Pond-Aquifer Flow and Water Availability in the 

Vicinity of Two Coastal Area Seepage Ponds, 

Glynn and Bulloch Counties, Georgia 

By John S. Clarke and Malek Abu Rumman 

Prepared in cooperation with the 

Georgia Department of Natural Resources 

Environmental Protection Division 

Scientific Investigations Report 2004-5260 

U.S. Department of the Interior

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

Gale A. Norton, Secretary 

U.S. Geological Survey

Charles G. Groat, Director 

U.S. Geological Survey, Reston, Virginia: 2004 

For sale by U.S. Geological Survey, Information Services


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Suggested citation: 

Clarke, John S., and Malek Abu Rumman, 2004, Pond-Aquifer Flow and Water Availability in the Vicinity of Two Coastal 

Area Seepage Ponds, Glynn and Bulloch Counties, Georgia: U.S. Geological Survey Scientific Investigations Report 

2004-5260, 76 p. 
iii 

Contents


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


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


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


Study Area. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2


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


Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3


Estimation of Ground-Water Seepage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3


Water Budget. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3


Pond Pumping Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5


Ground-Water Flow Models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5


Pond-Aquifer Flow and Water Availability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5


Glynn County Pond Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5


Pond-Aquifer Flow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7


Estimated Net Ground-Water Seepage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11


Water Budget. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11


Pond Pumping Test. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13


Ground-Water Flow Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15


Water Availability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17


Bulloch County Pond Site . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18


Pond-Aquifer Flow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18


Estimated Net Ground-Water Seepage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20


Water Budget. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20


Pond Pumping Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22


Ground-Water Flow Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28


Water Availability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29


Seepage Ponds as Sources of Irrigation Supply. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29


Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32


References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34


Appendix A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35


Appendix B. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55


Figures 

1-2. Maps Showing— 

1.	 Coastal Georgia area and location of Glynn County and Bulloch County pond sites. . . . . . . . . . . .2


2.	 Glynn County pond site at Brunswick, Georgia, and locations of test wells, stage recorder,


weather station, and bathymetry soundings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6


3.	 Graph showing mean monthly precipitation for 1971–2001, and total monthly precipitation for 


August 1999–October 2000, at the National Weather Service Station at Brunswick, Georgia . . . . . . . .7


4.	 Diagram showing subsurface geology at well 34H493 at the Glynn County pond site in Georgia . . . . .7


5.	 Map showing pond bathymetry and pond-bottom temperature, Glynn County pond site in 


Georgia, August 1999. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7


6.	 Regression models showing relation of pond stage to estimated surface area and estimated 


volume, Glynn County pond site in Georgia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

iv 

7. Conceptual model of the hydrogeologic framework and pond-aquifer flow for the Glynn County 

pond site in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9


8–9. Maps Showing—


8.	 Water-table surface of the surficial aquifer at the Glynn County pond site in Georgia, 


September 19, 1999. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10


9.	 Vertical head gradient in well clusters near the Glynn County pond site in Georgia, 

August 30, 2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11


10–13. Graphs Showing—


10.	 Mean daily water levels in wells 34H476 and 34H492 and pond stage and calculated 


cumulative net precipitation (precipitation minus evapotranspiration), Glynn County pond 


site in Georgia, November 1999–September 2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12


11.	 Daily measured precipitation and estimated evaporation and estimated daily net 


precipitation (precipitation minus evapotranspiration) at the Glynn County pond site in


Georgia, November1999–September 2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13


12.	 Cumulative daily net precipitation (precipitation minus evapotranspiration) and 


cumulative daily pond- volume changes; and estimated net 10-day average ground-water


seepage, Glynn County pond site in Georgia, November 1999–April 2000. . . . . . . . . . . . . . . . . . . . 14


13.	 Hourly water levels in wells 34H476, 34H485, and 34H496 and pond stage; and


cumulative daily net precipitation (precipitation minus evapotranspiration), Glynn County


pond site in Georgia, April 25, 2000–May 14, 2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15


14.	 Map showing simulated water-table contours of the surficial aquifer at the Glynn County pond 


site in Georgia prior to pumping test on May 1, 2000; and after pumping test on May 3, 2000 . . . . . . . 16


15.	 Estimated and simulated 10-day average net ground-water seepage at the Glynn County pond 


site in Georgia, November 1999–April 2000—graph and boxplot summarizing all values . . . . . . . . . . 17


16.	 Linear regression model showing relation between pond stage and simulated net ground-water


seepage, Glynn County pond site in Georgia, May 1–3, 2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18


17.	 Map showing Bulloch County pond site in Georgia and locations of test wells, stage recorder, 


and weather station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19


18.	 Graph showing mean monthly precipitation, 1971–2001, and total monthly precipitation,


October 2000–July 2001, at the National Weather Service Station at Brooklet, Georgia. . . . . . . . . . . . 20


19.	 Diagram showing subsurface geology at the Bulloch County pond site in Georgia. . . . . . . . . . . . . . . . . 20


20.	 Map showing pond bathymetry, Bulloch County pond site in Georgia, October 2000. . . . . . . . . . . . . . . 22 

21.	 Regression models showing relation of pond stage to estimated surface area and volume,


Bulloch County pond site in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23


22.	 Conceptual model of the hydrogeologic framework and pond-aquifer flow at the Bulloch County 

pond site in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23


23–24. Maps Showing—


23.	 Potentiometric surface of the surficial aquifer at the Bulloch County pond site in Georgia,


April 27, 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24


24.	 Vertical head gradient between the upper and lower part of the surficial aquifer, 


Bulloch County pond site in Georgia, June 6, 2001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

v 

25–28. Graphs Showing— 

25. Hourly water levels in wells 33R025 and 33R037 and pond stage; and cumulative daily net 

precipitation (precipitation minus evapotranspiration), Bulloch County pond site in 

Georgia, October 2000–July 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

26.	 Measured daily precipitation and estimated daily evaporation; and estimated daily net 

precipitation (precipitation minus evapotranspiration) at the Bulloch County pond site in 

Georgia, October 2000–July 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 

27.	 Cumulative daily net precipitation (precipitation minus evapotranspiration) and cumulative 

daily pond-volume changes; and estimated 7-day average net ground-water seepage, 

Bulloch County pond site in Georgia, October 2000–June 2001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

28.	 Hourly water levels in wells 33R025, 33R029, and 33R037 and pond stage; and cumulative 

net precipitation (precipitation minus evapotranspiration), Bulloch County pond site in 

Georgia, June 1–16, 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 

29.	 Maps showing simulated potentiometric surface of the surficial aquifer at the Bulloch County 

pond site in Georgia prior to pumping test on June 4, 2001; and after pumping test on June 5, 2001 30 

30.	 Estimated and simulated 7-day average net ground-water seepage at the Bulloch County pond 

site in Georgia, February–June 2001— graph and boxplot summarizing all values. . . . . . . . . . . . . . . . . 32 

31–32. Graphs Showing— 

31.	 Pond stage and simulated net ground-water seepage, Bulloch County pond site in 

Georgia, June 4–5, 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 

32.	 Hypothetical rate of depletion of pond storage at a pumping rate of 1,000 gallons per 

minute for 10 hours per day for climatic and hydrologic conditions similar to those observed 

during pond pumping tests at the Glynn County and Bulloch County pond sites in Georgia . . 33 

Tables 

1. Well construction and location information at the Glynn County pond site in Georgia . . . . . . . . . . . . . .8 

2. Well construction and location information at the Bulloch County pond site in Georgia . . . . . . . . . . 21 

Horizontal and Vertical Datums 

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

Historical data collected and stored as North American Datum 1927 have been converted to NAD 

83 for this publication. 

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

(NAVD 88). Historical data collected and stored as National Geodetic Vertical Datum of 1929 

have been converted to NAVD 88 for this publication. 

Pond-Aquifer Flow and Water Availability in the 

Vicinity of Two Coastal Area Seepage Ponds, 

Glynn and Bulloch Counties, Georgia 

By John S. Clarke1 and Malek Abu Rumman2 

Abstract 

Pond-aquifer flow and water availability at excavated seepage pond sites in Glynn County and in southern Bulloch County, 

Georgia, were evaluated to determine their potential as sources 

of water supply for irrigation. Excavated seepage ponds derive 

water primarily from ground water seeping into the pond, in a 

manner similar to a dug well completed in a surficial aquifer. 

The availability of water from seepage ponds is controlled by 

the permeability of surficial deposits, the amount of precipitation recharging the ground-water system, and the volume of 

water stored in the pond. The viability of seepage ponds as 

supplies for irrigation is limited by low seepage rates and high 

dependence on climatic conditions. Ponds will not refill unless 

there is adequate precipitation to recharge the surficial aquifer, 

which subsequently drains (seeps) into the pond. 

Ground-water seepage was estimated using a water-budget 

approach that utilized on-site climatic and hydrologic measurements, computing pond-volume changes during pond pumping 

tests, and by digital simulation using steady-state and transient 

ground-water flow models. From August 1999 to May 2000, 

the Glynn County pond was mostly losing water (as indicated 

by negative net seepage); whereas from October 2000 to June 

2001, the Bulloch County pond was mostly gaining water. 

At both sites, most ground-water seepage entered the pond 

following major rainfall events that provided recharge to the 

surficial aquifer. Net ground-water seepage, estimated using 

water-budget analysis and simulation, ranged from –11.5 to 

15 gallons per minute (gal/min) at the Glynn County pond site 

and from –55 to 31 gal/min at the Bulloch County pond site. 

Simulated values during pumping tests indicate that groundwater seepage to both ponds increases with decreased pond stage. 

At the Glynn County pond, simulated net ground-water seepage 

varied between 7.8 gal/min at the beginning of the test (high pond 

stage and low hydraulic gradient) and 103 gal/min at the end of 

the test (low pond stage and high hydraulic gradient). At the 

1

U.S. Geological Survey; 2Georgia Institute of Technology 

Bulloch County pond site, values ranged from –17.7 gal/min at 

the beginning of the test to 15 gal/min at the end of the test. 

Results at the two pond sites indicate that the use of excavated seepage ponds as sources for irrigation supply is 

limited by pond-storage volume and low net ground-water 

seepage rates during periods of low precipitation. Pumps 

withdrawing 1,000 gal/min for 10 hours per day—under 

climatic and hydrologic conditions similar to those observed 

during pond pumping tests at each site—would drain the 

Glynn County pond within 30 days and the Bulloch County 

pond within 3.5 days. Because the two pond sites are considered 

to represent the extremes of likely conditions to be encountered 

in the coastal Georgia area, it is likely that other seepage ponds 

would have similar storage-depletion rates. 

Introduction 

The Upper Floridan aquifer is the principal source of water 

in the coastal area of Georgia. Declining water levels and localized saltwater contamination have resulted in State-restricted 

(“capped”) withdrawals from the aquifer and have prompted 

interest in developing supplemental sources of ground water. 

