U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2010–5080

Prepared in cooperation with the U.S. Department of the Army

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
Cover. Drill rig used to install Lower Floridan production well, Hunter Army Airfield, Savannah, GA. 

Photo by O. Gary Holloway, USGS.
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

By John S. Clarke, Lester J. Williams, and Gregory C. Cherry

Prepared in cooperation with the U.S. Department of the Army

Scientific Investigations Report 2010–5080

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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reproduce any copyrighted materials contained within this report.

Suggested citation:

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.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................4

Site Description.....................................................................................................................................4

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

Hydrogeologic Setting ................................................................................................................5

Groundwater Flow .......................................................................................................................6

Well and Stream Site Identification...................................................................................................6

Methods of Data Collection and Analysis .................................................................................................8

Test Well Drilling ..................................................................................................................................8

Borehole Geophysical Logs ..............................................................................................................12

Flowmeter Testing...............................................................................................................................14

Water-Quality Sampling and Analysis.............................................................................................15

Core Hydraulic Analysis and Packer-Slug Tests ...........................................................................17

Aquifer Tests .......................................................................................................................................17

Groundwater-Flow Model .................................................................................................................18

Hydrogeology and Water Quality of the Floridan Aquifer System.......................................................19

Upper Floridan Aquifer ......................................................................................................................19

Lower Floridan Confining Unit ..........................................................................................................22

Lower Floridan Aquifer.......................................................................................................................23

Effect of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer .............................................25

Observed Water-Level Response.....................................................................................................25

Model Simulation................................................................................................................................28

Interaquifer Leakage and Drawdown Response..................................................................28

Upper Floridan Aquifer Drawdown Offset .............................................................................31

Effect of Pumping Offsets on Groundwater Levels at Hunter Army Airfield....................36

Effect of Pumping Offsets on Water Supply at Hunter Army Airfield................................40

Limitations of Analysis ..............................................................................................................42

Summary and Conclusions.........................................................................................................................42

Selected References...................................................................................................................................44

Appendix. Groundwater Model................................................................................................................47
iv

Figures

 1. Maps showing location of groundwater-flow model area, selected wells, 

and tidal station at Hunter Army Air Field and vicinity, coastal Georgia .............................3

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

in the coastal plain of Georgia (modified from Payne and others, 2005) .............................5

 3. Schematic diagram showing conceptual model of predevelopment and 

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

 4. Well-construction diagram for well HAAF 11 (36Q392) completed in the 

Lower Floridan aquifer, Hunter Army Airfield, Chatham County, Georgia, 2009 .................9

 5. Selected borehole geophysical data from well HAAF 11 (36Q392). Percentage 

flow contribution determined from flowmeter survey in open borehole, 

333 –1,168 feet, Hunter Army Airfield, Chatham County, Georgia .......................................12

 6. Borehole flowmeter data from well HAAF 11 (36Q392), Hunter Army Airfield, 

Chatham County, Georgia ..........................................................................................................14

 7–8. Graphs showing—

 7. Specific conductance of drilling fluids while drilling well HAAF 11 

(36Q392), Hunter Army Airfield, Chatham County, Georgia, 2009 ...............................15

 8. Distribution of selected chemical properties and constituent 

concentrations by sampled depth at well HAAF 11 (36Q392), 

Hunter Army Airfield, Chatham County, Georgia, July 2009........................................15

 9. Trilinear diagram showing composition of major ions at various depths at 

well HAAF 11 (36Q392), Hunter Army Airfield, Chatham County, Georgia, July 2009 ......17

 10–13. Graphs showing—

 10. Comparison of water levels at well HAAF 8 (36Q292) to background 

water levels at Morrison Plantation observation well (36Q020) and to 

stage and barometric pressure at tidal gage 02198773, February 12–25, 2009 .......20

 11. Effects of different time series components on development of a synthetic 

water level used to filter out extraneous trends from well HAAF 8 (36Q292), 

Hunter Army Airfield, Chatham County, Georgia ............................................................21

 12. Logarithmic plot of drawdown and time for observation well HAAF 8 (36Q292) 

during Upper Floridan aquifer test conducted in well HAAF 9 (36Q391), 

Hunter Army Airfield, Chatham County, Georgia, February 17 –18, 2009 ..................22

 13. Semi-logarithmic plots of drawdown and time and residual drawdown 

and t/t ' for aquifer test of Lower Floridan aquifer at well HAAF 11 (36Q392), 

Hunter Army Airfield, Chatham County, Georgia, August 13 –16, 2009 .....................24

 14. Map and graphs showing location and construction characteristics of wells 

used for aquifer tests at Hunter Army Airfield, Chatham County, Georgia .......................25

 15–16. Graphs showing—

 15. Water levels in the pumped Lower Floridan well (HAAF 11, 36Q392) and in 

Upper Floridan observation wells HAAF 8 (36Q292) and HAAF 5 (36Q288) 

before, during, and after the 72-hour aquifer test of the Lower Floridan aquifer, 

Hunter Army Airfield, Chatham County, Georgia, August 11–19, 2009 ......................26

 16. Estimated drawdown calculated in Upper Floridan wells (A) HAAF 8 

(36Q292) and (B) HAAF 5 (36Q288) during the 72-hour aquifer test of the 

Lower Floridan aquifer in well HAAF 11 (36Q392), Hunter Army Airfield, 

Chatham County, Georgia .................................................................................................27
v

17–18. Maps showing—

17. Simulated steady-state drawdown in the Lower Floridan aquifer 

resulting from pumping well HAAF 11 (36Q392) at 748 gallons per minute, 

Hunter Army Airfield and vicinity, coastal Georgia......................................................29

18. Simulated steady-state drawdown in the Upper Floridan aquifer 

resulting from pumping well HAAF 11 (36Q392) at 748 gallons per minute, 

Hunter Army Airfield and vicinity, coastal Georgia......................................................30

19. Block diagram showing change in simulated steady-state water budget 

resulting from initiation of pumping at well HAAF 11 (36Q392), Hunter Army Airfield, 

Chatham County, Georgia..........................................................................................................31

20. Maps showing percentage of interaquifer leakage from the Upper Floridan 

aquifer contributing flow to well HAAF 11 (36Q392) pumping at a rate of 

748 gallons per minute (1.08 million gallons per day), Hunter Army Airfield 

and vicinity, coastal Georgia.....................................................................................................32

21. Profile showing simulated steady-state drawdown in the Upper and Lower Floridan 

aquifers resulting from pumping in the Lower Floridan aquifer at well HAAF 11 

(36Q392) and from pumping the Upper Floridan aquifer at rates of 189 and 650 gallons 

per minute at a hypothetical Upper Floridan aquifer well at the site of well HAAF 11 

(36Q392), Hunter Army Airfield and vicinity, coastal Georgia..............................................33

22–25. Maps showing—

22. Simulated drawdown in the Upper Floridan aquifer resulting from 

pumping a hypothetical Upper Floridan aquifer well at the site of 

well HAAF 11 (36Q392) at rates of 189 and 650 gallons per minute, 

Hunter Army Airfield and vicinity, coastal Georgia......................................................34

23. Drawdown in the Upper Floridan aquifer for scenario A—effect of pumping 

the Lower Floridan aquifer at well HAAF 11 (36Q392) at a rate of 748 gallons 

per minute, and reducing pumping in existing Upper Floridan aquifer HAAF 

supply wells by 187 gallons per minute or 25 percent of well HAAF 11 (36Q392) 

pumping rate— Hunter Army Airfield and vicinity, coastal Georgia .........................38

24. Drawdown in the Upper Floridan aquifer for scenario B—effect of pumping 

the Lower Floridan aquifer at well HAAF 11 (36Q392) at a rate of 748 gallons 

per minute, and reducing pumping in existing Upper Floridan aquifer supply 

wells by 374 gallons per minute or 50 percent of the well HAAF 11 (36Q392) 

pumping rate—Hunter Army Airfield and vicinity, coastal Georgia .........................39

25. Simulated Upper Floridan aquifer potentiometric surfaces for the 

2005 base case and scenarios A, B, and C, Hunter Army Airfield and 

vicinity, coastal Georgia ...................................................................................................40
vi

Tables

1. Average daily groundwater withdrawal at Hunter Army Airfield, 2002–2008.....................4

 2. Well-construction and location information for selected wells at Hunter Army 

Airfield and vicinity.......................................................................................................................8

3. Chronology of drilling, logging, and testing in Lower Floridan Well HAAF 11 

(36Q392), Hunter Army Airfield, Chatham County, Georgia, 2009..........................................9

4. Lithologic description of rock cuttings, well HAAF 11 (36Q392), Hunter Army 

Airfield, Chatham County, Georgia...........................................................................................10

5. Water-quality analysis of samples collected at selected depths in well HAAF 11 

(36Q392), Hunter Army Airfield, Chatham County, Georgia, June and August 2009 ........16

 6. Vertical hydraulic conductivity and porosity of core samples collected from 

the Lower Floridan aquifer at well HAAF 11 (36Q392), Hunter Army Airfield, 

Chatham County, Georgia..........................................................................................................17

7. Horizontal hydraulic conductivity determined from packer-slug tests at 

well HAAF 11 (36Q392), Hunter Army Airfield, Chatham County, Georgia..........................17

8. Summary of aquifer tests conducted in Upper and Lower Floridan aquifer 

wells at Hunter Army Airfield, Chatham County, Georgia, 2009..........................................18

9. Simulated steady-state water budget for 2000 and after initiation of pumping 

748 gallons per minute (1.08 million gallons per day) at well HAAF 11 (36Q392), 

Hunter Army Airfield, Chatham County, Georgia...................................................................28

10. Simulated pumping and drawdown for the year 2005 permitted pumping rate 

and model scenarios A, B, and C.............................................................................................37

11. Projected reductions in Upper Floridan aquifer permitted capacity and net gain 

in total water capacity for various pumping periods, well HAAF 11 (36Q392), 

Hunter Army Airfield, Chatham County, Georgia...................................................................41

Acknowledgments

The authors appreciate the assistance of the U.S. Army, AECOM, Inc., and Oak Ridge 

National Laboratory for providing onsite support and data. Special thanks go to Stanley 

Thomas, Eric Stulpin, Tressa Rutland, and Nathaniel Williams, II, of the U.S. Army Environmental Protection and Compliance Branch. Peter Thibodeau and John Funk of AECOM, 

Inc., provided valuable technical support, review, and guidance regarding testing procedures 

employed at the site. Robert McManus of AECOM provided onsite logistical support and safety 

monitoring. O. Gary Holloway and Michael D. Hamrick, USGS, monitored drill site activities 

and collected hydrogeologic and water-quality data. Michael Register of Martin Drilling provided valuable assistance for onsite data collection. Debbie Warner Gordon and L. Elliott Jones, 

USGS, provided valuable support in conducting model simulations, and Jaime A. Painter, 

USGS, provided spatial analysis of data for model simulations and output. Lucy E. Edwards

and Colleen Durand (USGS, Reston, VA) provided information on the paleontology and

stratigraphic interval of core collected at the site. 
vii

Conversion Factors and Datums

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

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3

) 

gallon (gal) 3.785 cubic decimeter (dm3

) 

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)

gallon per day (gal/d) 0.003785 cubic meter per day (m3

/d)

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) 

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29) or 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.

*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.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).

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

By John S. Clarke, Lester J. Williams, and Gregory C. Cherry

Abstract

Test drilling and field investigations, conducted at 

Hunter Army Airfield (HAAF), Chatham County, Georgia, 

during 2009, were used to determine the geologic, hydraulic, 

and water-quality characteristics of the Floridan aquifer 

system and to evaluate the effect of Lower Floridan aquifer 

(LFA) pumping on the Upper Floridan aquifer (UFA). Field 

investigation activities included (1) constructing a 1,168-foot 

(ft) test boring and well completed in the LFA, (2) collecting 

drill cuttings and borehole geophysical logs, (3) collecting 

core samples for analysis of vertical hydraulic conductivity 

and porosity, (4) conducting flowmeter and packer tests in the 

open borehole within the UFA and LFA, (5) collecting depthintegrated water samples to assess basic ionic chemistry of 

various water-bearing zones, and (6) conducting aquifer tests 

in the new LFA well and in an existing UFA well to determine 

hydraulic properties and assess interaquifer leakage. Using data 

collected at the site and in nearby areas, model simulation was 

used to quantify the effects of interaquifer leakage on the UFA 

and to determine the amount of pumping reduction required in 

the UFA to offset drawdown resulting from the leakage.

Borehole-geophysical and flowmeter data indicate the 

LFA at HAAF consists of limestone and dolomitic limestone 

between depths of 703 and 1,080 ft, producing water from 

six major permeable zones: 723–731; 768–785; 818–837; 

917–923; 1,027–1,052; and 1,060–1,080 ft. Data from a 

flowmeter survey, conducted at a pumping rate of 748 gallons per minute (gal/min), suggest that the two uppermost 

zones contributed 469 gal/min or 62.6 percent of the total 

flow during the test. The remaining four zones contributed 

from 1.7 to 18 percent of the total flow. Grab water samples 

indicate that with the exception of fluoride, constituent 

concentrations in the LFA increased with depth; water from 

the deepest interval (1,075 ft) contained chloride and sulfate 

concentrations of 480 and 240 milligrams per liter (mg/L), 

respectively. These relatively high concentrations were 

interpreted to have little effect on the overall quality of the 

well because flowmeter results indicated that water from 

1,060 to 1,080 ft contributed less than 2 percent of the total 

flow to the completed well. 

Results of a 72-hour aquifer test indicate that pumping 

a LFA well at a rate of 748 gal/min produced a drawdown 

response of 0.76 ft in a well completed in the UFA located 

176 ft from the pumped well. A revised regional groundwaterflow model was used to simulate long-term (steady-state) 

leakage response of the UFA to pumping from the LFA and 

to estimate the equivalent amount of pumping from the UFA 

that would produce similar drawdown. Pumping the well at a 

rate of 748 gal/min (about 1 million gallons per day [Mgal/d]) 

resulted in a maximum simulated steady-state drawdown of 

36.2 ft in the LFA and was greater than 1 ft over a 146 squaremile area. Simulated steady-state drawdown in the overlying 

UFA that resulted from interaquifer leakage was greater than 

1 ft over a 141 square-mile area and was 2.03 ft at the pumped 

well. Flow to the pumped well was derived from increased 

lateral flow across the specified-head boundary (0.02 Mgal/d) 

and increased leakage from the UFA (0.52 Mgal/d), and by 

reductions in discharge to the Lower Floridan confining unit 

(0.53 Mgal/d) and to the lateral specified-head boundary 

(0.53 Mgal/d). Sixty-five percent of the leakage from the 

UFA occurred within 1 mile of the pumped well. This larger 

contribution results from a larger head gradient between the 

pumped well and the overlying aquifer in areas close to the 

pumped well.

The Georgia Environmental Protection Division 

interim permitting strategy for the LFA requires simulation 

of (1) aquifer leakage from the UFA to LFA resulting from 
2 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

pumping the new LFA well, and (2) the equivalent rate of 

UFA pumping that induces the identical maximum drawdown 

in the UFA that would be expected as a result of pumping the 

LFA. Results of this analysis can be used as a basis to reduce 

nearby UFA permitted pumping in the same general area 

(within a 5-mile radius) by an amount equal to or greater than 

the determined leakage rate. Results of model simulations 

indicate that these two requirements result in widely varying 

pumping offsets for the UFA. Simulated interaquifer leakage 

was 361 gal/min (0.52 Mgal/d), whereas the equivalent UFA 

pumping rate to offset maximum drawdown was 189 gal/min 

(0.27 Mgal/d). The simulated pumping rate to match the 

maximum drawdown in the UFA underpredicts the amount 

of pumping offset because the cone of depression formed in 

response to pumping the UFA is steeper near the pumped well 

and covers a smaller area than the area simulated in response to 

interaquifer leakage. Thus, the simulated leakage rate may be a 

more effective means of evaluating required pumping offsets.

Three groundwater-pumping scenarios were run to 

evaluate the effect of various pumping changes on groundwater levels at HAAF. For two of the scenarios, the LFA 

was pumped 24 hours per day at a rate of 748 gal/min, while 

pumping from UFA wells was reduced by 187 gal/min for 

the first scenario and 361 gal/min for the second scenario. 

The third scenario involved pumping the LFA well 24 hours 

per day at a rate of 374 gal/min and reducing pumping of the 

UFA by 187 gal/min. These pumpage reduction scenarios 

decreased the magnitude and extent of drawdown in the UFA 

in comparison to the scenario in which the LFA was pumped 

without adjusting withdrawal from the UFA. None of the 

scenarios resulted in noticeable changes in the configuration 

of the simulated potentiometric surface and related 

groundwater-flow directions for the UFA. 

Introduction

Hunter Army Airfield (HAAF) is located in the coastal 

area of Georgia near the city of Savannah (fig. 1). Water 

supply at the facility is derived from groundwater withdrawal 

from wells completed in the Upper Floridan aquifer (UFA). 

Concern over saltwater intrusion at Hilton Head Island, South 

Carolina, has resulted in increased restrictions by the Georgia 

Environmental Protection Division (GaEPD) on permitted 

groundwater withdrawals from the UFA. The current UFA 

groundwater permit at HAAF has been modified to reduce 

capacity from 1.03 to 0.98 million gallons per day (Mgal/d). 

To meet growing demands for water in the coastal Georgia 

area, GaEPD has encouraged use of alternative sources of 

water to the UFA, including wells completed in the Lower 

Floridan aquifer (LFA). HAAF seeks to offset reductions 

in permitted withdrawals from the UFA by making up the 

capacity using a well completed in the LFA. 

Pumping from the LFA may increase the head gradient locally between the UFA and LFA, lower water levels 

in the UFA, and induce leakage (groundwater flow) from 

the UFA to the LFA. In January 2003, GaEPD released an 

interim strategy for permitting Lower Floridan withdrawals 

in the 24-county coastal area of Georgia (Nolton Johnston, 

Georgia Environmental Protection Division, written commun., 

January 28, 2003). The GaEPD permitting strategy states: 

“The applicant must demonstrate, using detailed 

aquifer testing and standard hydrogeological 

methods, that their LF [Lower Floridan] aquifer 

withdrawal does not induce downward leakage 

from the UF [Upper Floridan] aquifer and will have 

no net negative impact on water levels in the UF 

aquifer, or:

“If the aquifer tests do show that there is an impact 

on water levels in the UF aquifer because of production from the LF aquifer, the applicant must calculate, using an approved hydrologic analysis, the UF 

contribution to the LF well. Then, using the information from the hydrogeological studies, the applicant 

must reduce nearby current UF withdrawals in an 

amount equal to any induced leakage from the UF 

into the LF. In other words, the applicant must offset 

the impact of the LF pumping by reducing nearby 

UF permitted pumping in the same general area 

(within a 5-mile (mi) radius) by an amount equal to 

or greater than the determined UF leakage. This will 

assure continued protection of the UF aquifer under 

the no net negative impact policy.”

As part of the interim permitting strategy, GaEPD 

provided a hydrogeological study protocol that states (Nolton 

Johnston, Georgia Environmental Protection Division, written 

commun., January 28, 2003): “The applicant must conduct 

site-specific hydrogeological testing to obtain the data 

needed for development of a groundwater model capable of 

determining the contribution from the UFA that would result 

from the proposed withdrawal from the LFA. This information 

would be used to develop a new groundwater model, which 

would be run to simulate 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.”

To assess the water-supply potential of the LFA, the 

U.S. Geological Survey (USGS), in cooperation with the 

U.S. Department of the Army, conducted an investigation 

during 2009 to determine the hydrogeology and water quality 

of the Floridan aquifer system and the potential effect that 

pumping from the LFA would have on the UFA. The study 

included construction of a test well in the LFA, detailed site 

investigations, and groundwater-modeling studies. 
Introduction 3

516

FORT STEWART

FLORIDA

GEORGIA

Model

boundary

Map area

at right

SOUTH

CAROLINA

PLAIN

COASTAL

516

16

95

95

EXPLANATION

Tidal station and identification

Hunter Army Airfield

Image from U.S. Geological Survey High Resolution Orthoimagery 

 for Coastal Georgia, 0.5-meter pixel resolution, 2007

Figure 1. Location of groundwater-flow model area, selected wells, and tidal station at Hunter Army Airfield 

and vicinity, coastal Georgia (potentiometric contours modified from Peck and others, 1999).

Base map from U.S. Geological Survey 

1:2,000,000-scale digital data 

Roads from TIGER 1:100,000-scale digital data (2000)

0 5 10 15 MILES

0 10 15 KILOMETERS 5

ATLANTIC

OCEAN

Location of

map below

Potentiometric contour May 1998—

Interval, 10 feet. Hachures indicate 

depression. Datum is NAVD 88

Well—Hunter Army Airfield identification; U.S. 

