Period length, days 1 55 1 6 1 36

P - Qo, inches per day 3 0 5 0 3 0

Evapotranspiration, 

inches per day

0 0.1 0 0.1 0 0.1

∂t

∂h

N′ P Qo ( ) – – ET Sy∂t

∂h = –

N′

QB

N′

adjusting model input. Calibration is generally accomplished by adjusting input values of hydraulic conductivity and recharge until an acceptable calibration 

criterion is achieved. The differences between simulated and measured ground-water levels and stream 

discharges are generally the bases for demonstrating 

the “goodness” or improvement of the calibration. 

Simulated water levels and discharges from a calibrated, deterministic ground-water model usually 

depart from measured water levels and discharges, 

even after a diligent calibration effort. The discrepancy between model results and measurements (model 

error) usually is caused by simplification of the conceptual model, grid scale, and the difficulty in obtaining sufficient measurements to account for all of the 

spatial variation in hydraulic properties throughout the 

model area. 

Calibration improvement was determined by 

decreases in sum-of-squares error (SS) which is 

defined by:

 , (6)

where

 is a simulated water level, in feet;

 is a measured water level, in feet; 

 is the number of water-level comparisons; 

 is a simulated discharge, in cubic feet per day; 

 is a measured discharge, in cubic feet per day; 

 is a weight which is the standard deviation of 

all measured water levels divided by the 

standard deviation of all measured flow 

rates (Watson and others, 1980); and 

 is the number of flow-rate comparisons. 

Although the sum-of-squares error serves as the 

objective function, root-mean-square (RMS) error is 

reported instead because RMS error is more directly 

comparable to actual values and serves as a composite 

of the average and the standard deviation of a set. 

Root-mean-square error is related to the sum-ofsquares error by:

. (7)

SS hˆ k hk ( ) – 2

k = 1

nwl

∑ qˆl ql ( ) – wl ( )2

l = 1

nq

= + ∑

hˆ k

hk

nwl

qˆl

ql

wl

nq

RMS SS

nwl wl

l = 1

nq

+ ∑

= ------------------------------
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 33

40

70

45

50

55

60

65

WATER LEVEL, IN FEET ABOVE SEA LEVEL

S-2300

D-2300

S-150

CREEK

AVERAGE EFFECTIVE RECHARGE RATE,

IN INCHES PER YEAR

TIME, IN DAYS

P Qo – – ET Sydt

dh –    dx

0

3000

∫

3000 -------------------------------------------------------------------------------

Sdt

dh

   dx

0

3000

∫ –

3000 -----------------------------------------

-10

30

0

10

20

0 25 50 75 100

QB

3000 ------------ N 6 in/yr =

Figure 23. Hypothetical time series that shows the expected water-level changes and average effective 

recharge rates for an aquifer system similar to the surficial-sand aquifer system.
34 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

Water-Level and Discharge Measurements

As measured water levels rarely coincide with 

the center of a cell, simulated water levels were interpolated laterally to points of measurement from the 

centers of surrounding cells. Simulated water levels 

were interpolated because they were assumed to be 

part of a continuous distribution. Vertical interpolation 

was not considered because of the discontinuity and 

associated refraction of potential fields from an aquifer across a confining unit. 

The model was calibrated to 697 water-level 

and 4 discharge measurements. The water levels were 

obtained from 212 wells during 4 synoptic surveys 

that spanned from April 6 to October 19, 1994 

(fig. 15). Eight discharge measurements from Rowell 

Creek and Black Creek were available for the four surveys, but only discharge measurements from the first 

two surveys were used for calibration. The discharge 

measurements from the later surveys were both made 

within a week of a large rainfall event and stream discharge appeared to contain a significant component of 

surface runoff (fig. 15). 

Because the study area only covered 20 percent 

(36 mi2

/177 mi2) of the Black Creek Basin, simulated 

ground-water discharge was multiplied by about five 

for comparison to the actual discharge measurements. 

This approach is based on the assumption that base 

flow was contributed uniformly across the basin. The 

base-flow results simulated by the regional model 

indicate that the entire study area contributes 21 percent (12 ft3

/s)/(58 ft3/s) of the base flow (fig. 20), 

which suggests that, regionally, base-flow discharges 

are contributed uniformly. In addition, the response of 

the Fiftone subbasin (2 mi2

) (fig. 14) and the Black 

Creek Basin (fig. 15) to rainfall events and the recession to base flow is very similar. 

Parameter Estimation

Model calibration is facilitated by a parameter 

estimation program (Halford, 1992). The parameter 

estimation process is initialized by using the model to 

establish the initial differences between simulated and 

measured water levels and discharges. These differences, or residuals, are then minimized by the parameter estimation program. To implement parameter 

estimation, the sensitivity coefficients, the derivatives 

of simulated water level or discharge change with 

respect to parameter change, are calculated by the 

influence coefficient method (Yeh, 1986) using the 

initial model results. Each parameter is changed a 

small amount and MODFLOW is used to compute 

new water levels and discharges for each perturbed 

parameter. The current arrays of sensitivity coefficients and residuals are used by a quasi-Newton procedure (Gill and others, 1981, p. 137) to compute the 

parameter changes that should improve the model. The 

model is updated to reflect the latest parameter estimates and a new set of residuals is calculated. The 

entire process of changing a parameter in the model, 

calculating new residuals, and computing a new value 

for the parameter is continued iteratively until model 

error or model-error change is reduced to a specified 

level or until a specified number of iterations are made 

(Halford, 1992). 

