Simulation of Selected Ground-Water Pumping Scenarios 

at Fort Stewart and Hunter Army Airfield, Georgia

By Gregory S. Cherry

Prepared in cooperation with the 

U.S. Department of the Army, Fort Stewart

Open-File Report 2006 -1148

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

P. Patrick Leahy, Acting Director

U.S. Geological Survey, Reston, Virginia: 2006

This report is a Web-only publication: http://ga.water.usgs.gov/pubs/ofr/2006/1148

For more information about the USGS and its products:

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Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Cherry, G.S., 2006, Simulation of selected ground-water pumping scenarios at Fort Stewart and Hunter Army Airfield, 

Georgia: U.S. Geological Survey Open-File Report 2006-1148, 16 p.
Contents

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

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

Ground-Water Flow Model............................................................................................................................3

Simulation of Ground-Water Pumping Scenarios.....................................................................................4

Model Limitations...........................................................................................................................................8

References Cited..........................................................................................................................................13

Figures

1. Map showing location of Fort Stewart and Hunter Army Airfield, Georgia,

and area of model..........................................................................................................................2

2. Schematic diagram showing model layers and boundary conditions.................................4

3–8. Maps Showing—

3. Well locations and model grid orientation in the vicinity of Fort Stewart and

Hunter Army Airfield, Georgia; and simulated ground-water pumpage for the

Upper Floridan aquifer, 2000...............................................................................................6

4. Simulated potentiometric surface of the Upper Floridan aquifer during

2000 using Base Case pumping rates for Fort Stewart and Hunter Army

Airfield, Georgia....................................................................................................................7

5. Simulated water-level change between Base Case conditions (2005 permitted

pumping for Fort Stewart and Hunter Army Airfield, Georgia) and Scenario A

(increase pumping by 1 million gallons per day at Hunter Army Airfield)...................9

6. Simulated water-level change between Base Case conditions (2005 permitted

pumping for Fort Stewart and Hunter Army Airfield, Georgia) and Scenario B

(increase pumping by 2 million gallons per day at Hunter Army Airfield).................10

7. Simulated water-level change between Base Case conditions (2005 permitted

pumping for Fort Stewart and Hunter Army Airfield, Georgia) and Scenario C

(decrease pumping by 1 million gallons per day at Fort Stewart and increase

pumping by 1 million)..........................................................................................................11

8. Simulated water-level change between Base Case conditions (2005 permitted

pumping for Fort Stewart and Hunter Army Airfield, Georgia) and Scenario D

(decrease pumping by 2 million gallons per day at Fort Stewart and increase

pumping by 2 million)..........................................................................................................12

Tables

1. Selected well data for production wells and 2005 permitted pumping rates

(Base Case) at Fort Stewart and Hunter Army Airfield, Georgia...........................................5

2. Simulated pumpage at Fort Stewart and Hunter Army Airfield, Georgia, and

maximum water-level changes in layer 5 of the ground-water flow model

(Upper Floridan aquifer)................................................................................................................8

iii
Conversion Factors

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 0.09290 square meter (m2)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

million gallons (Mgal) 3,785 cubic meter (m3)

Flow rate

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

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3/s)

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

iv
Abstract

A regional MODFLOW ground-water flow model of 

parts of coastal Georgia, Florida, and South Carolina was used 

to evaluate the effects of current and hypothetical groundwater withdrawal, and the relative effects of pumping in 

specific areas on ground-water flow in the Upper Floridan 

aquifer near Fort Stewart and Hunter Army Airfield (HAAF), 

coastal Georgia. Simulation results for four steady-state 

pumping scenarios were compared to each other and to a 

Base Case condition. The Base Case represents year 2000 

pumping rates throughout the model area, with the exception 

that permitted annual average pumping rates for the year 2005 

were used for 26 production wells at Fort Stewart and HAAF. 

The four pumping scenarios focused on pumping increases 

at HAAF resulting from projected future demands and additional personnel stationed at the facility and on reductions in 

pumping at Fort Stewart.

Scenarios A and B simulate 1- and 2-million-gallon-perday (Mgal/d) increases, respectively, at HAAF. Simulated 

water-level change maps for these scenarios indicate an area 

of influence that extends into parts of Bryan, Bulloch, Chatham, Effingham, and Liberty Counties, Ga., and Beaufort and 

Jasper Counties, S.C., with maximum drawdowns from 0.5 to 

2 feet (ft) for scenario A and 1 to 4 ft for Scenario B.