Geological Survey identification in parentheses

24-county coastal Georgia area 

as identified by the Georgia 

Environmental Protection Division

0

Well HAAF 1

(36Q285)

021989773

National Weather Service

 precipitation station 

 and identification

NWS 097847

Morrison Plantation 

 well (36Q020)

l

l l

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

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

l l l l l l l l l

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

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

l l

l l

l l l l l l

l l l l l l l

l

l

l l

l

l l l l l l l

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

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

l l

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

CHATHAM

TATTNALL

EVANS

JASPER

LONG

GA

SC

BEAUFORT

Savannah

Hinesville

Richmond 

Hill

–50

0

0

40

30

20

10

Salt Creek

River

Lower Ogeeche e

Hunter Army Airfield

0 0.5 1 1.5 MILES

0 0.5 1 1.5 KILOMETERS

021989773

35P110

35P125

36Q330

NWS 097847

Well HAAF 5

(36Q288)

Well HAAF 1

(36Q285)

Well HAAF 2

(36Q286)

Well HAAF 8

36Q292

Well HAAF 9

36Q391

Well HAAF 11

36Q392

81°0' 80°45'

 Hilton

 Head

Island

81°45' 81°30' 81°15'

32°0'

32°15'

31°45'

Figure 1. Location of groundwater-flow model area, selected wells, and tidal station at Hunter Army Air Field and vicinity, 

coastal Georgia (potentiometric contours modified from Peck and others, 1999).
4 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Purpose and Scope

This report documents results of field investigations and 

groundwater-model simulations conducted at HAAF during 

2009 to determine the hydrogeology and water quality of the 

Floridan aquifer system and to provide data needed to assess 

the effect of LFA pumping on the UFA, specifically to 

• Evaluate leakage response in a nearby well completed 

in the UFA to pumping from the LFA, and 

• Quantify the amount of pumpage reduction in the 

Upper Floridan well (or wells) required to offset leakage and the amount of drawdown induced by leakage.

Field investigations included:

• Constructing a 1,168-foot (ft) test boring and 

constructing a test well completed in the LFA;

• Collecting drill cuttings and borehole geophysical 

logs at the test well;

• Collecting core samples, in the test well, for analysis 

of vertical hydraulic conductivity and porosity;

• Conducting flowmeter and packer tests in the open 

borehole within the UFA and LFA in the test well; 

• Collecting depth-integrated water samples to assess 

basic ionic chemistry of various water-bearing zones 

in the test well; and

• Conducting aquifer tests in the LFA test well and 

in the existing UFA wells to determine hydraulic 

properties and assess interaquifer leakage. 

The effects of interaquifer leakage on the UFA were 

quantified through simulation using a modified groundwaterflow model of coastal Georgia (Payne and others, 2005). 

The groundwater model was used to quantify the amount of 

pumping reduction required in the UFA to offset drawdown 

from leakage resulting from pumping the LFA.

Site Description

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

Stewart and HAAF, Georgia, are home to the 3rd Infantry 

Division. HAAF is the focus of this investigation and is located

in the Georgia Coastal Plain in central Chatham County, 

Georgia (fig. 1). The site is characterized by flat topography, 

with sandy topsoil typical of the Georgia coastal area. 

The study area has a mild climate with warm, humid 

summers and mild winters. Long-term climatic patterns in 

the area are derived from records provided by the National 

Weather Service Station at Savannah International Airport 

(097847). During 1971–2000, precipitation at station 097847 

averaged about 49 inches per year (in/yr). Maximum monthly 

rainfall (exceeding 4 inches per month) generally occurs 

during June–September, with monthly rainfall totals averaging 

less than 4 inches during the rest of the year. Mean monthly 

pan evaporation at station 097847 during 1965–2003 ranged 

from 2.43 to 8.49 inches per month, with the greatest evaporation during April–August. 

Water Use

Water supply at HAAF is provided by four wells—

HAAF 1, 2, 8, and 9—completed in the UFA, with a GaEPD 

permit limit of 1.03 Mgal/d in 2009. During 2002–2008, average monthly withdrawal exceeded 1 Mgal/d during the peak 

period from April through July and annual averages ranged 

from 0.78 Mgal/d in 2005 to 1.0 Mgal/d in 2007 (table 1). 

GaEPD has requested that the 2009 permitted withdrawal 

rate be reduced by 0.05 Mgal/d to a rate of 0.98 Mgal/d by 

July 2010 (Stanley Thomas, U.S. Department of the Army, 

oral commun., February 19, 2010).

Table 1. Average daily groundwater withdrawal at Hunter Army Airfield, 2002–2008 (data from Julia Fanning, U.S. Geological Survey, 

written commun., February 2010).

Year

Average withdrawal, in million gallons per day

Jan Feb Mar Apr May June July Aug Sept Oct Nov Dec Annual 

average

2002 0.81 0.96 0.98 1.18 1.29 1.07 1.04 1.02 0.86 0.61 0.60 0.58 0.92

2003 0.62 0.77 0.86 0.77 0.89 0.84 0.95 0.93 0.93 0.95 0.80 0.75 0.84

2004 0.85 0.71 0.91 1.09 1.44 1.23 1.11 1.04 0.92 0.87 0.82 0.74 0.98

2005 0.67 0.65 0.60 0.84 0.82 0.75 0.97 0.92 0.99 0.75 0.80 0.62 0.78

2006 0.68 0.67 0.83 1.08 1.09 1.10 1.07 1.11 0.87 0.95 0.91 0.81 0.93

2007 0.91 0.70 0.87 1.26 1.28 1.24 1.12 1.11 0.95 1.03 0.92 0.63 1.00

2008 0.80 0.74 0.68 0.89 1.30 1.21 1.06 0.81 1.12 1.15 0.67 0.79 0.93

Average 0.76 0.74 0.82 1.01 1.16 1.06 1.05 0.99 0.95 0.90 0.79 0.70 0.91
Introduction 5

Hydrogeologic Setting

The 24-county coastal Georgia area 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 (Clarke and others, 1990). These 

sediments constitute three major aquifer systems, in order of 

descending depth: the surficial aquifer system, Brunswick 

aquifer system, and Floridan aquifer system (fig. 2). 

In the coastal area, the surficial aquifer system (fig. 2) 

consists of Miocene and younger age interlayered sand, clay, 

and thin limestone beds (Clarke, 2003). At HAAF, the surficial 

aquifer consists of fine sand at depths of less than 100 ft 

and largely is unconfined. Elsewhere in coastal Georgia, the 

aquifer system includes a water-table zone and two confined 

zones. 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 Brunswick aquifer system (fig. 2) 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, 

slightly phosphatic and dolomitic Miocene quartz sand and 

dense phosphatic limestone of the same age (Clarke and 

others, 1990). The lower Brunswick aquifer consists of poorly 

sorted, fine to coarse, phosphatic and dolomitic Oligocene 

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

Brunswick aquifer system consists of clayey fine sand and silt 

that has a much lower permeability than in the Brunswick, 

GA, area and can largely be considered a confining unit. 

Accordingly, in this study, sediments constituting 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 water for all uses (excluding 

thermoelectric) in the coastal area is the Floridan aquifer 

system, which is composed of carbonate rocks of varying permeability. The Floridan aquifer system is confined by overlying 

clay layers and separated into several permeable water-bearing 

zones by layers of relatively denser limestone that act as 

semiconfining units allowing vertical leakage of groundwater. 

In the Savannah–Hilton Head Island area, McCollum and 

Counts (1964) identified five water-bearing zones in strata that 

would later be defined as part of the Floridan aquifer system. 

The two shallowest of these water-bearing zones are part 

of the UFA, and the deeper three are part of the LFA (Krause 

and Randolph, 1989). In Beaufort County, SC, the term 

middle Floridan aquifer is used by the State of South Carolina 

(Ransom and White, 1999) for a water-bearing zone approximately 250–550 ft below land surface that is equivalent to 

zones 3 and 4 of McCollum and Counts (1964).

The UFA is overlain by a confining unit (fig. 2) consisting of layers of silty clay and dense phosphatic Oligocene 

dolomite that separate the aquifer from overlying permeable 

units of the Brunswick aquifer system. Reported vertical 

hydraulic conductivity of this confining unit, based on laboratory analysis of core, ranges from 2.3×10– 4 to 3.0 feet per day 

(ft/d) (Clarke and others, 2004).

The UFA (fig. 2) is highly productive and consists of 

Eocene to Oligocene limestone and dolomite. Miller (1986) 

reported ranges in aquifer thickness in Chatham County 

from 400 to 600 ft. Reported transmissivity of the UFA in 

Chatham County ranges from 20,000 to 80,000 feet squared 

per day (ft2/d) (Clarke and others, 2004). Zones of very high 

hydraulic conductivity exist within relatively thin intervals of 

the Floridan aquifer system, especially in the UFA (Clarke and 

others, 2004). 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

Lower Floridan aquifer

Fernandina 

permeable 

zone

Confining unit Not

modeled

Lower Floridan

confining unit

Lazaretto Creek Formation Upper Floridan confining unit

Geologic unit1 Hydrogeologic unit2

Confining

unit

Upper waterbearing zone

Upper 

Floridan semiconfining unit

Lower waterbearing zone

Lower Middle Upper

Series Model

layer Savannah Brunswick

1

GHB

(not

modeled)

2

3

4

5

6

7

2Modified from Randolph and others, 1991; Clarke and Krause, 2000. 

1Modified from Randolph and others, 1991; Weems and Edwards, 2001.

Upper

Brunswick

 aquifer

Lower

Brunswick

 aquifer

Upper waterbearing zone

Lower waterbearing zone

Tiger Leap 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]

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] 
6 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

The UFA is underlain by a confining unit of dense, 

recrystallized limestone and dolomitic limestone of middle 

to upper Eocene age that hydraulically separates, to varying 

degrees, the UFA from the LFA (fig. 2). Counts and Donsky 

(1963) reported the vertical hydraulic conductivity of this 

confining unit was 6.7×10– 4 ft/d on the basis of laboratory 

analysis of a single core.

The LFA is composed of Upper Cretaceous to middle 

Eocene limestone and dolomitic limestone. At Savannah, 

the aquifer consists mostly of middle Eocene limestone, and 

a thickness of 100–300 ft was reported in Chatham County 

(Miller, 1986). Reported transmissivity of the LFA in Chatham 

and Bryan Counties, GA, ranged from 8,200 to 8,300 ft2/d 

(Clarke and others, 2004). 

The LFA is underlain by a confining unit consisting of 

Upper Cretaceous to lower Eocene marl, which Falls and others (2005) describe as a semi-indurated, fine-grained mixture 

of carbonate, clay, silt, and sand that generally is dominated 

by clay and silt. The base of the Floridan aquifer system and 

the underlying marl is recognized on natural-gamma logs by 

a sharp increase in counts per second from carbonate strata to 

the marl (Falls and others, 2005).

The permeability of the Floridan aquifer system is 

reduced in the vicinity of the Gulf Trough (fig. 3)—a zone of 

relatively thick accumulations of fine-grained clastic sediments and clay-bearing carbonates where the permeability 

of coastal plain deposits decrease. In this area, groundwater 

flow is partially impeded by the juxtaposition of rocks of 

higher permeability updip and downdip from the trough, with 

rocks of lower permeability within the trough (Krause and 

Randolph, 1989).

Groundwater Flow

Groundwater flow in the Floridan aquifer system mainly 

is controlled by rates and distribution of recharge to and 

discharge from the system, the extent and effects of confinement, and the ability of aquifers to transmit and store water 

(Krause and Randolph, 1989). A schematic diagram of the 

conceptualized predevelopment (no pumping) and modern-day 

(2000) flow systems in coastal Georgia is shown in figure 3. 

Water recharges the aquifers in the northern part of the 

coastal area where they are exposed at or near land surface 

and flows mostly southeastward toward the coast where it 

discharges into overlying units and surface-water bodies. Prior 

to development, the flow system was considered to have been 

at dynamic equilibrium, and potentiometric surfaces were 

considered nearly static from year to year. 

The modern-day flow system reflects changes that have 

occurred as a result of groundwater development (withdrawal). 

Groundwater withdrawal has lowered water levels, induced 

additional recharge and reduced natural discharge, and 

degraded the quality of water in places along the coast. 

An extensive cone of depression has developed in the 

potentiometric surface of the UFA in the Savannah area 

(fig. 1). HAAF lies within this regional cone of depression. 

Here, the principal direction of groundwater flow is toward the 

center of the cone of depression near Savannah.

Saltwater contamination is restricting 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 substantial groundwater-level decline and subsequent saltwater intrusion from 

underlying strata containing highly saline water at Brunswick, 

GA, and encroachment of seawater at the northern end of 

Hilton Head Island, SC. Saltwater contamination at these 

locations has constrained further development of the UFA in 

the coastal area and has created competing demands for the 

limited supply of freshwater.

Well and Stream Site Identification

In this report, wells are identified by using a local site 

name, such as HAAF 11 and a USGS numbering system based 

on USGS topographic maps (such as 36Q392). In Georgia, 

each 71⁄2-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 HAAF 8 is the 292nd well inventoried in the 

Garden City quadrangle (map 36Q) and is designated 36Q292. 

Surface-water stations are identified by a numbering 

system used for hydrologic-station records in USGS publications since October 1, 1950. The order of listing stations is in 

a downstream direction along the main channel. All stations 

on a tributary entering upstream from a main stream station 

are listed before that station. Each surface-water station is 

assigned a unique 8- to 14-digit number. Each station number, 

such as 02352500, begins with the 2-digit identifier “02,” 

which designates the major river basin (for example, “02” is 

the South Atlantic Slope Basin), followed by the downstreamorder number, “352500,” which can range from 6 to 12 digits.
Introduction 7

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

Figure 3. Schematic diagram showing conceptual model of (A) predevelopment and (B) modernday (2000) flow system. Arrows indicate general direction of groundwater flow (modified from 

Payne and others, 2005; Priest, 2004).

Gulf

Trough

Figure 3. Schematic diagram showing conceptual model of (A) predevelopment and (B) modernday (2000) flow system (modified from Priest, 2004; Payne and others, 2005).
8 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Methods of Data Collection 

and Analysis

To assess the hydrogeology and water quality of the 

Floridan aquifer system at HAAF, site investigations were 

conducted during 2009, including construction of a 1,168-ft 

test boring and a new well (HAAF 11, 36Q392) completed in 

the LFA adjacent to existing HAAF Upper Floridan production 

well HAAF 8 (36Q292, fig. 1). Well construction and location 

information for all wells used during this study are listed in 

table 2. 

Test Well Drilling 

The LFA test well (HAAF 11, 36Q392) completed for 

this study was drilled at HAAF during May–June 2009. 

Drilling was conducted in several stages to accommodate 

collecting core, conducting geophysical logging, and 

completing various hydraulic tests (table 3). Prior to well 

completion, a pilot hole was drilled using (1) hydraulic 

mud-rotary methods and a bentonite-based drilling fluid 

through unconsolidated sediments to a depth of 333 ft, and 

(2) reverse-air rotary methods through consolidated limestone 

Table 2. Well-construction and location information for selected wells at Hunter Army Airfield and vicinity.

[NAD 83, North American Datum of 1983; NGVD 29, National Geodetic Vertical Datum of 1929; gal/min, gallon per minute; UF, Upper Floridan; 

LF, Lower Floridan; OW, not pumped (observation well)]

Well 

identifier Well name Site identification

Latitude Longitude Altitude 

(feet 

above 

NGVD 29)

Open interval

(feet below

land surface) Year of 

construction Aquifer Well yield (gal/min) Decimal degrees 

(NAD 83) Top Bottom

35P110 Richmond Hill 

UF TW

315443081185902 31.9119 –81.3164 10 314 441 2000 UF OW

35P125 CSSI Richmond 

Hill modified 

LF well

315443081185903 31.9120 –81.3163 12 1,010 1,095 2000 LF OW

36Q020 Morrison 

Plantation

320021081124801 32.0060 –81.2132 13 330 336 1948 UF OW

36Q285 U.S. Army, 

HAAF 1

320145081080601 32.0294 –81.1348 23 259 504 1940 UF 1,300

36Q286 U.S. Army, 

HAAF 2

320115081074501 32.0210 –81.1290 35 260 555 1942 UF 1,200

36Q288 U.S. Army, 

HAAF 5

320001081110401 32.0005 –81.1845 11 85 380 1956 UF OW

36Q292 U.S. Army, 

HAAF 8

320003081102301 32.0009 –81.1726 22 255 375 1956 UF 80

36Q330 Berwick Plantation Lower 

Floridan well

320139081134002 32.0275 –81.2278 11 718 1,080 2002 LF 800

36Q391 U.S. Army, 

HAAF 9

320018081100601 32.0049 –81.1759 16 270 425 2006 UF 600

36Q392 U.S. Army, 

HAAF 11

320005081102101 32.0014 –81.1725 20 703 1

1,112 2009 LF 750

1

 Original borehole depth 1,168 feet; backfilled to 1,112 feet.
Methods of Data Collection and Analysis 9

of the UFA and LFA to a depth of 1,168 ft. Packer tests, 

ambient/pumping flowmeter traverses, and grab water samples 

were collected in the open borehole. These tests were critical 

in determining depths of the confining unit and water-bearing 

zones, and groundwater quality. Using this information, casing 

was installed to a depth of 703 ft, and the well was backfilled 

to a total depth of 1,112 ft, leaving an open-hole interval of 

703–1,112 ft. Well-construction details are listed in table 2 and 

shown in figure 4.

During drilling, rock cuttings were collected every 10 ft 

from land surface to the bottom of the borehole, and changes 

in drill-bit penetration rates and the specific conductance of 

drilling fluids were monitored and recorded. Reflected-light 

microscopy was used to describe the mineralogy and texture 

of rock cuttings. Changes in drill-bit penetration rates were 

compared to lithologic changes observed in cuttings to assist 

in determining the depth intervals of contacts between rock 

units and voids, including solution cavities in the subsurface. 

A generalized description of rock cuttings is provided in 

table 4.

Table 3. Chronology of drilling, logging, and testing in Lower Floridan Well HAAF 11 (36Q392), Hunter Army Airfield, Chatham County, 

Georgia, 2009.

Drilling stage Testing/remarks

Stage 1: drilled a 32-inch hole with mud-rotary methods to 

109 feet; set and grouted 24-inch casing to 86 feet

No logs collected in this interval.

Stage 2: drilled a 23-inch hole with mud-rotary methods to 

350 feet; set and grouted 16-inch casing to 333 feet

Caliper and electric logs collected in mud-rotary hole prior to 

setting casing.

Stage 3: drilled an 8-inch pilot hole with reverse air-rotary 

methods to 1,168 feet; collected spot core at selected 

intervals during drilling

Caliper, electric, acoustic, and optical televiewer, and full-wave sonic logs 

collected in water-filled hole; ambient and pumping flowmeter traverses 

were run to determine the depth and yield of water-bearing zones and 

identify confining bed; packer tests completed in the confining bed.

Stage 4: reamed pilot hole from 8-inch to 16-inch to a depth 

of 705 feet and set and grouted 8-inch casing to 703 feet

Casing set to occlude Upper Floridan aquifer; no logs collected 

during this stage.

Stage 5: cleaned out bottom part of hole from 

703 to 1,112 feet

Removed cuttings from bottom part of hole for final development and 

well completion; ambient and pumping flowmeter traverses were run to 

confirm depth and yield of water-bearing zones in the completed well.

Figure 4. Well-construction diagram for well HAAF 11 

(36Q392) completed in the Lower Floridan aquifer, 

Hunter Army Airfield, Chatham County, Georgia, 2009.

23-inch inside diameter

surface casing

Depth to top of

Lower Floridan

confining unit

560 feet

Depth to bottom of

8-inch inner casing

703 feet (top of Lower

Floridan aquifer)

Depth to bottom of

16-inch inner casing

333 feet

Depth to top of

8-inch inner casing

313 feet

Land surface

Neat cement grout

Limestone

Completed well

depth is 1,112 feet

Backfill material

NOT TO SCALE

73/8-inch diameter 

open hole

8-inch inside diameter 

schedule 40 black steel 

inner casing

16-inch inside diameter 

schedule 40 black steel 

outer casing

Test borehole to 1,168 feet

Figure 4. Well-construction diagram for well HAAF 11 

(36Q392) completed in the Lower Floridan aquifer, 

Hunter Army Airfield, Chatham County, Georgia, 2009.
10 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Table 4. Lithologic description of rock cuttings, well HAAF 11 (36Q392), Hunter Army Airfield, Chatham County, Georgia. 