Logs of the parameters, log(x), are used because 

the estimated hydraulic conductivities, x, are usually 

log-normally distributed (Domenico and Schwartz, 

1990). Log-parameters also are better behaved from a 

numerical perspective, because estimates are restricted 

to positive values and are scaled to some degree. Logrecharge rates are also used, thus ensuring that all estimated values of are positive. Consequently, all sensitivities, covariances, and correlation coefficients are 

based on and , not and . 

Another benefit of this type of parameter 

estimation is the covariance matrix computed for the 

estimated parameters. This matrix is ranked by the 

magnitude of the main diagonal, because it is a rough 

indicator of the relative sensitivity of the model to a 

parameter. Specifically, the main diagonal is 

. The off-diagonal 

components, , describe the degree of interdependence between parameters, but evaluation is difficult 

without some sort of normalization (Gill and others, 

1981). 

Normalization is achieved by computing correlation coefficients (Hill, 1992), , similar to computed for a linear regression. If is 

, then xi

 is a dependent variable of xj. Alternatively, 

if is 0, then xi

 is an independent variable of xj. 

Correlation coefficients greater than 0.95 usually indicate a pair of parameters are highly correlated and cannot be estimated independently (Hill, 1992). 

N’

∂logx

∂ hˆ ∂logx

∂

qˆ ∂x

∂ hˆ ∂x∂

qˆ

Ci i , xi ∂log

∂hˆ k

2

k = 1

nwl

∑ xi ∂log

∂qˆl

2

l = 1

nq

= + ∑

Ci j ,

ρi j ,

Ci j ,

Ci i , Cj j ,

= ---------------------

r ρi j ,

±1

ρi j ,
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 35

Twelve parameters (table 4) were used as global 

multipliers that changed the value of either hydraulic 

conductivity or recharge by a fixed amount throughout 

the study area. The only exception is the hydraulic 

conductivity of the drains which were adjusted 

beneath the runways and flight-support areas only. 

The initial values of hydraulic conductivity came from 

the aquifer test results; lower confining unit vertical 

hydraulic conductivities were taken from Krause and 

Randolph (1989). An initial recharge rate of 6 in/yr 

was used for all four synoptic-survey periods. 

The minimum, maximum, average, and RMS 

errors of the calibrated model were -3.79, 4.36, 0.13, 

and 1.40 ft, respectively. A more detailed listing of the 

error statistics by layer and synoptic-survey period is 

provided in table 5. The greater number of water-level 

measurements available during synoptic-survey periods 3 and 4 did not overly bias model calibration 

toward those periods (table 5). The water-level residuals did not exhibit any apparent trend across the study 

area during any of the synoptic-survey periods 

(figs. 24-29). Simulated potentiometric surfaces and 

water-level residuals are only shown for synopticsurvey period 3 because the distribution of residuals is 

similar in all periods. 

Table 5. Water-level error statistics from calibrated Cecil Field Naval Air Station model by layer and synoptic-survey period

[in/yr, inches per year; minimum, maximum, average, and RMS in feet; n, number of samples; --, no observation] Layer

SYNOPTIC-SURVEY PERIOD

(1) (2) (3) (4)

April 6, 1994 May 7, 1994 August 22, 1994 October 19, 1994

N′ = 4.6 in/yr N′ = 4.6 in/yr N′ = 6.0 in/yr N′ = 7.5 in/yr

MinimumMaximumAverrage

RMS n Minimum

MaximumAverage

RMS n Minimum

MaximumAverage

RMS n Minimum

MaximumAverage

RMS n

1 -2.49 4.34 0.37 1.41 93 -2.44 4.36 0.43 1.42 93 -3.67 3.79 -0.01 1.40 109 -3.55 4.08 0.01 1.52 112

2 -1.62 3.71 .57 1.49 26 -1.54 3.76 .61 1.50 26 -2.83 3.33 .08 1.16 29 -3.31 2.53 .03 1.23 29

3 -3.74 1.64 -.34 1.59 25 -3.79 1.67 -.31 1.59 25 -3.06 1.71 -.07 1.10 39 -3.77 1.86 .01 1.37 36

4 -3.34 1.99 -.13 1.64 6 -3.35 2.03 -.10 1.81 5 -2.84 2.16 .03 1.33 8 -3.63 2.05 .00 1.53 8

5 -.40 .73 .24 .54 5 -.32 .69 .25 .52 5 -1.66 .62 -.22 .75 8 -1.79 .59 -.38 .87 7

6 -- -- -- -- -- -- -- -- -- -- -.37 .06 -.15 .26 2 .17 .17 .17 .17 1

All -3.74 4.34 .26 1.45 155 -3.79 4.36 .32 1.46 154 -3.67 3.79 -.02 1.28 195 -3.77 4.08 .00 1.43 193

Composite water level statistics for all periods -3.79 4.36 .13 1.40 697

Table 4. Initial and final values of parameters estimated to 

calibrate the model

[ft/d, feet per day; in/yr, inches per year]