For scenarios C and D, increases in pumping at HAAF 

were offset by decreases in pumping at Fort Stewart. Scenario 

C represents a 1-Mgal/d increase at HAAF and a 1-Mgal/d 

decrease at Fort Stewart; simulated water-level changes range 

from 0.4 to –2 ft. Scenario D represents a 2-Mgal/d increase at 

HAAF and 2-Mgal/d decrease at Fort Stewart; simulated waterlevel changes range from 0.8 to – 4 ft. The simulated waterlevel changes indicate an area of influence that extends into 

parts of Bryan, Bulloch, Candler, Evans, Chatham, Effingham, 

Liberty, Long, McIntosh, and Tattnall, Counties, Ga., and Jasper 

and Beaufort Counties, S.C. In general, decreasing pumping at 

Fort Stewart by an equivalent amount to pumping increases at 

HAAF reduced the magnitude and extent of drawdown resulting 

from the additional pumping. None of the scenarios resulted in 

large changes in the configuration of the simulated potentiometric surface and related ground-water flow directions.

The scenarios simulated vary from the original model 

only by increasing pumpage less than 1 percent of the total 

calibrated model withdrawals. The changes in pumpage are 

located near the center of the original model area. Thus, the 

scenarios described in this report are considered to be reasonable with no less uncertainty than the original calibrated model.

Introduction

The U.S. Department of the Army (U.S. Army) Garrison 

Fort Stewart and Hunter Army Airfield (HAAF), Georgia, is 

responsible for organizing, directing, coordinating, and controlling garrison support and service activities, including overall management of the garrison workforce. Also, the military 

installations house and train active military personnel and are 

capable of rapid deployment of the Armed Forces anywhere 

in the world. Fort Stewart and HAAF encompass an area of 

438 square miles (mi2) in southeastern Georgia and are located 

in parts of Bryan, Chatham, Evans, Liberty, Long, and Tattnall 

Counties (fig. 1). The U.S. Army, Fort Stewart, has plans to 

expand operations at HAAF, which would require construction 

of new housing facilities and additional quantities of water. 

Ground-water withdrawal in the Fort Stewart–HAAF 

area is regulated and permitted by the Georgia Environmental Protection Division (GaEPD). As part of an interim 

water-management strategy, GaEPD has capped permitted 

ground-water withdrawal from the Upper Floridan aquifer 

at 1997 rates in parts of the coastal area (including HAAF) 

during 1997–2005 to limit additional drawdown in the aquifer 

and to reduce the potential for further saltwater intrusion 

(Georgia Environmental Protection Division, 1997). To aid in 

ground-water permit applications to GaEPD, the U.S. Army 

is interested in determining the effect on ground-water levels 

of (1) increased pumping at HAAF and (2) redistribution of 

pumping capacity from Fort Stewart to HAAF. To answer 

these questions, the U.S. Geological Survey (USGS)—in 

cooperation with the U.S. Army, Fort Stewart—conducted a 

study using an existing regional USGS MODFLOW groundwater flow model of the coastal Georgia, Florida, and South 

Carolina area (Payne and others, 2005; fig. 1). 

Simulation of Selected Ground-Water Pumping Scenarios

at Fort Stewart and Hunter Army Airfield, Georgia

By Gregory S. Cherry
WARE

BURKE

CLINCH

DUVAL

COLLETON

JASPER

WAYNE

LAURENS

COFFEE

NASSAU

LONG

CHARLTON

BULLOCH

DODGE

SCREVEN

EMANUEL

CAMDEN

BRYAN

LIBERTY

IRWIN

APPLING

BAKER

GLYNN

HAMPTON

TELFAIR

DOOLY

BARNWELL

PIERCE

JEFFERSON

WILCOX TATTNALL

TOOMBS

JENKINS

BRANTLEY

EFFINGHAM

MCINTOSH

CHATHAM

BACON

BAMBERG

BERRIEN

WASHINGTON

ALLENDALE

ATKINSON

JOHNSON

TWIGGS

WHEELER

JEFF DAVIS

CRISP

PULASKI EVANS

ECHOLS

CANDLER

BEN HILL

COLUMBIA

LANIER

BLECKLEY

BEAUFORT

TURNER

TREUTLEN

WILKINSON

TIFT

GLASCOCK

HOUSTON

Location of map in figures 3–7

FORT

STEWART

N

0 25 50 MILES

0 25 50 KILOMETERS

Base from U.S. Geological Survey

1:100,000-scale digital data

Atlantic

Ocean

FLORIDA

GEORGIA SOUTH

CAROLINA

Model boundary

HUNTER

ARMY

AIRFIELD

Figure 1. Location of Fort Stewart and Hunter Army Airfield, Georgia,

and area of model (Payne and others, 2005) utilized in simulations.