Hydrogeologic

unit

Depth, in feet below 

land surface Lithologic description

Top Bottom

Surficial aquifer 0 60 Light gray silty fine to very fine sand

Upper Floridan 

confining unit

60 100 Greenish gray silty clay

100 115 Dark-greenish gray to greenish gray clayey sand and silt, trace shell fragments

115 195 Greenish gray, micaceous clay with minor siltstone

195 205 Dark greenish gray silty clay, abundant (5-15 percent) phosphate, sharks teeth, 

bone fragments

205 245 Dark-greenish gray phosphate bearing silty clay, some dolomite crystals

245 247 Dark greenish gray clay, includes calcium carbonate nodules, phosphate, and quartz

247 275 Fossiliferous limestone (calcarenite) with well-rounded quartz sand, black phosphate grains

275 285 Light gray poorly-sorted well rounded calcareous clayey sand

Upper Floridan 

aquifer

285 315 Light gray sandy limestone with disseminated phosphate and quartz sand; 

phosphate abundant between 295–300 feet

315 344 Light tan-very pale brown limestone with trace amounts phosphate

344 354 Tan to buff limestone (calcarenite). Note that in this interval the drill rig lost circulation, 

discharge water was dark brown, and that the driller thought this was clay; however, 

no clay was found in this interval

354 374 Speckled gray limestone (calcarenite) with black phosphate grains finely disseminated 

throughout, 0.5-foot thick shell bed (bivalves) at 362 feet

374 434 White limestone (calcarenite) and bryozoan hash, trace brown chert

434 454 White to light gray limestone (calcarenite), abundant bryozoans, trace glauconite

454 524 Same as above, very porous and loosely packed, poorly indurated

524 560 Light gray limestone (calcarenite) with fine glauconite, porous, abundant bryozoans, 

bivalves, and coral

Lower Floridan 

confining unit

560 595 White dolomitic limestone (calcarenite) and bryozoan hash; also light greenish-gray, 

glauconitic, chalky hash

595 635 White limestone (fine-calcarenite), slightly chalky and less porous than above

635 645 White to light gray dolomitic limestone (calcarenite), abundant bryozoans, trace glauconite

645 660 White chalky limestone (calcarenite) with a trace of carbonate mud and glauconite

660 675 White dolomitic limestone (fine to very fine calcarenite), abundant bivalves and oyster 

fragments at top of interval

675 703 White limestone (fine to very fine chalky calcarenite) with traces of silt, sand, 

and glauconite
Methods of Data Collection and Analysis 11

Table 4. Lithologic description of rock cuttings, well HAAF 11 (36Q392), Hunter Army Airfield, Chatham County, Georgia.—Continued 

Hydrogeologic

unit

Depth, in feet below 

land surface Lithologic description

Top Bottom

Lower Floridan 

aquifer

703 715 White shell hash of forams and bryozoans, fragments of chalky, glauconitic limestone

715 725 Light gray limestone (calcarenite), poorly indurated and porous, with some fine chalky 

glauconitic fragments

725 755 Light gray dolomitic limestone contains abundant bryozoans, porous, increase in 

grain size downward

755 785 White limestone (calcarenite), abundant shell fragments, trace glauconite, trace black opaques 

(possibly phosphate or pyrite)

785 792 Light gray dense limestone with molds and white poorly indurated porous bryozoan hash

792 825 Light gray to greenish gray porous limestone (calcarenite), porous, glauconitic, abundant 

bryozoan and shell fragments

825 865 Light gray to white, limestone (fine-grained calcarenite), slightly chalky and glauconitic, 

abundant shell, coral, and bryozoan fragments. Interval at 845–850 feet is dolomitic

865 900 Light grey-greenish grey glauconitic limestone (fine calcarenite) with chalky material in 

pore space. Abundant fossil hash

900 925 Light gray limestone (coarser-grained calcarenite, trace glauconite, fairly clean calcarenite 

with varying degrees of cementation

925 965 Light gray, chalky limestone (calcarenite), some glauconite

965 1,021 Light-greenish gray limestone (fine chalky calcarenite), trace glauconite. Interval at 

985–995 dolomitic

1,021 1,025 Mixture of light-greenish gray crystalline limestone and porous well- cemented mediumgrained calcarenite

1,025 1,052 Same as above but with glauconite disseminated throughout, abundant fossil hash 

1,025–1,045 feet

1,052 1,075 Same as above

1,075 1,080 Light gray glauconitic limestone (fine chalky calcarenite) with trace phosphate 

Confining unit

1,080 1,125 Light gray limestone (chalky, dense calcarenite) with abundant brown chert and beds 

of brown dolomitic limestone

1,125 1,168 Mixture of white and brown limestone (fine calcarenite) and dolomitic limestone, 

dense and well cemented. Abundant brown chert at bottom
12 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Borehole Geophysical Logs

Borehole geophysical logs were collected at various 

stages of drilling to characterize the physical properties of 

sediments penetrated. The first set of logs was collected in 

the 0–333 ft interval, where mud-rotary drilling was used 

to penetrate Miocene and younger clastic sediments. The 

second set of logs was collected in the 333–1,168 ft interval, 

following installation of 16-inch casing to a depth of 333 ft. 

Borehole geophysical logs include caliper; natural gamma; 

spontaneous potential; single-point, lateral, long- and shortnormal resistivity; borehole fluid resistivity and temperature; 

full-waveform sonic; acoustic televiewer (not shown in this 

report); and optical televiewer. Selected borehole geophysical 

data from well HAAF 11 (36Q392) are shown in figure 5. 

A variety of geophysical logs were used to indicate the 

locations of permeable zones in the hydrogeologic units. 

Figure 5. Borehole geophysical data from well HAAF 11 (36Q392). Percentage flow contribution 

determined from flowmeter survey in open borehole, 333–1,168 feet (see figure 6A), Hunter Army 

Airfield, Chatham County, Georgia (modified from Williams, 2010). [μS/cm, microsiemens per centimeter 

at 25 degrees Celsius] Upper Floridan aquifer Lower Floridan aquifer

Percentcontribution

Surficial 

aquifer Confining unit Confining unit Confining unit

100

200

300

400

500

600

700

800

900

1,000

1,100

Lateral

C-Marker

D-Marker

Packer test 

interval

Core

sample

Water

quality

sample

Spontaneous potential,

in millivolts

Specific conductance,

in μS/cm

Miocene

Oligocene Upper Eocene Middle Eocene Geology

Hydrogeology

Post

Miocene

6.7

44.9

27.2

1.8

3.0

4.3

1.5

5.4

1.8

3.0

0.6

Feet

Caliper,

in inches

0 500

0 600

0 5,000

0 50

60 90

0 100 5 20

Gamma, in APIU

(American Petroleum

Institute Units)

Fluid resistivity,

in ohm-meters

Fluid temperature,

in degrees Fahrenheit

Land

surface 64-inch 

normal

16-inch 

normal

0 500

0 500

Lateral resistivity,

in ohm-meters

Resistivity 16-inch normal,

in ohm-meters

Resistivity 64-inch normal,

in ohm-meters

Figure 5. Selected borehole geophysical data from well HAAF 11 (36Q392). Percentage flow contribution determined 

from flowmeter survey in open borehole, 333–1,168 feet (see figure 6A), Hunter Army Airfield, Chatham County, 

Georgia (modified from Williams, 2010). [μS/cm, microsiemens per centimeter at 25 degrees Celsius] 
Methods of Data Collection and Analysis 13

Figure 5. Selected borehole geophysical data from well HAAF 11 (36Q392). Percentage flow contribution determined 

from flowmeter survey in open borehole, 333–1,168 feet (see figure 6A), Hunter Army Airfield, Chatham County, 

Georgia (modified from Williams, 2010). [μS/cm, microsiemens per centimeter at 25 degrees Celsius]—Continued 

Sharp increases on formation resistivity logs (single-point, 

lateral, and long- and short- normal resistivity) were observed 

and often corresponded to locations of water-bearing intervals 

that were later identified through flowmeter testing. Additional 

information on the locations of water-bearing intervals was 

provided by optical televiewer and full-waveform sonic 

(variable density) logs. The optical televiewer provides a 

photographic snapshot inside the borehole, enabling visual 

identification of solution openings and less permeable intervals in the rock units. The full-waveform sonic log provides 

an indication of the relative velocity of soundwaves traveling 

through bedrock. In general, when travel times are relatively 

rapid, bedrock is denser; when travel times are relatively slow, 

more voids are present. Zones of secondary permeability 

shown in figure 5 were derived by examining images from 

the optical televiewer and full-waveform sonic log.

Figure 5. Borehole geophysical data from well HAAF 11 (36Q392). Percentage flow contribution 

determined from flowmeter survey in open borehole, 333–1,168 feet (see figure 6A), Hunter Army 

Airfield, Chatham County, Georgia (modified from Williams, 2010). [μS/cm, microsiemens per centimeter 

at 25 degrees Celsius]—Continued

Feet

Upper Floridan

 aquifer Lower Floridan aquifer

Surficial 

aquifer Confining unit Confining unit Confining unit Hydrogeology

100

200

300

400

500

600

700

800

900

1,000

1,100

Land

surface

352

Feet

B. Chalky calcarenite 

in contact with more 

permeable calcarenite 

in upper part of Lower 

Floridan aquifer.

A. Solution openings 

formed in the uppermost 

water-bearing zone of 

Upper Floridan aquifer.

C. Discrete solution 

openings formed along 

bedding planes in the 

Lower Floridan aquifer.

356

706

Feet

715

1,026

Feet

1,034

RX1-1A, 2A, 3A

0 2,048 0 90 180 270 0

Optical

televiewer

Azimuth

Zone of

secondary

permeability

Full-waveform

sonic
14 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Same flowmeter 

log plotted at 

different scales

Same flowmeter 

log plotted at 

different scales

Stationary

point 

measurement

Figure 6. Pumping flowmeter survey of (A) the Upper and Lower Floridan aquifer completed on June 17, 2009, prior 

to installation of casing in well HAAF 11 (36Q392) and (B) the Lower Floridan aquifer completed on August 15, 2009 

after installation of casing in well HAAF 11 (36Q392), Hunter Army Airfield, Chatham County, Georgia (modified from 

Williams, 2010).

7 23 0 0

Pumping flowmeter test—Upper 

and Lower Floridan aquifers 

333–1,168 feet borehole flow 

(gallons per minute)

Pumping flowmeter test—Lower 

Floridan aquifers 703–1,168 feet 

borehole flow (gallons per minute)

Caliper,

in inches

2,000 200

Flow is 

exaggerated 

through this 

zone probably 

because of 

turbulence 

resulting from 

variable 

borehole 

diameter in 

this interval

6.7

44.9

27.2

1.8

3.0

4.3

1.5

5.4

14.7

47.9

10.8

6.7

3.0

18.0

1.7

1.8

3.0

0.6

Bottom of 

16-inch 

casing is at 

333 feet

Bottom of 

8-inch 

casing is at 

703 feet

0 1,000 0 300

A B

Upper Floridan aquifer Lower Floridan aquifer

PercentcontributionPercentcontribution

Surficial 

aquiferConfining unit Confining unit Confining unit

100

200

300

400

500

600

700

800

900

1,000

1,100

Lateral

Hydrogeology

Feet

0 500 Land

surface 64-inch 

normal

16-inch 

normal

0 500

0 500

Lateral resistivity,

in ohm-meters

Resistivity 16-inch normal,

in ohm-meters

Resistivity 64-inch normal,

in ohm-meters

0 15

Caliper,

in inches

Flowmeter Testing

Flowmeter testing provided valuable insight regarding 

the relative contribution of flow from water-bearing zones in 

the UFA and LFA and confinement between the two aquifers 

(fig. 6A, B). This information ensured accurate placement of 

well casing in the completed well. Tests were conducted in 

two stages—(1) June 17, 2009, upon completion of drilling 

to a total depth of 1,168 ft and prior to installation of the 

8-inch casing to test flow in the 333–1,168 ft interval (UFA 

and LFA, fig. 6A), and (2) August 15, 2009, after installation of 

the 8-inch casing to test flow in the 703–1,112 ft interval (LFA 

only, fig. 6B). The first stage included assessment of borehole 

flow, pumping at a rate of 847 gallons per minute (gal/min), 

during the second stage borehole flow was measured at a rate 

of 748 gal/min. Testing was conducted by installing a test pump 

in the well and pumping while several traverses were made in 

the open borehole with an electromagnetic (EM) flowmeter to 

measure accumulated flow up the borehole (fig. 6A, B). 

Figure 6. Borehole flowmeter data from well HAAF 11 (36Q392), Hunter Army Airfield, Chatham County, Georgia: (A) pumping 

flowmeter survey of the Upper and Lower Floridan aquifers completed on June 17, 2009, prior to installation of casing; and (B) pumping 

flowmeter survey of the Lower Floridan aquifer completed on August 15, 2009, after installation of casing (modified from Williams, 2010). 
Methods of Data Collection and Analysis 15

1,000

1,075

875

700

525

1,000

1,075

875

700

525

1,000

1,075

875

700

525

1,000

1,075

875

700

525

SAMPLE DEPTH BELOW LAND SURFACE, IN FEET

MICROSIEMENS PER

CENTIMETER AT 

25 DEGREES CELSIUS

Magnesium

Sodium

Calcium

pH Specific conductance

Fluoride

Chloride

1 10 1,000 100 1 10 1,000 100

1001 10,000

0 21 3

40 2 6 8 10

Less than 1.0

Potassium

15105 20 25

UNITS

Sulfate

200100 300

1,000

1,075

875

700

525

Alkalinity as CaCO3

0 200100 300

0

MILLIGRAMS PER LITER

0

0 15105 20

2010 30 0

Figure 8. Distribution of selected chemical 

properties and constituent concentrations by 

sampled depth at well HAAF 11 (36Q392), 

Hunter Army Airfield, Chatham County, Georgia, 

July 2009.

Figure 8. Distribution of selected chemical 

properties and constituent concentrations 

by sampled depth at well HAAF 11 (36Q392), 

Hunter Army Airfield, Chatham County, 

Georgia, July 2009.

Figure 7. Specific conductance of drilling fluids 

while drilling well HAAF 11 (36Q392), Hunter Army 

Airfield, Chatham County, Georgia, 2009.

Figure 7. Specific conductance of drilling fluids 

while drilling well HAAF 11 (36Q392), Hunter Army 

Airfield, Chatham County, Georgia, 2009.

SPECIFIC CONDUCTANCE, IN MICROSIEMENS 

PER CENTIMETER 

AT 25 DEGREES CELSIUS

0 100 200 300 400 500 600 700 800 900 1,000 1,100

300

400

500

600

700

800

900

1,000

1,100

1,200

DEPTH BELOW LAND SURFACE, IN FEET

Upper Floridan

 aquifer

Lower Floridan 

aquifer

Confining

unit

Confining

unit

Water-Quality Sampling and Analysis

To assess vertical distribution of water quality, specific 

conductance of discharge water was monitored with a 

calibrated water-quality meter as reverse air drilling of the 

pilot hole progressed. After drilling the length of each 20-ft 

section of drill pipe, water samples were collected and specific 

conductance was measured (fig. 7). The pilot hole was terminated at a depth of 1,168 ft because a sharp rise in the specific 

conductance of drilling fluids to about 1,063 microsiemens 

per centimeter (μS/cm) at about 1,055 ft indicated that water 

quality had degraded from fresh to brackish. 

During borehole flowmeter testing, a wireline water 

sampler was used to collect water samples at five separate 

intervals in the open borehole (table 5; fig. 8). These samples 

represent a composite of water entering the borehole beneath 

that depth. For example, the sample at 1,000 ft represents a 

composite of water entering the borehole between 1,000 ft 

and the total depth of the borehole, 1,168 ft. Water samples 

were removed from the wireline sampler as it emerged from 

the borehole using a peristaltic pump to transfer the water 

into sample bottles. 

Upon completion of well construction, a water sample 

was collected and analyzed from the completed (703–1,112 ft) 

well near the end of the 72-hour aquifer test. Samples were 

analyzed for major ions, including chloride and sulfate, 

and for alkalinity as calcium carbonate, an indicator of 

hardness. Water samples for ion analysis were filtered using 

0.45-micrometer capsule filters and analyzed at Test America 

Laboratories, Savannah, GA. Water-quality analyses are summarized in table 5 and figure 8. A trilinear diagram showing 

the relative composition of major ions in water from the various depth intervals is shown in figure 9. In general, constituent 

concentrations increase with depth of sampled interval. 
16 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia Table 5. Water-quality analysis of samples collected at selected depths in well HAAF 11 (36Q392), Hunter Army Airfield, Chatham County, Georgia, June and August 2009. [Hydrogeologic unit: UF, Upper Floridan aquifer; LFA, Lower Floridan aquifer; —, not analyzed; <, less than] Collection date Sample depth, in feet Hydro- geologic unit pH laboratory Laboratory specific conductance, in micro- siemens per centimeter at 25 degrees Celsius Dissolved concentration, milligrams per liter Dissolved concentration, in micrograms per liter

Calcium

Magnesium

Sodium

Potassium

Alkalinity as 

CaCO3

Sulfate

Chloride

Fluoride

Dissolved solids, 

residue on 

evaporation

Iron

Manganese

U.S. Environmental Protection Agency Secondary Maximum 

Contaminant Level1 —

—

—

—

— 250 250 2

4 — 300 50

6/22/2009 525 UF 8.20 480 17 13 60 4.5 130 29 48 <1.0

—

—

—

6/22/2009 700 LFA 8.21 580 16 14 77 5.9 150 38 66 1.1

—

—

—

6/22/2009 875 LFA 8.25 1,100 13 16 170 11 190 92 170 1.8

—

—

—

6/22/2009 1,000 LFA 8.22 1,400 12 17 220 13 220 130 240 2.2

—

—

—

6/22/2009 1,075 LFA 8.17 2,400 17 24 440 20 250 240 480 0.59

—

—

—

8/16/2009 3705–1,112 LFA 8.20 520 17 14 67 5.3 150 32 51 <1.0 280 <50 <10

1 http://www.epa.gov/safewater/contaminants/index.html#mcls, accessed July 14, 2009.2 Primary standard.3 Completed well.
Methods of Data Collection and Analysis 17

Core Hydraulic Analysis and Packer-Slug Tests

To evaluate the hydraulic conductivity of the LFA, core 

samples were collected and analyzed for vertical hydraulic 

properties at a testing laboratory, and packer-slug tests 

were conducted in the borehole for horizontal hydraulic 

properties. Samples were collected at depths of 817.8–818.4; 

851.8–852.2; and 941.9–942.6 ft and submitted to S&ME 

Inc., Savannah, GA, for hydraulic testing of vertical 

hydraulic conductivity (Kv) and porosity (table 6). These 

largely consolidated core samples were preserved onsite 

using procedures described in ASTM D5079 and analyzed 

using a flexible wall permeameter following procedures 

described in ASTM D5084.

Additional information on the hydraulic conductivity 

of hydrogeologic units was obtained by conducting packerslug tests at selected intervals: 571–575, 641–645, 688–692, 

698–702, 802–806, and 812–816 ft (table 7). A description 

of the techniques used to deploy and test the integrity of the 

packer system is provided in Holloway and Waddell (2008). 

Each of the test intervals was isolated using straddle packers, and a slug of water was injected into the interval and 

the rate of head decline was recorded. Data were analyzed 

Table 6. Vertical hydraulic conductivity and porosity of core 

samples collected from the Lower Floridan aquifer at well HAAF 11

(36Q392), Hunter Army Airfield, Chatham County, Georgia.

[Analyses by S&ME, Inc., Savannah, Georgia]

Interval

(feet below 

land surface)

Vertical hydraulic 

conductivity 

(feet per day)

Porosity

(percent)

817.8–818.4 0.20 36.6

851.8–852.2 0.13 44.1

941.9–942.6 0.34 51.8

Table 7. Horizontal hydraulic conductivity determined from 

packer-slug tests at well HAAF 11 (36Q392), Hunter Army Airfield, 

Chatham County, Georgia (modified from Williams, 2010).

Hydrogeologic 

unit

Interval, 

in feet below 

land surface

Horizontal 

hydraulic 

conductivity, 

in feet 

per day1

Estimated 

vertical 

hydraulic 

conductivity, 

in feet 

per day2

Lower Floridan 

confining unit

571–575 1.1 0.13

641–645 3.1 0.36

688–692 0.16 0.02

698–702 0.20 0.02

Lower Floridan 

aquifer

802–806 1.7 0.19

812–816 1.7 0.20

1 Estimated using Bouwer and Rice (1976) method.

2 Estimated on the basis of the ratio of horizontal to vertical hydraulic 

conductivity of core from 817.8- to 818.4-foot interval (table 6) and packer 

test from 812- to 816-foot interval.

Figure 9. Trilinear diagram showing composition of major ions 

at various depths at well HAAF 11 (36Q392), Hunter Army Airfield, 

Chatham County, Georgia, July 2009. [Percentages are based on 

milliequivalents per liter]

 60 

 100 

 80 

 60 

 40 

 20 

 0 

 0 

 20 

 40 

 80 

 100 0 

 20 

 40 

 60 

 80 

 100 

 0 

 20 

 40 

 60 

 80 

 100 

 100 

 80 

 60 

 40 

 20 

 0 100 

 80 

 60 

 40 

 20 

 0 

 0 

 20 

 40 

 60 

 80 

 100 

 0 

 20 

 40 

 60 

 80 

 100 100 

 80 

 60 

 40 

 20 

 0 

 100 

 80 

 60 

 40 

 20 

 0 

PERCENT

Calcium

Magnesium

Chloride, fluoride, nitrite plus nitrate

Sulfate

Calcium plus magnesium

PERCENT 

PERCENT

Sodium plus potassium

Carbonate plus bicarbonate

Sulfate plus chloride

for horizontal hydraulic conductivity (Kh) using the Bouwer 

and Rice (1976) method. The Bouwer and Rice (1976) 

method assumes that the aquifer has an infinite areal extent, 

is homogeneous with a uniform thickness, the test well is 

fully or partially penetrating, effects of storage are negligible, 

flow to the well is quasi-steady-state, and the slug is introduced into the well instantaneously. 

Aquifer Tests 

Aquifer tests were conducted in an existing UFA well 

(HAAF 9, 36Q391) and in the newly constructed LFA well 

(HAAF 11, 36Q392) to determine hydraulic properties of 

the aquifers and whether pumping the LFA well produced 

a drawdown response in the UFA. Details on these tests are 

provided in Williams (2010); a brief description is provided 

here and in subsequent sections; results are summarized in 

table 8. 

EXPLANATION

Sampling depth, in feet

525

700

875

1,000

1,075

Completed well 

 (703–1,112 feet)
18 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

To determine transmissivity of the UFA, a 24-hour 

aquifer test was conducted February 17–18, 2009. For this 

test, well HAAF 9 (36Q391) was pumped at an average rate 

of 430 gal/min, and drawdown response was observed in the 

pumped well and in well HAAF 8 (36Q292), located about 

2,020 ft to the southwest (table 2; fig. 1). A similar 72-hour 

test was conducted August 13–16, 2009, in well HAAF 11 

(36Q392) to determine the transmissivity of the LFA. For 

this test, well HAAF 11 (36Q392) was pumped at an average 

rate of 748 gal/min, and water levels were recorded in the 

pumped well and in two observation wells (wells HAAF 5 

and HAAF 8), completed in the UFA, to assess interaquifer 

leakage response. For each test, data were collected and 

analyzed using the following procedures:

• Prior, during, and after each aquifer test, water levels 

were monitored to determine static water levels and 

assess ambient water-level fluctuations and trends. 

