U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2015–5072

Prepared in cooperation with the City of Rincon, Georgia

Simulated Effects of Lower Floridan Aquifer 

Pumping on the Upper Floridan Aquifer at 

Rincon, Effingham County, Georgia
Cover. Thunderstorm, over the Brunswick River Marsh, from Jekyll Island Causeway, Glynn County, Georgia (photograph by 

Alan M. Cressler, USGS).
Simulated Effects of Lower Floridan Aquifer 

Pumping on the Upper Floridan Aquifer at 

Rincon, Effingham County, Georgia

By Gregory S. Cherry and John S. Clarke

Prepared in cooperation with the City of Rincon, Georgia

Scientific Investigations Report 2015–5072

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Cherry, G.S., and Clarke, J.S., 2015, Simulated effects of Lower Floridan aquifer pumping on the Upper Floridan 

aquifer at Rincon, Effingham County, Georgia: U.S. Geological Survey Scientific Investigations Report 2015–5072, 

35 p., http://dx.doi.org/10.3133/sir20155072.

ISSN 2328–0328 (online)
iii

Contents

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

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

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

Previous Studies ...................................................................................................................................3

Site Description.....................................................................................................................................3

Water Use .....................................................................................................................................4

Hydrogeologic Setting ................................................................................................................4

Groundwater Flow .......................................................................................................................7

Well Identification........................................................................................................................8

Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer......................8

Observed Water-Level Response.......................................................................................................8

Model Simulation..................................................................................................................................8

Scenario A—Interaquifer Leakage and Drawdown Response .........................................10

Scenario B—Upper Floridan Aquifer Drawdown Offset.....................................................14

Scenario C—Effects of Pumping Offsets on Groundwater Levels at Rincon, Ga...........17

Limitations of Analysis .......................................................................................................................17

Summary........................................................................................................................................................22

Selected References...................................................................................................................................22

Appendix 1. Regional Groundwater Model..........................................................................................25

Revisions to Regional Model ............................................................................................................25

Refinement of Grid Resolution.................................................................................................28

Refinement of Hydraulic Conductivity Distribution ..............................................................28

Simulation of Observed Drawdown in Pumped Well...........................................................32

Comparison of Revised to Original Regional Model .....................................................................32
iv

Figures

1. Maps showing location of Rincon test site (well 36S048), observation well (36S047),

and production wells (34S034 and 34S035) south of Rincon, Georgia.................................2

2. Generalized correlation of geologic and hydrogeologic units and model layers

in the Coastal Plain of Georgia ...................................................................................................5

3. Borehole geophysical logs from test well 36S048 in the Lower Floridan aquifer

at Rincon, Effingham County, Georgia.......................................................................................6

4. Schematic diagram showing conceptual model of predevelopment, and 

modern-day (2000) flow system..................................................................................................7

5. Maps showing simulated steady-state drawdown in the Lower Floridan aquifer

(LFA) for scenario A—pumping LFA well 36S048 at 1,000 gallons per minute, 

Rincon and vicinity, Georgia .....................................................................................................11

6. Maps showing simulated steady-state drawdown in the Upper Floridan aquifer

for scenario A—pumping Lower Floridan aquifer well 36S048 at 1,000 gallons

per minute, Rincon and vicinity, Georgia................................................................................12

7. Maps showing simulated steady-state drawdown in the Upper Floridan aquifer

(UFA) for scenario B—pumping UFA well 36S047 at 292 gallons per minute,

Rincon and vicinity, Georgia .....................................................................................................15

8. Maps showing simulated steady-state drawdown in the Lower Floridan aquifer

(LFA) for scenario C—pumping LFA well 36S048 at 1,000 gallons per minute 

(gal/min) and reducing pumping at Upper Floridan aquifer wells 36S034 and

36S035 by 292 gal/min, Rincon and vicinity, Georgia ............................................................18

9. Maps showing simulated steady-state drawdown in the Upper Floridan aquifer (UFA)

for scenario C—pumping Lower Floridan aquifer well 36S048 at 1,000 gallons

per minute (gal/min) and reducing pumping at UFA wells 36S034 and 36S035

by 292 gal/min, Rincon and vicinity, Georgia..........................................................................19

10. Maps showing simulated Upper Floridan aquifer potentiometric surfaces for

Base Case (year 2000) and scenarios A and C, Rincon and vicinity, Georgia..................20

1–1. Maps showing location of selected wells, regional groundwater model and

boundary conditions, and revised model grid, City of Rincon, Fort Stewart,

and vicinity, Georgia...................................................................................................................26

1–2. Schematic diagram showing model layers and boundary conditions ..............................28

1–3. Maps showing simulated hydraulic property zones by model layer .................................29

1– 4. Maps showing difference between simulated and observed water levels by

model layer for 2000, revised regional flow model for Brunswick aquifer system 

(layer 3), Upper Floridan aquifer (layer 5), and Lower Floridan aquifer (layer 7) .............34

1– 5. Observed and simulated water levels in model layers 3, 5, and 7, revised

groundwater model ....................................................................................................................35
v

Tables

1. Summary of properties of the Lower Floridan aquifer in coastal Georgia..........................5

2. Description of model scenarios and simulated drawdown in the Upper and 

Lower Floridan aquifers for various pumping distributions at Rincon, Georgia.................9

3. Simulated steady-state water budgets for base case (year 2000) and for 

scenario A, after pumping 1,000 gallons per minute at Lower Floridan aquifer

well 36S048, Rincon, Georgia....................................................................................................13

4. Simulated steady-state water budgets for base case (year 2000) and for

scenario B, after pumping 292 gallons per minute at Upper Floridan aquifer

well 36S047, Rincon, Georgia....................................................................................................16

5. Simulated steady-state water budgets for base case (year 2000) and for

scenario C, after pumping 1,000 gallons per minute at Lower Floridan aquifer

well 36S048 and reducing pumping at Upper Floridan aquifer wells 36S034 

and 36S035 by 292 gallons per minute, Rincon, Georgia......................................................21

1–1. Horizontal and vertical hydraulic conductivity values assigned to hydraulicproperty zones for the original and revised groundwater-flow model..............................31

1–2. Water-level calibration statistics for the original and revised regional models, 

base case (year 2000) simulation.............................................................................................33

1–3. Comparison of simulated water budget by model layer between the original 

and revised regional models, base case (year 2000) simulation........................................33
vi

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square foot (ft2) 929.0 square centimeter (cm2)

square foot (ft2) 0.0929 square meter (m2)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

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

Flow rate

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

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)

Specific capacity

gallon per minute per foot 

[(gal/min)/ft)]

 0.207 liter per second per meter 

[(L/s)/m]

Hydraulic conductivity

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

Transmissivity*

foot squared per day (ft2/d) 0.0929 meter squared per day (m2/d) 

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

°C=(°F –32)/ 1.8
vii

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

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

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

Supplemental Information

*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.

Abbreviations

FAS Floridan aquifer system

GaEPD Georgia Environmental Protection Division

Kh horizontal hydraulic conductivity

Kv vertical hydraulic conductivity

LFA Lower Floridan aquifer

LFCU Lower Floridan confining unit

RMS root mean square

UFA Upper Floridan aquifer

USGS U.S. Geological Survey
viii

Acknowledgments

The authors thank Tim Baumgartner of EMC Engineering Services and Wesley Corbitt of the 

City of Rincon for helpful support throughout the study. Dr. Jim Kennedy, State Geologist, 

Georgia Environmental Protection Division (GaEPD), provided valuable feedback and comments 

during the review process. 

Lynn J. Torak, Gerard J. Gonthier, and Bruce G. Campbell, U.S. Geological Survey (USGS), 

provided valuable technical reviews and suggestions to improve this report. 
Simulated Effects of Lower Floridan Aquifer 

Pumping on the Upper Floridan Aquifer at 

Rincon, Effingham County, Georgia

By Gregory S. Cherry and John S. Clarke

Abstract

Steady-state simulations using a revised regional 

groundwater-flow model based on MODFLOW were run to 

assess the potential long-term effects on the Upper Floridan 

aquifer (UFA) of pumping the Lower Floridan aquifer (LFA) 

at well (36S048) near the City of Rincon in coastal Georgia 

near Savannah. Simulated pumping of well 36S048 at a rate 

of 1,000 gallons per minute (gal/min; or 1.44 million gallons 

per day [Mgal/d]) indicated a maximum drawdown of about 

6.8 feet (ft) in the UFA directly above the pumped well and at 

least 1 ft of drawdown within a nearly 400-square-mile area 

(scenario A). Induced vertical leakage from the UFA provided 

about 99 percent of the water to the pumped well. Simulated 

pumping of well 36S048 indicated increased downward 

leakage in all layers above the LFA, decreased upward leakage 

in all layers above the LFA, increased inflow to and decreased 

outflow from lateral specified-head boundaries in the UFA and 

LFA, and an increase in the volume of induced inflow from 

the general-head boundary representing outcrop units. Water 

budgets for scenario A indicated that changes in inflows and 

outflows through general-head boundaries would compose 

about 72 percent of the simulated pumpage from well 36S048, 

with the remaining 28 percent of the pumped water derived 

from flow across lateral specified-head boundaries. 

Additional steady-state simulations were run to evaluate a 

pumping rate in the UFA of 292 gal/min (0.42 Mgal/d), which 

would produce the equivalent maximum drawdown in the UFA as 

pumping from well 36S048 in the LFA at a rate of 1,000 gal/min 

(called the drawdown offset; scenario B). Simulated pumping 

in the UFA for the drawdown offset produced about 6.7 ft 

of drawdown, comparable to 6.8 ft of drawdown in the UFA 

simulated in scenario A. Water budgets for scenario B also 

provided favorable comparisons with scenario A, indicating that 

69 percent of the drawdown-offset pumpage (0.42 Mgal/d) in 

the UFA originates as increased inflow and decreased outflow 

across general-head boundaries from overlying units in the 

surficial and Brunswick aquifer systems and that the remaining 

simulated pumpage originates as flow across general- and 

specified-head boundaries within the UFA.

A steady-state simulation representing implementation of drawdown-offset-pumping reductions totaling 

292 gal/min at Rincon UFA production wells 36S034 and 

36S035 and pumping from the new LFA well 36S048 at 

1,000 gal/min (scenario C) resulted in decreased magnitude 

and areal extent of drawdown in the UFA compared with 

scenario A. In the latter scenario, the LFA well was pumped 

without UFA drawdown-offset-pumping reductions. Water 

budgets for scenario C yielded percentage contributions 

from flow components that were consistent with those from 

scenario B. Specifically, 69 percent of the increased pumping 

in scenario C originated from general-head boundaries from 

overlying units of the surficial and Brunswick aquifer systems 

and the balance of flow was derived from general- and 

specified-head boundaries in the UFA. In all scenarios, the 

placement of model boundaries and type of boundary exerted 

the greatest control on overall groundwater flow and interaquifer leakage in the system. 

Introduction

Increased pumping demands on limited freshwater 

resources available to the City of Rincon, Effingham County, 

Georgia (Ga.; fig. 1), primarily from the Upper Floridan 

aquifer (UFA), pose a threat of possible saltwater intrusion 

and lowering of groundwater levels (drawdown) in the area. 

To alleviate the potential for saltwater intrusion in coastal 

Georgia, the Georgia Environmental Protection Division 

(GaEPD) has restricted further development of the UFA in 

adjacent Chatham County and part of Effingham County 

and encouraged development of alternative water sources 

(Georgia Department of Natural Resources, 2006). On 

May 20, 2013, the GaEPD issued a policy statement with 

the determination that the Floridan aquifer system (FAS) “is 

really one aquifer with hydraulically connected upper and 

lower permeable zones” (Dr. Jim Kennedy, State Geologist, 

Georgia Environmental Protection Division, written commun., 

June 12, 2013).
2 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

BEAUFORT

BEAUFORT

JASPER

HAMPTON

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

CHATHAM

LONG

EVANS

CANDLER

TATTNALL

SCREVEN

GA

SC

N

Rincon

Base map from U.S. Geological Survey 

1:2,000,000-scale digital data

 Fort Stewart

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

ATLANTIC OCEAN

Hunter Army Airfield

Area

of map

above

FLORIDA

GEORGIA

SOUTH

CAROLINA

PLAIN

COASTAL

Southeast

Georgia

Embayment

EXPLANATION

24-county coastal Georgia area

 as identified by the Georgia

 Environmental Protection Division 

Potentiometric contour—Shows 

 altitude at which water level would

 have stood in tightly cased wells,

 May 1–26, 1998. Contour intervals

 10 and 50 feet. Hachures indicate

 depression. Datum is NAVD 88.

 (Modified from Peck and others, 1999)

Well and identification number

0

36S048

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 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 l l l

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

 Hilton

 Head

Island

Skidaway

Island

Savannah

Garden

City

Port Wentworth

Hinesville

Richmond

Hill

Pooler

Berwick

Area of map

below right

36Q392

35Q069

36Q330

36S047

36S048

36S034

36S035

36S047

Savannah

River

40

30

20

10

0

–10

–10

–30

–20

–40

–50

–60

–70

–80

–90

50

100

150

Figure 1. Location of Rincon test site (well 36S048), observation well (36S047), and production wells (34S034 and 34S035) 

south of Rincon, Georgia.

