U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5249

Prepared in cooperation with the City of Pooler, Georgia

Hydrogeology and Water Quality of the Floridan Aquifer 

System and Effect of Lower Floridan Aquifer Pumping 

on the Upper Floridan Aquifer, Pooler, Chatham County, 

Georgia, 2011–2012
Cover. Drilling test well in Lower Floridan aquifer, Pooler, Georgia. Photo by John S. Clarke, USGS.
Hydrogeology and Water Quality of the Floridan Aquifer 

System and Effect of Lower Floridan Aquifer Pumping 

on the Upper Floridan Aquifer, Pooler, Chatham County, 

Georgia, 2011–2012

By Gerard J. Gonthier 

Prepared in cooperation with the City of Pooler, Georgia

Scientific Investigations Report 2012–5249

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Gonthier, G.J., 2012, Hydrologeology and water quality of the Floridan aquifer system and effect of Lower Floridan 

aquifer pumping on the Upper Floridan aquifer, Pooler, Chatham County, Georgia, 2011–2012: U.S. Geological 

Survey Scientific Investigations Report 2012–5249, 62 p., available at http://pubs.usgs.gov/sir/2012/5249/.
iii

Contents

Acknowledgments ........................................................................................................................................x

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

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

Purpose and Scope ..............................................................................................................................2

Previous Studies ...................................................................................................................................2

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

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

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

Well Identification.................................................................................................................................5

Hydrogeology and Water Quality of the Floridan Aquifer System.........................................................6

Methods of Data Collection and Analysis ........................................................................................6

Test Drilling and Well Installation .............................................................................................6

Lithologic and Borehole Geophysical Logs...........................................................................10

Flowmeter Survey......................................................................................................................10

Water-Level Measurements ....................................................................................................10

Core Hydraulic Analysis and Packer-Isolated Slug Tests...................................................10

Aquifer Tests...............................................................................................................................14

Drawdown Estimation...............................................................................................................14

Water-Quality Sampling and Analysis....................................................................................16

Hydrogeology and Water Quality .....................................................................................................21

Upper Floridan Aquifer .............................................................................................................21

Flowmeter Survey.............................................................................................................21

Hydraulic Properties ........................................................................................................22

Water Quality.....................................................................................................................22

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

Flowmeter Survey.............................................................................................................23

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

Water Quality.....................................................................................................................23

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

Flowmeter Survey.............................................................................................................23

Hydraulic Properties ........................................................................................................24

Water Quality.....................................................................................................................24

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

Summary........................................................................................................................................................25

Selected References...................................................................................................................................27

Appendix 1. Slug tests, Pooler, Georgia, December 13–14, 2011........................................................30

Appendix 2. Aquifer Tests, Pooler, Georgia, March and April 2012....................................................45
iv

Figures

1. Map showing location of test site, Pooler, Georgia ...............................................................3

2. Chart showing generalized correlation of geologic and hydrogeologic units 

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

3. Log graphs showing lithology and geophysical properties of well 35Q069, 

Pooler, Georgia..............................................................................................................................7

4. Hydrogeologic units and well completion diagram of Lower Floridan aquifer 

well 35Q069, Pooler, Georgia ......................................................................................................8

5. Hydrogeologic units and well completion diagram of Upper Floridan aquifer 

well 35Q070, Pooler, Georgia ......................................................................................................9

6. Log graphs showing water-quality-sample collection locations in test hole 

35Q069 and flowmeter-survey data from pumping the Floridan aquifer system 

prior to installing an 8-inch-diameter casing in test hole 35Q069, and pumping 

the Lower Floridan aquifer after installing an 8-inch-diameter casing in test

hole 35Q069 and completing it as well 35Q069, Pooler, Georgia.........................................11

7. Diagram showing straddle-packer assembly in test hole 35Q069 as an 

example at the 612- to 618-feet packer-isolated interval, Pooler, Georgia, 

December 13, 2011......................................................................................................................13

8. Location and open interval of wells used for the 24-hour and 72-hour aquifer 

tests, Pooler, Georgia: diagram showing aquifer-test layout and schematic 

cross section showing major hydrogeologic units ...............................................................15

9. Graphs showing water levels in wells 35Q069 and 35Q070 before, during, and 

after the 24-hour Upper Floridan and 72-hour Lower Floridan aquifer tests, 

Pooler, Georgia, March 27–May 6, 2012 .................................................................................15

10. Graphs showing water-quality values by sample interval...................................................19

11. Piper diagram of water quality from sample intervals from test hole 35Q069, 

Pooler, Georgia, December 8, 2011..........................................................................................20

Appendix Figures 

1–1 to 1–4. Graphs showing—

1–1. Water-level response to inserting slug into the water column connected 

to the packer-isolated interval at 612 to 618 feet below land surface, 

Lower Floridan confining unit, test hole 35Q069, Pooler, Georgia, 

December 13, 2011 ......................................................................................................32

1–2. Semi-log plot of water-level recovery from inserting slug into the water 

column connected to the packer-isolated interval at 612 to 618 feet below 

land surface, Lower Floridan confining unit, test hole 35Q069, Pooler, 

Georgia, December 13, 2011 ......................................................................................33

1–3. Water-level response to pulling a slug from the water column connected

to the packer-isolated interval at 612 to 618 feet below land surface, 

Lower Floridan confining unit, test hole 35Q069, Pooler, Georgia, 

December 13, 2011 ......................................................................................................34

1–4. Semi-log plot of water-level recovery from pulling slug from the water 

column connected to the packer-isolated interval at 612 to 618 feet below 

land surface, Lower Floridan confining unit, test hole 35Q069, Pooler,

Georgia, December 13, 2011 ......................................................................................35
v

1–5 to 2–4. Graphs showing—

1–5. Water-level response to inserting a slug into the water column connected 

to the packer-isolated interval at 642 to 648 feet below land surface, 

Lower Floridan confining unit, test hole 35Q069, Pooler, Georgia, 

December 14, 2011 ......................................................................................................36

1–6. Semi-log plot of water-level recovery from inserting slug into the 

water column connected to the packer-isolated interval at 642 to 648 feet 

below land surface, Lower Floridan confining unit, test hole 35Q069, 

Pooler, Georgia, December 14, 2011.........................................................................37

1–7. Water-level response to pulling slug from the water column connected

to the packer-isolated interval at 642 to 648 feet below land surface,

Lower Floridan confining unit, test hole 35Q069, Pooler, Georgia, 

December 14, 2011 ......................................................................................................38

1–8. Semi-log plot of water-level recovery from pulling slug from the water

column connected to the packer-isolated interval at 642 to 648 feet below

land surface, Lower Floridan confining unit, test hole 35Q069, Pooler, 

Georgia, December 14, 2011 ......................................................................................39

1–9. Water-level response to inserting slug into the water column connected

to the packer-isolated interval at 690 to 696 feet below land surface,

Lower Floridan confining unit, test hole 35Q069, Pooler, Georgia, 

December 14, 2011 ......................................................................................................40

1–10. Water-level response to pulling slug from the water column connected

to the packer-isolated interval at 690 to 696 feet below land surface,

Lower Floridan confining unit, test hole 35Q069, Pooler, Georgia, 

December 14, 2011 ......................................................................................................41

1–11. Semi-log plot of water-level recovery from pulling slug from the water 

column connected to the packer-isolated interval at 690 to 696 feet 

below land surface, Lower Floridan confining unit, test hole 35Q069,

Pooler, Georgia, December 14, 2011.........................................................................42

1–12. Water-level response to inserting slug into then pulling slug from the 

water column connected to the Floridan aquifer system from 339 to 

1,130 feet below land surface, test hole 35Q069, Pooler, Georgia,

December 14, 2011 ......................................................................................................42

1–13. Measured and simulated water levels for inserting slug into water 

column open to the Floridan aquifer system from 339 to 1,130 feet 

below land surface, Pooler, Georgia, December 14, 2011 ....................................43

1–14. Measured and simulated water levels for pulling slug from water 

column open to the Floridan aquifer system from 339 to 1,130 feet 

below land surface, Pooler, Georgia, December 14, 2011 ....................................44

2–1. Water-level fluctuations in background wells and fluctuations in 

barometric pressure, gravity, and ocean tides used in the aquifer tests, 

Pooler, Georgia, March 27–May 6, 2012..................................................................47

2–2. Fit of synthetic water levels to measured water levels for wells 35Q070 

and 35Q069, Pooler, Georgia, April 28–May 6, 2012 ...............................................48

2–3. Measured and synthetic water levels and estimated drawdown in 

Upper Floridan aquifer well 35Q070 during the 24-hour aquifer test 

performed in the well, Pooler, Georgia, March 27–April 1, 2012 .........................49

2–4. Measured and synthetic water levels and estimated drawdown in 

Lower Floridan aquifer well 35Q069 during the 24-hour aquifer test 

performed in the Upper Floridan aquifer well 35Q070, Pooler, 

Georgia, March 27–April 1, 2012 ...............................................................................49
vi

2–5 to 2–11. Graphs showing—

2–5. Measured and synthetic water levels and estimated drawdown in 

Lower Floridan aquifer well 35Q069 during the 72-hour aquifer test 

performed in the well, Pooler, Georgia, April 16–May 5, 2012............................50

2–6. Measured and synthetic water levels and estimated drawdown in 

Upper Floridan aquifer well 35Q070 during the 72-hour aquifer test 

performed in the Lower Floridan aquifer well 35Q069, Pooler, Georgia,

 April 16–May 5, 2012 .................................................................................................50

2–7. Measured water levels at the start and end of the 24-hour aquifer

test in Upper Floridan aquifer well 35Q070, Pooler, Georgia,

March 27–28, 2012 .....................................................................................................52

2–8. Results of several different ways drawdown was estimated for 

Upper Floridan well 35Q070 in response to the 24-hour aquifer test 

in the well, Pooler, Georgia, March 27–28, 2012 ..................................................53

2–9. Semi-log graph of drawdown during the 24-hour aquifer test in 

well 35Q070 at a pumping rate of about 285 gallons per minute, 

Pooler, Georgia, March 27–28, 2012 ......................................................................53

2–10. Water level at the end of the 72-hour aquifer test in well 35Q069, 

Pooler, Georgia, April 19, 2012 .................................................................................55

2–11. Semi-log plot of drawdown in pumped well 35Q069 during the 

72-hour aquifer test, Pooler, Georgia, April 16–19, 2012 .....................................55

2–12. Diagram showing axisymmetric model for 72-hour aquifer test at pumped 

 well 35Q069, Pooler, Georgia, April 16–19, 2012................................................................57

2–13. Graphs showing resulting parameter values from MODOPTIM runs and 

RMS/MAX as a function of the preferred weight of model calibration on 

the Upper Floridan aquifer observation well 35Q070 relative to the weight 

on the Lower Floridan aquifer pumped well 35Q069........................................................60

2–14. Graphs showing simulated and measured water-level drawdown (pumping) 

and recovery at Lower Floridan aquifer pumped well 35Q069 and Upper 

Floridan aquifer observation well 35Q070 for the 72-hour aquifer test at 

pumped well 35Q069, Pooler, Georgia, April 16–19, 2012................................................62
vii

Tables

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

analyses, Pooler, Georgia, 2011–2012, and wells from previous studies ...........................6

2. Estimated vertical hydraulic conductivity and porosity of core samples 

collected from the Lower Floridan confining unit in test hole 35Q069, 

Pooler, Georgia, November 2011..............................................................................................10

3. Hydraulic conductivity values for depth intervals emphasizing the 

Lower Floridan confining unit in test hole 35Q069, Pooler, Georgia, 

December 13–14, 2011 ...............................................................................................................12

4. Summary of slug- and aquifer-test results, Pooler, Georgia, 

December 2011–April 2012 .......................................................................................................14

5. Intervals between water-sample collection depths in pumped test 

hole 35Q069 immediately following a flowmeter survey, Pooler, Georgia, 

December 8, 2011 .......................................................................................................................16

6. Water quality of composite samples and sample intervals immediately 

following a flowmeter survey in pumped test hole 35Q069, Pooler, Georgia, 

December 8, 2011........................................................................................................................17

7. Water-quality results for a water sample collected from well 35Q069, 

Pooler, Georgia, April 19, 2012. Water sample was collected 71 hours into 

the 72-hour aquifer test .............................................................................................................18

8. Flowmeter survey results in percent contribution of flow from the Upper Floridan 

aquifer, Lower Floridan confining unit, and Lower Floridan aquifer from selected 

studies in the vicinity of Pooler, Georgia, 2009–2012............................................................22

Appendix Tables

2–1. Disruptions in continuous monitoring of natural water-level during the study 

period, Pooler, Georgia, February through April 2012...........................................................45

2–2. Pumping rate through the 24-hour aquifer test in well 35Q070, Pooler, Georgia, 

March 27–28, 2012......................................................................................................................51

2–3. Pumping rate through the 72-hour aquifer test in well 35Q0690, Pooler, Georgia,

April 16–19, 2012 .........................................................................................................................54

2– 4. Hydraulic parameters used to simulate measured drawdown and recovery 

and estimated values of transmissivity and storativity for the 72-hour aquifer 

test at pumped well 35Q069, Pooler, Georgia, April 16–19, 2012.........................................56

2–5. Resulting parameter values and values of RMS/MAX from MODOPTIM runs 

with different weights of calibration in favor of Upper Floridan well 35Q070, 

relative to Lower Floridan pumped well 35Q069, simulating response to 

72-hour aquifer test, Pooler, Georgia, April 16–19, 2012 ......................................................61
viii

Conversion Factors and Datum

Inch/Pound to SI

Multiply By To obtain

Length

inch 25.4 millimeter (mm)

mile (mi) 1.609 kilometer (km)

Volume

cubic foot (ft3) 0.02832 cubic meter (m3)

acre-foot (acre-ft) 0.325851 million gallons (Mgal)

Flow rate

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

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

foot squared per day (ft2/day) 0.092903 meter squared per day (m2/d)

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

million gallons per day (Mgal/d) 0.0003785 million cubic meters per day (Mm3/d)

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.

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

°C=(°F–32)/ 1.8

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

at 25 °C). In this report, microsiemens per centimeter (μS/cm) is used for convenience.

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

(NAVD 88).

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

Abbreviations

ASTM American Society for Testing and Materials

EM electromagnetic

ft2/d foot squared per day

GaEPD Georgia Environmental Protection Division

gal/d gallon per day

gal/min gallon per minute

Kh horizontal hydraulic conductivity

Kv vertical hydraulic conductivity

LFA Lower Floridan aquifer

LFCU Lower Floridan confining unit

MAX maximum drawdown

NOAA National Oceanic and Atmospheric Administration

NTU nephelometric turbidity unit

PCU platinum-cobalt unit

RMS root mean square

SI sample interval

UFA Upper Floridan aquifer

USGS U.S. Geological Survey

μg microgram

μm micrometer

μg/L microgram per liter

mg/d milligram per day

mg/L milligram per liter
Acknowledgments

The following individuals provided abundant cooperation with the U.S. Geological Survey (USGS): 

Matt Saxon and Mark Williams of the Pooler Public Works Department provided helpful support 

throughout the study. Harry Jue and Holmes Bell of Hussey, Gay, Bell & DeYoung coordinated 

drilling and USGS testing activities at the test site. Rowe Drilling Company, Inc., drilled two 

boreholes and completed them as wells, provided and operated the packer assembly, and 

installed the test pump and provided assistance monitoring two aquifer performance tests. 

TestAmerica Laboratories, Inc., performed water chemistry analyses on collected water-quality 

samples and S&ME, Inc., performed hydraulic analyses on cores.

The author extends his appreciation for the dedicated work provided by O. (Gary) Holloway and 

Michael D. Hamrick, USGS. Gary Holloway fabricated the slug used for slug tests, designed 

and constructed the aquifer-test plumbing to remove discharge water from the site, and 

measured pump-discharge rate. Gary Holloway and Michael Hamrick provided the teamwork 

for monitoring drill-site activities and collecting hydrogeologic and water-quality data, which 

included drill cuttings, core samples, geophysical logging, flowmeter surveys, water-quality 

samples, and continuous and manual water-level measurements before, during, and after 

aquifer tests. 
Hydrogeology and Water Quality of the Floridan Aquifer 

System and Effect of Lower Floridan Aquifer Pumping 

on the Upper Floridan Aquifer, Pooler, Chatham County, 

Georgia, 2011–2012

By Gerard J. Gonthier 

Abstract

Two test wells were completed in Pooler, Georgia, in 

2011 to investigate the potential of using the Lower Floridan 

aquifer as a source of water for municipal use. One well 

was completed in the Lower Floridan aquifer at a depth 

of 1,120 feet (ft) below land surface; the other well was 

completed in the Upper Floridan aquifer at a depth of 486 ft 

below land surface. At the Pooler test site, the U.S. Geological 

Survey performed flowmeter surveys, packer-isolated slug 

tests within the Lower Floridan confining unit, slug tests of the 

entire Floridan aquifer system, and aquifer tests of the Upper 

and Lower Floridan aquifers.

Drill cuttings, geophysical logs, and borehole flowmeter 

surveys indicate that the Upper Floridan aquifer extends 

333 –515 ft below land surface, the Lower Floridan confining 

unit extends 515–702 ft below land surface, and the Lower 

Floridan aquifer extends 702–1,040 ft below land surface.

Flowmeter surveys indicate that the Upper Floridan 

aquifer contains two water-bearing zones at depth intervals 

of 339 –350 and 375–515 ft; the Lower Floridan confining 

unit contains one zone at a depth interval of 550–620 ft; 

and the Lower Floridan aquifer contains five zones at depth 

intervals of 702–745, 745–925, 925–984, 984–1,015, and 

1,015–1,040 ft. Flowmeter testing of the test borehole open 

to the entire Floridan aquifer system indicated that the Upper 

Floridan aquifer contributed 92.4 percent of the total flow 

rate of 708 gallons per minute; the Lower Floridan confining 

unit contributed 3.0 percent; and the Lower Floridan aquifer 

contributed 4.6 percent.

Horizontal hydraulic conductivity of the Lower Floridan 

confining unit derived from slug tests within three packerisolated intervals ranged from 0.5 to 10 feet per day (ft/d). 

Aquifer-test analyses yielded values of transmissivity for the 

Upper Floridan aquifer, Lower Floridan confining unit, and the 

Lower Floridan aquifer of 46,000, 700, and 4,000 feet squared 

per day (ft2/d), respectively. Horizontal hydraulic conductivity 

of 4 ft/d for the Lower Floridan confining unit, derived from 

aquifer-test analyses, is near the midrange for values derived 

from packer-isolated slug tests. The transmissivity of the entire 

Floridan aquifer system derived from aquifer-test analyses totals 

about 51,000 ft2/d, similar to the value of 58,000 ft2/d derived 

from open slug tests on the entire Floridan aquifer system.

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 

(less than 1 foot) water-level responses to aquifer-test pumping.

During the 72-hour aquifer test of pumping the Lower 

Floridan aquifer, a drawdown response of 51.7 ft was observed 

in the Lower Floridan pumped well and a drawdown response 

of 0.9 foot was observed in the Upper Floridan observation 

well located 85 ft from the pumped well.
2 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Introduction

The City of Pooler is located in western Chatham County, 

Georgia (Ga.), near the City of Savannah (fig. 1). Public water 

supply within the city is predominantly derived from groundwater withdrawn from two wells (36Q283 and 36Q348), 

completed in the Upper Floridan aquifer (UFA). Concern over 

saltwater intrusion at Hilton Head Island, South Carolina, has 

resulted in increased restrictions on groundwater withdrawal 

from the UFA by the Georgia Environmental Protection 

Division (GaEPD) in the Chatham County area. To meet the 

growing water demand in the 24-county coastal Georgia area, 

GaEPD has encouraged usage of alternative sources of water 

to the UFA, including wells completed in the Lower Floridan 

aquifer (LFA). The City of Pooler seeks to use the LFA for 

municipal water use.

Pumping from the LFA may locally increase the vertical 

hydraulic head gradient between the UFA and LFA, induce 

leakage (groundwater flow) from the UFA to the LFA, and 

lower water levels in the UFA. As a result, the GaEPD 

requires an assessment of these effects as a permitting requirement. In January 2003, GaEPD released an interim strategy 

for permitting LFA groundwater withdrawals in the 24-county 

coastal Georgia area (Nolton Johnston, Georgia Environmental 

Protection Division, written commun., January 28, 2003).

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

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

with the City of Pooler, performed an investigation during 

2011–2012 to determine the hydrogeology and water quality 

of the Floridan aquifer system and the potential effect that 

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

included construction of a test well in the UFA and a test well 

in the LFA, detailed site investigations, and hydraulic characterization of the Floridan aquifer system.

Purpose and Scope

This report documents results of field investigations 

completed at Pooler, Ga., during 2011–2012 to determine 

the hydrogeology and water quality of the Floridan aquifer 

system and to provide data needed to assess the effect of LFA 

pumping on the UFA, specifically to:

• Determine hydraulic and water-quality characteristics 

of the UFA, LFA, and the intervening Lower Floridan 

confining unit (LFCU), and

• Identify how pumping the LFA affects water levels 

in the UFA.

Field investigations included:

• Boring a 1,131-foot (ft)-deep test hole and 

constructing a 1,120-ft-deep test well completed 

in the LFA;

• Collecting drill cuttings and borehole geophysical 

logs at the test well;

• Sampling core within the LFCU for analysis of

vertical hydraulic conductivity (Kv) and porosity;

• Performing flowmeter surveys throughout the 

Floridan aquifer system in the open test hole and 

in the completed LFA well;

• Performing slug tests in packer-isolated intervals 

in the open test hole within the LFCU, and 

performing slug tests on the test hole open to 

the entire Floridan aquifer system;

• Collecting depth-integrated water samples to assess 

water quality of various water-bearing zones;

• Boring and constructing a 486-ft-deep observation 

well completed in the UFA; and

• Performing a 24-hour aquifer test at the test well

open to the UFA, and a 72-hour aquifer test at the

test well open to the LFA.

Data and subsurface samples collected during field 

investigations facilitated a hydrogeologic description of the 

subsurface at the test-well site by (1) determining transmissivity of the LFA, LFCU, and UFA; (2) storage coefficient 

of the LFCU and UFA; (3) specific capacity of the LFA well; 

and (4) horizontal hydraulic conductivity (Kh) and Kv of the 

LFCU. In conjunction with digital simulation and further 

analyses, this information can aid in determining the amount 

of pumping reduction in the UFA required by GaEPD to offset 

drawdown and leakage resulting from pumping a new LFA 

well located at Pooler, Ga. Results of this investigation add 

to the body of knowledge needed to characterize the Floridan 

aquifer system on a regional basis.

Previous Studies

Recent hydrologic investigations by the USGS, in 

cooperation with the U.S. Army, characterized the hydrogeology and groundwater flow in the Floridan aquifer system at 

Hunter Army Airfield in Chatham County, Ga., about 7 miles 

(mi) southeast of the Pooler test site (Clarke and others, 2010; 

Williams, 2010), and at Fort Stewart in Liberty County, Ga., 

about 23 mi southwest of the Pooler test site (Clarke and 

others, 2011; Gonthier, 2011). These investigations included 

data collection and groundwater-model simulations to 

determine the hydrogeology and water quality of the Floridan 

aquifer system and to provide information needed to assess 

the effect of LFA pumping on the UFA. The method of study 

of the current investigation at the Pooler test site is identical to 

the two earlier investigations at Hunter Army Airfield and 

Fort Stewart. 