• Upon completion of the pumping period, water-level 

recovery was monitored for a minimum period equal 

in length to the pumping period. 

• Test data were corrected to remove tidal and barometric 

fluctuations and influence of regional pumping trends. 

Aquifer test drawdown data were evaluated for local hydraulic 

properties using analytical solutions (Williams, 2010). 

Groundwater-Flow Model

A groundwater-flow model previously developed by the 

USGS (Payne and others, 2005) was modified for finer spatial 

resolution and site-specific data to assess (1) the amount 

of induced interaquifer leakage and drawdown in the UFA 

resulting from pumping the LFA at a rate of 748 gal/min, and 

(2) the equivalent amount of pumpage from the UFA that 

would replicate simulated drawdown in the UFA (that resulted 

from pumping the LFA) at two alternate locations (A) adjacent 

to the newly constructed LFA well and (B) at a distance of 

3,600 ft from the new well. The revised model also was used 

to simulate groundwater-pumping scenarios evaluating the 

effect of various pumping redistributions on groundwater 

conditions at HAAF. Modifications to the groundwater model 

are described in the appendix.

Table 8. Summary of aquifer tests conducted in Upper and Lower Floridan aquifer wells at Hunter Army Airfield, Chatham County, 

Georgia, 2009.

[—, not applicable]

Pumping 

well

Observation 

well

Hydrologic 

unit

Testing 

period

Test 

duration 

(hours)

Average 

pumping 

rate in 

gallons 

per minute

Transmissivity, 

in feet 

squared 

per day

Storage Remarks

HAAF 9 

(36Q391)

HAAF 8 

(36Q292)

Upper 

Floridan

February 

17–18, 

2009

24 430 39,700 2.54×10–04 Computed using Hantush 

(1960) leaky aquifer 

solution. Drawdown was 

corrected for tidal and other 

fluctuations prior to analysis.

HAAF 11 

(36Q392)

— Lower 

Floridan

August 

13–16, 

2009

72 748 10,700 

(drawdown), 

12,100 

(recovery)

— Single well test conducted 

upon completion of Lower 

Floridan well open from 

703 to 1,112 feet. Analysis 

used Theis (1935) solution. 

Drawdown was corrected for 

tidal and other fluctuations 

prior to analysis. Drawdown 

also monitored in Upper 

Floridan wells HAAF 5 and 

HAAF 8 to assess interaquifer leakage response.
Hydrogeology and Water Quality of the Floridan Aquifer System 19

Hydrogeology and Water Quality of 

the Floridan Aquifer System

Miller (1986) provided a regional definition of the 

Floridan aquifer system on the basis of stratigraphic and borehole geophysical data that were widely spaced over the coastal 

area of Georgia and South Carolina. New hydrogeologic and 

water-quality data were used as a basis to refine the definition 

of the Floridan aquifer system at HAAF. Initial estimates 

of the depth and thickness of units constituting the Floridan 

aquifer system were obtained using Miller’s (1986) structural 

contour maps. These maps indicated that the Floridan aquifer 

system at HAAF was approximately 760-ft thick, with the 

UFA between altitudes of –240 ft and –790 ft (NGVD 29) and 

the LFA between altitudes of –930 and –1,000 (NGVD 29). 

Given a land surface altitude of 20 ft (NGVD 29) at the HAAF 

drilling site, Miller’s (1986) projected depths below land 

surface to the UFA were 260 to 810 ft, and to the LFA were 

950 to 1020 ft. Subsequently, these depths were adjusted based 

on data collected from the newly constructed HAAF test well 

(HAAF 11, 36Q392). 

In general, hydrogeologic units of the Floridan aquifer 

system can be distinguished by differences in flow contribution, lithology, geophysical signatures, and water quality. The 

following sections describe the depths, hydraulic properties, 

and water quality of hydrogeologic units that constitute the 

Floridan aquifer system at HAAF.

Upper Floridan Aquifer 

The UFA at well HAAF 11 (36Q392) is present between 

depths of 285 and 560 ft and includes several high permeability zones in limestone (table 4; fig. 5). The top of the 

aquifer is composed of the Oligocene Suwannee Limestone 

and corresponds to a spike in the natural-gamma log called 

the “C-marker” (Wait, 1965; Gregg and Zimmerman, 1974; 

Clarke and others, 1990). The thickest part of the aquifer 

is composed of upper Eocene Ocala Limestone, which is 

characterized by a very low natural-gamma level, the top 

of which is called the “D-marker” (Wait, 1965; Gregg and 

Zimmerman, 1974; Clarke and others, 1990). The base of the 

aquifer (560 ft) is present in the uppermost part of middle 

Eocene limestone (Avon Park Formation) and was designated 

on the basis of flowmeter data that indicated a large reduction 

in borehole flow at that depth. 

At well HAAF 11, the uppermost part of the UFA at 

285–333 ft was isolated by well casing and not evaluated 

during flowmeter testing. Interpretation of borehole flowmeter 

data for June 17, 2009, indicates five main water-bearing 

intervals at the HAAF 11 (36Q392) well site: 380–395, 

405–435, 475–505, 515–520, and 545–560 ft (fig. 6A). These 

intervals seem to coincide with the development of secondary 

permeability in the aquifer in the form of solution cavities 

and voids that have a higher permeability than the native-rock 

matrix. Formation-resistivity logs indicate that each of these 

water-bearing intervals coincides with an increase in resistivity 

(fig. 5). Solution openings in the uppermost part of the UFA 

are evident for the 352–356 ft interval as shown on the optical 

televiewer log (fig. 5). Flowmeter data indicated that when 

pumping at a rate of 847 gal/min, 83.5 percent of the total flow 

came from the UFA, and the remaining 16.5 percent of the total 

flow came from the underlying confining unit and the LFA.

Water-bearing intervals of 405–435 ft and 475–505 ft 

in the UFA produced the highest percentage of accumulated 

flow with an estimated 610 gal/min or 72 percent of the total 

pumpage. The remaining three zones in the UFA produced 

a combined flow of 97 gal/min or 11.5 percent of the total 

pumpage. The base of the UFA, which is the top of the Lower 

Floridan confining unit, was designated at a depth of 560 ft, 

where a contribution to borehole flow was not detected. 

Water from the UFA can be distinguished from that of 

the LFA by lower specific conductance and concentrations 

of dissolved constituents, and by differences in the relative 

percentage of constituents (water type). Monitoring of 

reverse-air drilling fluids (formation water) indicates that 

water from the UFA, compared to deeper units, is relatively 

low in dissolved solids as indicated by lower specific 

conductance values, averaging 226 μS/cm (fig. 7). During 

flowmeter testing on June 22, 2009, a water-quality sample 

was collected using a wireline sampler in the open borehole 

from the UFA at a depth of 525 ft and analyzed for major 

ions (table 5; figs. 8, 9). Water from this interval represents 

a composite sample collected between 525 ft and the bottom 

of the borehole at 1,168 ft. Because of the relatively higher 

percentage flow contribution from the UFA, this sample is 

mostly representative of water from the UFA. Water from 

this sample is hard with an alkalinity value of 150 milligrams 

per liter (mg/L) and contains low concentrations of chloride 

(48 mg/L) and sulfate (29 mg/L). Each constituent analyzed 

was within U.S. Environmental Protection Agency (USEPA) 

primary maximum contaminant levels (MCL) and secondary 

maximum contaminant levels (SMCL; U.S. Environmental 

Protection Agency, 2009). Water from the UFA appears to be a 

slightly sodium-dominated mixed-anion type (fig. 9), whereas 

water from the LFA transitions to a sodium-chloride type in 

deeper parts of the aquifer. 

 To determine the transmissivity of the UFA, a 24-hour 

aquifer test was conducted February 17–18, 2009. For this 

test, well HAAF 9 (36Q291) was pumped at an average rate of 

430 gal/min, drawdown response was observed in the pumped 

well and in well HAAF 8 (36Q292), which is located about 

2,020 ft to the southwest (table 1; fig. 1). 

A well completed in the UFA at Morrison Plantation 

(36Q020), approximately 2.41 miles (mi) west of well HAAF 9 

(36Q291), was selected as a suitable background monitoring 

point for filtering unwanted water-level fluctuations from 

aquifer-test data. The background well shows similar regional 
20 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

trends to wells HAAF 8 (36Q292) and HAAF 9 (36Q291) 

and is influenced by tidal fluctuations. The well is located far 

enough away from well HAAF 9 (36Q291) not to be affected 

by pumping during the 24-hour aquifer test (fig. 10). Widerfrequency fluctuations seen on the hydrographs were attributed 

to barometric pressure changes during the 24-hour test period.

The amplitude and phase of each time series dataset 

(background, tide, barometric pressure) were adjusted to 

create a composite synthetic water-level hydrograph that 

would span the period of the test (fig. 11) using the method 

presented in Halford (2006). Adjustments to these individual 

time series components were made to minimize differences 

between synthetic and measured water levels (residuals). 

Estimated pumping induced drawdown was derived from the 

residuals during pumping and recovery periods. Visual examination and the root mean square (RMS) error of computed 

residuals were used to evaluate the effectiveness of each stage 

of filtering. The closer a value is to 0, the more representative 

the estimated value.

A trial-and-error approach was used to evaluate the 

effect of each time series component on the development of a 

synthetic water level for the aquifer test. During the trial-anderror analysis, each component was added iteratively, and the 

effect of that series on the filtering process was evaluated. The 

final filtering included several components that collectively 

improved the match. Figure 11 shows the effect of various 

components on the generation of synthetic water levels in 

well HAAF 8 (36Q292).

Using background water level time series alone yielded an 

RMS of residuals of 0.043 and a relatively poor fit to the tidal 

loading effects (fig. 11A). Using a 6-hour moving average of 

the background time series the fit was improved (RMS 0.033), 

but the effects of tidal loading still were evident in the resulting 

dataset (fig. 11B). Combining the raw background time series 

with the 6-hour moving average improved the quality of the 

data series fit, with an RMS of residuals of 0.027 (fig. 11C).

Barometric pressure had a slight influence on estimated 

water levels. Adding the barometric pressure time series to 

the analysis improved the fit to an RMS of 0.026 (fig. 11D). 

Similar attempts were made using the tidal stage time series 

alone, but this did not produce satisfactory results. Combining 

the tidal stage and barometric pressure time series gave an 

RMS of 0.039 and produced a poorer fit than by applying 

barometric pressure alone (fig. 11E).

The best fit (RMS 0.014) was obtained by combining 

all of the time series, including the raw and 6-hour moving 

average time series from the Morrison Plantation background 

well (36Q020), tidal stage, barometric pressure, gravity tides, 

and earth tides (fig. 11F). These data were used as a basis 

for analysis of the drawdown in well HAAF 8 (36Q292) 

in response to pumping at well HAAF 9 (36Q391) on 

February 17–18, 2009. 

Figure 10. Comparison of water levels at well HAAF 8 (36Q292) to background water levels 

at Morrison Plantation observation well (36Q020) and to stage and barometric pressure at 

tidal station 02198773, February 12–25, 2009 (modified from Williams, 2010).

Morrison Plantation Well

( 36Q020)

Aquifer

test

period Well HAAF 8

(36Q292)

Tidal stage Barometric 

pressure

FEBRUARY 2009

12 13 14 15 16 17 18 19 20 21 22 24 25

TIDAL STAGE BELOW OR 

ABOVE (–) NAVD 88, IN FEET

–8

–6

–4

–2

0

2

4

750

755

760

765

770

775

780

BAROMETRIC PRESSURE,

IN MILLIBARS

60.9

44.7

44.9

45.1

45.3

45.5

45.7

45.9

59.7

59.9

60.1

60.3

60.5

60.7

WATER LEVEL IN WELL 36Q292

WATER LEVEL IN WELL 36Q020

 BELOW LAND SURFACE, IN FEET

 BELOW LAND SURFACE, IN FEET

Figure 10. Comparison of water levels at well HAAF 8 (36Q292) to background water levels 

at Morrison Plantation observation well (36Q020) and to stage and barometric pressure at 

tidal gage 02198773, February 12–25, 2009 (modified from Williams, 2010).
Hydrogeology and Water Quality of the Floridan Aquifer System 21

Pumping

Fitting period

Recovery

Estimation period

RMS = 0.039

RMS = 0.043

RMS = 0.033

RMS = 0.027

RMS = 0.026

RMS = 0.014

WATER-LEVEL CHANGE, IN FEET

A

B

C

D

E

F

FEBRUARY 2009

–0.6

–0.4

–0.2

0

12 13 14 15 16 17 18 19

0.2

0.4

0.6

0.8

1.0

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

0.8

1.0

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

0.8

1.0

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

0.8

1.0

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

0.8

1.0

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

0.8

1.0

Figure 11. Effects of different time series components on development of a synthetic water level used to 

filter out extraneous trends for well HAAF 8 (36Q292), Hunter Army Airfield, Chatham County, Georgia: 

(A) raw water-level data for Morrison Plantation well 36Q020, (B) 6-hour moving average water level for 

well 36Q020, (C) raw and 6-hour moving average water-level data for well 36Q020, (D) raw and 6-hour 

moving-average water level for well 36Q020 and barometric pressure, (E) raw barometric pressure and 

raw tidal stage data, and (F) raw and 6-hour moving average water-level data for well 36Q020, tidal 

stage, barometric pressure, gravity tides, and earth tides (modified from Williams, 2010). [RMS, root 

mean square of differences between measured and synthetic water levels]

Measured well HAAF 8 (36Q392)

Synthetic well HAAF 8 (36Q392)

Difference between measured and synthetic 

Background Morrison 

Plantation well (36Q020)

Moving average in background 

Morrison Plantation well (36Q020)

Water-level change

Figure 11. Effects of different time series components on development of a synthetic water level used to filter out extraneous 

trends from well HAAF 8 (36Q292), Hunter Army Airfield, Chatham County, Georgia: (A) raw water-level data from well 36Q020, 

(B) 6-hour moving average water level from well 36Q020, (C) raw and 6-hour moving average water-level data from well 36Q020, 

(D) raw and 6-hour moving-average water level well 36Q020 and barometric pressure, (E) raw barometric pressure and raw tidal 

stage data, and (F) raw and 6-hour moving average water-level data from well 36Q020, tidal stage, barometric pressure, gravity tides, 

and earth tides (modified from Williams, 2010). [RMS, root mean square of differences between measured and synthetic water levels]
22 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Analysis of the drawdown data for well HAAF 8 

(36Q292) suggested a leaky artesian aquifer response with 

storage of water in the semiconfining unit (fig. 12). Both the 

early and late data do not fit the Theis (1935) type curve for 

an ideal confined aquifer. The departure of the drawdown 

response from the Theis curve after about 50 minutes suggests 

appreciable amounts of water are being released from storage, 

likely from the underlying Lower Floridan confining unit and 

the LFA.

A transmissivity of about 40,000 ft2/d and a storage coefficient of 2.5×10–4 (table 8) were computed for the UFA using 

the Hantush (1960) leaky aquifer solution (Williams, 2010). 

This solution assumes that (1) the aquifer is infinite in areal 

extent, homogeneous, and isotropic, (2) the pumping well is 

fully or partially penetrating and flow to the pumping well 

is horizontal, (3) the aquifer is leaky, and flow is unsteady, 

(4) the water is released from storage instantaneously, (5) the 

diameter of pumping well is very small, and the storage in the 

well can be neglected, (6) the confining unit is infinite in areal 

extent and has a uniform vertical hydraulic conductivity and 

thickness, (7) the confining unit has an adjacent constant-head 

source, and (8) flow in the confining unit is vertical. Although 

all of these conditions may not be absolutely satisfied for 

this site, the aquifer conditions approximate and satisfy the 

assumptions relevant for this solution approach. 

Lower Floridan Confining Unit

The thickness and hydraulic conductivity of the confining 

unit that separates the UFA and LFA controls the rate of interaquifer leakage. Thickness of the Lower Floridan confining unit 

was determined by evaluating rock cuttings, borehole geophysical logs, and results of flowmeter testing (figs. 5, 6; table 4). 

The Lower Floridan confining unit at well HAAF 11 

(36Q392) is interpreted to be present between depths of 

560 and 703 ft (fig. 5) and is composed of low permeability 

chalky and glauconitic limestone in the uppermost part of the 

middle Eocene Avon Park Formation (figs. 2, 5). This unit is 

similar in lithology to overlying and underlying rock units 

(table 4), which precluded the identification of the depth interval of the confining unit during drilling. Following completion 

of the pilot boring, thickness and location of the confining 

unit were assessed by using borehole geophysical logs and the 

results of a flowmeter survey. The Lower Floridan confining 

unit is about 143 ft thick at HAAF and similarly about 155 ft 

at the Berwick Plantation well (36Q330), which is 3.7 mi west 

of the HAAF site (fig. 1). The Lower Floridan confining unit 

can be distinguished as a zone of relatively higher naturalgamma radiation when compared to the very low naturalgamma radiation in the overlying Ocala Limestone of the UFA 

(fig. 5). The high natural gamma radiation likely is due to a 

high percentage of glauconite in the Avon Park Formation.

Borehole flowmeter testing in well HAAF 11 (36Q392) 

(fig. 6A) in the 333–1,168 ft open interval indicated the Lower 

Floridan confining unit contributed little to overall flow in 

the borehole. In particular, continuous vertical sections of 

limestone at 556–665 and 675–719 contributed no detectable 

amounts of water during the 847 gal/min flowmeter survey. 

Within the confining unit, a single water-bearing zone at 

665–675 ft yielded 36 gal/min or 4.3 percent of the total 

borehole flow during the flowmeter survey. This water-bearing 

zone corresponded to an increase in formation resistivity at 

that depth (fig. 5).

Prior to installation of 8-inch casing (table 7), the 

Kh of the Lower Floridan confining unit was determined 

by conducting packer-slug tests at four separate intervals 

within the confining unit: 571–575, 641–645, 688–692, and 

698–702 ft (fig. 5). Although the lithology in the confining 

unit interval was fairly consistent, two variations affected Kh: 

(1) fine-grained calcarenite with trace amounts of fine carbonate mud (chalk) disseminated throughout mostly granular limestone and (2) very fine-grained chalky and sandy limestone. 

The slug tests for the 571–575 ft and 641–645 ft intervals are 

representative of the first lithology variation with Kh of 1.1 and 

3.1 ft/d, respectively. In the second lithologic variation, the Kh

was 0.16 ft/d at 688–692 ft and 0.20 ft/d at 698–702 ft. 

An approximation of the Kv

 of the Lower Floridan 

confining unit was derived using Kh data obtained from 

packer-slug tests and Kv data obtained from core analysis 

(table 6). Sediments constituting the Lower Floridan confining 

unit and upper part of the LFA consist of middle Eocene 

Figure 12. Logarithmic plot of drawdown and time for 

observation well HAAF 8 (36Q292) during Upper Floridan 

aquifer test conducted in well HAAF 9 (36Q391), Hunter Army 

Airfield, Chatham County, Georgia, February 17–18, 2009 

(modified from Williams, 2010).

100 1 10 1,000 10,000

Note: Unknown interference 

that was not able to be filtered 

out of the raw data

0.001

0.01

0.1

1.0

TIME, IN MINUTES

DRAWDOWN, IN FEET

Theis (1935)

Leaky solution

Hantush (1960)

Figure 12. Logarithmic plot of drawdown and time for 

observation well HAAF 8 (36Q292) during Upper Floridan 

aquifer test conducted in well HAAF 9 (36Q391), Hunter 

Army Airfield, Chatham County, Georgia, February 17 –18, 

2009 (modified from Williams, 2010).
Hydrogeology and Water Quality of the Floridan Aquifer System 23

limestone that appears similar in lithologic composition. 

Because of this similarity, an approximation of the Kv of the 

Lower Floridan confining unit was determined using data from 

the uppermost part of the LFA. A Kh:Kv ratio of 8.5:1 was 

determined by comparing laboratory analyses for Kv (table 6) 

in the 817.8–818.4 ft interval (0.2 ft/d) to the packer test for 

Kh (table 7) in the nearby 812–816 ft interval (1.7 ft/d). Using 

this relation, the estimated Kv of the confining unit is estimated 

to range from 0.02 to 0.36 ft/day (table 7). These values are 

consistent with ranges for Kh reported by Freeze and Cherry 

(1979) for limestone and dolomite. 

Water in the Lower Floridan confining unit has a low 

specific conductance compared to deeper units as indicated by 

the monitoring of drilling fluids while completing the pilot hole 

in well HAAF 11 (36Q392). Water from the 570–685 ft interval 

had an average specific conductance of 225 μS/cm (fig. 7).

Lower Floridan Aquifer

The LFA at well HAAF 11 (36Q392) is present between 

depths of 703 and 1,080 ft and consists of several high permeability zones in limestone of the middle Eocene Avon Park 

Formation (fig. 2; table 4). A second round of flowmeter testing conducted in the completed LFA well on August 15, 2009, 

provided information on water-bearing intervals in the LFA. 