0 1 2 3 MILES

0 1 2 3 KILOMETERS

Figure 1. Location of Rincon test site (well 36S048), observation well (36S047), and production wells (34S034 and 34S035) 

south of Rincon, Georgia.
Introduction 3

To assess the water-supply development potential of 

the Lower Floridan aquifer (LFA) at Rincon, Ga., the U.S. 

Geological Survey (USGS), in cooperation with the City of 

Rincon, performed steady-state model simulations during 

2014 to determine the potential for long-term interaquifer 

leakage from the UFA into the LFA caused by pumping 

well 36S048 in the LFA. 

Purpose and Scope

This report documents results of steady-state groundwater-model simulations at Rincon, Ga., completed during 

2014 to assess the effect of LFA pumping on the UFA. The 

specific objectives were to 

• evaluate downward leakage response in a nearby 

well completed in the UFA to pumping from a 

proposed new well completed in the LFA;

• quantify the reduction in pumping in the UFA that 

would offset drawdown (called drawdown offset) 

in the UFA caused by pumping well 36S048 in the 

LFA; and

• identify changes in vertical leakage and lateral flow 

in the UFA and LFA resulting from implementing the 

proposed new pumping in the LFA and drawdownoffset pumping reductions in the UFA.

The scope of the current investigation at Rincon, Ga., 

emphasizes model simulations because field-data collection 

was completed as part of a previous investigation by Carter 

and Sloope (2004). The focus of this report is to analyze 

simulated changes in groundwater levels (drawdown) 

and interaquifer leakage on the UFA and LFA resulting 

from pumping well 36S048 in the LFA and implementing 

drawdown-offset pumpage reductions in the UFA. This 

report contains maps showing drawdown in response to 

pumping, and tables that list water-budget components of 

interaquifer and boundary flows and compare simulation 

results of the revised model to the original model of Payne 

and others (2005).

Previous Studies

Carter and Sloope (2004) completed detailed field 

studies at Rincon that included construction of well 36S048, 

collection of geophysical and flowmeter logs (by USGS), 

water sampling, and completion of an 8-hour UFA test and 

72-hour LFA test. Their report included some preliminary 

groundwater model simulations. A similar evaluation was 

completed at Berwick in western Chatham County by Jordan, 

Jones, and Goulding (2002).

The USGS completed several comprehensive field and 

modeling studies of the FAS in coastal Georgia. The simulated 

area is coincident with that used for a regional steady-state 

model developed by Payne and others (2005) that encompassed 42,155 square miles (mi2

) in Georgia and adjacent 

areas of South Carolina and Florida (fig. 1–1). Small-area 

studies at nearby Hunter Army Airfield (fig. 1; Clarke and 

others, 2010; Williams, 2010); Fort Stewart (Clarke and 

others, 2011; Gonthier, 2011), and Pooler, Ga. (Cherry and 

Clarke, 2013; Gonthier, 2012) involved field testing and 

groundwater-model simulations that evaluated the LFA as a 

possible alternative water source. Results of these studies, 

however, indicate a strong interaquifer connection between 

the UFA and LFA in the Chatham County area. These studies 

refined existing knowledge concerning the hydrogeology and 

water quality of the FAS in the Coastal Plain physiographic 

province and provided essential data and simulation results 

needed to assess the effect of LFA pumping on groundwater 

flow and water levels in the UFA. 

A revised hydrogeologic framework for the FAS was 

developed by Williams and Gill (2010) for eight northern 

coastal counties in Georgia and five coastal counties in 

South Carolina, including the area of Rincon, Ga. In this 

area, borehole geophysical and flowmeter log data collected 

during previous investigations were used to shift the position of internal boundaries of the UFA and LFA and of the 

individual permeable zones that compose these aquifers. 

These revised boundaries conform to those used at Rincon 

in the current investigation.

Site Description

Rincon, Ga., is located in the southeastern part of 

Effingham County, about 17 miles (mi) northwest of 

Savannah and 6.5 mi west of the Savannah River (fig. 1). 

During 2011, the City of Rincon had an estimated population 

of 8,865, representing a 100-percent increase since 2000 

(City-data.com, 2015). 

Because of concern about saltwater intrusion near the 

Savannah-Hilton Head Island, South Carolina, area (fig. 1), 

the GaEPD has implemented restrictions on groundwater 

withdrawal from the UFA and designated management 

zones in coastal Georgia. Rincon is located in the GaEPD 

“red zone,” where withdrawals from the UFA are capped 

at 2004 rates. In Chatham County, groundwater use has 

declined from 64.97 million gallons per day (Mgal/d) during 

2005 to 55.89 Mgal/d during 2010 (Fanning and Trent, 

2009; Steven J. Lawrence, U.S. Geological Survey, written 

commun., April 10, 2014).

The site is characterized by flat topography, with sandy 

topsoil that is typical of coastal Georgia. The site is underlain 

by geologic deposits consisting of sand, clay, limestone, and 

dolomite. The LFA well 36S048 is located at an altitude of 

about 70 feet (ft). Here, static (non-pumping) water levels in 

the UFA are at an altitude of about –5 ft, or a depth of 75 ft 

below land surface. 
4 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

Water Use

Groundwater use in nearby Chatham County totaled 

64.97 Mgal/d during 2005 (Fanning and Trent, 2009). 

About half of that, 33.52 Mgal/d, was for public supply. 

Most production wells are within the Savannah city limits; 

additional wells to the northwest serve Pooler, Garden City, 

and Port Wentworth (fig. 1; Fanning and Trent, 2009). Payne 

and others (2005) estimated that during 1980–2000, nearly 

95 percent of groundwater withdrawn from the county was 

pumped from the UFA and the remaining 5 percent was 

obtained from the LFA. Groundwater withdrawal from the 

FAS in Chatham County increased from 79.75 Mgal/d in 

1980 to 85.54 Mgal/d in 1990 and decreased to 68.15 Mgal/d 

in 2000, 64.97 Mgal/d in 2005, and 55.89 Mgal/d in 

2010 (Fanning and Trent, 2009; Steven J. Lawrence, U.S. 

Geological Survey, written commun., April 10, 2014).

The water supply for Rincon is provided by two wells 

(36S034 and 36S035) completed in the UFA located about 

2.5 mi southeast of the test site, with a permitted annual 

withdrawal rate of 0.87 Mgal/d in 2012 (H&K Engineering 

Group, 2013). During 2005, total groundwater use in 

Effingham County was 5.96 Mgal/d, with a withdrawal rate 

of 0.73 Mgal/d from the two production wells (Fanning and 

Trent, 2009). This represents an increase in total groundwater 

use from 5.29 Mgal/d during 2000 with a withdrawal rate of 

0.54 Mgal/d by the city of Rincon (Fanning, 2003).

Hydrogeologic Setting

Effingham County (fig. 1) is underlain by Coastal Plain 

strata consisting of consolidated to unconsolidated layers 

of sand and clay and semiconsolidated to very dense layers 

of limestone and dolomite (Miller, 1986; Clarke and others, 

1990; Williams and Gill, 2010). These sediments constitute 

three major aquifer systems, which are, in descending order, 

the surficial aquifer system, Brunswick aquifer system, and 

FAS. The Brunswick aquifer system near Rincon contains 

material having low permeability with no discernible waterbearing units (fig. 2).

In coastal Georgia, the surficial aquifer system consists 

of Miocene and younger interlayered sand, clay, and thin 

limestone beds (Clarke, 2003). At Rincon, the surficial aquifer 

system consists of fine-grained sands at depths of less than 

50 ft and is largely unconfined, with typical yields ranging 

from 2 to 25 gallons per minute (gal/min; Peck and others, 

2013). Elsewhere in coastal Georgia, the surficial aquifer 

system includes a water-table zone and two confined waterbearing zones; the areal extent of the confined water-bearing 

zones is unknown (Clarke, 2003). The surficial aquifer system 

is separated from the underlying Brunswick aquifer system by 

a confining unit consisting of silty clay and dense, phosphatic 

Miocene limestone.

The Oligocene to Miocene Brunswick aquifer system 

consists of two water-bearing zones—the upper Brunswick 

aquifer and the lower Brunswick aquifer (Clarke, 2003). The 

upper Brunswick aquifer consists of poorly sorted, fine- to 

coarse-grained, slightly phosphatic and dolomitic quartz sand 

and dense, phosphatic limestone (Clarke and others, 1990). 

The lower Brunswick aquifer consists of poorly sorted, 

fine- to coarse-grained, phosphatic and dolomitic Oligocene 

and Miocene sand (Clarke and others, 1990). At Rincon, the 

Brunswick aquifer system consists of clayey, fine-grained 

sand and silt that has much lower permeability than areas 

within the Southeast Georgia Embayment and can largely 

be considered a confining unit. For this study, the upper and 

lower Brunswick aquifers are considered a single unit, and the 

combined thickness and composite hydraulic properties are 

used for model simulations. 

The principal source of groundwater for all uses in 

coastal Georgia is the FAS, composed of carbonate rocks of 

varying permeability. In the coastal area, the system has been 

subdivided by the USGS into the UFA and LFA (Miller, 1986; 

Williams and Gill, 2010). 

The FAS is confined by overlying clay layers and is 

separated into several permeable water-bearing zones by 

layers of dense limestone or dolostone that act as semiconfining units that allow vertical leakage of groundwater 

into either the UFA or LFA, depending on the hydraulic 

conductivity and direction of the head gradient. According to 

Williams and Gill (2010), the UFA and LFA boundaries are at 

the following depths at the well 36S048 site:

• Top UFA: 290 ft

• Base UFA: 410 ft

• Top LFA: 565 ft

• Base of LFA: 1,000 ft

A detailed representation of the geophysical and flowmeter characteristics of the FAS at well 36S048 by Williams 

and Gill (2010) indicates five water-bearing zones within the 

FAS (fig. 3), which is similar to findings from an earlier study 

by McCollum and Counts (1964). The analysis of flowmeter 

data indicated zones 1 and 4 produce the majority of water, 

and zones 2, 3, and 5 produce lesser amounts. Zone 1 spans a 

30-ft interval within the UFA and zone 4 is present within the 

uppermost 10 to 15 ft of the LFA. Water production diminishes in the LFA below zone 5 at depths greater than 750 ft.

The LFA is at a shallower depth at Rincon than at the 

other three sites (table 1). Well 36S048 was pumped at 

1,000 gal/min, a higher rate than at the other three sites, 

resulting in larger drawdown in both the UFA and LFA at that 

well than at the other three sites. The observed drawdown 

in the UFA resulting from pumping the LFA was 4.5 ft at 

well 36S048 (Carter and Sloope, 2004) and ranged from 

0.37 to 0.8 ft at the other sites.
Introduction 5

Lower Middle Upper

Post-Miocene

Miocene

Eocene

Oligocene

Paleocene

Upper

Cretaceous

Undifferentiated

Suwannee Limestone

Ocala Limestone

Avon Park Formation

Oldsmar Formation

Cedar Keys Formation

Undifferentiated

Water-table zone Upper Floridan aquifer

Floridan aquifer system

Floridan aquifer system

Lower Floridan aquifer

Fernandina 

permeable 

zone

Confining unit Not

modeled

Lower Floridan

confining unit

Lazaretto Creek Formation Upper Floridan confining unit

Lower Coastal Plain3

Geologic unit4 Hydrogeologic unit

Barnwell Group

Undifferentiated

Santee Limestone

Upper Coastal Plain1

Congaree

Formation

Geologic unit

Snapp Formation

Ellenton Formation

(undifferentiated)

Upper 

Three 

Runs

aquifer

Confining

unit

Gordon

aquifer

Hydrogeologic

unit

Steel Creek 

Formation

Black Creek Group

(undifferentiated)

Confining unit 2

Upper Dublin 

aquifer

1Modified from Falls and others, 1997.

2In local areas includes Millers Pond aquifer.

Confining

unit

Upper waterbearing zone

Upper 

Floridan semiconfining unit

Lower waterbearing zone

Lower Middle Upper

Series Stage

Model

layer

Savannah Brunswick

1

GHB

(not

modeled)

2

3

4

5

6

7

Upper

Brunswick

 aquifer

Lower

Brunswick

 aquifer

Upper waterbearing zone

Lower waterbearing zone

Tiger Leap Member

of Parachucla Formation

Parachucla Formation

Marks Head Formation

Coosawhatchie Formation

Ebenezer Formation

Surficial aquifer system Brunswick aquifer system

Confining

unit

Confining

unit

Figure 2. Generalized correlation of geologic and hydrogeologic units and model layers in the Coastal Plain of Georgia 

(modified from Payne and others, 2005). [GHB, general-head boundary]

3Modified from Randolph and others, 1991; Clarke and Krause, 2000.

4Modified from Randolph and others, 1991; Weems and Edwards, 2001.

Figure 2. Generalized correlation of geologic and hydrogeologic units and model layers in the Coastal Plain of 

Georgia (modified from Payne and others, 2005). [GHB, general-head boundary].

Table 1. Summary of properties of the Lower Floridan aquifer in coastal Georgia.

