U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2011–1020

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

Summary of Hydrologic Testing of the Floridan 

Aquifer System at Fort Stewart, Georgia
Cover. Well drillers observing hydrologic monitoring equipment during the 72-hour aquifer test at 

Lower Floridan aquifer well 33P028, Fort Stewart, Liberty County, Georgia. Discharge from the aquifer 

test is being transported away from the well site. Fires in the background are control burning for 

brush control. Photo by Michael D. Hamrick, U.S. Geological Survey, 2010. 
Summary of Hydrologic Testing of the Floridan 

Aquifer System at Fort Stewart, Georgia

By Gerard J. Gonthier

Open-File Report 2011–1020

U.S. Department of the Interior

U.S. Geological Survey

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Gonthier, G.J., 2011, Summary of hydrologic testing of the Floridan aquifer system at Fort Stewart, Georgia: U.S. 

Geological Survey Open-File Report 2011–1020, 28 p.
iii

Contents

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

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

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

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

Methods of Study..................................................................................................................................5

Drilling, Core Collection, and Well Construction ....................................................................5

Geophysical Logs, Flowmeter Surveys, and Hydraulic Properties from Wells .................5

Hydrologic Testing .........................................................................................................................................8

Slug Tests ...............................................................................................................................................8

Test Results at the 726.5- to 733.5-Foot Interval....................................................................11

Test Results at the 766.5- to 773.5-Foot Interval....................................................................11

Test Results at the 816.5- to 823.5-Foot Interval....................................................................11

Test Results at the 876.5- to 883.5-Foot Interval....................................................................11

Aquifer Tests........................................................................................................................................15

Drawdown Estimation...............................................................................................................15

24-Hour Aquifer Test..................................................................................................................20

Drawdown Response.......................................................................................................21

Aquifer Test Analysis........................................................................................................22

72-Hour Aquifer Test..................................................................................................................22

Drawdown Response.......................................................................................................22

Model Simulation of the Lower Floridan Aquifer Test ................................................24

Summary........................................................................................................................................................27

References Cited..........................................................................................................................................28
iv

Figures

1. Map showing location of study, selected wells, and weather station

at Fort Stewart and vicinity, GA..................................................................................................2

2. Generalized correlation of geologic and hydrogeologic units in the 

Coastal Plain of Georgia ..............................................................................................................3

3. Well-construction diagrams for well 33P028 and well 33P029 at 

Fort Stewart, GA, 2010..................................................................................................................4

4–11. Graphs showing—

4. Selected borehole geophysical data and percent flow contribution from 

permeable zones in borehole 33P028, 460–1,300 feet at Fort Stewart, GA, 2010 ......6

5. Flowmeter survey data from pumping the Upper and Lower Floridan aquifers 

prior to installing a 14-inch casing in borehole 33P028, and pumping the 

Lower Floridan aquifer after installing a 14-inch casing in borehole 33P028 

and completing it as well 33P028 at Fort Stewart, GA...................................................7

6. Measured and simulated water levels and simulated water-injection 

volumes for packer-isolated interval 726.5- to 733.5-feet in borehole 33P028 

at Fort Stewart, GA, December 5, 2009, for Test 1, 5-gallon slug test, and 

Test 2, 10-gallon slug test .................................................................................................12

7. Slightly underdamped water-level responses to a 10-gallon slug of water 

injected into packer-isolated interval 766.5- to 773.5-feet in borehole 33P028 

at Fort Stewart, GA, December 6, 2009 ..........................................................................12

8. Measured and simulated water levels for test 1 in the packer-isolated 

interval 766.5- to 773.5-feet in borehole 33P028 at Fort Stewart, GA, 

December 6, 2009. Log of absolute value of displacement as a function 

of time, used to determine damping coefficient γ, and water displacement 

as a function of time, used to determine oscillation frequency ω.............................13

9. Measured and simulated water levels for test 2 in the packer-isolated 

interval 766.5- to 773.5-feet in borehole 33P028 at Fort Stewart, GA, 

December 6, 2009. Log of absolute value of displacement as a function 

of time, used to determine damping coefficient γ, and water displacement 

as a function of time, used to determine oscillation frequency ω.............................13

10. Measured and simulated water levels and simulated injection of water 

for packer-isolated interval 816.5- to 823.5-feet in borehole 33P028 at 

Fort Stewart, GA, December 6, 2009, Test 1 and Test 2 ...............................................14

11. Measured and simulated water levels and simulated injection of water 

for packer-isolated interval 876.5- to 883.5-feet in borehole 33P028 at 

Fort Stewart, GA, December 6, 2009, for Test 1 and Test 2 .........................................14
v

12. Diagrams showing location and open interval of wells used for the 72-hour 

aquifer test at Fort Stewart, GA. Diagram showing aquifer-test layout, and 

schematic cross section showing the open intervals of the wells in relation 

to major hydrogeologic units ....................................................................................................16

13–22. Graphs showing—

13. Raw, unfiltered water levels in wells 33P023, 33P029, and 33P025 before, 

during, and after the 24-hour Upper Floridan and 72-hour Lower Floridan 

aquifer tests at Fort Stewart, GA, March 1–15, 2010 ...................................................16

14. Water-level fluctuations in monitored wells and background wells and 

fluctuations in barometric pressure and gravity, used in the aquifer tests 

at Fort Stewart, GA, January 27–March 16, 2010, for fitting period for 

well 33P028, Upper Floridan 24-hour aquifer test, and Lower Floridan 

72-hour aquifer test...........................................................................................................17

15. Measured and synthetic water levels for well 33P028 during fitting period, 

January 31–March 1, 2010, at Fort Stewart, GA...........................................................18

16. Fit of synthetic water levels to measured water levels for wells 33P028, 

33P029, and 33P025, Fort Stewart, GA, February 25–March 2, 2010..........................19

17. Water levels at the start and end of the 24-hour aquifer test in well 33P029 

at Fort Stewart, GA, March 3–4, 2010.............................................................................20

18. Semi-log graph of drawdown during the 24-hour aquifer test in 

well 33P029 at a pumping rate of 387 gallons per minute at Fort Stewart, GA, 

March 3–4, 2010 .................................................................................................................20

19. Measured and synthetic water levels and estimated drawdown in 

wells 33P025 and 33P028 during the 24-hour aquifer test conducted 

in the Upper Floridan aquifer well 33P029 at Fort Stewart, GA, 

February 25–March 7, 2010 ..............................................................................................21

20. Effects on water level in well 33P028 at the end of the 72-hour aquifer test 

when a slug of water siphoned back into the well immediately after pumping 

stopped at Fort Stewart, GA, March 11, 2010................................................................22

21. Measured and synthetic water levels and estimated drawdown in 

wells 33P025 and 33P029 during the 72-hour aquifer test conducted in 

Lower Floridan aquifer well 33P028 at Fort Stewart, GA, March 5–15, 2010 ...........23

22. Semi-log plot of drawdown in pumped well 33P028 during the 72-hour 

aquifer test at Fort Stewart, GA, March 8–11, 2010 .....................................................23

23. Diagram showing axisymmetric radial model for 72-hour aquifer test at pumped 

well 33P028 at Fort Stewart, GA, March 8–11, 2010..............................................................25

 24. Graph showing simulated and measured water-level drawdown and recovery 

at pumped well 33P028 and observation wells 33P029 and 33P025 for the

72-hour aquifer test at Fort Stewart, GA, March 8–11, 2010................................................26
vi

Tables

1. Location and open intervals of wells used in the slug-test and aquifer-test 

analyses at Fort Stewart, GA, 2009–2010..................................................................................4

2. Estimated vertical hydraulic conductivity and porosity of core samples collected 

from the Lower Floridan confining unit in borehole 33P028 at Fort Stewart, GA...............8

3. Slug times for packer-isolated intervals in borehole 33P028 at Fort Stewart, GA, 

December 5–6, 2009 .....................................................................................................................9

4. Total injection volume and test-specific values for parameters used with 

the Bouwer and Rice (1976) method to estimate hydraulic conductivity..........................10

5. Hydraulic conductivity values for packer-isolated intervals in borehole 33P028 

in the Lower Floridan confining unit at Fort Stewart, GA, December 5–6, 2009...............10

6. Summary of aquifer test results, Fort Stewart, GA, March 2010 ........................................15

7. Hydraulic parameters that were used or estimated to fit simulated water levels 

to measured drawdown and recovery and estimated values of transmissivity 

and storativity for the 72-hour aquifer test at pumped well 33P028 and 

observation wells 33P029 and 33P025, Fort Stewart, GA, March 8–11, 2010....................26

Acknowledgments

Many people participated in the successful completion of this study. Special thanks go 

to Troy Funk and David Deloach of the U.S. Army Corps of Engineers, and Stanley Thomas, 

Eric Stulpin, Brent Rabon, and Tressa Rutland of the U.S. Army Environmental Protection and 

Compliance Branch. Troy Leeson of Layne–Atlantic Drilling provided valuable assistance for 

onsite data collection. 

O. Gary Holloway, U.S. Geological Survey (USGS), designed and constructed the 

aquifer-test plumbing to remove discharge water from the site and determine discharge rate. 

Gary Holloway and Michael D. Hamrick, USGS, monitored drill-site activities and collected 

hydrogeologic and water-quality data. John S. Clarke, USGS, helped perform the slug tests and 

provided figures for this report. Keith J. Halford, USGS, completed the model simulation of the 

Lower Floridan aquifer test, and Lester J. Williams, USGS, analyzed flowmeter survey data. 
vii

Conversion Factors

Multiply By To obtain

Length

inch 2.54 centimeter (cm)

inch 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

yard (yd) 0.9144 meter (m)

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3

) 

gallon (gal) 3.785 cubic decimeter (dm3

) 

Flow rate

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)

Hydraulic conductivity

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

Transmissivity*

foot squared per day (ft2

/d) 0.09290 meter squared per day (m2/d) 

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

°F =(1.8×°C)+ 32

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

°C= (°F–32)/ 1.8

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

(NAVD 88).

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

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

*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).

Summary of Hydrologic Testing of the Floridan 

Aquifer System at Fort Stewart, Georgia

By Gerard J. Gonthier

Abstract

Two test wells were completed at Fort Stewart, GA, in

January and February 2010 to investigate the potential of using 

the Lower Floridan aquifer as a source of water to satisfy 

anticipated increases in water use. One well was completed in 

the Lower Floridan aquifer at a depth of 1,255 feet below land 

surface; the other well was completed in the Upper Floridan 

aquifer at a depth of 560 feet below land surface. The U.S. 

Geological Survey conducted hydrologic testing at the well 

site including flowmeter surveys, slug tests within packerisolated intervals of the Lower Floridan confining unit, and 

aquifer tests of the Upper and Lower Floridan aquifers.

Flowmeter surveys at the study site indicate several 

permeable zones within the Floridan aquifer system. The 

Upper Floridan aquifer is composed of two water-bearing 

zones—the upper zone and the lower zone. The upper zone 

extends from 520 to 650 feet below land surface, contributes 

96 percent of the total flow, and is more permeable than the 

lower zone, which extends from 650 to 705 feet below land 

surface and contributes the remaining 4 percent of the flow. 

The Lower Floridan aquifer consists of three zones at depths 

of 912–947, 1,090–1,139, and 1,211–1,250 feet below land 

surface that are inter-layered with three less-permeable zones. 

The Lower Floridan confining unit includes a permeable 

zone that extends from 793 to 822 feet below land surface. 

Horizontal hydraulic conductivity values of the Lower 

Floridan confining unit derived from slug tests within four 

packer-isolated intervals were from 2 to 20 feet per day, 

with a high value of 70 feet per day obtained for one of the 

intervals. Aquifer testing, using analytical techniques and 

model simulation, indicated the Upper Floridan aquifer had 

a transmissivity of about 100,000 feet squared per day, and 

the Lower Floridan aquifer had a transmissivity of 7,000 feet 

squared per day. Flowmeter surveys, slug tests within 

packer-isolated intervals, and parameter-estimation results 

indicate that the hydraulic properties of the Lower Floridan 

confining unit are similar to those of the Lower Floridan 

aquifer. Water-level data, for each aquifer test, were filtered 

for external influences such as barometric pressure, earth-tide 

effects, and long-term trends to enable detection of small 

water-level responses to aquifer-test pumping of less than 

1 foot. During a 72-hour aquifer test of the Lower Floridan 

aquifer, a drawdown response of 0.3 to 0.4 foot was observed 

in two Upper Floridan aquifer wells, one of which was more 

than 1 mile away from the pumped well. 