In the coastal area, seepage ponds are sometimes excavated for 

golf courses, farms, or communities by digging through sandy 

surface soils until the water table is reached. Because these 

ponds are largely isolated from surface-water runoff, water is 

mostly derived from ground water seeping into the pond. Seepage ponds are often used to supply water for irrigation; however, the water-supply potential of such ponds is poorly understood. The U.S. Geological Survey (USGS)—in cooperation 

with the Georgia Department of Natural Resources, Environmental Protection Division (GaEPD)—conducted a study during 1999–2001 to evaluate pond-aquifer flow and water availability at two pond sites in coastal Georgia to determine their 

potential use as supplemental sources of water for irrigation. 
2 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

Purpose and Scope 

This report describes results of investigations to evaluate the 

water-supply potential at seepage pond sites at Brunswick, Glynn 

County, and in southern Bulloch County, Georgia. Included are 

descriptions of the hydrogeologic setting, estimates of a hydrologic budget and ground-water seepage rates, development and 

calibration of steady-state and transient ground-water flow 

models, and an assessment of water availability based on pond 

pumping tests and simulation results. The report is based on data 

collected during a period of drought in coastal Georgia—from 

August 1999 to October 2000 at the Glynn County pond site and 

from October 2000 to July 2001 at the Bulloch County pond site. 

Each pond site was characterized by constructing test wells; 

collecting core and cuttings samples and borehole geophysical 

logs; conducting pond bathymetric and bottom-temperature 

surveys; installing continuously monitored weather stations, 

pond-stage, and ground-water level recorders; and measuring 

periodic water levels from on-site and surrounding area wells. 

Long-term pumping tests were conducted at each pond to estimate ground-water seepage and to evaluate effects of pumping 

on ground-water levels. Steady-state and transient ground-water 

flow models were developed for each site to assess pond-aquifer 

relations and to estimate ground-water seepage rates and hydraulic properties of the surficial aquifer. 

Study Area 

To assess the water-supply potential of seepage ponds, two 

sites located in areas of contrasting hydrologic, physiographic, 

and soil conditions were selected to evaluate pond-aquifer relations and possible ranges of water availability (fig. 1). The Glynn 

County pond site is located in the Coastal Lowlands physiographic division (LaForge and others, 1925), near the Atlantic 

Ocean, in an area characterized by low relief and high-permeability sandy soils. The Bulloch County pond site is located in 

the Coastal Terraces physiographic division (LaForge and others, 

1925), in an area characterized by low relief and low-permeability clayey soils. During 1961–90, precipitation averaged about 

49 inches per year (in/yr) at the Glynn County pond site and about 

47 in/yr at the Bulloch County pond site (Sherlyn Priest, U.S. 

Geological Survey, written commun., 2002). The two sites are 

described in greater detail in subsequent sections of this report. 

Previous Studies 

Several publications have described ground-water flow near 

seepage ponds and lakes. Winter (1976, 1978) described numerical simulations of steady-state ground-water flow near lakes 

under various hydrologic conditions. Beck (1978) estimated 

water yield from shallow ponds in Coastal Plain settings by considering ponds as large-diameter wells and using hydraulic conductivity derived from grain-size analyses of surficial sands. 

Beck (1978) concluded that a yield of 100,000 gallons per day 

(gal/d), or 69.4 gallons per minute (gal/min), could be obtained 

from a 15-foot-deep pond having a 100-foot (ft) radius. 

BURKE 

EMANUEL 

SCREVEN 

BULLOCH 

EFFINGHAM 

TOOMBS 

TATTNALL 

EVANS 

CHATHAM 

LIBERTY 

APPLING LONG

MCINTOSH 

BRANTLEY 

PIERCE 

BACON 

CAMDEN 

pond site 

pond site 

Brunswick 

Statesboro 

ATLANTIC OCEAN 

0 20 MILES 

0 20 KILOMETERS

Base modified from U.S. Geological Survey 

1:5,000,000-scale digital data; 

COASTAL 

LOWLANDS 

COASTAL 

TERRACES

CANDLER 

JENKINS 

N 

GEORGIA 

Coastal 

area 

BRYAN 

WAYNE 

GLYNN 

WARE 

CHARLTON 

Glynn County 

Brunswick National 

Weather Service Station 

Bulloch County 

Brooklet National 

Weather Service Station 

physiography from LaForge and others, 1925 

Figure 1. Coastal Georgia area and location of Glynn County 

and Bulloch County pond sites. 

Krabbenhoft and others (1990) described three-dimensional simulation of ground-water exchange at Sparkling Lake, Wisconsin. 

The simulations indicated that ground-water recharge has a 

greater influence on inflow to the lake than outflow from the lake. 

Studies by Sacks and others (1992), Grubbs (1995), Lee (1996), 

Lee and Swancar (1997), and Metz and Sacks (2002) provided 

detailed descriptions of lake hydrologic budgets and lake-aquifer 

flow. Sacks and others (1992) described the hydrogeologic setting 

and a preliminary analysis of data to compute a hydrologic budget 

at Lake Barco, Florida. Grubbs (1995) described the development 

of a hydrologic budget and simulation of lake-aquifer flow at 

Lake Five-O, Florida, and indicated that ground-water flow to 

the lake was an important component of the lake hydrologic budget. Lee (1996) described hydrogeologic controls on lake-aquifer interaction at Lake Barco, Florida. Lee and Swancar (1997) 

described the importance of ground-water seepage and evapora­
3 

tion on hydrologic-budget computations for Lake Lucerne in 

Florida and provided a detailed error analysis of all components 

of the hydrologic budget. Metz and Sacks (2002) compared the 

hydrogeologic setting, ground-water flow patterns, water budgets, and water quality of three lakes in west-central Florida. 

Methods to estimate evaporation include studies by Sacks 

and others (1994) and Winter and Rosenberry (1995). Sacks and 

others (1994) compared evaporative losses from two morphometrically different seepage lakes in Florida using estimates 

derived using an energy-budget method. The study determined 

that the thermal regime of a lake must be considered to provide 

more accurate estimates of seasonal evaporation rates from a 

deep lake. Winter and Rosenberry (1995) evaluated 11 equations 

used for determining evaporation at a small lake in the northcentral United States. 

Acknowledgments 

The authors appreciate the assistance of Dr. Gene Kieferl, 

Coastal Georgia Community College, and Mr. Chuck Lee, 

landowner, for providing access to the Glynn County and 

Bulloch County pond sites, respectively. John Wentworth, 

Georgia Department of Transportation, provided access to drilling 

sites located along Interstate Highway 16 in Bulloch County. 

Wes Harris, Georgia Cooperative Extension Service, helped 

evaluate potential pond test sites in Bulloch County. 

Dr. Aris Georgakakos, Georgia Institute of Technology, provided support and technical advice during the course of the study. 

Dr. Gerrit Hoogenboom, The University of Georgia, provided 

assistance in the installation of climatic monitoring stations at each 

pond site. Joe Sheridan and George Barfield, U.S. Department of 

Agriculture, provided assistance in the construction of test wells 

at the Glynn County pond site. Test wells at the Bulloch County 

pond site were constructed with the help of Mark Hall, Harold 

Baker, and John Sides of the Georgia Geologic Survey. 

Michael F. Peck, Alan M. Cressler, David C. Leeth, Welby L. 

Stayton, Donald R. Dowling, and Christopher B. Walls, USGS, 

provided valuable assistance in site setup and in monitoring hydrologic conditions during pond pumping tests. DáVette Taylor-Harris, 

Michael T. Laitta, and Keith W. McFadden, USGS, developed the 

project database and applied Geographic Information System 

techniques to plot hydrologic data and compute estimates of pond 

volume. Douglas D. Nagle and Paul D. Ankcorn, USGS, conducted bathymetric surveys at the pond sites. 

Estimation of Ground-Water Seepage 

Ground-water seepage represents water either entering or 

leaving a pond as the result of hydraulic gradients between the 

aquifer and the pond. Net seepage is the difference between 

ground-water inflow minus outflow—when positive, more 

ground water enters than leaves the pond; when negative, more 

ground water leaves than enters the pond. The following 

relation (Darcy’s Law) applies: 

Q = K I A (1) 

Estimation of Ground-Water Seepage 

where 

Q is the seepage rate in cubic feet per day, 

K is the hydraulic conductivity in feet per day, 

I is the hydraulic gradient in feet per foot, and 

A is the cross-sectional area in square feet. 

Hydraulic conductivity is a temporal constant; both hydraulic 

gradient and cross-sectional area (area in which aquifer is in contact with pond bottom) may change as pond stage or groundwater levels change. Hydraulic gradients can be either toward 

the pond or away from the pond. Rates and directions of groundwater seepage vary depending on ground-water levels and pond 

stage and related changes in hydraulic gradient and cross-sectional area. Decreased pond stage or increased ground-water 

levels result in an increased hydraulic gradient toward the pond 

and increased rates of seepage to the pond. Increased pond stage 

results in a decreased or reversed hydraulic gradient, whereby 

seepage inflow is either decreased or reversed. 

Ground-water seepage was estimated (1) using a waterbudget approach that utilized on-site climatic and hydrologic 

measurements, (2) computing pond-volume changes during a 

pond pumping test, and (3) by digital simulation using steadystate and transient ground-water flow models. These estimates 

were used to assess the availability of water at both sites. 

Water Budget 

The water-budget approach used measurements of precipitation, pond evaporation, and pond-volume changes to estimate 

net ground-water seepage. Because surface-water inflow and 

outflow do not occur at either pond site and overland runoff is 

negligible because of permeable soils and low relief, these terms 

were omitted from the water budget. The method used to compute net ground-water seepage was virtually the same as that 

used by Pollman and others (1991), whereby the volumetric 

hydrologic budget for the pond is given by: 

∆V– ε∆V = (P +/– εP) – (E +/– εE) + (Gi +/– εGi) – (Go +/– εGo)(2) 

where 

∆V is the pond volume change, 

P is precipitation, 

E is lake evaporation, 

Gi is ground-water seepage into to the pond, 

Go is ground-water seepage out from the pond, and 

εθ is the error associated with the estimate of hydrologic 

variable θ. 

Equation 2 can be rearranged to solve for net ground-water seepage: 

Gnet = (Gi +/– εGi) – (Go +/– εGo) 

Gnet = (∆V +/– ε∆V) – (P +/– εP) + (E +/– εE) (3) 
4 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

With the exception of the error terms, all the terms on the right 

side of equation 3 were measured directly or indirectly. The 

“Area and Volume Statistics” function of the ARCVIEW3.1TM 

3D-Analyst Tool was used to compute pond volume and area at 

each site using results of bathymetric surveys and projecting for 

a range of pond stages. Bathymetric surveys featured a location 

accuracy within a few feet, and bathymetry accuracy within 

0.5 percent of indicated depth (D.D. Nagle, U.S. Geological 

Survey, written commun., 2004). Continuous relations between 

pond stage, pond volume, and pond area were developed using 

polynomial regression of pond stage and pond volume and pond 

stage and pond area. Changes in pond volume were computed 

by comparing volumes at different pond stages. 

Precipitation and evaporation estimates were derived using 

data from automated weather stations installed at each site by the 

University of Georgia, Department of Biological and Agricultural 

Engineering, as part of the Georgia Automated Environmental 

Monitoring Network (http://www.griffin.peachnet.edu/bae). 

Sensors at each weather station measure air temperature, relative humidity, wind speed and direction, net and total solar radiation, barometric pressure, precipitation, and soil temperature at 

2-, 4-, and 8-inch depths. Volumetric estimates of precipitation 

and evaporation were computed by multiplying total precipitation 

or evaporation (in volume per unit surface area units) by area. 