[HAAF, Hunter Army Airfield; LFA, Lower Floridan aquifer; UFA, Upper Floridan aquifer]

Property Rincon

(well 36S048)

Pooler

(well 35Q069)

Berwick

(well 36Q330)

HAAF

(well 36Q392)

LFA depth interval (feet) 565 –1,000 702 –1,040 700–1,080 703 –1,080

LFA test yield (gallons per minute) 1,000 782 718 748

LFA specific capacity (gallons per minute 

per foot of drawdown)

13.8 15 23.1 20.8

Maximum drawdown LFA (feet) 72.3 52 31 36

Maximum drawdown UFA (resulting 

from LFA pumping, in feet)

4.5 0.8 0.37 0.76
6 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

Figure 3. Borehole geophysical logs from test well 36S048 in the Lower Floridan at Rincon, Effingham County, Georgia.

Black bars denote water-bearing zones. Zones 1 through 5 correlate to water-bearing zones previously defined in the

area by McCollum and Counts (1964). [gal/min, gallon per minute; API, American Petroleum Institute; °F, degrees

Fahrenheit; ft, feet; T.D., total depth; LN, long normal resistivity; SN, short normal resistivity]. From Williams and Gill (2010). 

Upper

Floridan 

aquifer

Lower

Floridan 

aquifer

Surficial aquifer

Upper 

confining

unit

Middle

confining 

unit

Geophysical 

Marker D

Hydrogeologic

unit

Base of Floridan aquifer system

Top of Floridan aquifer system

Lower

confining

unit

Geologic

age

Post

Miocene

Miocene

Oligocene

Upper

Eocene

Middle

Eocene

Paleocene?

3

T.D.=1,000

Temperture and 

fluid inflection

at water zones

0 500

SN LN

2

1

4

Gamma, in API

0 100

Caliper, in inches

Land

surface

300

600

700

100

200

400

500

800

900

1,000

5

ZONE

Flowmeter,

in gal/min

0 35

0 1,500

First 

logging

run

Second

logging

run

Flowmeter

test

11/13/2003

1,000 gal/min

Flowmeter test

1/13/2004

433 gal/min

scaled 0 to

1,000 gal/min

Flowmeter test

1/13/2004

433 gal/min

scaled 0 to 

200 gal/min1,000 gal/min

Geophysical Marker C

Bottom of

production

casing

(565 ft)

Major inflow zone

Major

inflow

zone

Resistivity,

in ohm-meters

Depth, in feet below land surface

Small

inflow

zone

Fluid resistivity,

in ohm-meters

0 50

60 90

Fluid temperature, °F “Dead” water in borehole below last water zone 

Figure 3. Borehole geophysical logs from test well 36S048 in the Lower Floridan aquifer at Rincon, Effingham County, 

Georgia. Black bars denote water-bearing zones. Zones 1 through 5 correlate to water-bearing zones previously defined 

in the area by McCollum and Counts (1964). [gal/min, gallon per minute; API, American Petroleum Institute; °F, degrees 

Fahrenheit; ft, foot; T.D., total depth; LN, long normal resistivity; SN, short normal resistivity]. From Williams and Gill (2010).
Introduction 7

Groundwater Flow

Groundwater flow in the FAS is controlled by the rate 

and distribution of recharge to and discharge from the system, 

the extent and effectiveness of confinement, and the ability of 

aquifers to transmit and store water (Krause and Randolph, 

1989). The conceptualized predevelopment (no pumping) 

and modern-day (2000) groundwater-flow systems in coastal 

Georgia (fig. 4) receive water from precipitation and downward leakage through shallow geologic units that recharge the 

aquifers in the northern part of the coastal area where the units 

are exposed at or near land surface. Groundwater then flows 

mostly southeastward toward the coast where it discharges 

into overlying units and surface-water bodies. During the 

1880s prior to development, the flow system was considered to 

be in dynamic equilibrium, with recharge balancing discharge, 

and potentiometric surfaces were considered nearly static 

from year to year with water-level altitudes of 30 to 40 ft at 

Savannah (Krause and Clarke, 2001). In general, predevelopment potentiometric surfaces were higher than current 

groundwater levels, and test wells tapping the FAS flowed at 

the surface along the coast (fig. 4A).

GEORGIA

COASTAL PLAIN

Regional flow system

Regional flow system

?

?

?

?

NOT TO SCALE

Gordon aquifer

Upper Three Runs aquifer

Confining unit

Upper Floridan aquifer

Brunswick aquifer system

Lower Floridan aquifer

Saltwater

?

?

?

?

Surficial aquifer system

Wetlands

Water

table

Flowing

Altamaha well

River

Unsaturated zone

Ocmulgee

River

Little

Ocmulgee

River

A. Predevelopment 

Pumping

well

B. Modern day

Gulf

Trough

NORTHWEST SOUTHEAST

SE

NW

Figure 4. Schematic diagram showing conceptual model

of (A) predevelopment and (B) modern-day (2000) flow

system. Arrows indicate general direction of groundwater

flow (modified from Priest, 2004; Payne and others, 2005).

Figure 4. Schematic diagram showing conceptual model 

of A, predevelopment, and B, modern-day (2000) flow 

system. Arrows indicate general direction of groundwater 

flow (modified from Priest, 2004; Payne and others, 2005).
8 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

The modern-day (2000) flow system reflects changes that 

have occurred as a result of groundwater pumping (fig. 4B). 

Groundwater withdrawals have lowered water levels, induced 

additional recharge from vertical leakage and regional flow, 

reduced natural discharge, and allowed seawater to migrate 

into the UFA in places along the coast. Groundwater pumping 

has caused an extensive cone of depression to develop in 

the potentiometric surface of the UFA in the Savannah area 

(fig. 1). This cone of depression has affected groundwater 

flow near Rincon, as evidenced by the shape of potentiometric 

contours, indicating groundwater flow is toward the center of 

the cone of depression at Savannah.

Saltwater contamination restricts the development of 

groundwater supply in coastal Georgia and adjacent parts 

of South Carolina and Florida (Krause and Clarke, 2001). 

Pumping from the UFA has resulted in (1) substantial groundwater-level decline and subsequent saltwater intrusion into 

the UFA at Brunswick, Ga., from underlying strata containing 

highly saline water and (2) seawater encroachment into the 

UFA at the northern end of Hilton Head Island, S.C. (fig. 1–4). 

Saltwater contamination at these locations has constrained 

further development of the UFA in the coastal area and has 

increased the competition for limited freshwater resources.

Well Identification

In this report, wells are identified by means of a USGS 

numbering system based on the index of USGS topographic 

maps (such as 36S048). In Georgia, each 7-1/2-minute 

topographic quadrangle map has been given a number and 

letter designation beginning at the southwestern corner of 

the State. Numbers increase eastward through 39, and letters 

increase alphabetically northward through Z and then become 

double-letter designations AA through PP. The letters “I” 

and “O” are not used. Wells inventoried in each quadrangle 

are numbered sequentially beginning with 1. For example, 

well 36S048 is the 48th well inventoried in the Rincon 

quadrangle (map 36S).

Simulated Effects of Lower Floridan 

Aquifer Pumping on the Upper 

Floridan Aquifer

Numerical simulation of groundwater flow within the 

study area provides a quantitative estimate of the long-term 

(steady-state) leakage and drawdown response of the UFA 

to pumping from the LFA. Estimates of the amount of UFA 

pumpage that would produce the equivalent drawdown in the 

UFA as that caused by pumping in the LFA can be determined 

through simulation to obtain the drawdown-offset pumping 

rate for the UFA. Aquifer testing at Rincon, Ga., provided 

(1) the basis to simulate drawdown response in nearby UFA 

wells to pumping in the LFA and (2) a dataset of hydrologic 

conditions that were used to estimate hydraulic properties and 

assess simulation accuracy. Drawdown response in the UFA to 

pumping the LFA was evaluated by the simulation of several 

hypothetical pumping scenarios in the UFA and LFA. 

Observed Water-Level Response

LFA well 36S048 was pumped at a rate of 1,000 gal/min 

during a 72-hour period beginning on January 20, 2004. 

During the 72-hour test, 72.3 ft of drawdown was observed in 

pumped well 36S048, and 4.5 ft of drawdown was observed in 

UFA well 36S047. The wells are located within 100 ft of each 

other (Carter and Sloope, 2004) but their screened intervals are 

separated vertically by about 155 ft of fine-grained material 

identified as the Lower Floridan confining unit (LFCU). 

Model simulation was used to determine long-term, steadystate drawdown and leakage effects on the FAS beyond the 

aquifer-test period. Simulations by Payne and others (2005) 

in coastal Georgia indicate that it could take 5 years or more 

to reach a steady-state condition in the Chatham County area. 

Because a 5-year aquifer test period is impractical, model 

simulation provided a practical means to estimate the longterm steady-state drawdown and leakage conditions in the FAS 

resulting from pumping in the LFA.

Model Simulation

A groundwater-flow model was used to simulate requirements described in the GaEPD interim strategy for permitting 

LFA groundwater withdrawals in the 24-county coastal 

Georgia area (fig. 1). These requirements are to (1) quantify 

the aquifer leakage from the UFA to LFA resulting from 

pumping LFA well 36S048 and (2) calculate “the equivalent 

Upper Floridan pumping that induces the identical maximum 

drawdown in the Upper Floridan that would be expected as 

a result of pumping the Lower Floridan” (Nolton Johnston, 

Georgia Environmental Protection Division, written commun., 

January 28, 2003). 

The regional groundwater-flow model of Payne and 

others (2005) was modified so that steady-state model 

simulations would comply with the requirements of the 

GaEPD interim strategy for permitting LFA groundwater 

withdrawal. The spatial resolution of the model was increased 

and model inputs integrated recently acquired local hydrogeologic information obtained from field investigations at 

Rincon (described earlier) and from nearby existing wells, 

as described in appendix 1. Simulations to evaluate drawdown and leakage in the FAS resulting from pumping LFA 

well 36S048 were performed using the revised model, which 

incorporated average pumping rates for 2012 (0.87 Mgal/d) at 

Rincon wells and pumping rates for 2000 elsewhere as a base 

case for the evaluations. For the revised model, general- and 

specified-head boundaries were not modified from those used 

in the original regional groundwater-flow model of Payne 

and others (2005). General-head boundaries represent aquifer 
Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 9

outcrop areas in model layers 1, 2, and 5, allowing recharge 

to enter into the groundwater-flow system. Specified-head 

boundaries represent lateral boundaries for model layers 5, 6, 

and 7, located along the southwest edge of the model domain 

where the FAS extends beyond the domain boundary.

Steady-state simulations of the following groundwaterwithdrawal conditions were used to quantify the long-term 

response of the FAS to changes in pumpage in both the UFA 

and LFA (table 2).

• Scenario A—LFA well 36S048 was pumped at a 

rate of 1,000 gal/min, approximately the same rate 

used during the 72-hour aquifer test of Carter and 

Sloope (2004). 

• Scenario B—UFA well 36S047 was pumped at a rate 

292 gal/min, which would produce a UFA drawdown 

equivalent to that simulated by pumping LFA 

well 36S048 at a rate of 1,000 gal/min (simulated in 

scenario A). Several trial-and-error simulations were 

performed to obtain a drawdown-offset pumping rate 

(292 gal/min) for the UFA that achieved a drawdown 

equivalent to that obtained by the simulation of LFA 

well 36S048 pumping at a rate of 1,000 gal/min.

• Scenario C—LFA well 36S048 was pumped at 

1,000 gal/min, and total pumpage from existing UFA 

wells at Rincon was reduced by a drawdown-offset 

pumping rate of 292 gal/min. The reduction in UFA 

pumpage represents the amount of UFA pumping that 

would produce a maximum drawdown in the UFA 

equivalent to that resulting from pumping the LFA at 

1,000 gal/min (results of scenario B).

Table 2. Description of model scenarios and simulated drawdown in the Upper and Lower Floridan aquifers for various pumping 

distributions at Rincon, Georgia.

[LFA, Lower Floridan aquifer; UFA, Upper Floridan aquifer; gal/min, gallon per minute; —, not applicable]

Scenario

Pumping change,

in gallons per minute UFA drawdown LFA drawdown

Upper Remarks

Floridan 

aquifer

Lower 

Floridan 

aquifer

Maximum,

in feet

Area of 

1-foot 

contour, 

in square 

miles

Area of 

0.5-foot 

contour, 

in square 

miles

Maximum,

in feet

Area of 

1-foot 

contour, 

in square 

miles

Area of 

0.5-foot 

contour, 

in square 

miles

A 0 +1,000 6.78 396 1,156 72.0 395 1,156 Pumping LFA well 36S048 at 

approximate rate used during

the 72-hour aquifer test 

(1,000 gal/min)

B +292 0 6.73 8.35 105 — — — Pumping UFA well 36S047 at a 

rate that would replicate the 

amount of UFA drawdown 

simulated as the result of

pumping LFA well 36S048 at 

a rate of 1,000 gal/min (as was 

simulated in scenario A). Note 

that several model runs were 

completed until a drawdown 

match was obtained.

C –292 +1,000 5.99 190 733 71.2 190 732 Pumping LFA well 36S048 at

approximate rate used during

the 72-hour aquifer test 

(1,000 gal/min) and reducing 

flow from existing Rincon 

UFA wells by 292 gal/min. 