Introduction

The U.S. Geological Survey (USGS) is assessing the 

potable water-supply potential of the Floridan aquifer system 

at Fort Stewart, GA (fig. 1), in cooperation with the U.S. 

Department of the Army. As development at Fort Stewart 

increases, water use is anticipated to increase. To meet 

growing demand, the U.S. Army is installing additional wells 

at Fort Stewart, including one production well completed in 

the Lower Floridan aquifer (well 33P028). 

Regional drawdown of the Upper Floridan aquifer has 

caused water-supply concerns in coastal Georgia. As a result 

of these concerns, the Georgia Environmental Protection 

Division has instituted pumping restrictions in order to prevent 

further drawdown in the Upper Floridan aquifer that might 

accelerate saltwater intrusion in the area of Hilton Head 

Island, SC (Georgia Environmental Protection Division, 

2006). These restrictions include limiting pumping from 

the Lower Floridan aquifer that may cause drawdown in the 

Upper Floridan aquifer. Hydrogeologic units in the study area 

(Payne and others, 2005) are shown in figure 2.
2 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

Fort Stewart

FLORIDA

GEORGIA

Map area

at right

SOUTH

CAROLINA

PLAIN

COASTAL

LIBERTY

BRYAN

LONG

EVANS CHATHAM

TATTNALL

Location of

map below

Map detail

 Hilton

 Head

Island

FORT STEWART 516

16

95

95

l l l l l

l l

l

l l

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

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

l l l l l l l l l

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

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

l l l l

l l l l l l

l l l l l l l

l

l

l l

l

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

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

l l

l l l l l l l l l

l l l l

l

l

l l l l l l l l

l l l l

l l

40

30

20

10

0

0

–50

0

EXPLANATION

Hunter Army Airfield

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

Potentiometric contour May 1998—

Hachures indicate depression. Contour 

interval, 10 feet. Datum is NAVD 88

24-county coastal Georgia area 

as identified by the Georgia 

Environmental Protection Division

0

0 2 4 MILES

0 2 4 KILOMETERS

Fort Stewart

33M004 Well and identifier

Image from U.S. Geological Survey High Resolution Orthoimagery 

 for Coastal Georgia, 0.5-meter pixel resolution, 2007

National Weather Service precipitation 

 station and identifier

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

CHATHAM

TATTNALL

EVANS

JASPER

LONG

GA

SC

BEAUFORT

WAYNE

MCINTOSH

Savannah

Hinesville

Ludowici

Richmond 

Hill

33M004

34N089

34N089

81°45' 81°30' 81°15' 81°00' 80°45'

32°00'

32°15'

31°45'

0

1

1 2 MILES

0 2 KILOMETERS

33P025

33P029

33P028

KLHW

KLHW

Figure 1. Location of study area, selected wells, and weather 

station at Fort Stewart and vicinity, GA (potentiometric contours 

modified from Peck and others, 1999).
Introduction 3

Purpose and Scope

Site-specific hydrologic testing was conducted in late 

2009 and early 2010 to determine the hydraulic characteristics 

of the Upper Floridan aquifer, Lower Floridan confining unit, 

and Lower Floridan aquifer in the vicinity of the production 

well proposed at Fort Stewart, GA. This report provides a 

brief description of geophysical logging and flowmeter 

surveys and summarizes:

• Slug tests conducted within packer-isolated intervals 

of the Lower Floridan confining unit between the 

Upper and Lower Floridan aquifers before the 

Lower Floridan well was completed,

• A 24-hour aquifer test of the Upper Floridan aquifer, and

• A 72-hour aquifer test of the Lower Floridan aquifer.

Also summarized within this report are test analyses and 

results. Location and construction details for the wells used 

in the slug tests and aquifer tests at Fort Stewart are included 

in table 1.

Two new wells were drilled as part of this study at Fort 

Stewart, Georgia—one in the Lower Floridan aquifer and one 

in the Upper Floridan aquifer (fig. 3). Data collection included 

geophysical logging, flowmeter surveys, slug tests within 

packer-isolated intervals of the Lower Floridan confining 

unit, a 24-hour aquifer test in the Upper Floridan aquifer, and 

a 72-hour aquifer test in the Lower Floridan aquifer. Water 

levels in both newly drilled wells and an existing third well 

in the Upper Floridan were monitored during both aquifer 

tests. In addition to the three wells used to directly monitor the 

aquifer tests, background water levels from two distant wells 

in the Upper Floridan were used to filter non-pump water-level 

influences from drawdown, discussed later. The two distant 

wells (33M004 and 34N089) are part of the Georgia statewide 

continuous monitoring network and are distant enough to not 

be affected by aquifer-test pumping but still within the vicinity 

of the study site, with water-level fluctuations that are similar 

to those at the study site. 

Site Description

The aquifer-test site is located in the south-central part 

of Fort Stewart about 3.5 miles (mi) north of Hinesville in 

Liberty County, GA (fig. 1). Wells 33P028 and 33P029 are 

40 feet apart, with a land-surface altitude of approximately 

80 feet (ft) above sea level (NAVD88). Observation well 

33P025 is located about 9,600 ft to the east of the study site. 

Background well 33M004 is located 18 mi due south of 

pumped well 33P028; background well 34N089 is located 

about 13 mi east of pumped well 33P028.

The study site is underlain by sedimentary rocks and 

unconsolidated sediments as old as the Cretaceous age (Miller, 

1986; fig. 2). Clarke and others (1990) describe geologic units 

in the area of the study site from Paleocene to Holocene.

The region around the study site is underlain by shallow 

aquifers and the Floridan aquifer system (Miller, 1986). Clarke 

and others (1990) describe the groundwater resources in the 

area of the study site. The shallowest aquifers include the 

surficial aquifer, upper Brunswick aquifer, and lower Brunswick aquifer, which consist of Miocene and younger sand, 

clay and carbonate deposits. The base of the lower Brunswick 

aquifer is composed of a relatively tight, impermeable carbonate layer. The Floridan aquifer system is divided into the 

Upper and Lower Floridan aquifers in the study area. Miller 

(1986) noted that components of the Floridan aquifer system 

are not restricted to stratigraphic boundaries but are almost 

completely composed of carbonate deposits. Most flow in the 

Floridan aquifer system occurs in discrete permeable zones 

that are separated by layers of denser carbonate deposits. 

The Upper Floridan aquifer consists of Oligocene limestone and late Eocene limestone and dolostone, and the uppermost part of the aquifer generally is the most permeable. 

Figure 2. Generalized correlation of geologic and hydrogeologic 

units in the Coastal Plain of Georgia. 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

Lower Floridan

confining unit

Lazaretto Creek Formation Upper Floridan confining unit

Lower Coastal Plain

Geologic unit1 Hydrogeologic unit2

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

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

Confining

unit

Upper waterbearing zone

Upper Floridan semiconfining unit

Lower waterbearing zone

Lower Middle Upper

Series

Savannah Brunswick

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
4 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

However, the Upper Floridan aquifer as a whole consists 

of several permeable water-bearing zones separated by 

layers of dense limestone or dolostone. The Upper Floridan 

aquifer thickness ranges from 200 to 700 ft in the study area 

(Miller, 1986). 

The Upper and Lower Floridan aquifers generally are 

separated by a confining unit that consists of middle Eocene 

dense, dolomitic limestone. Thickness of the confining unit 

that separates the Upper and Lower Floridan aquifers is 

mapped to be about 200 ft in the study area (Miller, 1986). 

The Lower Floridan aquifer consists of middle and early 

Eocene dolomitic limestone but can include Paleocene and 

Upper Cretaceous carbonate units. As with the Upper Floridan 

aquifer, the Lower Floridan aquifer consists of several permeable water-bearing zones that are separated by layers of dense 

carbonate deposits.

Figure 3. Well-construction diagrams for (A) well 33P028 and (B) 33P029 at Fort Stewart, GA, 2010. 

B. Well 33P029 (Upper Floridan aquifer)

14-inch outside diameter

surface casing

Depth to top of

Lower Floridan

confining unit

705 feet

Depth to bottom of

8-inch outer casing

460 feet

Depth to bottom of

14-inch surface casing

Neat cement grout

Limestone

Completed well

depth is 560 feet

NOT TO SCALE

7.25-inch diameter 

8-inch outside diameter

schedule 40 PVC

outer casing 

Depth to top of

Upper Floridan

aquifer 460 feet

26-inch outside diameter

Depth to top of

Lower Floridan

confining unit

Depth to bottom of

14-inch inner casing

895 feet (top of Lower

Floridan aquifer)

Depth to bottom of

20-inch inner casing

460 feet

Depth to bottom of

26-inch surface casing

150 feet

Neat cement grout

Depth of completed 

well 1,255 feet

Backfill material

12.25-inch diameter 

open hole

14-inch outside diameter 

inner casing

20-inch outside diameter 

schedule 40 black steel 

schedule 40 black steel 

outer casing

Depth of borehole 1,300 feet

surface casing

705 feet

Limestone

Land surface 81.76 feet above NAVD 1988 Land surface 81.36 feet above NAVD 1988

A. Well 33P028 (Lower Floridan aquifer)

NOT TO SCALE

155 feet

open hole

Figure 3. Well-construction diagrams for (A) well 

33P028 and (B) well 33P029 at Fort Stewart, GA, 2010 

(John S. Clarke, U.S. Geological Survey, written 

commun., January 2011).

Table 1. Location and open intervals of wells used in the slug-test and aquifer-test analyses at Fort Stewart, GA, 2009–2010. 

[See fig. 1 for site locations; USGS, U.S. Geological Survey; latitude and longitude are in degrees north and east, respectively; UFA, Upper Floridan aquifer; 

FAS, Floridan aquifer system; LFA, Lower Floridan aquifer; well 33P028 was a bore hole for testing prior to being completed in the LFA; NA, not applicable]

USGS 

identifier

USGS 

site 

number

Land 

elevation 

above 

NAVD 1988

Latitude Longitude Depth, in feet below land surface

Aquifer (Decimal degrees)

Static 

water level 

March 2, 2010

Top of 

open 

interval

Bottom 

of open 

interval

33M004 313845081361701 60.3 31.64833 –81.60111 54.34 538 870 UFA

33P025 315447081345601 89 31.91315 –81.58227 86.52 420 520 UFA

33P028 315434081364701 81.76 31.90953 –81.61294 NA 460 1,300 FAS

33P028 315434081364701 81.76 31.90953 –81.61294 78.25 895 1,255 LFA

33P029 315434081364702 81.36 31.90946 –81.61293 77.49 460 560 UFA

34N089 315214081235301 16.01 31.87056 –81.39806 23.85 410 789 UFA
Introduction 5

Methods of Study

Hydraulic characteristics of the Floridan aquifer system 

at Fort Stewart, GA, were determined by installing two wells 

and performing geophysical logging, flowmeter surveys, slug 

tests, aquifer tests, and analyses. Method details concerning 

drilling and well construction, geophysical logging, flowmeter 

surveys, and hydrologic testing are described below.

Drilling, Core Collection, and Well Construction

An initial borehole was drilled to 1,300 ft during midNovember 2009 and cased to near the top of the Floridan 

aquifer system at a depth of 460 ft below land surface (fig. 3). 

Drill cores were collected at four discrete depths within the 

Lower Floridan confining unit and tested for vertical hydraulic 

conductivity and porosity using standard laboratory methods. 

Geophysical logs and flowmeter surveys were conducted 

during late November 2009 in the carbonate deposits of the 

borehole from 460 to 1,300 ft. Slug tests were completed at 

four packer-isolated intervals in the Lower Floridan confining 

unit in early December 2009. Borehole 33P028 was completed 

to a depth of 1,255 ft in mid-January 2010 as well 33P028 

in the Lower Floridan aquifer. A 14-inch diameter carbonsteel casing was installed to a depth of 895 ft, and the well 

was completed as a 12.25-inch open hole in limestone. The 

borehole was backfilled in the 1,255 to 1,300-ft interval with a 

bentonite-grout mixture. Water-level monitoring began in well 

33P028 on January 27, 2010.

Well 33P029 in the Upper Floridan aquifer was constructed

40 ft southwest of well 33P028 in mid-February 2010 (fig. 3). 

The casing was 8-inch diameter schedule 40 polyvinyl 

chloride (PVC) installed to a depth of 460 ft, and the well was 

completed as a 7.25-inch open hole in limestone. Water-level 

monitoring began in well 33P029 in the Upper Floridan aquifer on February 24, 2010, and in a previously installed well 

(33P025) on February 25, 2010. Background wells 33M004 

and 34N089 have been monitored for water levels for years 

prior to the study.