Estimated parameter Initial Final

Kxy of surficial-sand aquifer, ft/d 5 2.8

Kz of surficial-sand aquifer, ft/d .4 .048

Kz of blue-marl confining unit, ft/d .01 .0096

Kxy of upper-rock aquifer, ft/d 40 180

Kz of gray-marl confining unit, ft/d .002 .0027

Kz of lower confining unit, ft/d .002 .0012

KRB of riverbeds, ft/d .05 .7

K of drains beneath runways, ft/d .005 .035

N′ during survey period 1, in/yr 6 4.6

N′ during survey period 2, in/yr 6 4.6

N′ during survey period 3, in/yr 6 6.0

N′ during survey period 4, in/yr 6 7.5
36 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 24. Simulated potentiometric surface of the top third of the surficial-sand aquifer (layer 1) on August 22, 1994.
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 37

Figure 24a. Simulated potentiometric surface of the top third of the surficial-sand aquifer (layer 1) on 

August 22, 1994, inset A.
38 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 24b. Simulated potentiometric surface of the top third of the surficial-sand aquifer (layer 1) on 

August 22, 1994, inset B.
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 39

Figure 25. Simulated potentiometric surface of the middle third of the surficial-sand aquifer (layer 2) on August 22, 

1994. 
40 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 25a. Simulated potentiometric surface of the middle third of the surficial-sand aquifer 

(layer 2) on August 22, 1994, inset A.
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 41

Figure 25b. Simulated potentiometric surface of the middle third of the surficial-sand aquifer (layer 2) on 

August 22, 1994, inset B.
42 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 26. Simulated potentiometric surface of the bottom third of the surficial-sand aquifer (layer 3) on August 22, 

1994.
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 43

Figure 26a. Simulated potentiometric surface of the bottom third of the surficial-sand aquifer (layer 3) on 

August 22, 1994, inset A.
44 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 26b. Simulated potentiometric surface of the bottom third of the surficial-sand aquifer (layer 3) on 

August 22, 1994, inset B.
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 45

Figure 27. Simulated potentiometric surface of the blue-marl confining unit (layer 4) on August 22, 1994.
46 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 27a. Simulated potentiometric surface of the blue-marl confining unit (layer 4) on August 22, 1994, inset A.
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 47

Figure 27b. Simulated potentiometric surface of the blue-marl confining unit (layer 4) on August 22, 1994, inset A.
48 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 28. Simulated potentiometric surface of the upper-rock aquifer (layer 5) on August 22, 1994.
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 49

Figure 29. Simulated potentiometric surface of the lower-rock aquifer (layer 6) on August 22, 1994. 
50 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

Simulated water levels for all four synoptic-survey 

periods approximate the measured levels well over the 

range of approximately 40 ft observed in the study area 

(fig. 30). The measured water levels that were compared 

range from 48 to 85 ft or 67 percent of the range of water 

levels in the surficial-aquifer system. The histogram of 

water level residuals (fig. 30) shows that 57 percent of the 

simulated water levels were within 1 ft and 85 percent 

were within 2 ft of measured water levels. 

Although the model can simulate the ground-water 

flow system beneath Cecil Field NAS fairly well, there 

are a few areas where the model is deficient. The simulated water table north of SR 228, along the western edge 

of the study area, is 2 to 3 ft less than the measured water 

table, probably because the drains that specify land surface are assigned at too low a level (see residuals of -3 

and -2 ft in fig. 24). The simulated water table south of 

Lake Fretwell, on the west bank of Rowell Creek, is 3 to 

4 ft above the measured water table because an 

40

90

50

60

70

80

CALCULATED WATER LEVEL, IN FEET

40 50 60 70 80 90

MEASURED WATER LEVEL, IN FEET

PERIOD 1

PERIOD 2

PERIOD 3

PERIOD 4

0

250

50

100

150

200

NUMBER OF OBSERVATIONS

-4 5 -3 -2 -1 0 1 2 3 4

RESIDUAL, IN FEET

Figure 30. Comparison of simulated to measured water levels and histogram showing residual 

distribution for the calibrated model.

unmapped drain probably exists (see residuals of 3 and 

4 ft in fig. 24, inset A). The lack of vertical interpolation 

when calculating heads for comparison to measured 

values is another source of error (see residual of 3 ft along 

Rowell Creek in fig. 25, inset A). This is only significant 

in the surficial-sand aquifer near the creeks where both 

simulated and measured water levels show head differences of about 10 ft between the top and bottom of the 

surficial-sand aquifer. 

The simulated discharges for the Black Creek 

Basin compare favorably with measured values obtained 

during synoptic-survey periods 1 and 2 (table 6). The 

simulated discharge measurements from the later periods 

could not be directly compared to measured rates because 

surface runoff is not simulated. However, the measured 

water levels for these periods were reasonably well simulated by calibrating to an effective-recharge rate (figs. 24-

29) and simulated ground-water discharge to the Black 

Creek Basin was less than half the measured values. 
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 51

Most of the estimated parameters are not highly 

correlated to one another (table 7). The only exception 

is the strong inverse correlation, -0.97, between the 

vertical hydraulic conductivity of the gray-marl and 

lower confining units. The average vertical hydraulic 

conductivity of the gray-marl and lower confining 

units must be about 0.0015 ft/d or less to simulate the 

measured water levels in the surficial aquifer system. 