A revised hydrogeologic framework for the Floridan 

aquifer system was developed by Williams and Gill (2010) 

for eight northern coastal counties in Georgia and five coastal 

counties in South Carolina, including the area surrounding 

Pooler, Ga. Borehole-geophysical and flowmeter-survey logs 

collected during previous investigations were used by Williams 
Introduction 3

Pooler

BULLOCH

BRYAN

LIBERTY

EFFINGHAM

TATTNALL

EVANS

JASPER

LONG

GA

SC

BEAUFORT

Hinesville

Richmond 

Hill

81°0' 80°45'

 Hilton

 Head

Island

81°45' 81°30' 81°15'

32°0'

32°15'

31°45'

0 0.5 1.0 1.5 MILES

0 0.5 1.0 1.5 KILOMETERS

Image from U.S Geological Survey High Resolution Orthoimagery

for Coastal Georgia, 0.5-meter pixel resolution, 2007 

ATLANTIC

OCEAN

Figure 1. Location of Pooler test site (wells 35Q069 and 35Q070) south of Pooler, Georgia.

Area

of map

below

right

Area

of map

above

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 5 10 15 KILOMETERS

FLORIDA

GEORGIA

SOUTH CAROLINA

PLAIN

COASTAL

Southeast

Georgia

embayment

Skidaway

Island

CHATHAM

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

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

l

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

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

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

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

–10

–10

–30

–20

–40

–50

–60

–70

–80

–90

Savannah

Pooler

16

95

95

95

16

Garden

City

Port

Wentworth

Brunswick

33R045

36Q330 36Q397

33P028

33P029 33P025

35P110

35P125

36Q283

36Q348

35Q069

35Q070

02198980

36Q020 36Q392

Streamgage and identification number

EXPLANATION

24-county coastal Georgia area

 as identified by the Georgia

 Environmental Protection Division 

Potentiometric contour May 1998—

 Contour interval is 10 feet. 

 Hachures indicate depression. 

 Datum is NAVD 88 

Weather station and number

Well and identification number

0

097847

02198980

33P025

Hunter Army Airfield

Fort Stewart

N

097847

Figure 1. Location of test site (wells 35Q069 and 35Q070), Pooler, Georgia. 
4 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

and Gill (2010) to shift the position of hydrogeologic vertical 

boundaries of the Upper and Lower Floridan aquifers and of 

individual permeable zones contained within these aquifers. 

The investigation included a deep test well (36Q330) drilled at 

Berwick Plantation in Chatham County, Ga., located 3.73 mi 

southeast of the Pooler test site, which provided data that helped 

correlate hydrogeologic-unit depths at the Pooler test site. 

Site Description

The Pooler test site is characterized by flat topography 

capped with sandy topsoil typical of the Georgia coastal 

area (Clark and Zisa, 1976). Land-surface altitude is about 

19 ft above the North American Vertical Datum of 1988 

(NAVD 88). The LFA well at the Pooler test site (well 35Q069) 

is located within Pooler city limits, about 1.7 mi southwest of 

the intersection of Interstates 16 and 95 (fig. 1).

The study area has a mild climate with warm, humid 

summers and mild winters. Long-term climatic patterns in 

the area are derived from records provided by the National 

Weather Service Station at Savannah International Airport 

(climatological station “Savannah WSO Airport, Georgia 

[097847],” accessed at http://www.sercc.com/cgi-bin/sercc/

cliMAIN.pl?ga7847, on September 27, 2011). During 

1971–2000, precipitation at station 097847 averaged about 

49 inches per year (in/yr).

Water Use

Groundwater use in Chatham County totaled 

64.97 million gallons per day (Mgal/d) during 2005 (Fanning 

and Trent, 2009). About 50 percent (33.52 Mgal/d) was 

withdrawn for public supply. Groundwater is supplied by 

about 60 wells in Chatham County (Fanning and Trent, 2009). 

Although most of these wells are located in Savannah, several 

are located to the northwest, serving Pooler, Garden City, and 

Port Wentworth (Fanning and Trent, 2009). Payne and others 

(2005) estimated that during 1980–2000, close to 95 percent 

of groundwater withdrawn from the county was derived from 

the UFA, with the remaining 5 percent obtained from the LFA. 

Groundwater withdrawal from the Floridan aquifer system 

in Chatham County increased from 79.75 Mgal/d in 1980 to 

85.54 Mgal/d in 1990, and decreased to 68.15 Mgal/d in 2000 

(Payne and others, 2005). 

Water demand for the City of Pooler has increased with 

increasing population. Pooler’s population has increased from 

4,453 in 1990 and 6,162 in 2000 to 19,140 in 2010 (U.S. Census, 

2002; U.S. Census accessed February 21, 2012, at http://2010.

census.gov/2010census/popmap/ipmtext.php?fl=13:1362104). 

To satisfy its increasing water demand, Pooler began purchasing 

water from the City of Savannah in 1997. Pooler currently has two 

production wells (36Q283 and 36Q348; fig. 1) and is permitted 

to withdraw an annual daily average of 697,000 gallons per day 

(gal/d) from the Floridan aquifer system (Carol Couch, Director 

of the Georgia Environmental Protection Division, written 

commun., December 30, 2008). 

Hydrogeologic Setting

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

strata consisting of consolidated to unconsolidated layers 

of sand and clay and semiconsolidated to dense layers of 

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

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

three major aquifer systems, in order of descending depth: 

the surficial aquifer system, the Brunswick aquifer system, 

and the Floridan aquifer system (fig. 2). Within the vicinity of 

Pooler, near Savannah, the Brunswick aquifer system has low 

permeability, with no discernible water-bearing units.

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

consists of Miocene and younger interlayered sand, clay, 

and thin limestone beds (Clarke, 2003). Near the eastern 

boundary of the Hunter Army Airfield, about 8 mi southeast 

of the Pooler test site, the surficial aquifer system is about 

100 ft thick and consists of an unconfined to semiconfined 

upper sand at a depth of 11–20 ft below land surface, and a 

confined lower sand at a depth of 37–56 ft below land surface. 

Well 36Q397, completed in the lower confined sand of the 

surficial aquifer system at the eastern boundary of Hunter 

Army Airfield, was pumped at a rate of 50 gallons per minute 

(gal/min) in 2011 (Gonthier, 2012). At Hunter Army Airfield, 

a confining unit consisting of silty clay and dense, phosphatic 

Miocene limestone (Clarke and others, 2011) transitions 

the base of the surficial aquifer system into the underlying 

Brunswick aquifer system.

The Brunswick aquifer system (fig. 2) consists of two 

water-bearing zones in the Brunswick, Ga. area—the upper 

Brunswick aquifer and the lower Brunswick aquifer (Clarke, 

2003). The upper Brunswick aquifer consists of poorly sorted, 

fine to coarse, slightly phosphatic and dolomitic quartz sand, 

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

The lower Brunswick aquifer consists of upper Oligocene and 

lower Miocene poorly sorted, fine to coarse, phosphatic and 

dolomitic sand (Clarke and others, 1990). The productivity 

of the Brunswick aquifer system is greatest in the vicinity of 

the Southeast Georgia embayment (Clarke, 2003), which is 

centered around St. Marys, Ga., near the Atlantic Coast, just 

north of the Georgia–Florida border (Miller, 1986; fig. 1). In 

Chatham County, the lower Brunswick aquifer is about 16 ft 

thick (Weems and Edwards, 2001), compared to 121 ft thick in 

the City of Brunswick. At well 36Q397 at the eastern boundary 

of Hunter Army Airfield, the Brunswick aquifer system is about 

200 ft thick and consists of mostly fine-grain material having 

no major water-bearing units (Gonthier, 2012).

The principal source of water for all uses (excluding 

thermoelectric) in the coastal area of Georgia is the Floridan 

aquifer system (fig. 2). Williams and Gill (2010) noted that the 

Floridan aquifer system consists of a fairly thick sequence of 

carbonate rocks of mostly upper and middle Eocene age. The 

Floridan aquifer system is about 800 ft thick at the Berwick 

Plantation well (36Q330), about 3.73 mi southeast of the 

Pooler test site (Williams and Gill, 2010). The Floridan aquifer 

system is overlain by the silty clay and dense phosphatic lower 
Introduction 5

Oligocene dolomite of the “upper confining unit” that separates the aquifer system from the overlying permeable units 

of the Brunswick aquifer system. Reported Kv of the upper 

confining unit, based on laboratory analysis of core, ranges 

from 2.3×10–  4 to 3 feet per day (ft/d; Clarke and others, 2004) 

with two values reported for Chatham County of 8×10– 4 and 

4×10– 3 ft/d (Furlow, 1969). The base of the Floridan aquifer 

system is underlain by low-permeability carbonate and clastic 

rocks of the “lower confining unit” (Williams and Gill, 2010).

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

upper Eocene to lower Oligocene limestone and dolomite. 

Williams and Gill (2010) reported an aquifer thickness in 

Figure 2. Generalized correlation of geologic and hydrogeologic 

units in the Coastal Plain of Georgia.

Post-Miocene

Miocene

Eocene

Oligocene

Paleocene

Upper

Cretaceous

Undifferentiated

Suwannee Limestone

Ocala Limestone

Avon Park Formation

Oldsmar Formation

Cedar Keys Limestone

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

Coastal Plain

Geologic unit 1 Hydrogeologic unit 2

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

Lower waterbearing zone

Upper 

Floridan semiconfining unit

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

Lower Middle Upper Lower Middle Upper

Lower

Upper

western Chatham County near Pooler, Ga., between 200 and 

250 ft. Reported transmissivity of the UFA in Chatham County 

ranges from 20,000 to 80,000 feet squared per day (ft2/d; 

Clarke and others, 2004). Zones of high hydraulic conductivity 

exist within relatively thin intervals of the Floridan aquifer 

system, especially in the UFA (Clarke and others, 2004).

The UFA is underlain by the LFCU, consisting of dense, 

recrystallized middle Eocene limestone and dolomitic limestone that hydraulically separates, to varying degrees, the UFA 

from the LFA (fig. 2). Counts and Donsky (1963) reported that 

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

laboratory analysis of a single core from Chatham County. The 

position and thickness of the LFCU was recently remapped 

in the area on the basis of flowmeter surveys and borehole 

geophysical logs (Williams and Gill, 2010). The LFCU is 

between 150 to 200 ft thick in western Chatham County in the 

vicinity of the Pooler test site and lies at an altitude between 

about –520 and –680 ft NAVD 88.

The LFA consists of middle Eocene limestone and 

dolomitic limestone but can include Paleocene and Upper 

Cretaceous carbonate units. As with the UFA, the LFA consists 

of several permeable water-bearing zones that are separated 

by layers of dense carbonate deposits. In western Chatham 

County in the vicinity of the Pooler test site, the LFA exists at 

altitudes ranging from about –680 to –1,120 ft NAVD 88, and 

has a reported thickness of about 450 ft (Williams and Gill, 

2010). Transmissivity of the LFA at Berwick Plantation well 

36Q330, 3.73 mi southeast of the Pooler test site, in Chatham 

County, Ga., is 8,200 ft2/d, (Clarke and others, 2004). Well 

36Q392 completed in the LFA at Hunter Army Airfield, 7 mi 

southeast of the Pooler test site in Chatham County, has a 

transmissivity of 11,000 ft2/d (Williams, 2010).

The LFA is underlain by a lower confining unit (not to 

be confused with the LFCU), which consists of lower Eocene 

marl of low permeability (Williams and Gill, 2010; fig. 2). 

Falls and others (2005) describe the lithology of the lower 

confining unit as a semi-indurated, fine-grained mixture of 

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

by clay and silt. In parts of the coastal area, the base of the 

Floridan aquifer system and the underlying marl is recognized 

on natural-gamma logs by a sharp increase in counts per 

second (Falls and others, 2005).

Well Identification

Wells in this report are identified by a USGS numbering 

system based on the index of USGS topographic maps (such 

as 35Q069). In Georgia, each 7-1/2-minute topographic 

quadrangle map has been given a number and letter designation 

beginning at the southwestern corner of the State. Numbers 

increase eastward through 39, and letters increase alphabetically 

northward through “Z” and then become double-letter designations “AA” through “PP.” The letters “I” and “O” are not used. 

Wells inventoried in each quadrangle are numbered sequentially 

beginning with “1.” For example, well 35Q069 is the 69th well 

inventoried in the Meldrim SE quadrangle (map 35Q).

Figure 2. Generalized correlation of geologic and hydrogeologic 

units in the Coastal Plain of Georgia.
6 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Hydrogeology and Water Quality 

of the Floridan Aquifer System

To assess the hydrogeology and water quality of the 

Floridan aquifer system at the Pooler test site, multidiscipline 

site investigations were performed during 2011–2012 to 

collect and analyze geologic, geophysical, hydrologic, meteorological, and water-chemistry data. Analysis of these data 

provided a basis for refining the depth, thickness, hydraulic 

properties, and water quality of hydrogeologic units that 

compose the Floridan aquifer system in Chatham County, Ga. 

Methods of Data Collection and Analysis

Hydrogeology and water quality of the Floridan aquifer 

system at the Pooler test site were assessed by installing two 

wells and performing geophysical logging, flowmeter surveys, 

water-quality sample collection and analyses, collection and 

hydraulic analysis of cores, packer-isolated and open slug 

tests, a 24-hour aquifer test of the UFA, and a 72-hour aquifer 

test of the LFA. Well installation included drilling a 1,131-ftdeep test hole followed by constructing a new 1,120-ft-deep 

well (35Q069) completed in the LFA (fig. 1). An observation 

well was constructed in the UFA (35Q070) 85 ft north of the 

new LFA well. Well construction information for all wells 

used during this study is listed in table 1. Data collection in 

the new test wells included borehole geophysical logging, 

flowmeter surveys, water-quality sampling and analysis, core 

hydraulic analysis, packer-isolated slug tests, and aquifer tests. 

Test Drilling and Well Installation

Mud-rotary drilling extended a borehole 302 ft into the 

sediments above the Floridan aquifer system at the Pooler 

test site. Drilling extended the borehole to 339 ft below land 

surface, just below the top of the Floridan aquifer system 

(figs. 3 and 4). A 24-inch-diameter surface casing was installed 

into the open hole to a depth of 80 ft; a 16-inch-diameter 

casing was installed to a depth of 339 ft. 

Air-rotary drilling extended the borehole to a depth 

of 1,131 ft. An 8-inch-diameter casing was installed in 

the 318 –705 ft depth interval, and the well (35Q069) was 

completed as an open hole to the LFA within the depth interval 

of 705–1,120 ft (fig. 4). The bottom 11 ft of the original 

1,131 ft most likely was backfilled with native material.

The UFA well (35Q070) was drilled at a location 85 ft 

to the north of well 35Q069. Following installation of a 

14-inch-diameter casing to a depth of 80 ft, mud-rotary 

drilling extended the borehole to a depth of 340 ft, just below 

the top of the Floridan aquifer system (fig. 5). A 6-inchdiameter casing was installed to 340 ft below land surface. 

Air-rotary drilling completed the boring into the UFA to a 

depth of 486 ft. The well was left as an open hole to the UFA 

from 340 to 486 ft below land surface.

Table 1. Location and open intervals of wells used in the slug-test and aquifer-test analyses, Pooler, Georgia, 2011–2012, 

and wells from previous studies.

[Site locations, fig. 1; USGS, U.S. Geological Survey; NAVD 88, North American Vertical Datum of 1988; aquifer: FAS, Floridan aquifer system; 

UFA, Upper Floridan aquifer; LFA, Lower Floridan aquifer; SAS, surficial aquifer system; well 35Q069 was a test hole prior to being completed in 

the LFA; —, no data]

USGS

identifier

USGS

site number

Land-surface 

altitude

(feet above

NAVD 88)

Depth (feet below land surface)

Static water level Aquifer

March 26, 2012

Top of

open interval

Bottom of

open interval

35Q069 320356081162001 19 — 339 1,131 FAS

41.99 705 1,120 LFA

35Q070 320357081162001 19 41.51 340 486 UFA

36Q283 320656081145801 23 — 280 610 UFA

36Q348 320650081144001 24 — 250 600 UFA

33R045 320754081364301 85 17.82 745 994 LFA

35P110 315443081185902 9.52 –19.62 315 441.25 UFA

35P125 315443081185903 11 –17.91 1,010 1,095 LFA

36Q020 320021081124801 13 –31.02 330 336 UFA

33P028 315434081364701 81.76 — 895 1,255 LFA

33P029 315434081364702 81.36 — 460 560 UFA

33P025 315447081345601 89 — 420 520 UFA

36Q330 320139081134002 11 — 718 1,080 LFA

36Q392 320005081102101 20 — 703 1,112 LFA

36Q397 320146081073701 24 — 35 70 SAS
Hydrogeology and Water Quality of the Floridan Aquifer System 7

Figure 3. Lithology and geophysical properties of well 35Q069, Pooler, Georgia. [c.u., confining unit; μS/cm, microsiemens 

per centimeter at 25 degrees Celsius]

Caliper,

in inches

Gamma, in APIU

(American Petroleum

Institute Units)

Land surface is 19 feet above NAVD 88

8 12.5 17

Spontaneous

potential, in millivolts

0 50 100 150 –50 0 50 100 150 200

Lithology 0 375 750

0 375 750

0 375 750

Lateral resistivity,

in ohm-meters

Resistivity 64-inch

normal, in ohm-meters

Resistivity 16-inch 

normal, in ohm-meters

160 210 260

70 75 80

Drill fluid specific

conductance, μS/cm

Drill fluid temperature,

in degrees Fahrenheit

Hydrogeologic

 units

Geologic

 units

C-marker

D-marker

Test-hole

construction

Casing Open hole

50

Land

surface

100

350

400

600

150

200

250

300

450

500

550

650

700

750

800

850

900

950

1,000

1,050

1,100

Depth, in feet below land surface

Mudded borehole

Core sample

Packer-isolated

slug-test interval

Brunswick aquifer system

Surficialaquifer

system

Upper

Floridan

aquifer

Lower

Floridan

confining

unit

PostMiocene

Miocene

Oligocene

Upper

Eocene

Middle

Eocene

Lower

Floridan

aquifer

Lower

confining

unit

Upper Floridan

confining unit

Upper

Brunswick

confining

unit

Upper 

Brunswick

aquifer

Lower

Brunswick

aquifer

Lower Brunswick

confining unit

Sand and gravel

Clay

Sand and gravel 

with clay layers

Shells, gravel

and sand mix

Sand with 

hard clay layers

Sand, very fine,

and clay

Clay

Clay

Clay, sandy

Sand, silty

Sand, fine gravel Sand, with some hard clay

Limestone

Limestone

Limestone,

loose shelly

Limestone

Limestone

Limestone, loose

Limestone and gypsum

Limestone

Limestone

Limestone

Limestone

Shells and

limestone

Limestone,

porous

Limestone, porous

Limestone and gypsum

Limestone and gypsum

Limestone

Limestone

Limestone

Limestone, porous

Limestone, porous

Limestone

and silt

Figure 3. Lithology and geophysical properties of well 35Q069, Pooler, Georgia. [c.u., confining unit; μS/cm, microsiemens 

per centimeter at 25 degrees Celsius]

Hydrogeology and Water Quality of the Floridan Aquifer System 7
8 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Land

surface

Land surface is 19 feet above NAVD 88

SCALE HORIZONTALLY EXAGGERATED

Lithology 24-inch-diameter casing Grout

80 feet

318 feet

339 feet

16-inch-diameter

casing

8-inchdiameter

casing

8- to

11-inchdiameter

open

hole

Backfill

50

100

350

400

600

Hydrogeologic

 unit

Brunswick

aquifer

system

Surficial 

aquifer

system

150

200

250

300

450

500

550

650

700

750

800

850

900

950

1,000

1,050

1,100

1,150

Upper

Floridan

aquifer

Lower

Floridan

confining

unit

Lower

Floridan

aquifer

Lower

confining

unit

Upper Floridan

confining unit

Depth, in feet below land surface

705 feet

1,120 feet

Figure 4. Hydrodrogeologic units and well completion diagram of Lower Floridan aquifer

well 35Q069 at Pooler, Georgia. Lithology patterns are labeled in figures 3 and 7. Figure 4. Hydrogeologic units and well completion diagram of Lower 

Floridan aquifer well 35Q069, Pooler, Georgia. Lithology patterns are labeled 

in figures 3 and 7.
Hydrogeology and Water Quality of the Floridan Aquifer System 9

80 feet

486 feet

340 feet

6-inchdiameter

open

hole

Brunswick

aquifer

system

Surficial 

aquifer

system

Upper

Floridan

aquifer

Lower

Floridan

confining

unit

Lower

Floridan

aquifer

Upper Floridan

confining unit

Figure 5. Hydrogeologic units and well completion diagram of Upper Floridan aquifer 

well 35Q070 at Pooler, Georgia. Lithology patterns are labeled in figures 3 and 7. Grout

Land surface is 19 feet above NAVD 88

SCALE HORIZONTALLY EXAGGERATED

Hydrogeologic Lithology

 unit

Land

surface

50

100

350

400

600

150

200

250

300

450

500

550

650

700

750

800

850

900

950

1,000

1,050

Depth, in feet below land surface

14-inch-diameter casing

6-inchdiameter

casing

Figure 5. Hydrogeologic units and well completion diagram of Upper 

Floridan aquifer well 35Q070, Pooler, Georgia. Lithology patterns are labeled 

in figures 3 and 7.

Hydrogeology and Water Quality of the Floridan Aquifer System 9
10 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Lithologic and Borehole Geophysical Logs

Drill cuttings collected every 10 ft from test hole 35Q069 

were identified for grain size and mineral content; within the 

carbonate sequence, the amount of induration (cementation 

between grains) and the percentage of shells and recognized 

shell bits were also recorded. Although it is likely that the 

carbonate sequence contains dolostone, cuttings were not 

specifically tested for the presence of dolomite, and so the 

carbonate sequence is referred to as “limestone” in this 

report. In addition to drill cuttings, limestone cores were 

extracted and examined for the depth intervals of 564.3–569.8, 

600 – 605, and 640–648.4 ft (fig. 3). Secondary porosity in 

the form of fractures and dissolution features present within 

the core was not discernible in drill cuttings from the same 

depth intervals. Drilling fluid also was sampled every 10 ft for 

specific conductance and temperature.

Borehole-geophysical logs were collected at various 

stages of drilling well 35Q069 to characterize the physical 

properties of penetrated sediments, rock, and interstitial fluid. 

The first set of logs was collected in the 0–339 ft interval 

where mud-rotary drilling penetrated clastic sediments overlying the Floridan aquifer system. The second set of logs was 

collected in the 339–1,131 ft interval where air-rotary drilling 

was used to penetrate the carbonates of the Floridan aquifer 

system. In both intervals, the following logs were collected: 

caliper (shown on figure 3 as the inner diameter of the casing 

from 0 –339 ft); natural gamma; spontaneous potential, and 

single-point lateral, long- and short-normal resistivity. In 

the deeper carbonate interval, borehole-fluid resistivity and 

temperature were collected.