	 	 Simulation of Selected Ground-Water Pumping Scenarios
This report describes simulated water-level changes by 

comparing a Base Case condition with four hypothetical steadystate pumping scenarios using the regional flow model of Payne 

and others (2005). The Base Case represents year 2000 pumping rates throughout the model area, with the exception that 

permitted annual average pumping rates for the year 2005 were 

used for 26 production wells at Fort Stewart and HAAF. For 

each scenario, the pumping distribution and simulated waterlevel changes relative to Base Case conditions are described. 

The limitations of the model analysis also are provided.

Ground-Water Flow Model

The model used in this study is described in detail in 

Payne and others (2005); only a brief description is included 

herein. Regional ground-water flow was simulated using 

MODFLOW-2000 (Harbaugh and others, 2000), a finitedifference, constant-density flow simulator that is widely 

used and is appropriate for modeling local- and regionalscale ground-water flow systems. The model boundaries 

encompass an area of about 42,155 mi2 (fig. 1). 

The model (Payne and others, 2005) was calibrated for 

1980 and 2000 pumping conditions assuming steady-state 

flow. The model consists of seven aquifers and confining 

units. These include, in descending order:

•	 the confined upper and lower water-bearing zones of 

the surficial aquifer system, grouped together as unit 1; 

•	 the Brunswick aquifer system confining unit (unit 2);

•	 the upper and lower Brunswick aquifers, grouped as 

the Brunswick aquifer system (unit 3); 

•	 the Upper Floridan aquifer confining unit (unit 4);

•	 the Upper Floridan aquifer (unit 5); 

•	 the Lower Floridan aquifer confining unit (unit 6); and

•	 the Lower Floridan aquifer (unit 7).

These units crop out to the northwest of the study area and 

generally dip and thicken to the southeast. The thickness, 

extent, and other hydraulic properties of these units, as well 

as the model development process are described in detail in 

Payne and others (2005). A schematic diagram showing model 

layers and boundary conditions is shown in figure 2.

The finite-difference technique used by MODFLOW 

requires that the simulated area be divided into discrete cells, 

with uniform properties throughout each cell. The MODFLOW model is horizontally discretized using a variably 

spaced grid, with cell sizes ranging from about 4,000 by 

5,000 feet (ft) (0.7 mi2) to 16,500 by 16,500 ft (9.8 mi2; Payne 

and others, 2005). Grid density is higher at Savannah, Ga., to 

enable simulation of steeper head gradients near areas of concentrated pumping and to facilitate linkage with smaller-scale 

solute transport models being developed in those areas (fig. 3). 

At HAAF, mesh resolution is 5,000 by 5,200 ft, whereas at 

Fort Stewart the resolution is 14,900 by 16,100 ft. Each unit is 

represented with one layer of grid cells in the 

vertical dimension. 

With the exception of units 5, 6, and 7 (Upper and 

Lower Floridan aquifers and intervening confining unit), 

lateral boundaries for all layers are designated as no-flow. 

For layers 5–7, the lateral boundaries on all sides of the 

model, except for the southern and southwestern sides, also 

are designated as no-flow. The southern and southwestern 

lateral boundaries are set as specified head for the layers 

representing the Upper Floridan aquifer (unit 5), the Lower 

Floridan aquifer (unit 7), and the intervening confining unit 

(unit 6). For these three units, the value assigned to specifiedhead cells is based on Upper Floridan aquifer head estimated 

from potentiometric-surface maps. 

The bottom boundary of the model is no-flow, whereas 

the top boundary is set as a head-dependent flux (or general 

head) boundary condition, with a controlling specified head 

and a conductance term that regulates the flux into the top 

layer of the model. The controlling head is the water-table 

altitude in the onshore area, and the freshwater equivalent of 

the saltwater head in the offshore area. In the onshore area, 

the conductance is set to limit the amount of recharge entering the system in any given grid cell to less-than-maximum 

estimated recharge from baseflow estimates (Priest, 2004). 

For the purpose of simplification and because little is known 

about hydraulic properties in the offshore area, the conductance imposed in the offshore area is large, posing minimal 

resistance to flow in or out of the system. 