Table 6. Simulated and measured discharges from Rowell 

and Black Creeks during the four synoptic-survey periods

[Discharge in cubic feet per second; , simulated; , measured]

Date

Synopticsurvey 

period

Discharge

Rowell Creek Black Creek

a

April 6, 1994 1 0.5 0.4 51 48

May 7, 1994 2 .5 .3 50 47

August 22, 1994b 3 .6 6 62 180

October 19, 1994b 4 .7 12 73 270

a

 Simulated discharge was multiplied by 177 mi2/36 mi2 for comparison to measured discharge. b Discharges during periods 3 and 4 were not considered as part of 

the calibration criteria.

qˆ q

qˆ q qˆ q

Table 7. Correlation coefficients between parameters from the calibrated model

[--, not applicable]

Estimated parameters Correlation coefficients, 

Kxy surficial-sand aquifer 1.00 -- -- -- -- -- -- -- -- -- -- --

N′ during period 1 -.27 1.00 -- -- -- -- -- -- -- -- -- --

N′ during period 2 -.26 .00 1.00 -- -- -- -- -- -- -- -- --

N′ during period 3 -.47 .00 .00 1.00 -- -- -- -- -- -- -- --

N′ during period 4 -.42 .00 .00 .00 1.00 -- -- -- -- -- -- --

Kz surficial-sand aquifer .67 -.16 -.16 -.31 -.26 1.00 -- -- -- -- -- --

Kz blue-marl confining unit .44 -.12 -.09 -.13 -.05 .27 1.00 -- -- -- -- --

Kz lower confining unit .30 -.16 -.15 -.33 -.18 .14 .15 1.00 -- -- -- --

Kxy upper-rock aquifer -.64 .40 .34 .34 .25 -.54 -.56 -.33 1.00 -- -- --

K runway drains .22 -.07 -.07 -.25 -.38 .18 .12 .12 -.15 1.00 -- --

Kz gray-marl confining unit -.20 .03 .03 .26 .12 -.07 -.09 -.97 .18 -.05 1.00 --

KRB riverbeds .82 -.12 -.11 -.47 -.44 .70 .32 .45 -.62 .17 -.37 1.00

Kxy surficial-sand aquifer

N′ during period 1

N′ during period 2

N′ during period 3

N′ during period 4

Kz surficial-sand aquifer

Kz blue-marl confining unitKz lower confining unitKxy upper-rock aquifer

K runway drainsKz gray-marl confining unit

KRB riverbeds

Main diagonal, 31,000 15,000 14,000 4,300 4,000 3,500 3,300 3,000 2,500 1,900 1,700 400

ρi j ,

ρi j ,

Ci j ,

Ci i , Cj j ,

= ---------------------

Ci i ,

The high degree of correlation between the two 

parameters is due to a sparsity of water-level measurements in the lower-rock aquifer which does not 

allow for differentiating between the confining 

effects of the gray-marl and the lower confining 

units. The second highest degree of correlation of 

0.82 is between the lateral hydraulic conductivity of 

the surficial-sand aquifer and the riverbed conductance. 

Sensitivity Analysis

Each estimated parameter was varied, independently, from 0.2 to 5 times their calibrated value to 

determine how these parameters affected simulation 

results. This range was greater than the uncertainties 

associated with the parameters, but provided a more 

complete perspective on parameter sensitivity. Model 

sensitivity was described in terms of RMS error 

using only water-level measurements. The sensitivity 

of the model to changing one parameter while all others are held at their calibrated values is shown in 

figure 31. Model error was determined to be most
52 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

1

10

2

3

4

5

7

Recharge

River bed conductances

Runway drain conductances 

Lateral hydraulic conductivity, surficial-sand aquifer

Vertical hydraulic conductivity,

1

3

1.5

2

2.5

0.2 0.5 1 2 5

Vertical hydraulic conductivity, blue-marl confining unit

Lateral hydraulic conductivity, upper-rock aquifer 

Vertical hydraulic conductivity, gray-marl confining unit

Vertical hydraulic conductivity, lower confining unit

ROOT-MEAN-SQUARE ERROR, IN FEET

MULTIPLIER OF CALIBRATED VALUE

surficial-sand aquifer

Figure 31. Model sensitivity to independent changes in model calibration parameters.
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 53

sensitive to changes in recharge and the lateral 

hydraulic conductivity of the surficial-sand aquifer. 

The model displays an intermediate degree of sensitivity to changes in the vertical hydraulic conductivity of 

the surficial-sand aquifer, the vertical hydraulic conductivity of the blue-marl confining unit, and the runway drain conductances (fig. 31). Generally, the 

model is less sensitive to changes in the hydraulic 

properties of the deeper aquifers and confining units 

(fig. 31) due to fewer observations at depth and less 

circulation through the deeper aquifers. 

The model sensitivity to riverbed conductance 

was asymmetrical, with model error increasing for 

values less than the calibrated value and remaining 

constant for values greater than the calibrated value 

(fig. 31). Any large value of riverbed conductance 

would have sufficed; it did not need to be estimated, 

given the lack of model sensitivity to increasing riverbed conductance and the degree of correlation with the 

lateral hydraulic conductivity of the surficial-sand 

aquifer. This result indicates that stream and lake 

interaction was controlled by the hydraulic conductivity of the surficial-sand aquifer instead of the riverbed 

conductance. 

A post-calibration sensitivity analysis of the 

model boundaries was not performed, because the 

results of the regional model simulations serve as a 

sensitivity analysis of the no-flow lateral boundaries 

of the study-area model. The original test of the noflow boundaries is still valid because the hydraulic 

properties and boundaries used in the regional model 

are similar to those used in the calibrated study-area 

model. 

Simulation of Ground-Water Flow

The analysis of ground-water flow and potential 

movement of contaminants within the surficial aquifer 

system was addressed using the calibrated model 

driven by the long-term, average recharge rate. This 

rate was assumed to be 6 in/yr because it is bracketed 

by the effective-recharge rates of 4.6 and 7.5 in/yr estimated for dry and wet conditions, respectively. The 

results of the cross-sectional model also suggest that 

the long-term rate should fall between the two 

extremes (fig. 23). 