Geophysical Logs, Flowmeter Surveys, and 

Hydraulic Properties from Wells

Borehole geophysical logs were collected at various 

stages of drilling to characterize the physical properties of 

the sediments penetrated (fig. 4). Borehole geophysical logs 

include caliper, natural gamma, spontaneous potential, lateral 

long-normal (64-inch) and short-normal (16-inch) resistivity, 

borehole fluid resistivity, and temperature. Not shown in this 

report but also collected were single-point resistivity, fullwaveform sonic acoustic televiewer, and optical televiewer logs. 

Flowmeter surveys were used to determine the relative 

contribution of flow from water-bearing zones and confinement between the Upper and Lower Floridan aquifers (fig. 5). 

Surveys were 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. This information ensured 

accurate placement of the well casing in the completed well. 

The EM flowmeter surveys were conducted in two stages. 

The first was conducted during November 19–20, 2009, upon 

completion of drilling to a total depth of 1,300 ft, and prior 

to installing the 14-inch casing, to test flow in the 460- to 

1,300-ft interval (Upper and Lower Floridan aquifers, fig. 5A). 

The second survey was conducted on January 26, 2010, after 

installing the 14-inch casing, to test flow in the 895- to 1,300-

ft interval (Lower Floridan aquifer only, fig. 5B). The first 

stage included assessment of borehole flow at a pumping rate 

of 770 gallons per minute (gal/min); during the second stage, 

borehole flow was measured at a pumping rate of 740 gal/min. 

Horizontal hydraulic conductivity estimates of the Lower 

Floridan confining unit were obtained by performing slug 

tests at four depth intervals (726.5–733.5 ft, 766.5–773.5 ft, 

816.5–823.5 ft, and 876.5–883.5 ft). Special simulations of 

slug-test water levels were made to account for the effects 

of prolonged slug events. Concepts from Bouwer and Rice 

(1976) were used to determine hydraulic conductivity of 

three of the intervals, and the van der Kamp (1976) method 

was used in the other because of higher permeability and 

an oscillatory water-level response. More detail on testing 

and analytical procedures for the slug tests is provided in a 

subsequent section of this report.

Aquifer tests were conducted to estimate the transmissivity of the Upper and Lower Floridan aquifers and to determine 

the effects of pumping one aquifer on water levels in the 

other aquifer. For each test, water levels were monitored in 

the pumping well and in two observation wells. Each of three 

wells was instrumented with a vented transducer. Transducer 

measurements are recorded as feet of water above the probe 

and were transformed to show depth to water below land 

surface. Drawdown for the three monitored wells was estimated by using a procedure developed by Halford (2006a) to 

filter water-level data for effects of barometric pressure, earth 

tide, and long-term trends. 

A 24-hour aquifer test was performed in the Upper 

Floridan aquifer well 33P029 during March 3–4, 2010, with 

the well pumping at an average rate of 387 gal/min. The 

Cooper-Jacob, straight-line method (Cooper and Jacob, 1946) 

was applied to the drawdown data from this well to determine 

the transmissivity of the Upper Floridan aquifer.

A 72-hour aquifer test was performed in the Lower 

Floridan aquifer well 33P028 during March 8–11, 2010, 

when the well was pumped at an average rate of 740 gal/min. 

Transmissivity of the Upper and Lower Floridan aquifers 

was estimated by using the numerical model MODFLOW-96 

(McDonald and Harbaugh, 1988; Harbaugh and McDonald, 

1996) with the calibration tool MODOPTIM (Halford, 2006b). 

A more complete description of the 24- and 72-hour aquifer 

tests and analytical procedures is provided in subsequent 

sections of the report.
6 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

Figure 4. Selected borehole geophysical data and percent flow contribution from permeable zones 

in borehole 33P028, 460–1,300 feet at Fort Stewart, GA, 2010. Percent flow contribution was 

determined from flowmeter survey in borehole 33P028 (see figure 5A) [μS/CM, microsiemens per 

centimeter at 25 degrees centigrade].

Spontaneous potential,

in millivolts

Specific conductance,

in μS/cm

GeologyHydrogeology

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

0 500

0 500

Lateral resistivity,

in ohm-meters

Resistivity 16-inch normal,

in ohm-meters

Resistivity 64-inch normal,

in ohm-meters

Percentcontributionof flow

Upper Floridan aquifer Lower Floridan aquifer

Surficial 

aquifer Lower Floridan confining unit Upper Floridan confining unit

100

200

300

400

500

600

700

800

900

1,000

1,100

Lateral

Packer test 

interval

Core

sample

Waterquality

sample

Miocene

Oligocene Upper Eocene Middle Eocene

Post

Miocene

surface

64-inch 

normal

16-inch 

normal

1,200

1,300

79.3

3.9

3.1

1.8

1.0

1.8

9.1

Figure 4. Selected borehole geophysical data and percent flow contribution from permeable zones in 

borehole 33P028, 460–1,300 feet at Fort Stewart, GA, 2010. Percent flow contribution was determined from 

flowmeter survey in borehole 33P028 (see fig. 5A; John S. Clarke, U.S. Geological Survey, written commun., 

January 2011). [μS/cm, microsiemens per centimeter at 25 degrees Celsius]
Introduction 7

Figure 5. Flowmeter survey data from (A) pumping the Upper and Lower Floridan aquifers prior 

to installing a 14-inch casing in borehole 33P028, and (B) pumping the Lower Floridan aquifer after 

installing a 14-inch casing in borehole 33P028 and completing it as well 33P028 at Fort Stewart, GA 

(John S. Clarke, U.S. Geological Survey, written commun., January 2011).

Figure 5. Flowmeter survey data from (A) pumping the Upper and Lower Floridan aquifers prior to 

installing a 14-inch casing in borehole 33P028; and (B) pumping the Lower Floridan aquifer after 

installing a 14-inch casing in borehole 33P028 and completing it as well 33P028 at Fort Stewart, GA.

100

200

300

400

500

600

700

800

900

1,000

1,100

1,200

1,300

A

Upper and Lower Floridan aquifers 

460–1,300 feet 

0 300 600 900 2001000 0 300 600 900

Caliper,

in inches

5 20

GeologyHydrogeology

Feet

Borehole flow 

(gallons per minute)

B

Lower Floridan aquifer

 895–1,300 feet 

Borehole flow 

(gallons per minute)

Land

surface

Stationary

point 

measurements

Stationary

point 

measurements

Post Surficial 

Miocene aquifer

Bottom of

20-inch

casing is at 

460 feet

Bottom of 

14-inch 

casing is at 

895 feet

79.3

3.9

3.1

1.8

1.0

1.8 32

18

50

9.1

Percentcontributionof flowPercentcontributionof flow

Upper Floridan aquifer Lower Floridan aquifer Lower Floridan confining unit Upper Floridan confining unit

Miocene

Oligocene Upper Eocene Middle Eocene

Same flowmeter 

log plotted at 

different scales
8 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

Hydrologic Testing

Geophysical logs and flowmeter surveys at borehole 33P028 

delineate several water-bearing zones bounded by less-permeable 

zones throughout the Floridan aquifer system (John S. Clarke, 

USGS, written commun., September 13, 2010). The Upper 

Floridan aquifer occurs at depths between 440 and 705 ft and 

is composed of two water-bearing zones—the 520- to 650-ft 

upper zone, which is more permeable than the 650- to 705-ft 

lower zone (figs. 4, 5). Flowmeter surveys indicate that the 

upper zone contributes 96 percent of the flow in the Upper 

Floridan aquifer; the lower zone contributes the remaining 

4 percent (fig. 5).

At borehole 33P028 is a unit that is mapped by Williams 

and Gill (2010) as the Lower Floridan confining unit. Hydrologic properties of this unit, however, are similar to those of 

the Lower Floridan aquifer as described later. For discussion 

in this report, this mapped hydrogeologic unit will be referred 

to as the Lower Floridan confining unit.

The Lower Floridan confining unit extends from the 

base of the Upper Floridan to about a depth of 912 ft and is 

composed of three zones; the middle zone is much more permeable than the top and bottom zones. The middle permeable, 

relatively high-yielding zone of the Lower Floridan confining 

unit extends at depths from 793 to 822 ft (fig. 5). Drill cores 

were collected within the Lower Floridan confining unit and 

tested for vertical hydraulic conductivity and porosity. Laboratory tests of cores at depths from 703 to 855 ft indicate vertical 

hydraulic conductivities ranging from 0.26 to 0.79 ft per day 

(ft/d) and porosities ranging from 0.25 to 0.38 (table 2).

The Lower Floridan aquifer extends from the base 

of the Lower Floridan confining unit to at least a depth of 

1,300 ft and contains three permeable zones—912–947 ft, 

1,090–1,139 ft, and 1,211–1,250 ft—interlayered with three 

less-permeable zones. Abnormally high water temperatures in 

the bottom 200 ft of the borehole (fig. 4) indicate the possibility of upward flow from deeper units (John S. Clarke, U.S. 

Geological Survey, written commun., September 13, 2010).

Slug Tests

Horizontal hydraulic conductivity estimates of the Lower 

Floridan confining unit were obtained by conducting slug tests 

within four packer-isolated intervals in borehole 33P028 at 

depths of 726.5–733.5 ft, 766.5–773.5 ft, 816.5–823.5 ft, and 

876.5–883.5 ft during December 5–6, 2009 (fig. 4; table 3). 

Two slug tests were conducted for each interval. Each interval 

was isolated by first using straddle packers and then injecting 

a slug of water, usually 10 gallons in volume, into the 7-ft 

interval through a 3-inch diameter pipe. The change in head 

over time was recorded. Pressure transducers were used to 

monitor water-level response within the isolated interval as 

well as above and below the zone being tested. The general 

procedures for conducting these tests were as follows:

1. Lower packer assembly to the target depth.

2. Attach pressure transducers to dataloggers and start 

recording at 1-second intervals.

3. Inflate upper packer to full seating pressure.

4. Inflate lower packer to full seating pressure.

5. Allow water level in the test interval to stabilize 

(typically 10 minutes).

6. Inject water into the isolated interval through the drill 

stem to raise water level within the test interval.

7. Monitor water level before, during, and after injection 

until water levels return to pre-test conditions.

8. Review pressure data from above and below the 

isolated interval to check for indications of leakage 

across the straddle packer; re-inflate or reposition 

packers if necessary.

9. Deflate packers and move to the next depth interval.

The caliper log was used to position the packers in 

smooth sections of the borehole that were selected to be more 

likely to have the greatest potential for good packer seating. 

Vugs, solution cavities, and irregular borehole enlargements 

were identified so the rubber packers would not be inflated 

along these irregular surfaces. Data from upper and lower 

pressure transducers indicated that the packers adequately 

sealed the test intervals. 

A normal slug test involves the nearly instant insertion or 

withdrawal of a slug to or from the water column. The period 

of the slug event is very brief and the ensuing recovery is then 

related to the hydraulic conductivity of the surrounding aquifer 

material. Peak water level as discussed in this report is the 

highest water level attained during the insertion of slug water. 

In this study, complete placement of water into the 

water column for the slug test (hereafter referred to as the 

slug event) took at least 15 to 30 seconds. This conservative estimate of the duration of the slug event is based on a 

Table 2. Estimated vertical hydraulic conductivity and porosity 

of core samples collected from the Lower Floridan confining 

unit in borehole 33P028 at Fort Stewart, GA.

[Analyses by Geotechnics, Inc., East Pittsburgh, PA]

Core sample

interval 

(feet below 

land surface)

Vertical hydraulic 

conductivity 

(feet per day)

Porosity 

702.9–703.8 0.26 0.37

750.9–751.4 0.37 0.25

803.5–804.4 0.40 0.26

854.0–854.8 0.79 0.38

Average 0.45 0.32
Hydrologic Testing 9

gradual increase in water levels to a peak water level resulting 

from the slug event (table 3). These prolonged slug events 

complicated the determination of hydraulic conductivity of the 

surrounding aquifer material, because of ongoing water-level 

recovery. At some point during the slug test, water was still 

entering the water column in the well, but at a rate that was 

less than the recovery rate. As a result, part of the recession 

curve represents the recovery being slowed down by the 

continued insertion of water during the slug event. This part of 

the recession could not be evaluated for hydraulic conductivity 

using standard techniques. The actual time that this type of 

recession occurred initially was not known.

To account for the effect of the prolonged slug events, 

slug-test water levels of the three packer-isolated intervals 

through time were simulated. Another packer-isolated interval 

could not be simulated due to its response (discussed later). 