Flowmeter Survey

Flowmeter surveys were performed to quantify the 

relative contributions of flow from water-bearing zones within 

the Floridan aquifer system including confinement between 

the UFA and LFA (fig. 6). Two flowmeter surveys were 

performed in well 35Q069 at two different stages of the well’s 

construction. The first flowmeter survey was performed when 

the well was still a test hole open to the entire Floridan aquifer 

system. The second flowmeter survey was performed after the 

well was complete and was open only to the LFA. To perform 

a flowmeter survey, the well was pumped while several 

traverses were made in the open borehole with an electromagnetic (EM) flowmeter that measured accumulated flow up the 

borehole. The first flowmeter survey pumped the test hole for 

well 35Q069 at a rate of 708 gal/min on December 8, 2011. 

Information from this flowmeter survey (1) ensured accurate 

placement of the well casing below the LFCU during 

completion of the LFA well, and (2) was used to calculate the 

concentrations of constituents in water from intervals between 

composite grab samples. The second flowmeter survey 

pumped the completed LFA well 35Q069 at a rate of about 

783 gal/min on April 18, 2012, in conjunction with a 72-hour 

aquifer test. The second flowmeter survey did not affect the 

results of the 72-hour aquifer test. The water-level hydrograph of the pumped well during the flowmeter survey was 

inspected. There was no indication that the flowmeter survey 

created water-level fluctuations during the aquifer test.

Water-Level Measurements

Continuous and intermittent groundwater-level measurements were made at the borehole according to USGS standard 

procedures (Garber and Koopman, 1968; Stallman, 1971; and 

Cunningham and Schalk, 2011). Manual intermittent waterlevel measurements were made for calibration of groundwaterlevel recorder readings and for direct monitoring of background water-level trends during the performance of the slug 

tests. Manual measurements were made to the nearest 0.01 ft 

using an electric tape and following procedures described in 

Garber and Koopman (1968). Continuous groundwater-level 

recorders were equipped with submerged, vented pressure 

transducers. For long-term monitoring, pressure transducers 

collected water-level measurements every 15 minutes; for the 

slug-test monitoring, pressure transducers collected waterlevel measurements every second to every 15 minutes.

Core Hydraulic Analysis and Packer-Isolated 

Slug Tests

Core samples were collected and analyzed for Kv and 

porosity at a testing laboratory. Packer-isolated slug tests were 

completed in the borehole to estimate horizontal hydraulic 

properties. Relatively undisturbed core samples were collected 

at depth intervals of 564.3–569.8, 600–605, and 640–648.4 ft 

and submitted to S&ME, Inc. (Atlanta, Ga.) for hydraulic 

testing of Kv and porosity (table 2). To retain the undisturbed 

nature of these largely consolidated core samples, the samples 

were preserved onsite using procedures described in American 

Society for Testing and Materials (ASTM) D5079 (2008) 

and analyzed using a flexible wall permeameter following 

procedures described in ASTM D5084 (2010).

Table 2. Estimated vertical hydraulic conductivity and porosity 

of core samples collected from the Lower Floridan confining unit 

in test hole 35Q069, Pooler, Georgia, November 2011.

[Analyses by S&ME, Inc., Atlanta, Georgia; ft/d, foot per day]

Core sample interval 

(feet below

land surface)

Vertical hydraulic 

conductivity

(ft/d)

Porosity

564.3–569.8 1.67 0.34

600–605 1.08 0.33

640–648.4 0.57 0.33
Hydrogeology and Water Quality of the Floridan Aquifer System 11

SI-1

SI-2

SI-3

SI-4

SI-5

3.6

40.0

12.6

31.4

3.3

7.4

1.7

6.3

3.0

2.7

0.9

0.8

0.2

86.1

Figure 6. Water-quality-sample collection locations in test hole 35Q069 and flowmeter survey data from (A) pumping 

the Floridan aquifer system prior to installing an 8-inch casing in test hole 35Q069, and (B) pumping the Lower Floridan 

aquifer after installing an 8-inch casing in test hole 35Q069 and completing it as well 35Q069 near Pooler, Georgia. 

[SI, sample interval between water-quality samples]

Upward borehole flow (gallons per minute)

Land surface is 19 feet above NAVD 88

0 354 708 0 30 60

Sand and gravel

Clay

Sand and gravel 

with clay layers

Shells, gravel

and sand mix

Lithology

0 391.5 783

Upward borehole flow

(gallons per minute)

50

Land

surface

100

350

400

600

Hydrogeologic

 units

Geologic

 units

Brunswick aquifer system

Surficialaquifer

system

Sand with 

hard clay layers

Sand, very fine,

and clay

Clay

Clay

Clay, sandy

Sand, silty

Sand, fine gravel Sand, with some hard clay

150

200

250

300

450

500

550

650

700

750

800

850

900

950

1,000

1,050

1,100

Limestone

Limestone

Limestone,

loose shelly

Limestone

Limestone

Limestone, loose

Limestone and gypsum

Limestone

Same flowmeter

log plotted at

different scales

Stationary

point

measurements

Water-quality

sample

Open circles

and bars

represent

interpreted

flow

Individual

stationary

flowmeter

sample point

Bottom of 

16-inch-diameter

casing

Bottom of 

8-inch-diameter

casing

Limestone

Limestone

Limestone

Shells and

limestone

Limestone,

porous

Limestone, porous

Upper

Floridan

aquifer

Lower

Floridan

confining

unit

PostMiocene

Miocene

Oligocene

Upper

Eocene

Middle

Eocene

Lower

Floridan

aquifer

Lower

confining

unit

Upper Floridan

confining unit

Limestone and gypsum

Limestone and gypsum

Limestone

Limestone

Limestone

Limestone

and silt

Limestone, porous

Limestone, porous

Depth, in feet below land surface

Upper

Brunswick

confining

unit

Upper Brunswick aquifer

Lower

Brunswick

aquifer

Lower Brunswick

confining unit

Percent

contribution

of flow

Percent

contribution

of flow

A. Floridan aquifer system

Flowmeter survey range: 339–1,110 feet

B. Lower Floridan aquifer

Flowmeter survey range: 705–1,110 feet

Figure 6. Water-quality-sample collection locations in test hole 35Q069 and flowmeter-survey data from (A) pumping the 

Floridan aquifer system prior to installing an 8-inch-diameter casing in test hole 35Q069, and (B) pumping the Lower Floridan 

aquifer after installing an 8-inch-diameter casing in test hole 35Q069 and completing it as well 35Q069, Pooler, Georgia. 

[SI, sample interval between water-quality samples]
12 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Packer-isolated slug tests at selected intervals—

612–618 ft, 642– 648 ft, and 690–696 ft (fig. 3)—during 

December 13–14, 2011, were performed in the test hole for 

well 35Q069 to obtain a sample of values of Kh of the LFCU 

(table 3). Each test interval was isolated using a straddle 

packer (fig. 7). The packer assembly sealed off a 6.05-ft depth 

interval of the aquifer and hydraulically connected the aquifer 

to a 3.45-inch-inner-diameter casing. A slug was inserted 

(falling-head test) or removed (rising-head test) from the water 

column within this connected casing. The slug consisted of 

a 10.17-ft-long, 2.37-inch-diameter polyvinyl-chloride pipe 

filled with sand and capped on each end. The volume of the 

slug totaled 0.31 cubic foot (ft3) and displaced 4.8 ft of the 

water column, which fluctuated within the 3.45-inch-innerdiameter casing during the test. The change in head (water 

level) with time was recorded during the tests.

Pressure transducers monitored water-level response 

within the isolated interval as well as above and below the 

zones being tested. The lower transducer was connected to the 

packer assembly just below the lower packer, 10.48 ft below 

the middle transducer. The upper transducer was lowered into 

the borehole, roughly 150 ft below land surface, within the 

part of the borehole that was cased off from the Miocene to 

post-Miocene sediments. Both packers in the packer assembly 

were connected to the same air line for inflation. The general 

procedures for conducting the packer-isolated slug 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 both packers to a specific pressure.

4. Retrieve and review water-level data to confirm reestablishment of static conditions after packer inflation.

5. Perform falling-head slug test by submerging slug into 

water column connected to packer-isolated interval to 

raise hydraulic head within the test interval.

6. Retrieve and review water-level data to confirm return 

to pre-test conditions and to assess any leakage beyond 

the packer-isolated interval.

7. Increase pressure to further inflate packers.

8. Retrieve and review water-level data to confirm reestablishment of static conditions after packer inflation.

9. Perform rising-head slug test by removing slug from 

water column connected to packer-isolated interval to 

lower hydraulic head within the test interval.

10. Retrieve and review water-level data to confirm return 

to pre-test conditions and to assess any leakage beyond 

the packer-isolated interval.

Table 3. Hydraulic conductivity values for depth intervals emphasizing the Lower Floridan confining unit in test hole 35Q069, Pooler, 

Georgia, December 13–14, 2011.

[ft/d, feet per day; ft2/d, foot squared per day; hydrogeologic unit: LFCU, Lower Floridan confining unit; FAS, Floridan aquifer system; methods by Bouwer 

and Rice (1976) or van der Kamp (1976) are contained in spreadsheets from Halford and Kuniansky (2002); —, not applicable]

Depth interval 

(feet below 

land surface)

Hydrogeologic 

unit Type Month and day Time Analysis method

Horizontal 

hydraulic

conductivity 

(ft/d)

Transmissivity 

(ft2/d)a

612–618 LFCU Falling head December 13 3:29:26 p.m. Bouwer and Rice 10 —

612–618 LFCU Rising head December 13 3:39:32 p.m. Bouwer and Rice 10 —

642–648 LFCU Falling head December 14 8:46:48 a.m. Bouwer and Rice 3 —

642–648 LFCU Rising head December 14 9:01:08 a.m. Bouwer and Rice 3 —

690–696 LFCU Rising head December 14 12:08:45 p.m. Bouwer and Rice 0.5 —

339–1,130 FAS Falling head December 14 1:11:42 p.m. van der Kamp — 54,000

339–1,130 FAS Rising head December 14 1:17:58 p.m. van der Kamp — 62,000

a

A storage coefficient of 1.7×10–3 was used for the Floridan aquifer system.
Hydrogeology and Water Quality of the Floridan Aquifer System 13

The caliper log was used to position the packers in 

smooth sections of the borehole for good packer seating. The 

flowmeter survey and geophysical logs were used to locate 

parts of the borehole (penetrating the LFCU) that were especially tight or nonpermeable (fig. 3); these nonpermeable parts 

of the LFCU were targeted for slug tests. Packer pressure was 

increased from one test to the next at a given depth to better 

detect any possible leakage within the packers. A decrease in 

recovery rate with increased pressure to inflate packers would 

indicate the presence of leakage.

The Kh of the surrounding material of the LFCU was 

calculated using water-level-recovery data following a slug 

test and the Bouwer and Rice method (Bouwer and Rice, 

1976), which was contained in the spreadsheet developed 

by Halford and Kuniansky (2002). The log of the fraction of 

remaining water-level displacement to the initial displacement 

was plotted on the y-axis, and corresponding time was plotted 

on the x-axis. This semi-log plot depicts the exponential decay 

of the fractional displacement in water level, or water-level 

recovery, with time as a straight line. The spreadsheet by 

Halford and Kuniansky (2002) utilizes two analytical points 

on the water-level recovery line in an analysis plot. The slope 

of these two aligned points and the geometries of the well 

opening and aquifer are then used to estimate the Kh.

In addition to slug tests performed on packer-isolated 

intervals, falling-head and rising-head slug tests were 

performed in the test hole when it was open to the entire 

Floridan aquifer system prior to completion as well 35Q069. 

Water levels in the well exhibited underdamped responses 

(oscillations) during the slug tests. Transmissivity of the 

Floridan aquifer system was calculated using the underdamped 

water-level-recovery data obtained from the two slug tests, 

and the van der Kamp method (van der Kamp, 1976), which 

was contained in the spreadsheet developed by Halford and 

Kuniansky (2002). This spreadsheet allows positioning of 

analytical points on an analysis plot to define the frequency 

and damping rate of the oscillations. The frequency and 

damping rate of the oscillations, test-hole diameter, and a 

value of the coefficient of storage are used to estimate the 

transmissivity (table 3). Details and results of the slug tests 

are discussed in appendix 1.

6.05 ft

Middle

transducer

616-ft depth

Lower

transducer

625-ft depth

Upper

transducer

150-ft depth

Airline to

both packers

3.45-inchdiameter casing

Upper

packer

Lower

packer

Packer

isolated

interval

Figure 7. Straddle-packer assembly in test hole 

35Q069 as an example at the 612- to 618-feet packerisolated interval, Pooler, Georgia, December 13, 2011.

Casing Borehole

Clay with

some sand Limestone

NOT TO SCALE

Figure 7. Straddle-packer assembly in test hole 

35Q069 as an example at the 612- to 618-feet packerisolated interval, Pooler, Georgia, December 13, 2011.
14 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Aquifer Tests

Aquifer tests performed at the Pooler test site estimated 

the transmissivity of the Upper and Lower Floridan aquifers 

and quantified the effects of pumping one aquifer on water 

levels in the other aquifer (table 4 and fig. 8). Water levels 

were monitored in wells 35Q069 and 35Q070 during both 

aquifer tests (fig. 9). A 24-hour aquifer test was performed 

during March 27–28, 2012, in well 35Q070 open to the 

UFA, and a 72-hour aquifer test was performed during 

April 16–19, 2012, in well 35Q069 open to the LFA. 

Drawdown response to the 24-hour aquifer test was 

analyzed using the Cooper and Jacob (1946) method to 

determine the transmissivity of the UFA. Drawdown 

response to the 72-hour aquifer test was simulated using 

MODOPTIM (Halford, 2006b) to determine the transmissivity of the UFA, LFCU, and LFA, as well as the storage 

coefficient of the UFA and the LFCU. Aquifer test analyses 

and results are discussed in detail in appendix 2.

Drawdown Estimation

Drawdown for the two monitored wells was estimated by 

using a tool developed by Halford (2006a) to filter water-level 

data for effects of barometric pressure, earth tide, ocean tide, 

and long-term trends. The filtering tool has a time-series 

spreadsheet that is used to match (fit) synthetic water levels to 

the measured water levels during a period that is unaffected 

by an aquifer test or local pumping event (hereinafter referred 

to as the “fitting period”). Data and parameters that were used 

to fit the synthetic water levels to measured water levels are 

then used to fit synthetic water levels to measured water levels 

before the start of aquifer-test pumping and after the end of 

aquifer-test recovery. 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 yielded the drawdown (maximum 

drawdown listed in table 4). After filtering the water-level 

data for nonaquifer-test influences, the data were culled to 

be spaced evenly with log (time) to minimize the number of 

data points used to match simulated drawdown to measured 

drawdown. Water level used to estimate drawdown is in feet 

above NAVD 88. Drawdown estimation methods and results 

are discussed in detail in appendix 2.

Table 4. Summary of slug- and aquifer-test results, Pooler, Georgia, December 2011–April 2012.

[Hydrogeologic unit: FAS, Floridan aquifer system; UFA, Upper Floridan aquifer; LFCU, Lower Floridan confining unit; LFA, Lower Floridan aquifer; 

ft, foot; gal/min, gallon per minute; ft2/d, foot squared per day; —, not applicable]

Hydrogeologic 

unit

Slug test Aquifer test

Open hole 24-hour 72-hour

Test information

Location — Test hole 35Q069 Well 35Q070 Well 35Q069

Interval tested (ft) — FAS UFA LFA

Test date — December 14, 2011 March 27–28, 2012 April 16–19, 2012

Pumping rate (gal/min) — — 285 783

Analysis — van der Kamp Cooper-Jacob MODOPTIM with

MODFLOW

Maximum drawdown (feet)

Well 35Q070 UFA — 11.1 0.9

Well 35Q069 LFA — 0.09 51.7

Coefficients

Transmissivity (ft2/d) FAS 58,000 — 51,000

Transmissivity (ft2/d) UFA — 30,000 46,000

Storage UFA — — 5.90×10–4

Transmissivity (ft2/d) LFCU — — 700

Storage LFCU — — 4.50×10–4

Transmissivity (ft2/d) LFA — — 4,000
Hydrogeology and Water Quality of the Floridan Aquifer System 15

Figure 9. Water levels in wells 35Q069 and 35Q070 before, during, and after the 24-hour 

Upper Floridan and 72-hour Lower Floridan aquifer tests, Pooler, Georgia, March 27–

May 6, 2012. A, Pumping phase of Upper Floridan 24-hour aquifer test. B, Recovery phase 

of Upper Floridan 24-hour aquifer test. C, Early fitting period. D, Failed Lower Floridan 

aquifer test. E, Pumping phase of Lower Floridan 72-hour aquifer test. F, Recovery phase 

of Lower Floridan 72-hour aquifer test. G, Late fitting period. The fitting period is used to 

describe the match (or fit) of synthetic water levels to those that were measured.

–120

–100

–80

–60

–40

–20

0

–27

–26

–25

–24

–23

–22

–21

27 1

A B C D E F G

6 11 16 21 26 1 6

EXPLANATION

2012

Pumping phase of aquifer test

Recovery phase of aquifer test

Fitting period

Data gap in at least one well

Other pumping event

This graph is larger vertical scale from graph above

Water level in wells 35Q069 and 35Q070, in feet below NAVD 88

March April May

 35Q069

(Lower

Floridan

aquifer)

 35Q070

(Upper

Floridan

aquifer)

B. Schematic cross section

100

200

300

400

500

600

700

800

900

1,000

1,100

Land

surface

1,200

1,300

Depth, in feet below land surface

NOT TO SCALE

1,120 feet

486 feet

Well 35Q069Well

Brunswick aquifer

 35Q070

system

Upper Floridan c.u.

Surficial

aquifer 

Upper Floridan

aquifer

Lower Floridan

confining unit

Lower Floridan

aquifer

Lower 

confining

unit 

Well open interval

EXPLANATION

A. Aquifer-test layout

85 feet

Well 35Q070

(Upper Floridan

aquifer)

Well 35Q069

(Lower Floridan

aquifer)

N

Figure 8. Location and open interval of wells 

used for the 24-hour and 72-hour aquifer tests, 

Pooler, Georgia. A, Diagram showing aquifer-test 

layout. B, Schematic cross section showing major 

hydrogeologic units. [c.u., confining unit]
16 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Water-Quality Sampling and Analysis

To assess vertical distribution of water quality, the 

specific conductance and temperature of drilling fluid were 

measured at 10-ft depth intervals during air-rotary drilling of 

the 360 –1,131 ft interval in the test hole (fig. 3). The measurement procedure consisted of capturing a sample of drilling 

fluid as it emerged at land surface and measuring the specific 

conductance after every 10 ft of drilling progression. Although 

discharge water is a composite of all units exposed above a 

given depth, changes in specific conductance and temperature 

provide an indication of changes in water quality with depth in 

the test hole. Water was added to assist drilling the 360–370 ft 

depth interval. The added water resulted in lower values for 

specific conductance and temperature associated with the 

360 – 400-ft depth interval compared to all other depths. As 

a result, the data for the 360–410-ft depth interval were not 

used and are not shown in figure 3. 

Water samples were collected in the test hole for well 

35Q069 on December 8, 2011, immediately following a 

borehole-flowmeter survey using a wireline grab sampler at five 

distinct depths (table 5 and fig. 6). The test hole was pumped 

for at least an hour during the borehole-flowmeter survey before 

water samples were collected. Sample-collection points were 

located at depths of 339, 745, 845, 985, and 1,050 ft. Water 

collected at each sample-collection point is a composite of 

all water entering the well below the sample-collection point. 

These grab samples were collected with the pump set within the 

casing above all water-bearing intervals and pumping at a rate 

of 708 gal/min; therefore, each of these samples represents a 

composite of the water entering the test hole beneath a specific 

sampling depth. Water was transferred from the grab sampler 

to sample bottles using a peristaltic pump. Samples were 

analyzed for specific conductance, pH, alkalinity reported as 

calcium carbonate, and total dissolved solids, in additional to 

major ions, including calcium, magnesium, sodium, potassium, 

iron, manganese, sulfate, chloride, and fluoride (table 6). Water 

collected for major ions was filtered using a capsule filter with 

a 0.45-micrometer (μm) pore medium. Samples for cations 

were preserved with nitric acid. Samples were analyzed at 

TestAmerica Laboratories, Inc., Savannah, Ga. Cations were 

analyzed using induced coupled plasma; anions were analyzed 

using ion chromatography. Bicarbonate concentrations were 

calculated from values of alkalinity.

Table 5. Intervals between water-sample collection depths in pumped test hole 35Q069 immediately following a flowmeter survey, 

Pooler, Georgia, December 8, 2011. 