The UFA reduction represents 

the results of scenario B—the 

equivalent amount of UFA 

pumping that would produce 

an equivalent drawdown in 

the UFA as that resulting 

from pumping the LFA at 

1,000 gal/min.
10 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

Scenario A—Interaquifer Leakage and 

Drawdown Response

For scenario A, simulated pumping of LFA well 36S048 

at a rate of 1,000 gal/min resulted in a maximum steadystate drawdown of 72.0 ft (table 2, fig. 5), which is nearly 

identical to the 72.3-ft maximum drawdown observed during 

the 72-hour aquifer test (table 1; Carter and Sloope, 2004). 

Because changes in water levels over time were approaching 

zero by the end of the 72-hour test, the steady-state simulation 

provided a reasonable representation of field conditions in 

the LFA during the Carter and Sloope (2004) aquifer test. 

Simulated steady-state drawdown in the LFA of at least 1 ft 

occurred within 396-mi2 area for scenario A (fig. 5, table 2).

Simulated pumping of LFA well 36S048 at a rate of 

1,000 gal/min (scenario A) caused leakage through the LFCU, 

which resulted in drawdown in the overlying UFA (fig. 6). 

Because water levels in the UFA had not stabilized by the 

end of the 72-hour test, observed drawdown in well 36S047 

represented a lower limit for steady-state model calibration. 

Comparison of simulated drawdown and drawdown 

determined from water-level data at the end of the 72-hour 

aquifer test in LFA well 36S048 (Carter and Sloope, 2004) 

indicated that the simulated steady-state UFA drawdown near 

well 36S047 of about 6.8 ft exceeded the observed value of 

4.5 ft by about 2.3 ft. This difference is expected because 

the long-term steady-state response to pumping simulated 

by the model represents an aquifer condition that contains 

no time-varying changes in groundwater level in response to 

hydrologic stress; that is, all hydrologic stresses on the aquifer 

(such as pumping) are allowed to operate on the flow system 

for enough time until no further changes to groundwater levels 

occur. Simulations by Payne and others (2005) indicated 

steady state conditions were reached in about 5 years. If 

groundwater-level decline (drawdown) in response to pumping 

is measured only 72 hours following the inception of pumping 

in well 36S048, the observed drawdown is 2.3 ft less than 

the 6.8 ft maximum observed during steady-state conditions. 

Model results for scenario A indicate that drawdown in the 

UFA exceeded 1 ft over a 396-mi2 area (table 2, fig. 6).

 Steady-state water budgets derived from simulations of 

2000 conditions with and without pumping at well 36S048 

were compared to compute the difference in groundwater 

flows in the UFA and LFA resulting from additional pumping 

in the LFA (table 3). Although rounding errors in values 

for water-budget components prevents an exact accounting 

of flows in the FAS, comparison of component values for 

the two simulations provides some insight into the relative 

contribution of groundwater flow to LFA well 36S048. 

Pumping 1.44 Mgal/d (1,000 gal/min) at well 36S048 caused 

small changes to the 2000 regional water budget and resulted 

in the following redistribution of groundwater flow among 

model layers: 

• Layer 1 (surficial aquifer) —a 0.79-Mgal/d gain in 

inflow from, and a 0.12-Mgal/d reduction in outflow 

to, the general-head boundary for a net inflow of 

+ 0.91 Mgal/d.

• Layer 2 (Brunswick aquifer system confining unit) —a 

0.058-Mgal/d-gain in inflow from, and a 0.005-Mgal/d 

reduction in outflow to, the general-head boundary for 

a net inflow of + 0.063 Mgal/d.

• Layer 5 (UFA)—a 0.045-Mgal/d gain in inflow from, 

and a 0.026-Mgal/d reduction in outflow to, the 

general-head boundary and a 0.30-Mgal/d gain in 

inflow from, and 0.097-Mgal/d reduction in outflow 

to, lateral specified-head boundaries for a net inflow 

of + 0.468 Mgal/d. 

• Layer 7 (LFA)—a 0.009-Mgal/d gain in inflow 

from, and a 0.008-Mgal/d reduction in outflow to, 

lateral specified-head boundaries for a net inflow

of + 0.017 Mgal/d.

For scenario A, 9 percent of the simulated 1.44-Mgal/d 

pumping rate in well 36S048 was supplied by increased 

leakage from overlying layers, and 1 percent was derived 

from the lateral specified-head boundary of the LFA 

(layer 7) (table 3). A combined 90.8 percent of the inflow to 

well 36S048 from overlying layers was derived from generalhead boundaries in the surficial aquifer (layer 1, 63.2 percent) 

and from the specified-head boundary in the UFA (layer 5, 

27.6 percent). The remainder of flow from overlying layers 

(9.3 percent) was contributed from the general-head boundary 

in the Brunswick aquifer system confining unit (layer 2, 

4.4 percent) and in the UFA (layer 5, 4.9 percent).
Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 11

BEAUFORT

BEAUFORT

JASPER

HAMPTON

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

CHATHAM

LONG

EVANS

CANDLER

TATTNALL

SCREVEN

36S048

36S034

36S035

GA

SC

N

N

10

15

20

25

30

Rincon

Base map from U.S. Geological Survey 

1:2,000,000-scale digital data

0 250 500 FEET

0 75 150 METERS

36S048

GEORGIA

Model area

SOUTH

CAROLINA

FLORIDA

Map

area

above

10

36S048

EXPLANATION

Specified-head boundary

No-flow boundary

Line of equal simulated drawdown—

 Interval, in feet, is variable

Well and identifier

Figure 5. Simulated steady-state drawdown in the Lower Floridan aquifer for scenario A—pumping

Lower Floridan aquifer well 36S048 at 1,000 gallons per minute, Rincon and vicinity, Georgia. Maximum

drawdown in well 36S048 is 72.0 feet. 

 Fort Stewart

0.5

1

2

3

4

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

ATLANTIC OCEAN

Hunter

Army

 Airfield

Figure 5. Simulated steady-state drawdown in the Lower Floridan aquifer (LFA) for scenario A—pumping 

LFA well 36S048 at 1,000 gallons per minute, Rincon and vicinity, Georgia. Maximum drawdown in well 36S048 

is 72.0 feet.
12 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

BEAUFORT

BEAUFORT

JASPER

HAMPTON

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

CHATHAM

LONG

EVANS

CANDLER

TATTNALL

SCREVEN

GA

SC

N

Rincon

Base map from U.S. Geological Survey 

1:2,000,000-scale digital data

0 0.25 0.5 MILE

0 0.25 0.5 KILOMETER

6.0

36S048

EXPLANATION

Line of equal simulated

 drawdown—Interval, 

 in feet, is variable

Well and identifier

Figure 6. Simulated steady-state drawdown in the Upper Floridan aquifer for scenario A—pumping

Lower Floridan aquifer well 36S048 at 1,000 gallons per minute, Rincon and vicinity, Georgia. Maximum

drawdown in well 36S047 is 6.78 feet. 

 Fort Stewart

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

ATLANTIC OCEAN

1

2

3

4

5

36S048

36S047

36S034

36S035

0.5

4.5

N

5.0

5.5

6.0

6.5

36S048

36S047

Hunter

Army

 Airfield

Figure 6. Simulated steady-state drawdown in the Upper Floridan aquifer for scenario A—pumping 

Lower Floridan aquifer well 36S048 at 1,000 gallons per minute, Rincon and vicinity, Georgia. Maximum 

drawdown in well 36S047 is 6.78 feet.
Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 13

Table 3. Simulated steady-state water budgets for base case (year 2000) and for scenario A, after pumping 1,000 gallons per minute (1.44 million gallons per day) at Lower Floridan aquifer well 36S048, Rincon, Georgia.[Values may not sum to totals because of independent rounding; <, less than]

Hydrogeologic

unit

Model

layer

Simulated flow, in million gallons per day

Pumpage Inflow from general-head boundary Outflow to general-head boundary Inflow along lateral boundary Outflow along lateral boundary

2000

Well

36S048 

added

Difference 2000

Well

36S048 

added

Difference 2000

Well

36S048 

added

Difference 2000

Well 

36S048 

added

Difference 2000

Well 

36S048 

added

Difference

Surficial 

aquifer system

1 <0.001 <0.001 <0.001 278.25 279.04 0.79 100.13 100.01 –0.12 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Confining 

unit

2 <0.001 <0.001 <0.001 32.880 32.938 0.058 2.049 2.043 –0.005 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Brunswick 

aquifer system

3 0.241 0.241 0.000 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Confining 

unit 

4 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Upper 

Floridan aquifer

5 674.45 674.45 0.00 147.34 147.38 0.045 19.568 19.542 –0.026 843.11 843.41 0.30 385.64 385.54 –0.097

Confining 

unit 

6 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.004 0.004 0.000 <0.001 <0.001 <0.001

Lower 

Floridan aquifer

7 132.02 133.45 1.44 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 15.094 15.103 0.009 2.607 2.600 –0.008

Total all layers 806.71 808.14 1.44 458.47 459.36 0.888 121.75 121.60 –0.151 858.21 858.52 0.309 388.25 388.14 –0.108
14 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

Scenario B—Upper Floridan Aquifer 

Drawdown Offset

The amount of equivalent UFA pumping to offset the 

drawdown effect in the UFA of pumping LFA well 36S048 

was obtained by simulating pumping reductions at existing 

UFA production wells located within a 5-mi radius of 

well 36S048. Simulations to determine the pumping rate in the 

UFA that would produce the equivalent maximum drawdown 

in the UFA as pumping from the LFA (drawdown offset), 

constituted scenario B. For this scenario, various pumping 

rates were assigned to a hypothetical UFA well positioned 

above the model location that simulated pumping in LFA 

well 36S048 until the amount of UFA drawdown matched 

that simulated for the UFA by scenario A (table 2, fig. 7). 

Simulations indicated that pumping at a rate of 292 gal/min 

approximated the 6.8-ft drawdown in the UFA simulated by 

scenario A. 

The drawdown offset for the UFA produces an identical 

maximum drawdown above LFA well 36S048 as that 

produced by pumping LFA well 36S048. The drawdown-offset 

pumping rates for wells in the UFA, however, do not produce 

the same areal drawdown pattern in the UFA as produced by 

pumping the LFA well and deriving UFA drawdown from 

induced leakage to the pumped LFA well. For example, 

the 1-ft water-level contour in the UFA corresponding to 

scenario B (simulating drawdown-offset pumping in UFA 

wells) encompassed an area of about 8.4 mi2 (fig. 7), whereas 

the 1-ft contour encompassed 396 mi2 in the UFA when the 

LFA was pumped (scenario A, fig. 6). The large difference 

in affected area between scenarios involving pumping UFA 

and LFA wells results from differences in (1) the hydraulic 

properties of the aquifers and (2) the manner by which groundwater flow to the simulated wells in either the UFA or LFA as 

leakage and from lateral boundaries contributes to the pumped 

volumes. Drawdown in the LFA resulting from pumping 

well 36S048 occurs over a large area that induces leakage 

from (and drawdown within) an equally large area of the UFA. 

In contrast, drawdown resulting from offset-pumping wells 

located in the UFA occurs within a steep cone of depression 

that is smaller in area than the broad cone of depression 

generated in the LFA from pumping LFA well 36S048. 

Comparison of water-budget values for components 

derived from simulating 2000 conditions without LFA 

pumpage with similar component values corresponding to 

drawdown-offset pumpage at UFA well 36S047 (scenario B; 

table 4) indicated small changes to the 2000 regional water 

budget and the following redistribution of groundwater flow 

among model layers: 

• Layer 1 (surficial aquifer)—a 0.23-Mgal/d gain in 

inflow from, and a 0.04-Mgal/d reduction in outflow 

to, the general-head boundary for a net inflow of 

+ 0.27 Mgal/d.

• Layer 2 (Brunswick aquifer system confining unit)—a 

0.017-Mgal/d gain in inflow from, and a 0.002-Mgal/d 

reduction in outflow to, the general-head boundary for 

a net inflow of + 0.019 Mgal/d.

• Layer 5 (UFA)—a 0.015-Mgal/d gain in inflow from, 

and a 0.008-Mgal/d reduction in outflow to, the 

general-head boundary and a 0.07-Mgal/d gain in 

flow from, and 0.022-Mgal/d reduction in outflow to, 

lateral specified-head boundaries for a net inflow of 

+ 0.115 Mgal/d.

• Layer 7 (LFA)—a 0.003-Mgal/d gain in inflow from, 

and a 0.002-Mgal/d reduction in outflow to, lateral 

specified-head boundaries for a net inflow of 

+ 0.005 Mgal/d.

For scenario B, 69 percent of the 0.42-Mgal/d pumping 

rate in well 36S047 was derived from increased leakage from 

overlying layers, and 27 percent was derived from generaland specified-head boundaries for the UFA (layer 5; table 4). 

About 64 percent of the inflow to the pumped UFA well from 

overlying layers was derived from the general-head boundary 

in the surficial aquifer (0.27 Mgal/d; layer 1); the remaining 

inflow to well 36S047 from overlying layers was contributed 

by the general-head boundary in the Brunswick aquifer system 

confining unit (layer 2).
Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 15

BEAUFORT

BEAUFORT

JASPER

HAMPTON

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

CHATHAM

LONG

EVANS

CANDLER

TATTNALL

SCREVEN

GA

SC

N

Rincon

Base map from U.S. Geological Survey 

1:2,000,000-scale digital data

N

4.0

36S047

EXPLANATION

Line of equal simulated

 drawdown—Interval 

 is 0.5 foot

Well and identifier

Figure 7. Simulated steady-state drawdown in the Upper Floridan aquifer for scenario B—pumping

Upper Floridan aquifer well 36S047 at 292 gallons per minute, Rincon and vicinity, Georgia. Maximum

drawdown in well 36S047 is 6.74 feet. 