For land areas, potential evapotranspiration was estimated 

by using the Priestly-Taylor method (Stewart and Rouse, 1976), 

as reported by the Georgia Automated Environmental Monitoring Network (http://www.griffin.peachnet.edu/bae). In the area 

of each pond, potential evapotranspiration was estimated by 

using the Penman method (Winter and Rosenberry, 1995): 

PET = (s/(s + γ ))(Qn – Qx) + (γ /(s + γ )) * 

[(15.36(0.5+0.01U2))(e0–ea)] (4) 

where 

PET is the potential evaporation (in calories per square 

centimeter per day), 

s/(s + γ ) and γ /(s + γ ) are parameters derived from the 

slope of the saturated vapor pressure-temperature 

curve at the mean air temperature, 

γ is the psychrometric constant equal to 0.66, 

Qn is net radiation (in calories per square centimeter per 

day), 

Qx is change in heat stored in the pond (in calories per 

square centimeter per day), 

U2 is wind speed at 2 meters above surface (in meters per 

second), 

e0 is saturated vapor pressure (in millibars), and 

ea is vapor pressure at air temperature and relative 

humidity (in millibars). 

Data from the weather stations were used to compute evaporation estimates. In addition, to determine the change in heat 

stored in the pond (Qx), temperature probes were installed at 

varying pond depths to record changes in daily temperatures. At 

the Glynn County pond site, temperature probes were installed 

at depths of 1, 2.5, 22, and 24 ft, with an average temperature 

difference between the uppermost and deepest probe of 

5.2 degrees Celsius (o

C). At the Bulloch County pond site, temperature probes were installed at 1-ft depth increments between 

0 and 5 ft, with an average temperature difference between the 

uppermost and lowermost probe of 0.4o

C. In each pond, the 

temperature data from all depths were averaged and used to calculate the term Qx as follows: 

Qx = ρ *Cp* ∆ T* D, (5) 

where 

ρ is the water density (grams per cubic centimeter), 

Cp is the specific heat of water at constant pressure taken 

as 1.007 calories/gram/degree Kelvin, 

–T is the pond temperature change on a biweekly basis in 

degrees Kelvin, and 

D is the average water depth in the pond (centimeters). 

The influence of measurement errors of each water-budget 

component is important to the interpretation of the final water 

budget (Winter, 1981). Error estimates provide an indication of 

how well individual hydrologic fluxes are understood and measured, and are a measure of the reliability or accuracy of the 

hydrologic budget as a predictive tool (Lee and Swancar, 1997). 

Although the error, ε Gnet, associated with a given net groundwater seepage estimate is unknown, the uncertainty of Gnet can 

be characterized by computing a confidence interval as follows: 

Gnet ± Zα/2 σ Gnet, (6) 

where 

Zα/2 is the standard normal deviate for an α/2 level of 

significance, and 

σε Gnet is the standard deviation of ε Gnet. 

This approach assumes that the error, ε Gnet, associated with 

Gnet follows a normal distribution with mean equal to zero 

(Gnet is assumed to be unbiased) and has a variance equal to σ2 

ε Gnet. The standard deviation of the error associated with the net 

ground-water seepage estimate, σε Gnet, was computed as follows (Pollman and others, 1991): 

σε Gnet = √ [(P)(CVP)]2 + [(E)(CVE)]2 + [(∆ V)(CV∆ V)]2, (7) 

where 

CVθ = σεθ /θ is the coefficient of variation for the hydrologic variable θ (expressed as a fraction). 
5 

Pond-volume changes were assigned a coefficient of variation of 5 percent, and monthly precipitation was assigned a 

coefficient of variation of 15 percent based on studies by Winter 

(1981). Pond-evaporation estimates were assigned a coefficient 

of variation of 15 percent, based on studies by Sacks and others 

(1994), who determined that the monthly coefficient of variation for pond-evaporation estimates computed using an energybudget method had a mean of 15 percent, a lower quartile of 

11 percent, and an upper quartile of 17 percent. 

Pond Pumping Tests 

To assess the effect of changing pond stage on ground-water 

seepage rates, a pond pumping test was conducted at each site 

during a period of dry hydrologic conditions, considered to be 

the most likely period when the ponds would be utilized for irrigation. These tests involved pumping water out of the pond to 

lower stage by 1–2 ft while monitoring pumping rates, climatic 

conditions, and ground-water levels. Recovery rates were compared to changes in pond storage to determine estimates of net 

ground-water seepage. Water levels in wells surrounding each 

pond were monitored to determine changes in ground-water 

gradients near the pond and provide data for model calibration. 

Water pumped from each pond was discharged at distances 

sufficiently away from the pond to ensure no re-infiltration. 

Ground-Water Flow Models 

Pond-aquifer flow was simulated using the USGS digital, 

three-dimensional, finite-difference ground-water flow model, 

MODFLOW (McDonald and Harbaugh, 1988). Steady-state 

and transient simulations were used to evaluate changes in 

ground-water levels and seepage prior to and during pond 

pumping tests. The steady-state models provided initial hydraulic property values and boundary conditions for the transient 

models. Changes in recharge, pond stage, ground-water levels, 

and seepage over time were simulated using the transient models. Model development, calibration, and sensitivity analyses 

are described in Appendix A for the Glynn County pond site 

and in Appendix B for the Bulloch County pond site. 

Following calibration, the ground-water flow models were 

used to determine rates of net ground-water seepage at each 

pond under a range of pond stages. Flows into and out of each 

pond were determined using the MODFLOW postprocessor 

ZONEBUDGET (Harbaugh, 1990). 

Pond-Aquifer Flow and 

Water Availability 

Pond-aquifer flow and water availability were evaluated 

at two pond test sites in areas of contrasting hydrologic, physiographic, and soil conditions in coastal Georgia—one at 

Pond-Aquifer Flow and Water Availability 

Brunswick in Glynn County, and one in southern Bulloch 

County. Information obtained from testing and analysis at the 

two sites can be used to assess the potential of seepage ponds as 

supplemental sources of water for irrigation throughout the 

coastal region. The testing program at each of the two sites was 

conducted during a period of prolonged drought in coastal 

Georgia—August 1999 through July 2001. 

Glynn County Pond Site 

The Glynn County site is a 5-acre pond located on the campus of Coastal Georgia Community College at Brunswick, Ga. 

(fig. 2). The area is generally urbanized and includes paved 

parking lots and streets, buildings and homes, open fields, and 

stands of pine trees. A second pond is located about 750 ft 

northeast of the Glynn County pond. Field studies at the site 

were conducted during August 1999 through October 2000. 

For 9 of the 15 months during this period, monthly precipitation 

at the nearby National Weather Service Station at Brunswick 

was below the long-term average for 1971–2001 (fig. 3). 

To characterize the hydrogeology of the site, 24 test wells 

were installed and completed at various depths in the surficial 

aquifer (table 1)—three wells were completed at depths of 

48.5–53.8 ft (from 36.0 to 41.8 ft below NAVD 88), one well 

at a depth of 23.0 ft (10.8 ft below NAVD 88), and 18 wells at 

depths of 8.4–10.4 ft (1.9 to 3.8 ft above NAVD 88). In addition, one well tapping the surficial aquifer (well 34H506) was 

completed in the pond at a depth of 15.0 ft (0.4 ft below 

NAVD 88). Pond stage and ground-water levels in selected 

wells were monitored continuously. 

The Glynn County pond was constructed during the early 

1970s by excavating to a maximum depth of about 21 ft (about 

13 ft below NAVD 88) into the upper part of a 50- to 55-ft-thick 

sequence of fine quartz sand that is part of the surficial aquifer 

(fig. 4). A dense, low-permeability clay layer was penetrated at 

about 50 ft below land surface during drilling of several deep 

wells. Natural gamma logs, from wells located within 0.75 mile 

(mi) of the pond site, indicate that the clay layer is at similar 

depths throughout the area. Beck (1978) estimated that the permeability of surficial sands at two locations near Brunswick, 

Glynn County, ranged from 4.7 to 7.5 gallons per day per square 

foot (from 0.6 to 1.0 foot per day). Pond-bottom sediments are 

similar to the surficial aquifer, with the exception that a layer of 

organic material has been deposited over the years. 

A bathymetric survey of the pond using a fathometer and a 

Global Positioning System (GPS) device was conducted during 

August 1999—the pond bottom ranges in altitude from about 

8 ft above to about 13 ft below NAVD 88 (fig. 5). At the highest 

observed stage during October 1999 (8.52 ft above NAVD 88), 

the volume of water in the pond was about 2,343,000 cubic feet 

(ft3) with a surface area of about 222,000 square feet (ft2). 

The volume decreased to about 1,767,000 ft3 and surface 

area decreased to about 172,000 ft2 when observed pond stage 

was lowest (5.34 ft above NAVD 88) during June 2000. 
6 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

0 100 200 300 400 500 FEET

N 

Stage recorder 

Weather 

station 

Pond temperature profile 

34H499 

34H480 

34H493 

34H498 

34H481 

34H482 

34H483 

34H494 

34H497 

34H490 

34H484 

34H488 

34H487 

34H485 34H478 34H479 

34H486 

34H489 

34H477 

34H476 

34H496 34H506 

34H491 34H492 

Base from U.S. Geological Survey digital orthophoto 

Weather 

station 

Stage recorder

Pond temperature profile

Brunswick East 1:24,000; 1983 

0 50 100 METERS 

EXPLANATION 

34H484 WELL AND SITE NAME — See table 1 for well information 

POND BATHYMETRY SOUNDING—August 1999 

0 1,000 FEET 

1000 200 METERS

500 

N 

Brunswick 

Off-site

pond

Area of map above Coastal 

Georgia 

Community 

College Altama Avenue 

Cyprus Mill Creek 

GEORGIA 

Base from U.S. Geological Survey 

digital orthophoto 

Brunswick East 1:24,000; 1983 

Figure 2. Glynn County pond site at Brunswick, Georgia, and locations of test wells, stage recorder, weather station, 

and bathymetry soundings. 
7 

10 

20 

30 

Pond-Aquifer Flow and Water Availability 

12 Pond-Aquifer Flow

10 

The sandy surficial aquifer is recharged by rainfall near 8 

the pond, and ground water flows toward the pond and dis­ 6 

charges into it mostly along its western shoreline, with some 

flow entering along the pond bottom (fig. 7). Because of low 

relief and permeable soils, overland runoff is minimal at the site 

and water flows into the pond from the surficial aquifer. Water 

discharges from the pond primarily through evaporation or 

through ground-water seepage along the eastern shore of the 

Mean monthly 

Total monthly 

August 1999 – 

1971– 2001 

O ctober 2000 

 DEPTH BELOW LAND SURFACE, IN FEET	 PRECIPITATION,

IN INCHES

4 

2 

0 

AUG

SEP

OCT

NOV

DEC

JAN

FEB

MAR

APR

MAY

JUNE

JULY

AUG

SEP

OCT 

MONTH 

Figure 3. Mean monthly precipitation for 1971– 2001, and 

pond toward an unnamed off-site pond and Cyprus Mill Creek. 

Regionally, ground-water flow in the surficial aquifer is a 

total monthly precipitation for August 1999 – October 2000, at	 subdued replica of land-surface topography. Water flows 

the National Weather Service Station at Brunswick, Georgia.	 eastward from a topographic high located along Altama Boulevard, west of the pond site, toward Cyprus Mill Creek, a 

tidal estuary with elevations that are at or below NAVD 88 

(see locations, fig. 2). 