Pumping distribution was assigned based on countyaggregate and site-specific data, which were used to estimate 

average annual pumpage for 1980, 1997, and 2000. Pumpage 

is assigned to units 3 (Brunswick aquifer system), 5 (Upper 

Floridan aquifer), and 7 (Lower Floridan aquifer). The sum 

of site-specific and nonsite-specific pumping rates for 1980 

and 2000 were assigned to the model grid cell in which their 

respective assigned locations and aquifers were situated. This 

report focuses on the pumping distribution during 2000, with 

modifications made to pumping rates at Fort Stewart and 

HAAF within the Upper Floridan aquifer. The total estimated 

pumping in the model area during 2000 was about 810 Mgal/d. 

According to water-use estimates (Fanning, 2003), pumping 

from the Upper Floridan aquifer near Fort Stewart and HAAF 

was 0.7 Mgal/d in Evans, 0.7 Mgal/d in Long, 1.6 Mgal/d in 

Bryan, 15.7 Mgal/d in Liberty, and 68.2 Mgal/d in Chatham 

Counties, Ga. The single largest concentration of pumping 

in Georgia was at Jesup, Wayne County, at a rate of about 

60 Mgal/d from the Upper Floridan aquifer during 2000 

(Fanning, 2003).

Ground-Water Flow Model	 	 
EXPLANATION

General-head boundary—Represents water table or equivalent 

 freshwater head that provides flow to underlying confined aquifers. 

 Flow is restricted by a conductance term, which is equivalent to 

 hydraulic conductivity

No-flow boundary

No-flow or fixed-head boundary

Offshore area—Equivalent freshwater head 

 of overlying saltwater column

Onshore area—Water-table head

Aquifer

Confining unit

Onshore Offshore

Layer 3

Layer 2

CONFINED ZONE OF SURFICIAL AQUIFER SYSTEM Layer 1

BRUNSWICK

AQUIFER SYSTEM

UPPER FLORIDAN AQUIFER

LOWER FLORIDAN AQUIFER

Layer 4

Layer 5

Layer 6

Layer 7

Figure 2. Schematic diagram showing model layers and boundary conditions (from Payne and others, 2005).

Simulation of Ground-Water

Pumping Scenarios

The calibrated MODFLOW model (Payne and others, 

2005) was used to simulate the Base Case condition or 

scenario and to provide insight into the potential effects of 

four different pumping scenarios on water-levels in the 

Upper Floridan aquifer. The scenarios were designed to simulate (1) the effect of incremental pumping increases at HAAF 

(Scenarios A and B); and (2) the relative effects of transferring 

pumping from Fort Stewart to HAAF (Scenarios C and D). 

All pumping was from the Upper Floridan aquifer.

The Base Case steady-state scenario represents year 2000 

pumping rates throughout the model area, with the exception that permitted annual average pumping rates for the year 

2005 were used for 26 production wells at Fort Stewart and 

HAAF. Table 1 includes information about well depth, pump 

capacity, and pumping rate used in the Base Case simulation; 

figure 3 shows the spatial distribution of production wells and 

the variation in grid-cell size from the finer mesh near the 

City of Savannah to the coarser mesh on Fort Stewart. Note 

that in some cases, the Base Case pumping rate assigned to 

an individual well may be higher than the rated pump capacity; this results from distributing the total permitted capacity 

among the several wells, which in some cases exceeds the 

rated capacity of an individual well.

The Base Case simulates permitted pumping rates of 

4.5 Mgal/d at Fort Stewart and 1.03 Mgal/d at HAAF, which 

is substantially higher than the actual pumping rates for 2005 

of 2.3 and 0.8 Mgal/d, respectively. Simulated pumping at 

Fort Stewart was evenly distributed to the several wells 

(0.3 Mgal/d) with the exception of wells FS11, FS12, and 

FS13, which were held within a range of 0.11 to 0.12 Mgal/d 

owing to their lower pumping capacity (table 1). The simulated pumping distribution among the nine wells on HAAF 

was relatively even ranging from 0.1 to 0.12 Mgal/d.

The Base Case was used as a basis for comparison of 

simulated heads from each of the four pumping scenarios. 

The simulated Base Case potentiometric-surface map of the 

Upper Floridan aquifer (fig. 4) indicates HAAF is within the 

regional cone of depression created by ground-water pump-

	 	 Simulation of Selected Ground-Water Pumping Scenarios
age in the Chatham County area. The principal direction of 

ground-water flow in the study area is toward the cone of 

depression. The simulated water-level changes, presented for 

each scenario, represent additional drawdown within the area 

influenced by the regional cone of depression (fig. 4). None 

of the scenarios resulted in large changes in the configuration 

of the simulated potentiometric surface and related groundwater flow directions.