[<, less than]

Sample interval

(SI) number

Depth of sample interval

(feet below land surface)

Hydrogeologic unit(s) to which

 the sample interval is open

Percent

contribution

of flow Top

(also depth of

grab sample)

Bottom

(also depth of grab 

sample for the lower 

sample interval)

SI-1 339 745 Upper Floridan aquifer; Lower Floridan confining 

unit; small, upper part of Lower Floridan aquifer

96.3

SI-2 745 845 Central-upper part of Lower Floridan aquifer 3.0

SI-3 845 985 Central-lower part of Lower Floridan aquifer 0.5

SI-4 985 1,050 Lower part of Lower Floridan aquifer; small, 

upper part of lower confining unit

0.2

SI-5 1,050 1,120 Lower confining unit <0.01
Hydrogeology and Water Quality of the Floridan Aquifer System 17

Table 6. Water quality of composite samples and sample intervals immediately following a flowmeter survey in pumped test hole 35Q069, Pooler, Georgia, December 8, 2011.[Range of fluoride concentrations caused by censored values from composite samples. Composite-sample results and flow contribution from screens were used to calculate water quality in sample intervals. Criteria type: SDWR, Secondary Drinking Water Regulation; HBVs, health-based value for individuals on a 500 milligrams per day (mg/d) restricted sodium diet; μS/cm, microsiemens per centimeter at 25 degrees Celsius; SU, pH standard units; mg/L, milligram per liter; μg/L, microgram per liter; —, no water-quality criteria for this constituent or did not calculate concentration because of censored values from composite samples; <, less than]

Constituent

parameter Units

Water-quality values of 

composite samples with depth range 

(feet below land surface)

a Water quality of individual

sample intervals (SI) by depth range

(feet below land surface) Waterquality 

criteriak

Criteria

type 1,120–

1,050b

1,120–

985c

1,120–

845d

1,120–

745e

1,120–

339f

1,120–

1,050b

1,050–

985 g

985–

845h

845–

745i

745–

339j

SI-5 SI-4 SI-3 SI-2 SI-1

Specific conductance μS/cm 277 238 234 230 235 277 238 232 229 235

—

—

pH SU 8.17 8.04 8.02 8.05 8.00 8.17 8.04 8.01 8.06 8.00 6.5–8.5 SDWRAlkalinity mg/L 134 113 110 119 108 134 113 109 121 108

—

—

Total dissolved solids mg/L 204.0 182.0 184.0 168.0 199.0 204.0 182.0 184.8 164.2 200.2 500 SDWRCalcium mg/L 19.80 28.80 29.40 26.60 28.70 19.80 28.80 29.63 25.93 28.78

—

—

Magnesium mg/L 5.85 8.22 8.22 8.63 7.56 5.85 8.22 8.22 8.73 7.52

—

—

Sodium mg/L 30.50 10.90 10.10 9.05 8.50 30.50 10.90 9.80 8.80 8.48 20 HBVsPotassium mg/L 3.08 2.28 2.51 2.05 1.87 3.08 2.28 2.60 1.94 1.86

—

—

Iron μg/L 153 <100 <100 <100 <100 153

—

—

—

— 300 SDWR

Manganese μg/L <10 <10 <10 <10 <10 <10

—

—

—

— 50 SDWR

Bicarbonate mg/L 163 138 134 145 132 163 138 133 148 131

—

—

Sulfate mg/L 11.90 5.97 5.78 5.71 5.37 11.90 5.97 5.71 5.69 5.36 500 HBVChloride mg/L 8.05 5.22 5.26 5.09 5.07 8.05 5.22 5.28 5.05 5.07 250 SDWRFluoride mg/L 0.50 0.36 0.34 0.33 0.30 0.50 0.36 0.33 0.33 0.30 2 SDWRContribution of flow percent <0.01 0.2 0.7 3.7 100.0 <0.01 0.2 0.5 3.0 96.3

—

—

a Water-quality values of composite samples and flowmeter-survey data (contribution of water-bearing zones) were used to calculate water-quality value of sample interval.b Lower confining unitc Lower confining unit and small, lower part of Lower Floridan aquifer.d Lower confining unit and lower half of the Lower Floridan aquifer.e Lower confining unit and most of the Lower Floridan aquifer.f Lower confining unit and Floridan aquifer system.g Small lower part of Lower Floridan aquifer.h Central lower part of Lower Floridan aquifer.i Central upper part of Lower Floridan aquifer.j Small upper part of Lower Floridan aquifer, Lower Floridan confining unit, and Upper Floridan aquifer.k U.S. Environmental Protection Agency, 2011.
18 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

A simple mixing equation and the flow contribution from 

water-bearing units from the borehole-flowmeter survey were 

used to convert composite water-sample concentrations into 

concentrations of individual sample intervals between samplecollection points (fig. 10). Water was assumed to be flowing 

from adjacent hydrogeologic units and completely mixing 

before reaching the collection point. The top four sample 

intervals represent the Floridan aquifer system (table 5). The 

UFA, LFCU, and the upper part of the LFA are represented by 

sample interval one or SI-1; the central-upper part of the LFA 

is represented by SI-2; the central-lower part of the LFA is 

represented by SI-3; and the lower part of the LFA associated 

with high resistivity values is represented by SI-4. The lower 

confining unit, underlying the Floridan aquifer system, is 

represented by SI-5. The mixing equation from Kendall and 

Caldwell (1998, p. 80) was applied to sample intervals in the 

test hole as follows:

QT ,nCT ,n = QT ,n−1CT ,n−1 + QI,nCI,n , (1)

where

QT,n is the composite discharge at samplecollection point n, contributed to or 

flowing up the borehole from all waterbearing intervals below sample-collection 

point n, in gallons per minute;

CT,n is the concentration of a specific conservative 

constituent in discharge water QT,n , 

expressed in a linear-unit value that varies 

with constituent, but represents the mass 

of the constituent per volume of water;

QT,n–1 is the composite discharge at samplecollection point n–1, contributed to or 

flowing up the borehole from all waterbearing intervals below sample-collection 

point n–1, in gallons per minute;

CT,n–1 is the concentration of a specific conservative 

constituent in discharge water QT,n–1 , 

expressed in a linear-unit value that varies 

with constituent, but represents the mass 

of the constituent per volume of water;

QI,n is the discharge entering the well from the 

interval between sample-collection points 

n and n –1, in gallons per minute; and

CI,n is the concentration of a specific conservative 

constituent in discharge water QI,n , 

expressed in a linear-unit value that varies 

with constituent, but represents the mass 

of the constituent per volume of water.

Discharge rates are known from the borehole-flowmeter 

survey, and the composite water-sample concentrations 

at sample-collection points are known from sampling and 

analysis. Therefore, equation 1 can be rearranged to solve for 

the concentration, CI,n , of the specific conservative constituent in discharge water (QI,n ) entering the well between the two 

sample-collection points n and n–1:

I n

T n T n T n T n

I n Q

Q C Q C

C

,

, , , 1 , 1

,

− − − = . (2)

In addition to the grab water samples collected in the 

test borehole, a composite water sample was collected from 

completed LFA well 35Q069 after 71 hours of pumping during 

the 72-hour aquifer test on April 19, 2012. The water sample 

was analyzed for pH, total dissolved solids, color, alkalinity, 

dissolved carbon dioxide, turbidity, hardness as calcium 

carbonate, iron, manganese, zinc, sulfate, chloride, fluoride, 

and nitrate plus nitrite (table 7). Bicarbonate concentration was 

calculated from the value of alkalinity. Over 600 well volumes 

were pumped from the well prior to sampling water from the 

LFA. Samples were analyzed at TestAmerica Laboratories, 

Inc., Savannah, Ga. Water type for sample intervals from the 

test borehole and the composite sample for well 35Q069 was 

determined from the percentage of equivalents of sodium 

plus potassium, calcium, magnesium, chloride plus fluoride, 

sulfate, and carbonate plus bicarbonate that were plotted on a 

piper diagram (fig. 11; Piper, 1944).

Table 7. Water-quality results for a water sample collected 

from well 35Q069, Pooler, Georgia, April 19, 2012. Water sample 

was collected 71 hours into the 72-hour aquifer test.

[SU, standard pH units; mg/L, milligram per liter; PCU, platinum-cobalt unit; 

NTU, nephelometric turbidity unit; μg/L, microgram per liter; <, less than]

Constituent parameter Value Unit

pH 8.15 SU

Total dissolved solids 190 mg/L

Color < 5.0 PCU

Alkalinity 110 mg/L

Dissolved carbon dioxide < 5.0 mg/L

Turbidity 0.13 NTU

Hardness as calcium carbonate 81 mg/L

Iron < 50 μg/L

Manganese < 10 μg/L

Zinc < 20 μg/L

Bicarbonate 134 mg/L

Sulfate 14 mg/L

Chloride 6.4 mg/L

Fluoride 0.68 mg/L

Nitrate plus nitrite as nitrogen < 0.05 mg/L
Hydrogeology and Water Quality of the Floridan Aquifer System 19

235

235

229

232

238

277

200 220 240 260 280 300

All

SI-1

SI-2

SI-3

SI-4

SI-5

All

SI-1

SI-2

SI-3

SI-4

SI-5

All

SI-1

SI-2

SI-3

SI-4

SI-5

All

SI-1

SI-2

SI-3

SI-4

SI-5

All

SI-1

SI-2

SI-3

SI-4

SI-5

Specific conductance (μS/cm)

8.00

8.00

8.06

8.01

8.04

8.17

7.00 8.00 9.00

pH (standard pH units)

199.0

200.2

164.2

184.8

182.0

204.0

140 160 180 200 220

Total dissolved solids (mg/L)

28.70

28.78

25.93

29.63

28.80

19.80

0 10 20 30 40

Calcium (mg/L)

EXPLANATION

SI-1—Upper Floridan aquifer, Lower Floridan confining unit, 

 and upper part of Lower Floridan aquifer

SI-2—Central-upper part of Lower Floridan aquifer

SI-3—Central-lower part of Lower Floridan aquifer

SI-4—Lower part of Lower Floridan aquifer

SI-5—Lower confining unit

All—Average of SI-1 through SI-5, weighted for contribution of flow

SI-1 through SI-5

Sample interval

7.56

7.52

8.73

8.22

8.22

5.85

0 5 10 15

Magnesium (mg/L)

8.50

8.48

8.80

9.80

10.90

30.50

0 10 20 30 40 50

Sodium (mg/L)

1.87

1.86

1.94

2.60

2.28

3.08

0 1 2 3 4 5 6

Potassium (mg/L)

0 50 100 150 200

<100

<100

<100

<100

<100

Iron (μg/L)

0 5 10 15 20 25

Manganese (μg/L)

<10

<10

<10

<10

<10

<10

132

131

148

133

138

163

0 50 100 150 200

Bicarbonate (mg/L)

5.37

5.36

5.69

5.71

5.97

11.90

0 5 10 15

Sulfate (mg/L)

5.07

5.07

5.05

5.28

5.22

8.05

0 5 10

Chloride (mg/L)

0.30

0.30

0.33

0.33

0.36

0.50

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Fluoride (mg/L)

153

Figure 10. Water-quality values by sample interval. Samples were collected December 8, 2011, immediately following a 

flowmeter survey of the Floridan aquifer system at test hole 35Q069. Composite water-sample lab-analysis results and 

flowmeter-survey data (contribution of water-bearing zones) were used to calculate water quality from sample intervals. 

Sample-interval locations are shown in figure 7. Sample-interval specifications are listed in table 5. [μS/cm, microsiemens 

per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; μg/L, micrograms per liter; SI, sample interval]

Figure 10. Water-quality values by sample interval. Samples were collected December 8, 2011, immediately following 

a flowmeter survey of the Floridan aquifer system at test hole 35Q069, Pooler, Georgia. Composite water-sample labanalysis results and flowmeter-survey data (contribution of water-bearing zones) were used to calculate water quality 

from sample intervals. Sample-interval locations are shown in figure 7. Sample-interval specifications are listed in table 5. 

[μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; μg/L, micrograms per liter; 

SI, sample interval]
20 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Calcium Chloride

Magnesium

0

20

40

60

80

100

0

20

40

60

80

0

100

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

100

80

60

40

20

0

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

EXPLANATION

Dominant ion

Calcium (cation)

Sodium (cation)

Bicarbonate (anion)

Sample interval (SI) and depth, 

 in feet below land surface

SI-1, 339 to 745

Well composite

SI-2, 745 to 845

SI-3, 845 to 985

SI-4, 985 to 1,050

SI-5, 1,050 to 1,120

Calcium + Magnesium

PERCENT OF EQUIVALENTS

PERCENT OF EQUIVALENTS

Sodium + Potassium

Bicarbonate + Carbonate

Sulfate

Chloride + Sulfate

Figure 11. Piper diagram of water-quality from sample intervals from test hole 35Q069 near Pooler, Georgia, 

December 8, 2011. Composite water-sample lab-analysis results and flowmeter-survey data (contribution of 

water-bearing zones) were used to calculate water quality from sample intervals. 

PERCENT OF EQUIVALENTS

Figure 11. Piper diagram of water quality from sample intervals from test hole 35Q069, Pooler, Georgia, 

December 8, 2011. Composite water-sample lab-analysis results and flowmeter-survey data (contribution 

of water-bearing zones) were used to calculate water quality from sample intervals.
Hydrogeology and Water Quality of the Floridan Aquifer System 21

Hydrogeology and Water Quality

Hydrogeologic units of the Floridan aquifer system were 

distinguished by differences in flow contribution, lithology, 

geophysical characteristics, and water quality. Miller (1986) 

provided a regional definition of the Floridan aquifer system 

on the basis of widely spaced stratigraphic and borehole 

geophysical data in the coastal area of Georgia and South 

Carolina. Newly collected hydrogeologic and water-quality 

data were used to help refine Miller’s (1986) regional definition of the Floridan aquifer system (Williams and Gill, 2010). 

The following sections describe the depths and hydraulic 

characteristics of hydrogeologic units that form the Floridan 

aquifer system at the Pooler test site.

Drillers and geophysical logs indicate a sediment 

sequence within the top 262 ft of test hole 35Q069 consisting 

of clay, sand, and some gravel. This clastic sequence represents a combination of the surficial and Brunswick aquifer 

systems and is part of the confining unit above the Floridan 

aquifer system (fig. 3). Varying amounts of phosphate were 

identified in the drill cuttings in the upper 262 ft of sediment. 

Below the 262 ft of clastic deposits is a 39-ft transition zone to 

limestone, consisting of a mix of shells, gravel, and sand. The 

limestone extends from 301 to 1,040 ft in depth and is typically moderately indurated and fossiliferous. Zones of poor 

induration exist at depths of 333–370, 430–470, 505–515, 

560–570, 705–750, 900–910, and 960–980 ft. Limestone was 

found to contain some gypsum deposits at depths of 630–640, 

780–790, and 820–830 ft. Drill cuttings collected from depths 

between 1,040 and 1,090 ft exhibit a mixed lithology of 

poorly indurated limestone and brown silt. This deeper unit 

is associated with increased natural-gamma radiation and 

decreased resistivity that extends to near the bottom of the 

hole at 1,120 ft. Williams and Gill (2010) interpreted these 

geophysical-log signatures in a test hole in Effingham County, 

Ga., to be the lower confining unit, not included as part of the 

Floridan aquifer system. Drill cuttings from depths between 

1,090 and 1,120 ft were not inspected.

Based on the flowmeter survey, driller cuttings, and 

geophysical logs, constituent limestone units of the Floridan 

aquifer system at the Pooler test site extend in depth from 

333 to 1,040 ft at the following depth intervals: Upper 

Floridan confining unit, 301–333 ft; UFA, 333–515 ft; 

LFCU, 515–702 ft; and LFA, 702–1,040 ft.

Upper Floridan Aquifer 

The UFA at well 35Q069 mostly consists of lower 

Oligocene and upper Eocene carbonate units that include high 

permeability zones (fig. 3). The top of the UFA (at a depth of 

333 ft) consists of the lower Oligocene Suwannee Limestone 

and corresponds to the top of loose, shelly limestone. An 

impermeable limestone unit of the lower Oligocene Lazaretto 

Creek Formation overlies the top of the aquifer and functions 

as the Upper Floridan confining unit. The base of the Tiger 

Leap Formation serves as the top of Upper Floridan confining 

unit, which lies just below the top of the Oligocene deposits. 

The top of the Oligocene corresponds to a spike in the naturalgamma log called the “C-marker” (Wait, 1965; Gregg and 

Zimmerman, 1974; Clarke and others, 1990). The thickest part 

of the aquifer consists of the upper Eocene Ocala Limestone, 

which is characterized by low natural-gamma radiation, the 

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

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

UFA (at a depth of 515 ft) is close to the contact between the 

upper Eocene Ocala Limestone and the middle Eocene Avon 

Park Formation, and corresponds to the base of a thin, porous 

limestone and the main water-bearing unit. 

Regional maps showing the depth and thickness of the 

geologic units that form the Floridan aquifer system (Williams 

and Gill, 2010) indicate that the UFA underlying the Pooler 

test site has a depth interval of 370–540 ft below land surface. 

Lithologic, geophysical, and flowmeter data collected from 

well 35Q069 (fig. 3) were used to refine this depth interval to 

333–515 ft.

Flowmeter Survey

On December 8, 2011, a borehole flowmeter survey 

was completed in the test hole for well 35Q069 in the depth 

interval between 339 and 1,110 ft below land surface at the 

Pooler test site while pumping at a rate of 708 gal/min. The 

survey indicated that 92.4 percent (654.2 gal/min) of the total 

flow originated from the UFA, and the remaining 7.6 percent 

(53.8 gal/min) was derived from the underlying LFCU and 

the LFA (fig. 6). Of the 654.2 gal/min contributed by the 

UFA, 6.3 percent of the flow was produced in the 339–350 ft 

interval, and 86.1 percent of the flow was produced in the 

375–515 ft interval. The dominance of water-bearing potential 

in the UFA also was found at Hunter Army Airfield (Clarke 

and others, 2010) and Fort Stewart (table 8; Clarke and others, 

2011; Gonthier, 2011). At all three sites, the majority of the 

water-bearing potential is located below the “D-marker” 

within the Ocala Limestone. Unlike test holes at Hunter Army 

Airfield and Fort Stewart, the test hole for well 35Q069 at 

the Pooler test site was cased above the “D-marker” and 

contained a relatively minor water-bearing zone just above 

the “D-marker.” Relatively thin, distinct water-bearing zones 

in the UFA (below the “D-marker”) were observed at Hunter 

Army Airfield. In contrast, more diffuse zones were observed 

at Fort Stewart and Pooler. One subtle difference between 

the main water-bearing zones in the UFA at Fort Stewart and 

Pooler is that water-bearing potential decreases with depth 

at Fort Stewart (Gonthier, 2011, p. 6) whereas water-bearing 

potential does not appear to change with depth at Pooler.
22 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Hydraulic Properties

The transmissivity and storage coefficient of the 

UFA were estimated by analyzing water-level data 

collected during the 24-hour aquifer test performed during 

March 27–28, 2012, by pumping UFA well 35Q070 at a rate 

of 285 gal/min; and from the 72-hour aquifer test performed 

during April 16–19, 2012, by pumping LFA well 35Q069 at a 

rate of about 783 gal/min. Details of the aquifer tests, analysis, 

and results are discussed in appendix 2, and aquifer-test results 

are listed in table 4. Drawdown within pumped UFA well 

35Q070 during the 24-hour aquifer test exhibited turbulence 

and an instantaneous water-level decline. The typical, gradual 

increases in drawdown with time that were used to determine 

hydraulic properties of the aquifer were small compared to 

nonaquifer-test influences that remained after filtering, making 

the drawdown data difficult to analyze with accuracy. A 

transmissivity estimate for the UFA of 30,000 ft2/d (fig. 2–9 

in appendix 2, table 4) was computed using the Cooper-Jacob 

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

Transmissivity and storage coefficient were also estimated by simulating the response of both wells to the 72-hour 

aquifer test at pumped LFA well 35Q069 (MODFLOW-96 and 

MODOPTIM as discussed in appendix 2). Estimated transmissivity and storage coefficient of the UFA at observation well 

35Q070 were 46,000 ft2/d and 5.9×10–4, respectively.

The accepted transmissivity for the UFA is the value 

that was estimated from the simulation of the response to the 

72-hour aquifer test of LFA well 35Q069 (46,000 ft2/d). The 

drawdown data for UFA observation well 35Q070 in response 

to the 72-hour aquifer test contains much less noise than the 

drawdown data in response to the 24-hour aquifer test. Therefore, the results of the simulation of the 72-hour aquifer test 

are more reliable and less error prone than those of the 24-hour 

aquifer test. Also, the estimated transmissivity from the slug 

tests open to the entire Floridan aquifer system (58,000 ft2/d, 

see appendix 1) is close to the estimated transmissivity of the 

entire Floridan aquifer system (51,000 ft2/d), based on the 

simulation of the 72-hour aquifer test. Considering a model 

thickness of 183 ft, the Kh and specific storage of the UFA 

totaled 250 ft/d and 3.2×10–6 ft–1, respectively.

Water Quality

Water quality in the UFA is determined from sample 

interval one (SI-1), which is the interval between two 

composite grab samples taken at a depth of 339 and 745 ft and 

the specific conductance of drilling fluid. SI-1 spans all of the 

UFA and LFCU and a small part of the LFA thickness. Greater 

than 96 percent of the flow in this interval originates from 

the UFA (table 5). As a result, it is relatively safe to assume 

that two water-bearing zones below the base of the UFA do 

not influence the water chemistry of SI-1. Furthermore, the 

three sample intervals directly below SI-1 (SIs 2–4) have 

water quality similar to that of SI-1 (fig. 10, table 6). Values 

of specific conductance of drilling fluid fluctuated about 

240 microsiemens per centimeter at 25 degrees Celsius 

(μS/cm) from the top of data collection at the 410 ft depth to 

the central part of the LFA at about the 890 ft depth (fig. 3).

Water type for SI-1 and the total composite flow is 

calcium bicarbonate (fig. 11). Specific conductance in water 

from SI-1 was 235 μS/cm, similar to that of the previously 

sampled drilling fluid and that of SIs 2-4. Major ions were 

present in low concentrations in water from SI-1, and the 

concentration of total dissolved solids was 200.2 milligrams 

per liter (mg/L). Concentrations of major ions, iron, and 

manganese within the UFA were well below water-quality 

criteria established by the U.S. Environmental Protection 

Agency for drinking water and health advisories (table 6; 

U.S. Environmental Protection Agency, 2011).

Lower Floridan Confining Unit

The LFCU at well 35Q069 consists of chalky and 

glauconitic limestone in the uppermost part of the middle 

Eocene Avon Park Formation between depths of 515 and 

702 ft (figs. 2 and 3). Thickness and hydraulic conductivity 

of the confining unit control the rate of interaquifer leakage 

between the UFA and LFA.

The LFCU is 187 ft thick at the Pooler test site compared 

to 207 ft thick at Fort Stewart (Clarke and others, 2011) 

located 23 mi to the southwest, and 143 ft thick at Hunter 

Army Airfield (Clarke and others, 2010) located 7 mi to the 

east-southeast. The LFCU contains carbonate sediments of 

slightly lower permeability than those of the LFA. This unit 

is similar in lithology to overlying and underlying rock units, 

which precluded identification of the confining unit during 

drilling. Following construction of the test hole for well 

35Q069, the thickness and depths of the confining unit were 

assessed by using borehole geophysical logs, the results of a 

flowmeter survey, and the deepest packer-isolated slug test.

Table 8. Flowmeter survey results in percent contribution of 

flow from the Upper Floridan aquifer, Lower Floridan confining 

unit, and Lower Floridan aquifer from selected studies in the 

vicinity of Pooler, Georgia, 2009–2012.

[—, no measurement]

Hydrogeologic unit

Percent contribution of flow

Fort

Stewarta

Pooler

(this study)

Hunter Army 

Airfieldb

Upper Floridan aquifer 92.3 92.4 83.6

Lower Floridan confining unit 3.1 3.0 4.3

Lower Floridan aquifer 4.6 4.6 11.7

Lower confining — 0 0.6

a

 Gonthier, 2011.

b

 Clarke and others, 2010.
Hydrogeology and Water Quality of the Floridan Aquifer System 23

Flowmeter Survey

The flowmeter survey in the test hole for well 35Q069 

(fig. 6) indicated that the LFCU contributed little water to the 

overall flow in the test hole. In particular, continuous vertical 

sections of limestone between depths of 520 and 550 ft and 

between 620 and 702 ft contributed no detectable amounts 

of water during test pumping at 708 gal/min. Within the 

confining unit, a single water-bearing zone between depths of 

550 and 620 ft yielded 21.3 gal/min, or 3.0 percent of the total 

test-hole flow, during the flowmeter survey.

Hydraulic Properties

The vertical hydraulic conductivity (Kv) and porosity of 

the LFCU were determined by hydraulic analysis of the core at 

three intervals: 564.3–569.8, 600.0–605.0, and 640.0–648.4 ft 

(table 2). Values of Kv ranged from 0.57 to 1.67 ft/d. These 

values compare with the lower end of the full range of 

reported hydraulic-conductivity values for karst limestone 

(Freeze and Cherry, 1979; 0.21 to 3,850 ft/d). Porosity values 

for all three cores were about 0.33 and are within the reported 

ranges for limestone and sandy clay (Heath, 1983). The Kv of 

the cores may not fully represent the Kv of the confining unit 

because of the small volumes that the cores represent.