The water-table configuration is strongly influenced by almost all of the drainage features, even the 

smallest of drains (fig. 24). The simulated lateral flow 

direction near the top of the surficial-sand aquifer is 

usually perpendicular to the nearest drainage feature. 

The simulated potentiometric surface at the bottom of 

the surficial-sand aquifer is only influenced by the 

larger surface drainage features (fig. 26). In the deeper 

upper-rock and lower-rock aquifers, only the deeply 

incised lower reach of Sal Taylor Creek and Yellow 

Water Creek influence the simulated potentiometric 

surfaces of these aquifers (figs. 28 and 29). Simulated 

lateral flow in the upper-rock and lower-rock aquifers 

generally is from the northern and eastern edges of the 

study area towards Yellow Water Creek in the southwestern part of the study area. 

The simulated volumetric flow budget for the 

surficial aquifer system and the intermediate confining 

unit is schematically shown in figure 32. The arrows 

indicate the flow across the boundaries between 

layers. Only 4.6 of the 6 in/yr of applied recharge is 

available to circulate through the ground-water system 

because 1.4 in/yr is rejected to additional evapotranspiration and runoff (fig. 32). The rates of recharge 

rejection are not spatially uniform; most recharge is 

rejected north of SR 228 and east of the runways in the 

Sal Taylor Creek Basin at rates of 3 in/yr or more 

(fig. 33). These are areas where the water table is 

perennially near or at land surface. 

Most of the simulated ground-water flow 

(78 percent after discounting the 1.4 in/yr of rejected 

recharge) circulates solely within the surficial-sand 

aquifer (fig. 32), which indicates that most of the 

contaminant migration can be expected to be through 

the surficial-sand aquifer. The remaining 1.0 in/yr 

(22 percent) passes to the upper-rock aquifer with 

0.2 in/yr (4 percent of the 4.6 in/yr of total circulation) 

continuing downward to the lower-rock aquifer. Most 

of the 0.8 in/yr of flow that discharges from the upperrock aquifer to the surficial-sand aquifer does so along 

Yellow Water, Rowell, and Sal Taylor Creeks 

(fig. 34). Simulated deep infiltration from the study 

area to the Upper Floridan aquifer amounts to 

0.2 in/yr. The higher rates of deep infiltration occur 

along the eastern half of the study area (fig. 35).
54 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

Simulation of Movement of Contaminants

The advective movement of conservative contaminants from selected sites to discharge points 

was simulated using a particle-tracking routine, 

MODPATH (Pollock, 1989). For this analysis, the 

dissolved contaminant was assumed not to appreciably alter the density of the ground water. A column 

of three particles that extended from 10 to 40 ft 

4.4

0.2 1.0

SURFICIAL-SAND AQUIFER (1, 2, and 3)

UPPER-ROCK AQUIFER (5)

LOWER-ROCK AQUIFER (6)

1.4

RECHARGE

BLUE-MARL CONFINING UNIT (4)

0.2

0.2

0.2

0.2

1.0

0.2

0.2

6.0

0.0

0.0

0.8

0.8

Deep Infiltration to Upper Floridan aquifer

NET FLOW ACROSS A BOUNDARY

FLOW ACROSS A BOUNDARY

FLOW RATE, IN INCHES PER YEAR

LAYER NUMBER

0.2

INTERMEDIATE 

CONFINING UNIT SURFICIAL AQUIFER SYSTEM

(5)

BASE-FLOW

DISCHARGE RECHARGE

REJECTED

EXPLANATION

Figure 32. Simulated volumetric flow budget using a recharge rate of 6 inches per 

year. 

below the water table was used to approximate the 

initial position of contaminants at selected sites. For 

half of the sites, the different initial depths below 

the water table did not affect the particle paths 

(fig. 36, sites 1, 2, 3, or 8). However, at sites 5, 7, 

13, 16, and the fuel farms, the particle paths 

diverged more with depth because the nearest drain 

or creek was not a major point of discharge, and the 

deeper flow paths were captured by a more remote 

surface-water outlet at a lower stage. 
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 55

Figure 33. Locations of simulated rejected recharge in the study area. 
56 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 34. Average simulated head difference between the bottom of the surficial-sand aquifer (layer 3) and upper-rock 

aquifer (layer 5) potentiometric surfaces.
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 57

Figure 35. Simulated deep infiltration to the Upper Floridan aquifer (layer 7) from the lower-rock aquifer (layer 6) using 

a recharge rate of 6 inches per year.
58 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 36. Simulated flow paths from selected sites to their respective discharge points using a recharge rate of 6 

inches per year. 
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 59

Figure 36a. Simulated flow paths from selected sites to their respective discharge points using a recharge 

rate of 6 inches per year, inset A 
60 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Figure 36b. Simulated deep infiltration to the Upper Floridan aquifer (layer 7) from the lower-rock aquifer 

(layer 6) using a recharge rate of 6 inches per year, inset B.
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 61

Most of the particles traveled 1,500 ft or less 

before being discharged from the ground-water system. For an assumed effective porosity of 20 percent, 

typical traveltimes are 40 years or less. If the effective 

porosity ranges from 10 to 30 percent, traveltimes will 

range from half as long to 1.5 times longer than those 

computed with an effective porosity of 20 percent. 