 Fort Stewart

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

0 50 100 FEET

0 15 30 METERS

ATLANTIC OCEAN

3.0

3.5

4.0

4.5

36S047

36S047

36S034

36S035

0.5

1

Hunter 

Army

 Airfield

Figure 7. Simulated steady-state drawdown in the Upper Floridan aquifer (UFA) for scenario B—pumping 

UFA well 36S047 at 292 gallons per minute, Rincon and vicinity, Georgia. Maximum drawdown in well 36S047 

is 6.74 feet.
16 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga. Table 4. Simulated steady-state water budgets for base case (year 2000) and for scenario B, after pumping 292 gallons per minute (0.42 million gallons per day) at Upper Floridan aquifer well 36S047, Rincon, Georgia. [Values may not sum to totals because of independent rounding; <, less than] Hydro- geologic unit Model layer Simulated flow, in million gallons per day Pumpage Inflow from general-head boundary Outflow to general-head boundary Inflow along lateral boundary Outflow along lateral boundary 2000 Well 36S047 added Difference 2000 Well 36S047 added Difference 2000 Well 36S047 added Difference 2000 Well 36S047 added Difference 2000 Well 36S047 added Difference Surficial aquifer system

1 <0.001 <0.001 <0.001 278.25 278.48 0.23 100.13 100.09 –0.04 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Confining 

unit

2 <0.001 <0.001 <0.001 32.880 32.897 0.017 2.049 2.047 –0.002 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Brunswick 

aquifer system

3 0.241 0.241 0.000 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Confining 

unit 

4 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Upper 

Floridan aquifer

5 674.45 674.87 0.42 147.34 147.35 0.015 19.568 19.560 –0.008 843.11 843.18 0.07 385.64 385.62 –0.022

Confining 

unit 

6 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.004 0.004 0.000 <0.001 <0.001 <0.001

Lower 

Floridan aquifer

7 132.02 132.02 0.000 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 15.094 15.097 0.003 2.607 2.605 –0.002

Total all layers 806.71 807.13 0.42 458.47 458.73 0.262 121.75 121.70 –0.050 858.21 858.28 0.073 388.25 388.22 –0.024
Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 17

Scenario C—Effects of Pumping Offsets on 

Groundwater Levels at Rincon, Ga.

Scenario C evaluated the combined effect of simulating pumping at 1,000 gal/min (1.44 Mgal/d) from LFA 

well 36S048 and simultaneously decreasing pumping in 

existing UFA wells (36S034 and 36S035) at Rincon by the 

drawdown-offset pumping rate of 292 gal/min (0.42 Mgal/d, 

scenario B) (table 2, fig. 8). Maximum simulated drawdown 

totaled 71.2 ft at LFA well 36S048 (fig. 8) and 5.99 ft near 

well 36S047 in the UFA (fig. 9).

Applying the drawdown offset at Rincon reduced the 

magnitude and areal extent of simulated drawdown in the UFA 

(scenario C) relative to scenario A, which did not simulate 

drawdown offsets for the existing UFA wells at Rincon 

(table 2). For scenario A, pumping LFA well 36S048 at a 

rate of 1,000 gal/min without drawdown offsets in the UFA 

resulted in UFA drawdown that exceeded 1 ft over a 395-mi2

area, with a maximum drawdown of 6.8 ft near well 36S048 

(fig. 6). The drawdown offset simulated by scenario C resulted 

in UFA drawdown exceeded 1 ft over a 190 mi2 area, with 

a maximum drawdown of 5.99 ft near well 36S047 (fig. 9; 

table 2). 

The simulated potentiometric surface of the UFA for 

scenarios A and C differed slightly from the year-2000 base case 

in the regional configuration of implied groundwater-flow directions in the UFA (fig. 10). The −20- and −30-ft potentiometric 

contours moved slightly to the northwest, upgradient from their 

base case positions, indicating lowering of the potentiometric 

surface because of pumping of well 36S048. The −20-ft contour 

contains a distinctive bend toward well 36S048, indicating 

slight convergence of flow toward the well. Groundwater flow 

in the region, however, remained dominated by a large cone of 

depression centered over the Savannah, Ga., area.

Comparing simulated water-budget components from 

scenario C to that obtained from base case conditions for 2000 

indicated that (table 5) adjustments to UFA pumping rates for 

the drawdown offset resulted in small changes to the base-case 

water budget, with the following redistribution of groundwater 

flow among model layers: 

• Layer 1 (surficial aquifer)—a 0.56-Mgal/d gain in 

inflow from, and a 0.09-Mgal/d reduction in outflow 

to, the general-head boundary for a net inflow of 

+ 0.65 Mgal/d.

• Layer 2 (Brunswick aquifer system confining unit)—a 

0.044-Mgal/d gain in inflow from, and a 0.005-Mgal/d 

reduction in outflow to, the general-head boundary for 

a net inflow of + 0.049 Mgal/d.

• Layer 5 (UFA)—a 0.03-Mgal/d gain in inflow from, 

and a 0.02-Mgal/d reduction in outflow to, the 

general-head boundary and a 0.22-Mgal/d gain in 

inflow from, and 0.07-Mgal/d reduction in outflow to, 

lateral specified-head boundaries for a net inflow of 

+ 0.34 Mgal/d.

• Layer 7 (LFA)—a 0.003-Mgal/d gain in inflow from, 

and a 0.005-Mgal/d reduction in outflow to, lateral 

specified-head boundaries for a net inflow of 

+ 0.008 Mgal/d.

As a result of the 1.02-Mgal/d net increase in the FAS 

pumping rate simulated in scenario C, increased leakage from 

overlying layers supplied about 69 percent of the flow to LFA 

well 36S048, and general- and specified-head boundaries for 

the UFA supplied about 30 percent (layer 5). The general-head 

boundary in the surficial aquifer (layer 1) contributed most 

of the inflow to the pumped well (about 0.65 Mgal/d) from 

overlying layers. The remainder of inflow from overlying 

layers was contributed by the general-head boundary in the 

Brunswick aquifer system confining unit (layer 2).

Limitations of Analysis

Analyses of the effects of pumping the LFA on water 

levels in the UFA are limited by the accuracy of field data, 

including possible errors and uncertainty in water-level 

measurements, hydraulic properties, and groundwater-use 

estimates based on 2000 pumping rates (not quantified in 

this report). For example, available water-level data were not 

adjusted to minimize or eliminate effects of local interferences, such as tides, barometric pressure, and pumping, which 

could affect recorded water levels and the computed hydraulic 

properties based upon them. 

Model results are limited by the same model assumptions 

and design as described by Payne and others (2005). Potential 

errors are inherent in (1) the conceptual model of groundwater 

flow; (2) approximations made in representing the physical 

properties of the flow system and spatial distribution of these 

properties; (3) approximations made in the formulation and 

application of model boundary and initial conditions; (4) numerical approximation and solution of the mathematical model of 

the flow system; and (5) assumptions made in using the models 

to predict the future behavior of the flow system, such as no 

variations in recharge rates or boundary heads. Although the 

revised groundwater-flow model reasonably simulates steadystate conditions, use of transient simulation would provide 

insight into drawdown and groundwater leakage over time.

The variably spaced grid used in the revised model 

contains aspect ratios (relative lengths between row and 

column dimensions) as large as 1,011:1, which can lead to 

inaccurate representation of hydraulic gradients parallel 

to the long axis of the grid cell (de Marsily, 1986, p. 351). 

Fortunately, these large-aspect-ratio grid cells are only in 

areas distant from Rincon and have little effect on simulated 

results in the area, as indicated by the calibration results in 

appendix 1. Although the vertical hydraulic conductance of the 

LFCU was held constant at the highest measured value in the 

area, 1.67 feet per day (ft/d), it is likely that this parameter had 

little effect on simulation results because of low sensitivity, 

as indicated by the sensitivity analysis performed as part of a 

study for the City of Pooler, Ga. (Cherry and Clarke, 2013).
18 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

BEAUFORT

BEAUFORT

JASPER

HAMPTON

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

CHATHAM

LONG

EVANS

CANDLER

TATTNALL

SCREVEN

GA

SC

N

Rincon

Base map from U.S. Geological Survey 

1:2,000,000-scale digital data

N

10

36S048

EXPLANATION

Line of equal simulated

 drawdown—Interval, 

 in feet, is variable

Well and identifier

Figure 8. Simulated steady-state drawdown in the Lower Floridan aquifer for scenario C—pumping

Lower Floridan aquifer well 36S048 at 1,000 gallons per minute and reducing pumping at Upper Floridan

aquifer wells 36S034 and 36S035 by 292 gal/min, Rincon and vicinity, Georgia. Maximum drawdown in

well 36S048 is 71.2 feet. 

 Fort Stewart

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

0 250 500 FEET

0 75 150 METERS

ATLANTIC OCEAN

0.5

1

2

3

36S048

36S034

36S035

10

15

20

25

30

36S048

Hunter

 Army

 Airfield

Figure 8. Simulated steady-state drawdown in the Lower Floridan aquifer (LFA) for scenario C—

pumping LFA well 36S048 at 1,000 gallons per minute (gal/min) and reducing pumping at Upper Floridan 

aquifer wells 36S034 and 36S035 by 292 gal/min, Rincon and vicinity, Georgia. Maximum drawdown in 

well 36S048 is 71.2 feet. 
Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 19

BEAUFORT

BEAUFORT

JASPER

HAMPTON

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

CHATHAM

LONG

EVANS

CANDLER

TATTNALL

SCREVEN

GA

SC

N

Rincon

Base map from U.S. Geological Survey 

1:2,000,000-scale digital data

5.0

36S048

EXPLANATION

Line of equal simulated

 drawdown—Intervals 

 are 0.5 and 1 foot

Well and identifier

Figure 9. Simulated steady-state drawdown in the Upper Floridan aquifer for scenario C—pumping

Lower Floridan aquifer well 36S048 at 1,000 gallons per minute and reducing pumping at Upper Floridan

aquifer wells 36S034 and 36S035 by 292 gal/min, Rincon and vicinity, Georgia. Maximum drawdown in

well 36S047 is 5.99 feet. 

 Fort Stewart

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

0 0.25 0.5 MILE

0 0.25 0.5 KILOMETER

ATLANTIC OCEAN

1

2

36S048 3

36S047

36S034

36S035

0.5

4.5

5.0

5.5

4.0

3.5

N

36S047

Hunter 

Army

 Airfield

Figure 9. Simulated steady-state drawdown in the Upper Floridan aquifer (UFA) for scenario C—

pumping Lower Floridan aquifer well 36S048 at 1,000 gallons per minute (gal/min) and reducing pumping 

at UFA wells 36S034 and 36S035 by 292 gal/min, Rincon and vicinity, Georgia. Maximum drawdown in 

well 36S047 is 5.99 feet.
20 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

N

Base map from U.S. Geological Survey 1:2,000,000-scale digital data

50

36S048

EXPLANATION

Simulated potentiometric contour—Intervals 10 and 20 feet.  Hachures indicate depression. Datum is NAVD 88

Well and identifier

Base Case Scenario A

Scenario C 0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

 Fort Stewart

Hunter

Army

 Airfield

0

50

100

–20

40 –

60 –

–80 –100

0

50

100

–20

40 –

–60

–80

–100

0

50

100

–20

40 –

60 –

–80

Rincon

–100

Figure 10. Simulated Upper Floridan aquifer potentiometric surfaces for Base Case (year 2000) and scenarios A and C, Rincon and vicinity, Georgia.

36S048

36S034

36S035

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 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 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 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

BEAUFORT

BEAUFORT JASPER

HAMPTON

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

CHATHAM

LONG

EVANS

CANDLER

TATTNALL

SCREVEN

GA

SC

Figure 10. Simulated Upper Floridan aquifer potentiometric surfaces for Base Case (year 2000) and scenarios A and C, Rincon and vicinity, Georgia.
Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 21

Table 5. Simulated steady-state water budgets for base case (year 2000) and for scenario C, after pumping 1,000 gallons per minute (1.44 million gallons per day) at Lower Floridan aquifer well 36S048 and reducing pumping at Upper Floridan aquifer wells 36S034 and 36S035 by 292 gallons per minute (0.42 million gallons per day), Rincon, Georgia.[Values may not sum to totals because of independent rounding; <, less than]

Hydrogeologic

unit

Model

layer

Simulated flow, in million gallons per day

Pumpage Inflow from general-head boundary Outflow to general-head boundary Inflow along lateral boundary Outflow along lateral boundary

2000

Well

36S048 

added

Difference 2000

Well

36S048 

added

Difference 2000

Well

36S048 

added

Difference 2000

Well 

36S048 

added

Difference 2000

Well 

36S048 

added

Difference

Surficial 

aquifer system

1 <0.001 <0.001 <0.001 278.25 278.81 0.56 100.13 100.04 –0.09 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Confining 

unit

2 <0.001 <0.001 <0.001 32.880 32.924 0.044 2.049 2.044 –0.005 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Brunswick 

aquifer system

3 0.241 0.241 0.000 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Confining 

unit 

4 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Upper 

Floridan aquifer

5 674.45 674.03 –0.42 147.34 147.37 0.030 19.568 19.548 –0.020 843.11 843.33 0.22 385.64 385.57 –0.070

Confining 

unit 

6 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.004 0.004 0.000 <0.001 <0.001 <0.001

Lower 

Floridan aquifer

7 132.02 133.45 1.440 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 15.094 15.100 0.003 2.607 2.602 –0.005

Total all layers 806.71 807.72 1.02 458.47 459.10 0.634 121.75 121.63 –0.115 858.21 858.43 0.225 388.25 388.17 –0.075
22 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

Simulated rates of interaquifer leakage and drawdown 

in the UFA can differ from actual rates because of the influence of specified- and general-head boundaries, which allow 

water exchange between the model and the aquifer region 

that extends beyond the model grid. This additional water 

supply can affect simulated drawdown and rates of interaquifer leakage. Lastly, the revised groundwater-flow model 

reasonably simulates steady-state conditions; use of transient 

simulation would provide insight into drawdown and the 

effects of aquifer storage in the FAS over time.