Scenario A represents a 1-Mgal/d increase in pumping 

rate at HAAF that was distributed evenly among the nine productions wells located at HAAF (table 2, fig. 5). The simulated 

water-level changes indicate an area of influence that extends 

into parts of Effingham, Bryan, and Chatham Counties, Ga., 

and Jasper County, S.C. In Chatham County, the water-level 

changes range from approximately –0.5 to –2.1 ft and the 

–1.5-ft line of equal simulated water-level change approximates the boundaries at HAAF (table 2 and fig. 5).

Scenario B represents a 2-Mgal/d increase in pumping rate 

at HAAF that was distributed evenly among the nine production 

wells located at HAAF (table 2, fig. 6). The simulated waterlevel changes are wider in aerial extent than in Scenario A, with 

approximately double the drawdown. For example, the –0.5-ft 

line of equal simulated water-level change in Scenario A (fig. 4) 

has been replaced by the –1-ft line in Scenario B (fig. 6). In 

Chatham County, the water-level changes range from approximately –0.9 to –4.1 ft and the –3-ft line of equal simulated 

water-level change approximates the boundaries at HAAF.

Scenario C represents a 1-Mgal/d increase of pumping 

at HAAF and a decrease of 1 Mgal/d at Fort Stewart (table 2, 

fig. 7). The 1-Mgal/d increase was distributed evenly among 

the nine production wells located at HAAF and the 1-Mgal/d 

decrease was subtracted evenly among the 17 production wells 

located at Fort Stewart. The maximum drawdown in Chatham 

County for Scenario C ranged from 0.3 to 1.8 ft. Reducing 

pumping at Fort Stewart resulted in a water-level recovery 

exceeding 0.2 ft across the base (maximum 0.4 ft), and helped 

reduce the area influenced by increased pumping at HAAF. 

For example, the area in which simulated declines exceeded 

0.5 ft decreased appreciably when compared with Scenario A 

(fig. 5), which involved a 1-Mgal/d pumping increase at 

HAAF without an accompanying reduction at Fort Stewart. 

Although the 1-Mgal/d reduction resulted in some recovery 

at Fort Stewart, the recovery is lower in magnitude than the 

amount of drawdown resulting from the same amount of pumping at HAAF (Scenario A, fig. 3). The smaller recovery could 

be attributed to wider spacing of pumping wells at Fort Stewart, 

to a coarser model grid size, or to the relatively higher hydraulic conductivity of the Upper Floridan aquifer near the base.

Table 1. Selected well data for production wells and

2005 permitted pumping rates (Base Case) at Fort Stewart

and Hunter Army Airfield, Georgia.

[ft, foot; gal/min, gallon per minute; Mgal/d, million gallons per day; 

—, no data. In some cases, the Base Case pumping rate assigned to an 

individual well may be higher than the rated pump capacity; this results 

from distributing the total permitted capacity among the wells, which in 

some cases exceeds the rated capacity of an individual well]

Well

number

Well depth

below land

surface (ft)

Pump

capacity

(gal/min)

Base Case assigned

pumping rate

(Mgal/day)

Fort Stewart

FS1 816 2,100 0.3

FS2 808 1,400 0.3

FS3 750 1,400 0.3

FS4 802 1,600 0.3

FS5 779 1,100 0.3

FS6 472 500 0.3

FS7 508 500 0.3

FS8 706 400 0.3

FS9 560 175 0.3

FS10 600 190 0.3

FS11 500 75 0.1

FS12 605 80 0.1

FS13 605 80 0.1

FS14 705 535 0.3

FS15 — — 0.3

FS16 — — 0.3

FS17 — 300 0.3

Total 4.5

Hunter Army Airfield

HAA1 504 1,300 0.1

HAA2 555 1,300 0.1

HAA3 370 30 0.1

HAA4 360 80 0.1

HAA5 380 30 0.1

HAA6 450 70 0.1

HAA7 375 80 0.1

HAA8 623 1,000 0.1

HAA9 — 180 0.1

Total 1.0

Simulation of Ground-Water Pumping Scenarios	 	 
16

95

95

JASPER

WAYNE

LONG

BULLOCH

BRYAN

LIBERTY

SCREVEN

HAMPTON

EFFINGHAM

MCINTOSH

CHATHAM

JENKINS

TATTNALL

GLYNN

EVANS

BEAUFORT

CANDLER

COLLETON

BRANTLEY

PIERCE

ALLENDALE

SOUTH CAROLINA

GEORGIA

FS8 FS9

FS17

FS16

FS11

FS4

FS2 FS1

FS3

FS5

FS15

FS10 FS13

FS12

FS14

HAA6

HAA9 HAA8

HAA7

HAA1

HAA2

HAA5 HAA3

HAA4

FS6A

FS6B

Savannah

Hinesville

Richmond Hill

Jesup

0

Base from U.S. Geological Survey 1:100,000-scale digital data

5

N

10 15

0 5 10 15 20 KILOMETERS

20 MILES

FORT STEWART

HUNTER ARMY

AIRFIELD

EXPLANATION

Well and number

Pumping rates per grid cell— 

 In million gallons per day,

 during 2000, layer 5 

 (Upper Floridan aquifer)