Traveltimes for particles released 10 ft below the 

water table were shorter compared to those released 

40 ft below the water table for any given site (table 8). 

The pathline with the shortest traveltime (2 years) 

originated at site 16; the pathline with the longest 

traveltime (300 years) originated at the north fuel farm 

(table 8 and fig. 36). 

The ground-water flow patterns around many of 

the sites of contamination are similar to the flow pattern around site 3 and can be illustrated by examining 

site 3 in detail. Site 3 is located about 1,200 ft west of 

Rowell Creek and just south of Lake Fretwell (fig. 37). 

The model simulates the flow field at site 3 reasonably 

well, but not perfectly, as shown by the discrepancy 

between the simulated particle paths and the measured 

TCE plume (U.S. Department of the Navy, 1995). 

Table 8. Approximate traveltimes of particle 

movement from selected sites to their respective

discharge points 

Site identifier

Approximate 

traveltime, in years, 

for particles released 

below the water tablea

10 feet 40 feet

1 45

2 8 11

3 20 30

5 5 20

7 11 40

8 40 40

11 50 70

13 4 40

16 2 50

17 12 30

North Fuel Farm 20 300

South Fuel Farm 5 60

a

 Traveltimes are based on an effective porosity of 

20 percent throughout the study area. Associated particle 

paths are displayed in figure 36.

Ground-water flow from site 3 is toward the east-southeast and has a downward component (fig. 37). As 

ground water flows away from site 3, the lateral direction changes little, but the vertical gradients reverse and 

are upward about 500 ft away from Rowell Creek. All 

flow from around site 3 converges on Rowell Creek and 

is discharged (fig. 37). 

The effects of the flow field on plume development and contaminant transport were shown by tracing 

a panel of 112 particles from site 3 and examining their 

positions 5, 10, 15, and 20 years after release (fig. 37). 

The initial positions of the particles were defined by a 

panel with a 120-ft-long trace along the land surface 

and a vertical extent of about 45 ft (see 0 yr in fig. 37). 

The difference in velocity between the shallowest and 

deepest particles became noticeable after 10 years of 

travel and pronounced after 20 years when the distance 

between the leading and trailing edges of the panel was 

about 300 ft (fig. 37). The velocity difference between 

the shallowest and deepest particles is due to addition of 

recharge to the flow system. This difference becomes 

more exaggerated within 500 ft of Rowell Creek 

because the upward gradients force the water added by 

recharge in this area to circulate near the surface. 

Ground-water flow from the north fuel farm is 

atypical compared to the other sites of contamination. 

At most of the other sites, the flow field is fairly uniform. Particles released 100 to 200 ft perpendicular to 

the paths shown in figure 36 will have paths that 

roughly parallel those shown initially at these sites. The 

flow field near the north fuel farm is divergent because 

it is just east of a ground-water divide (fig. 36), and 

because small displacements of the particle release 

location, either laterally or vertically, can greatly alter 

the particle path and terminus. 

The effects of the divergent flow field can be 

illustrated best by tracing eight particles from the north 

fuel farm and examining their paths in plan and section 

(fig. 38). Particles that were started about 15 ft below 

the water table all moved downward initially but 

reversed direction and were captured by nearby creeks. 

The pathlines and traveltimes for these particles were 

relatively short and ranged from 15 to 34 years. The 

particles that were started about 40 ft below the water 

table all moved downward across the blue-marl confining unit and into the upper-rock aquifer (fig. 38). After 

entering the upper-rock aquifer, these particles traveled 

about 2,000 to 4,000 ft south of the north fuel farm 

before emerging from the upper-rock aquifer into the 

surficial-sand aquifer (fig. 38). 
62 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cecil Field Naval Air Station, Jacksonville, Florida

Rowell Creek

70

68

66

64

62

60

70

68

66

64

62

60

58

54

56

72

Lake

Fretwell

N

SITE 3

C

C C

C

52

UPPER-ROCK

SURFICIALSAND

AREA OF 

DETAIL

VERTICAL EXAGGERATION 9X

0 500 FEET

0 yr

5 yr

10 yr

15 yr

20 yr

0 yr 5 yr 10 yr 15 yr

20 yr

EXPLANATION

PARTICLE -- in the top of surficial-sand aquifer (layer 1)

PARTICLE -- in the middle or bottom of surficial-sand aquifer (layers 2 or 3)

SHALLOWEST PARTICLE PATH

DEEPEST PARTICLE PATH 

NOTE: All particles and paths are projected 

POTENTIOMETRIC CONTOUR--shows altitude at which water level would have stood in 

tightly cased wells. Contour interval 2 feet. Datum is sea level

68

54

52

54

56

58

56

TCE plume, 1994

Rowell

Creek

0 100 METERS 

DEPTH ABOVE OR BELOW SEA LEVEL

5 mg/l

75

50

25

SEA

LEVEL

-25

-50

75

50

25

SEA

LEVEL

-25

-50

Figure 37. Simulated flow paths, particles, traveltimes, and potentiometric contours near site 3, using a 

recharge rate of 6 inches per year.
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 63

D

E

D

E

AREA OF 

DETAIL

SURFICIALUPPER-R

O

C

K

S

U

RF CI AI L S- A

N

D

UPPER-ROCK

SAND

0 2,000 FEET

EXPLANATION

PARTICLE ORIGIN

PARTICLE PATH--originating from the top of 

surficial-sand aquifer (layer 1)