The simulation involved injecting water into the water column 

on a per-second basis. While water was being injected, a 

recovery constant was applied to the water column in the well 

to control the rate of recovery. By calibrating a simulation to 

the measured water levels of a slug test, two key pieces of 

information were obtained: (1) the recovery constant used 

for successful calibration that can be used to determine the 

hydraulic conductivity of the surrounding aquifer material, and 

(2) simulated water injection for successful calibration can be 

used to determine when the slug event was complete. Once 

the completion of the slug event is known, based on the results 

of the simulation, the recovery curve can be used in standard 

analysis to determine the hydraulic conductivity of the surrounding aquifer material. The value determined from standard 

analysis can be compared with that derived from the simulation. The following equation governs the slug-test simulation:

H H

H I H

e

t f

t t f

m t ( ) ( )

( ) ( ) ( )

' = + 

+ – ( − − 1 1 )

( ) ∆

, (1)

where 

H( t ) is the simulated water level at time t, in feet;

H(  f  ) is the final water level, which is the same 

as the stable or background water level 

because the duration of the slug tests are 

on the order of minutes, in feet;

H( t–1) is the simulated water level at time t–1, 

in feet;

I( t–1) is the simulated volume of slug water that 

connects to and is injected into the water 

column in the well casing at time t–1, 

in feet;

m' is the decay constant for the difference 

between H( t–1) and H(  f  ), in seconds–1; and

Δt is the amount of time between time t and 

time t–1 or 1 second.

Figures presented in this report set the H(  f  ) to zero.

Simulated water levels had a very close fit to measured 

water levels in the three packer-isolated intervals. Also, the 

total simulated injection volumes were closer to the reported 

slug volumes than estimated volumes based on peak water 

levels times the area of the casing cross section (table 4).

The decay constant m' used to match the simulated water 

levels to the measured water levels, physical dimensions of the 

packer-isolated interval, and concepts from Bouwer and Rice 

(1976) were then used to determine the hydraulic conductivity 

of the packer-isolated interval (table 5). From Bouwer and 

Rice (1976) and equation 1:

Table 3. Slug times for packer-isolated intervals in borehole 33P028 at Fort Stewart, GA, December 5–6, 2009. 

[Pk/90r, the ratio of time for the water level to rise to peak (Pk) water level to the 90-percent recovery time (90r) after peak water level; 

*, slug recovery oscillates, time in seconds for 90-percent damping of oscillation amplitude]

Packer-isolated 

interval

(feet below 

land surface)

Test

Start of slug test Time for 

water to rise

to peak

(seconds)

90-percent

recovery 

after peak

(seconds)

Pk/90r Date

(2009)

Time

 (EST)

726.5–733.5 1 Dec 05 17:03:31 30 53 0.57

2 Dec 05 17:33:06 18 44 0.41

766.5–773.5 1 Dec 06 9:37:23 15 32* 0.47

2 Dec 06 9:58:09 17 38* 0.45

816.5–823.5 1 Dec 06 11:32:27 16 41 0.39

2 Dec 06 11:43:41 20 39 0.51

876.5–883.5 1 Dec 06 13:16:10 19 358 0.05

2 Dec 06 13:36:56 23 379 0.06
10 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

K

m r R

r

L

c

e

w =

' , ln86 400

2

2

, (2)

where

K is the hydraulic conductivity, in feet per day;

rc is the radius of the well casing where the 

water-level change occurs, 0.0625 ft;

Re is the effective radial distance from the 

well over which the head difference is 

dissipated, in feet;

rw is the radial extent of the well to the 

undisturbed material (borehole radius), 

between 0.491 and 0.517 ft;

L is the length of the packer-isolated interval 

open to the material, 7 ft; and

m' is the decay constant, in seconds–1, used 

to fit simulated water levels to measured 

water levels.

Values of ln(Re  /rw ) are determined assuming full penetration 

of the aquifer (minimal vertical groundwater movement). 

The rw and m' values for each test are listed in table 4.

When the simulation indicated the slug event was 

completed, the water-level recovery known to be after the 

slug event was used in a spreadsheet from Halford and 

Kuniansky (2002) based on the Bouwer and Rice (1976) 

method to determine the hydraulic conductivity of the 

surrounding aquifer material (table 5). For intervals 

726.5–733.5 ft, 816.5–823.5 ft, and 876.5–883.5 ft, K values 

resulting from the simulation were essentially the same as 

those from the Halford and Kuniansky (2002) spreadsheet 

based on the Bouwer and Rice (1976) method.

Table 4. Total injection volume and test-specific values for parameters used with the Bouwer and Rice (1976) 

method to estimate hydraulic conductivity. 

[Inner diameter of casing is 3 inches]

Packer-isolated 

interval

(feet below 

land surface)

Test

Total injection volume (gallons) Test-specific values

Reported

Estimated

from

simulation

Estimated from 

peak water 

level

Peak

water

level

(feet)

Well-bore

radius

r

w

(feet)

Decay

constant

m'

(sec–1)

726.5–733.5 1 5 2.3 1.1 2.96 0.508 0.048

2 10 3.4 2.3 6.38 0.508 0.052

816.5–823.5 1 10 2.9 1.8 5.00 0.492 0.062

2 10 3.7 2.2 5.86 0.492 0.062

876.5–883.5 1 10 4.2 3.9 10.56 0.517 0.0068

2 10 4.1 3.8 10.35 0.517 0.0067

Table 5. Hydraulic conductivity values for packer-isolated intervals in borehole 33P028 in the Lower Floridan 

confining unit at Fort Stewart, GA, December 5–6, 2009. 

[ft/d, feet per day; HSBR, values determined using the Halford and Kuniansky (2002) spreadsheet based on Bouwer and Rice (1976); 

MSBR, values determined using the decay constant that matched simulated water levels to measured water levels, physical dimensions 

of the packer-isolated interval, and concepts from Bouwer and Rice (1976); HVDK, values determined using the Halford and Kuniansky 

(2002) spreadsheet based on van der Kamp (1976); —, not applicable]

Packer-isolated 

interval

(feet below 

land surface)

Test 1—Hydraulic conductivity (ft/d) Test 2—Hydraulic conductivity (ft/d) Final

values

HSBR MSBR HVDK HSBR MSBR HVDK (ft/d)

726.5–733.5 18.8 18.2 — 20.5 19.7 — 20

766.5–773.5 — — 65.0 — — 74.3 70

816.5–823.5 24.2 23.7 — 23.5 23.9 — 20

876.5–883.5 2.3 2.6 — 2.3 2.6 — 2
Hydrologic Testing 11

Test Results at the 726.5- to 733.5-Foot Interval

Water levels in two slug tests in the first packer-isolated 

interval in well 33P028 at 726.5–733.5 ft were evaluated by 

using the Bouwer and Rice (1976) method (fig. 6; tables 3–5). 

The initial water level was 78.10 ft below land surface. For 

the first test, a 5-gallon slug of water was introduced into the 

packer-isolated interval through the 3-inch diameter drill stem. 

Water injection took at least 30 seconds to enter the packerisolated interval based on the time water levels rose to a peak. 

The peak water level was 2.96 ft, and 90-percent recovery 

after peak was reached after 53 seconds. For the second test, a 

10-gallon slug of water was injected over a period of at least 

18 seconds. The peak water level was 6.38 ft, and 90-percent 

recovery after peak was reached after 44 seconds. 

Simulated water levels were a close fit to measured water 

levels for both tests (fig. 6). Simulation results indicate that 

the water injection for tests 1 and 2 took 39 and 21 seconds, 

respectively. Total simulated volume of water injected for 

tests 1 and 2 was 2.3 and 3.4 gallons, respectively. Results of 

both tests indicated a hydraulic conductivity of about 20 ft/d 

(table 5).

Test Results at the 766.5- to 773.5-Foot Interval

Slug tests for the 766.5- to 773.5-ft packer-isolated 

interval were characterized by an oscillatory response indicative of high permeability (fig. 7; table 3). Before tests 1 and 2, 

water levels were 77.88 and 78.08 ft below land surface, 

respectively. For each test, a 10-gallon slug of water was 

injected into the packer-isolated interval through the 3-inch 

diameter drill stem. Water injection for tests 1 and 2 took at 

least 15 and 17 seconds, respectively, to enter the packerisolated interval. The peak water levels for tests 1 and 2 were 

4.97 and 5.10 ft, respectively. The response for both tests 

was slightly underdamped (oscillating water levels, fig. 7). 

Tests 1 and 2 initially recovered from peak water levels to 

pre-test water levels in 15 and 17 seconds, respectively. 

Following this initial recovery, water levels for tests 1 and 

2 declined 0.20 and 0.17 ft beneath pre-test water levels, 

respectively. Water levels for tests 1 and 2 then swung back 

up to a second peak of about 0.08 and 0.19 ft, respectively, 

about 40 seconds after the first peak in water levels. Following 

these oscillations, water levels finally stabilized roughly 

80 seconds after the initial peak water level.

Because of the water-level oscillations that occurred at 

this interval, the van der Kamp (1976) method was used to 

determine hydraulic conductivity. An underdamped response 

typically has an oscillation of a constant frequency that 

decreases in amplitude over time with the rate of decrease 

being constant. A spreadsheet from Halford and Kuniansky 

(2002) based on van der Kamp (1976) was used to analyze 

the data for Transmissivity. A 5-point moving average was 

used in the water-level data to reduce noise. With an assumed 

storativity of 2.15E-05, (based on results of 72-hour aquifertest model), results of both slug tests within this isolated 

interval indicate a hydraulic conductivity of approximately 

70 ft/d (figs. 8, 9; table 5). The estimates are not very accurate 

because the water-level oscillation barely completed one full 

cycle before stabilizing, precluding the use of several peaks to 

accurately determine the damping coefficent. 

Test Results at the 816.5- to 823.5-Foot Interval

Water levels in two slug tests in the third packer-isolated 

interval in well 33P028 at 816.5–823.5 ft were evaluated by 

using the Bouwer and Rice (1976) method (fig. 10; tables 3–5). 

Initial water levels before tests 1 and 2 were 77.98 and 78.02 ft 

below land surface, respectively. To initiate each test, a 

10-gallon slug of water was injected into the packer-isolated 

interval through the 3-inch diameter drill stem. Water injection 

for tests 1 and 2 took at least 16 and 20 seconds, respectively, 

to enter the interval. The peak water levels for tests 1 and 2 

were 5.00 and 5.86 ft, respectively. After peak water levels 

were achieved, 90-percent recovery for tests 1 and 2 was 

reached at 41 and 39 seconds, respectively. 

Simulated water levels were a close fit to measured water 

levels (fig. 10). Model results indicate that the water injections 

for tests 1 and 2 took 24 and 26 seconds, respectively. Total 

volume of injected water for tests 1 and 2 was estimated at 

2.9 and 3.7 gal, respectively (table 4). Results of both tests 

indicated a hydraulic conductivity of about 20 ft/d (table 5).

Test Results at the 876.5- to 883.5-Foot Interval

Water levels in two slug tests in the fourth packer-isolated 

interval at 876.5–883.5 ft in well 33P028 were evaluated 

by using the Bouwer and Rice (1976) method (fig. 11; 

tables 3–5). Water levels before tests 1 and 2 were 77.03 and 

77.15 ft below land surface, respectively. To initiate each test, 

a 10-gallon slug of water was injected into the packer-isolated 

interval through the 3-inch diameter drill stem. Water injection 

for tests 1 and 2 took at least 19 and 23 seconds to enter the 

packer-isolated interval, respectively. The peak water levels 

for tests 1 and 2 were 10.56 and 10.35 ft, respectively. After 

peak water levels were achieved, 90-percent recovery for 

tests 1 and 2 was reached 5 minutes 58 seconds and 6 minutes 

19 seconds, respectively. 