Phosphatic(?) 

sand? p 

p 

Fine 

sand 

p p

p p 

Confining 

unit	 Clay Surficial aquifer 

12 

2 

N –13 

–11 

–11 

–11 

–9 

–9 

–7

–7 

–5 

5 

–3 

–1

1 

3

0 100 200 FEET

 ALTITUDE, IN FEET ABOVE OR BELOW (–) NAVD 88 

NAVD 

88 

–8

–18 

40 –28 

50 –38 

40 60 80 100 

NATURAL GAMMA, IN AMERICAN 

PETROLEUM INSTITUTE UNITS 

Figure 4. Subsurface geology at well 34H493 at the 

Glynn County pond site in Georgia. 

To facilitate water-budget computations across a wide range 

of pond stages, polynomial regression models (fig. 6) were 

used to provide continuous relations between pond stage and 

pond volume, and between pond stage and pond area across a 

wide range of projected stages (from 14 ft below to 10 ft 

above NAVD 88). The relation for pond stage and pond area 

applies a second-order polynomial regression, whereas the 5 

0 25 50 METERS 

EXPLANATION 

POND-BOTTOM TEMPERATURE—

 In degrees Celsius 

18.8 to 19.0

19.0 to 19.2

19.2 to 19.3

19.3 to 19.5

19.5 to 19.7

19.7 to 19.9

19.9 to 20.0

20.0 to 20.2

20.2 to 20.4

BATHYMETRY CONTOUR— 

relation for pond stage and pond volume applies a third-order Shows altitude of pond bottom.

polynomial regression (fig. 6). Contour interval 2 feet. Datum 

is NAVD 88 (–, below datum) 

Figure 5. Pond bathymetry and pond-bottom temperature, 

Glynn County pond site in Georgia, August 1999. 
Table 1. Well construction and location information at the Glynn County pond site in Georgia. [degrees, °; minute, '; second, "] 

Well 

number Well name Model layer Latitude (NAD 83) Longitude (NAD 83) Altitude of land surface, in feet (NAVD 88) Well depth in feet below land surface Casing diameter (inches) Open in feet bel

land surfa

Top 

inter

B

34H476 CSSI Glynn Pond Test Site P-1 A1 31°10'59.15" 81°28'57.75" 12.12 8.4 2 3.4 734H477 CSSI Glynn Pond Test Site P-2 A1 31°10'57.90" 81°28'58.63" 12.60 9.8 2 4.8 734H478 CSSI Glynn Pond Test Site P-3 A1 31°11'00.36" 81°28'57.50" 13.63 9.8 2 4.8 734H479 CSSI Glynn Pond Test Site P-4 A1 31°11'00.37" 81°28'54.26" 11.79 10.0 2 5.0 634H480 CSSI Glynn Pond Test Site P-5 A1 31°11'01.76" 81°28'52.96" 11.36 9.8 2 4.8 534H481 CSSI Glynn Pond Test Site P-6 A1 31°11'00.83" 81°28'49.49" 12.11 10.0 2 5.0 634H482 CSSI Glynn Pond Test Site P-7 A1 31°10'58.20" 81°28'49.86" 12.44 10.3 2 5.3 634H483 CSSI Glynn Pond Test Site P-8 A1 31°10'57.46" 81°28'51.91" 11.79 10.0 2 5.0 634H484 CSSI Glynn Pond Test Site P-9 A1 31°10'55.15" 81°28'55.30" 13.39 9.8 2 4.8 634H485 CSSI Glynn Pond Test Site P-10 A1 31°11'00.03" 81°28'58.84" 13.82 10.0 2 5.0 734H486 CSSI Glynn Pond Test Site P-11 A1 31°11'00.76" 81°28'56.18" 12.34 10.4 2 5.4 634H487 CSSI Glynn Pond Test Site P-12 A1 31°10'56.91" 81°28'56.90" 12.82 10.4 2 5.4 634H488 CSSI Glynn Pond Test Site P-13 A1 31°10'45.97" 81°28'57.62" 12.84 10.2 2 5.2 634H489 CSSI Glynn Pond Test Site P-14 A1 31°11'01.75" 81°28'56.26" 12.99 10.0 2 5.0 634H490 CSSI Glynn Pond Test Site P-15 A1 31°10'55.96" 81°28'57.58" 11.87 9.9 2 4.9 634H491 CSSI Glynn Pond Test Site P-16 A5 31°10'59.34" 81°28'57.86" 12.21 23.0 2 18.0 734H492 CSSI Glynn Pond Test Site P-17 A8 31°10'59.37" 81°28'57.76" 12.54 48.5 2 43.5 734H493 CSSI Glynn Pond Test Site P-18 A8 31°11'00.76" 81°28'49.61" 11.96 53.8 2 48.8 634H494 CSSI Glynn Pond Test Site P-19 A8 31°10'57.62" 81°28'52.11" 12.00 49.6 2 44.6 634H496 CSSI Glynn Pond Test Site P-20 A1 31°10'58.75" 81°28'57.49" 9.81 10.0 2 5.0 634H497 CSSI Glynn Pond Test Site P-21 A1 31°10'58.05" 81°28'52.71" 10.03 10.0 2 5.0 6

34H498 CSSI Glynn Pond Test Site P-22 A1 31°10'59.91" 81°28'49.62" 9.39 10.0 2 5.0 634H499 CSSI Glynn Pond Test Site P-23 A1 31°10'59.90" 81°28'54.20" 10.60 10.0 2 5.0 434H506 CSSI Glynn Pond Test Site Dock Well A3 31°10'58.82" 81°28'57.01" 14.59 15.0 2 11.0 7

*Water level, in feet below top of casing, measured November 6, 1999 
Pond-Aquifer Flow and Water Availability 9 

250,000 

NW SE 

Water table 

SURFICIAL AQUIFER 

Study pond 

Off-site 

pond 

CLAY CONFINING UNIT 

Cyprus Mill Creek 

Study pond

Direction of ground-water flow 

Pond evaporation 

Pond stage 

ESTIMATED POND VOLUME, ESTIMATED POND SURFACE AREA, 

IN CUBIC FEET IN SQUARE FEET 

200,000


150,000


100,000


50,000


0


2,500,000


2,000,000 

1,500,000 

1,000,000 

500,000 

0 

–20 –15 –10 –5 NAVD 88 +5 +10 +15 

Minimum observed 

stage = – 5 .34 

Minimum observed 

stage = – 5.34 

Maximum observed 

stage = +8.52 

Maximum observed 

stage = +8.52 

y = 36.612x3 + 273.95x2 + 6491.3x + 124045 

y = 3896.3x2 + 127085x + 977425 

R2 = 0.996 

R2 = 0.996 

POND STAGE, IN FEET BELOW (–) OR ABOVE (+) NAVD 88 

Figure 6. Regression models showing relation of pond stage to estimated surface area and 

estimated volume, Glynn County pond site in Georgia. 

Recharge area 

NAVD 88 

15 feet 

38 feet 

NOT TO SCALE 

Figure 7. Conceptual model of the hydrogeologic framework and pond-aquifer flow for the Glynn County 

pond site in Georgia. 
10 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

A water-table map for September 19, 1999, was constructed 

using data from 14 test wells completed in the uppermost part 

of the surficial aquifer (fig. 8). The map indicates that locally 

pond water recharges the aquifer along the northern, southern, 

and eastern shores, and aquifer water seeps into the pond along 

the western shore (fig. 8). A bottom-temperature survey of the 

pond water during August 1999 was conducted using a YSI Inc. 

temperature-specific conductance probe that was trolled at a 

depth of about 0.5 ft above the pond bottom with position 

recorded using a GPS device. Data from the survey indicate 

that pond-bottom temperatures ranged from 18.8 to 20.4°C 

and suggest that cooler ground water seeps into the pond along 

parts of the western shore (fig. 5). During September 1999, a 

ground-water high located about 50 ft west of the pond resulted 

in development of a local ground-water divide. Periodic and 

continuous water-level data at the pond site indicate that this 

divide is a temporary feature that is not present during most 

of the year. 

Paired test wells completed in the upper and lower parts of 

the surficial aquifer indicate that vertical head gradients generally are small within the study area; thus, horizontal gradients to 

the pond are more dominant. This relation is demonstrated on a 

map showing vertical head differences at three paired well sites 

adjacent to the pond on August 30, 2000 (fig. 9). 

7.0 

7.0

6.9 

6.8 

6.9 

7.6 

7.5 

7.5 

7.4 

7.3 

7.2 

7.1 

7.1 

7.0 

7.0 

7.7 

7.7 

7.1 

7.1 

7.2

7.2 

7.2 

7.3 

7.3 

7.3 

7.4 

7.4 

7.4 

7.5 

Pond stage = 7.56 feet

To 

off-site 

pond 

N 

Base from U.S. Geological Survey 0 100 200 300 400 500 FEET 

digital orthophoto 

Brunswick East 1:24,000; 1983 0 50 100 METERS 

EXPLANATION 

7.0	 WATER-TABLE CONTOUR— Shows altitude at which water level would 

have stood in tightly cased wells. Dashed where approximately located. 

Contour interval 0.1 foot. Datum is NAVD 88 

GROUND-WATER FLOW


GROUND-WATER DIVIDE


OBSERVATION WELL


Figure 8. Water-table surface of the surficial aquifer at the Glynn County pond site in Georgia, September 19, 1999 

(from Peck and others, 2001). 
Pond-Aquifer Flow and Water Availability 11 

0.003 

34H481 

34H493 

34H483 

34H494 

34H492 

0.003 

0.004 

34H491 

34H476 

N 

Base from U.S. Geological Survey 0 100 200 300 400 500 FEET 

digital orthophoto

Brunswick East 1:24,000; 1983 0 50 100 METERS 

EXPLANATION 

34H483 WELL AND SITE NAME 

0.003 VERTICAL HEAD GRADIENT, IN FEET PER FOOT— 

Up arrow indicates upward gradient, down arrow 

indicates downward gradient 

Figure 9. Vertical head gradient in well clusters near the Glynn County pond site in Georgia, August 30, 2000. 

Pond stage and ground-water levels show a rapid response 

to precipitation events and a decline during dry periods in 

response to increased evapotranspiration. These responses are 

illustrated on hydrographs showing mean daily water levels in 

wells 34H476 and 34H492, pond stage, and cumulative net precipitation (precipitation minus evapotranspiration) (fig. 10). 

Estimated Net Ground-Water Seepage 

Net ground-water seepage rates were computed using 

water-budget analysis, model simulation, and results of a pond 

pumping test. The water-budget analysis was conducted at 

10-day intervals from November 6, 1999, through April 24, 

2000 (pretest period). The pond pumping test was conducted 

during May 1–3, 2000. Model simulations covered the period 

November 6, 1999, through May 3, 2000. 

Water Budget 

During November 1999–April 2000, maximum daily rainfall was 1.94 inches in March 2000; and average daily evapotranspiration ranged from 0.01 inch during March 2000 to 

0.20 inch during April 2000 (average daily rate 0.08 inch) 

(fig. 11). Pond-stage altitude prior to the pumping test ranged 

from 7.73 ft during January 2000 to 8.39 ft during November 

1999 (fig. 10), with a corresponding estimated volume change 

of about 125,000 ft3 (based on polynomial regression, fig. 6). 
12 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia CUMULATIVE NET
 PRECIPITATION,
 IN INCHES MEAN DAILY WATER LEVEL, IN FEET ABOVE NAVD 88


10 

9 

8 

7 

6 

5 

0 

–5 

–10 

–15 

–20 

NOV DEC 

B. 

End test 

A. 