PARTICLE PATH--originating from the middle 

of surficial-sand aquifer (layer 2)

NOTE: All paths are projected 

DIRECTION OF PARTICLE MOVEMENT VERTICAL EXAGGERATION 25X

VERTICAL EXAGGERATION 25X

N

D E

D

E

0 500 METERS

FEET

FEET

80

40

SEA

LEVEL

-40

-80

80

40

SEA

LEVEL

-40

-80

80

40

SEALEVEL

-40

-80

80

40

-40

-80

SEA

LEVEL

Figure 38. Simulated flow paths from the north fuel farm to their respective discharge points, using a recharge rate 

of 6 inches per year.
64 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

The sharp bends and sudden changes in particle direction, seen in plan view (fig. 38), are clearly the effects 

of movement between the surficial-sand and upperrock aquifers when seen in section (fig. 38). Traveltimes for the particles that were started about 40 ft 

below the water table ranged from 340 to 480 years. 

The domestic supply wells near the base do not 

substantially influence ground-water flow through the 

surficial aquifer system but concern exists about where 

the water originates, what path it takes, and how long it 

takes to travel from land surface to a well. The effects 

of the wells were simulated by assuming that they were 

screened in the upper-rock aquifer and were pumped at 

a rate of 750 gal/d. The cluster of wells along SR 228 

had the greatest effect on the potentiometric surface of 

the upper-rock aquifer and created a drawdown of 

about 0.06 ft in the vicinity of the wells (fig. 39). 

The contributing areas and traveltimes to the 

domestic supply wells were determined by seeding the 

perimeter of each well with 800 particles and backtracking upgradient to the water table. The contributing areas are not very extensive (fig. 39). The shortest 

traveltimes for particles to reach wells were for those 

along the western side of the base and ranged from 70 

to 80 years. The shortest traveltimes for particles to 

reach wells along the eastern edge of the base ranged 

from 90 to 200 years. 

Model Limitations

The study-area model addresses questions about 

the advective movement of contaminants through the 

surficial aquifer system beneath Cecil Field NAS 

fairly well, but it cannot mimic the true system 

exactly. The study-area model, or any other model, is 

limited by simplification of the conceptual model, 

discretization effects, difficulty in obtaining sufficient 

measurements to account for all of the spatial variation 

in hydraulic properties throughout the model area, and 

limitations in the vertical accuracy of land surface representations. 

The conceptual model has been simplified by 

assuming most of the current questions raised by site 

assessments and remediation plans can be addressed 

with a steady-state ground-water flow model. However, available data indicate that during any given 

year, water levels will fluctuate seasonally about 3 ft, 

and the surficial-sand aquifer infrequently approaches, 

at best, a quasi-steady state condition. A steady-state 

model is adequate to estimate the advective movement 

of contaminants over a few years or more, but might 

not be adequate to estimate the advective movement of 

contaminants over less than a few years. 

Lateral discretization of the study area into a 

rectangular grid of cells and vertical discretization into 

layers forced an averaging of hydraulic properties. 

Each cell represents a homogeneous block or some 

volumetric average of the aquifer medium. Discretization errors occurred in even the smallest model cells, 

which were 50 ft on a side and about 15 ft thick, 

because the surficial-sand aquifer contains clay lenses 

that are about 1 ft thick interbedded in silty-sand. Due 

to the averaging of the hydraulic properties, the model 

cannot simulate the dispersive effects of aquifer heterogeneity. 

The model of a heterogeneous aquifer system 

was simplified further by describing the hydraulic 

conductivities of the aquifers and confining units as 

global multipliers that changed the values by a fixed 

amount throughout the study area. The lack of sufficient measurements to account for all of the spatial 

variation in hydraulic properties throughout the model 

area necessitated this gross simplification. Simplifying 

the model to this degree does not invalidate it, but does 

mean model results should only be interpreted at 

scales larger than the representative elemental volume 

of hydraulic conductivity. 

Vertical accuracy of land surface representations was limited by the source material used. Locations and elevations of streams were obtained from 

1:24,000 USGS quadrangle maps of the area. Elevations of drains that defined land surface elevations 

greater than 75 ft were obtained from the Jacksonville 

1:250,000, 1-degree, Digital Elevation Model (DEM). 

Both land surface representations have a limited vertical accuracy. Elevations from the quadrangle sheets 

are assumed to be within ±5 ft, half of the contour 

interval. The DEM reports elevations to the nearest 

3.3 ft but was based on the source sheets for 1:250,000 

USGS maps which were published with a 25-ft contour interval. 

Because the land surface models are not perfect 

and do not always adequately represent the topography, a few areas of anomalously high discharge occur 

through the surface drains. Most of these areas are 

located close to SR 228, along the western edge of 

Cecil Field NAS. Although the flux from these cells is 

high, the excess discharge from any one cell is less 

than 1 gal/min and the total excess discharge is no 

more than 4 percent of the total budget. 
Simulation of Ground-Water Flow and Advective Transport of Contaminants in the Surficial Aquifer System 65

Figure 39. Simulated contributing areas for domestic supply wells screened in the upper-rock aquifer (layer 5) and 

drawdowns associated with producing 750 gallons per day per well.
66 Ground-Water Flow in the Surficial Aquifer System and Potential Movement of Contaminants from Selected Waste-Disposal 

Sites at Cedil Field Naval Air Station, Jacksonville, Florida

SUMMARY

As part of the Installation Restoration Program, 

Cecil Field NAS in Jacksonville, Florida, is considering remedial-action alternatives to control the movement of contaminants from sites that could discharge 

contaminants to the surface. This requires an understanding of how the ground-water flow system 

responds to current conditions and any future stresses 

or alterations imposed on the system. Numerical simulation provides the most tractable way of achieving 

this level of understanding. 