Simulated water levels were a close fit to measured water 

levels (fig. 11). Model results indicate that the water injection 

for tests 1 and 2 took 25 and 23 seconds, respectively. Total 

volume of water injected for tests 1 and 2 was estimated at 

4.2 and 4.1 gal, respectively. Results of both tests indicated a 

hydraulic conductivity of about 2 ft/d, the lowest of the four 

intervals tested (table 5).
12 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

Simulated water injection, in gallons 

Water displacement, in feet

Time, in seconds

Time, in seconds

1,900 1,950 2,000 2,050 2,100 

–2 

0 

2 

4 

6 

8 

10 

B. Test 2, 726.5–733.5 feet

Initial water level = 78.10 feet below land surface 

Horizontal hydraulic conductivity = 19.7 feet per day 

Simulated injection = 3.42 gallons 

0 

0.5

0 

0.5 

–0.5 

0 

0.5 

1.0 

1.5 

2.0 

2.5 

3.0 

3.5 

4.0 

120 140 160 180 200 220 240 260 280 300 320 

A. Test 1, 726.5–733.5 feet

Simulated water injection Measured water level Simulated water level 

Initial water level = 78.10 feet below land surface 

Horizontal hydraulic conductivity = 18.2 feet per day 

Simulated injection = 2.26 gallons 

EXPLANATION

Time, in seconds

Time, in seconds

B. Test 2, 766.5–773.5 feet

A. Test 1, 766.5–773.5 feet

–1 

0 

1 

2 

3 

4 

5 

6 

1,650 1,700 1,750 1,800 1,850 

Raw data 

5-point moving average 

Initial water level = 78.08 feet below land surface 

–1 

0 

1 

2 

3 

4 

5 

6 

400 450 500 550 600 

Corrected for slope 

EXPLANATION

EXPLANATION

5-point moving average

 corrected for slope 

Initial water level = 77.88 feet below land surface 

Slope correction of –0.0006 feet per second 

 was added for clarity

Water displacement, in feet

Figure 6. Measured and simulated water levels and simulated water-injection volumes for packer-isolated 

interval 726.5- to 733.5-feet in borehole 33P028 at Fort Stewart, GA, December 5, 2009. Both tests roughly yield 

a hydraulic conductivity of 20 feet per day. (A) Test 1, 5-gallon slug test, (B) Test 2, 10-gallon slug test.

Figure 7. Slightly underdamped water-level responses to a 10-gallon slug of water injected into 

packer-isolated interval 766.5- to 773.5-feet in borehole 33P028 at Fort Stewart, GA, December 6, 2009.
Hydrologic Testing 13

0.01

0.1

1

10

Absolute displacement, in feet Water displacement, in feet

Measured

Simulated

Trend

–1

0

1

2

3

4

0 50 100

0 50 100

Time, in seconds

A. Estimating damping coefficient γ

B. Estimating frequency ω

ω = 0.15087563 second–1

γ = 0.11906377 second–1

EXPLANATION

Figure 8. Measured and simulated water levels for 

test 1 in the 766.5–773.5-foot packer-isolated 

interval in borehole 33P028 at Fort Stewart, GA, 

December 6, 2009. (A) Log of absolute value of 

displacement as a function of time, used to 

determine damping coefficient γ, (B) Water 

displacement as a function of time, used to 

determine oscillation frequency ω. (Simulation is 

based on van der Kamp, 1976, method. Graphs are 

modified from spreadsheet from Halford and 

Kuniansky, 2002).

Figure 9. Measured and simulated water levels for 

test 2 in the 766.5–773.5-foot packer-isolated 

interval in borehole 33P028 at Fort Stewart, GA, 

December 6, 2009. (A) Log of absolute value of 

displacement as a function of time, used to 

determine damping coefficient γ, (B) Water 

displacement as a function of time, used to 

determine oscillation frequency ω. (Simulation is 

based on van der Kamp method, 1976. Graphs are 

modified from spreadsheet from Halford and 

Kuniansky, 2002).

Absolute displacement, in feet

0.01

Water displacement, in feet

Time, in seconds

0.1

1

10

A. Estimating damping coefficient γ

γ = 0.095103 second–1

–1

0

1

2

3

4

B. Estimating frequency ω

ω = 0.154728 second–1

0 50 100

0 50 100

Measured

Simulated

Trend

EXPLANATION

Figure 8. Measured and simulated water levels for 

test 1 in the packer-isolated interval 766.5- to 773.5-feet in 

borehole 33P028 at Fort Stewart, GA, December 6, 2009. 

(A) Log of absolute value of displacement as a function of 

time, used to determine damping coefficient γ, (B) Water 

displacement as a function of time, used to determine 

oscillation frequency ω. Simulation is based on van der 

Kamp (1976) method. Graphs are modified from spreadsheet from Halford and Kuniansky (2002).

Figure 9. Measured and simulated water levels for 

test 2 in the packer-isolated interval 766.5- to 773.5-feet in 

borehole 33P028 at Fort Stewart, GA, December 6, 2009. 

(A) Log of absolute value of displacement as a function of 

time, used to determine damping coefficient γ, (B) Water 

displacement as a function of time, used to determine 

oscillation frequency ω. Simulation is based on van der 

Kamp (1976) method. Graphs are modified from spreadsheet from Halford and Kuniansky (2002).
14 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

Simulated water injection, in gallons 

Water displacement, in feet

Time, in seconds

–1

0

1

2

3

4

5

6

7

240 290 340 390 440

A. Test 1, 816.5–823.5 feet

Initial water level = 77.98 feet below land surface

Horizontal hydraulic conductivity = 23.7 feet per day

Simulated injection = 2.89 gallons

0

0.5

1.0

0

0.5

1.0

–1

0

1

2

3

4

5

6

7

910 960 1,010 1,060 1,110

B. Test 2, 816.5–823.5 feet

Initial water level = 78.02 feet below land surface

Horizontal hydraulic conductivity = 23.9 feet per day

Simulated injection = 3.74 gallons

Time, in seconds

EXPLANATION

Simulated water injection Measured water level Simulated water level 

0 

0.5 

1.0 

0 

0.5 

1.0 

–2 

0 

2 

4 

6 

8 

10 

12 

14 

16 

120 220 320 420 520 620 720 820 920 

Initial water level = 77.03 feet below land surface

Horizontal hydraulic conductivity = 2.6 feet per day 

Simulated injection = 4.17 gallons 

–2 

0 

2 

4 

6 

8 

10 

12 

14 

16 

1,350 1,450 1,550 1,650 1,750 1,850 1,950 2,050 2,150 

Simulated water injection, in gallons 

Initial water level = 77.15 feet below land surface 

Horizontal hydraulic conductivity = 2.6 feet per day 

Simulated injection = 4.12 gallons 

Water displacement, in feet

Time, in seconds

A. Test 1, 876.5–883.5 feet

B. Test 2, 876.5–883.5 feet

Time, in seconds

EXPLANATION

Simulated water injection Measured water level Simulated water level 

Figure 10. Measured and simulated water levels and simulated injection of water for packer-isolated interval 

816.5- to 823.5-feet in borehole 33P028 at Fort Stewart, GA, December 6, 2009. Both tests were reported to be 

10-gallon slug tests and both roughly yielded a hydraulic conductivity of 20 feet per day. (A) Test 1, (B) Test 2.

Figure 11. Measured and simulated water levels and simulated injection of water for packer-isolated interval 

876.5- to 883.5-feet in borehole 33P028 at Fort Stewart, GA, December 6, 2009. Both tests were reported to be 

10-gallon slug tests and both roughly yielded a hydraulic conductivity of 2 feet per day. (A) Test 1, (B) Test 2.
Hydrologic Testing 15

Aquifer Tests

Aquifer tests were conducted at Fort Stewart to estimate 

the transmissivity of the Upper and Lower Floridan aquifers 

and to determine the effects of pumping one aquifer on 

water levels in the other aquifer. A 24-hour aquifer test was 

performed during March 3–4, 2010, in well 33P029 open to 

the Upper Floridan aquifer, and a 72-hour aquifer test was 

performed during March 8–11, 2010, in well 33P028 open to 

the Lower Floridan aquifer.

The Upper Floridan aquifer well 33P025, less than 2 mi 

to the east, was used as an observation well for both aquifer 

tests (fig. 12). During the 24-hour aquifer test at well 33P029, 

wells 33P028 and 33P025 were observation wells; during the 

72-hour aquifer test at well 33P028, wells 33P029 and 33P025 

were observation wells. As a result, each aquifer test had one 

pumped well and two observation wells. 

Wells 33P028 and 33P029 were monitored at least from 

February 24 to March 16, 2010. This period included a pre-test 

time of slightly more than 4.5 days, the 24-hour aquifer test 

period, a recovery period of about 4 days, the 72-hour aquifer 

test period in the Lower Floridan aquifer, and a post-test 

period of approximately 4 days (fig. 13). Well 33P025 was 

monitored from February 25 to March 11, 2010. Details on 

these tests are provided in subsequent sections, and results are 

summarized in table 6.

Drawdown Estimation

Drawdown for the three monitored wells was estimated 

by using a procedure developed by Halford (2006a) to filter 

water-level data for effects of barometric pressure, earth tide, 

and long-term trends. Time series related to known influences 

on water levels are used as explanatory variables to synthesize 

a water-level time series. The synthetic water levels are 

matched (fit) to measured water levels during periods that are 

unaffected by an aquifer test (hereinafter referred to as the 

fitting period) by adjusting the multiplier and phase shift of 

each explanatory time-series component. To improve the fit to 

measured water levels the resulting synthetic water levels also 

can be adjusted by a constant offset and slope with respect 

to time. Differences between the synthetic and measured 

water levels are minimized with a sum-of-squares objective 

function. With the synthetic water levels matching measured 

water levels during the fitting period, the synthetic water levels 

represent non-pumping water levels during the aquifer test. 

With the synthetic water level being the same as the measured 

water level, at the start of the aquifer test, the synthetic water 

level minus the measured water level during the aquifer test 

can be taken as the amount of occurring drawdown. 

The filtering procedure was applied to water levels to 

determine drawdown curves in response to the 24-hour and 

72-hour aquifer tests with emphasis on the observation wells. 

Filtering caused negligible modification to the drawdown of 

the pumped well because the magnitude of the drawdown 

signal, more than 4 ft, obscured any signals corresponding to 

non-pumping influences, which probably ranged from a few 

hundredths of a foot to less than 0.4 ft (fig. 14). Pre-test water 

levels indicated a prominent (up to 0.25 ft) cyclic diurnal fluctuation in water levels, however, characteristic of earth tides.

Barometric pressure and gravity along with water levels 

from two background wells (33M004 and 34N089) were 

used to filter out non-pump influences (fig. 14). Barometricpressure data were compiled from an on-site barometer and 

from hourly barometric-pressure data from National Oceanic 

and Atmospheric Administration (NOAA) weather station 

KLHW, located 3.35 mi southeast of the aquifer-test site 

(KLHW location shown in fig. 1). The time series spreadsheet 

from Halford (2006a) generates gravity time series from 

entered values of latitude, longitude, and land-surface altitude. 

Gravity is used as a surrogate for earth tides and is expressed 

as microgravity deviation from average. Both background 

wells are open to the Upper Floridan aquifer. Pre-aquifer-test 

water levels in the background wells were similar to water 

levels in wells 33P025, 33P028, and 33P029 (fig. 14).

Table 6. Summary of aquifer test results, Fort Stewart, GA, 

March 2010. 

[UFA, Upper Floridan aquifer; LFCU, Lower Floridan confining unit; 

LFA, Lower Floridan aquifer; gal/min, gallon per minute; ft2/d, square foot 

per day; PE, parameter estimation; —, not applicable] 

Aquifer Aquifer test

24-hour 72-hour

Test information

Location Well 33P029 Well 33P028

Pumped aquifer UFA LFA

Test date, 2010 March 3–4 March 8–11

Pumping rate (gal/min) 387 740

Analysis method Cooper-Jacob PE numerical 

model

Coefficients

Transmissivity (ft2

/d) UFA 100,000* 90,000

Storativity (dimensionless) — 7.50E-04

Transmissivity (ft2

/d) LFCU — 4,000

Storativity (dimensionless) — 6.80E-04

Transmissivity (ft2

/d) LFA — 7,000

Storativity (dimensionless) — 1.50E-03

Maximum drawdown (feet)

Well 33P025 UFA <0.1 0.3

Well 33P029 4.1 0.4

Well 33P028 LFA 0.2 38.8

*

Rounds a transmissivity of 120,000 ft2/d to one significant figure.
16 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

Well 33P028

Well 33P028

Well 33P029

Well 33P029

Well 33P025

Well 33P025

Figure 13. Raw, unfiltered water levels in wells 33P028, 33P029, and 33P025 before, during, and after 

24-hour Upper Floridan and 72-hour Lower Floridan aquifer tests at Fort Stewart, GA, March 1–16, 2010: 

(A) before aquifer-test pumping, (B) failed aquifer test and well development (intermittent pumping), 

(C) well recovery, (D) Upper Floridan 24-hour aquifer test, (E) well recovery, (F) Lower Floridan 72-hour 

aquifer test, (G) dominant recovery, and (H) dominant non-pumping influences.