0 to 0.5

0.5 to 1

1 to 5

5 to 10

10 to 51

FS14

Figure 3. Well locations and model grid orientation in the vicinity of Fort Stewart and Hunter Army Airfield, Georgia

(see table 1); and simulated ground-water pumpage for the Upper Floridan aquifer, 2000 (from Payne and others, 2005).

	 	 Simulation of Selected Ground-Water Pumping Scenarios
Figure 4. Simulated potentiometric surface of the Upper Floridan aquifer during 2000 using Base Case pumping

rates for Fort Stewart and Hunter Army Airfield, Georgia (see table 2).

16

95

95

JASPER

WAYNE

LONG

BULLOCH

BRYAN

LIBERTY

SCREVEN

HAMPTON

EFFINGHAM

MCINTOSH

CHATHAM

JENKINS

TATTNALL

GLYNN

EVANS

BEAUFORT

CANDLER

COLLETON

BRANTLEY

PIERCE

ALLENDALE

SOUTH CAROLINA

GEORGIA

l

l l l l l l l l l l l

l

l

l

l

l l l l l l

l l l l l l l l l l l l

l

l l l l l l l l

l l l l l l l l l

l

l

l

l l l l

l l l l l l l l l l l

l

l l l l

l l l l l l l

l l

l l l l l l l l l

l l

l

l

l

l l l l l

l

l l l

l l l l Savannah

Hinesville

Richmond 

 Hill

Jesup 0

0

10

40

20

60

80

–10

100

120

–20

140

–40

160

–60

–80

–100

0

–10

10

20

Base from U.S. Geological Survey 0

1:100,000-scale digital data

5

N

10 15

0 5 10 15 20 KILOMETERS

20 MILES

FORT

STEWART

HUNTER 

ARMY

AIRFIELD

EXPLANATION

Well

Simulated potentiometric 

 contour—Shows simulated 

 potentiometric surface during 

 2000. Hachures indicate 

 depression. Contour 

 intervals 10 and 20 feet.

 Datum is NAVD 88

10

Simulation of Ground-Water Pumping Scenarios	 	 
Table 2. Simulated pumpage at Fort Stewart and Hunter Army Airfield, Georgia, and maximum water-level changes in layer 5

of the ground-water flow model (Upper Floridan aquifer).

[Mgal/d, million gallons per day; –, water-level decline]

Designated

area

1

2000 

pumpage

(Mgal/d)

Base

Case 2005 

permitted

pumpage

(Mgal/d)

Scenario

A B C D

Pumpage

(Mgal/d)

Maximum

water-level

change

(feet)

Pumpage

(Mgal/d)

Maximum

water-level

change

(feet)

Pumpage

(Mgal/d)

Maximum

water-level

change

(feet)

Pumpage

(Mgal/d)

Maximum

water-level

change

(feet)

Fort Stewart 2.3 4.5 4.5 –0.7 4.5 –1.4 3.5 –0.4 to 0.4 2.5 –0.8 to 0.8

Hunter Army 

Airfield 0.3 1.0 2.0 – 2.1 3.0 –4.1 2.0 –1.8 3.0 –3.7

  Total 2.6 5.5 6.5 7.5 5.5 5.5

1

Payne and others (2005).

Scenario D represents a 2-Mgal/d increase in pumping 

at HAAF and a decrease of 2-Mgal/d at Fort Stewart (table 2, 

fig. 8). The 2-Mgal/d increase was distributed evenly among 

the nine productions wells located at HAAF and the 2-Mgal/d 

decrease was subtracted evenly among the 17 production wells 

located at Fort Stewart. The maximum drawdown in Chatham 

County for Scenario D ranged from 0.6 to 3.7 ft. As was the 

case for Scenario C, reducing pumping at Fort Stewart by an 

amount equivalent to pumping increases at HAAF, reduced 

the extent of water-level decline resulting from the HAAF 

increase (fig. 8). For example, the area in which simulated 

declines exceeded 0.5 ft decreased appreciably when compared 

with Scenario B (fig. 6), which involved a 2-Mgal/d pumping 

increase at HAAF without an accompanying reduction at Fort 

Stewart. The 2-Mgal/d reduction at Fort Stewart reduced the 

extent of drawdown resulting from the HAAF increase, the 

reduction resulted in twice the recovery at Fort Stewart than in 

Scenario C, which involved one-half the pumping rate. 