Well 34H492 

Start test 

Pond stage 

Well 34H476 

JAN FEB MAR APR MAY JUNE JULY AUG SEPT 

1999 2000 

Figure 10. (A) Mean daily water levels in wells 34H476 and 34H492 and pond stage; and 

(B) calculated cumulative net precipitation (precipitation minus evapotranspiration), 


Glynn County pond site in Georgia, November 1999 – September 2000.


Ground-water recharge to the shallow ground-water flow 

system can be approximated from net precipitation. Because 

overland runoff is negligible at the site, part of the precipitation 

remaining after evaporative loss (net precipitation) enters the 

shallow ground-water system, with an unknown amount lost to 

plant transpiration. The relation between cumulative net precipitation and ground-water levels is demonstrated in the hydrographs shown in figure 10. In general, when cumulative net precipitation is high, ground-water levels are high (indicating groundwater recharge), and when cumulative net precipitation is low, 

ground-water levels are low. The ground-water component of 

the water budget is illustrated by comparing plots showing 

cumulative daily net precipitation (precipitation minus evapotranspiration) and cumulative daily change in pond volume over 

time (fig. 12). With the assumption that surface runoff is negligible and errors in estimating atmospheric fluxes and pond volume 

changes are reasonably small and unbiased, the difference between 

the two lines represents cumulative net ground-water seepage. 
Pond-Aquifer Flow and Water Availability 13 

INCHES INCHES 

3.5 

2.5 

1.5 

0.5 

0 

–0.5 

3.5 

2.5 

1.5 

0.5 

0 

–0.5 

Daily net precipitation 

Precipitation 

1999 

DEC FEB MAR APR JUNE SEPT

2000 

B. 

A. 

Pretest period Test and posttest period 

Evaporation 

NOV JAN MAY JULY AUG 

Figure 11. (A) Daily measured precipitation and estimated evaporation; and (B) estimated daily 

net precipitation (precipitation minus evapotranspiration) at the Glynn County pond site in Georgia, 

November 1999–September 2000. 

The temporal distribution of net ground-water seepage 

computed using the water-budget method prior to the pumping test is related to patterns in precipitation (fig. 12). Net 

seepage values above the zero line on figure 12 represent 

periods of net seepage into the pond, whereas values below 

the zero line represent periods of net seepage out of the 

pond. Net seepage generally is positive during periods of 

high rainfall, indicating more ground water is entering the 

pond than leaving it. Conversely, net seepage generally is 

negative during periods of low rainfall, indicating that more 

ground water is leaving the pond than entering it. In general, 

net ground-water seepage was negative during the pretest 

period, corresponding to the observed decrease in pond 

stage. Computed net seepage per 10-day period ranged from 

–1,600 ft3/d (-8.3 gal/min) during January 15–25, 2000, to 

3,000 ft3/d (15 gal/min) during March 26–April 4, 2000 

(fig. 12). Confidence intervals shown on figure 12 were 

derived using equation 7 and error estimates derived from 

the hydrologic literature. 

Pond Pumping Test 

To estimate rates of net ground-water seepage under 

extreme conditions (dry, low pond stage), a pumping test 

was conducted in the pond during May 1–3, 2000. A largecapacity pump was used to lower the water level in the pond 

by 2 ft during a 32-hour period. Discharge water was piped 

about 0.1 mi away from the pond to a drainage ditch south 

of the pond. Following cessation of pumping, rates of 

recovery were monitored for 48 hours (fig. 13). During the 

test and recovery period, there was no precipitation and estimated evaporation was about 0.2 inch per day (in/d). Thus, 

changes in pond stage during the test were mainly due to the 

volume of water removed by pumping and contributed by 

ground-water seepage. Seepage estimates are limited by the 

accuracy of precipitation and discharge measurements, 

evaporation estimates, and pond-volume estimates determined using pond-stage and bathymetric data. Determination of seepage rates during the pumping test was not possible because the flowmeter measuring pumping rates was 

highly inaccurate and variable; however, estimated rates of 

seepage were computed following cessation of pumping. 
14 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia ESTIMATED NET GROUND-WATER CUMULATIVE VOLUME, SEEPAGE, IN CUBIC FEET PER DAY IN CUBIC FEET PER DAY 

0 

–10,000 

–30,000 

–50,000 

–70,000 

–90,000 

5,000 

4,000 

3,000 

2,000 

1,000 

0 

–1,000 

–2,000 

–3,000

net precipitation 

1999 

DECEMBER MARCH APRIL 

2000 

10 16 26 6 5 15 25 4 5 4 14 24

Estimated net 

l 

– 95 percent 

l 

B. 

A. 

Cumulative daily 

Cumulative daily 

pond-volume change 

NOVEMBER JANUARY FEBRUARY 

16 26 14 24 15 25 

ground-water seepage 

+ 95 percent 

confidence interva

confidence interva

Figure 12. (A) Cumulative daily net precipitation (precipitation minus evapotranspiration) and cumulative daily 

pond-volume changes; and (B ) estimated net 10-day average ground-water seepage, Glynn County pond site in 

Georgia, November 1999 – April 2000. 

Net ground-water seepage was estimated based on observed 

pond-volume changes during recovery and the relation: 

Net ground-water seepage = change in pond 

volume + evaporation – precipitation (8) 

Rates of ground-water seepage vary depending on pond 

stage and related changes in hydraulic gradient and crosssectional area. Decreasing pond stage results in an increased 

hydraulic gradient toward the pond and increased rates of 

seepage to the pond. Following cessation of pumping, pond 

stage recovered about 0.1 ft in 48 hours (fig. 13), corresponding 

to a rate of about 8,100 ft3/d (42 gal/min), which when combined with the estimated evaporation rate of about 4,300 ft3

/d 

(22 gal/min), equals a net seepage rate of about 12,400 ft3

/d 

or 64 gal/min. Applying an error analysis of precipitation, 

evaporation, and volume using equations 6 and 7 indicates that 

the net ground-water seepage estimate has a margin of error 

of +/–1,300 ft3/d (6.8 gal/min). Values estimated using the 

recovery analysis compare favorably with the estimated 

seepage rate reported by Beck (1978) for a hypothetical 

15-ft-deep, 100-ft-radius pond (69 gal/min) in a similar 

hydrogeologic setting. 

In monitoring wells, the water-level response to lowered 

pond stage varied depending on distance from the pond. 

Wells located directly adjacent to the pond showed waterlevel declines of 1.0–1.5 ft during the test (well 34H496, 

fig. 13), whereas wells located farther away from the pond 

showed less pronounced responses of 0.5 ft or less (wells 

34H476 and 34H485, fig. 13). Because there was no appreciable rainfall in the area for several weeks following the cessation 

of pumping, water levels in many of the wells continued to 

decline, reflecting evapotranspiration and transient drainage of 

the water table in response to the lowered pond stage (fig. 10). 
Pond-Aquifer Flow and Water Availability 15 

CUMULATIVE DAILY 

NET PRECIPITATION, 

IN INCHES 

HOURLY WATER

LEVEL, IN FEET

ABOVE NAVD 88 

10 

9 

8 

7 

6 

5 

–1 

–2 

–3 

–4 

–5 

End test 

Blank where data missing 

APRIL 2000 

25 

B. 

A. 

27 29 1 3 5 7 9 

Well 34H485 

Well 34H476 

Well 34H496 

Start test 

Pond stage 

11 13 

MAY 2000 

Figure 13. (A) Hourly water levels in wells 34H476, 34H485, and 34H496 and pond stage; and 

(B) cumulative daily net precipitation (precipitation minus evapotranspiration), Glynn County 

pond site in Georgia, April 25, 2000–May 14, 2000. 

Ground-water levels continued to decline until several rainfall 

events during July 2000—almost 2 months after the pumping 

test—caused water levels to rise. Although pond stage showed 

some recovery following the test in response to rainfall during 

August and September, as of December 2000, pond stage 

remained about 0.8 ft below pretest conditions. Conversely, 

ground-water levels had recovered to pretest conditions by 

early September 2000. 

Ground-Water Flow Model 

The Glynn County pond site model utilized steady-state and 

transient simulations to evaluate changes in ground-water level 

and seepage to and from the pond prior to and during a 32-hour 

pumping test. Initial conditions were simulated as steady state, 

followed by simulation of transient changes in recharge, pond 

stage, ground-water levels, and seepage. Model development, 

calibration, and sensitivity are described in Appendix A. 

For the initial steady-state simulation of pretest conditions, 

ground-water flow directions are from the western boundary 

and into the pond along the western and northern shores 

(fig. 14A). Pond water recharges the aquifer along the southeastern shoreline. Some water discharging from the pond seeps 

into the second pond site, and the remaining water moves 

toward Cyprus Mill Creek, east of the simulated area. These 

flow patterns compare favorably to the conceptual model of 

ground-water flow (fig. 7), regional ground-water flowpaths 

as inferred from topography, and locally to water-table maps 

derived from test-well data (fig. 8). Some differences are apparent between the simulated and observed water-table maps. 

The observed-conditions map for November 1999 shows 

water flowing toward the pond from the western shore and discharging from all other directions, whereas the simulated map 

shows water entering from the western, northern, and southern 

shores, and discharging from the east-southeastern shores. 

In addition, the divide west of the pond site is evident on the 

observed-conditions map but not on the simulated map. These 

differences may reflect changes in water-table conditions during the two periods—water levels were lower during November 1999 than during May 2000, resulting in different flow 

conditions near the pond. 

A map showing the simulated water table after 32 hours of 

pumping indicates a depression surrounding the study pond, 

with a steepened hydraulic gradient that captures flow along all 

shorelines (fig. 14B). This depression resulted in the development of a ground-water divide between the study pond and the 

off-site pond located east of the site. 

The MODFLOW post-processor ZONEBUDGET 

(Harbaugh, 1990) was used to enable determination of simulated net ground-water seepage at the pond. In this procedure, 

the pond is designated as a separate “zone,” and flows into and 

out of that zone are summarized. For the pretest period, simulated 

10-day-average net seepage values compare favorably with 

values estimated using the water-budget analysis, ranging from 

a low of –2,200 ft3/d (–11.4 gal/min) during December 26, 

1999–January 5, 2000, to a high of 2,600 ft3/d (13.5 gal/min) 

during March 25–April 4, 2000, with a median rate of –360 ft3/d 

(–1.9gal/min) (fig. 15). An exception occurs during January 

15–25, 2000, when simulated net seepage rises 10 days earlier 

than determined through water-budget estimates. Despite this 

discrepancy, simulated values generally showed a good match 

to estimated values (see bar graph, fig. 15). 
16

A

pumpi B

N 

0 

0 

0.2 MILES 

0.2 KILOMETERS 

0.1 

0.1 

10.5 — 

A. 

B. 

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

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

10.5 

10.0 

9.5 

9.0 

8.5 

8.0 

7.5 

7.0 

6.5 

11.0 

10.5 

10.0 

9.5 

9.0 

8.5 

8.0 

7.5 

7.0 

6.5 

Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

Figure 14. Simulated water-table contours of the surficial aquifer at the Glynn County pond site in Georgia ( ) prior to 

ng test on May 1, 2000; and ( ) after pumping test on May 3, 2000. 

EXPLANATION 

DIRECTION OF GROUND-WATER FLOW 

GROUND-WATER DIVIDE 

Base from U.S. Geological Survey digital orthophoto 

Brunswick East 1:24,000; 1983 

SIMULATED WATER-TABLE CONTOUR Shows 

altitude of water table in surficial aquifer. 