The Kh of the LFCU was determined by completing 

packer-isolated slug tests at three separate intervals—

612–618, 642– 648, and 690–696 ft (fig. 3)—prior to installation of the 8-inch-diameter casing to complete the test hole, as 

LFA well 35Q069. The Kh values for the three intervals ranged 

from 0.5 to 10 ft/d (table 3). The packer-isolated interval 

690–696 ft had a Kh of 0.5 ft/d. Somewhere between the low 

hydraulic conductivity at 696 ft and the top of a water-bearing 

unit at 705 ft is the base of the LFCU. The base of the LFCU 

in the Pooler test site is determined to be at a depth of 702 ft, 

about midway between 696 and 705 ft.

The transmissivity and storage coefficient of the LFCU 

was estimated by analyzing water-level data collected during 

the 72-hour aquifer test performed on April 16–19, 2012, by 

pumping LFA well 35Q069 at a rate of about 783 gal/min. The 

response of both UFA well 35Q070 and LFA well 35Q069 

to the aquifer test was simulated using MODFLOW-96 and 

MODOPTIM (as discussed in appendix 2). A transmissivity 

value and a storage coefficient for the LFCU of 700 ft2/d 

and 4.5×10–4, respectively, were used to calibrate the model. 

Considering a model thickness of 182 ft, the Kh and specific 

storage of the LFCU were 4 ft/d and 2.5×10–6 ft–1, respectively. 

The value of Kh estimated from the simulation is in the middle 

range of values determined from the packer-isolated slug tests 

(table 3).

Water Quality

The UFA, LFCU, and the upper part of the LFA are 

located between two water-sample-collection depths. As a 

result, all flow contributed from the LFCU is mixed with the 

UFA and the upper part of the LFA to create water chemistry 

of SI-1. Because of the mixture of water in SI-1, determining 

the quality of water that is derived from the LFCU is not 

possible. Based on the similarity of water chemistry of the top 

four sample intervals (fig. 10), however, the quality of water 

derived from the LFCU (depth interval 550–620 ft) is most 

likely similar to that of the UFA and LFA. Values of specific 

conductance of drilling fluid within the LFCU fluctuated about 

240 μS/cm (fig. 3).

Lower Floridan Aquifer

The LFA at well 35Q069 consists of chalky and glauconitic limestone in the upper part of the middle Eocene Avon 

Park Formation that is similar in lithology to overlying units 

(fig. 2). At well 35Q069, the top of the LFA is at a depth of 

702 ft and extends to a depth of 1,040 ft. Williams and Gill 

(2010) described the hydrogeology of LFA test well 36Q330, 

located at Berwick Plantation in Chatham County 3.73 mi 

southeast of the Pooler test site. Geophysical logs indicate 

that the LFA at the Berwick Plantation well (36Q330) is 

between depths of 712 and 1,080 ft. The LFA is 338 ft thick at 

the Pooler test site, which is similar to thicknesses measured 

at Fort Stewart (388 ft), Hunter Army Airfield (377 ft), and 

Berwick Plantation (368 ft).

Flowmeter Survey

Water-bearing zones were identified in the LFA using 

two flowmeter surveys. Results of flowmeter testing of the 

Floridan aquifer system in the test hole for well 35Q069 

performed on December 8, 2011, identified four water-bearing 

depth intervals in the LFA: 702 –745, 745–800, 800–925, 

984–1,015 ft (fig. 6). The LFA contributed about 32.5 gal/min, 

or 4.6 percent of the total flow. 

A second flowmeter survey was performed in well 

35Q069 on April 18, 2012, during the 72-hour aquifer test 

and extended from the base of the casing at a depth of 705 ft 

to near the bottom of the well at a depth of 1,110 ft. This 

survey identified five water-bearing depth intervals within 

the LFA: 705 –745, 745–925, 925–984, 984–1,015, and 

1,015–1,040 ft. The main water-bearing zone was identified 

at a depth interval of 745–925 ft, yielding 84.0 percent of the 

total flow. Water-bearing zones generally were diffuse. No 

flow was detected below a depth of 1,040 ft where the lower 

confining unit to the Floridan aquifer system is located.
24 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

U.S. Environmental Protection Agency, 2011). Water-quality 

results of SIs 2– 4 were similar to those of SI-1, indicating that 

water quality in the Floridan aquifer system is homogenous.

Results from SI-5 indicate that water chemistry in the 

lower confining unit is distinct from that of the Floridan 

aquifer system (figs. 10 and 11; table 6). Compared to sample 

intervals in the Floridan aquifer system, SI-5 contained higher 

concentrations of potassium, sodium, chloride, fluoride, 

sulfate, bicarbonate, and total dissolved solids; higher values 

of pH and specific conductance; and lower concentrations 

of calcium and magnesium. The 1,050-ft grab sample was 

the only sample to have a detectable concentration of iron 

(153 micrograms per liter [μg/L]), which was below the U.S. 

Environmental Protection Agency (2011) secondary drinking 

water regulation of 300 μg/L. The water type within SI-5 is 

sodium bicarbonate. 

The chemical results of SI-5 may not fully represent 

the water quality in the lower confining unit because of the 

low contribution of flow during the flowmeter surveys. Some 

water from above may have mixed with water from the 

lower confining unit during collection of the grab sample. 

Despite these limitations, the chemical analyses indicate that 

water from the lower confining unit should not have a strong 

effect on the water quality in LFA well 35Q069. Despite 

higher concentrations of most major ions compared to those 

from the Floridan aquifer system, concentrations of major 

ions, iron, and manganese in water from SI-5 were below 

U.S. Environmental Protection Agency secondary drinkingwater regulations or normal health-based criteria for (U.S. 

Environmental Protection Agency, 2011). The one minor 

exception was a sodium concentration of 30.5 mg/L, which 

exceeded the health-based value of 20 mg/L for individuals on 

a 500-milligram-per-day restricted sodium diet.

A composite water sample collected from completed 

LFA well 35Q069 near the end of the 72-hour aquifer test on 

April 19, 2012, was tested for a limited number of analytes 

(table 7). The total dissolved solids concentration measured 

190 mg/L. Concentrations of analyzed constituents were 

similar though slightly higher than concentrations of analyzed 

constituents contained in water samples collected from 

SIs 1– 4. Chloride concentrations in the water sample from the 

aquifer test measured 6.4 mg/L compared to about 5.1 mg/L 

in SIs 1– 4. Fluoride and sulfate concentrations in the water 

sample from the 72-hour aquifer test were 0.68 and 14 mg/L, 

respectively. Maximum fluoride and sulfate concentrations in 

water samples collected immediately following the flowmeter 

survey on December 8, 2011, were 0.5 and 11.9 mg/L, 

respectively. Despite higher concentrations of some analytes, 

concentrations of major ions were well below U.S. Environmental Protection Agency secondary drinking-water regulations and health-based criteria (table 6; U.S. Environmental 

Protection Agency, 2011). Analytes that were not detected 

included iron, manganese, zinc, and nitrate plus nitrite.

Hydraulic Properties

The transmissivity of the LFA was estimated by analyzing 

water-level data collected during the 72-hour aquifer test 

performed April 16 –19, 2012, by pumping LFA well 35Q069 

at a rate of about 783 gal/min. The response of both UFA well 

35Q070 and LFA well 35Q069 to the aquifer test was simulated using MODFLOW-96 and MODOPTIM (as discussed in 

appendix 2). A transmissivity of 4,000 ft2/d was used for the 

LFA to calibrate the model. Considering a model thickness of 

338 ft, the Kh of the LFA was estimated to be 12 ft/d.

Water Quality

The quality of water in the LFA was initially evaluated 

during drilling of the test hole for well 35Q069 by measuring 

specific conductance of air-rotary drilling fluid from the 

410–1,120-ft-depth interval (fig. 3). Values of specific conductance of drilling fluid between depths of 900 and 1,080 ft 

were slightly lower (fluctuating about 225 μS/cm) compared 

to those between depths of 410 and 890 ft (fluctuating about 

240 μS/cm). Between depths of 1,080 and 1,120 ft, values 

of specific conductance of drilling fluid slightly increased, 

fluctuating about 230 μS/cm.

More detailed water quality of the LFA was determined 

from four composite grab samples taken at depths of 

745, 845, 985, and 1,050 ft after the flowmeter survey, 

which was completed in the test hole for well 35Q069 on 

December 8, 2011. Using the water-quality results of these 

composite grab samples and mixing equation 2, water-quality 

properties were evaluated for flow contributed to the sample 

intervals between the depths from which composite grab samples 

were collected. Results of SI-2, SI-3, and SI-4 (table 6) encompass 88 percent of the flow contributed to the LFA and represent 

the water quality of the LFA. The remaining 12 percent of 

the flow contributed to the LFA went into SI-1. SI-5 provides 

some insight on the water quality of the lower confining unit, 

located directly below the Floridan aquifer system.

Water chemistry for SI-2, SI-3, and SI-4 are nearly 

identical (figs. 10 and 11). The three sample intervals had 

nearly the same concentrations of fluoride (0.33–0.36 mg/L) 

and pH (8.01– 8.06). For the three sample intervals, concentrations ranged as follows: calcium 25.93–29.63 mg/L; 

magnesium 8.22–8.73 mg/L; chloride 5.05–5.28 mg/L; sulfate 

5.69–5.97 mg/L; bicarbonate 133–148 mg/L; and specific 

conductance 229–238 μS/cm. The water type for SI-2, SI-3, 

and SI-4 is calcium bicarbonate (fig. 11).

Major ions were present in low concentrations in water 

from the three sample intervals. The maximum concentration 

of total dissolved solids measured 184.8 mg/L. Concentrations 

of major ions, iron, and manganese within the LFA were well 

below U.S. Environmental Protection Agency secondary 

drinking-water regulations or health-based criteria (table 6; 
Summary 25

Effect of Lower Floridan Aquifer 

Pumping on the Upper Floridan Aquifer

The effect of pumping LFA well 35Q069 on water 

levels in UFA well 35Q070 were evaluated by monitoring 

drawdown response during the 72-hour aquifer test performed 

during April 16–19, 2012. Observed water-level responses 

in the UFA and LFA wells as a result of pumping the LFA 

were determined by using water-level data that were filtered 

for tidal, barometric, and long-term trends and by following 

procedures described in appendix 2. In response to pumping 

LFA well 35Q069 for 72 hours at a rate of about 783 gal/

min, drawdown in LFA well 35Q069 and UFA well 35Q070 

reached 51.7 and 0.9 ft, respectively (figs. 2–5 and 2–6, 

appendix 2). This drawdown response created a hydraulic 

head gradient with the water level approximately 50 ft lower 

in the LFA than in the UFA. Such a hydraulic-head gradient 

leads to the potential for flow of water from the UFA into the 

LFA; however, the actual flow rate was not determined from 

aquifer-test data and is beyond the scope of this report. 

Summary

To assess the water-supply potential of the Lower 

Floridan aquifer (LFA) at Pooler, Georgia, the U.S. Geological 

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

performed an investigation during 2011–2012 to determine the 

hydrogeology and water quality of the Floridan aquifer system 

and the potential effect that pumping from the LFA would 

have on the Upper Floridan aquifer (UFA). The study included 

construction of a test well in the UFA and a test well in the 

LFA, detailed site investigations, and hydraulic and waterquality characterization of the Floridan aquifer system. Based 

on the flowmeter survey, driller cuttings, and geophysical logs, 

hydrogeologic units of the Floridan aquifer system are present 

at the following depth intervals: Upper Floridan confining 

unit, 301–333; UFA 333–515; Lower Floridan confining unit 

(LFCU), 515–702; and LFA, 702–1,040 feet (ft).

The UFA at well 35Q069 mostly consists of lower 

Oligocene and upper Eocene carbonate units that include high 

permeability zones. A borehole flowmeter survey completed 

in the test hole for well 35Q069 open to the entire Floridan 

aquifer system indicated that 92.4 percent of the total flow 

of 708 gallons per minute (gal/min) originated from the 

UFA, and the remaining 7.6 percent was derived from the 

underlying LFCU and LFA. Most of the flow (86.1 percent) in 

the UFA was produced in the 375–515 ft interval. During the 

spring of 2012, 24- and 72-hour aquifer tests were performed 

to determine hydraulic properties of the UFA. Estimates of 

transmissivity were derived using the Cooper-Jacob method 

for the 24-hour aquifer test and simulated with MODOPTIM 

with MODFLOW for the 72-hour aquifer test. Transmissivity 

ranged from 30,000 to 46,000 feet squared per day (ft2/d) for 

the 24-hour and 72-hour aquifer tests, respectively, with the 

higher value considered to be more representative of the actual 

transmissivity. Considering a model thickness of 187 ft, the 

horizontal hydraulic conductivity (Kh) and specific storage of 

the UFA were 250 feet per day (ft/d) and 3.2×10– 6 per foot 

(ft – 1), respectively.

UFA water quality was determined from two borehole 

grab samples collected immediately following the flowmeter 

survey. The UFA has a calcium bicarbonate water type with 

low concentrations of major ions and a total dissolved solids 

concentration of 200.2 milligrams per liter (mg/L). Concentrations of major ions, iron, and manganese within the UFA 

were below U.S. Environmental Protection Agency secondary 

drinking-water regulations and health-based criteria.

The LFCU at well 35Q069 consists of chalky and 

glauconitic limestone in the uppermost part of the middle 

Eocene Avon Park Formation between depths of 515 and 

702 ft. Thickness and hydraulic conductivity of the confining 

unit control the rate of leakage between the UFA and LFA. A 

flowmeter survey indicated that the LFCU contributed little 

to the overall flow in the test hole, with a single water-bearing 

zone at 550–620 ft yielding 21.3 gal/min, or 3.0 percent of the 

total test-hole flow. Values of vertical hydraulic conductivity 

(Kv ) and porosity of the LFCU were determined by analyzing 

the core for hydraulic analysis at three intervals. Values of 

Kv from the sample ranged from 0.57 to 1.67 ft/d; porosity 

values for the three cores were about 0.33. A sample of values 

of horizontal hydraulic conductivity (Kh ) of the LFCU was 

determined by completing packer-isolated slug tests at three 

separate intervals. The Kh values for the three intervals ranged 

from 0.5 to 10 ft/d. 

The transmissivity and storage coefficient of the LFCU 

was estimated from the 72-hour aquifer test by pumping 

LFA well 35Q069 at a rate of about 783 gal/min. A transmissivity and storage coefficient for the LFCU of 700 ft2

/d and 

4.5×10– 4, respectively, were used to calibrate the model. 

Considering a model thickness of 182 ft, the Kh and specific 

storage of the LFCU was 4 ft/d, and 2.5×10– 6 ft – 1, respectively. The value of Kh

 estimated from the model simulation 

is in the middle of the range of values determined from the 

packer-isolated slug tests.

The LFA at well 35Q069 consists of chalky and 

glauconitic limestone in the upper part of the middle Eocene 

Avon Park Formation that is similar in lithology to overlying 

units. Results of flowmeter testing identified five waterbearing depth intervals within the LFA: 705–745, 745–925, 

925–984, 984–1,015, and 1,015–1,040 ft, with the interval 

745–925 ft contributing 84.0 percent of the total flow of about 

783 gal/min. Water-bearing zones generally were diffuse. No 

flow was detected below a depth of 1,040 ft where the lower 

confining unit is located. Transmissivity of the LFA was 

estimated through model simulation using data from a 72-hour 

aquifer test where LFA well 35Q069 was pumped at a rate of 

about 783 gal/min. Estimated transmissivity for the LFA was 

4,000 ft2/d. Considering a model thickness of 338 ft, the Kh of 

the LFA was estimated at 12 ft/d.
26 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

The quality of water in the LFA was determined from 

four grab samples collected during flowmeter testing of the 

test hole and from a composite sample collected from the 

completed LFA well near the end of the 72-hour aquifer test. 

Water from the LFA is a calcium bicarbonate type, with low 

concentrations of major ions, and a total dissolved solids 

concentration of 190 mg/L. Concentrations of all constituents 

were below U.S. Environmental Protection Agency secondary 

drinking-water regulations and health-based criteria.

The effect of pumping the LFA well 35Q069 on water 

levels in UFA well 35Q070 were evaluated by monitoring 

drawdown response during the 72-hour aquifer test. Observed 

water-level responses in the UFA and LFA wells as a result of 

pumping the LFA were determined by using water-level data 

that were filtered for tidal, barometric, and long-term trends. 

Drawdown in LFA pumped well 35Q069 was 51.7 ft, whereas 

drawdown in UFA well 36Q070 was 0.9 ft after 72 hours of 

pumping at a rate of about 783 gal/min.
Selected References 27

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2002–197, 51 p.

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flow model: U.S. Geological Survey Open-File Report 

96 – 486, 220 p.

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Aquifer-System Analysis, Professional Paper 1403-B, 

91 p., 33 pls.

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Appendixes 1 and 2
30 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Appendix 1. Slug tests, Pooler, Georgia, December 13–14, 2011

Slug tests were performed in the 10.3-inch-diameter test 

hole for well 35Q069 at three packer-isolated intervals within 

the Lower Floridan confining unit (LFCU) and within the 

entire test hole open to the Floridan aquifer system. Horizontal 

hydraulic conductivity (Kh) estimates of the LFCU within 

three packer-isolated intervals ranged from 0.5 foot per day 

(ft/d) in the deepest interval with a depth of 690–696 feet (ft; 

table 3), to 10 ft/d in the shallowest interval with a depth of 

612–618 ft. Packer seals for each of the tests appeared tight, 

with leakage around the packer possibly affecting results of 

the falling-head slug test of the deepest interval. The estimated 

transmissivity of the Floridan aquifer system from the open 

hole slug tests totaled 58,000 feet squared per day (ft2/d).

Test Results at the 612- to 618-Foot Interval

Analysis of two slug tests were performed on the 

612- to 618-ft packer-isolated interval on December 13, 2011, 

and resulted in comparable estimates of Kh of about 10 ft/d. 

For the first test, the initial static water level stood 46.2 ft 

below land surface. The slug was inserted into the water 

column at 3:29:26 p.m. In response to slug insertion, the water 

level immediately spiked to about 6 ft above static water level 

followed by a drop to 3.5 ft above static water level within 

5 seconds (fig. 1–1). Water-level recovery was within the 

resolution of the transducers (0.02 ft) about 4.5 minutes after 

the slug insertion. Packer seals appeared tight, as water levels 

above and below the packer-isolated interval did not exhibit a 

detectable response to the slug test. Within the packer-isolated 

interval, the smooth recovery typified the expected exponential 

decay of water-level displacement that was analyzed using the 

Bouwer and Rice (1976) method with a straight line on the 

analysis plot (fig. 1–2). The horizontal hydraulic conductivity 

(Kh ) derived from these data was 9.6 ft/d.

The second slug test for the 612- to 618-ft packer-isolated 

interval was performed about 10 minutes after the slug was 

initially inserted by removing the slug from the water column 

at 3:39:32 p.m. (fig. 1–3). In response to slug removal, the 

water level dropped to 3.74 ft below static water level after 

about 8 seconds. Water-level recovery was within 0.02 ft a 

little more than 5 minutes after slug removal. Packer seals 

appeared tight, as water levels above and below the packerisolated interval did not exhibit a detectable response to 

the slug test. Within the packer-isolated interval, smooth 

recovery typified the expected exponential decay of waterlevel displacement that was analyzed using the Bouwer and 

Rice (1976) method with a straight line on the analysis plot 

(fig. 1–4). The Kh derived from these data was 9.4 ft/d.

Test Results at the 642- to 648-Foot Interval

Analysis of two slug tests were performed on the 

642- to 648-ft packer-isolated interval on December 14, 2011, 

and resulted in comparable estimates of Kh of about 3 ft/d. For 

the first test, the initial static water level stood 46.1 ft below 

land surface. The slug was inserted into the water column 

at 8:46:48 a.m. In response to slug insertion, the water level 

immediately spiked to 7.4 ft above static water level followed 

by a drop to 4.2 ft above static water level after 9 seconds 

(fig. 1–5). Water-level recovery was within 0.02 ft of static 

conditions about 13 minutes after slug insertion. Packer 

seals appeared tight, as water levels above and below the 

packer-isolated interval did not exhibit a detectable response 

to the slug test. Within the packer-isolated interval, the smooth 

recovery typified the expected exponential decay of waterlevel displacement that was analyzed using the Bouwer and 

Rice (1976) method with a straight line on the analysis plot 

(fig. 1–6). The Kh derived from these data was 3.4 ft/d.

The second slug test for the 642- to 648-ft interval 

was performed about 15 minutes after the slug was initially 

inserted by removing the slug from the water column at 

9:01:08 a.m. (fig. 1–7). In response to slug removal, the 

water level dropped to 4.52 ft below static water level after 

9 seconds. Water-level recovery was within 0.02 ft of static 

water level about 11 minutes after slug removal. Packer 

seals appeared tight, as water levels above and below the 

packer-isolated interval did not exhibit a detectable response 

to the slug test. Within the packer-isolated interval, smooth 

recovery nearly typified the expected exponential decay of 

water-level displacement that was analyzed using the Bouwer 

and Rice (1976) method. Only a slight negative concavity in 

the water-level-recovery curve was exhibited on the analysis 

plot (fig. 1–8). The slight negative concavity in the curve is 

only apparent about 3.5 minutes after the start of the slug test 

and did not affect the estimate of Kh, which was determined to 

be 3.3 ft/d.

Test Results at the 690- to 696-Foot Interval

Analysis of a rising-head slug test were performed on the 

690- to 696-ft packer-isolated interval on December 14, 2011, 

and resulted in an estimate of Kh of 0.5 ft/d. For the first test, 

the initial static water level stood 46.1 ft below land surface. 

The slug was inserted into the water column at 11:51:11 a.m. 

In response to the slug insertion, the water level immediately 

spiked to 5.7 ft above static water level and then oscillated for 

about 22 seconds (fig. 1–9). Water-level recovery was within 
Appendix 1. Slug tests, Pooler, Georgia, December 13–14, 2011 31

0.02 ft about 18 minutes after slug insertion. The lower packer 

seal appeared to leak. Water levels above the packer-isolated 

interval did not exhibit a response to the slug test; however, in 

response to the slug insertion, water levels below the packerisolated interval increased by about 0.02 ft above static water 

level for more than 5 minutes. This slight water-level increase 

indicates leakage between the packer-isolated interval and the 

lower transducer. Due to the response of the lower transducer, 

no hydraulic conductivity was estimated from this slug test. 

The air pressure to the packers was increased at the noon 

hour to prevent leakage around the lower packer during slug 

removal. The air-pressure increase caused a slight temporary 

water-level increase that occurred late in the slug-test recovery.

The second slug test for the 690- to 696-ft packer-isolated 

interval was performed about 18 minutes after the slug was 

initially inserted by removing the slug from the water column 

at 12:08:45 p.m. (fig. 1–10). In response to slug removal, the 

water level dropped to 4.53 ft below static water level after 

10 seconds. Water-level recovery was within 0.02 ft of static 

water level about 51 minutes after the slug removal. Packer 

seals appeared tight, as water levels above and below the 

packer-isolated interval did not exhibit a detectible response to 

the slug test. Within the packer-isolated interval, the recovery 

did not typify a perfect exponential decay of water-level 

displacement that was analyzed using the Bouwer and Rice 

(1976) method, but rather had a slightly negative concavity 

within the curve on the analysis plot (fig. 1–11). Consequently, 

the Kh was estimated by aligning analytical points to the 

recovery data during 5.5–15.7 minutes following slug 

removal. The estimated Kh for this slug test was 0.5 ft/d.