Summary

• Simulation of long-term pumping at a rate of 

1,000 gallons per minute (gal/min) from the Lower 

Floridan aquifer (LFA) (scenario A) caused a 

maximum drawdown of about 6.8 feet in the Upper 

Floridan aquifer (UFA), with drawdown exceeding 

1 foot over a 396-square-mile area. Induced vertical 

leakage from the UFA supplied about 99 percent of the 

pumped water to the LFA. Simulated drawdown in the 

UFA, although slight with regard to drawdown in the 

LFA, extended into the Coastal Plain beyond Rincon 

because of the relatively large (about 4:1) contrast in 

the water-transmitting ability, or transmissivity, of the 

UFA compared with the LFA.

• Simulated pumping changed regional water-budget 

components and redistributed flow among model 

layers, specifically increasing inflow to, and decreasing 

outflow from, lateral boundaries in the UFA and LFA, 

and increasing inflow (recharge) from the generalhead boundary to outcrop areas in the surficial aquifer, 

underlying semiconfining unit, and UFA.

• Simulations that addressed the leakage offset 

(scenario A) or drawdown offset (scenario B) of the 

Georgia Environmental Protection Division interim 

permitting strategy identified widely varying pumping 

offsets for the UFA, depending on whether the leakage 

or the drawdown offset for UFA pumping was evaluated. For example, when pumping the LFA at a rate of 

1,000 gal/min, the drawdown offset (292 gal/min) was 

three times less than the leakage offset (990 gal/min).

• Simulating pumping of LFA well 36S048 at a rate of 

1,000 gal/min and simultaneously pumping existing 

Rincon UFA supply wells according to the reduced 

UFA drawdown offset of 292 gal/min resulted in 

decreased magnitude and extent of drawdown in the 

UFA relative to scenario A, which did not contain drawdown offsets for pumping the UFA wells at Rincon. 

• Simulated rates of interaquifer leakage and 

drawdown in the UFA can differ from actual rates 

because the model-imposed specified-head and 

general-head boundaries that contributed flow 

to these leakage and drawdown conditions are 

controlled by the hydraulic head and gradients 

computed in the model area near these boundaries. 

As actual hydrologic stresses change the head and 

gradient conditions near these boundaries, leakage 

and drawdown in the UFA will vary from that 

simulated by the model. In addition, these model 

boundaries can simulate an unlimited supply of 

water to the groundwater system—water that may 

not be available to meet actual, additional pumping 

demand from the LFA. 

• Simulations with the revised regional model of 

Payne and others (2005) developed for this study, 

have contributed toward improving regional 

characterization of the Floridan aquifer system. 

Model results could be improved, however, by 

(1) replacing lateral specified-head boundaries 

with a natural boundary, such as a groundwater 

divide, which would imply expanding the model 

area; and, (2) actively simulating the surficial aquifer 

with a model layer that would contain hydrologic 

stresses and horizontal flow. 

Selected References

Carter and Sloope, 2004, Hydrogeologic characterization 

Lower Floridan aquifer, Effingham County, Georgia: 

Savannah, Ga., Carter and Sloope Consulting Engineers, 

Report for City of Rincon.

Cherry G.S., and Clarke, J.S., 2013, Simulated effects of 

Lower Floridan aquifer at Pooler, Chatham County, 

Georgia: U.S. Geological Survey Scientific Investigations 

Report 2013–5004, 46 p., accessed May 4, 2015, 

at http://pubs.usgs.gov/sir/2013/5004/.

City-data.com, 2015, Rincon, Georgia: Web site accessed 

May 4, 2015, at http://www.city-data.com/city/RinconGeorgia.html. 

Clarke, J.S., 2003, The surficial and Brunswick aquifer 

systems—Alternative ground-water resources for coastal 

Georgia, in Hatcher, K.J., ed., Proceedings of the 2003 

Georgia Water Resources Conference, April 23–24, 2003: 

The University of Georgia, Institute of Ecology, Athens, 

Ga., CD–ROM.
Selected References 23

Clarke, J.S., Cherry, G.C., and Gonthier, G.J., 2011, 

Hydrogeology and water quality of the Floridan aquifer 

system and effects of Lower Floridan aquifer pumping 

on the Upper Floridan aquifer at Fort Stewart, Georgia:

U.S. Geological Survey Scientific Investigations 

Report 2011–5065, 59 p., accessed May 4, 2015, 

at http://pubs.usgs.gov/sir/2011/5065/.

Clarke, J.S., Hacke, C.M., and Peck, M.F., 1990, Geology 

and ground-water resources of the coastal area of Georgia: 

Georgia Geologic Survey Bulletin 113, 106 p.

Clarke, J.S., and Krause, R.E., 2000, Design, revision, and 

application of ground-water flow models for simulation of 

selected water-management scenarios in the coastal area of 

Georgia and adjacent parts of South Carolina and Florida: 

U.S. Geological Survey Water-Resources Investigations 

Report 00–4084, 93 p.

Clarke, J.S., Williams, L.J., and Cherry, G.C., 2010, 

Hydrogeology and water quality of the Floridan aquifer 

system and effect of Lower Floridan aquifer pumping on 

the Upper Floridan aquifer at Hunter Army Airfield, 

Chatham County, Georgia: U.S. Geological Survey 

Scientific Investigations Report 2010–5080, 56 p., accessed 

May 4, 2015, at http://pubs.usgs.gov/sir/2010/5080/.

de Marsily, Ghislain, 1986, Quantitative hydrogeology: 

Orlando, Fla., Academic Press, Inc., 440 p.

Falls, W.F., Baum, J.S., Harrelson, L.G., Brown, L.H., and 

Jerden, J.L., 1997, Geology and hydrogeology of Cretaceous and Tertiary strata, and confinement in the vicinity of 

the U.S. Department of Energy Savannah River Site, South 

Carolina and Georgia: U.S. Geological Survey WaterResources Investigations Report 97–4245, 125 p.

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, 180 p., 

accessed May 4, 2015, at http://ga.water.usgs.gov/

publications/other/ggs-ic106/.

Fanning, J.L., and Trent, V.P., 2009, Water use in Georgia 

by county for 2005; and water-use trends, 1980–2005: 

U.S. Geological Survey Scientific Investigations 

Report 2009–5002, 186 p., accessed May 4, 2015, 

at http://pubs.usgs.gov/sir/2009/5002/.

Georgia Department of Natural Resources, 2006, Coastal 

Georgia Water & Wastewater Permitting Plan for Managing 

Salt Water Intrusion: Accessed September 7, 2012, at 

http://www1.gadnr.org/cws/Documents/saltwater_

management_plan_june2006.pdf.

Gonthier, G.J., 2011, Summary of hydrologic testing of 

the Floridan aquifer system at Fort Stewart, Georgia: 

U.S. Geological Survey Open-File Report 2011–1020, 

40 p., accessed May 4, 2015, at http://pubs.usgs.gov/

of/2011/1020/.

Gonthier, G.J., 2012, Hydrogeology and water quality of the 

Floridan aquifer system and effect of Lower Floridan 

aquifer pumping on the Upper Floridan aquifer, Pooler, 

Chatham County, Georgia, 2011–2012: U.S. Geological

Survey Scientific Investigations Report 2012–5249, 62 p.,

accessed May 4, 2015, at http://pubs.usgs.gov/sir/2012/5249/.

Halford, K.J., 2006, Documentation of a spreadsheet for timeseries analysis and drawdown estimation: U.S. Geological 

Survey Scientific Investigations Report 2006–5024, 38 p.

Harbaugh, A.W., 1990, A computer program for calculating 

subregional water budgets using results from the U.S. Geological Survey modular three-dimensional finite-difference 

ground-water flow model: U.S. Geological Survey OpenFile Report 90–392, 46 p.

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

2000, MODFLOW–2000, The U.S. Geological Survey 

modular ground-water model—User guide to modularization concepts and the ground-water flow process: U.S. 

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

accessed May 5, 2015, at http://water.usgs.gov/nrp/

gwsoftware/modflow2000/ofr00-92.pdf.

Hill, M.C., 1998, Methods and guidelines for effective model 

calibration: U.S. Geological Survey Water-Resources 

Investigation Report 98–4005, 90 p.

H&K Engineering Group, 2013, Proposed 1.0 MGD Lower 

Floridan aquifer withdrawal request, City of Rincon, 

Effingham County, Georgia: Savannah, Ga., H&K Engineering Group, Permit request to Georgia Environmental 

Protection Division, variously paged.

Krause, R.E., and Clarke, J.S., 2001, Coastal ground water at 

risk—Saltwater contamination at Brunswick, Georgia, and 

Hilton Head Island, South Carolina: U.S. Geological Survey 

Water-Resources Investigations Report 01–4107, 1 sheet.

Krause, R.E., and Randolph, R.B., 1989, Hydrology of the 

Floridan aquifer system in southeast Georgia and adjacent 

parts of Florida and South Carolina: U.S. Geological Survey 

Professional Paper 1403–D, 65 p., 18 pl.

McCollum, M.J., and Counts, H.B., 1964, Relation of saltwater encroachment to the major aquifer zones, Savannah 

area, Georgia and South Carolina: U.S. Geological Survey 

Water-Supply Paper 1613–D, 26 p.
24 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

Miller, J.A., 1986, Hydrogeologic framework of the Floridan 

aquifer system in Florida and in parts of Georgia, Alabama, 

and South Carolina: U.S. Geological Survey Professional 

Paper 1403–B, 91 p., 33 pl.

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—Predevelopment, 1980, and 2000: U.S. Geological Survey Scientific Investigations Report 2005–5089, 91 p., accessed 

May 4, 2015, at http://pubs.usgs.gov/sir/2005/5089/.

Peaceman, D.W., 1983, Interpretation of well-block pressures in numerical reservoir simulation with nonsquare grid 

blocks and anisotropic permeability: Society of Petroleum 

Engineers Journal, v. 23, no. 3, p. 531–543.

Peck, M.F., Clarke, J.S., Ransom, Camille, III, and Richards, C.J.,

1999, Potentiometric surface of the Upper Floridan aquifer in Georgia and adjacent parts of Alabama, Florida, and 

South Carolina, May 1998, and water level-trends in 

Georgia, 1990–98: Georgia Geologic Survey Hydrologic 

Atlas 22, 1 sheet, scale 1:100,000, 1977–81.

Peck, M.F., Gordon, D.W., and Painter, J.A., 2013, Groundwater conditions in Georgia, 2010–2011: U.S. Geological Survey Scientific Investigations Report 2013–5084, 

63 p., accessed May 4, 2015, at http://pubs.usgs.gov/

sir/2013/5084/. 

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.

Randolph, R.B., Pernik, Maribeth, and Garza, Reggina, 1991, 

Water-supply potential of the Floridan aquifer system in the 

coastal area of Georgia—A digital model approach: Atlanta, 

Georgia Geologic Survey Bulletin 116, 30 p.

Southeast Regional Climate Center, 2010, Savannah 

WSO Airport, Georgia—Climate summary: Accessed 

September 3, 2010, at http://www.sercc.com/cgi-bin/sercc/

cliMAIN.pl?ga7847.

Weems, R.E., and Edwards, L.E., 2001, Geology of Oligocene, Miocene, and younger deposits in the coastal area of 

Georgia: Atlanta, Georgia Geologic Survey Bulletin 131, 

124 p.

Williams, L.J., 2010, Summary of hydrologic testing of 

the Floridan aquifer system at Hunter Army Airfield, 

Chatham County, Georgia: U.S. Geological Survey 

Open-File Report 2010–1066, 30 p., accessed 

May 4, 2015, at http://pubs.usgs.gov/of/2010/1066/. 

Williams, L.J., and Gill, H.E., 2010, Revised hydrogeologic 

framework of the Floridan aquifer system in the northern 

coastal area of Georgia and adjacent parts of South Carolina: U.S. Geological Survey Scientific Investigations 

Report 2010–5158, 103 p., 3 pl., accessed May 4, 2015, 

at http://pubs.usgs.gov/sir/2010/5158/.
Appendix 1. Regional Groundwater Model

A regional groundwater-flow model (herein referred to 

as “regional model”) developed by Payne and others (2005) 

for the coastal region of Georgia and adjacent parts of South 

Carolina and Florida was modified and used to simulate the 

effects of pumping from the Lower Floridan aquifer (LFA) at 

Rincon, Georgia. The regional model is described in detail in 

Payne and others (2005); a brief description follows. 