The geologic units in the study area consist of 

sediments of Holocene to Miocene age that extend 

from land surface to the base of the Hawthorn Group. 

Previous investigators have defined this sequence as 

two regional hydrogeologic units: the surficial aquifer 

system and the intermediate confining unit. This report 

further subdivided the sequence into six local hydrogeologic units because this study was primarily concerned with ground-water movement near the surface. 

The surficial aquifer system was subdivided into three 

hydrogeologic units: the surficial-sand aquifer, the 

blue-marl confining unit, and the upper-rock aquifer. 

The intermediate confining unit was also subdivided 

into three hydrogeologic units: the gray-marl confining unit, the lower-rock aquifer, and the lower confining unit. The hydrogeologic structure within the study 

area was defined by depth and thickness data from 

gamma-ray and geologists’ logs. 

The recession-curve-displacement method was 

used to estimate the long-term recharge rate in the 

study area using discharge measurements at Black 

Creek near Middleburg (0224600) from 1932 to 1994. 

Estimates of recession index and time-base values at 

station 0224600 ranged from 25 to 210 d/logQ and 

from 3 to 9 days, respectively. Potential recharge rates 

to the study area ranged from 4 to 10 in/yr, using the 

given ranges of recession index and time-base values. 

Ground-water flow through the surficial aquifer 

system was simulated with a seven-layer, finite-difference model that extended vertically from the water 

table to the top of the Upper Floridan aquifer. Initial 

estimates of the hydraulic properties of the aquifers 

and confining units came from a series of aquifer tests 

conducted on the base. The distribution of surfacewater features controls the directions and rates of flow 

in the surficial aquifer system. All the lateral model 

boundaries in each layer are assumed to be groundwater divides that coincide with surface-water 

features. 

The effective-recharge approach was used to 

calibrate a series of independent steady-state models 

to the available data which consisted of “snapshot” 

images of a transient system. This approach assumes 

recharge, evapotranspiration, and the water released 

from specific yield can be approximated by one term, 

the effective-recharge rate. To apply this approach, 

necessity and practicality force the assumptions that 

the change in head with respect to time is constant 

during each survey, that survey periods are short, that 

the spatial distribution of effective recharge is 

spatially uniform, and that no water is contributed to 

the ground-water system from compressibility-based 

storage. These assumptions were tested with a 

transient, cross-sectional model which demonstrated 

that the needed assumptions will probably be met. 

The model was calibrated to 697 water-level 

and four discharge measurements. The water levels 

were obtained from 212 wells during four synoptic 

surveys that spanned from April 6 to October 19, 

1994. Eight discharge measurements from Rowell and 

Black Creeks were available for the four survey 

periods, but only discharge measurements that 

represented base-flow conditions were used. Model 

calibration was facilitated by a parameter estimation 

program that estimated the log-hydraulic conductivities. Ten parameters were used as global multipliers 

that changed the value of either hydraulic conductivity 

or effective recharge by a fixed amount. 

The minimum, maximum, average, and rootmean-square errors of the calibrated model were -3.79, 

4.36, 0.13, and 1.40 ft, respectively, and they did not 

exhibit any apparent trend across the study area. The 

simulated flow rates for the Black Creek Basin 

compared favorably with measured values. Most of 

the estimated parameters are not highly correlated to 

one another. Model error was determined to be the 

most sensitive to changes in recharge and to the lateral 

hydraulic conductivity of the surficial-sand aquifer. 

The ground-water flow system and potential 

contaminant movement within the study area is driven 

by an estimated long-term recharge rate of 6 in/yr. 

Most of the ground-water flow, 78 percent after 

discounting the 1.4 in/yr of rejected recharge, circulates within the surficial-sand aquifer, which indicates 

most of the contaminant movement can be expected to 

move through the surficial-sand aquifer alone. Deep 

infiltration from the study area to the Upper Floridan 

aquifer is about 0.2 in/yr, with higher rates of infiltration along the eastern half of the study area. 
References 67

The advective movement of conservative contaminants from selected sites to discharge points was 

simulated, using a particle-tracking routine, MODPATH. For this analysis it was assumed that the dissolved contaminant does not appreciably alter the 

density of the ground water. A column of three particles that extended from 10 to 40 ft below the water 

table was used to approximate the initial position of 

contaminants at each site. Most of the particles traveled 1,500 ft or less before being discharged from the 

ground-water system. If an effective porosity of 

20 percent is assumed, typical traveltimes are 40 years 

or less. Particles released 10 ft below the water table 

have shorter traveltimes than those released 40 ft 

below the water table for any given site. The pathline 

with the shortest traveltime (2 years) originated at site 

16, and the pathline with the longest traveltime 

(300 years) originated at the north fuel farm. 

The recharge areas and traveltimes to the 

domestic supply wells were determined by seeding the 

perimeter of each well with 800 particles and backtracking upgradient to the water table. The recharge 

areas of the domestic supply wells are not very extensive. The shortest traveltimes for particles to reach the 

domestic supply wells ranged from 70 to 200 years, if 

an effective porosity of 20 percent is assumed. 

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