Hachures indicate depression. Contour interval 

0.1 and 0.5 feet. Datum is NAVD 88 
NET GROUND-WATER SEEPAGE, 

IN CUBIC FEET PER DAY 

0 

–2,000 

–4,000 

Pond-Aquifer Flow and Water Availability 17 

A. 

4,000 

2,000 

Estimated 

1999 

DECEMBER MARCH APRIL 

2000 

10 16 26 6 5 15 25 4 5 4 14 24

Simulated 

NOVEMBER JANUARY FEBRUARY 

16 26 14 24 15 25 

NET GROUND-WATER 

SEEPAGE, IN CUBIC

FEET PER DAY 

B. 

2,000 EXPLANATION 

90 PERCENTILE—Percentage 

of values equal or less than 75 0 indicated values 

50 

Mean 

25 

–2,000 

ESTIMATED SIMULATED 10 

Figure 15. Estimated and simulated 10-day average net ground-water seepage at the Glynn County 

pond site in Georgia, November 1999 –April 2000 —( A) graph and (B) boxplot summarizing all values. 

Rates of ground-water seepage vary depending on pond 

stage and related changes in hydraulic gradient and crosssectional area. To evaluate the impact of changing hydraulic 

gradients on ground-water seepage, simulated transient net 

seepage was plotted with measured pond stage during the 

pumping test (fig. 16). Decreasing pond stage results in an 

increased hydraulic gradient toward the pond and increased 

rates of ground-water seepage into the pond. Simulated net 

ground-water seepage varied from 1,500 ft3

/d (7.8 gal/min) 

at the beginning of the test (high pond stage and low hydraulic 

gradient) to 19,900 ft3/d (103 gal/min) at the end of the test 

(low pond stage and high hydraulic gradient). Median simulated net ground-water seepage during the pumping test was 

12,100 ft3/d (63 gal/min). 

The calibrated ground-water flow model provided 

estimates of the vertical and horizontal hydraulic conductivity of the surficial aquifer. Initial values of hydraulic conductivity were adjusted during calibration based on observed 

head and calculated net ground-water seepage into the pond. 

Because estimates of hydraulic properties in the surficial 

aquifer are sparse, these estimates are useful to help guide 

development of the surficial aquifer and provide information 

to help development of other ground-water models in the 

area. Model-calibrated horizontal hydraulic conductivity 

ranged from 0.5 to 50 ft/d, and vertical hydraulic conductivity 

ranged from 0.5 to about 40 ft/d. 

Water Availability 

Available storage in the Glynn County pond ranged from 

about 1.8 to 2.4 million cubic feet (from 13.5 to 18 million 

gallons—Mgal) under the observed range of stage (from 

5.34 to 8.75 ft) during 1999–2000 (figs. 6 and 10). Net 

ground-water seepage rates determined using the water-budget 

method and through flow-model simulation, indicate that the 

Glynn County pond was mostly losing water during November 1999–April 2000—more water moved from the pond 

than entered the pond (fig. 15). Most ground-water seepage 

entered the pond following major rainfall events that provided 

recharge to the surficial aquifer. This recharge raised water 

levels in the surficial aquifer and produced steepened hydraulic gradients toward the pond. Graphs of cumulative daily net 

precipitation, cumulative daily pond volume change, and estimated seepage demonstrate the relation between recharge 

events and estimated net ground-water seepage (fig. 12). 

Net ground-water seepage rates vary in response to 

changing pond stage. Simulated values during a 32-hour 

pumping test indicate that ground-water seepage to the 

pond increases with decreased pond stage (fig. 16). A 2-ft 

drop in pond stage during the test resulted in an additional 

18,400 ft3

/d (95 gal/min) of ground-water seepage into the 

pond. Note that these estimates are only appropriate for 

climatic and ground-water-level conditions similar to those 

observed during the test period. Different conditions could 

cause significant differences in hydraulic gradients near the 

pond and affect seepage rates. 
18 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

SIMULATED NET GROUND-WATER 

SEEPAGE, IN CUBIC FEET PER DAY 

30,000 

20,000 

10,000 

0 

End test 

Begin 

test 

5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 

POND STAGE, IN FEET ABOVE NAVD 88 

Figure 16. Pond stage and simulated net ground-water 

seepage, Glynn County pond site in Georgia, May 1– 3, 2000. 

Bulloch County Pond Site 

The Bulloch County site is a 5-acre pond located in 

southern Bulloch County about 20 mi southeast of Statesboro, 

Ga. (figs. 1 and 17). The area generally is rural, and the site is 

located adjacent to a plowed cotton field adjacent to Interstate 

Highway 16. Field studies at the site were conducted during 

October 2000 through July 2001. For 8 of the 10 months 

during this period, monthly precipitation at the nearby National 

Weather Service Station at Brooklet was at or below the longterm average for 1971–2001 (fig. 18). 

The pond was excavated as a borrow pit for road-fill 

material during construction of nearby Interstate Highway 16 

during the 1970s. The pond was excavated into layers of clay 

and clayey sand and is underlain by a clayey sand layer that 

comprises the uppermost part of the surficial aquifer (fig. 19). 

The surficial aquifer is underlain by layers of clay and sandy 

clay that act as a semiconfining unit. 

To characterize geologic and hydrologic conditions, eight 

shallow wells (from 5 to 16 ft deep) were installed, two of which 

were equipped with recorders to monitor ground-water levels 

continuously (table 2). In addition to the shallow wells, eight 

deeper (greater than 19 ft depth) test wells and one 62-ft-deep 

corehole were drilled around the pond to better define the site 

hydrogeology. Drill cuttings were collected and examined from 

each well, and borehole geophysical logs were collected in wells 

33R036 and 33R037 to characterize subsurface geology (fig. 19). 

A bathymetric survey conducted during October 2000, 

using a fathometer and GPS device, indicates pond-bottom altitude ranges from about 81 to 88 ft above NAVD 88 (fig. 20). 

The estimated volume of water in the pond at the highest 

observed stage during April 2001 (86.5 ft above NAVD 88) was 

about 774,000 ft3 (5.79 Mgal) with a surface area of about 

216,000 ft2

. When observed pond stage was lowest (83.7 ft 

above NAVD 88) during June 2001, the volume decreased to 

about 263,000 ft3 (1.97 Mgal) and surface area decreased to 

about 149,000 ft2 (fig. 21). To facilitate water-budget computations across a wide range of pond stages, polynomial regression 

models (fig. 21) were used to provide continuous relations 

between pond stage and pond volume, and between pond stage 

and pond area across a wide range of projected stages. The relation for pond stage and pond area applies a third-order polynomial regression, whereas the relation for pond stage and pond 

volume applies a second-order polynomial regression (fig. 21). 

Pond-Aquifer Flow 

The surficial aquifer is recharged by rainfall near the pond; 

ground water flows toward the pond and discharges into it 

where clayey surficial deposits are breached or thin (fig. 22). 

Water leaves the pond primarily through evaporation or 

through ground-water seepage. 

Regionally, ground-water flow in the surficial aquifer is 

a subdued replica of land-surface topography. Water flows 

northward from a topographic high located along Interstate 

Highway 16, south of the pond site, toward Lower Black 

Creek (see locations, fig. 17). 

Water-level data from 12 wells were used to construct a 

potentiometric-surface map of the surficial aquifer for April 27, 

2001 (fig. 23); the map indicates that ground-water flow generally was toward the pond south and east of the pond or northward toward Lower Black Creek (see location, fig. 17), north 

of the pond. Steepened gradients, as indicated by closely 

spaced contours along the southern part of the map, may reflect 

modifications to the original topography of the area—fill material, used to build up depressed areas along Interstate Highway 

16, increased land-surface altitudes and corresponding watertable altitudes, producing steepened hydraulic gradients. Despite 

this topography, overland runoff to the pond is minimal because 

of a berm surrounding the pond, drainage ditches along the 

eastern and southern shores, and an intermittent stream channel 

along the western shore. In addition, a plowed field to the north 

and east of the pond reduces quantities of runoff due to higher 

permeability and infiltration capacity of tilled soil. Two depressions are evident on the potentiometric-surface map—one 

beneath the western shore of the pond and one in the farmed area 

northeast of the pond (fig. 23). The depression beneath the pond 

is in an area where the course of an unnamed tributary was 

altered when the borrow pit was excavated. The depression in the 

farmed area occurs beneath a topographically low area. 

The pond is hydraulically separated from the surficial aquifer across most of the pond area due to the low permeability of 

the upper soil layer; however, this low permeability layer was 

breached at the eastern end of the pond. At that location, an 

excavation was cut through the sandy clay layer and into an 

underlying clayey sand layer of higher permeability, which 

resulted in flooding of, and eventual abandonment of, the borrow pit. During the construction of shallow well 33R028 (see 

location, fig. 17), a sand layer was penetrated at a depth of 

about 9 ft and the water level in the well rose above land surface, indicating confined conditions near the pond. The clayey 

soil apparently acts as a confining or semiconfining unit to the 

uppermost part of the surficial aquifer in the vicinity of the pond. 
33R042 33R035

Pond-Aquifer Flow and Water Availability 19 

Creek 

16 

Lower Black 

33R042 

33R030 

33R033 

33R032 

33R035 

33R034 

33R031 

33R041 

33R029 

33R028 

33R040 

33R025 

33R036 

33R037 

33R027 

33R043 

33R044 

33R026 

33R024 

33R023 

33R038 

33R039 

Stage 

recorder 

Weather 

station 

N 

33R033 

33R032 

33R035 

33R034 

33R042

33R030

33R031 

33R041 

33R029

33R028 

33R040 

33R025 

33R036 

33R037 

33R027 

33R043 33R039 

33R044 

33R026

33R024

33R023

33R023

Stage 

recorder 

Weather 

station 

Base from U.S. Geological Survey digital orthophoto 0 500 1,000 FEET 

Lanier 1:24,000; 1993 

EXPLANATION 0 100 200 METERS 

WELL AND SITE NAME—See table 2 for well information 

33R042 Test well 33R035 Existing well 

Figure 17. Bulloch County pond site in Georgia and locations of test wells, stage recorder, and weather station. 
4 

6 

0 

10 

20 

30 

40 

50 

60 

20	 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

October 2000 – Mean monthly

1971– 2001 

Total monthly 

July 2001 

upward gradient of 0.02 ft/ft. Several of the paired wells are 8 

located within 20 ft of the pond, with downward gradients indicating that the “hinge line” separating areas of recharge (down­

 DEPTH BELOW LAND SURFACE, IN FEET	 PRECIPITATION,

ward gradient) from areas of discharge (upward gradient) is close 

to the pond. 

Pond stage and ground-water levels rise in response to precipitation and decline during dry periods in response to increased 

evapotranspiration. This relation is illustrated on a plot showing 

ground-water levels in wells 33R025 and 33R037, pond stage, 

and cumulative net precipitation (fig. 25). During March–June 

2001 (immediately prior to a pumping test), pond stage declined 

about 1.1 ft and ground-water levels in well 33R025 declined 

about 3 ft, corresponding to a period when evaporation mostly 

exceeded precipitation (as indicated by decreasing cumulative 

net precipitation values). Water-level rises during March, June, 

and July 2001, corresponded to precipitation events. 