Water level in wells

33P028 or 33P029,

in feet above NAVD 88

Water level in wells 33P025, 

33P028, or 33P029, in feet 

above or below NAVD 88

Water level in well 33P025, 

in feet below NAVD 88

–40

–30

–20

–10

0

10

15

–7.1

–6.1

–5.1

–4.1

2

3

4

5

A

This graph is expanded vertical scale from graph above

B C D E F G H

1 3 5 7 9 11 13

March 2010

Well open interval

NOT TO SCALE

EXPLANATION

1,255 feet

520 feet

560 feetWell 33P029

Well 33P028

Well 33P025

Upper Floridan

confining unit

Surficial 

aquifer

Upper Floridan

 aquifer

Lower Floridan

confining unit

Lower Floridan 

aquifer

9,600 feet

A. Aquifer test layout B. Schematic cross section

100

200

300

400

500

600

700

800

900

1,000

1,100

Land

surface

Well 33P029

(Upper Floridan aquifer)

Well 33P025

(Upper Floridan aquifer)

1,200

1,300

Well 33P028

(Lower Floridan aquifer)

40 feet

Depth below land surface, in feet

N

Figure 13. Raw, unfiltered water levels in wells 33P023, 33P029, and 33P025 before, during, and 

after the 24-hour Upper Floridan and 72-hour Lower Floridan aquifer tests at Fort Stewart, GA, 

March 1–15, 2010. (A) Before aquifer-test pumping, (B) Failed aquifer test and well development 

(intermittent pumping), (C) Well recovery, (D) Upper Floridan 24-hour aquifer test, (E) Well recovery, 

(F) Lower Floridan 72-hour aquifer test, (G) Dominant recovery, (H) Dominant non-pumping influences.

Figure 12. Location and open interval of 

wells used for the 72-hour aquifer test at Fort 

Stewart, GA. (A) Diagram showing aquifer-test 

layout, (B) Schematic cross section showing 

the open intervals of the wells in relation to 

major hydrogeologic units (John S. Clarke, 

U.S. Geological Survey, written commun., 

January 2011).
Hydrologic Testing 17

Figure 14. Water-level fluctuations in monitored wells and background wells, and 

fluctuations in barometric pressure and gravity, used in aquifer tests at Fort Stewart, GA, 

January 27–March 16, 2010: (A) fitting period for well 33P028, (B) Upper Floridan 24-hour 

aquifer test, and (C) Lower Floridan 72-hour aquifer test.

A B C

–9.0

–8.5

–8.0

–7.5

–7.0

–6.5

–6.0

5

6

7

8

Water level, well 33P025 or 34N089, above NAVD 88, in feet

Well 33M004

–8.1

–7.1

–6.1

–5.1

–4.1

1

2

3

4

5

Water level, wells 33P028, 33P029, or 33M004 above NAVD 88, in feet 

Well 33P028

–200

–150

–100

–50

0

50

100

150

33.0

33.2

33.4

33.6

33.8

34.0

34.2

34.4

27 29 2 431 6 8 10 12 16 1814 20 22 24 26 28 2 4 6 8 10 12 1614

Gravity

Well 34N089

Well 33P029

Well 33P025

2010

January February March

Microgravity deviation 

from average, normalized

 to maximum value

Barometric 

pressure

Barometric pressure 

in feet of water

Offsite (background) wells

Onsite wells

Figure 14. Water-level fluctuations in monitored wells and background wells and fluctuations 

in barometric pressure and gravity, used in the aquifer tests at Fort Stewart, GA, January 27–

March 16, 2010. (A) Fitting period for well 33P028, (B) Upper Floridan 24-hour aquifer test, 

(C) Lower Floridan 72-hour aquifer test.
18 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

Water-level and barometric-pressure data for well 33P028 

and background wells were available for the month prior 

to pumping associated with the aquifer tests (fig. 14). The 

pre-aquifer-test fitting period to match synthetic water levels 

with measured water levels in well 33P028 spanned from noon 

on January 31 to 3:00 a.m. on March 2. The goodness of fit 

was expressed with the root mean square error (RMSE) of the 

difference between the synthetic and measure water levels. 

Synthetic water levels match measured water levels for well 

33P028 well with a RMSE of 0.0195 ft (fig. 15). Multipliers 

and phase shifts of explanatory time-series components that 

matched synthetic water levels with measured water levels at 

well 33P028 during the fitting period were used to synthesize 

water levels for well 33P028 during the aquifer tests. There 

are 10 explanatory time-series components—each of the 

two background wells and barometric pressure have three 

components: the data, 6-hour moving average of the data, 

and a residual (data minus 6-hour moving average). The tenth 

explanatory time-series component was microgravity deviation from normal.

Water-level data for wells 33P029 and 33P025 were 

available for a limited period of about 4.5 days before the 

pumping associated with the aquifer tests (fig. 14). The 

period was too short to be a fitting period for these wells. 

The similarity of water levels in wells 33P025 and 33P029 to 

those in well 33P028 during this time allowed use of the same 

multipliers and phase shifts developed to match synthetic 

water levels with measured water levels for well 33P028 for 

use in matching synthetic water levels with measured water 

levels for wells 33P025 and 33P029. It was assumed that 

water levels in the two wells were similar to those in pumped 

well 33P028 during the 1-month pre-aquifer-test fitting period. 

The offset and slope were independently adjusted for 

each of the three wells (33P028, 33P029, and 33P025) for 

each time period. As a result, each synthetic water-level time 

series has the same multipliers and phase shifts but different 

offsets and slopes. The synthetic water level for a well w

during a given time period j at a given time t, is:

Syn C m t t a V w j t w j w j , , , , i i t, – ( ) ( ) ( ) + = + × ( ) + × 0 φi i

n

( ) =

∑1

, (3)

where

Syn(w, j, t ) is the synthetic water level of well w during 

time period j at time t, in feet;

C(w, j ) is the offset for well w during time period j, 

in feet;

m(w, j ) is the linear water-level trend with respect 

to time (slope) for well w during time 

period j, in feet per day;

t

0 is an arbitrary reference time; (t–t0 )is in days;

n is the number of explanatory time-series 

components, 10 in this study;

ai is the amplitude of the multiplier for the 

i

th explanatory time-series component, 

in feet per unit of the explanatory 

time-series component;

V(i, t +φ ) is the value of the i

th explanatory time-series 

component at time t +φ; and

φi is the phase shift for the i

th explanatory 

time-series component, in days.

2010

Figure 15. Measured and synthetic water levels for well 33P028 during fitting period, 

January 31–March 1, 2010 at Fort Stewart, GA.

31 2 4 6 8 10 12 14 16 18 20 22 24 26 28 1

January

February March

–1.0 

–0.5 

0 

0.5 

1.0 

1.5 

2.0 

Water-level change, in feet 

Measured

EXPLANATION

Synthetic Synthetic minus measured

Figure 15. Measured and synthetic water levels for well 33P028 during fitting period, 

January 31–March 1, 2010, at Fort Stewart, GA. Root mean square error of the difference 

between synthetic and measured water levels is 0.0195 foot.
Hydrologic Testing 19

Values of ai and φi are used to best fit synthetic water 

levels to measured water levels for well 33P028 during 

the 1-month fitting period (fig. 15). Each time period (j) is 

either a fitting period or an aquifer-test period. Adjustments 

to the offset and slope were made to either evaluate the fit 

of synthetic water levels to measured water levels during a 

fitting period or to set the synthetic water level to be that of 

the measured water level just prior to the start of the aquifer 

tests. Thus, the synthetic water level minus the measured 

water level then represented drawdown induced by pumping at 

well 33P029 during the 24-hour aquifer test or at well 33P028 

during the 72-hour aquifer test.

Adjusting the offset and slope allowed synthetic water 

levels, originally from well 33P028, to fit measured water 

levels in wells 33P029 and 33P025. As a quality-assurance 

check, synthetic water levels were adjusted to best fit 

measured water levels during the 4.5-day fitting period for 

each observation well. Synthetic water levels from 33P028 

also were fit to measured water levels of a short fitting period, 

similar to the short fitting period of wells 33P025 and 33P029 

for comparison. RMSE values of the difference between 

synthetic and measured water levels were small, less than or 

equal to 0.022 ft for the three wells (fig. 16).

Figure 16. Fit of synthetic water levels to measured water levels for wells 33P028, 33P029, and 

33P025 at Fort Stewart, GA, February 25–March 2, 2010.

1.4

1.5

1.6

1.7

1.8

1.9

2.0

–0.2

–0.1

0

0.1

0.2

0.3

0.4

Measured and synthetic 

water-level change, in feet 

Measured and synthetic water-level change and 

synthetic minus measured water level, in feet 

RMSE = 0.015 foot

Well 33P028

–0.2

–0.1

0

0.1

0.2

0.3

0.4

RMSE = 0.014 foot

Well 33P029

–0.2

–0.1

0

0.1

0.2

0.3

0.4

February March

2010

25 26 27 28 1 2

Measured

EXPLANATION

Synthetic Synthetic minus measured

RMSE = 0.022 foot

Well 33P025

Synthetic minus measured

water level, in feet 

Figure 16. Fit of synthetic water levels to measured water levels for wells 33P028, 33P029, and 

33P025, Fort Stewart, GA, February 25–March 2, 2010. Root mean square error (RMSE) shown is 

for the difference between synthetic and measured water levels.
20 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

24-Hour Aquifer Test

The 24-hour aquifer test at well 33P029 was first 

attempted on March 2, 2010, at 10:00 a.m. Heavy drilling mud 

in the water pumped from the well affected water levels and 

pumping rates. After about 3 hours 45 minutes, the aquifer 

test was aborted and the well was developed by intermittently 

pumping until midnight (about 10 hours). Well 33P029 was 

then allowed 9 hours of recovery before the aquifer test was 

re-started at 9:00 a.m. on March 3, 2010. Time constraints did 

not allow further well recovery.

Aquifer-test pumping at Upper Floridan aquifer well 

33P029 began on March 3, 2010, at 9:00 a.m. and continued 

for 24 hours 5 minutes to March 4, 2010, at 9:05 a.m. The well 

was pumped at a rate of 387 gal/min. In response to the start 

of pumping, the water level in well 33P029 dropped almost 

instantly with some oscillation. The oscillation damped to zero 

within 90 seconds, and water levels seemed to stabilize in the 

short term (fig. 17). From about 90 seconds to about 2.5 hours, 

drawdown consistently increased with log time (constant log 

cycle). From 2.5 to 24 hours, drawdown increased at a lower 

rate than had occurred during the first 2.5 hours (fig. 18). 

At the end of the 24-hour pumping period, the water level 

rose almost instantly with some oscillation that damped to 

zero within 100 seconds. The oscillating period was about 

20 seconds (fig. 17).

Figure 17. Water levels at the (A) start and (B) end of the 24-hour aquifer test in well 33P029 at Fort Stewart, GA, 

March 3 and 4, 2010.

Water level below land 

surface, in feet

Water level below land 

surface, in feet

A. Start of aquifer test B. End of aquifer test

70

75

80

85

70

75

80

85

8:58:34 AM8:59:17 AM9:00:00 AM9:00:43 AM9:01:26 AM9:04:19 AM9:05:02 AM9:05:46 AM9:06:29 AM9:07:12 AM

March 3, 2010 March 4, 2010

Time between data points is 5 seconds

Figure 18. Semi-log graph of drawdown during the 

24-hour aquifer test in well 33P029 at a pumping 

rate of 387 gallons per minute at Fort Stewart, GA, 

March 3–4, 2010

3.5

3.7

3.9

4.1

4.3

4.5

0.00001 0.0001 0.001 0.01 0.1 1 10

Drawdown, in feet

Time after start of 24-hour aquifer test, in days

Slope on semi-log plot:

Trend

= 0.1124

∆ log (t)

∆s

Well 33P029

Figure 17. Water levels at the (A) start and (B) end of the 24-hour aquifer test in well 33P029 at Fort Stewart, GA, 

March 3–4, 2010.

Figure 18. Semi-log graph of drawdown during 

the 24-hour aquifer test in well 33P029 at a pumping 

rate of 387gallons per minute at Fort Stewart, GA, 

March 3–4, 2010.
Hydrologic Testing 21

Drawdown Response

Total decline in water level (drawdown plus nonpumping influences) during the 24-hour aquifer test at well 

33P029 equaled about 4.26 ft, from 76.24 to 80.50 ft below 

land surface. Adjusting for a minor linear trend indicates a 

corrected drawdown of 4.1 ft (fig. 19). 