Model Limitations

The four steady-state scenarios presented in this report 

are believed to reasonably depict changes in ground-water 

levels resulting from pumping changes ranging from 1 to 

2 Mgal/d at Fort Stewart and HAAF, but results are limited 

by model assumptions and design. Model results must be 

interpreted in light of uncertainties and approximations 

inherent in the formulation of the model. 

This ground-water flow model used in this study is 

subject to the limitations described by Payne and others (2005). 

These limitations include error and uncertainty in field measurements of water level and in estimates of pumping; limitations of the conceptual models; approximations made in representing the physical properties of the flow system and errors 

inherent in estimating the spatial distribution of these properties; approximations made in the formulation and application 

of model boundary and initial conditions; errors associated 

with numerical approximation and solution of the mathematical 

model of the flow system; and assumptions made in using the 

models to predict the future behavior of the flow system. 

Simulated water-level changes may be influenced by 

the difference in mesh resolution at the two sites. The larger 

mesh size at Fort Stewart will result in a more generalized 

water-level response that is averaged across the larger cell 

area. Water-level changes at HAAF will have higher resolution because of the finer mesh. It is possible that the model 

was more responsive to pumping near HAAF as a result of the 

finer grid-cell size, which was intended to simulate the steep 

cone of depression located near Savannah, Ga. The increase 

in pumpage for these four scenarios is less than 1 percent of 

the total withdrawals for the 2000 calibrated model (Payne 

and others, 2005). Additionally, the location of the pumpage is 

near the center of the calibrated model.

	 	 Simulation of Selected Ground-Water Pumping Scenarios
–1

–1.5

–0.5

Savannah

Hinesville

Jesup

Richmond 

 Hill

JASPER

WAYNE

LONG

BULLOCH

BRYAN

LIBERTY

SCREVEN

HAMPTON

EFFINGHAM

MCINTOSH

CHATHAM

JENKINS

TATTNALL

GLYNN

EVANS

BEAUFORT

CANDLER

COLLETON

BRANTLEY

PIERCE

ALLENDALE

SOUTH CAROLINA

GEORGIA

16

95

95

Base from U.S. Geological Survey 0

1:100,000-scale digital data

5

N

10 15

0 5 10 15 20 KILOMETERS

20 MILES

FORT

STEWART

HUNTER 

ARMY

AIRFIELD

EXPLANATION

Well

–1

–0.5 to –1

–1 to –1.5

–1.5 to –2.1

Line of equal simulated 

 water-level change—

 Interval, in feet, is variable

Simulated water-level 

 change, in feet

Figure 5. Simulated water-level change between Base Case conditions (2005 permitted pumping for Fort Stewart and

Hunter Army Airfield, Georgia) and Scenario A (increase pumping by 1 million gallons per day at Hunter Army Airfield).

Model Limitations	 	 
–2

–3

–1

–0.5

JASPER

WAYNE

LONG

BULLOCH

BRYAN

LIBERTY

SCREVEN

HAMPTON

EFFINGHAM

MCINTOSH

CHATHAM

JENKINS

TATTNALL

GLYNN

EVANS

BEAUFORT

CANDLER

COLLETON

BRANTLEY

PIERCE

ALLENDALE

SOUTH CAROLINA

GEORGIA

16

95

95

Savannah

Hinesville

Richmond 

 Hill

Jesup

Base from U.S. Geological Survey 0

1:100,000-scale digital data

5

N

10 15

0 5 10 15 20 KILOMETERS

20 MILES

FORT

STEWART

HUNTER 

ARMY

AIRFIELD

–1 to –2

–2 to –3

–3 to –4.1

–0.5 to –1

EXPLANATION

Well

–1 Line of equal simulated 

 water-level change—

 Interval, in feet, is variable

Simulated water-level 

 change, in feet

Figure 6. Simulated water-level change between Base Case conditions (2005 permitted pumping for Fort Stewart and

Hunter Army Airfield, Georgia) and Scenario B (increase pumping by 2 million gallons per day at Hunter Army Airfield).