Estimated Net Ground-Water Seepage 

Net ground-water seepage rates were computed using waterIN INCHES 

2 

0 

OCT NOV DEC JAN FEB MAR APR MAY JUNE JULY 

MONTH 

Figure 18. Mean monthly precipitation, 1971–2001, and 

total monthly precipitation, October 2000–July 2001, at the 

National Weather Service Station at Brooklet, Georgia. 

WELL WELL


33R036 33R037


Confining 

unit 

Confining 

unit 

sand 

sand 

Sandy 

Surficial aquifer 

Sandy

Clayey 

Clayey 

clay 

clay	

92 budget analysis, model simulation, and results of a pond pumping 

test. The water-budget analysis was conducted during October 

2000–June 2001, and was followed by a pond pumping test dur82 ing June 4–5, 2001. 

Water Budget 

 ALTITUDE IN FEET ABOVE NAVD 88 

72 

The water-budget analysis computed net ground-water seepage for 7-day periods during October 2000–June 2001, prior to 

62 the pumping test. During this period, average daily evaporation 

was 0.08 in/d, with a maximum daily rate of 0.22 in/d during 

May 2001 and minimum daily rate of 0.0003 in/d during January 

52 2001 (fig. 26). Rainfall during this period had a daily maximum 

of 1.5 inches during December 2000. Pond-stage altitude ranged 

from 86.5 ft during March 2001 to 85.1 ft during June 2001 

42 (figs. 21 and 25), with a corresponding volume change of 

272,600 ft3

 (2 Mgal). 

Ground-water recharge to the shallow ground-water flow 

32 system can be approximated from net precipitation. Because Interbedded 20 40 60 80 100 120 140 160 

clay and sand NATURAL GAMMA, IN 

AMERICAN PETROLEUM 

INSTITUTE UNITS 

Figure 19. Subsurface geology at the Bulloch County 

pond site in Georgia.

Water levels from paired test wells completed in the upper 

and lower parts of the surficial aquifer during June 2001 indicate 

that vertical head gradients generally are downward within the 

study area, indicating a potential for aquifer recharge (fig. 24). 

Downward head gradients ranged from 0.02 to 0.06 feet per foot 

(ft/ft); only one site in the southwestern part of the area had an 

overland runoff is negligible at the site, part of the precipitation 

remaining after evaporative loss (net precipitation) enters the 

shallow ground- water system, with an unknown amount lost to 

plant transpiration. The relation between cumulative net precipitation and ground-water levels is demonstrated on the hydrographs shown in figure 25. In general, when cumulative net precipitation is high, ground-water levels are high (indicating 

ground-water recharge), and when cumulative net precipitation is 

low, ground-water levels are low. The ground-water component 

of the water budget is illustrated by comparing plots showing 

cumulative net precipitation (precipitation minus evapotranspiration) and cumulative pond-volume change over time (fig. 27). 

Assuming that surface runoff is negligible and errors in estimating atmospheric fluxes and lake volume changes are reasonably 

small and unbiased, the difference between the two lines represents cumulative net ground-water seepage. 
Table 2. Well construction and location information at the Bulloch County pond site in Georgia. [°, degrees; ', minute; ", second; —, no data] 

Well 

number Well name Model layer Latitude (NAD 83) Longitude (NAD 83) 

Altitude of 

land surface, 

in feet 

(NAVD 88) 

Well depth 

in feet 

below land 

surface 

Casing 

diameter 

(inches) 

Open inte

below land surfac

Top 

rval in fe

Bottom 

33R023 CSSI Bulloch Pond Test Site P-1 A3 32°13'48.3" 081°36'19.9" 91.94 12.4 2.0 7.4 12.4 33R024 CSSI Bulloch Pond Test Site P-2 A3 32°13'48.8" 081°36'21.2" 89.15 5.4 2.0 2.4 5.4 33R025 CSSI Bulloch Pond Test Site P-3 A3 32°13'50.3" 081°36'16.0" 93.22 11.5 2.0 6.5 11.5 33R026 CSSI Bulloch Pond Test Site P-4 A3 32°13'47.9" 081°36'23.0" 96.59 14.1 2.0 9.1 14.1 33R027 CSSI Bulloch Pond Test Site P-5 A3 32°13'46.9" 081°36'16.7" 91.13 10.2 2.0 5.2 10.2 33R028 CSSI Bulloch Pond Test Site P-6 A3 32°13'47.6" 081°36'11.2" 87.27 8 2.0 6.0 8.0 33R029 CSSI Bulloch Pond Test Site P-7 A3 32°13'53.30" 081°36'09.80" 93.56 16.30 2.0 11.3 16.3 33R030 CSSI Bulloch Pond Test Site P-8 A3 32°13'46.04" 081°36'06.01" 91.79 14.1 2.0 9.1 14.1 33R031 CSSI Bulloch Pond Test Site 4-inch deep well — 32°13'58.5" 081°36'17.3" 93.91 185 4.0 110.0 185.0 33R033 Ricky Denmark A3 32°14'06" 081°36'08" 97.06 20.7 32.0 — 20.7 33R034 Leland Gregory A3 32°13'58" 081°35'58" 98.35 20.2 30.0 — 20.2 33R035 Earl Nelson A3 32°14'07" 081°35'54" 99.83 22.3 1.5 — 22.3 33R036 CSSI Bulloch Pond Test Site P-9 (Corehole) — 32°13'50" 081°36'16" 92.43 62.00 — — 62.0 33R037 CSSI Bulloch Pond Test Site P-10 A4 32°13'50" 081°36'16" 92.52 28 2.0 23.0 28.0 33R038 CSSI Bulloch Pond Test Site P-11 A4 32°13'49" 081°36'20" 90.82 29.20 2.0 24.20 29.2 33R039 CSSI Bulloch Pond Test Site P-12 A4 32°13'47" 081°36'17" 90.45 28.7 2.0 23.7 28.7 33R040 CSSI Bulloch Pond Test Site P-13 A4 32°13'48" 081°36'11" 90.59 27.8 2.0 22.8 27.8 33R041 CSSI Bulloch Pond Test Site P-14 A4 32°13'59" 081°36'17" 93.66 30.8 2.0 25.8 30.8 33R042 CSSI Bulloch Pond Test Site P-15 A4 32°13'42" 081°35'57" 96.08 28.4 2.0 23.4 28.4 33R043 CSSI Bulloch Pond Test Site P-16 A4 32°13'44" 081°36'26" 99.40 28.6 2.0 23.6 28.6 33R044 CSSI Bulloch Pond Test Site P-17 A3 32°13'44" 081°36'26" 99.40 19.3 2.0 18.9 19.3 
22 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

N	

EXPLANATION 

85.5 BATHYMETRY CONTOUR—Shows 

altitude of pond bottom. Contour 

interval is 0.5 feet. Datum is 

NAVD 88 

DATA POINT 

Base from U.S. Geological Survey 0 250 500 FEET 

85 

84 

83 

82 

85.5 

84.5 

83.5 

82.5 

81 

81.5 

80 

82.5 

82.5 83 

80.5 

80.5 

81 

0 100 METERS

0 100 200 FEET 

N 

50 

digital orthophoto, 

Brunswick East 1:24,000; 1992 

0 	25 50 METERS 

Figure 20.	 Pond bathymetry, Bulloch County pond site in Georgia, October 2000. 

The temporal distribution of net ground-water seepage 

computed using the water-budget method prior to the 

pumping test is consistent with precipitation patterns (fig. 27). 

Net seepage values above the zero line in figure 27 represent 

periods of net seepage into the pond, whereas values below 

the zero line represent periods of net seepage out of the pond. 

Net seepage generally is positive during periods of high rainfall, 

indicating more ground water is entering the pond than leaving 

it. Conversely, net seepage generally is negative during periods 

of low rainfall, indicating that more ground water is leaving the 

pond than entering it. In general, net ground-water seepage was 

positive during the pretest period, ranging from –10,670 ft3/d 

(–55 gal/min) immediately before the test during May 25 – 

June 1, 2001, to 5,690 ft3

/d (30 gal/min) during March 16 –23, 

2001 (fig. 27). Confidence intervals shown on figure 27 were 

derived using equation 7 and error estimates derived from 

the literature. 

Pond Pumping Test 

To estimate rates of net ground-water seepage under 

extreme conditions (dry, low pond stage), a long-term 

pumping test was conducted in the pond during June 4–5, 2001 

(fig. 28). A large-capacity pump was used to lower the water 

level in the pond by 1.74 ft during a 28-hour period. Discharge 

water was piped about 50 ft south of the pond to a concrete 

drainage culvert that passed beneath Interstate Highway 16. 

Following cessation of pumping, recovery was monitored for 

72 hours (fig. 28). During the same period, there was no 

precipitation and estimated evaporation was about 0.23 in/d. 

Thus, changes in pond stage during the pumping test mainly 

were due to the volume of water removed by pumping and 

contributed by ground-water seepage. Seepage estimates 

are limited by the accuracy of precipitation measurements 

and evaporation estimates, and of pond-volume estimates 

determined using pond-stage and bathymetric data. 
Pond-Aquifer Flow and Water Availability 23 

0 

50,000 

100,000 

150,000 

200,000 

250,000 

0 

200,000 

400,000 

600,000 

800,000 

1,000,000 

1,200,000 

stage = 86.5 

stage = 83.7 

stage = 83.7 

stage = 86.5 

IN CUBIC FEET 

y = 498.53x3 – 131090.07x2 + 11503366.69x – 336632587.71 

y = 11673.99x2 + 1804634.80x + 69526945.43 

R2 = 0.99 

R2 = 0.98

ESTIMATED POND

SURFACE AREA,

 IN SQUARE FEET 

Maximum observed 

Minimum observed 

Minimum observed 

Maximum observed 

ESTIMATED POND VOLUME,

81 82 83 84 85 86 87 88


POND STAGE, IN FEET ABOVE NAVD 88


Figure 21. Regression models showing relation of pond stage to estimated surface area and volume, 

Bulloch County pond site in Georgia. 

Altitude stream ditch 

Direction of ground-water flow 

Study pond CONFINING UNIT 

Recharge area Recharge area 

90 feet 

above 

NAVD 88 

60 feet 

SURFICIAL AQUIFER 

Pond evaporation 

Intermittent Drainage 

INTERBEDDED CLAY AND SAND CONFINING UNIT 

Clayey sand 

Pond stage 

SANDY CLAY 

NOT TO SCALE 

Figure 22. Conceptual model of the hydrogeologic framework and pond-aquifer flow at the Bulloch County 

pond site in Georgia. 

89 
24 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

16 

l l l l l 

l l l l l l l l 

l

l 

l 

91 

88

86 

8 5 

87 

92 

90 

89 

89 

87 

8 7 

88

88

86 

85 

92 

91 

91 

9 1 

90 

90 

93 

8 6 

8 6 85.82

estimated 

85.78 

86.63 

86.38 

86.67 

87.06 

93.72. 