The regional model uses MODFLOW–2000 (Harbaugh 

and others, 2000) to simulate flow in the surficial, Brunswick, 

and Floridan aquifer systems. To account for natural hydrologic boundaries, the model encompasses a 42,155-squaremile (mi2

) area that includes the Coastal Plain of Georgia, 

northeastern Florida, southwestern South Carolina, and the 

adjacent offshore area (fig. 1–1).

The regional model consists of the following seven model 

layers and corresponding hydrogeologic units (fig. 1–2) in 

descending order:

• Layer 1: Confined upper and lower water-bearing 

zones of the surficial aquifer system.

• Layer 2: Brunswick aquifer system confining unit.

• Layer 3: Upper and lower Brunswick aquifers, 

composing the Brunswick aquifer system. 

• Layer 4: Upper Floridan confining unit.

• Layer 5: Upper Floridan aquifer (UFA). 

• Layer 6: Lower Floridan confining unit (LFCU). 

• Layer 7: LFA. 

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). 

The regional model was discretized in the areal 

dimensions using a variably spaced grid and cell sizes 

range from approximately 4,000×5,000 feet (ft; 0.7 mi2) to 

16,500×16,500 ft (9.8 mi2). At Rincon, the mesh resolution 

was 14,900×16,100 ft, requiring 10×10-ft refinement for the 

current model application. Each hydrogeologic unit was represented by one layer of grid cells in the vertical dimension. 

Lateral boundaries for all layers of the regional model 

were designated as no flow, with the exception of the southern 

and southwestern sides of layers 5, 6, and 7 (UFA, LFA, and 

intervening confining unit), which were set as specified-head 

cells. Values assigned to specified-head cells were based 

on estimates of UFA head derived from the potentiometricsurface map for 1998 developed by Peck and others (1999). 

The lowermost boundary of the regional model was 

designated as no flow, corresponding with the lower confining 

unit of the Floridan aquifer system; the uppermost boundary 

was set as a head-dependent flow (or general-head) boundary 

representing the confined zone of the surficial aquifer system 

(fig. 1–2). The general-head boundary required a controlling 

specified-head and a conductance term to regulate groundwater flow between the top two layers of the model. The 

controlling head represented the water table in the onshore 

area and the freshwater equivalent of the saltwater head in the 

offshore area. In the onshore area, the conductance was set to 

limit the amount of recharge entering the system in any given 

grid cell for the 1980 and 2000 simulation periods to less-thanmaximum recharge derived from base-flow estimates (Priest, 

2004). The conductance established in the offshore area was 

set arbitrarily large, posing minimal resistance to flow in or 

out of the system, because little is known about hydraulic 

properties in this area. 

Estimates of average annual pumpage were assigned 

in the regional model on the basis of county-aggregate and 

site-specific data. These data were used to develop pumpage 

distributions for the assumed steady-state conditions of 1980 

and 2000 used for calibration. Pumpage was assigned to model 

layers 3 (Brunswick aquifer system), 5 (UFA), and 7 (LFA) on 

the basis of the open interval of wells. Pumping rates within a 

model cell were obtained by summing site-specific and nonsite-specific pumping rates corresponding to that model cell. 

Pumpage simulated by the model totaled 692 million gallons 

per day (Mgal/d) for 1980 and 798 Mgal/d for 2000. Because 

pumpage during 2010 (799 Mgal/d) was about the same as 

during 2000, the revised model used to evaluate groundwater 

flow near Rincon was within the same range of calibrated 

pumping conditions as the regional model. 

Revisions to Regional Model

The regional model of Payne and others (2005) was 

modified using hydrogeologic information obtained from field 

investigations at Rincon (Carter and Sloope, 2004) and from 

existing wells in the vicinity of Pooler, Ga. (Gonthier, 2012), 

Hunter Army Airfield (Clarke and others, 2010), and Fort 

Stewart (fig. 1; Clarke and others, 2011). These modifications 

involved adding new hydraulic-property zones for vertical 

and horizontal hydraulic conductivity in the UFA and LFA 

and intervening confining unit. Modifications to the regional 

model that were made during previous investigations at 

Pooler (Cherry and Clarke, 2013) and Hunter Army Airfield 

in Chatham County (Clarke and others, 2010), and at Fort 

Stewart in Liberty County (Clarke and others, 2011) also were 

applied to the model developed for the current study. The 

revised model retained the layering and boundary conditions 

of the original regional model.

Appendix 1. Regional Groundwater Model 25
26 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

 Fort

Stewart

Rincon 36S048

36S034

HAAF

FLORIDA

GEORGIA

Model

boundary

SOUTH

CAROLINA

Figure 1–1. Location of selected wells, regional groundwater model and boundary conditions, and

revised model grid, City of Rincon, Fort Stewart, and vicinity, Georgia.

N

EXPLANATION

Specified head (model

 layers 5, 6, and 7)

No-flow (all model layers)

Model boundary

Well and identifier

Model grid with row-column

 spacing of 16,181×16,311 feet 

Area of dense model grid—

 Row-column spacing ranges 

 from 10×10 feet to 5,858×5,397 feet 

0 30 60 MILES

0 30 60 KILOMETERS

Base from U.S. Geological Survey

1:100,000-scale digital data

Map area on

facing page

36S035

36S048

Effingham

County

Figure 1–1. Location of selected wells, regional groundwater model and boundary conditions, and 

revised model grid, City of Rincon, Fort Stewart, and vicinity, Georgia. [HAAF, Hunter Army Airfield]
Appendix 1. Regional Groundwater Model 27

Map area

Figure 1–1. Location of selected wells, regional groundwater model and boundary conditions, and

revised model grid, City of Rincon, Georgia.—Continued 

22.9

10.0

15.3

22.9

Grid cell size, in feet

15.3

N

EXPLANATION

Model grid

36S048 Well and identifier

22.9

15.3

10.0

22.9

15.3

Grid cell size, in feet

Well area grid detail

36S048

0 500 1,000 FEET

0 100 200 METERS

Effingham

County

Figure 1–1. Location of selected wells, regional groundwater model and boundary conditions, and 

revised model grid, City of Rincon, Fort Stewart, and vicinity, Georgia. [HAAF, Hunter Army Airfield]—

Continued
28 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

Onshore Offshore

Layer 3

Layer 2

Layer 1 Confined zone of surficial aquifer system

Brunswick aquifer system

Upper Floridan aquifer

Lower Floridan aquifer

Layer 4

Layer 5

Layer 6

Layer 7

Confining unit

No-flow or fixed-head boundary

EXPLANATION

No-flow boundary

Offshore area—Equivalent freshwater head 

 of overlying saltwater column

Onshore area—Water-table head

Aquifer

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

Figure 1–2. Schematic diagram showing model layers and boundary conditions (from Payne and others, 2005). Figure 1–2. Schematic diagram showing model layers and boundary conditions (from Payne and others, 2005).

Refinement of Grid Resolution

Grid-cell dimensions were modified to variably spaced 

dimensions (fig. 1–1) that increase from the smallest cell size 

of 10×10 ft near well 36S048 to a maximum size of about 

10,113 ft in the row direction and 10,128 ft in the column 

direction. The grid orientation was rotated 325 degrees 

counterclockwise to accommodate the finer mesh in the 

Rincon area. The revised model consists of 590 rows and 

649 columns. The variably spaced grid used in the revised 

model contains aspect ratios between row and column dimensions as large as 1,011:1, which can lead to numerical errors 

(de Marsily, 1986, p. 351). Fortunately, these large-aspectratio grid cells are only in areas distant from Rincon and have 

little effect on simulation results in the study area.

Refinement of Hydraulic Conductivity Distribution

Previous studies at Rincon, Pooler, Hunter Army 

Airfield and Fort Stewart provided the basis for revising 

hydraulic-conductivity values assigned to these areas in 

the regional model (Payne and others, 2005). In the area 

outside of Rincon, values of vertical and horizontal hydraulic 

conductivity (Kv and Kh

, respectively) correspond to values 

used (1) in the regional model (Payne and others, 2005); (2) in 

the area of Hunter Army Airfield, as simulated by Clarke and 

others (2010); (3) in the area of Fort Stewart, as simulated 

by Clarke and others (2011), and (4) in the area of Pooler, as 

simulated by Cherry and Clarke (2013) (fig. 1–3; table 1–1). 

Field testing at Rincon (Carter and Sloope, 2004) provided 

new information about the hydraulic properties of the UFA 

(layer 5) and LFA (layer 7) and enabled refinement of values 

that were used in the regional model. In addition, results of a 

72-hour aquifer test conducted in the LFA provided information about drawdown in the LFA and overlying UFA, which 

guided revisions to Kv

 and Kh values from previous calibrated 

values near Rincon. Hydraulic conductivity of the LFCU was 

not measured during previous studies at Rincon, so results 

from core analyses and packer-slug tests at Hunter Army 

Airfield and Pooler were applied to the model (Williams, 

2010; Gonthier, 2012).
Appendix 1. Regional Groundwater Model 29

Pooler

Brunswick

Rincon

Figure 1–3. Simulated hydraulic property zones by model layer. See table A–1 for hydraulic conductivity 

values assigned to zones. 

Base from U.S. Geological Survey

1:100,000-scale digital data

Hydraulic-property zone

Well and identifier

EXPLANATION

N

SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

Fort

Stewart

Hunter Army 

Airfield

SC

AG

GA

FL

25 50 KILOMETERS

 Surficial aquifer system

 (layer 1)

 Upper Brunswick confining unit

 (layer 2)

 Brunswick aquifer system

 (layer 3)

 Upper Floridan confining unit

 (layer 4)

AT LANT IC O CE AN

0

250 50 MILES

Upper Floridan aquifer

 (layer 5)

C1

F6

F11

F7

F5

F1

F10

F9

F4

F12

F13

F14

C1

C3

C5

C2

F2

F8

C2 C2

C4

C1

C3

C4

B1

C1

C3

C4

36S048

F15

36S048

36S048

C5

assigned to zones.

Figure 1–3. Simulated hydraulic property zones by model layer. Table 1–1 lists hydraulic conductivity values 
30 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

Lower Floridan confining unit

 (layer 6)

Lower Floridan aquifer

 (layer 7)

LFC2

LFC3

LFC1

LF2

LF3

LF1

LFC4 LFC4

Figure 1–3. Simulated 

hydraulic-property 

zones by model layer. 

See table A–1 for 

hydraulic conductivity 

values assigned to 

zones.—Continued

Figure 1–3. Simulated hydraulic property zones by model layer. Table 1–1 lists hydraulic conductivity values 

assigned to zones.—Continued

New hydraulic-property zones were developed for 

the study on the basis of field data collected by Carter and 

Sloope (2004), Gonthier (2012), and Clarke and others (2010, 

2011), as described previously. Zones were added as follows 

(fig. 1–3; table 1–1):

• UFA (layer 5)—zone F13 added at Hunter Army 

Airfield and expanded outward to include Pooler, 

zone F14 added at Fort Stewart, and zone F15 

added at Rincon.

• LFCU (layer 6)—zone LFC2 added at Hunter Army 

Airfield and expanded outward to include Pooler, 

zone LFC3 added at Fort Stewart, and zone LFC4 

added at Rincon.

• LFA (layer 7)—zone LF2 added at Hunter Army 

Airfield and expanded outward to include Pooler, 

zone LF3 added at Fort Stewart, and zone LF4 

added at Rincon.

The hydraulic-property zones surrounding Rincon in 

layers 5 through 7 each encompass a 34-mi2 area that includes 

the area of highest grid resolution and wells evaluated by 

model simulations (fig. 1–3). Each zone was initially assigned 

a Kh and Kv value on the basis of results of field testing at or 

near each site. With the exception of Kv for the LFCU, values 

were adjusted slightly to calibrate to water-level changes in 

the UFA and LFA observed during 72-hour aquifer tests at 

the two sites. The highest value for Kv of the LFCU that was 

determined during a previous investigation at Hunter Army 

Airfield, 1.67 feet per day (ft/d; Williams, 2010), was assigned 

to this model parameter for layer 6, zone LFC4, located at 

Rincon. This value is about two orders of magnitude larger 

than the previous value of Kv (0.02 ft/d) used in the regional 

model by Payne and others (2005) for the Rincon area. The 

new Kv value was held constant at the request of the Georgia 

Environmental Protection Division to ensure simulation of 

conservative leakage rates; that is, rates that would yield 

the highest possible leakage, because Kv was not measured 

at Rincon. Areal hydraulic conductivity in and near Rincon 

for zone LFC4 was assigned a value of 14.2 ft/d, which 

was obtained by applying the 8.5:1 ratio of areal to vertical 

hydraulic conductivity reported at Hunter Army Airfield to 

the Kv value. 

For the UFA (layer 5), a value of 45 ft/d was assigned 

to the areal hydraulic conductivity of zone 15. This value is 

slightly lower than the 70-ft/d value assigned in the original 

regional model; however, the product of this value and the 

simulated thickness of the aquifer yielded an estimated 

transmissivity of 15,500 feet squared per day (ft2/d) at Rincon, 

which is 6.9 percent greater than the 14,500 ft2/d value 

obtained from field testing by Carter and Sloope (2004). 