Test Results at the 339- to 1,130-Foot Interval

After slug tests were completed within packer-isolated 

intervals, the packers were deflated while the packer assembly 

remained at the 690- to 696-ft depth. The water column that 

was connected to the packer assembly was then connected 

to the Upper Floridan aquifer, LFCU, and Lower Floridan 

aquifer (the entire Floridan aquifer system) between depths 

of 339 and 1,130 ft. All three transducers were, therefore, 

hydraulically connected to the water column. Analysis of two 

slug tests performed on the entire Floridan aquifer system 

resulted in comparable estimates of transmissivity of about 

58,000 ft2/d. For the first test, the initial static water level 

stood 46.0 ft below land surface. The slug was inserted into 

the water column on December 14, 2011, at 1:11:42 p.m. 

Slug insertion caused identical water-level oscillations at the 

middle and lower transducers having an initial and maximum 

displacement of about 0.25 ft above static water level and a 

period of about 29 seconds.

The upper transducer measured a damped water-level 

response to slug insertion, attaining a maximum water-level 

displacement of about 0.10 ft, or about 40 percent to that of 

the middle and lower transducers. Furthermore, water levels at 

the upper transducer responded immediately to slug insertion 

with oscillations having the same period (29 seconds) as that 

of the water levels measured with the middle and lower transducers. Maximum water-level displacement, however, was 

achieved about 5 seconds after maximum water-level displacements were recorded at the middle and lower transducers.

The second slug test for the 339- to 1,130-ft interval 

was performed when the amplitude of the oscillation from 

the previous test damped to an undetectable amount (by 

1:17:58 p.m.), slightly more than 4 minutes after slug insertion. Removing the slug at this time resulted in a water-level 

response that virtually mirrored that of the slug insertion 

(fig. 1–12).

The two slug tests for the 339- to 1,130-ft open interval 

were analyzed using the van der Kamp (1976) method 

(figs. 1–13 and 1–14). A storage coefficient value for the 

Floridan aquifer system of 1.7×10–3 was used based on the 

72-hour aquifer-test results (appendix 2). A borehole diameter 

of 10.2 inches was used based on the average borehole 

diameter for the open interval. The transmissivity estimates 

from the rising (insertion) and falling (removal) slug tests 

were 54,000 and 62,000 ft2/d, respectively.

The estimate of transmissivity is affected by parameter 

sensitivity and by likely attenuation of the slug influence. 

Transmissivity estimates are sensitive to both the storage 

coefficient and the borehole diameter that is used in the 

van der Kamp (1976) method. As an example, increasing the 

borehole diameter from 10.2 to 10.5 inches and decreasing the 

storage coefficient from 1.7×10–3 to 1.1×10–3 in the fallinghead slug test increases the estimated transmissivity from 

54,000 to 60,000 ft2/d. The lower maximum displacement 

response of the upper transducer, compared to the transducers 

that are closer to the slug influence near the center of the 

packer assembly, indicates that the slug influence does not 

evenly affect the entire borehole. As a result, the estimate of 

transmissivity does not necessarily reflect an even measurement of the entire Floridan aquifer system.
32 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012 Figure 1–1. Water-level response to inserting slug into the water column connected to the packer-isolated interval at 612 to 618 feet below land surface, Lower Floridan confining unit, test hole 35Q069 near Pooler, Georgia, December 13, 2011. Static water level was 46.2 feet below land surface. 

A. Complete vertical extent of water-level response. B. Larger vertical scale of water-level response.

–

765432101

150 200 250 300 350 400 450

Slug-induced hydraulic head, in feet above static water level 

Lower transducer

EXPLANATION

Middle transducerUpper transducer

A

–0.04–

0.100.080.060.040.020.000.02

100 150 200 250 300 350 400 450 500

Time, in seconds after December 13, 2011, at 3:26 p.m.

B

100 500

Figure 1–1. Water-level response to inserting slug into the water column connected to the packer-isolated interval at 612 to 618 feet below land surface, Lower Floridan confining unit, test hole 35Q069, Pooler, Georgia, December 13, 2011. Static water level was 46.2 feet below land surface. A, Complete vertical extent of water-level response. 

B, Larger vertical scale of water-level response.
Appendix 1. Slug tests, Pooler, Georgia, December 13–14, 2011 33

0.01

0.10

1.00

0 30 60 90 120 150 180

Remaining proportion of initial displacement 

Time, in seconds

Measured displacement

Representation line

EXPLANATION

Figure 1–2. Semi-log plot of water-level recovery from inserting slug into the water column connected

to the packer-isolated interval at 612 to 618 feet below land surface, Lower Floridan confining unit, 

test hole 35Q069 near Pooler, Georgia, December 13, 2011. Representation line was used to determine

the horizontal hydraulic conductivity of the packer-isolated interval, using the Bouwer and Rice (1976) 

method. Modified from a graph on a spreadsheet from Halford and Kuniansky (2002). 

Figure 1–2. Semi-log plot of water-level recovery from inserting slug into the water column connected 

to the packer-isolated interval at 612 to 618 feet below land surface, Lower Floridan confining unit, 

test hole 35Q069, Pooler, Georgia, December 13, 2011. Representation line was used to determine the 

horizontal hydraulic conductivity of the packer-isolated interval, using the Bouwer and Rice (1976) 

method. Modified from a graph on a spreadsheet from Halford and Kuniansky (2002).
34 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012 Figure 1–3. Water-level response to pulling a slug from the water column connected to the packer-isolated interval at 612 to 618 feet below land surface, Lower Floridan confining unit, test hole 35Q069 near Pooler, Georgia, December 13, 2011. Static water level was 46.2 feet below land surface. 

A. Complete vertical extent of water-level response. B. Larger vertical scale of water-level response.

–4.00–3.50–3.00–2.50–2.00–1.50–1.00–0.50

0

0.50

0 50 100 150 200 250 300 350 400 450 500

–0.10–0.08–0.06–0.04–0.02

0

0.040.02

0 50 100 150 200 250 300 350 400 450 500

Slug-induced hydraulic head, in feet above static water level 

Lower transducer

EXPLANATION

Middle transducerUpper transducer

Time, in seconds after December 13, 2011, at 3:37 p.m.

AB

Figure 1–3. Water-level response to pulling a slug from the water column connected to the packer-isolated interval at 612 to 618 feet below land surface, Lower Floridan confining unit, test hole 35Q069, Pooler, Georgia, December 13, 2011. Static water level was 46.2 feet below land surface. A, Complete vertical extent of water-level response. 

B, Larger vertical scale of water-level response.
Appendix 1. Slug tests, Pooler, Georgia, December 13–14, 2011 35

0.01

0.10

1.00

Remaining proportion of initial displacement 

0 30 60 90 120 150 180

Time, in seconds

Figure 1–4. Semi-log plot of water-level recovery from pulling slug from the water column connected

to the packer-isolated interval at 612 to 618 feet below land surface, Lower Floridan confining unit, 

test hole 35Q069 near Pooler, Georgia, December 13, 2011. Representation line was used to determine

the horizontal hydraulic conductivity of the packer-isolated interval, using the Bouwer and Rice (1976) 

method. Modified from a graph on a spreadsheet from Halford and Kuniansky (2002). 

Measured displacement

Representation line

EXPLANATION

Figure 1–4. Semi-log plot of water-level recovery from pulling slug from the water column connected 

to the packer-isolated interval at 612 to 618 feet below land surface, Lower Floridan confining unit, 

test hole 35Q069, Pooler, Georgia, December 13, 2011. Representation line was used to determine the 

horizontal hydraulic conductivity of the packer-isolated interval, using the Bouwer and Rice (1976) 

method. Modified from a graph on a spreadsheet from Halford and Kuniansky (2002). 
36 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012 Figure 1

–5. Water-level response to inserting a slug into the water column connected to the packer-isolated interval at 642 to 648 feet below land 

surface, Lower Floridan confining unit, test hole 35Q069 near Pooler, Georgia, December 14, 2011. Static water level was 46.1 feet below land surface. A. Complete vertical extent of water-level response. B. Larger vertical scale of water-level response.

–1

876543210

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

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

–0.04–0.02

0

0.100.080.060.040.02

Slug-induced hydraulic head, in feet above static water level 

Time, in seconds after December 14, 2011, at 8:43 a.m.

AB

Lower transducer

EXPLANATION

Middle transducerUpper transducer

Figure 1–5. Water-level response to inserting a slug into the water column connected to the packer-isolated interval at 642 to 648 feet below land surface, Lower Floridan confining unit, test hole 35Q069, Pooler, Georgia, December 14, 2011. Static water level was 46.1 feet below land surface. A, Complete vertical extent of water-level response. 

B, Larger vertical scale of water-level response.
Appendix 1. Slug tests, Pooler, Georgia, December 13–14, 2011 37

0 120 240 360 480

0.01

0.10

1.00

Remaining proportion of initial displacement 

Time, in seconds

Figure 1–6. Semi-log plot of water-level recovery from inserting slug into the water column connected

to the packer-isolated interval at 642 to 648 feet below land surface, Lower Floridan confining unit, 

test hole 35Q069 near Pooler, Georgia, December 14, 2011. Representation line was used to determine

the horizontal hydraulic conductivity of the packer-isolated interval, using the Bouwer and Rice (1976)

method. Modified from a graph on a spreadsheet from Halford and Kuniansky (2002). 

Measured displacement

Representation line

EXPLANATION

Figure 1–6. Semi-log plot of water-level recovery from inserting slug into the water column connected 

to the packer-isolated interval at 642 to 648 feet below land surface, Lower Floridan confining unit, 

test hole 35Q069, Pooler, Georgia, December 14, 2011. Representation line was used to determine the 

horizontal hydraulic conductivity of the packer-isolated interval, using the Bouwer and Rice (1976) 

method. Modified from a graph on a spreadsheet from Halford and Kuniansky (2002).
38 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012 Figure 1–7. Water-level response to pulling slug from the water column connected to the packer-isolated interval at 642 to 648 feet below land surface, Lower Floridan confining unit, test hole 35Q069 near Pooler, Georgia, December 14, 2011. Static water level was 46.1 feet below land surface.

A. Complete vertical extent of water-level response. B. Larger vertical scale of water-level response.

–5.0–4.5–4.0–3.5–3.0–2.5–2.0–1.5–1.0–0.5

0

0.5

100 200 300 400 500 600 700 800 900

100 200 300 400 500 600 700 800 900

–0.10–0.08–0.06–0.04–0.02

0

0.040.02

Slug-induced hydraulic head, in feet above static water level 

Lower transducer

EXPLANATION

Middle transducerUpper transducer

Time, in seconds after December 14, 2011, at 8:58 a.m.

AB

Figure 1–7. Water-level response to pulling slug from the water column connected to the packer-isolated interval at 642 to 648 feet below land surface, Lower Floridan confining unit, test hole 35Q069, Pooler, Georgia, December 14, 2011. Static water level was 46.1 feet below land surface. A, Complete vertical extent of water-level response. 

B, Larger vertical scale of water-level response.
Appendix 1. Slug tests, Pooler, Georgia, December 13–14, 2011 39

0.01

0.10

1.00

Remaining proportion of initial displacement 

0 120 240 360 480

Time, in seconds

Figure 1–8. Semi-log plot of water-level recovery from pulling slug from the water column connected

to the packer-isolated interval at 642 to 648 feet below land surface, Lower Floridan confining unit, 

test hole 35Q069 near Pooler, Georgia, December 14, 2011. Representation line was used to determine

the horizontal hydraulic conductivity of the packer-isolated interval, using the Bouwer and Rice (1976) 

method. Modified from a graph on a spreadsheet from Halford and Kuniansky (2002). 

Measured displacement

Representation line

EXPLANATION

Figure 1–8. Semi-log plot of water-level recovery from pulling slug from the water column connected 

to the packer-isolated interval at 642 to 648 feet below land surface, Lower Floridan confining unit, 

test hole 35Q069, Pooler, Georgia, December 14, 2011. Representation line was used to determine the 

horizontal hydraulic conductivity of the packer-isolated interval, using the Bouwer and Rice (1976) 

method. Modified from a graph on a spreadsheet from Halford and Kuniansky (2002). 
40 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012 Figure 1–9. Water-level response to inserting slug into the water column connected to the packer-isolated interval at 690 to 696 feet below land surface, Lower Floridan confining unit, test hole 35Q069 near Pooler, Georgia, December 14, 2011. Static water level was 46.1 feet below land surface. 

A. Complete vertical extent of water-level response. B. Larger vertical scale of water-level response.

–1

6543210

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

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

–0.10–0.05

0

0.300.250.200.150.100.05

Increased air pressure

in packers

Time, in seconds after December 14, 2011, at 11:44 a.m.

Lower transducer

EXPLANATION

Middle transducerUpper transducer

Slug-induced hydraulic head, in feet above static water level 

Figure 1–9. Water-level response to inserting slug into the water column connected to the packer-isolated interval at 690 to 696 feet below land surface, Lower Floridan confining unit, test hole 35Q069, Pooler, Georgia, December 14, 2011. Static water level was 46.1 feet below land surface. A, Complete vertical extent of water-level response. 

B, Larger vertical scale of water-level response.
Appendix 1. Slug tests, Pooler, Georgia, December 13–14, 2011 41 Figure 1–10. Water-level response to pulling slug from the water column connected to the packer-isolated interval at 690 to 696 feet below land surface, Lower Floridan confining unit, test hole 35Q069 near Pooler, Georgia, December 14, 2011. Static water level was 46.1 feet below land surface. 

A. Complete vertical extent of water-level response. B. Larger vertical scale of water-level response.

–5

–4

–3–2

–

101

100 300 500 700 900 1,100 1,300 1,500 1,700 1,900 2,100 2,300 2,500 2,700 2,900 3,100 3,300

100 300 500 700 900 1,100 1,300 1,500 1,700 1,900 2,100 2,300 2,500 2,700 2,900 3,100 3,300

–0.10–0.08–0.06–0.04–0.02

0

0.040.02

Slug-induced hydraulic head, in feet above static water level 

Time, in seconds after December 14, 2011, at 12:05 p.m.

Lower transducer

EXPLANATION

Middle transducerUpper transducer

Figure 1–10. Water-level response to pulling slug from the water column connected to the packer-isolated interval at 690 to 696 feet below land surface, Lower Floridan confining unit, test hole 35Q069, Pooler, Georgia, December 14, 2011. Static water level was 46.1 feet below land surface. A, Complete vertical extent of water-level response. 

B, Larger vertical scale of water-level response.
42 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

0 15 30

Time, in minutes

Remaining proportion of initial displacement 

Figure 1–11. Semi-log plot of water-level recovery from pulling slug from the water column connected

to the packer-isolated interval at 690 to 696 feet below land surface, Lower Floridan confining unit, 

test hole 35Q069 near Pooler, Georgia, December 14, 2011. Representation line was used to determine

the horizontal hydraulic conductivity of the packer-isolated interval, using the Bouwer and Rice (1976) 

method. Modified from a graph on a spreadsheet from Halford and Kuniansky (2002). 

Measured displacement

Representation line

EXPLANATION

Slug submerged

–0.3

–0.2

–0.1

0

0.1

0.2

0.3

13:09:00 13:11:53 13:14:46 13:17:38 13:20:31 13:23:24

December 14, 2011

Slug-induced hydraulic head, in

feet above static water level

Lower transducer

EXPLANATION

Middle transducer

Upper transducer

Figure 1–11. Semi-log plot of water-level recovery from pulling slug from the water column connected 

to the packer-isolated interval at 690 to 696 feet below land surface, Lower Floridan confining unit, 

test hole 35Q069, Pooler, Georgia, December 14, 2011. Representation line was used to determine the 

horizontal hydraulic conductivity of the packer-isolated interval, using the Bouwer and Rice (1976) 

method. Modified from a graph on a spreadsheet from Halford and Kuniansky (2002). 

Figure 1–12. Water-level response to inserting slug into then pulling slug from 

the water column connected to the Floridan aquifer system from 339 to 1,130 feet 

below land surface, test hole 35Q069, Pooler, Georgia, December 14, 2011. Static 

water level was 46.0 feet below land surface.
Appendix 1. Slug tests, Pooler, Georgia, December 13–14, 2011 43

0.01

0.1

1.0

0 50 100 150 200 250

Absolute displacement, in feet

Time, in seconds

Time, in seconds

–0.25

–0.20

–0.15

–0.10

–0.05

0

0.05

0.10

0.15

0.20

0.25

0.30

0 20 40 60 80 100

Displacement, in feet

Figure 1–13. Measured and simulated water levels for inserting slug into water column 

open to the Floridan aquifer system from 339 to 1,120 feet below land surface near Pooler, 

Georgia, December 14, 2011. 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).

A. Estimating damping coefficient γ

B. Estimating frequency ω

γ=0.0161598 second–1

ω=0.217411 second–1

EXPLANATION

Simulated

Trend

Measured

Figure 1–13. Measured and simulated water levels for inserting slug into water column 

open to the Floridan aquifer system from 339 to 1,130 feet below land surface, Pooler, 

Georgia, December 14, 2011. 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).
44 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

0.01

0.1

1.0

0 50 100 150 200 250

–0.30

–0.25

–0.20

–0.15

–0.10

–0.05

0

0.05

0.10

0.15

0.20

0.25

0 20 40 60 80 100

Time, in seconds

Time, in seconds

Absolute displacement, in feet Displacement, in feet

Figure 1–14. Measured and simulated water levels for pulling slug from water column 

open to the Floridan aquifer system from 339 to 1,120 feet below land surface near Pooler, 

Georgia, December 14, 2011. 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).

A. Estimating damping coefficient γ

B. Estimating frequency ω

γ=0.014071 second–1

ω=0.216662 second–1

EXPLANATION

Simulated

Trend

Measured

Figure 1–14. Measured and simulated water levels for pulling slug from water column 

open to the Floridan aquifer system from 339 to 1,130 feet below land surface, Pooler, 

Georgia, December 14, 2011. 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).
Appendix 2. Aquifer Tests, Pooler, Georgia, March and April 2012 45

Appendix 2. Aquifer Tests, Pooler, Georgia, March and April 2012

Aquifer tests were performed at the Pooler test site 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 27–28, 2012, in well 35Q070 open 

to the Upper Floridan aquifer (UFA), and a 72-hour aquifer 

test was performed during April 16–19, 2012, in well 35Q069 

open to the Lower Floridan aquifer (LFA).

Wells 35Q069 and 35Q070 were monitored at least from 

March 26 to May 8, 2012. This timeframe included a pre-test 

period of slightly more than 16 hours, a 24-hour aquifer test 

period in the UFA, a failed 72-hour aquifer test period in the 

LFA, a 72-hour aquifer test period in the LFA, and a post-test 

period of at least 10 days. Numerous disruptions occurred 

in the continuous water-level record during the study period 

(table 2–1, fig. 9), caused by pumping events in well 35Q069 and 

data gaps in both wells due to temporary removal of transducers.

Drawdown Estimation

Drawdown for the two wells was estimated by using a 

procedure developed by Halford (2006a) to filter water-level 

data for effects of barometric pressure, earth and ocean tides, 

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 (fig. 2–1). The synthetic 

water levels closely approximate (are matched to) measured 

water levels during periods that are unaffected by an aquifer 

test (hereinafter referred to as the “fitting period”; fig. 2–2). 

Measured water level during the fitting period is used to 

determine phase shifts and multipliers of explanatory variables 

that will synthesize a water level that matches the measured 

water levels. Differences between the synthetic and measured 

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

function. The synthetic water level is further adjusted using 

the offset and slope with respect to time to match the synthetic 

water level with the measured water level just before the start 

of the aquifer test and just after full recovery from the aquifer 

test. The difference between synthetic and measured water 

level during the aquifer test and recovery represents drawdown 

induced by pumping; the synthetic water levels represent 

nonpumping water levels.

Four drawdown conditions were evaluated during the 

aquifer tests: (1) response of the UFA well to the 24-hour 

aquifer test in the UFA (fig. 2–3); (2) response of the LFA 

well to the 24-hour aquifer test in the UFA (fig. 2–4); 

Table 2–1. Disruptions in continuous monitoring of natural water-level during the study period, Pooler, Georgia, February through 

April 2012.

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

Well

Start time End time

Event description (Month day time)

35Q069 February 23 1 p.m. NA Start monitoring

35Q069 March 4 3:30 p.m. March 5 0:45 a.m. Unnatural spike

35Q069 March 20 6:15 p.m. March 26 4:45 p.m. Unnatural daily fluctuations characteristic of a clogged vent

35Q070 March 26 4:30 p.m. NA Start monitoring

Both March 27 9:00:03 a.m. March 28 9:05:03 a.m. 24-hour aquifer test (pumping UFA well 35Q070 at 285 gal/min)

Both March 28 9:05:03 a.m. March 31 12 a.m. 24-hour aquifer test recovery

35Q070 April 2 11:50 a.m. April 4 8:15 a.m. No data

36Q069 April 3 1 p.m. April 4 5:30 p.m. No data

36Q069 April 4 6:30 p.m. April 4 7:15 p.m. Pumping event in LFA well 35Q069 and recovery

36Q069 April 5 2:45 p.m. April 5 8:15 p.m. Pumping event in LFA well 35Q069 and recovery

36Q069 April 10 6 a.m. April 10 11:36 a.m. No data

Both April 10 12:00:44 p.m. April 10 3:20 p.m. Failed 72-hour aquifer test (pumping LFA well 35Q069 at 

1,200 gal/min)

Both April 10 3:20 p.m. April 11 12 p.m. Failed 72-hour aquifer test recovery

36Q069 April 12 7 a.m. April 13 12 p.m. Pumping event in LFA well 35Q069 and recovery

Both April 16 12:00:04 p.m. April 19 12:05:10 p.m. 72-hour aquifer test (pumping LFA well 35Q069 at 783 gal/min)

Both April 19 12:05:10 p.m. April 28 12 a.m. 72-hour aquifer test recovery

Both NA May 8 2 p.m. End of monitoring
46 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

(3) response of the LFA well to the 72-hour aquifer test in 

the LFA (fig. 2–5); and (4) response of the UFA well to the 

72-hour aquifer test in the LFA (fig. 2–6).

Barometric pressure and gravity along with water levels 

from four background wells (36Q020, 33R045, 35P110, and 

35P125) and ocean tides from Savannah River stream gage 

(station number 02198980) were used as explanatory timeseries components to filter out nonpumping influences during 

the aquifer tests (locations shown in fig. 1). Barometric-pressure 

data were compiled from hourly barometric-pressure data from 

National Oceanic and Atmospheric Administration (NOAA) 

weather station 097847, located about 6 miles northeast of the 

Pooler test site (097847 location shown in fig. 1). Ocean-tide 

data were provided from the Savannah River stream gage 

02198980, located at the mouth of the Savannah River where 

it enters the Atlantic Ocean. Gravity is used as a surrogate for 

earth tides and is expressed as microgravity deviation from 

average. The time-series spreadsheet from Halford (2006a) 

generates gravity time series from entered values of latitude, 

longitude, and land-surface altitude. Background wells are far 

enough away from the Pooler test site so that they do not show 

a response (zero drawdown) to the aquifer tests but have a 

response to nonaquifer-test influences that are similar to those of 

the test wells. Background wells 36Q020 and 35P110 are open 

to the UFA; background wells 33R045 and 35P125 are open to 

the LFA. Background well 36Q020 is strongly affected by earth 

tides. Water levels in background wells and barometric pressure, 

gravity, and ocean tides are shown in figure 2–1.