Total water-level declines in observation wells 33P028 

and 33P025 were obscured by barometric-pressure changes 

and earth tides and complicated by pumping during the failed 

aquifer test and well development in well 33P029 (fig 19). 

Water levels in well 33P025 could not be adequately filtered to 

remove fluctuations in the value of synthetic water level minus 

measured water level. Values of synthetic water level minus 

measured water level for well 33P025 actually increased hours 

before the start of the failed aquifer test on March 2, then 

increased again soon after the start of the 24-hour aquifer test 

on March 3. Based on the amplitude in the fluctuations in synthetic water level minus measured water level, whatever actual 

response that might have occurred in well 33P025 to pumping 

at well 33P029 would be less than 0.1 ft. Water levels in well 

33P028 were adequately filtered, revealing drawdown during 

the failed aquifer test and subsequent well development on 

March 2, and the 24-hour aquifer test on March 3–4. The 

value of synthetic water level minus measured water level, 

set at zero at the beginning of the failed aquifer test, was 

about 0.12 ft during the failed aquifer test and subsequent 

well development on March 2. Drawdown started to decrease 

about 5 hours 30 minutes after the cessation of well development; however, it is unsure whether this was actual recovery 

or a temporary fluctuation. During the 24-hour aquifer test, 

the value of synthetic water level minus measured water 

level increased from 0.12 to 0.24 ft. Even though recovery 

during the 9-hour break between the two pumping periods 

is uncertain, it is assumed that at least some recovery from 

pumping on March 2 occurred during the 24-hour aquifer test, 

as indicated by the sag in the lower dashed line in fig. 19B. 

Such recovery would mean that drawdown in response to the 

24-hour aquifer test was greater than 0.12 ft and probably 

close to 0.2 ft.

2010

Water-level change, in feet 

Well 33P025

0.2 foot of drawdown

24-hour

aquifer

test

Well development

4.1 feet of drawdown

Well 33P028

Well 33P029

Figure 19. Measured and synthetic water levels and estimated drawdown in wells 33P025 

and 33P028 during the 24-hour aquifer test conducted in the Upper Floridan aquifer well 

33P029 at Fort Stewart, GA, February 25–March 7, 2010.

–0.5 

0 

0.5 

1.0 

1.5 

2.0 

2.5 

–0.5 

0 

0.5 

1.0 

1.5 

2.0 

2.5 

February March

25 26 27 28 1 2 3 4 5 6 7

–1 

0 

1 

2 

3 

4 

5 

Measured

EXPLANATION

Synthetic Synthetic minus measured (estimated

 drawdown during 72-hour aquifer test)

Figure 19. Measured and synthetic water levels and estimated drawdown in wells 33P025 

and 33P028 during the 24-hour aquifer test conducted in the Upper Floridan aquifer 

well 33P029 at Fort Stewart, GA, February 25–March 7, 2010.
22 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

Aquifer Test Analysis

The Cooper-Jacob, straight-line method (Cooper and 

Jacob, 1946) was applied to the drawdown data from pumped 

well 33P029 to determine the transmissivity of the Upper 

Floridan aquifer (fig. 18). The uncertainty associated with 

various factors indicates that the transmissivity probably is 

between 100,000 and 140,000 square feet per day (ft2/d). The 

best fit of the log cycle of drawdown to the drawdown data 

between 90 seconds and 2.5 hours is a slope of about 0.1124, 

yielding a transmissivity value of about 120,000 ft2/d. The 

log cycle still fits these data well with slopes ranging from 

0.1247 to 0.1033, yielding transmissivity values from 110,000 

to 130,000 ft2/d. Therefore, uncertainty as a result of curve 

fit is about ± 10,000 ft2/d. Drawdown data between 2.5 hours 

and 1 day seem to be affected by a recharge boundary or some 

large-scale aspect of the Upper Floridan aquifer and were not 

used to determine transmissivity. The results are within the 

range of values reported by Clarke and others (2004) for the 

Upper Floridan aquifer, although they are near the lower end 

of that range. Clarke and others (2004) report transmissivity 

values for the Upper Floridan aquifer in Liberty County from 

124,000 ft2

/d to 160,000 ft2/d. The value of 124,000 ft2/d was 

from earlier work done at Fort Stewart (Warren, 1944).

Some recovery from the failed aquifer test and well 

development might have caused a slight overestimation in the 

transmissivity of the Upper Floridan aquifer; however, the 

offset used to synthesize water levels might have neutralized 

this overestimation. As indicated in figure 17, most of the 

drawdown in response to pumping or recovery in response to 

cessation of pumping occurred almost immediately. The subsequent drawdown, while minor, was crucial in determining 

transmissivity. This minor drawdown took hours to occur. As 

this minor, but crucial, drawdown in response to pumping the 

Upper Floridan aquifer occurred over many hours, the recovery from such pumping so, too, may have occurred over many 

hours. Simulations of water levels using the simple Theis 

model indicate that some recovery from the failed aquifer test 

and well development might have caused an overestimation 

of transmissivity by about 10,000 ft2

/d. However, the slope 

correction of 0.03 ft/d that was added to the synthesized water 

levels neutralizes this overestimation. The final estimation of 

transmissivity for the Upper Floridan aquifer from the 24-hour 

aquifer test, rounded to one significant digit, is 100,000 ft2/d.

72-Hour Aquifer Test

Pumping at Lower Floridan aquifer well 33P028 began 

on March 8, 2010, at 10:00 a.m. and continued for 72 hours 

5 minutes to March 11 at 10:05 a.m. The well was pumped 

at a rate of 740 gal/min. The pump lacked a check valve; 

therefore, when aquifer-test pumping ceased, a slug of water 

siphoned back into the well. As a result, the water level has 

a slug signature during the first minute of recovery (fig. 20). 

Within minutes, the effect of the slug was no longer apparent 

in recovery. Within about 30 hours, the water level in pumped 

well 33P028 recovered to where non-pump influences 

dominated the hydrograph. 

Drawdown Response

Total decline in water level (drawdown plus external 

influences) during the 72-hour aquifer test at pumped well 

33P028 equaled about 38.4 ft, from 78.5 to 116.9 ft below 

land surface. Adjusting for a minor linear trend indicates a 

corrected drawdown of 38.8 ft (fig. 21). Drawdown at the 

pumped well as a function of log (time) was nonlinear as 

indicated by the continuously decreasing log cycle of drawdown with time (fig. 22). The nonlinear nature of drawdown 

on the semi-log plot precluded the viable use of an analytical 

method for estimating the transmissivity of the Lower 

Floridan aquifer.

Total drawdown rates at the end of the 72-hour aquifer 

test in observation wells 33P025 and 33P029 in Upper 

Floridan aquifer were 0.3 and 0.4 ft, respectively (fig. 21). 

Some fluctuations in the value of synthetic water level minus 

measured water level remained after filtering, but water levels 

from both observation wells adequately filtered influences of 

barometric pressure and earth tides. The main fluctuation in 

the value of synthetic water level minus measured water level 

that seemed to escape filtering for both observation wells had 

a wavelength of 1 day, reaching a maximum and minimum in 

the early afternoon and early morning, respectively, with an 

amplitude up to about 0.05 ft. Offsets and slopes were set for 

the observation wells so that the synthetic water level was the 

same as the measured water level at the start of the 72-hour 

80

90

100

110

120

10:04:05 AM10:04:48 AM10:05:31 AM10:06:14 AM10:06:58 AM

10:07:41 AM

Water level below land surface,

 in feet

March 11, 2010

Back-siphon slug

Figure 20. Graph showing the effects on water level in 

well 33P028 at the end of the 72-hour aquifer test when 

a slug of water siphoned back into the well 

immediately after pumping stopped at Fort Stewart, 

GA, March 11, 2010.

Well 330028

Time between data points is 5 seconds

Figure 20. Effects on water level in well 33P028 at 

the end of the 72-hour aquifer test when a slug of 

water siphoned back into the well immediately after 

pumping stopped at Fort Stewart, GA, 

March 11, 2010.
Hydrologic Testing 23

aquifer test. For well 33P029, the offset and slope were set 

with the assumption that minor recovery was still occurring 

1 day after the 24-hour aquifer test in the Upper Floridan 

aquifer but was negligible by the start of the 72-hour aquifer 

test. As a result, values of the synthetic water level minus the 

measured water level for well 33P029 are set by the offset and 

slope to recede from about 0.1 ft on March 5, 2010, to zero on 

March 8, 2010. 

No record of recovery is available for well 33P025 

because of equipment malfunction. Most drawdown in well 

33P029 recovered within about 3 days following the end of 

the 72-hour aquifer test. 

March 2010

Water-level change, in feet

0.3 foot of drawdown

Recorder malfunction

72-hour aquifer test

Figure 21. Measured and synthetic water levels and estimated drawdown in wells 33P025 

and 33P029 during 72-hour aquifer test conducted in Lower Floridan aquifer well 33P028 at 

Fort Stewart, GA, March 5–15, 2010.

0.4 foot of drawdown

Maximum

drawdown

38.8 feet

–0.5 

0 

0.5 

1.0 

1.5 

Well 33P025

–0.5 

0 

0.5 

1.0 

1.5 

Well 33P029 

0 

0.5 

–0.5 

1.0 

1.5 

2.0 

2.5 

3.0 

5 6 7 8 9 10 11 12 13 14 15

Well 33P028 

Measured

EXPLANATION

Synthetic Synthetic minus measured (estimated

 drawdown during 72-hour aquifer test)

0

10

20

30

40

0.00001 0.0001 0.001 0.01 0.1 1 10

Drawdown, in feet

Elapsed time, in days

Figure 22. Semi-log plot of drawdown in pumped 

well 33P028 during the 72-hour aquifer test at 

Fort Stewart, GA, March 8–11, 2010.

Well 33P028

Figure 21. Measured and synthetic water levels and estimated drawdown in wells 33P025 

and 33P029 during the 72-hour aquifer test conducted in Lower Floridan aquifer well 33P028 

at Fort Stewart, GA, March 5–15, 2010.

Figure 22. Semi-log plot 

of drawdown in pumped 

well 33P028 during the 

72-hour aquifer test at 

Fort Stewart, GA, 

March 8–11, 2010.
24 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

Model Simulation of the Lower Floridan Aquifer Test

Because of the nonlinear nature of the semi-log plot, 

transmissivity of the Upper and Lower Floridan aquifers was 

estimated by using the numerical model MODFLOW-96 

(McDonald and Harbaugh, 1988; Harbaugh and McDonald, 

1996) with the calibration tool MODOPTIM (Halford, 2006b). 

Hydraulic properties were estimated by minimizing the 

weighted sum-of-squares of differences between simulated 

and measured drawdown. Estimated drawdown, discussed 

in previous sections, will be referred to here as “measured 

drawdown.” Drawdown values in observation wells were 

weighted slightly more than drawdown in the pumped well 

during calibration to compensate for the larger measured 

drawdown in the pumped well relative to that in the observation wells. In addition to weighting favorably to the drawdown 

of observation wells, a constant amount of drawdown was 

subtracted from drawdown in the pumped well. This equalization of drawdown magnitude prevents the observation-well 

drawdown values from being superfluous during the calibration process (Halford 2006b).

The aquifer system was simulated with a two-dimensional, axisymmetric, radial, transient groundwater-flow model 

that incorporated pumped well 33P028 and observation wells 

33P029 and 33P025 (fig. 23). Horizontal hydraulic conductivity was assumed to be isotropic and homogeneous within 

each hydrogeologic unit. Specific storage was assumed to be 

homogeneous through the entire aquifer system. A vertical 

anisotropy ratio (vertical hydraulic conductivity divided by 

the horizontal hydraulic conductivity) of 0.1 was assumed for 

each hydrogeologic unit (table 7).

Five hydrogeologic units are represented in the model—

the surficial aquifer, Upper Floridan confining unit, Upper 

Floridan aquifer, Lower Floridan confining unit, and Lower 

Floridan aquifer (fig. 23). The model domain is discretized 

into 114 rows representing aquifer thickness and 59 columns 

representing the radial distance from center of pumped well 

33P028 to the external model boundary. The model radially 

extends 200,000 ft from well 33P028 and represents the 

subsurface depth interval between 77 ft (the estimated watertable depth) and 1,400 ft. Radial grid spacing (column width) 

increases from 0.51 ft at pumped well 33P028 to 36,640 ft at 

the edge of the model. Each row height represents a vertical 

thickness of 11.6 ft of the simulated aquifers and intervening 

confining layers. The thin, unconfined, surfical aquifer was 

represented as a single layer and row in the model.