Hydraulic properties for model layer 7, which represents 

the LFA, were represented with a single zone in the regional 

model of Payne and others (2005) having uniform Kh and 

Kv values of 10 ft/d. At Rincon, zone LF4 was assigned a Kh

value of 20 ft/d and a Kv value of 2.4 ft/d. Multiplying the 

Kh value by the simulated thickness of the aquifer yields an 

estimated transmissivity of 3,830 ft2/d, 55 percent greater 

than the 2,470-ft2/d value derived from field testing at Rincon 

(Carter and Sloope, 2004). 
Appendix 1. Regional Groundwater Model 31

Table 1–1. Horizontal and vertical hydraulic conductivity values assigned to hydraulic-property zones for the original (Payne and 

others, 2005) and revised groundwater-flow model.

[––, not applicable]

Unit Model

layer

Payne and others, 2005 Revised model

Hydraulic

property

zone

Hydraulic conductivity,

in feet per day

Hydraulic

property

zone

Hydraulic conductivity,

in feet per day

Horizontal Vertical Horizontal Vertical

Surficial aquifer 1 –– 70 70 1 70 70

Confining unit 2 C1 0.00002 0.00002 C1 0.00002 0.00002

C2 0.20000 0.20000 C2 0.20000 0.20000

C3 0.00001 0.00001 C3 0.00001 0.00001

C4 0.00010 0.00010 C4 0.00010 0.00010

C5 0.00010 0.00010 C5 0.00010 0.00010

Brunswick aquifer system 3 B1 50 50 B1 50 50

C1 0.00002 0.00002 C1 0.00002 0.00002

C2 0.20000 0.20000 C2 0.20000 0.20000

C3 0.00001 0.00001 C3 0.00001 0.00001

C4 0.00010 0.00010 C4 0.00010 0.00010

Confining unit 4 C1 0.00002 0.00002 C1 0.00002 0.00002

C2 0.20000 0.20000 C2 0.20000 0.20000

C3 0.00001 0.00001 C3 0.00001 0.00001

C4 0.00010 0.00010 C4 0.00010 0.00010

C5 0.00010 0.00010 C5 0.00010 0.00010

Upper Floridan aquifer 5 F1 34 34 F1 34 34

F2 2 2 F2 2 2

F3 100 100 F3 100 100

F4 70 70 F4 70 70

F5 394 394 F5 394 394

F6 2,819 2,819 F6 2,819 2,819

F7 150 150 F7 150 150

F8 2,727 2,727 F8 2,727 2,727

F9 100 100 F9 100 100

F10 56 56 F10 56 56

F11 94 94 F11 94 94

F12 25 25 F12 25 25

— — — F13 90 90

— — — F14 398 398

— — — F15 45 45

Confining unit 6 — 0.02000 0.02000 LFC1 0.02000 0.02000

— — — LFC2 2.00000 0.2

— — — LFC3 10.00000 0.2

— — — LFC4 14.20000 1.67

Lower Floridan aquifer 7 — 10.0000 1 LF1 10 10

— — — LF2 87 10

— — — LF3 15.80000 1.6

— — — LF4 20 2.4
32 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

Simulation of Observed Drawdown in 

Pumped Well

Drawdown calculated by the revised model represents the 

average drawdown for a node located at the areal center of the 

grid cell containing the pumped well. This average drawdown 

under-represents the observed drawdown in the actual pumped 

well 36S048 because the area of the grid cell containing the 

pumped well (10 square feet [ft2]) is much larger than the area 

defined by the 12-inch well diameter (0.78 ft2). Simulated 

drawdown at the pumped well can be adjusted on the basis 

of the proportional increase in area attributed to the grid cell 

relative to the area of the pumped well using the following 

equation (Peaceman, 1983): 

Sp=Sb+[Q×ln(re /rw )/2π(TxxTyy )0.5 ], (1)

where

Sp is adjusted drawdown in the pumped well, 

in feet;

Sb is simulated drawdown in the pumped well, 

in feet;

Q is pump discharge, in cubic feet per day 

(192,499);

re is equivalent well block radius, in feet (5);

rw is well radius, in feet (0.5);

Txx is transmissivity in the x direction, in feet 

squared per day (3,826); and 

Tyy is transmissivity in the y direction, in feet 

squared per day (3,826).

Results obtained using this equation indicated that simulated 

values of drawdown at the center of the grid cell containing 

pumped well 35S048 would under-represent the observed 

drawdown in the well by 18.4 ft for a pumping rate of 

1,000 gallons per minute (gal/min). A similar analysis of a 

UFA well pumped at a rate of 292 gal/min, having the same 

radius as well 35S048 but with a value of 15,500 ft2/d for 

transmissivity indicates that the simulated drawdown in the 

grid cell containing the pumped well under-represents the 

observed drawdown at the pumped well by about 1.33 ft. 

Simulated drawdown for the UFA and LFA were adjusted 

using these correction factors and compared with observed 

data for model calibration.

Comparison of Revised to Original Regional Model

Because the regional model of Payne and others (2005) 

was modified by changing grid-cell sizes and assigning different 

hydraulic properties in and near Rincon, a favorable comparison 

of results from the two models ensures that the revised model 

is an accurate representation of groundwater flow. Simulation 

results indicate that water-level residuals and the water budget 

for the revised regional model are similar to those for the original 

model of Payne and others (2005), and thus, both models provide 

similar simulation of the hydrologic system (tables 1–2, 1–3). 

These results were expected because model revisions were 

limited to areas covering 114.5 mi2 at Fort Stewart, 221-mi2

at Hunter Army Airfield and Pooler, and 34 mi2 at Rincon, 

representing less than 1 percent of the model area. 

The mean water-level residual for layer 3 (Brunswick 

aquifer system) shifted from a positive bias in the original 

model (1.79 ft) to a negative bias in the revised model 

(–2.59 ft), as shown on the residuals map and graphs 

(table 1–2, figs. 1–4A, 1–5). For layer 5 (UFA), the mean 

residual remained negative in the revised model, changing 

from –0.84 ft to –3.55 ft. The mean residual for layer 7 (LFA) 

remained positive in the revised model but was closer to zero 

than the value from the original model, changing from 5.2 ft to 

3.05 ft. The root mean square (RMS) of residuals for layer 5 

was similar for the original (9.94 ft) and revised (10.5 ft) 

models. For layer 7, the RMS of residuals decreased from 

9.15 ft in the regional model to 8.87 ft in the revised model. 

The RMS of water-level residuals for layer 3 (11.1 ft) in 

the revised model is nearly double that of the original regional 

model (5.91 ft) but is considered acceptable for the purpose 

of the modified model, which is to simulate flow in the UFA 

and LFA. Most of the increase in the RMS of residuals for 

layer 3 can be attributed to two wells in the Brunswick aquifer 

system that contained water-level residuals of nearly –19 ft 

each (figs. 1–4A, 1–5). These wells are located adjacent to one 

another in the same model cell and in an area where the grid 

size of the revised model is more than four times larger than 

in the original regional model. This larger grid size reduced 

the capability of the model to simulate steep gradients in the 

vicinity of the Savannah area cone of depression and resulted 

in a large residual. This relatively large grid size and corresponding large water-level residuals were distant from the area 

of high grid resolution and the focus of the study at Rincon, 

and are assumed to only slightly influence simulated heads 

in the Rincon area. In the Rincon area, the revised model 

simulated groundwater levels for the Brunswick aquifer that 

were within 5 ft of measured water levels (fig. 1–4A), attesting 

to the relatively high accuracy of the revised model in the area 

of interest to this study. 

Simulated water budgets for the regional and revised 

models compared favorably with most variation occurring 

in layers 1, 2, and 5 (table 1–3). The revised model showed 

a decrease in recharge from, and discharge to, the overlying 

general-head boundary in the surficial aquifer and Brunswick 

aquifer system confining unit (layers 1 and 2). In the UFA 

(layer 5), the revised model showed an increase in recharge 

from, and decrease in discharge to, the general-head boundary, 

and increased outflow and inflow along lateral specified-head 

boundaries. In the LFA (layer 7), inflow from the lateral 

specified-head boundary was slightly lower, and outflow to the 

lateral specified-head boundary was slightly higher.
Appendix 1. Regional Groundwater Model 33

Table 1–2. Water-level calibration statistics for the original (Payne and others, 2005) and revised regional models, base case (year 

2000) simulation.

[Residual equals simulated minus observed head]

Aquifer Model

layer

Number

of

 observations

Minimum residual,

in feet

Maximum residual,

in feet

Mean of residuals,

in feet

Root mean square of

residuals, in feet

Original

model

Revised

model

Original

model

Revised

model

Original

model

Revised

model

Original

model

Revised

model

Brunswick 

aquifer 

system

3 10 –7.67 –18.5 13.3 14.4 1.79 –2.59 5.91 11.1

Upper 

Floridan 

aquifer

5 155 –44.4 –33.9 36.4 28.0 –0.84 –3.55 9.94 10.5

Lower 

Floridan 

aquifer

7 11 –3.62 –5.10 21.5 25.0 5.2 3.05 9.15 8.87

Table 1–3. Comparison of simulated water budget by model layer between the original (Payne and others, 2005) and revised regional 

models, base case (year 2000) simulation.

[Values reported to three significant digits and may not sum to totals because of independent rounding; <, less than]

Hydrogeologic

unit

Model 

layer

Simulated flow, in million gallons per day

Pumpage Recharge from

general head boundary

Discharge to

general head boundary

Inflow along

lateral boundary

Outflow along

lateral boundary

Original 

model

Revised 

model

Original 

model

Revised 

model

Original 

model

Revised 

model

Original 

model

Revised 

model

Original 

model

Revised 

model

Surficial 

aquifer 

system

1 <0.001 <0.001 310 278 132 100 <0.001 <0.001 <0.001 <0.001

Confining 

unit

2 <0.001 <0.001 46.6 32.9 3.62 2.05 <0.001 <0.001 <0.001 <0.001

Brunswick 

aquifer 

system

3 0.241 0.241 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Confining 

unit 

4 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Upper 

Floridan 

aquifer

5 669 669 141 147 22.3 19.6 712 843 268 386

Confining 

unit 

6 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0.004 0.005 <0.001 <0.001

Lower 

Floridan 

aquifer

7 129 129 <0.001 <0.001 <0.001 <0.001 15.5 15.1 2.32 2.62

Total all layers 798 798 498 458 158 122 728 858 270 389
34 Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.

GA

FL

GA

SC

Figure 1–4. Difference between simulated and observed water levels (residuals) by model layer for 2000,

revised regional flow model: (A) Brunswick aquifer system (layer 3), (B) Upper Floridan aquifer (layer 5),

and (C) Lower Floridan aquifer (layer 7). 

0 50 MILES 25

0 50 KILOMETERS 25

C. Lower Floridan aquifer

 (layer 7)

A. Brunswick aquifer system

 (layer 3)

Base from U.S. Geological Survey

1:100,000-scale digital data

B. Upper Floridan 

 aquifer

 (layer 5)

Approximate 

limit of 

Brunswick 

aquifer 

system

N

N

A

TLANTIC

OCEAN

Hunter Army

Airfield 

EXPLANATION

Water-level residual, in feet—

Represents difference of 

simulated minus observed 

water level

< –10

–10 to –5

–5 to 0

0 to 5

5 to 10

< –10

–10 to –5

–5 to 0

0 to 5

>5 0 25

0 25 50 KILOMETERS

50 MILES

GA

FL

GA

SC

Fort

Stewart

Pooler

Brunswick

Rincon

SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

revised regional flow model. A, Brunswick aquifer system (layer 3), B, Upper Floridan aquifer (layer 5), and

C, Lower Floridan aquifer (layer 7). 

Figure 1– 4. Difference between simulated and observed water levels (residuals) by model layer for 2000, 
Appendix 1. Regional Groundwater Model 35

Brunswick aquifer system (layer 3)

Upper Floridan aquifer (layer 5)

Lower Floridan aquifer (layer 7)

Observed water level above or below NAVD 88, in feet

Simulated water level above or below NAVD 88, in feet

Figure 1–5. Observed and simulated water levels in model layers 3, 5, and 7, 

revised groundwater model.

R2=0.5114

–30

–20

–10

0

10

20

30

–30 –20 –10 0 10 20 30

R2=0.9611

–100

–50

0

50

100

150

200

250

–100 –50 0 50 100 150 200 250

R2=0.9831

–50

0

50

100

150

–50 0 50 100 150

groundwater model.

Figure 1–5. Observed and simulated water levels in model layers 3, 5, and 7, revised 
Manuscript approved May 11, 2015

Prepared by the USGS Science Publishing Network

Raleigh Publishing Service Center

Edited by Michael Deacon

Illustrations and layout by Caryl J. Wipperfurth

For more information concerning this report, contact:

Director, South Atlantic Water Science Center

North Carolina–South Carolina–Georgia

720 Gracern Road, Suite 129

Columbia, SC 29210

Phone: (803) 750-6100

http://ga.water.usgs.gov/
Cherry and Clarke—Simulated Effects of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer at Rincon, Effingham County, Ga.—Scientific Investigations Report 2015–5072

ISSN 2328–0328 (online)

http://dx.doi.org/10.3133/sir20155072