Test results indicated that there are six main water-bearing 

intervals in the LFA: 723–731; 768–785; 818–837; 917–923; 

1,027–1,052; and 1,060–1,080 ft (fig. 6B). These intervals 

seem to coincide with the development of secondary permeability in the aquifer in the form of solution cavities and voids 

that have a higher permeability than the native-rock matrix 

(fig. 5). Formation-resistivity logs indicate that each of these 

water-bearing intervals coincides with an increase in resistivity 

(fig. 5). Flowmeter-test data indicated that when pumping 

at a rate of 748 gal/min, the upper two water-bearing zones 

produced 62.6 percent of the total flow, with 110 gal/min from 

the 723- to 731-ft interval and 359 gal/min from the 768- to 

785-ft interval. The remaining four zones contributed from 

1.7 to 18 percent of the flow during the flowmeter test (fig. 6). 

Flow was not detected beneath a depth of 1,080 ft by the 

flowmeter. Based on the low contribution from the interval 

beneath 1,080 ft, the base of the LFA was assigned a depth 

of 1,080 ft at the HAAF 11 (36Q392) well site. 

The quality of water in the LFA was evaluated during 

drilling by monitoring the specific conductance of reverseair drilling fluids, collecting grab water samples during 

flowmeter testing, and collecting a composite water sample 

from the completed well HAAF 11. Monitoring of drilling 

fluids indicated that the specific conductance in the LFA 

between depths of 710 and 1,045 ft averaged 262 μS/cm. 

At a depth of 1,055 ft, the specific conductance increased 

abruptly to 1,063 μS/cm and averaged 751 μS/cm for the 

1,055–1,165 interval. These higher specific-conductance 

values suggest an increase in dissolved-solids-concentration 

that could affect the suitability of the well for water supply. 

Drilling was halted at a depth of 1,168 ft to avoid further 

penetration into zones containing water with a high total 

dissolved-solids concentration.

Grab water samples were collected using a wireline 

sampler from five discrete depths on June 22, 2009, and were 

analyzed for major ions (table 5; figs. 8, 9). Water samples 

collected in the UFA and LFA indicate a transition with depth 

(fig. 9) from a slightly sodium-dominated mixed-anion type 

water in the UFA at a depth of 525 ft to a sodium-chloride 

type water in the LFA at the 1,075 ft depth. 

Water from the LFA is hard to very hard with an alkalinity of 150–250 mg/L (fig. 8; table 5). Data indicate that with 

the exception of fluoride, constituent concentrations increase 

continuously with depth. Water from the deepest interval 

(1,075 ft) has a chloride concentration of 480 mg/L, which 

exceeds the USEPA SMCL of 250 mg/L (U.S. Environmental 

Protection Agency, 2009). The 240 mg/L sulfate concentration 

of water from the same interval is slightly below the USEPA 

SMCL of 250 mg/L (U.S. Environmental Protection Agency, 

2009). Flowmeter testing indicates that water from the 

1,075-ft zone contributes less than 2 percent of the total flow 

to the well (fig. 6B); thus, the relatively higher concentrations 

of chloride and sulfate in the deepest interval will not likely 

affect the quality of the water in the completed well. 

A composite sample of water in the LFA was collected 

and analyzed from well HAAF 11 (36Q392) on August 16, 

2009 (table 5), near the completion of a 72-hour aquifer 

test. Constituent concentrations of the composite sample 

(703 –1,112-ft interval) of the completed well were generally 

below values for each of the grab samples in the LFA and all 

were below USEPA MCLs and SMCLs (U.S. Environmental 

Protection Agency, 2009). 

To determine the transmissivity of the LFA, well 

HAAF 11 (36Q392) was pumped for a 72-hour period at 

an average rate of 748 gal/min, during August 13–16, 2009 

(table 8). Water-level and discharge data were recorded in the 

pumped well, and water levels were recorded in two observation wells (wells HAAF 5 and HAAF 8) completed in the 

UFA to assess interaquifer leakage response. 

Pumping from well HAAF 9 (36Q391), completed in the 

UFA about 2,000 ft to the north, was identified as a possible 

source of interference for the HAAF 11 (36Q392) aquifer test. 

Because of water-supply needs, pumping at well HAAF 9 

(36Q391) could not be discontinued during the 72-hour test and 

background/recovery periods. For this reason, pumping at the 

well was maintained at a generally constant rate starting a few 

days before the start of the aquifer test (August 10, 2009) and 

was operated on that schedule throughout the duration of the 

test and recovery period (August 22, 2009). Small fluctuations 

in the water-system pressure caused slight variations in pumping 

at well HAAF 9 (36Q391). In addition to adjusting for pumping 

influences, aquifer test data were corrected for regional trends, 

barometric pressure, and tidal influences prior to aquifer-test 

analysis by using the same filtering procedures described earlier 

for the test conducted in the UFA (Williams, 2010). 
24 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Transmissivity of the LFA was estimated from drawdown 

and recovery data from the pumped well (well HAAF 11) 

because no other nearby LFA wells were available for drawdown observations (table 8). Transmissivity calculated using 

the Theis (1935) solution was 11,000 ft2/day (Williams, 2010). 

The Theis (1935) solution assumes the aquifer has an infinite 

areal extent; is homogeneous, isotropic, and has a uniform 

thickness; the pumping well is fully penetrating; flow to the 

well is horizontal; the aquifer is confined; flow is unsteady 

and water is released instantaneously from storage with 

decline of hydraulic head; and the effects of borehole storage 

are small enough to be neglected. The Theis (1935) solution 

also assumes the aquifer is confined; however, the aquifer test 

indicates a leaky artesian aquifer response with a flattening 

of the drawdown curve starting several hundred minutes into 

the test (fig. 13). Thus, the type curve for this analysis was 

only matched to early-time data between 7 and 90 minutes. 

The observed values plotting below the Theis (1935) curve 

between 1 and 7 minutes are probably the result of borehole 

storage that were not matched in this analysis.

The Theis (1935) solution for recovery data in a confined 

aquifer also was used to calculate transmissivity for the 

aquifer test (Williams, 2010). In the Theis recovery method, 

the residual drawdown is plotted against time over elapsed 

time since pumping began (t/t') on a semilogarithmic plot 

(fig. 13B). A straight-line fit through the observed residual 

drawdown gives a transmissivity of about 12,000 ft2

/day 

(table 8), which is comparable to the value determined using 

the drawdown data. 

Vertical hydraulic conductivity of the LFA was 

determined from core samples at depths of 817.8–818.4, 

851.8–852.2, and 941.9–942.6 ft (table 6). These samples 

originally were to have been collected from the Lower 

Floridan confining unit; however, this interval was not readily 

identified during drilling, and sampled intervals were designated on the basis of projected depths of the Lower Floridan 

confining unit according to maps by Miller (1986). Field 

testing after samples were collected indicated that the samples 

actually were collected from intervals within the upper part 

of the LFA. This discrepancy resulted from differing local 

geology and the coarse resolution of Miller’s (1986) maps. 

Vertical hydraulic conductivity of the LFA, determined by 

using a flexible wall permeameter following procedures 

described in ASTM D5084, ranged from 0.13 to 0.34 ft/d. 

Figure 13. Semi-logarithmic plots of (A) drawdown and time 

and (B) residual drawdown and t/t' for aquifer test of Lower 

Floridan aquifer at well HAAF 11 (36Q392), Hunter Army Airfield, 

Chatham County, Georgia, August 13–16, 2009 (modified from 

Williams, 2010).

1 10 100 1,000

1 10 100 1,000 10,000

DRAWDOWN, IN FEET RESIDUAL DRAWDOWN, IN FEET

Theis (1935)

Theis (1935)

A. Drawdown and time

B. Residual drawdown

TIME t/t'

Time, in minutes

20

24

28

32

36

40

0

4

8

2

6

12

10

Figure 13. Semi-logarithmic plots of (A) drawdown 

and time and (B) residual drawdown and t/t' for aquifer 

test of Lower Floridan aquifer at well HAAF 11 (36Q392), 

Hunter Army Airfield, Chatham County, Georgia, 

August 13 –16, 2009.
Effect of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 25

Effect of Lower Floridan Aquifer Pumping 

on the Upper Floridan Aquifer

Potential effects of pumping the LFA on water levels in 

the UFA were evaluated by monitoring drawdown response in 

nearby UFA wells and through model simulation. Observed 

water-level response in the UFA as a result of pumping the 

LFA was determined by using water-level data that were 

filtered for tidal, barometric, and pumping interferences and 

by following procedures described earlier. These measured 

drawdowns were used to provide a basis for evaluating the 

accuracy of model simulations. Model simulation was used 

to determine long-term (steady-state) leakage response of the 

UFA to pumping from the LFA, and to estimate the equivalent amount of UFA pumping that would produce similar 

drawdown. 

Observed Water-Level Response

During the 72-hr LFA test in well HAAF 11 (36Q392), 

36.3 ft of drawdown were observed in the pumped well, 

and small amounts of drawdown were observed in two UFA 

observation wells. The closest of these observation wells 

(HAAF 8, 36Q292) is located 176 ft from the pumping 

well and the farthest (HAAF 5, 36Q288) is located 3,600 ft 

from the pumping well (fig. 14). Well HAAF 9 (36Q391), 

located 1,995 ft from the pumping well, was not used in the 

drawdown analysis because this well was operational and 

was being pumped at a generally constant rate during the test. 

Small fluctuations in water-system pressure caused variations 

in pumping at well HAAF 9 (36Q391) that masked any 

observable drawdown resulting from LFA pumping at 

well HAAF 11.

Note: Distance between well HAAF 11 and well HAAF 8 is 176 feet

5,298 feet

3,600 feet

1,995 feet

2,020 feet

Figure 14. Location and construction characteristics of 

wells used for aquifer tests at Hunter Army Airfield, 

Chatham County, Georgia: (A) diagram showing aquifer 

test layout and (B) schematic section showing the open 

intervals of the wells in relation to major hydrogeologic 

units (modified from Williams, 2010).

A. Aquifer test layout

B. Schematic cross section

Upper Floridan

 aquifer

Lower Floridan 

aquifer

Confining

unit

Confining

unit

Surficial 

aquifer

Confining unit

100

200

300

400

500

600

700

800

900

1,000

1,100

Land

surface

1,112 feet

Well open interval

DEPTH BELOW LAND SURFACE, IN FEET

NOT TO SCALE

Well HAAF 9

36Q391

Well HAAF 8

36Q292 Well HAAF 11

36Q392

(pumped well)

Well HAAF 5

(36Q288)

Well HAAF 9

36Q391

Well HAAF 8

36Q292

Well HAAF 11

36Q392

Well HAAF 5

(36Q288)

EXPLANATION

Figure 14. Location and construction characteristics 

of wells used for aquifer tests at Hunter Army Airfield, 

Chatham County, Georgia: (A) diagram showing aquifer 

test layout and (B) schematic section showing the open 

intervals of the wells in relation to major hydrogeologic 

units (modified from Williams, 2010).
26 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Figure 15. Water-levels in the pumped Lower 

Floridan well HAAF 11 (36Q392) and in Upper 

Floridan observation wells HAAF 8 (36Q292) and 

HAAF 5 (36Q288) before, during, and after the 

72-hour aquifer test of the Lower Floridan aquifer, 

Hunter Army Airfield, Chatham County, Georgia, 

August 11–19, 2009 (modified from Williams, 2010).

12 14 16 18

49.0

49.5

50.0

63.5

64.0

64.5

65.0

60

70

80

90

100

110

50.5

WATER LEVEL BELOW LAND SURFACE, IN FEET

Well HAAF 8 (36Q292; 176 feet from pumping well)

Well HAAF 5 (36Q288; 3,600 feet from pumping well)

Well HAAF 11 (36Q392)

Pump on 

(747 gallons 

per minute) 

Pump off

AUGUST 2009

Raw, unfiltered water-level data from the 72-hour LFA 

test and the response at the two UFA observation wells are 

depicted in figure 15. Visual examination of unfiltered data 

indicates that drawdown at well HAAF 8 (36Q292) is readily 

evident, whereas drawdown in well HAAF 5 (36Q288) is less 

evident but still distinguishable. 

Water-level data from wells HAAF 5 (36Q288) and 

HAAF 8 (36Q292) were filtered to reduce the effects of tidal 

loading, barometric pressure, regional trends, and pumping 

at well HAAF 9 (36Q391), following the method presented 

in Halford (2006). To ensure that local interferences were 

adequately filtered in order to accurately depict small drawdown values resulting from LFA pumping, the fitting period 

for the filtering included a week before and several weeks 

after the test was completed (fig. 16). The amplitude and phase 

of each time series (background, tide, barometric pressure) 

were adjusted to create a composite synthetic water-level 

hydrograph for each observation well that spanned the period 

of the test, following procedures described earlier for the UFA 

24-hour aquifer test.

During the filtering process, the effects of pumping well 

HAAF 9 (36Q391) on the drawdown response had to be taken 

into account. For well HAAF 8 (36Q292), the synthetic and 

measured water levels (fig. 16A) matched fairly well during 

the week prior to pumping at well HAAF 9 (36Q391). During 

this pre-test period, well HAAF 9 (36Q391) was taken offline 

in order to obtain natural background fluctuations. Pumping 

at well HAAF 9 (36Q391) was started 2 days prior to the 

startup of the LFA test and was maintained at an average rate 

of 400 gal/min until 5 days following the end of the test. At 

well HAAF 8 (36Q292), the drawdown and recovery response 

of pumping both at wells HAAF 9 (36Q391) and HAAF 11 

(36Q392) are shown in figure 16A. After removing the effects 

of well HAAF 9 (36Q391) pumping, the total drawdown and 

recovery response attributed to pumping at well HAAF 11 

(36Q392) are estimated to be 0.76 ft (fig. 16A). At well 

HAAF 5 (36Q288), which is located 3,600 ft away from the 

LFA well, the total pumping-induced drawdown attributed to 

well HAAF 11 (36Q392) is estimated to be 0.43 ft (fig. 16B).

Figure 15. Water levels in the pumped Lower 

Floridan well (HAAF 11, 36Q392) and in Upper 

Floridan observation wells HAAF 8 (36Q292) and 

HAAF 5 (36Q288) before, during, and after the 

72-hour aquifer test of the Lower Floridan aquifer, 

Hunter Army Airfield, Chatham County, Georgia, 

August 11–19, 2009 (modified from Williams, 2010).
Effect of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 27

Figure 16. Estimated drawdown calculated in Upper Floridan wells (A) HAAF 8 (36Q292) and (B) HAAF 5 (36Q288) 

during the 72-hour aquifer test of Lower Floridan aquifer in well HAAF 11 (36Q392), Hunter Army Airfield, 

Chatham County, Georgia (modified from Williams, 2010).

AUGUST 2009

AUGUST 2009

13

3 4 5 6 7 8 9 13

14

14

15

15

16

10 11 12 16 20 21 22 23 24 25 26 27

17

17

18

AUGUST 2009

13 14 15 16 17 18 19

18 19

AUGUST 2009

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

0

0

0

–0.6

–0.4

–0.8

–1.0

–0.2

0.2

0.4

0.6

0.8

1.0

0.2

0.4

0.6

0.8

1.0

–1.5

–1.0

–0.5

0.5

–0.6

–0.4

–0.8

–1.0

–0.2

0

1.0

–1.5

–1.0

–0.5

0.5

1.0

0.76 foot of drawdown

0.43 foot of drawdown

A. Well HAAF 8 (36Q292)

B. Well HAAF 5 (36Q288)

RMS = 0.199

RMS = 0.404

RMS = 0.454

RMS = 0.821

EXPLANATION

WATER-LEVEL CHANGE, IN FEET WATER-LEVEL CHANGE, IN FEET

Measured 

Synthetic

Difference between 

 measured and synthetic

Water-level change

HAAF 9 pump on

HAAF 11

HAAF 9 Pump on Pump on

HAAF 11

pump on HAAF 11 

pump off HAAF 9 pump off

HAAF 11 

pump off

HAAF 9 pump off

Figure 16. Estimated drawdown calculated in Upper Floridan wells (A) HAAF 8 (36Q292) and 

(B) HAAF 5 (36Q288) during the 72-hour aquifer test of the Lower Floridan aquifer in well HAAF 11 

(36Q392), Hunter Army Airfield, Chatham County, Georgia (modified from Williams, 2010).
28 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Model Simulation

A revised regional groundwater-flow model was used 

to simulate the water-level response of the UFA in selected 

observation wells to pumping from the LFA at well HAAF 11. 

The revised model was also used to simulate the equivalent 

pumping rate in a hypothetical UFA well at the same location 

as HAAF 11 (36Q392) that would produce a comparable 

drawdown to that resulting from pumping well HAAF 11. 

To simulate the steady-state effect on groundwater flow 

of pumping the new LFA well at HAAF, the existing USGS 

MODFLOW model of Payne and others (2005) was modified 

based on hydrogeological information obtained from field 

investigations (described earlier) and from existing wells in 

the area. The model grid was modified to a finer 10-ft by 10-ft 

cell size near well HAAF 11 (36Q392) to enable more accurate 

simulations at smaller scale. Revisions to the regional model 

of Payne and others (2005) are described in the appendix. 

The location of the regional model is shown on in figure 17.

The revised model used regional pumping and boundary 

conditions during 2000 as a “base case” for comparing 

drawdown and water-budget changes simulated in response to 

pumping well HAAF 11 (36Q392). Pumping well HAAF 11 

(36Q392) at a rate of 748 gal/min resulted in a simulated 

maximum steady-state drawdown of 36.2 ft in the LFA, which 

is nearly identical to the 36.3 ft of drawdown observed in well 

HAAF 11 (36Q392) during the 72-hour aquifer test. Simulated 

steady-state drawdown in the LFA was greater than 1 ft over a 

146-square-mile (mi2) area (fig. 17). 

Interaquifer Leakage and Drawdown Response

Pumping from the LFA at well HAAF 11 (36Q392) also 

resulted in a simulated drawdown response in the overlying 

UFA because of leakage through the Lower Floridan confining 

unit (fig. 18). Model simulations indicate that steady-state 

drawdown in the UFA was greater than 1 ft over a 141 mi2 area 

and was 1.9 ft near well HAAF 5 (36Q288) and 2.03 ft near 

wells HAAF 11 (36Q392) and HAAF 8 (36Q292). 

The GaEPD interim permit strategy for the LFA (Nolton 

Johnston, Georgia Environmental Protection Division, written 

commun., January 28, 2003) states that “the applicant must 

offset the impact of the LF [Lower Floridan] pumping by 

reducing nearby UF [Upper Floridan] permitted pumping in 

the same general area (within a 5 mi radius) by an amount 

equal to or greater than the determined UF leakage.” To assess 

the amount of leakage resulting from pumping in the Lower 

Floridan aquifer, the steady-state water budget before and 

after the initiation of pumping at well HAAF 11 (36Q392) 

was compared (table 9; fig. 19). 

Table 9. Simulated steady-state water budget for 2000 and after initiation of pumping 748 gallons per minute (1.08 million gallons per 

day) at well HAAF 11 (36Q392), Hunter Army Airfield, Chatham County, Georgia.

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

Hydrogeologic 

unit

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

Year 

2000

Well 

HAAF 11 

added

Year 

2000

Well 

HAAF 11 

added

Year 

2000

Well 

HAAF 11 

added

Year 

2000

Well 

HAAF 11 

added

Year 

2000

Well 

HAAF 11 

added

Surficial 

aquifer 

system

1 0 0.00 274 275 102 102 <0.001 <0.001 <0.001 <0.001

Confining 

unit

2 0 0.00 46.1 46.1 4.7 4.70 <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 0.00 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

Upper 

Floridan 

aquifer

5 669 669 145 145 22.1 22.1 715 742 266 288

Confining 

unit

6 0 0.00 <0.001 <0.001 <0.001 <0.001 0.004 <0.001 <0.001 <0.001

Lower 

Floridan 

aquifer

7 129 130 <0.001 <0.001 <0.001 <0.001 16.6 <0.001 1.97 <0.001

Total all layers 798 800 465 466 129 129 732 742 268 288
Effect of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 29

FORT STEWART

81°45' 81°30' 81°15' 81°00' 80°45'

32°00'

ATLANTIC

OCEAN

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

CHATHAM

TATTNALL

EVANS JASPER

LONG

GA

SC

31°45'

32°15'

Map

area

above

GEORGIA

SOUTH

CAROLINA

Model

boundary

BEAUFORT

Well HAAF 11

(36Q391) Well HAAF 5

(36Q288)

Well HAAF 8

(36Q292)

Well HAAF 1

(36Q285)

Well HAAF 2

(36Q286)

3

4

5

2

10

1

2

HUNTER

ARMY

AIRFIELD

N

EXPLANATION

5

0 5 10 15 MILES

0 10 15 KILOMETERS 5

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

1 2 MILES

0

0

1 2 KILOMETERS

Figure 17. Simulated steady-state drawdown in the Lower Floridan aquifer resulting from pumping 

well HAAF 11 (36Q392) at 748 gallons per minute, Hunter Army Airfield and vicinity, coastal Georgia.

Line of equal simulated drawdown—Interval, 1 foot

Well—Hunter Army Airfield identification; U.S. 

Geological Survey identification in parentheses

Well HAAF 5

(36Q288)

Figure 17. Simulated steady-state drawdown in the Lower Floridan aquifer resulting from pumping well HAAF 11 

(36Q392) at 748 gallons per minute, Hunter Army Airfield and vicinity, coastal Georgia.
30 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

FORT STEWART

81°45' 81°30' 81°15' 81°00' 80°45'

32°00'

ATLANTIC

OCEAN

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

TATTNALL

EVANS

LONG

SC

BEAUFORT

31°45'

32°15'

GEORGIA

SOUTH

CAROLINA

Model

boundary

GA

N

EXPLANATION

Line of equal simulated drawdown—Interval, 0.1 foot

Well—Hunter Army Airfield identification; U.S. 