10	 	 Simulation of Selected Ground-Water Pumping Scenarios
Figure 7. Simulated water-level change between Base Case conditions (2005 permitted pumping for Fort Stewart

and Hunter Army Airfield, Georgia) and Scenario C (decrease pumping by 1 million gallons per day at Fort Stewart

and increase pumping by 1 million gallons per day at Hunter Army Airfield).

–1

–0.5

–1.5

0

0.25

JASPER

WAYNE

LONG

BULLOCH

BRYAN

LIBERTY

SCREVEN

HAMPTON

EFFINGHAM

MCINTOSH

CHATHAM

JENKINS

TATTNALL

GLYNN

EVANS

BEAUFORT

CANDLER

COLLETON

BRANTLEY

PIERCE

ALLENDALE

SOUTH CAROLINA

GEORGIA

16

95

95

Savannah

Hinesville

Richmond 

 Hill

Jesup

Base from U.S. Geological Survey 0

1:100,000-scale digital data

5

N

10 15

0 5 10 15 20 KILOMETERS

20 MILES

FORT

STEWART

HUNTER 

ARMY

AIRFIELD

EXPLANATION

Well

–1

0.4 to 0.25

0.25 to –0.5

Line of equal simulated 

 water-level change—

 Interval, in feet, is variable

Simulated water-level 

 change, in feet

–0.5 to –1

–1 to –1.5

–1.5 to –1.8

Model Limitations	 	 11
–1.0

–2.0

–0.5

0.25

0.50

–3.0

JASPER

WAYNE

LONG

BULLOCH

BRYAN

LIBERTY

SCREVEN

HAMPTON

EFFINGHAM

MCINTOSH

CHATHAM

JENKINS

TATTNALL

GLYNN

EVANS

BEAUFORT

CANDLER

COLLETON

BRANTLEY

PIERCE

ALLENDALE

SOUTH CAROLINA

GEORGIA

16

95

95

Savannah

Hinesville

Richmond 

 Hill

Jesup

Base from U.S. Geological Survey 0

1:100,000-scale digital data

5

N

10 15

0 5 10 15 20 KILOMETERS

20 MILES

FORT

STEWART

HUNTER 

ARMY

AIRFIELD

Well

EXPLANATION

–1 Line of equal simulated water-level 

 change—Interval, in feet, is variable

Simulated water-level change, in feet

0.25 to 0

0.8 to 0.25

0 to –0.5

–0.5 to –1

–1 to –2

–2 to –3

–3 to –3.7

Figure 8. Simulated water-level change between Base Case conditions (2005 permitted pumping for Fort Stewart

and Hunter Army Airfield, Georgia) and Scenario D (decrease pumping by 2 million gallons per day at Fort Stewart

and increase pumping by 2 million gallons per day at Hunter Army Airfield).

12   Simulation of Selected Ground-Water Pumping Scenarios
References Cited

Fanning, J.L., 2003, Water use in Georgia by county for 2000 

and water-use trends for 1980–2000: Georgia Geologic 

Survey Information Circular 106, 176 p., accessed January 

18, 2006, at http://ga.water.usgs.gov/pubs/other/ggs-ic106/

Georgia Environmental Protection Division, 1997, Interim 

strategy for managing saltwater intrusion in the Upper 

Floridan aquifer of southeast Georgia, April 23, 1997: 

Atlanta, Ga., Georgia Environmental Protection Division, 

19 p.

Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, 

M.G., 2000, MODFLOW-2000, The U.S. Geological Sur-

vey modular ground-water model—User guide to modu-

larization concepts and the ground-water flow process: 

U.S. Geological Survey Open-File Report 00-92, 121 p., 

accessed January 18, 2006, at http://pubs.er.usgs.gov/

usgspubs/ofr/ofr0092

Payne, D.F., Abu Rumman, Malek, and Clarke, J.S., 2005, 

Simulation of ground-water flow in coastal Georgia and 

adjacent parts of South Carolina and Florida—Predevelop-

ment, 1980, and 2000: U.S. Geological Survey Scientific 

Investigations Report 2005-5089, 91 p., accessed January 

18, 2006, at http://pubs.usgs.gov/sir/2005/5089/

Priest, Sherlyn, 2004, Stream-aquifer relations in the coastal 

area of Georgia and adjacent parts of Florida and South 

Carolina: Georgia Geologic Information Circular 108, 40 p.

References Cited	 	13
Manuscript approved for publication, May 16, 2006

Prepared by USGS Georgia Water Science Center

Edited by Patricia L. Nobles

Graphics by Caryl J. Wipperfurth

For more information concerning the research in this report, contact

USGS Georgia Water Science Center, Atlanta

Telephone: 770-903-9100