The synthetic water level for a well w during a given 

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

i

n

, , , , 0 ,

1 i

= + ( ) − + ( ) ( ) ( ) ( ) +

=

× × (3)

where

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

during time period j at time t, in feet 

above datum;

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

in feet above datum;

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 ) in days;

n is the number of explanatory time-series 

components, ranging from 3 to 12 in 

this study;

ai is the amplitude of the multiplier for the 

ith explanatory time-series component, 

in feet per unit of the explanatory timeseries component;

V(i,t+φi) is the value of the ith explanatory timeseries component at time t+φi

; and

φi is the phase shift of the i th explanatory 

time-series component, in days.

The parameters ai and φi are adjusted for each explanatory 

time-series component and m(w,j) and C(w,j) are applied to the 

synthetic water-level time series. The time period j is either 

the specific fitting period or the period of time from before to 

after the aquifer test and recovery. The data set that makes up 

j is the time period plus a phase shift buffer before the start 

and after the end of j. The buffers allow the start and end of 

the explanatory time series to shift (φi  ) beyond the bounds of 

j. For a given combination of explanatory time-series components and fitting period, the time-series spreadsheet is used to 

adjust the amplitude of multipliers, phase shifts, slope with 

respect to time, and offset to fit the synthetic water level to the 

measured water level during the fitting period. The spreadsheet 

and supporting program are used to fit the synthetic water 

level to the measured water level by minimizing their differences with a sum-of-squares objective function.

Numerous disruptions in the water-level data of the 

test wells limit the times that can be used as fitting periods 

(table 2–1). Two fitting periods ultimately were used: an early 

fitting period beginning midnight on March 31, 2012, and 

ending at 11 a.m. on April 2, 2012; and a late fitting period 

beginning midnight on April 28, 2012, and ending at 9 a.m. on 

May 6, 2012. Halford (2006a) indicates that the fitting period 

ideally would include the period immediately antecedent of 

the aquifer test and should extend at least four times longer 

than the prediction period. Given that the prediction period 

should include the pumping and recovery periods, the required 

fitting period would extend about 2 weeks for the 24-hour 

aquifer test and about 4 weeks for the 72-hour aquifer test. 

Unfortunately, the available fitting periods could not extend as 

long as required; therefore, some nonaquifer-test influence was 

not removed from the water-level data.

For each well-aquifer-test situation, a combination of 

explanatory time series and a fitting period were selected to 

best filter nonpumping influences from the water-level data. 

The process begins by selecting a candidate combination from 

the pool of available explanatory time-series components. 

The amplitudes of the multipliers and phase shifts are 

adjusted for each explanatory time series to minimize the 

sum-of-squares objective. If the synthesized water level for 

a candidate combination of explanatory time series matches 

closely to the selected fitting period (fig. 2–2), the amplitudes 

of multipliers and phase shifts used to match the fitting period 

are used to synthesize water levels from several hours before 

the aquifer test to about a day after the aquifer-test recovery. 

The offset and slope with respect to time were adjusted so that 

the synthetic water level matched the measured water for the 

time just before the aquifer test and just after the aquifer-test 

recovery. Different combinations of explanatory time series 

were used with both fitting periods to find the best match 

before and after the aquifer test and recovery (best matches 

shown in figs. 2–3 to 2–6). For the best match of synthetic 

water level with measured water level before the aquifer test 

and after the aquifer-test recovery, the synthetic water level 

during the aquifer test was taken as the water level not being 

affected by the aquifer test. The drawdown was the synthetic 

water level minus the measured water level.
Appendix 2. Aquifer Tests, Pooler, Georgia, March and April 2012 47

–3

–2

–1

0

1

2

3

4

5

Water level, in feet

above NAVD 88 

29.4

29.5

29.6

29.7

29.8

29.9

30.0

30.1

30.2

30.3

30.4

Barometric pressure,

in inches of mercury 

–1.0

–0.8

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

0.8

1.0

–8

–6

–4

–2

0

2

4

6

Ocean tide, Savannah

River at Fort Pulaski,

in feet above NAVD 88

Microgravity, deviation

from average, normalized

to minimum value

Figure 2–1. Water-level fluctuations in background wells and fluctuations in barometric pressure, gravity, 

and ocean tides used in the aquifer tests near Pooler, Georgia, March 27–May 6, 2012. A. Pumping phase 

of Upper Floridan 24-hour aquifer test. B. Recovery phase of Upper Floridan 24-hour aquifer test. C. Early 

fitting period. D. Failed Lower Floridan aquifer test. E. Pumping phase of Lower Floridan 72-hour aquifer test. 

F. Recovery phase of Lower Floridan 72-hour aquifer test. G. Late fitting period. The fitting period is used 

to describe the match (or fit) of synthetic water levels to those that were measured.

A B C D E F G

27 1 6 11 16 21 26 1 6

2012

March April May

EXPLANATION

Pumping phase of aquifer test

Recovery phase of aquifer test

Fitting period

Data gap in at least one well

Other pumping event

Water level (top graph only)

Lower Floridan aquifer 

 Well 33R045 – 14 feet

 Well 35P125 + 21 feet

Upper Floridan aquifer 

 Well 35P110 + 22 feet

 Well 36Q020 + 33 feet

Figure 2–1. Water-level fluctuations in background wells and fluctuations in barometric pressure, gravity, 

and ocean tides used in the aquifer tests, Pooler, Georgia, March 27–May 6, 2012. A, Pumping phase of Upper 

Floridan 24-hour aquifer test. B, Recovery phase of Upper Floridan 24-hour aquifer test. C, Early fitting period. 

D, Failed Lower Floridan aquifer test. E, Pumping phase of Lower Floridan 72-hour aquifer test. F, Recovery 

phase of Lower Floridan 72-hour aquifer test. G, Late fitting period. The fitting period is used to describe the 

match (or fit) of synthetic water levels to those that were measured.
48 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

– 0.40

– 0.35

– 0.30

– 0.25

– 0.20

– 0.15

– 0.10

– 0.05

0

0.05

0.10

Water-level change, in feet Water-level change, in feet

Measured

EXPLANATION

Synthetic

Synthetic minus measured

–0.25

–0.20

–0.15

–0.10

–0.05

0

0.05

0.10

0.15

0.20

0.25

March 31 April 1

28 29

April May

30 1 2 3 4 5 6

April 2

2012

2012

–1.60

–1.40

–1.20

–1.00

–0.80

–0.60

–0.40

–0.20

0

0.20

0.40

Figure 2–2. Fit of synthetic water levels to measured water levels for 

wells 35Q070 and 35Q069 near Pooler, Georgia, April 28–May 6, 2012. The 

early fitting period was used to estimate drawdown in well 35Q070 in response 

to the 24- and 72-hour aquifer tests and in well 35Q069 in response to the 

24-hour aquifer test. The late fitting period was used to estimate the drawdown 

in well 35Q069 in response to the 72-hour aquifer test.

Well 35Q070 (early fitting period)

Well 35Q069 (early fitting period)

Well 35Q069 (late fitting period)

Figure 2–2. Fit of synthetic water levels to measured water levels for wells 

35Q070 and 35Q069, Pooler, Georgia, April 28–May 6, 2012. The early fitting 

period was used to estimate drawdown in well 35Q070 in response to the 

24- and 72-hour aquifer tests and in well 35Q069 in response to the 24-hour 

aquifer test. The late fitting period was used to estimate the drawdown in 

well 35Q069 in response to the 72-hour aquifer test.
Appendix 2. Aquifer Tests, Pooler, Georgia, March and April 2012 49

Figure 2–3. Measured and synthetic water levels and estimated drawdown in Upper Floridan aquifer well 35Q070 

during the 24-hour aquifer test performed in the well, near Pooler, Georgia, March 27–April 1, 2012.

This graph is larger vertical scale from graph above

24-hour

aquifer test

24-hour

aquifer test

–15

–10

–5

0

5

10

15

Water-level change, in feet –0.5

–0.4

–0.3

–0.2

–0.1

0

0.1

0.2

0.3

0.4

0.5

March 27 March 28 March 29 March 30 March 31 April 1

2012

Measured

EXPLANATION

Synthetic Synthetic minus measured (estimated drawdown)

24-hour

aquifer test

Recovery

–0.2

0

0.2

0.4

0.6

0.8

1.0

Water-level change, in feet 

Figure 2–4. Measured and synthetic water levels and estimated drawdown in Lower Floridan aquifer well 35Q069 

during the 24-hour aquifer test performed in the Upper Floridan aquifer well 35Q070, near Pooler, Georgia, 

March 27–April 1, 2012.

March 27 March 28 March 29 March 30 March 31 April 1

2012

Measured

EXPLANATION

Synthetic Synthetic minus measured (estimated drawdown)

Figure 2–3. Measured and synthetic water levels and estimated drawdown in Upper Floridan aquifer well 35Q070 

during the 24-hour aquifer test performed in the well, Pooler, Georgia, March 27–April 1, 2012.

Figure 2–4. Measured and synthetic water levels and estimated drawdown in Lower Floridan aquifer well 35Q069 

during the 24-hour aquifer test performed in the Upper Floridan aquifer well 35Q070, Pooler, Georgia, March 27–

April 1, 2012.
50 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Figure 2–5. Measured and synthetic water levels and estimated drawdown in Lower Floridan aquifer well 35Q069 

during the 72-hour aquifer test performed in the well, near Pooler, Georgia, April 16–May 5, 2012.

72-hour

aquifer test

Recovery

72-hour Recovery

aquifer test

–60

–40

–20

0

20

40

60

–3.0

–2.5

–2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

2.0

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5

This graph is larger vertical scale from graph above

Water-level change, in feet 

April May

2012

Measured

EXPLANATION

Synthetic Synthetic minus measured (estimated drawdown)

April May

72-hour

aquifer test

Recovery

?

?

?

?

Figure 2–6. Measured and synthetic water levels and estimated drawdown in Upper Floridan aquifer well 35Q070 

during the 72-hour aquifer test performed in the Lower Floridan aquifer well 35Q069, near Pooler, Georgia, 

April 16–May 5, 2012.

–4

–3

–2

–1

0

1

2

16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 1 2 3 4 5

2012

Measured

EXPLANATION

Synthetic Synthetic minus measured (estimated drawdown)

Water-level change, in feet 

Figure 2–5. Measured and synthetic water levels and estimated drawdown in Lower Floridan aquifer well 35Q069 

during the 72-hour aquifer test performed in the well, Pooler, Georgia, April 16–May 5, 2012.

Figure 2–6. Measured and synthetic water levels and estimated drawdown in Upper Floridan aquifer well 35Q070 

during the 72-hour aquifer test performed in the Lower Floridan aquifer well 35Q069, Pooler, Georgia, April 16–

May 5, 2012.
Appendix 2. Aquifer Tests, Pooler, Georgia, March and April 2012 51

24-Hour Aquifer Test

Pumping for the 24-hour aquifer test at UFA well 35Q070 

began at 9:00 a.m. on March 27, 2012, and continued for 

24 hours and 5 minutes until 9:05 a.m. on March 28, 2012 

(fig. 2–3). The well was pumped at a rate that varied slightly 

(table 2–2). The pumping rate for the first 15 hours was 

285 gallons per minute (gal/min). Starting at midnight on 

March 28, 2012, the rate decreased to about 281 gal/min and 

continued to decrease to slightly less than 280 gal/min. From 

early morning on March 28, 2012, until pumping was stopped, 

the discharge rate slowly increased back to 285 gal/min.

explanatory time-series components was effective in filtering 

out much nonaquifer-test influence on aquifer-test water 

levels. The drawdown time series resulting from applying 

several different filters to mitigate the nonaquifer-test noise 

produced similar results (fig. 2–8). 

During the 15 minutes to 15 hours aquifer testing, 

drawdown consistently increased with log time (constant log 

cycle; fig. 2–9). At midnight on March 28, 2012, drawdown 

decreased. At this time, the discharge rate suddenly decreased 

from 285 to 281 gal/min, which might have caused the 

decrease in drawdown.

The decline in water level (caused by aquifer-test and 

other influences) during the 24-hour aquifer test at UFA 

pumped well 35Q070 equaled about 11.2 ft, from 41.7 to 

about 52.9 ft below land surface. The water-level decline 

was obscured by barometric-pressure change. Adjusting 

the drawdown for barometric-pressure change indicates a 

corrected drawdown, in response to the aquifer test, of about 

11.1 ft as of midnight March 28, 2012, (fig. 2–3).

The water-level decline in LFA well 35Q069, in response 

to the 24-hour aquifer test in UFA pumped well 35Q070, was 

obscured by barometric-pressure change, earth tides, and 

ocean tides. Twelve explanatory time-series components, 

including all four background wells and the fitting period from 

midnight on March 31, 2012, to 11:00 a.m. on April 2, 2012, 

were used to filter out nonaquifer-test influences. Synthetic 

water levels matched well with measured water levels before 

and after the 24-hour pumping period and recovery (fig. 2–4). 

Drawdown began to increase approximately 20 minutes after 

the start of the 24-hour aquifer test. From about 20 minutes 

to about 8 hours after the start of the 24-hour aquifer test, 

drawdown increased from 0 to 0.09 ft. From 8 hours after 

the start of the aquifer test to the end of the pumping period, 

drawdown fluctuated slightly, but stayed at about 0.09 ft.

Aquifer-Test Analysis

The Cooper-Jacob, straight-line method (Cooper 

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

pumped well 35Q070 to determine the transmissivity of the 

UFA (fig. 2–9). Using the laminar drawdown (as described 

by Batu, 1998) between 15 minutes and 15 hours after the 

start of the aquifer test, the transmissivity was estimated to 

be 30,000 feet squared per day (ft2/d). The data points used 

to determine the log-cycle of drawdown consist of clusters 

on either end of the line, which created uncertainty in the 

selection of distinct analytical end points to the straight line 

and therefore the slope. Transmissivities for different fits to the 

drawdown data range from about 27,000 to 33,000 ft2/d and 

round to 30,000 ft2/d.

Table 2–2. Pumping rate through the 24-hour aquifer test in 

well 35Q070, Pooler, Georgia, March 27–28, 2012.

Time of start of

change in pump rate

(Month day time)

Measured pump rate

 (gallons per minute)

March 26 4:30 p.m. 0

March 27 9:00:03 a.m. 287

March 27 9:15 a.m. 285

March 28 12 a.m. 281

March 28 3 a.m. 280

March 28 7 a.m. 281

March 28 8 a.m. 283

March 28 8:45 a.m. 285

March 28 9:05:03 a.m. 0

Drawdown Response

In response to aquifer-test pumping, the water level in 

well 35Q070 declined by 10.4 feet (ft) almost instantaneously, 

exhibiting random water-level fluctuations (noise) in the 

drawdown signal (fig. 2–7). In response to the end of the 

24-hour pumping period, the water level rose to near zero 

drawdown almost instantaneously with some noise that 

diminished after about 100 seconds. The near-instantaneous 

decline in water level represents the turbulent component of 

drawdown (Batu, 1998). The laminar component of drawdown 

is a consistent increase in drawdown with time, which is used 

to estimate the transmissivity of the UFA. 

Filtering the aquifer-test water-level response for 

barometric-pressure change seemed to best filter out erratic 

water-level fluctuations; however, no combination of 
52 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Start of aquifer test

End of aquifer test

Time between data points is 5 seconds

March 27, 2012

Pumping

Pumping

March 28, 2012

Figure 2–7. Measured water levels at the start and end of the 24-hour aquifer test in Upper Floridan 

aquifer well 35Q070 near Pooler, Georgia, March 27–28, 2012.

Water level below land surface, in feet Water level below land surface, in feet

38

40

42

44

46

48

50

52

54

56

08:58:36 08:59:26 09:00:16 09:01:06 09:01:56 09:02:46

38

40

42

44

46

48

50

52

54

56

09:03:36 09:04:26 09:05:16 09:06:06 09:06:56 09:07:46

Figure 2–7. Measured water levels at the start and end of the 24-hour aquifer test in Upper Floridan 

aquifer well 35Q070, Pooler, Georgia, March 27–28, 2012.
Appendix 2. Aquifer Tests, Pooler, Georgia, March and April 2012 53

Figure 2–9. Semi-log graph of drawdown during the 24-hour aquifer test in well 35Q070 at a 

pumping rate of about 285 gallons per minute near Pooler, Georgia, March 27–28, 2012.

Slope on semi-log plot: = 0.3400 ∆s

∆ log (t)

–0.4

–0.2

0

0.2

0.4

0.6

0.8

0.00001 0.0001 0.001 0.01 0.1 1 10

Estimated laminar drawdown, in feet 

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

Trend Measured drawdown

EXPLANATION

10.3

10.4

10.5

10.6

10.7

10.8

10.9

11.0

11.1

11.2

11.3

11.4

11.5

March 27

07:12:00

March 27

19:12:00

March 28

07:12:00

Estimated drawdown, in feet

2012

24-hour aquifer test

EXPLANATION

Uncorrected

Estimated drawdown

Corrected using barometric-pressure change

Average of all attempts

Other attempted corrections using different 

 combinations of explanatory time-series data

Figure 2–8. Results of several different 

ways drawdown was estimated for Upper 

Floridan well 35Q070 in response to the 

24-hour aquifer test in the well, Pooler, 

Georgia, March 27–28, 2012.

Figure 2–9. Semi-log graph of drawdown during the 24-hour aquifer test in well 35Q070 at a 

pumping rate of about 285 gallons per minute, Pooler, Georgia, March 27–28, 2012.
54 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

72-Hour Aquifer Test

The 72-hour aquifer test at LFA well 35Q069 was first 

attempted at noon on April 10, 2012. The well was pumped 

at a rate of 1,200 gal/min, much greater than the planned 

pumping rate for the proposed public-supply well for the City 

of Pooler, Georgia. Because the discharge rate was too high, 

the aquifer test was discontinued 3 hours and 20 minutes after 

pumping had started. The well was then allowed 6 days to 

recover from the failed aquifer test.

The second aquifer-test pumping at LFA well 35Q069 

began at noon on April 16, 2012, and continued for 72 hours 

and 5 minutes until 12:05 p.m. on April 19, 2012. The well 

was pumped at rates of 780 to 788 gal/min (table 2–3). The 

pump lacked a check value; therefore, when the aquifer-test 

pumping ceased, a slug of water siphoned back into the well. 

As a result, the water level shows a slug signature during 

the first 3 minutes of recovery (fig. 2–10). Within 3.5 days, 

the water level in pumped well 35Q069 was dominated by a 

long-term decline in background water level (figs. 9 and 2–5).

Drawdown Response

Filtering caused negligible modification to the drawdown 

of well 35Q069 when it was pumped, because the magnitude of 

the drawdown signal (greater than 50 ft) obscured any signals 

corresponding to nonpumping influences, which probably 

ranged from a few hundredths of a foot to about 0.4 ft (fig. 9). 

Pre-test data indicated a minor (up to 0.1 ft) cyclic diurnal 

fluctuation in water level, which is characteristic of earth tides, 

and more substantial (up to 0.4 ft) fluctuations in water levels, 

which is a characteristic of barometric-pressure change.

Decline in water level (caused by aquifer-test and other 

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

35Q069 equaled 52.0 ft, from 43.4 to 95.4 ft below land surface. 

Adjusting nonaquifer-test influences indicates a corrected 

drawdown of 51.7 ft in response to the aquifer test (fig. 2–5). 

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. 2–11). The nonlinear nature 

of drawdown on the semi-log plot precluded the viable use 

of an analytical method for estimating the transmissivity of 

the LFA. The rate of drawdown slightly increased 0.007 day 

(5 minutes) after the start of the aquifer test. No discharge-rate 

change was documented at the time. This slight change might 

be caused by lateral heterogeneity in the hydraulic properties 

of the Floridan aquifer system at the Pooler test site. 

Water-level decline in UFA well 35Q070 in response 

to the 72-hour aquifer test in LFA pumped well 35Q069 was 

obscured by daily fluctuations and long-term water-level 

trends. Water-level decline in UFA well 35Q070 (caused by 

aquifer test and other influences) during the 72-hour aquifer 

test at the LFA pumped well was 0.8 ft, from 43.1 to 43.9 ft 

below land surface (–24.1 to –24.9 ft above NAVD 88; fig. 9). 

Adjusting for nonaquifer-test influences indicates a corrected 

drawdown of 0.9 ft (fig. 2–6). Drawdown was obscured mostly 

by a change in long-term water-level trend on April 23, 2012, 

from relatively unchanging to declining and by daily fluctuations with an amplitude of about 0.2 ft (fig. 2–6). Explanatory 

time-series components that were used to filter out non-aquifertest influences were barometric-pressure change and water 

levels from all four background wells. Some nonaquifer-testrelated fluctuations in the value of synthetic water level minus 

measured water level remained after filtering; however, water 

levels from UFA well 35Q070 during the aquifer test were 

adequately filtered of barometric-pressure change and longterm water-level trend. Recovery data were dominated by noise 

that could not be adequately filtered out, and as a result, the 

recovery data were not used for aquifer-test analysis.

Table 2–3. Pumping rate through the 72-hour aquifer test in 

well 35Q0690, Pooler, Georgia, April 16–19, 2012.

Time of start of

change in pump rate

(Month day time)

Measured pump rate

 (gallons per minute)

April 12 1:30 p.m. 0

April 16 12:00:04 p.m. 788

April 16 12:55 p.m. 788

April 16 3 p.m. 783

April 16 6 p.m. 783

April 16 7 p.m. 780

April 19 12:05:09 p.m. 780

April 19 12:05:10 p.m. 0
Appendix 2. Aquifer Tests, Pooler, Georgia, March and April 2012 55

Figure 2–10. Water level at the end of the 72-hour aquifer test in well 35Q069 near Pooler, Georgia, 

April 19, 2012.

Pump

shut-off

Additional water-level rise 

due to lack of check valve 

on pump riser pipe

45

47

49

51

53

55

57

59

12:00:00 12:07:12 12:14:24 12:21:36 12:28:48

April 19, 2012

Pumping

Water level below land surface, in feet

Figure 2–11. Semi-log plot of drawdown in pumped well 35Q069 during the 72-hour aquifer test near Pooler, 

Georgia, April 16–19, 2012.

Well-bore storage effects

0

5

10

15

20

25

30

35

40

45

50

55

0.00001 0.0001 0.001 0.01 0.1 1

Drawdown, in feet

Time after start of 72-hour aquifer test, in days

Figure 2–10. Water level at the end of the 72-hour aquifer test in well 35Q069, Pooler, Georgia, 

April 19, 2012.