Geological Survey identification in parentheses

1.5

Well HAAF 5

(36Q288)

0 5 10 15 MILES

0 10 15 KILOMETERS 5

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

1 2 MILES

0

0

1 2 KILOMETERS

Figure 18. Simulated steady-state drawdown in the Upper Floridan aquifer resulting from pumping 

well HAAF 11 (36Q392) at 748 gallons per minute, Hunter Army Airfield and vicinity, coastal Georgia.

HUNTER

ARMY

AIRFIELD

JASPER

1.5

1.4

1.4

1.8

1.7

1.6

1.9

2.0

0.5

1.0

1.5

Well HAAF 11

(36Q391) Well HAAF 5

(36Q288) Well HAAF 8

(36Q292)

Well HAAF 1

(36Q285)

Well HAAF 2

(36Q286)

CHATHAM

Map

area

above

Figure 18. Simulated steady-state drawdown in the Upper Floridan aquifer resulting from pumping well HAAF 11 (36Q392) 

at 748 gallons per minute, Hunter Army Airfield and vicinity, coastal Georgia.
Effect of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 31

General-head boundary

Interunit leakage

Lateral specified-head boundary

Pumpage

+0.01

–0.18

+1.08

+1.08

0.00

0.00

Surficial aquifer system (layer 1)

Lower Floridan aquifer (layer 7)

Upper Floridan aquifer (layer 5)

Lower Floridan confining unit (layer 6)

Upper Floridan confining unit (layer 4)

Brunswick aquifer system (layer 3)

Brunswick aquifer system 

 confining unit (layer 2)

+0.36

–0.08

+0.36

–0.08

+0.45

–0.02

–0.01

+0.39

0.00

+0.02 +0.52

–0.52

+0.52

–0.52

+0.02

0.00

+0.43

–0.01

+0.46

–0.01

–0.53

–0.01

0.00

–0.01

+0.02

–0.01

+0.46

+0.52

NOT TO SCALE

Change in groundwater flow, in million gallons per day, and budget component

Figure 19. Change in simulated steady-state water budget resulting from initiation of pumping at well 

HAAF 11 (36Q392), Hunter Army Airfield, Chatham County, Georgia (modified from Payne and others, 2005).

EXPLANATION

Pumping 1 Mgal/d (748 gal/min) at well HAAF 11 

(36Q392) resulted in small changes and redistribution of 

flow among model layers, including increased downward and 

decreased upward leakage in all layers, and increased inflow 

and decreased outflow from lateral boundaries in the UFA 

and LFA. In the surficial aquifer, pumping at well HAAF 11 

(36Q392) resulted in increased inflow (recharge) from the 

general head boundary (0.36 Mgal/d). Flow to well HAAF 11 

(36Q392) was derived from increased lateral flow from the 

specified-head boundary (0.02 Mgal/d), from increased leakage through the Lower Floridan confining unit (0.52 Mgal/d), 

and by reductions in discharge to the Lower Floridan confining unit (0.52 Mgal/d) and to lateral specified-head boundaries 

(0.01 Mgal/d).

The simulated steady-state water budget was further evaluated using the MODFLOW postprocessor ZONEBUDGET 

(Harbaugh, 1990). ZONEBUDGET enables summation of 

flow entering and leaving a designated subarea of the model 

domain. For this evaluation, three zones surrounding well 

HAAF 11 (36Q392) were designated: 0–0.5 mi, 0.5–1 mi, 

and greater than 1 mi. The percentage of leakage from the 

UFA that contributed flow to the LFA at well HAAF 11 

(36Q392) is shown on figure 20. About 65 percent of the water 

that was contributed to well HAAF 11 (36Q392) came from 

within 1 mi of the pumped well, of which 36 percent came 

from within 0.5 mi of the pumped well. The larger contribution of water resulted from a larger head gradient between the 

pumped well and the overlying aquifer in areas close to the 

pumped well.

Upper Floridan Aquifer Drawdown Offset

As part of the interim permitting strategy, GaEPD provided 

a hydrogeological study protocol, that states that a groundwater 

model shall be used (Nolton Johnston, Georgia Environmental 

Protection Division, written commun., January 28, 2003) “...

to simulate 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.”

To replicate simulated drawdown in the UFA in response to 

pumping in the LFA at well HAAF 11, a series of steady-state 

simulations were conducted using the revised model. Each 

simulation involved applying a different pumping rate to a 

hypothetical UFA well located at the site of well HAAF 11 

(36Q392) and running the model under steady-state conditions. Profiles (fig. 21) and maps (fig. 22) show the simulated 

drawdown in the UFA (1) resulting from LFA pumping in 

well HAAF 11 (36Q392) at 748 gal/min and (2) resulting 

from pumping a hypothetical UFA well at the same location 

as well HAAF 11 (36Q392) at rates of 189 and 650 gal/min.

Figure 19. Change in simulated steady-state water budget resulting from initiation of pumping at well HAAF 11 

(36Q392), Hunter Army Airfield, Chatham County, Georgia (modified from Payne and others, 2005).
32 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Well HAAF 11

Well HAAF 5 (36Q392)

(36Q288)

Well HAAF 8

(36Q292)

Well HAAF 1

(36Q285)

Well HAAF 2

(36Q286)

0 5 10 15 MILES

0 10 15 KILOMETERS 5

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

Figure 20. Map showing percentage of interaquifer leakage from the Upper Floridan aquifer contributing flow to 

well HAAF 11 (36Q392) pumping at a rate of 748 gallons per minute (1.08 million gallons per day), Hunter Army Airfield 

and vicinity, coastal Georgia.

HUNTER

ARMY

AIRFIELD

N

EXPLANATION

Well HAAF 5

(36Q288)

1 2 MILES

0

0

1 2 KILOMETERS

Well—Hunter Army Airfield identification; U.S. 

Geological Survey identification in parentheses

Percentage contribution of interaquifer leakage 

from the Upper Floridan aquifer to flow in 

well HAAF 11 (36Q392)

35

28

37

FORT STEWART

81°45' 81°30' 81°15' 81°00' 80°45'

32°00'

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

CHATHAM

TATTNALL

EVANS JASPER

LONG

GA

SC

31°45'

32°15'

Map

area

above

GEORGIA

SOUTH

CAROLINA

Model

boundary

BEAUFORT

ATLANTIC

OCEAN

Figure 20. Percentage of interaquifer leakage from the Upper Floridan aquifer contributing flow to well HAAF 11 (36Q392) 

pumping at a rate of 748 gallons per minute (1.08 million gallons per day), Hunter Army Airfield and vicinity, coastal Georgia.
Effect of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 33

0 20 40 MILES 10 30

SC

GA

ATLANTIC

OCEAN

0 20 40 KILOMETERS 10 30

SIMULATED DRAWDOWN, IN FEET

A A’

MILES

MILES

Maximum drawdown in 

Well HAAF 11 is 36.17 feet

Maximum drawdown in 

Well HAAF 11 is 36.17 feet

N

Base map from U.S. Geological Survey 

 1:2,000,000-scale digital data

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115

0

1

2

3

4

5

6

7

8

9

10

59.7 59.8 59.9 60.0 60.1 60.2 60.3 60.4 60.5 60.6 60.7 60.8 60.9 61.0 61.1 61.2 61.3 61.4 61.5 61.6 61.7

EXPLANATION

Simulated drawdown, in feet, in Upper 

Floridan aquifer resulting from pumping 

Upper Floridan aquifer well 

Simulated drawdown, in feet, in Upper 

Floridan aquifer resulting from 

pumping Lower Floridan aquifer well 

at 748 gallons per minute

Simulated drawdown, in feet, in Lower 

Floridan aquifer resulting from 

pumping Lower Floridan aquifer well 

at 748 gallons per minute

Line of section

650 gallons per minute

189 gallons per minute

A’A

Well—Hunter Army Airfield identification; U.S. 

Geological Survey identification in parentheses

JASPER

WAYNE

BULLOCH

LONG

SCREVEN

BRYAN

LIBERTY

GLYNN

TATTNALL

PIERCE

BRANTLEY

EFFINGHAM

MCINTOSH

EVANS

CANDLER

BEAUFORT

CHATHAM

A

A’

Well HAAF 11

(36Q392)

Well HAAF 11

(36Q392)

Figure 21. Profile showing simulated steady-state drawdown in the Upper and Lower Floridan aquifers resulting from 

pumping in the Lower Floridan aquifer at well HAAF 11 (36Q392) and from pumping the Upper Floridan aquifer at rates 

of 189 and 650 gallons per minute at a hypothetical Upper Floridan aquifer well at the site of well HAAF 11 (36Q392).

Figure 21.  Profile showing simulated steady-state drawdown in the Upper and Lower Floridan aquifers resulting from 

pumping in the Lower Floridan aquifer at well HAAF 11 (36Q392) and from pumping the Upper Floridan aquifer at rates 

of 189 and 650 gallons per minute at a hypothetical Upper Floridan aquifer well at the site of well HAAF 11 (36Q392), 

Hunter Army Airfield and vicinity, coastal Georgia. 
34 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

CHATHAM

Map

area

above FORT STEWART

81°45' 81°30' 81°15' 81°00' 80°45'

32°00'

ATLANTIC

OCEAN

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

TATTNALL

EVANS JASPER

LONG

GA

SC

31°45'

32°15'

GEORGIA

SOUTH

CAROLINA

Model

boundary

BEAUFORT

0.6

0.7

0.8

0.5

1.0

0.5

Well HAAF 11

Well HAAF 5 (36Q392)

(36Q288)

Well HAAF 9

(36Q391)

Well HAAF 1

(36Q285)

Well HAAF 2

(36Q286)

Well HAAF 8

(36Q292)

N

EXPLANATION

1 2 MILES

0

0

1 2 KILOMETERS

Line of equal simulated drawdown— Interval, 0.1 foot. 

Datum is NAVD 88

Well—Hunter Army Airfield identification; U.S. 

Geological Survey identification in parentheses

0.5

HUNTER

ARMY

AIRFIELD

Figure 22. Simulated drawdown in the Upper Floridan aquifer resulting from pumping a hypothetical Upper 

Floridan aquifer well at the site of well HAAF 11 (36Q392) at rates of (A) 189 and (B) 650 gallons per minute, 

Hunter Army Airfield and vicinity, Coastal Georgia.

A

Well HAAF 5

(36Q288)

0 5 10 15 MILES

0 10 15 KILOMETERS 5

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

Figure 22. Simulated drawdown in the Upper Floridan aquifer resulting from pumping a hypothetical Upper Floridan 

aquifer well at the site of well HAAF 11 (36Q392) at rates of (A) 189 and (B) 650 gallons per minute, Hunter Army Airfield 

and vicinity, coastal Georgia.
Effect of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 35

FORT STEWART

81°45' 81°30' 81°15' 81°00' 80°45'

32°00'

ATLANTIC

OCEAN

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

TATTNALL

EVANS JASPER

LONG

SC

31°45'

32°15'

GEORGIA

SOUTH

CAROLINA

Model

boundary

0.5

BEAUFORT

1.0

1.4

1.3

1.2

1.2

1.5

2.0

1.6

1.7

1.8

1.9

2.5 3.0

3.5

GA

Well HAAF 11

Well HAAF 5 (36Q392)

(36Q288)

Well HAAF 9

(36Q391)

Well HAAF 1

(36Q285)

Well HAAF 2

(36Q286)

Well HAAF 8

(36Q292)

N EXPLANATION

1 2 MILES

0

0

1 2 KILOMETERS

Line of equal simulated drawdown— Interval, 0.1 foot. 

Datum is NAVD 88

Well—Hunter Army Airfield identification; U.S. 

Geological Survey identification in parentheses

0.5

HUNTER

ARMY

AIRFIELD

B

Well HAAF 5

(36Q288)

0 5 10 15 MILES

0 10 15 KILOMETERS 5

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

Figure 22. Simulated drawdown in the Upper Floridan aquifer resulting from pumping a hypothetical Upper 

Floridan aquifer well at the site of well HAAF 11 (36Q392) at rates of (A) 189 and (B) 650 gallons per minute, 

Hunter Army Airfield and vicinity, Coastal Georgia.—Continued

Map

area

above

CHATHAM

Figure 22. Simulated drawdown in the Upper Floridan aquifer resulting from pumping a hypothetical Upper Floridan 

aquifer well at the site of well HAAF 11 (36Q392) at rates of (A) 189 and (B) 650 gallons per minute, Hunter Army Airfield 

and vicinity, coastal Georgia.—Continued
36 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Simulated water-level profiles indicate that drawdown 

resulting from pumping the hypothetical UFA well at a rate of 

189 gal/min approximates the maximum drawdown resulting 

from pumping the LFA at well HAAF 11, but is less at 

distances greater than 0.1 mi from the pumped well (fig. 21). 

Conversely, drawdown resulting from pumping at a rate of 

650 gal/min approximates that resulting from pumping in the 

LFA at distances greater than 0.1 mi from the pumped well, 

but is greater closer to the pumped well. The poor overall 

match to drawdown resulting from interaquifer leakage results 

from differences in how the model simulates stress. Leakage 

stress occurs over a wide area, not at a localized position as 

is the case with an individual pumping well. Thus, drawdown 

resulting from leakage shows a more gradual lateral gradient 

and covers a wider area than drawdown resulting from a well 

being pumped directly from the UFA. 

The area in which simulated drawdown exceeded 1 ft 

further illustrates differences in how the model simulates 

stress in the form of interaquifer leakage from the simulation 

of pumping a single UFA well (fig. 22). Simulated drawdown 

in the UFA that resulted from pumping the LFA covered a 

141-mi2 area (fig. 18), whereas drawdown that resulted from 

pumping an UFA well at rates of 189 and 650 gal/min covered 

areas of 0.003 mi2 and 88.7 mi2, respectively (fig. 22). In general, the cone of depression formed in response to pumping in 

the UFA was much steeper near the pumped well, but diminished in magnitude away from the pumped well, whereas the 

cone of depression formed in response to pumping in the LFA 

was less steep, but covered a larger area. Neither pumping rate 

matched the 0.52 Mgal/d (361 gal/min) simulated leakage rate.

Effect of Pumping Offsets on Groundwater Levels 

at Hunter Army Airfield

To assess the effect of pumping redistribution on current 

groundwater conditions at HAAF, three model scenarios 

were run:

• Scenario A—Effect of pumping the LFA at well 

HAAF 11, 24 hours per day at a rate of 748 gal/min 

(1.08 Mgal/d), and reducing pumping in existing UFA 

HAAF supply wells by 187 gal/min (0.27 Mgal/d) 

or 25 percent of the pumping rate at well HAAF 11 

(36Q392). The reduction in withdrawals from the UFA 

is close to the 189 gal/min rate required to match the 

maximum drawdown simulated near well HAAF 11.

• Scenario B—Effect of pumping the LFA at well 

HAAF 11, 24 hours per day at a rate of 748 gal/min 

(1.08 Mgal/d), and reducing pumping in existing UFA 

HAAF supply wells by 374 gal/min (0.54 Mgal/d) 

or 50 percent of the pumping rate at well HAAF 11 

(36Q392). The reduction in withdrawals from the UFA 

is close to the simulated steady-state leakage rate of 

361 gal/min.

• Scenario C—Effect of pumping the LFA at well 

HAAF 11 (36Q392) at a rate of 374 gal/min 

(0.54 Mgal/d), and reducing pumping in existing UFA 

HAAF supply wells by 187 gal/min (0.27 Mgal/d) 

or 50 percent of the pumping rate at well HAAF 11 

(36Q392). Note that all withdrawals from the UFA at 

wells HAAF 8 (36Q292) and HAAF 9 (36Q391) were 

eliminated for this scenario.

For each scenario, permitted withdrawals for the UFA at 

HAAF during 2005 were apportioned on the basis of the 

reported well yield of the four main withdrawal wells—

HAAF 1 (36Q285), HAAF 2 (36Q286), HAAF 9 (36Q391), 

and HAAF 8 (36Q292)—and reduced by a designated 

percentage (table 10). Withdrawal from well HAAF 11 

(36Q392) was held constant at 1.08 Mgal/d (748 gal/min) for 

scenarios A and B, and was decreased by one-half for scenario C (374 gal/min or 0.54 Mgal/d). For scenarios A and B, 

pumping reductions in the UFA were uniformly distributed 

among wells HAAF 1 (36Q285), HAAF 2 (36Q286), HAAF 8 

(36Q292), and HAAF 9 (36Q391) on the basis of the well’s 

contribution to the overall yield (table 10). For scenario C, 

pumping was eliminated at wells HAAF 8 (36Q292) and 

HAAF 9 (36Q391), with the balance of pumping reductions at 

wells HAAF 1 (36Q285) and HAAF 2 (36Q286). Simulated 

drawdown at selected well sites for scenarios A, B, and C is 

listed in table 10. Maps showing water-level changes in the UFA 

resulting from scenarios A and B are provided in figures 23–24. 

A map showing drawdown for scenario C is not provided 

because drawdown was less than 0.5 ft throughout the area. 

For each scenario, pumping reductions in the existing 

HAAF UFA wells resulted in decreased magnitude and extent 

of drawdown when compared to a scenario in which a well 

in the LFA was pumped without reducing withdrawals in the 

UFA (fig. 18). Pumping at well HAAF 11 (36Q392) in the LFA 

without reducing pumping in the UFA resulted in a simulated 

drawdown in the UFA of 2.03 ft near well HAAF 11, with 

drawdown exceeding 1 ft over a 141-mi2 area. For scenario 

A, simulated drawdown was 1.59 ft near well HAAF 11 

(36Q392), and the area in which drawdown exceeded 1 ft 

covered 40.5 mi2 (fig. 23; table 10). Simulated drawdown 

in the UFA for scenario B was 1.15 ft at well HAAF 11 

(36Q392), and the simulated 1-ft drawdown contour covered 

an area of 4.37 mi2 (fig. 24; table 10). For scenario C, simulated drawdown in the UFA was less than 0.5 ft throughout the 

area and was 0.42 ft near well HAAF 11 (36Q392) (table 10). 

None of the scenarios resulted in noticeable changes in the 

configuration of the simulated potentiometric surface and 

related groundwater-flow directions for the UFA (fig. 25). 
Effect of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 37

Table 10.   Simulated pumping and drawdown for the year 2005 permitted 

[Mgal/d, million gallons per day; gal/min, gallons per minute; —, not applicable]

pumping rate and model scenarios A, B, and C.

Aquifer Well 

Simulated 2005 permitted 

pumping rate 

Simulated scenario 

pumping rate, 

in Mgal/d

Simulated scenario 

water-level decline in the 

Upper Floridan aquifer, in feet 

Pump rate 

(gal/min) Percent Mgal/d A1 B2 C3 A1 B2 C3

Upper HAAF 1 

Floridan (36Q285)

Upper HAAF 2 

Floridan (36Q286)

Upper HAAF 5 

Floridan (36Q288)

Upper HAAF 8 

Floridan (36Q292)

Upper HAAF 9 

Floridan (36Q391)

Upper Floridan aquifer 

total

Lower HAAF 11 

Floridan (36Q392)

Grand total

1,300

1,200

—

80

600

2,500

—

—

40.9

37.7

—

2.5

18.9

100

—

—

0.42

0.39

—

0.03

0.19

1.03

—

 1.03

0.31

0.29

—

0.02

0.14

0.76

1.08

1.84

0.20

0.19

—

0.01

0.09

0.49

1.08

1.57

0.40

0.37

—

0.00

0.00

0.76

0.54

1.30

0.81

0.82

1.51

1.59

1.45

—

1.59

—

0.13

0.12

1.13

1.14

0.93

—

1.15

—

0.31

0.31

0.49

0.40

0.12

—

0.42

—

1 Scenario A—Effect of pumping the Lower Floridan aquifer at well HAAF 11, 24 hours per day at a rate of 748 gal/min (1.08 Mgal/d), and reducing 

pumping in existing Upper Floridan aquifer HAAF supply wells by 187 gal/min (0.27 Mgal/d) or 25 percent of well HAAF 11 pumping rate. The reduction 

in Upper Floridan pumping is close to the 189 gal/min rate required to match the maximum drawdown simulated near well HAAF 11. 

2 Scenario B—Effect of pumping the Lower Floridan aquifer at well HAAF 11, 24 hours per day at a rate of 748 gal/min (1.08 Mgal/d), and reducing 

pumping in existing Upper Floridan aquifer HAAF supply wells by 374 gal/min (0.54 Mgal/d) or 50 percent of well HAAF 11 pumping rate. The reduction 

in Upper Floridan aquifer pumping is close to the simulated steady state leakage rate of 361 gal/min. 

3 Scenario C—Effect of pumping the Lower Floridan aquifer at well HAAF 11 at a rate of 374 gal/min (0.54 Mgal/d), and reducing pumping in existing 

Upper Floridan aquifer HAAF supply wells by 187 gal/min (0.27 Mgal/d) or 50 percent of well HAAF 11 pumping rate. Note all Upper Floridan aquifer 

pumping eliminated at wells HAAF 8 and HAAF 9 for this scenario.
38 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

FORT STEWART

81°45' 81°30' 81°15' 81°00' 80°45'

32°00'

ATLANTIC

OCEAN

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

TATTNALL

EVANS JASPER

LONG

GA

SC

31°45'

32°15'

GEORGIA

SOUTH

CAROLINA

Model

boundary

BEAUFORT

1.4

1.5

1.1

1.3

1.2

0.9

0.9

1.0

0.5

1.0

1.5 Line of equal simulated drawdown— Interval, 0.1 foot

Well—Hunter Army Airfield identification; U.S. 