The radial distal edges to the well and the base of the 

model are simulated as no-flow boundaries; the upper boundary represents the water table. The aquifer-test stress period 

is represented by 53 time steps totaling slightly more than 

3 days. Time steps ranged from about 0.47 seconds to 14 hours 

25 minutes, with each succeeding time step increasing from 

the previous time step by a multiplier of 1.25.

MODOPTIM (Halford, 2006b) was used to estimate 

horizontal hydraulic conductivity in the Upper Floridan aquifer, 

Lower Floridan confining unit, and Lower Floridan aquifer and 

to estimate a single value for the composite specific storage of 

all five hydrogeologic units represented in the model (fig. 23; 

table 7). Hydraulic properties of the surficial aquifer and Upper 

Floridan confining unit were not sensitive to parameter estimation; the horizontal hydraulic conductivity of these units was 

unchanged from the initial value of 0.01 ft/d used for calibration. The goodness of fit of simulated drawdown or recovery to 

measured drawdown indicates a successful calibration for wells 

33P028 and 33P029 (fig. 24). Simulated values for drawdown 

for well 33P025 were about half the measured values for drawdown, which may be due to poor drawdown estimation or 

actual heterogeneity within the Upper Floridan aquifer that 

was not accounted for within the model.

Simulated transmissivity of the Lower Floridan aquifer 

was about 7,000 ft2/d (table 7), almost twice the transmissivity values simulated by a regional model of coastal Georgia 

(Payne and others, 2005) but within the range of values 

reported by Clarke and others (2004). Regional estimates of 

transmissivity derived from models usually are less than local 

estimates because of larger variability of aquifer properties 

compared with limited variability at local, aquifer-test scales 

(Lynn Torak, U.S. Geological Survey, written commun., 

2010). Simulated transmissivity of the Upper Floridan aquifer 

was nearly 90,000 ft2/d, slightly less than the transmissivity 

that was determined by using the straight-line method (Cooper 

and Jacob, 1946) on drawdown data from a 24-hour aquifer 

test at well 33P029. Using the vertical anisotropy (0.1) and 

the horizontal hydraulic conductivity for the Lower Floridan 

confining unit (17 ft/d) from the model, vertical hydraulic 

conductivity for the Lower Floridan confining unit was 

estimated to be 1.7 ft/d. This vertical hydraulic conductivity 

estimate is about four times the median of values directly 

determined from cores.

Parameter estimation and model simulation, in addition to 

collected data, indicate that the Lower Floridan confining unit 

at the study site has hydraulic properties similar to those of 

the Lower Floridan aquifer. The MODOPTIM estimated value 

of horizontal hydraulic conductivity of the Lower Floridan 

confining unit is essentially the same as that of the Lower 

Floridan aquifer. Further, the model parameter-estimation 

results indicate a high measure of redundancy in the horizontal 

hydraulic conductivity of the Lower Floridan confining unit 

and the Lower Floridan aquifer. The high measure of redundancy for these two parameters indicates that MODOPTIM 

would provide a better estimate of the horizontal hydraulic 

conductivity for both the Lower Floridan confining unit 

and the Lower Floridan aquifer as a single unit. The results 

of flowmeter surveys and slug tests support the hydrologic 

similarity between the two units at the study site. 
Hydrologic Testing 25

Well 33P028

–1,400

–1,200

–1,000

–800

–600

–400

–200

0

0 0.05 0.1 1 10 100 1,000 10,000 100,000

Well

33P029

Screen

at bottom

of well

Well

33P025

Distance from center of pumped well 33P028, in feet

Linear scale Log scale Casing

Pumped well bore (1 column) Aquifer system (56 columns)

33P028

33P029

33P025

Model grid area below

114 Rows

Column boundaries

Open to aquifer

Inactive cells

Surficial aquifer (1 row)

Upper Floridan confining unit (32 rows)

Upper Floridan aquifer (21 rows)

Lower Floridan confining unit (19 rows)

Lower Floridan aquifer (41 rows)

Row boundaries

EXPLANATION

Depth below land surface, in feet

Annular space

Figure 23. Axisymmetric radial model for 72-hour aquifer test at pumped well 33P028 

at Fort Stewart, GA, March 8–11, 2010.
26 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

A B

0

2

4

6

8

10

0.001 0.01 0.1 1 10

Drawdown, in feet

0

0.1

0.2

0.3

0.4

0.5

0.6

0.001 0.01 0.1 1 10

Drawdown, in feet

Time, in days Time, in days

Well 33P029 drawdown

EXPLANATION

Well 33P025 drawdown

Well 33P029 recovery

Simulated

Measured

Simulated

Measured

Well 33P028 drawdown

Well 33P028 recovery

Simulated

Measured

Simulated

Measured

Simulated

Measured

Figure 24. Simulated and measured water-level drawdown and recovery at (A) pumped well 33P028 and 

(B) observation wells 33P029 and 33P025 (simulated and measured water-level drawdown only, at well 

33P025) for the 72-hour aquifer test at Fort Stewart, GA, March 5–11, 2010.

Figure 24. Simulated and measured water-level drawdown and recovery at (A) pumped well 33P028 

and (B) observation wells 33P029 and 33P025 (simulated and measured water-level drawdown only, at 

well 33P025) for the 72-hour aquifer test at Fort Stewart, GA, March 8–11, 2010.

Table 7. Hydraulic parameters that were used or estimated to fit simulated water levels to measured drawdown and 

recovery and estimated values of transmissivity and storativity for the 72-hour aquifer test at pumped well 33P028 and 

observation wells 33P029 and 33P025, Fort Stewart, GA, March 8–11, 2010. 

[Kx, horizontal hydraulic conductivity; Kz, vertical hydraulic conductivity; Kz/Kx, vertical anisotropy; Ss, specific storage; ft/d, foot per day; 

ft2/d, foot squared per day; bracketed numbers were estimated with a parameter-estimation program MODOPTIM (Halford, 2006b); NA, not 

applicable; *, the surficial aquifer was represented as one layer] 

Hydrogeologic unit

Hydraulic parameter

Kx 

(ft/d)

Kz/Kx

(dimensionless)

Ss

(1/feet)

Thickness 

(feet)

Transmissivity

(ft2

/d)

Surficial aquifer 0.01 0.1 [3.07E-06] 11.6* NA

Upper Floridan 

confining unit

0.01 0.1 [3.07E-06] 371 NA

Upper Floridan aquifer [355.785] 0.1 [3.07E-06] 244 86,812

Lower Floridan 

confining unit

[17.0321] 0.1 [3.07E-06] 220 3,747

Lower Floridan aquifer [14.2962] 0.1 [3.07E-06] 476 6,802
Summary 27

Summary

Site-specific hydrologic testing was conducted at Fort 

Stewart in Liberty County, GA, in late 2009 and early 2010 to 

determine the hydraulic characteristics of the Upper Floridan 

aquifer, Lower Floridan confining unit, and Lower Floridan 

aquifer. Geophysical and flowmeter surveys in a borehole 

33P028 constructed in mid-November 2009 delineate several 

water-bearing zones bounded by less-permeable zones 

throughout the Floridan aquifer system. The Upper Floridan 

aquifer occurs at depths between 440 and 705 feet (ft) and 

is composed of two water-bearing zones—the 520- to 650-ft 

upper zone contributes 96 percent of the flow in the Upper 

Floridan aquifer; the 650- to 705-ft lower zone contributes 

the remaining 4 percent. The Lower Floridan confining unit 

extends from the base of the Upper Floridan to about 912 ft 

and consists of three zones; the middle zone from 793 to 822 ft 

depth is much more permeable than the top and bottom zones. 

Drill cores from the Lower Floridan confining unit at depths 

from 703 to 855 ft indicate vertical hydraulic conductivities 

ranging from 0.26 to 0.79 feet per day (ft/d) and porosities 

ranging from 0.25 to 0.40. The Lower Floridan aquifer extends 

from the base of the Lower Floridan confining unit to at least 

1,300 ft below land surface and contains three permeable 

zones—912–947 ft, 1,090–1,139 ft, and 1,211–1,250 ft—

interlayered with three less-permeable zones. 

 Horizontal hydraulic conductivity estimates of the Lower 

Floridan confining unit were obtained from slug tests at four 

packer-isolated intervals in borehole 33P028—726.5–733.5 ft, 

766.5–773.5 ft, 816.5–823.5 ft, and 876.5–883.5 ft. Hydraulic 

conductivity in three of the four packer-isolated intervals was 

estimated by using a method for constant decay response back 

to static conditions. The isolated interval from 766.5–773.5 ft 

depth had a slightly underdamped response (oscillation), and 

hydraulic conductivity was estimated using a method for oscillating responses. Horizontal hydraulic conductivity for each of 

the four intervals, with increasing depth, was 20, 70, 20, and 

2 ft/d, respectively.

Aquifer tests were conducted to estimate the transmissivity of the Upper and Lower Floridan aquifers and to 

determine the effects of pumping one aquifer on water levels 

in the other. For each test, drawdown was monitored in the 

pumping well and in two observation wells. Drawdown for the 

three monitored wells was estimated by using a procedure to 

filter water-level data for effects of barometric pressure, earth 

tide, and long-term trends. 

A 24-hour aquifer test in which Upper Floridan aquifer 

well 33P029 was pumped at an average rate of 387 gallons 

per minute (gal/min) resulted in a total drawdown of 4.1 ft in 

the pumped well and 0.2 ft in Lower Floridan well 33P028. 

Analysis of drawdown data from the pumped well using a 

straight-line analytical method indicated the transmissivity, 

rounded to one significant figure, was 100,000 ft2

/d. 

A 72-hour aquifer test in which Lower Floridan aquifer 

well 33P028 was pumped at an average rate of 740 gal/min 

resulted in a maximum drawdown of 38.8 ft in the pumped 

well and of 0.3 and 0.4 ft in Upper Floridan aquifer observation well 33P025 (more than a mile from the pumped well) 

and well 33P029, respectively. 

Drawdown data from the 72-hour test were used as 

a basis to estimate transmissivity of the Upper and Lower 

Floridan aquifers using the numerical model MODFLOW-96 

with the calibration tool MODOPTIM. The aquifer system 

was simulated with a two-dimensional, axisymmetric, radial, 

transient groundwater-flow model that incorporated pumped 

well 33P028 and observation wells 33P029 and 33P025 and 

extended 200,000 ft from the pumped well and to a depth 

of 1,400 ft. Five hydrogeologic units are represented in the 

model—the surficial aquifer, Upper Floridan confining unit, 

Upper Floridan aquifer, Lower Floridan confining unit, and 

Lower Floridan aquifer. MODOPTIM was used to estimate 

horizontal hydraulic conductivity in the Upper Floridan 

aquifer, Lower Floridan confining unit, and Lower Floridan 

aquifer, and to estimate a single value for the composite 

specific storage of all five hydrogeologic units represented in 

the model. Transmissivity of the Lower Floridan aquifer was 

estimated to be about 7,000 ft2

/d. Transmissivity of the Upper 

Floridan aquifer was estimated to be 90,000 ft2/d, slightly less 

than the transmissivity that was determined by using drawdown data from a 24-hour aquifer test at well 33P029. 

Parameter estimation and model simulation, in addition 

to collected data, indicate that the Lower Floridan confining 

unit at the study site has hydraulic properties similar to those 

of the Lower Floridan aquifer. The MODOPTIM estimated 

value of horizontal hydraulic conductivity of the Lower 

Floridan confining unit is essentially the same as that of the 

Lower Floridan aquifer. Further, model-parameter estimation 

results indicate a high measure of redundancy in the horizontal 

hydraulic conductivity of the Lower Floridan confining unit 

and the Lower Floridan aquifer. The high measure of redundancy for these two parameters indicates that MODOPTIM 

would provide a better estimate of the horizontal hydraulic 

conductivity for both the Lower Floridan confining unit 

and the Lower Floridan aquifer as a single hydrologic unit. 

The results of flowmeter surveys and slug tests support the 

hydrologic similarity between the two units at the study site.
28 Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia

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Manuscript approved for publication, January 5, 2011

Edited by Rebecca J. Deckard

Illustrations by Bonnie J. Turcott

Layout by Caryl J. Wipperfurth

For more information concerning the research in this report, contact

USGS Georgia Water Science Center

3039 Amwiler Road

Atlanta, Georgia 30360

telephone: 770-903-9100

http://ga.water.usgs.gov
Gonthier—Summary of Hydrologic Testing of the Floridan Aquifer System at Fort Stewart, Georgia—Open-File Report 2011–1020