Figure 2–11. Semi-log plot of drawdown in pumped well 35Q069 during the 72-hour aquifer test, Pooler, 

Georgia, April 16–19, 2012.
56 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Model Simulation of the Lower Floridan 

Aquifer Test

Because of the nonlinear nature of drawdown on the 

semi-log plot, transmissivity of the UFA and LFA 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 using the parameterestimation program PEST (Doherty, 2005) minimizing the 

weighted sum-of-squares of differences between simulated 

and measured drawdown (hereinafter, these differences are 

referred to as “residuals”). Estimated drawdown, discussed in 

previous sections, is referred to as “measured drawdown.” 

The aquifer system was simulated with a twodimensional, axisymmetric, radial, transient groundwater-flow 

model that incorporated LFA pumped well 35Q069 and UFA 

well 35Q070 (fig. 2–12; Harbaugh and others, 2000). Full 

description of the derivation of two-dimensional radial models 

using a single layer or multiple layers are provided in the 

following references: Rutledge (1991); Reily and Harbaugh 

(1993); and Langevin (2008). The method of computation 

of flow observations for parameter estimation with radial 

models is described in Clemo (2002). Horizontal hydraulic 

conductivity and specific storage were assumed to be isotropic 

and homogeneous within each hydrogeologic unit. Model 

simulation results were not sensitive to the vertical anisotropy 

ratio (vertical hydraulic conductivity divided by the horizontal 

hydraulic conductivity). A vertical anisotropy ratio of 0.1 was 

assumed for each hydrogeologic unit (table 2–4). A pumping 

rate of 783 gal/min was used in the aquifer-test simulations.

The model domain was discretized into 120 rows representing the different aquifer thicknesses and 60 columns representing the radial distance from LFA pumped well 35Q069 to 

the external boundary (fig. 2–12). The model radially extends 

200,000.22 ft from the center of well 35Q069, and represents 

the subsurface depth interval between 43.38 (the estimated 

water-table depth) to 1,162.06 ft. Radial grid spacing (column 

width) consists of two increments of 0.333 ft to represent the 

borehole of LFA pumped well 35Q069. Column width beyond 

the borehole increases by a factor of 1.293, from 0.02 ft 

adjacent to the well to 45,282 ft at the edge of the model. Each 

row height represents a vertical thickness of 9.639 ft for the 

simulated aquifers and convening confining units.

Hydrogeologic units are represented in the model as five 

layers (fig. 2–12):

• Layer 1, represents the surficial aquifer system, 

Brunswick aquifer system, and Upper Floridan 

confining unit, undifferentiated;

• Layer 2, represents the UFA;

• Layer 3, represents the LFCU;

• Layer 4, represents the LFA; and

• Layer 5, represents the lower confining unit 

underlying the Floridan aquifer system.

Storage is assigned separately to parts of layer 1. The top 

model row represents water-table conditions and was assigned 

a specific yield of 0.1, within the range of specific yield for 

unconfined aquifers (Freeze and Cherry, 1979). All other 

model rows in layer 1 represent confined conditions and were 

assigned a specific storage of 2.55×10–6 ft– 1. 

Table 2– 4. Hydraulic parameters used to simulate measured drawdown and recovery and estimated values of transmissivity and 

storativity for the 72-hour aquifer test at pumped well 35Q069, Pooler, Georgia, April 16–19, 2012.

[ft/d, foot per day; ft –1, per foot; ft2/d, foot squared per day; NA, not applicable]

Hydrogeologic unit Layer Model 

rows

Horizontal 

hydraulic 

conductivity 

(ft/d)

Specific 

storage 

(ft –1)

Vertical

anisotropy

(dimensionless)

Thickness 

(feet)

Transmissivity

(ft2/d)

Storage

coefficient

(dimensionless)

Water-table row 1 1 a0.491857 a1.000×10–1 a0.1 9.639 NA NA

Overlying confining unit 1 29 a0.491857 a2.550×10– 6 a0.1 279.531 NA NA

Upper Floridan aquifer 2 19 252.768000 3.206×10– 6 a0.1 183.141 46,292 5.872×10–4

Lower Floridan 

confining unit

3 19 3.563630 2.484×10– 6 a0.1 183.141 653 4.549×10–4

Lower Floridan aquifer 4 35 11.935000 a1.190×10– 7 a0.1 337.365 4,026 NA

Lower confining unit 5 17 a0.100000 a2.370×10– 6 a0.1 163.863 NA NA

Floridan aquifer system 2–4 73 NA NA NA 704 50,971 NA

aNot estimated but assigned to the model.
Appendix 2. Aquifer Tests, Pooler, Georgia, March and April 2012 57

43

100

200

300

400

500

600

700

800

900

1,000

1,100

1,162

0 0.2 0.4 0.6 0.8 1 10 100 1,000 10,000 100,000

Layer 1: Water table (1 row)

Layer 1: Overlying confining unit

Layer 2: Upper Floridan aquifer

Layer 3: Lower Floridan confining unit

EXPLANATION

Well construction Lithologic subunits

Inactive cells (casing)

Borehole Layer 4: Lower Floridan aquifer

Layer 5: Lower confining unit

Linear scale Log scale

Land surface

(19 feet above

NAVD 88) 

Distance from center of pumped well 35Q069, in feet

120 rows

Observation

well 35Q070

Hydrogeologic units beyond pumped well (57 columns)

Figure 2–12. Axisymmetric model for 72-hour aquifer test at pumped well 35Q069 near Pooler, Georgia, 

April 16–19, 2012. Observation well 35Q070 and pumped well 35Q069 are included in the top diagram. Curved

arrows in the top diagram represent transforming three-dimensional location of well opening to the twodimensional model grid. The model is surrounded by no-flow boundaries.

Pumped

well

35Q069

35Q069

35Q070

Model grid

Depth, in feet below land surface

Pumped well casing or wellbore

(3 columns)

Figure 2–12. Axisymmetric model for 72-hour aquifer test at pumped well 35Q069, Pooler, Georgia, April 16–19, 2012. 

Observation well 35Q070 and pumped well 35Q069 are included in the top diagram. Curved arrows in the top diagram 

represent transforming three-dimensional location of well opening to the two-dimensional model grid. The model is 

surrounded by no-flow boundaries.
58 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

The edges of the model are simulated as no-flow boundaries. This includes the proximal edge which is the center 

of the well, the lower boundary which is within the lower 

confining unit, the distal edge, and the upper boundary which 

represents the water table. The distal edge being 200,000 ft 

from the pumping well was beyond the radius of influence of 

the pumping well. The model is run with two stress periods 

that represent the aquifer test and recovery. The aquifer-test 

stress period is represented by 60 timesteps totaling slightly 

more than 3 days. Timesteps ranged from 0.92 second to 

12 hours 52 seconds, with each succeeding timestep increasing 

from the previous timestep by a multiplier of 1.2. The recovery 

stress period is represented by 70 timesteps totaling slightly 

more than 5 days. Timesteps ranged from 0.3 second to 

20 hours 2 minutes 24 seconds, with each succeeding time step 

increasing from the previous timestep by a multiplier of 1.2.

The model simulated the drawdown in response to the 

72-hour aquifer test. No other influences were simulated so 

that initial heads and flow within the model were zero. During 

model simulation, water was injected at the same rate that 

water was withdrawn at the pumped well (783 gal/min). The 

water was injected into the model at the top cell representing 

the inner borehole (column and row 1) using the MODFLOW 

WEL package (McDonald and Harbaugh, 1988; Harbaugh and 

McDonald, 1996). High values for hydraulic conductivity and 

storage coefficient were assigned to the cells representing 

the borehole.

MODOPTIM computes the relative sensitivity of parameters and the “measure-of-redundancy” between parameters 

(Halford, 2006b). Parameter sensitivity indicates how adjustments to a parameter value will affect the objective function 

(sum-of-squares of the residuals or the goodness of fit between 

the simulated and measured data), and provides the basis for 

comparing simulated and measured drawdown. Parameters 

with very low sensitivity, relative to the most sensitive parameters (less than or equal to 0.01; Hill, 1998) are not estimated, 

but rather, are assigned a general value in the model.

The measure-of-redundancy is made between a pair of 

parameters. It describes how similar two parameters appear 

to a given objective function. A high measure-of-redundancy 

between parameters of the same hydraulic property usually 

means that one parameter should be estimated for multiple 

hydrogeologic units as a single merged hydrogeologic unit. 

A high measure-of-redundancy between parameters of 

different hydraulic properties (example, one parameter is 

horizontal hydraulic conductivity (Kh) and the other parameter 

is specific storage) means that one of the parameters should be 

eliminated from the estimation process. Values of measureof-redundancy of greater than 0.99 indicate that the two 

parameters cannot be estimated independently (Poeter and 

Hill, 1997; Halford, 2006b). The relative sensitivities of 

parameters and values of measure-of-redundancy between 

parameter pairs in a MODOPTIM run were then used to 

determine which, if any, parameters should be eliminated from 

the estimation process and be assigned a general value 

in subsequent MODOPTIM runs.

The flowmeter-survey data indicated that no flow was 

coming from the lower confining unit in the bottom 80 ft of 

the open hole of well 35Q069. Assuming equal hydraulic 

head in all units and pumping during the flowmeter survey, 

no-flow contribution from a hydrogeologic unit equates to 

zero hydraulic conductivity for that unit. Both zero hydraulic 

conductivity and no flow are unlikely. The permeability of 

the lower confining unit is so much lower than that of the 

hydrogeologic units of the Floridan aquifer system, that a 

greater pumping rate would be required to observe water being 

produced from the lower confining unit. The Kh of the lower 

confining unit (layer 5) was not estimated in the final series of 

MODOPTIM runs but was set to a value of 0.1 ft/d.

The root-mean-square of the residuals divided by the 

maximum drawdown (RMS/MAX) was used to compare the 

success of runs in simulating well response to the aquifer 

test and recovery. The lower the RMS/MAX, the closer the 

simulated drawdown and recovery matched the measured 

drawdown and recovery. The residuals of the drawdown and 

recovery from LFA pumped well 35Q069 and the residuals 

of drawdown from UFA well 35Q070 were used to determine 

goodness of fit. MODOPTIM runs did not calibrate to the 

residuals of the recovery of UFA well 35Q070, because 

these data were poor due to failure to filter out nonaquifertest influences as mentioned earlier in the report. Also, 

MODOPTIM runs did not calibrate to the residuals of about 

the first 10 minutes after the start and stop of pumping in 

LFA pumped well 35Q069 to avoid wellbore-storage effects 

during the start of the aquifer test and slug effects during the 

end of the aquifer test. Given that no estimated parameters 

have a relative sensitivity of less than 0.01 or a value of 

measure-of-redundancy with another estimated parameter of 

greater than 0.99, the values of parameters that make the best 

match (lowest RMS/MAX) of simulated response to measured 

response are the estimated values of those parameters.

The relative weights applied to residuals of UFA well 

35Q070 compared to similar weights applied to residuals of 

LFA pumped well 35Q069 were adjusted in MODOPTIM 

simulations to best match the simulated response to measured 

response of both wells. The term “relative weight” means the 

weight on the residuals associated with UFA well 35Q070 

relative to the weight on the residuals associated with LFA 

pumped well 35Q069. For example, a relative weight of 919 

means that the weight applied to UFA well 35Q070 residuals 

was 919 times greater than the weight applied to LFA pumped 

well 35Q069 residuals. Initial MODOPTIM runs indicated 

that applying equal weights to the residuals of LFA pumped 

well 35Q070 and UFA well 35Q070 data led to a poor match 

of simulated drawdown to measured drawdown of UFA well 

35Q070. The relative weight applied to UFA well 35Q070 
Appendix 2. Aquifer Tests, Pooler, Georgia, March and April 2012 59

residuals was increased in simulations that represented the 

72-hour aquifer test in an attempt to calibrate MODOPTIM 

results for hydraulic flow properties of the UFA and LFA. 

Increasing the relative weight negatively affected calibrating 

to the LFA model parameters, and matching simulated 

drawdown and recovery to measured drawdown and recovery 

at LFA pumped well 35Q069. Increasing the relative weight 

applied to UFA well 35Q070 residuals affected net improvements in model accuracy in the UFA at the expense of actually 

decreasing the model’s ability to accurately represent flow and 

aquifer-test water-level response in LFA pumped well 35Q069. 

An acceptable tradeoff was achieved where model improvement by increasing the relative weights associated with UFA 

well 35Q070 data outweighed the negative effects on model 

accuracy in LFA pumped well 35Q069 (and the model’s ability 

to represent flow and water levels in the LFA).

Initial MODOPTIM runs provided preliminary estimates 

of parameters and determined which parameters should be 

estimated and which parameters should be assigned a set 

value. The Kh and specific storage of the confining units above 

and below the Floridan aquifer system (layers 1 and 5, respectively) were relatively insensitive, often had high measures 

of redundancy, and therefore, could not be estimated well 

with MODOPTIM runs. Values for these parameters changed 

little from initial values during the estimation process. In 

initial MODOPTIM runs, the value of specific storage of the 

LFA was estimated to be 1.19×10– 7, more than an order of 

magnitude lower than the value estimated for the other layers, 

and represents an unrealistic value for this aquifer parameter. 

The value for specific storage was set to its low preliminary 

value and was not estimated in final runs. Initial MODOPTIM 

runs also indicated that anisotropy within the Floridan aquifer 

system was not sensitive in the estimation process. Anisotropy 

was set to be 0.1 in all layers.

The final series of MODOPTIM runs was performed 

to estimate the Kh of the UFA, LFCU, and LFA (layers 2–4, 

respectively) and the specific storage of the UFA and LFCU 

(layers 2–3, respectively). Initial values of parameters used in 

the final series of MODOPTIM runs were based on the results 

of initial MODOPTIM runs. Relative weights in favor of the 

UFA ranged from 1 to 3,676 (fig. 2–13). The base or starting 

value for relative weight was the ratio of the maximum 

drawdown of the LFA pumped well over the maximum 

drawdown of the UFA observation well (57.44). Other values 

of relative weight were 57.44 times 2n, where n ranges from 

about –6 to 6. These multiples were used so that relative 

weights could plot on a log scale.

The final series of MODOPTIM runs indicated good 

matches of simulated response (drawdown and recovery) 

to measured response for relative weights from 1 to 919 

(table 2–5). The only MODOPTIM runs that did not have 

good matches used relative weights of 1,838 and 3,676. The 

MODOPTIM run with a relative weight of 1 had a good 

match, because initial values of parameters based on the 

results from previous runs were already close to the values 

that provide the best match.

Parameter values were determined from the MODOPTIM 

run that used a relative weight of 105.35 (table 2–5). This 

MODOPTIM run has the lowest RMS/MAX without high 

values of the measure-of-redundancy. Despite the lowest 

RMS/MAX values for MODOPTIM runs with relative weights 

114.88 to 919.04, a high value of the measure-of-redundancy 

between specific storage and Kh of the UFA and erratic fluctuations in parameter values precludes selecting results from this 

range of relative weights (fig. 2–13). The Kh values for the 

UFA, LFCU, and LFA were 253, 3.6, and 11.9 ft/d, respectively. These values round to 250, 4, and 12 ft/d, respectively. 

Specific storage of the UFA and LFCU were 3.2×10–

 

6

 and 

2.5×10–6 ft –1, respectively. These parameter values lead to a 

good fit between simulated and measure responses (fig. 2–14). 

Given the model thicknesses of the UFA, LFCU, and LFA 

(183, 183, and 337 ft, respectively), which are similar to actual 

thicknesses, the transmissivities of the UFA, LFCU, and LFA 

are estimated to be 46,000, 700, and 4,000 ft2/d, respectively, 

and the values of storage coefficient of the UFA and LFCU are 

estimated to be 5.9×10– 4 and 4.5×10– 4, respectively.
60 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

Upper Floridan aquifer

Lower Floridan confining unit

18

17

16

15

14

13

12

11

4.0

3.8

3.6

3.4

3.2

200

270

220

210

260

240

230

250

1×106

2×106

3×106

4×106

5×106

6×106

7×106

8×106

1 10

Relative weight

Figure 2–13. Resulting parameter values from MODOPTIM runs and RMS/MAX as a function of the 

preferred weight of model calibration on the Upper Floridan aquifer observation well relative to the 

weight on the Lower Floridan aquifer pumped well (relative weight). Data are in table B4. RMS/MAX 

is the root mean squared of the differences between the simulated and measured drawdown values 

divided by the 72-hour (maximum) drawdown.

RMS/MAX Specific storage, in feet–1 Horizontal hydraulic conductivity, in feet per day

Lower Floridan confining unit

Upper Floridan aquifer

Lower Floridan aquifer

100 1,000 10,000

0.0236

0.024

0.025

0.03

0.04

0.1

0.3

Figure 2–13. Resulting parameter values from MODOPTIM runs and RMS/MAX as a function of the 

preferred weight of model calibration on the Upper Floridan aquifer observation well 35Q070 relative 

to the weight on the Lower Floridan aquifer pumped well 35Q069 (relative weight). Data are in 

table 2– 4. RMS/MAX is the root mean squared of the differences between the simulated and 

measured drawdown values divided by the 72-hour (maximum) drawdown.
Appendix 2. Aquifer Tests, Pooler, Georgia, March and April 2012 61

Table 2–5. Resulting parameter values and values of RMS/MAX from MODOPTIM runs with different weights of calibration in favor 

of Upper Floridan well 35Q070, relative to Lower Floridan pumped well 35Q069, simulating response to 72-hour aquifer test, Pooler, 

Georgia, April 16–19, 2012. 

[Relative weight, weight for model calibration of Upper Floridan well 35Q070, relative to the weight of Lower Floridan aquifer pumped well 35Q069; 

Kh, horizontal hydraulic conductivity; ft/d; foot per day; Ss, specific storage; ft –1, per foot; RS, sensitivity of a parameter to model calibration relative to the 

most sensitive parameter. RMS/MAX is the root-mean-square of the difference in simulated and measured response, in feet, divided by the maximum drawdown in response to the aquifer test. Data used for the RMS/MAX were the drawdown, after wellbore-storage effects, and recovery, after back-syphon slug 

effects for well 35Q069 and drawdown for well 35Q070; UFA, Upper Floridan aquifer; LFCU, Lower Floridan confining unit; LFA, Lower Floridan aquifer]

Relative 

weight

Horizontal hydraulic conductivity 

(Kh, ft/d)

Specific storage

(Ss, ft –1) RMS/MAX RS Kh

 UFA

Value of measure-of-redundancy 

of Kh UFA with other parameters

UFA LFCU LFA UFA LFCU Kh LFCU Kh LFA Ss UFA Ss LFCU

1.00 248.253 3.90132 11.9002 3.07×10–6 0.000002 0.029259 0.010625 0.57 0.26 0.85 – 0.36

3.59 248.430 3.89985 11.9004 3.07×10–6 0.000002 0.029167 0.023713 0.06 0.12 0.89 0.12

7.18 248.979 3.89498 11.9012 3.07×10–6 0.000002 0.028880 0.044367 – 0.26 0.07 0.89 0.41

14.36 250.949 3.87748 11.9039 3.08×10–6 0.000002 0.027927 0.086956 – 0.57 0.06 0.90 0.60

15.00 251.169 3.87550 11.9042 3.08×10–6 0.000002 0.027828 0.090737 – 0.58 0.06 0.90 0.61

15.66 262.997 3.68467 11.9189 3.11×10– 6 0.000002 0.024878 0.091445 – 0.59 0.06 0.90 0.61

17.08 262.875 3.69785 11.9177 3.11×10– 6 0.000002 0.024872 0.099827 – 0.62 0.06 0.90 0.62

20.31 263.083 3.71622 11.9158 3.11×10– 6 0.000002 0.024893 0.118735 – 0.67 0.06 0.90 0.65

24.15 262.959 3.72780 11.9146 3.11×10– 6 0.000002 0.024901 0.141242 – 0.72 0.06 0.90 0.66

26.34 262.874 3.73137 11.9143 3.11×10– 6 0.000002 0.024902 0.154053 – 0.74 0.07 0.90 0.67

28.72 247.781 3.42567 11.9472 3.15×10–6 0.000003 0.024584 0.16483 – 0.77 0.07 0.90 0.68

40.62 250.212 3.49150 11.9398 3.17×10–6 0.000003 0.024531 0.23294 – 0.82 0.09 0.90 0.70

57.44 251.757 3.53371 11.9360 3.18×10–6 0.000002 0.024520 0.328515 – 0.84 0.12 0.90 0.71

81.23 252.541 3.55579 11.9342 3.19×10–6 0.000002 0.024518 0.457878 – 0.86 0.16 0.90 0.71

96.60 252.744 3.56194 11.9346 3.20×10–6 0.000002 0.024517 0.541934 – 0.86 0.19 0.90 0.71

105.35 252.768 3.56363 11.9350 3.21×10–6 0.000002 0.024517 0.589144 – 0.86 0.20 0.90 0.71

114.88 234.224 3.28968 11.9940 5.21×10–6 0.000002 0.023915 0.619642 – 0.88 0.24 0.91 0.66

229.76 219.559 3.42362 11.9930 7.57×10–6 0.000001 0.023956 1 – 0.89 0.45 0.92 0.63

459.52 240.028 3.44858 11.9446 4.91×10–6 0.000002 0.024061 1 – 0.88 0.69 0.91 0.68

919.04 222.496 3.47916 11.9953 7.32×10–6 0.000001 0.023952 1 – 0.89 0.88 0.92 0.64

1,838.08 238.380 3.46175 11.3982 5.48×10–6 0.000002 0.035769 1 – 0.88 0.97 0.91 0.68

3,676.16 207.875 3.67805 17.4864 5.53×10–6 0.000002 0.172299 1 – 0.89 0.99 0.91 0.68
62 Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012

0

5

10

15

20

25

30

35

40

45

50

55

0.00001 0.0001 0.001 0.01 0.1 1 10

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.001 0.01 0.1 1 10

Drawdown, in feet

Recovery

Pumping response

Figure 2–14. Simulated and measured water-level drawdown (pumping) and recovery at Lower Floridan aquifer pumped 

well 35Q069 and Upper Floridan aquifer observation well 35Q070 for the 72-hour aquifer test at pumped well 35Q069 near 

Pooler, Georgia, April 16–19, 2012.

EXPLANATION

Simulated Measured

Pumping response

Lower Floridan aquifer pumped well 35Q069 Upper Floridan aquifer observation well 35Q070

Time after the start or stop of the 72-hour aquifer test, in days

Figure 2–14. Simulated and measured water-level drawdown (pumping) and recovery at Lower Floridan aquifer pumped 

well 35Q069 and Upper Floridan aquifer observation well 35Q070 for the 72-hour aquifer test at pumped well 35Q069, Pooler, 

Georgia, April 16–19, 2012.
Manuscript approved on November 6, 2012

Edited by Twila Darden Jones

Illustrations and layout by Caryl J. Wipperfurth

Science Publishing Network, Raleigh PSC

For more information about this publication, contact:

USGS Georgia Water Science Center

3039 Amwiler Road

Atlanta, GA 30360

telephone: 770-903-9100

http://ga.water.usgs.gov/
Gonthier—Hydrogeology and Water Quality of the Floridan Aquifer System, Pooler, Chatham County, Georgia, 2011–2012—Scientific Investigations Report 2012–5249