Geological Survey identification in parentheses

Figure 23. Drawdown in the Upper Floridan aquifer for scenario A—effect of pumping the Lower Floridan aquifer 

at well HAAF 11 (36Q392) at a rate of 748 gallons per minute, and reducing pumping in existing Upper Floridan 

aquifer HAAF supply wells by 187 gallons per minute or 25 percent of well HAAF 11 (36Q392) pumping rate—

Hunter Army Airfield and vicinity, coastal Georgia.

HUNTER

ARMY

AIRFIELD

N

EXPLANATION

0 5 10 15 MILES

0 10 15 KILOMETERS 5

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

1 2 MILES

0

0

1 2 KILOMETERS

Well HAAF 5

(36Q288)

Well HAAF 11

(36Q391) Well HAAF 5

(36Q288)

Well HAAF 8

(36Q292)

Well HAAF 1

(36Q285)

Well HAAF 2

(36Q286)

CHATHAM

Map

area

above

Figure 23. Drawdown in the Upper Floridan aquifer for scenario A—effect of pumping the Lower Floridan aquifer at 

well HAAF 11 (36Q392) at a rate of 748 gallons per minute, and reducing pumping in existing Upper Floridan aquifer HAAF 

supply wells by 187 gallons per minute or 25 percent of well HAAF 11 (36Q392) pumping rate— Hunter Army Airfield and 

vicinity, coastal Georgia.
Effect of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 39

FORT STEWART

81°45' 81°30' 81°15' 81°00' 80°45'

32°00'

ATLANTIC

OCEAN

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

TATTNALL

EVANS JASPER

LONG

GA

SC

31°45'

32°15'

GEORGIA

SOUTH

CAROLINA

Model

boundary

BEAUFORT

Figure 24. Drawdown in the Upper Floridan aquifer for scenario B—effect of pumping the Lower Floridan aquifer 

at well HAAF 11 (36Q392) at a rate of 748 gallons per minute, and reducing pumping in existing Upper Floridan 

aquifer supply wells by 374 gallons per minute or 50 percent of the well HAAF 11 (36Q392) pumping rate—

Hunter Army Airfield and vicinity, Coastal Georgia.

0 5 10 15 MILES

0 10 15 KILOMETERS 5

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

N

EXPLANATION

0.5

1 2 MILES

0

0

1 2 KILOMETERS

Line of equal simulated drawdown— Interval, 0.1 foot

Well—Hunter Army Airfield identification; U.S. 

Geological Survey identification in parentheses

HUNTER

ARMY

AIRFIELD

Well HAAF 5

(36Q288)

0.7

1.0

1.0

0.8

0.6

0.9

0.5

0.5

1.1

CHATHAM

Map

area

above

Well HAAF 11

(36Q392)

Well HAAF 5

(36Q288)

Well HAAF 9

(36Q391)

Well HAAF 1

(36Q285)

Well HAAF 2

(36Q286)

Well HAAF 8

(36Q292)

Figure 24. Drawdown in the Upper Floridan aquifer for scenario B—effect of pumping the Lower Floridan aquifer at 

well HAAF 11 (36Q392) at a rate of 748 gallons per minute, and reducing pumping in existing Upper Floridan aquifer supply 

wells by 374 gallons per minute or 50 percent of the well HAAF 11 (36Q392) pumping rate—Hunter Army Airfield and 

vicinity, coastal Georgia. 
40 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

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

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l l l l l l l l l l l

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0

50

100

–50

0

0

0

50

100

–50

0

0

50

100

–50

–50

0

50

0

100

Base from U.S. Geological Survey

1:100,000-scale digital data

EXPLANATION

Simulated potentiometric contour— Interval, 10 feet. 

Hachures indicate depression. Datum is NAVD 88

Base case Scenario A

Figure 25. Simulated Upper Floridan aquifer potentiometric surfaces for the 2005 base case and scenarios A, B, and C, 

Hunter Army Airfield and vicinity, coastal Georgia.

Scenario B

50

SC

GA

ATLANTIC

OCEAN

Scenario C

0 20 40 MILES 10 30

0 20 40 KILOMETERS 10 30

N

Model boundary

Jasper

Wayne

Bulloch

Long

Bryan

Liberty

Glynn

Tattnall Pierce

Brantley

Effingham

Chatham

Mcintosh

Candler

Beaufort

Evans

Effect of Pumping Offsets on Water Supply at 

Hunter Army Airfield

The GaEPD interim strategy for permitting withdrawals 

from the LFA (Nolton Johnston, Georgia Environmental 

Protection Division, written commun., January 28, 2003) 

requires an applicant to 

• “offset the impact of the LF [Lower Floridan] 

pumping by reducing nearby UF [Upper Floridan] 

permitted pumping in the same general area (within a 

5 mile radius) by an amount equal to or greater than 

the determined UF leakage” and

• “simulate 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.” 

Results of model scenarios indicate that these two requirements result in widely varying simulated pumping offsets 

for the UFA. Simulated interaquifer leakage from the UFA 

through the Lower Floridan confining unit into the LFA 

was 361 gal/min (0.52 Mgal/d), whereas a pumping rate of 

189 gal/min (0.27 Mgal/d) in a hypothetical UFA well was 

needed in order to match simulated maximum drawdown in 

the UFA that resulted from interaquifer leakage. The simulated 

pumping rate needed to match the maximum drawdown in the 

UFA underpredicts the amount of pumping offset because the 

cone of depression formed in response to pumping the UFA is 

steeper near the pumped well and covers a smaller area than 

the area simulated in response to interaquifer leakage. Thus, 

the simulated leakage rate may be a more effective means of 

evaluating required pumping offsets.

Figure 25. Simulated Upper Floridan aquifer potentiometric surfaces for the 2005 base case 

and scenarios A, B, and C, Hunter Army Airfield and vicinity, coastal Georgia.
Effect of Lower Floridan Aquifer Pumping on the Upper Floridan Aquifer 41

Changes in total permitted capacity at HAAF would 

vary based on the amount of reduction mandated by GaEPD. 

Described below are the required pumping reduction and 

net gain in production capacity at HAAF that would result 

if (1) the June 2010 revised UFA permit rate is assumed 

(0.98 Mgal/d), (2) pumping from the LFA is for a 24-hour 

period at a rate of 748 gal/min (1.08 Mgal/d), and (3) the 

simulated leakage response (361 gal/min or 0.51 Mgal/d) is 

used as the basis of the reduction:

• Because the simulated leakage rate is 0.51 Mgal/d, 

the June 2010 UFA permit at HAAF would need to 

be reduced by 0.26 Mgal/d to a revised permitted 

withdrawal rate of 0.72 Mgal/d,

Table 11. Projected reductions in Upper Floridan aquifer permitted capacity and net gain in total water capacity for various pumping 

periods, well HAAF 11 (36Q392), Hunter Army Airfield, Chatham County, Georgia.

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

Lower Floridan pumping 

(well HAAF 11)

Estimated Upper 

Floridan aquifer 

pumping reduction 

to offset Lower 

Floridan aquifer 

pumping effects

June 2010 

permitted 

Upper Floridan 

aquifer withdrawal rate 

Revised Upper 

Floridan aquifer 

withdrawal rate 

(accounting 

for pumping 

reduction)

Total pumping 

capacity 

(Upper and 

Lower Floridan 

aquifers 

combined)

Net increase 

in pumping 

capacity 

(Upper and 

Lower Floridan 

aquifers 

combined)

Remarks

Daily 

pumping 

period 

(hours)

Gal/min Mgal/d Gal/min Mgal/d Mgal/d Mgal/d Mgal/d Mgal/d

To offset simulated interaquifer leakage response in Upper Floridan aquifer in response to Lower Floridan aquifer pumping in well HAAF 11

8 748 0.36 361 0.17 0.98 0.81 1.17 0.19

12 748 0.54 361 0.26 0.98 0.72 1.26 0.28 See model 

scenario C

16 748 0.72 361 0.35 0.98 0.63 1.35 0.37

24 748 1.08 361 0.52 0.98 0.46 1.54 0.56 See model 

scenario B

To offset simulated maximum drawdown in the Upper Floridan aquifer in response to pumping a 

hypothetical Upper Floridan aquifer well at the location of well HAAF 11

8 748 0.36 189 0.09 0.98 0.89 1.25 0.27

12 748 0.54 189 0.14 0.98 0.84 1.38 0.40

16 748 0.72 189 0.18 0.98 0.80 1.52 0.54

24 748 1.08 189 0.27 0.98 0.71 1.78 0.80 See model 

scenario A

• The new LFA permit would be based on a 24-hour 

per day pumping period at a withdrawal rate of 

748 gal/min (1.08 Mgal/d),

• The resulting water production would be the 

combined permitted rate of the UFA and LFA or 

0.72 Mgal/d+ 1.08 Mgal/d = 1.8 Mgal/d.

Pumping offsets and net gain in water production capacity for a variety of pumping periods are listed in table 11. For a 

12-hour daily pumping period, the net gain in capacity would 

range from 0.28 Mgal/d (to meet the leakage requirement) to 

0.4 Mgal/d (to meet the maximum drawdown requirement). 

For a 24-hour pumping period, the net gain in capacity would 

range from 0.56 Mgal/d (to meet the leakage requirement) to 

0.8 Mgal/d (to meet the maximum drawdown requirement).
42 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Limitations of Analysis

The effect of pumping the LFA on water levels in the 

UFA was analyzed using model simulation. These analyses are 

limited by the accuracy of field data, including possible errors 

and uncertainty in water-level measurements, hydraulic properties, and pumping. Although water-level data were filtered 

(Halford, 2006) in an attempt to eliminate local interferences, 

such as tidal and pumping effects, it is possible that some of 

these interferences still affect recorded levels and, thus, computed hydraulic properties and measured drawdown response.

A revised regional-flow model was used to simulate longterm (steady-state) changes in groundwater flow. Additional 

insight into changes in water levels over time could be gained 

by using transient simulation. The revised model reasonably 

depicts changes in groundwater levels resulting from pumping 

the LFA at HAAF at a rate of 748 gal/min. Results are limited 

by the same model assumptions and design as described by 

Payne and others (2005). In addition to limitations of field 

data accuracy as described above, the revised model may have 

inaccuracies in the conceptual model of groundwater flow. For 

example, approximations made in representing the physical 

properties of the flow system and errors inherent in estimating 

the spatial distribution of these properties; approximations 

made in the formulation and application of model boundary 

and initial conditions; errors associated with numerical 

approximation and solution of the mathematical model of the 

flow system; and assumptions made in using the models to 

predict the future behavior of the flow system. The variably 

spaced mesh used in the revised model has an aspect ratio 

between row and column dimensions of as high as 1,646:1, 

which can lead to numerical errors (de Marsily, 1986, p. 351). 

Fortunately, such high aspect ratios occur only in areas distant 

from HAAF and will have little effect on simulated results in 

the area. 

Simulated rates of interaquifer leakage and drawdown 

may be lower than actual because of the influence of specifiedhead and general-head boundaries. These boundaries supply 

an unlimited amount of water to the groundwater system, 

which may result in lower than actual simulated drawdown 

and related rates of interaquifer leakage. Simulation results 

could be improved by eliminating these types of boundaries 

and using active simulation of the surficial aquifer and by 

using lateral specified-flux boundaries. 

Summary and Conclusions

To assess the hydrogeology and water quality of the 

Floridan aquifer system and the potential effect of Lower Floridan 

aquifer (LFA) pumping on the Upper Floridan aquifer (UFA), 

the U.S. Geological Survey, in cooperation with the Department 

of the Army, conducted an investigation at Hunter Army Airfield 

(HAAF) in Chatham County, Georgia, during 2009. The study 

included construction of a test well completed in the LFA, 

aquifer-performance testing, geophysical logging, water-quality 

sampling and analysis, and digital groundwater modeling. 

Results of test drilling and field tests indicate the LFA at 

HAAF consists of limestone and dolomitic limestone between 

depths of 703 and 1,080 feet (ft). Six major permeable zones 

were identified through borehole-geophysical logging and 

flowmeter testing: 723–731; 768–785; 818–837; 917–923; 

1,027–1,052; and 1,060–1,080 ft. The two uppermost zones

contribute about 469 gallons per minute (gal/min) or 62.6 percent of the total flow of 748 gal/min. The remaining four zones 

each contributed from 1.7 to 18 percent of the total flow.

Grab water samples indicate water from the LFA is hard 

to very hard with a bicarbonate alkalinity of 150 to 250 milligrams per liter (mg/L). With the exception of fluoride, 

constituent concentrations increased with depth. Water from 

the deepest interval (1,075 ft) has a chloride concentration of 

480 mg/L and a sulfate concentration of 240 mg/L, above and 

close to the U.S. Environmental Protection Agency (USEPA) 

secondary maximum contamination level for drinking water, 

respectively. Flowmeter testing indicates that water from the 

1,075-ft zone contributes less than 2 percent of the total flow 

to the well, so the relatively higher concentrations of chloride 

and sulfate in the deepest interval will not adversely affect the 

water quality in the completed well. A composite water-quality 

sample from the completed LFA well indicated constituent 

concentrations of the composite sample were all below 

USEPA primary and secondary maximum contaminant levels 

(U.S. Environmental Protection Agency, 2009).

To assess the effect of LFA pumping on the UFA, 

a 72-hour aquifer test was conducted in which LFA well 

HAAF 11 (36Q392) was pumped at an average rate of 

748 gal/min, and drawdown response was monitored in nearby 

UFA wells HAAF 5 (36Q288) and HAAF 8 (36Q292). At 

the end of 72 hours, total drawdown—after filtering data to 

remove the effects of regional pumping, tides, and barometric 

pressure—was 0.76 ft in well HAAF 8 (36Q292) and 0.43 ft 

in well HAAF 5 (36Q288). 

A revised regional groundwater-flow model was used to 

simulate the long-term (steady-state) leakage response of the 

UFA to pumping from the LFA, and to estimate the equivalent 

amount of UFA pumping that would produce similar 
Summary and Conclusions 43

drawdown. The model incorporated data obtained during field 

testing at the site of well HAAF 11 and applied a fine grid 

resolution near that well to enable more accurate simulation 

at small scale. 

Pumping well HAAF 11 (36Q392) at a rate of 

748 gal/min (about 1 million gallons per day [Mgal/d] resulted 

in a maximum steady-state drawdown of 36.2 ft in the LFA 

and was greater than 1 ft over a 146-square-mile (mi2) area. 

Simulated steady-state drawdown in the overlying UFA that 

resulted from interaquifer leakage was greater than 1 ft over a 

141-mi2 area and was 2.03 ft at the site of well HAAF 11. 

Pumping 1 Mgal/d at well HAAF 11 (36Q392) had a 

minor influence on the overall simulated water budget, with 

most of the change reflected by an increase in recharge from 

the overlying general head boundary. Although the overall 

budget changed little, there was a redistribution of flow in 

individual model layers. Flow to well HAAF 11 (36Q392) 

was derived by increased lateral flow from specified-head 

boundary (0.02 Mgal/d), increased leakage from the UFA 

through the Lower Floridan confining unit (0.52 Mgal/d), 

and reductions in discharge to the Lower Floridan confining 

unit (0.53 Mgal/d) and to lateral specified-head boundaries 

(0.53 Mgal/d). Sixty-five percent of the leakage from the UFA 

occurred within 1 mile of well HAAF 11. This larger contribution results from a larger head gradient between the pumped 

well and overlying aquifer in areas close to the pumped well.

As part of the interim permitting strategy for the LFA, 

the Georgia Environmental Protection Division (GaEPD) 

required simulation of (1) aquifer leakage from the UFA to 

LFA resulting from pumping the new LFA well, and (2) “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.” Results 

of this analysis can be used as a basis to reduce nearby UFA 

permitted pumping in the same general area (within a 5-mi 

radius) by an amount equal to or greater than the determined 

leakage rate. Results of model simulations indicate that these 

two requirements result in widely varying pumping offsets 

for the UFA. Simulated interaquifer leakage was 361 gal/min 

(0.52 Mgal/d), whereas the equivalent UFA pumping rate to 

offset maximum drawdown was 189 gal/min (0.27 Mgal/d). 

The simulated pumping rate necessary to match the maximum 

drawdown in the UFA underpredicts the amount of pumping 

offset because the cone of depression formed in response to 

pumping the UFA is steeper near the pumped well and covers a 

smaller area than the area simulated in response to interaquifer 

leakage. Thus, the simulated leakage rate may be a more 

effective means of evaluating required pumping offsets.

Three groundwater pumping scenarios were run to 

evaluate the effect of various pumping changes on groundwater levels at HAAF:

• Scenario A—Effect of pumping the LFA at well 

HAAF 11, for 24 hours per day at a rate of 748 gal/min 

(1.08 Mgal/d), and reducing pumping in existing UFA 

HAAF supply wells by 187 gal/min (0.27 Mgal/d) 

or 25 percent of the pumping rate at well HAAF 11 

(36Q392). The reduction in withdrawals from the UFA 

is close to the 189 gal/min rate required to match the 

maximum drawdown simulated near well HAAF 11.

• Scenario B—Effect of pumping the LFA at well 

HAAF 11, for 24 hours per day at a rate of 748 gal/min 

(1.08 Mgal/d), and reducing pumping in existing UFA 

HAAF supply wells by 374 gal/min (0.54 Mgal/d) 

or 50 percent of the pumping rate at well HAAF 11 

(36Q392). The reduction in withdrawals from the UFA 

is close to the simulated steady state leakage rate of 

361 gal/min.

• Scenario C—Effect of pumping the LFA at well 

HAAF 11 (36Q392) at a rate of 374 gal/min 

(0.54 Mgal/d), and reducing pumping in existing UFA 

HAAF supply wells by 187 gal/min(0.27 Mgal/d) 

or 50 percent of the pumping rate at well HAAF 11 

(36Q392). Note that all withdrawals from the UFA at 

wells HAAF 8 (36Q292) and HAAF 9 (36Q391) were 

eliminated for this scenario.

For each scenario, pumping reductions in existing HAAF 

UFA wells resulted in decreased magnitude and extent of 

drawdown when compared to a scenario in which the LFA 

was pumped without adjusting withdrawals from the UFA. 

Pumping from the LFA well HAAF 11 (36Q392) without 

adjusting pumping from the UFA resulted in drawdown 

exceeding 1 ft over a 141 mi2, whereas drawdown exceeding 

1 ft covered areas of 40.5 mi2 for scenario A and 4.37 mi2 for 

scenario B. Drawdown was less than 0.5 ft for scenario C. 

None of the scenarios resulted in noticeable changes in the 

configuration of the simulated potentiometric surface and 

related groundwater flow directions for the UFA. 

GaEPD requires a reduction in permitted UFA withdrawals in order to offset simulated rates of leakage and 

maximum drawdown in the UFA. Once these reductions 

are accounted for, the net gain in water production capacity 

would be the difference between the maximum pumping rate 

in well HAAF 11 (36Q392) and the pumping offset. Changes 

in total permitted capacity at HAAF would vary on the basis 

of which amount of reduction is mandated by GaEPD. For a 

12-hour daily pumping period, the net gain in capacity would 

range from 0.28 Mgal/d (to meet the leakage requirement) to 

0.4 Mgal/d (to meet the maximum drawdown requirement). 

For a 24-hour pumping period, the net gain in capacity would 

range from 0.56 to 0.8 Mgal/d.
44 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

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

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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.

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 July 25, 

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

Peaceman, D.W., 1983. Interpretation of well-block pressures 

in mumerical reservoir simulation with nonsquare grid 

blocks and anisotropic permeability: Society of Petroleum 

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

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.

Ransom, Camille, III, and White, J.I., 1999, Potentiometric 

surface of the Floridan aquifer system in southern South 

Carolina: South Carolina Department of Health and 

Environmental Control, Bureau of Water Publication 

No. 02B–99, 1 sheet.

Reilly, T.E., and Harbaugh, A.W., 2004, Guidelines for evaluating groundwater flow models: U.S. Geological Survey 

Scientific Investigations Report 2004–5038, 30 p.

Theis, C.V., 1935, Relation between the lowering of the 

piezometric surface and the rate and duration of discharge 

of a well using groundwater storage: American Geophysical 

Union Transcripts, pt. 2, p. 519–524; dupl., as U.S. Geological Survey Groundwater Note 5, 1952.

U.S. Environmental Protection Agency, 2009, Drinking water 

contaminants, accessed August 19, 2009, at http://www.epa.

gov/safewater/contaminants/index.html#listmcl.

Wait, R.L., 1965, Geology and occurrence of fresh and 

brackish water in Glynn County, Georgia: U.S. Geological 

Survey Water-Supply Paper 1613–E, 94 p.

Warner, Debbie, and Aulenbach, B.T., 1999, Hydraulic characteristics of the Upper Floridan aquifer in the Savannah 

and St. Marys areas of coastal Georgia: Georgia Geologic 

Survey Information Circular 105, 23 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. 

Appendix. Groundwater Model

Figures

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

and boundary conditions, and revised model grid Hunter Army Airfield 

and vicinity, coastal Georgia.....................................................................................................48

A–2. Schematic diagram showing model layers and boundary conditions ..............................50

A–3. Schematic diagram showing simulated hydraulic property zones by model layer.........52

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

(residuals) by model layer for 2000, revised regional flow model: Brunswick 

aquifer system (layer 3), Upper Floridan aquifer (layer 5), and Lower Floridan 

aquifer (layer 7)...........................................................................................................................55

A–5. Graphs showing observed and simulated water levels in model layers 3, 5, 

and 7 revised groundwater model ...........................................................................................56

Tables

A–1. Comparison of water-level calibration statistics between the original and 

revised regional models, year 2000 steady-state simulation ..............................................54

A–2. Comparison of simulated water budget by model layer between the original 

and revised regional models, year 2000 simulation ..............................................................54

Contents

Regional Groundwater Model ...................................................................................................................48

Revisions to Regional Model......................................................................................................................51

Refinement of Hydraulic Conductivity Distribution .......................................................................51

Simulation of Observed Drawdown in Pumped Well....................................................................53

Comparison of Revised to Original Regional Model ..............................................................................53

References Cited..........................................................................................................................................56
48 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Regional Groundwater Model

The existing USGS regional groundwater-flow model 

(herein termed “regional model” ) of Payne and others 

(2005) was modified using hydrogeological information 

obtained from field investigations and from existing wells 

in the vicinity of Hunter Army Airfield (HAAF) to simulate 

pumping from the newly constructed Lower Floridan aquifer 

well (HAAF 11, 36Q392) and the effect on the Upper Floridan 

aquifer. The regional model is described in detail in Payne and 

others (2005); only a brief description is included here. The 

regional model was calibrated for predevelopment, 1980, and 

2000 pumping and boundary conditions. Because pumpage 

during 2009 was within the range simulated by the earlier 

regional model, the revised model was deemed suitable for 

the evalution of groundwater flow at HAAF.

EXPLANATION

Specified head

No-flow

FORT

STEWART

0 20 40 MILES

0 20 40 KILOMETERS

Base from U.S. Geological Survey

1:100,000-scale digital data

N

Model grid with uniform spacing of Model boundary

 16,461 feet by 16,461 feet

Area of dense model grid—Size progressively 

 increased by a factor of 1.5 from the smallest 

 grid spacing of 10 feet to a spacing of 16,461 feet

HUNTER

ARMY

AIRFIELD

Facing page

detailed

map area

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

and revised model grid, Hunter Army Airfield and vicinity, coastal Georgia.

FLORIDA

GEORGIA

Model

boundary

SOUTH

CAROLINA

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

and revised model grid Hunter Army Airfield and vicinity, coastal Georgia.
Regional Groundwater Model 49

N

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

and revised model grid, Hunter Army Airfield and vicinity, coastal Georgia.—Continued

EXPLANATION

Model grid

Well—Hunter Army Airfield identification; U.S. Geological Survey 

 identification in parentheses

Well HAAF 9

(36Q391)

200 METERS

500 1,000 FEET

100

0

0

Well HAAF 11

(36Q392)

Well HAAF 9

(36Q391)

Well HAAF 5

(36Q288)

Well HAAF 8

(36Q292)

Map

area

HUNTER

ARMY

AIRFIELD

50.6

22.5

15.0

22.5

33.8

15.0

10.0

33.8

GRID CELL SIZE, IN FEET

22.5

15.0

10.0

15.0

GRID CELL SIZE, IN FEET

0 1 2 MILES

0 1 2 KILOMETERS

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

and revised model grid Hunter Army Airfield and vicinity, coastal Georgia.—Continued

Regional groundwater flow was simulated using 

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

used and is appropriate for modeling local- and regional-scale 

groundwater-flow systems. To account for natural hydrologic 

boundaries, the model area covers a 42,155 square-mile (mi2) 

area extending inland in Georgia and into northeastern Florida 

and southwestern South Carolina, and the adjacent offshore 

area (fig. A–1).
50 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

The regional model (Payne and others, 2005) was 

calibrated for 1980 and 2000 pumping conditions assuming 

steady-state flow. The regional model consists of seven hydrogeologic units (fig. A–2), which include, in descending order:

• the confined upper and lower water-bearing zones of the 

surficial aquifer system, grouped together as layer 1; 

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

• the upper and lower Brunswick aquifers, grouped as 

the Brunswick aquifer system (layer 3); 

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

• the Upper Floridan aquifer (layer 5); 

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

• the Lower Floridan aquifer (layer 7). 

These units crop out to the northwest of the study area 

and generally dip and thicken to the southeast. The thickness, 

extent, and other hydraulic properties of these units as well 

as the model development process are described in detail in 

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

layers and boundary conditions is shown in figure A–2.

The original regional model was horizontally discretized 

using a variably spaced grid, with cell sizes ranging 

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

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

is 5,000 × 5,200 ft, whereas at Fort Stewart the resolution is 

14,900 × 16,100 ft. Each hydrogeologic unit is represented 

with one layer of grid cells in the vertical dimension. 

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

Lower Floridan aquifers and intervening confining unit), 

lateral boundaries for all layers are designated as noflow. For 

layers 5, 6, and 7, the lateral boundaries on all sides of the 

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

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

(from Payne and others, 2005).

Confining unit

No-flow or fixed-head boundary

EXPLANATION

General-head boundary—Represents water table or equivalent 

 freshwater head that provides flow to underlying confined aquifers. 

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

 hydraulic conductivity

No-flow boundary

Offshore area—Equivalent freshwater head 

 of overlying saltwater column

Onshore area—Water-table head

Aquifer

Figure A–2. Schematic diagram showing model layers and boundary conditions (from Payne and others, 2005).
Revisions to Regional Model 51

model, except for the southern and southwestern sides, also 

are designated as noflow. The southern and southwestern 

lateral boundaries are set as specified head for the layers 

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

Floridan aquifer (layer 7), and the intervening confining 

unit (layer 6). For these three layers, the value assigned to 

specified-head cells is based on Upper Floridan aquifer head 

estimated from potentiometric-surface maps for 1998 (Peck 

and others, 1999). 

The bottom boundary of the regional model is noflow, 

corresponding to the lower confiing unit of the Floridan aquifer 

system, whereas the top boundary is set as a head-dependent 

flux (or general head) boundary condition with a controlling 

specified head and a conductance term that regulates the flux 

into the top layer of the model. The controlling head is the 

water-table altitude in the onshore area and the freshwater 

equivalent of the saltwater head in the offshore area. In the 

onshore area, the conductance is set to limit the amount of 

recharge entering the system in any given grid cell to lessthan-maximum estimated recharge from baseflow estimates 

(Priest, 2004). For the purpose of simplification and because 

little is known about hydraulic properties in the offshore area, 

the conductance imposed in the offshore area is large, posing 

minimal resistance to flow in or out of the system. 

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

average annual pumpage for 1980 and 2000. Pumpage was 

assigned to model layers 3 (Brunswick aquifer system), 

5 (Upper Floridan aquifer), and 7 (Lower Floridan aquifer) 

based on the open interval of wells located within a model 

cell. The sum of site-specific and nonsite-specific pumping 

rates for 1980 and 2000 were assigned to the model cell in 

which their respective assigned locations and aquifers were 

situated. Total pumpage simulated by the model during 1980 

was 692 Mgal/d, by 2000 this had risen to 798 Mgal/d. 

Revisions to Regional Model

A steady-state simulation was conducted using the model 

of Payne and others (2005) in which well HAAF 11 (36Q392) 

was pumped at a rate of 748 gallons per minute (gal/min) and 

drawdown response was recorded. Simulated Upper Floridan 

aquifer drawdown was 4.13 ft at well HAAF 8 (36Q292) and 

3.08 ft at well HAAF 5 (36Q288); Lower Floridan aquifer 

drawdown was 222 ft at well HAAF 11 (36Q392). The amount 

of drawdown simulated in the Lower Floridan aquifer was 

more than six times greater than the 36.3 ft of drawdown 

observed in well HAAF 11 (36Q392) during a 72-hour aquifer 

test in which drawdown appeared to have mostly stabilized. 

This excessive drawdown indicated that modification of the 

model was needed. 

The regional model was modified to enable more detailed 

simulation in the vicinity of the new Lower Floridan aquifer 

well (HAAF 11, 36Q392) and surrounding area. Grid-cell 

dimensions were modified to a variably spaced grid (fig. A–1) 

that was progressively increased by a factor of 1.5 from the 

smallest grid spacing of 10 ft near well HAAF 11 (36Q392) to 

a spacing of 16,461 ft, which was held constant to the lateral 

extent of the model. The revised model consists of 452 rows 

and 530 columns. Model layering and boundary conditions 

remained unchanged from the original regional model.

Refinement of Hydraulic Conductivity Distribution

Values of vertical and horizontal hydraulic conductivity 

(Kv, Kh) in the revised model corresponded to values used in 

the original regional model (Payne and others, 2005) in the 

area outside of HAAF. Field testing at HAAF provided new 

information on the hydraulic properties of the Upper Floridan 

aquifer (layer 5), Lower Floridan confining unit (layer 6), and 

Lower Floridan aquifer (layer 7), and enabled refinement of 

values in the original regional model. In addition, results of a 

72-hour aquifer test conducted in the Lower Floridan aquifer 

provided information on drawdown in the Lower Floridan 

aquifer and in the overlying Upper Floridan aquifer, which 

guided a revised calibration of the model in the vicinity of 

HAAF within the larger regional model. Maps showing the 

distribution of hydraulic- property zones by model layer are 

shown in figure A–3.

To incorporate refined hydraulic-property information, 

new hydraulic-property zones were created in the Upper 

Floridan aquifer (zone F13), Lower Floridan aquifer 

(zone LF2) and intervening Lower Floridan confining layer 

(zone LFC2). These zones cover a 114.5 mi2

 common area 

that includes HAAF and the area of highest grid resolution, 

and extend from western Chatham County into eastern Bryan 

County (fig. A–3). 

 For the Upper Floridan aquifer (layer 5), zone F13 

was subdivided from existing regional model zone F4, and 

assigned a Kv and Kh value of 76 feet per day (ft/d), which is 

slightly higher than the 70 ft/d value assigned in the original 

regional model. This modification was based on results 

of aquifer testing conducted in wells HAAF 9 (36Q291) 

and HAAF 8 (36Q292). Multiplying this Kh

 value by the 

simulated thickness of the aquifer (526 ft) gives an estimated 

transmissivity of 40,000 square feet per day (ft2/d), matching 

results of field testing at HAAF.

In the original regional model (Payne and others, 2005), 

the Lower Floridan confining unit (layer 6) was assigned a 

constant Kh and Kv value of 0.02 ft/d; in the revised model, zone 

LFC2, covering an identical area as zone F13, was subdivided 

from this single zone and assigned a Kh value of 0.2 ft/d and 

a Kv value of 0.02 ft/d (fig. A–3). These values were based on 

adjustments made during model calibration and are at the lower 

end of the range of estimated values derived from packer-slug 

tests and core analysis at well HAAF 11 (36Q392). 
52 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

F3

F4

F5

F6

F1

F2

F12

F13

F11

F8

F7

F9

C4

C1

B1

C2

C3

C1

C2

C3

C5

C4

C1

C2

LFC2

LF2

C3

C4

C5

Upper Brunswick 

confining unit

(layer 2)

Surficial aquifer system

(layer 1)

Lower Floridan confining unit (layer 6)

Lower Floridan aquifer (layer 7)

Upper Floridan confining unit

(layer 4)

Brunswick aquifer system

(layer 3)

Upper Floridan aquifer

(layer 5)

N

FL

GA

SC

GA

LFC1

LF1

F10

Hydraulic-property zone

Model-calibrated horizontal 

 hydraulic conductivity,

 in feet per day

EXPLANATION

0.00001 to 0.00017

0.02 to 0.66

2 to 94

100 to 394

2,500 to 3,200

C1

Horizontal Vertical

1 N/A 70 70

2, 4 C1 0.00017 0.00017

C2 0.2 0.2

C3 0.00001 0.00001

C4 0.0001 0.0001

C5 0.0001 0.0001

3 B1 50 50

C1 0.00017 0.00017

C2 0.2 0.2

C3 0.00001 0.00001

C4 0.0001 0.0001

5 F1 34 34

F2 2 2

F3 100 100

F4 70 70

F5 394 394

F6 2,819 2,819

F7 150 150

F8 2,727 2,727

F9 100 100

F10 56 56

F11 94 94

F12 25 25

F13 76 76

6 LFC1 0.02 0.02

10

LFC2 0.20 0.02

7 LF1 10 10

LF2 100

Hydraulic conductivity

Layer Zone (feet per day)

Figure 1–3. Schematic diagram showing simulated hydraulic property zones by model layer. Zone added for updated 

model shown in magenta (modified from Payne and others, 2005).

Figure A–3. Schematic diagram showing simulated hydraulic property zones by model layer. Zone added for updated 

model shown in magenta (modified from Payne and others, 2005).
Comparison of Revised to Original Regional Model 53

In model layer 7, which represents the Lower Floridan 

aquifer, zone LF2 was subdivided from a single zone that was 

previously assigned a uniform Kh and Kv value of 10 ft/d in 

the original regional model of Payne and others (2005). Zone 

LF2 covers an identical area as zones F13 and LFC2 and 

was assigned a Kh value of 100 ft/d and a Kv value of 10 ft/d 

on the basis of results of aquifer testing conducted in well 

HAAF 11 (36Q392). Multiplying the Kh value by the regionalmodel-assigned thickness for the Lower Floridan aquifer of 

70 ft, gives a transmissivity of 7,000 ft2/d, which is similar in 

magnitude to the 11,000 ft2/d value computed from an aquifer 

test conducted in well HAAF 11 (36Q392). Kv of the Lower 

Floridan aquifer in zone LF2 was 10 ft/d based on an assumed 

10:1 ratio of horizontal to vertical properties.

Simulation of Observed Drawdown in 

Pumped Well

Drawdown calculated by the MODFLOW model 

represents the average drawdown for the grid cell, which is 

less than that observed in an individual pumped well. Because 

the grid cell size used in the MODFLOW simulation at the 

pumped well (10 ft2) is much larger than the well diameter 

(0.33 ft), the drawdown calculated by the MODFLOW model 

at the pumped well needs to be proportionately increased on 

the basis of well diameter to provide a more precise estimate 

of drawdown. Simulated drawdown was adjusted using the 

following equation (Peaceman, 1983):

Sp=Sb+[Q*ln(re/r

w)/2π(TxxTyy)0.5] (1)

where:

Sp is the adjusted drawdown in the 

pumped well, in feet;

Sb is the MODFLOW simulated drawdown 

in the pumped well, in feet;

Q is the pump discharge, in cubic feet 

per day (144,000);

re is the equivalent well block radius, 

in feet (5);

r

w is the well radiusin feet (0.33);

Txx is the transmissivity in the x-direction, 

in feet squared per day (7,000); and 

Tyy is the transmissivity in the y-direction, 

in feet squared per day (7,000).

Use of this equation indicated that simulated values in the 

model cell containing well HAAF 11 (36Q392) when pumped 

at a rate of 748 gal/min would would be 8.9 ft less than the 

observed drawdown. A similar analysis for an Upper Floridan 

aquifer well pumped at a rate of 650 gal/min and using the same 

parameters, with the exception of a value of 40,000 ft2

/d for 

transmissivity, indicates that simulated values for a pumped well 

in this aquifer would require an adjustment of about 1.36 ft.

Comparison of Revised to Original 

Regional Model

Because the original regional model of Payne and others 

(2005) was modified by changing grid size and by assigning 

different hydraulic properties in the vicinity of HAAF, a 

comparison of the two models is provided to ensure the 

model is still an accurate representation of groundwater flow. 

Summaries of water-level residuals (simulated minus observed 

head) and simulated water budgets for the two models are 

presented (tables A–1 and A–2). Water-level residuals for the 

Upper and Lower Floridan aquifers are shown on maps plotted 

in figure A–4 and graphically in figure A–5. 

Model results indicate that the revised regional model 

has water-level residuals and a simulated water budget similar 

to the original model of Payne and others (2005), and thus, 

the model remains equally representative of the hydrologic 

system (tables A–1, A–2). These results are expected because 

model revisions are limited to a 114.5 mi2 area representing 

less than 1 percent of the model area. Water-level residuals 

for the revised regional model were slightly higher than those 

calculated from the original model for layers 5 and 7, with 

root mean square (RMS) of 10.2 and 9.91 ft, respectively 

(table A–1). Eighty percent of the residuals for layer 5 derived 

from the revised model were within –3.8 to 2.9 ft of the 

original regional model residuals. 

The RMS of water-level residuals for layer 3 (11 ft) 

in the revised model was nearly double that in the original 

regional model (5.91 ft) but is acceptable for the purpose of 

the modified model. Simulated water levels in 8 of 10 wells in 

layer 3 showed residuals that increased by 0.34 to 3.4 ft when 

compared to the orginal regional model. Most of the increase 

in the RMS for layer 3 can be attributed to two wells that had 

water-level residuals of –19 ft (figs. A–4, A–5). These wells 

are located adjacent to one another in the same model cell, 

in an area where the grid size of the revised model was more 

than four times greater than in the original regional model. This 

larger grid size reduced the capability of the model to simulate 

steeper gradients in the vicinity of the Savannah area cone of 

depression and resulted in a greater residual. Because the larger 

grid size and related increase in RMS for layer 3 occurred away 

from the area of higher grid resolution in the vicinity of HAAF, 

model simulations at HAAF were not affected. 

Simulated water budgets for the regional and revised 

models were similar, with only slight variation (table A–2). 

The revised model showed a slight increase in interaction with 

lateral boundaries and a slight decrease in interaction with 

the overlying general head boundary, specifically, (1) overall 

decreases in recharge from and discharge to the overlying 

general-head boundary, and (2) decreased outflow and 

increased inflow along lateral specified-head boundaries. 
54 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

Table A–1. Comparison of water-level calibration statistics between the original (Payne and others, 2005) and revised regional 

models, year 2000 steady-state simulation.

[Residual equals simulated minus observed head]

Aquifer Model 

layer

Number 

of 

observations

Minimum residual 

(feet)

Maximum residual 

(feet)

Mean of residuals 

(feet)

Root mean square 

of residuals (feet)

Original 

model1

Revised 

model

Original 

model1

Revised 

model

Original 

model1

Revised 

model

Original 

model1

Revised 

model

Brunswick 

aquifer 

system

3 10 –7.67 –19.1 13.3 14.0 1.79 –1.87 5.91 11

Upper 

Floridan 

aquifer

5 155 – 44.4 –30.2 36.4 33 –0.84 –1.78 9.94 10.2

Lower 

Floridan 

aquifer

7 11 –3.62 –5.7 21.5 29 5.2 3.84 9.15 9.91

1 Original model by Payne and others, 2005.

Table A–2. Comparison of simulated water budget by model layer between the original (Payne and others, 2005) and revised 

regional models, 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 

model1

Revised 

model

Original 

model1

Revised 

model

Original 

model1

Revised 

model

Original 

model1

Revised 

model

Original 

model1

Revised 

model

Surficial 

aquifer 

system

1 <0.001 <0.001 310 274 132 102 <0.001 <0.001 <0.001 <0.001

Confining 

unit

2 <0.001 <0.001 46.6 46.1 3.62 4.70 <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

5 669 669 141 145 22.3 22.1 712 715 268 266

Confining 

unit

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

Lower 

Floridan 

aquifer

7 129 129 <0.001 <0.001 <0.001 <0.001 15.5 16.6 2.32 1.97

Total all layers 798 798 498 465 158 129 728 732 270 268
Comparison of Revised to Original Regional Model 55

SOUTH

GEORGIA CAROLINA

FLORIDA

Model

area

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).

Base from U.S. Geological Survey

1:100,000-scale digital data

N

EXPLANATION

< –10

–9 to –5

–4 to 0

1 to 5

6 to 10

> 10

Water-level residual, in feet—

Represents difference of 

simulated minus observed 

water level

SC

GA

GA

FL

0 50 MILES 25

0 50 KILOMETERS 25

C. Lower Floridan aquifer

 (layer 7)

Base from U.S. Geological Survey

1:100,000-scale digital data

Approximate

 limit of Brunswick 

 aquifer system

A. Brunswick aquifer system

 (layer 3)

0 25 50 MILES

0 25 50 KILOMETERS

B. Upper Floridan 

 aquifer

 (layer 5)

Figure A–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).
56 Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia

References Cited

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 March 23, 2010, at http://pubs.er.usgs.gov/

usgspubs/ofr/ofr2000-92.

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 March 23, 

2010, 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.

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.

Figure 1–5. Graphs showing observed and simulated 

water levels in model layers, 3, 5, and 7, revised 

groundwater model.

–30

–30

–10

–10

–20

–20

10

10

20

20

30

30

Brunswick aquifer system (layer 3)

–100

–100

–50

–50

–50

0

0

0

50

50

50

100

100

150

150

–50 0 50 100 150

200

250

100 150 200 250

Upper Floridan aquifer (layer 5)

Lower Floridan aquifer (layer 7)

0

0

R2 = 0.5289

R2 = 0.9602

R2 = 0.984

OBSERVED WATER LEVEL ABOVE OR BELOW (–) NAVD 88, IN FEET

SIMULATED WATER LEVEL ABOVE

 OR BELOW (–) NAVD 88, IN FEET

Figure A–5. Observed and simulated water levels in 

model layers 3, 5, and 7 revised groundwater model.
Manuscript approved on April 13, 2010

For more information about this publication, contact:

USGS Georgia Water Science Center

3039 Amwiler Road

Atlanta, GA 30360

telephone: 770-903-9100

http://ga.water.usgs.gov/

Edited by Kay E. Hedrick

Illustrations by Bonnie J. Turcott

Layout by Caryl J. Wipperfurth
Clarke, Williams, and Cherry—Hydrogeology and Water Quality of the Floridan Aquifer System, Hunter Army Airfield, Chatham County, Georgia—Scientific Investigations Report 2010–5080