85.91 

91.41 

86.18 

85.16 

85.58 

86.11 

87.54 

85.58

85.82

estimated

85.78 

86.63 

86.38 

86.67

87.06 

93.72

estimated 

87.54 

85.91 

91.41 

86.18 

86 

86 

N 

Base from U.S. Geological Survey digital orthophoto 0 500 1,000 FEET 

Lanier 1:24,000; 1993 

EXPLANATION 0 100 200 METERS 

90 POTENTIOMETRIC CONTOUR—Shows altitude at which water table would have stood in tightly cased 

wells on April 27, 2001. Hachures indicate depression. Contour interval 1 foot. Datum is NAVD 88 

INTERMITTENT UNNAMED TRIBUTARY TO ASH BRANCH CREEK 

DIRECTION OF GROUND-WATER FLOW 

91.41	 WELL—Number is altitude of potentiometric surface in feet above NAVD 88. Value estimated by 

comparing to water-level fluctuations in nearby well 

Figure 23. Potentiometric surface of the surficial aquifer at the Bulloch County pond site in Georgia, April 27, 2001. 
0.003

Pond-Aquifer Flow and Water Availability 25


16 

0.05 

0.02 

0.02 

0.02 

0.04 

0.02 

0.06 

N 

0 500 1,000 FEET 

0 100 200 METERS

0.05 

0.02 

0.02 

0.04 

0.02 

0.06 

EXPLANATION 

WELL AND NUMBER—Vertical head gradient in feet 

per foot. Up arrow indicates upward gradient; down 

arrow indicates downward gradient 

Base from U.S. Geological Survey 

digital orthophoto 

Lanier 1:24,000; 1993 

Figure 24. Vertical head gradient between the upper and lower part of the surficial aquifer, Bulloch County pond site 

in Georgia, June 6, 2001. 
88 

26 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

End test 

A. 

Start test 

Well 33R025 

Well 33R037 

Pond stage 

HOURLY WATER CUMULATIVE DAILY NET LEVEL, IN FEET INCHES INCHES PRECIPITATION, IN INCHES ABOVE NAVD 88 – 

– 

87 

86 

85 

84 

83 

4 

3 

2 

1 

0 

–1 

–2 

–3 

–4 

–5 

1.6 

1.2 

0.8 

0.4 

0 

0.4 

1.6 

1.2 

0.8 

0.4 

0 

0.4 

2000 2001 

OCT DEC FEB MAR APR JUNE 

B. 

NOV JAN MAY JULY 

Figure 25. (A) Hourly water levels in wells 33R025 and 33R037 and pond stage; and (B) cumulative daily net 

precipitation (precipitation minus evapotranspiration), Bulloch County pond site in Georgia, October 2000 – July 2001. 

2001 

Net precipitation 

Precipitation 

OCT DEC FEB MAR APR JUNE

2000 

B. 

A. 

Pretest period 

Test and 

posttest period 

Evaporation 

NOV JAN MAY JULY 

Figure 26. (A) Measured daily precipitation and estimated daily evaporation; and (B) estimated daily net 

precipitation (precipitation minus evapotranspiration) at the Bulloch County pond site in Georgia, October 2000 – July 2001. 
Pond-Aquifer Flow and Water Availability 27 

net precipitation 

– 50,000 

–100,000 

50,000 

0 

100,000 

150,000 

200,000 

A. 

Cumulative daily 

Cumulative daily 

pond-volume changes 

CUMULATIVE VOLUME, 

IN CUBIC FEET PER DAY 

OCT DEC FEB MAR 

JUNE

APRIL 

2000 2001

0 

4,000 

8,000 

seepage 

l 

– l 

B. 

NOV JAN MAY 

ESTIMATED NET GROUND-WATER 

SEEPAGE, IN CUBIC FEET PER DAY 

–8,000 

–12,000 

–4,000 

Estimated 7-day 

average net 

ground-water 

+ 95 percent confidence interva

95 percent confidence interva

Figure 27. (A) Cumulative daily net precipitation (precipitation minus evapotranspiration) and cumulative 

daily pond-volume changes; and (B) estimated 7-day average net ground-water seepage, Bulloch County 

pond site in Georgia, October 2000–June 2001. 

End test 

90 

89 

88 

87 

86 

85 

84 

83 

82 

A. 

Well 33R037 

Well 33R029 Well 33R025 Pond stage Start test 

HOURLY WATER LEVEL, 

IN FEET ABOVE NAVD 88 

1 

–1 

2 

–2 

0

IN INCHES 

B.

CUMULATIVE NET 

PRECIPITATION,

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 

JUNE 2001 

Figure 28. (A) Hourly water levels in wells 33R025, 33R029, and 33R037 and pond stage; and (B) cumulative 

net precipitation (precipitation minus evapotranspiration), Bulloch County pond site in Georgia, June 1–16, 2001. 
28 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

Determination of seepage rates during the pumping test was 

not possible because pumping rates measured by flowmeter 

were highly inaccurate and variable; however, estimated rates 

of seepage were computed following cessation of pumping. 

Net ground-water seepage was estimated based on observed 

pond-volume changes during recovery using equation 8. During 

June 7–10, 2001, pond stage rose about 0.09 ft, corresponding 

to a volume change of 13,500 ft3. Evaporation removed about 

6,100 ft3, and precipitation added about 22,800 ft3. Using equation 8, the computed net ground-water seepage rate is about 

2,000 ft3/d (10 gal/min). Applying an error analysis of precipitation, evaporation, and volume using equations 6 and 7 indicates that the net ground-water seepage estimate has a margin 

of error of +/–2,000 ft3

/d (10 gal/min). This high margin of 

error corresponded to a precipitation event that resulted in a 

large coefficient of variation for precipitation, limiting the 

utility of estimates derived using the recovery analysis. 

Water-level responses in monitoring wells varied depending on distance from the pond. Wells located directly adjacent 

to the pond showed water-level declines of 0.82 ft during the 

test (wells 33R025 and 33R037, fig. 28), whereas wells located 

farther away from the pond showed less pronounced declines 

of 0.24 ft or less (well 33R029, fig. 28). Following the pumping 

test, water levels in the monitoring wells recovered to prepumping levels within 7–9 days. Several storms provided more than 

3.4 inches of rain during this period, which quickly recharged 

the surficial aquifer (fig. 28). Despite this rainfall, pond stage 

recovered only slightly during this period. By the end of 

July 2001, the pond surface remained 0.6 ft below pretest stage. 

Ground-Water Flow Model 

The Bulloch County pond site model utilized steady-state 

and transient simulations to evaluate changes in ground-water 

level and seepage to and from the pond prior to and during the 

28-hour pumping test. Initial conditions were simulated as steady 

state, followed by simulation of transient changes in recharge, 

pond stage, ground-water levels, and seepage. Model development, calibration, and sensitivity are described in Appendix B. 

For the initial steady-state simulation of pretest conditions 

(June 4, 2001), ground water flows from the southern boundary 

northward (fig. 29A). Some water seeps into the pond along the 

southern shoreline and exits the pond along the northern shoreline. These flow patterns compare favorably to the conceptual 

model of ground-water flow (fig. 22), regional ground-water 

flowpaths as inferred from topography, and locally to potentiometric-surface maps derived from test-well data (fig. 23). Some 

differences are apparent between the simulated and observedconditions maps. The observed-conditions map for April 2001 

shows two cones of depression—one beneath the pond and one 

beneath a small depression located about 330 ft northeast of the 

pond—that do not appear on the simulated surface for June 

2001. In addition, water levels generally are lower and the 

hydraulic gradient is steeper north of the pond on the simulated 

map than on the observed map. These differences may reflect 

changes in hydrologic conditions during the two periods— 

water levels were lower during June 2001 than during April 

2001, resulting in different flow conditions near the pond. 

In particular, the relation between observed pond stage and 

ground-water levels in wells north of the pond was different 

during the two periods (fig. 25). During April 2001, pond stage 

was lower than ground-water levels in nearby wells north of the 

pond resulting in a more pronounced water-level gradient 

toward the pond as indicated on the observed-conditions map 

(fig. 23). Conversely, during June 2001, pond stage was higher 

than ground-water levels in nearby wells, resulting in a more 

pronounced gradient away from the pond, as indicated on the 

simulated map (fig. 29A). 

A map showing the simulated potentiometric surface after 

28 hours of pumping indicates a depression beneath the study 

pond, with a steepened hydraulic gradient along the southern 

shoreline (fig. 29B). The overall direction of ground-water 

flow remains northward toward Lower Black Creek. 

The MODFLOW post-processor ZONEBUDGET (Harbaugh, 1990) was used to enable determination of simulated 

net ground-water seepage at the pond. In this procedure, the 

pond is designated as a separate “zone,” and flows into and out 

of that zone are summarized. For the pretest period, simulated 

net seepage compares favorably with values estimated using 

the 7-day water-budget analysis, ranging from –10,400 ft3

/d 

(–54 gal/min) immediately prior to the pumping test during 

May 25–June 1, 2001, to 6,000 ft3/d (31 gal/min) during 

March 30–April 6, 2001, with a median rate of 800 ft3

/d 

(4.2 gal/min) (fig. 30). Rates of ground-water seepage vary 

depending on pond stage and related changes in hydraulic gradient and cross-sectional area. To evaluate the impact of changing hydraulic gradients on ground-water seepage, simulated net 

seepage was plotted with pond stage during the pumping test 

(fig. 31). Decreasing pond stage results in an increased hydraulic gradient toward the pond and increased rates of seepage to 

the pond. Simulated net ground-water seepage varied from net 

losing conditions at the beginning of the test (high pond stage 

and low hydraulic gradient) to net gaining conditions at the end 

of the test (low pond stage and high hydraulic gradient). Values 

ranged from –3,420 ft3

/d (–18 gal/min) at the beginning of the 

test to 2,880 ft3

/d (15 gal/min) at the end of the test. Median 

simulated net ground-water seepage during the pumping test 

was 1,270 ft3/d (6.6 gal/min). 

The calibrated ground-water flow model provided estimates 

of the vertical and horizontal hydraulic conductivity of the 

surficial aquifer. Initial values of hydraulic conductivity were 

adjusted during calibration based on observed head and calculated net ground-water seepage into the pond. Because hydraulic-property data for the surficial aquifer are sparse, these 

estimates are useful to help guide development of the surficial 

aquifer and provide information to help development of other 

ground-water models in the area. Calibrated horizontal hydraulic conductivity was lowest in the shallow, clayey soil zone with 

most values near 0.0005 ft/d, and highest in the surficial aquifer, 

with most values near 70 ft/d. Calibrated vertical hydraulic conductivity ranged from 0.05 in pond bed sediments to 10 ft/d in 

the surficial aquifer. 
Water Availability 

Available storage in the Bulloch County pond site ranged 

from 263,000 to 774,000 ft3

 (from 1.97 to 5.79 Mgal) under 

the observed range of stage (from 83.7 to 86.5 ft) during 

October 2000–June 2001 (fig. 21). Ground-water seepage provides a net increase in water to the pond during part of the year. 

Net ground-water seepage rates determined using the waterbudget method and flow- model simulation indicate that the 

Bulloch County pond was mostly gaining water during the study 

period—more water flowed into the pond than flowed out of the 

pond (fig. 30). Most ground-water seepage entered the pond following major rainfall events that provided recharge to the surficial aquifer. This recharge raised water levels in the surficial 

aquifer and produced slightly steepened hydraulic gradients 

toward the pond. The relation between recharge events and estimated net ground-water seepage is illustrated by graphs of 

cumulative daily net precipitation, cumulative daily pond-volume change, and estimated seepage (fig. 27). 

Net ground-water seepage rates vary in response to changing 

pond stage. Simulated values during a 28-hour pumping test 

indicate that ground-water seepage to the pond increases with 

decreased pond stage (fig. 31). A 1.74-ft drop in pond stage 

during the test resulted in a pronounced change in flow conditions near the pond, transitioning ground-water seepage from 

a net losing to a net gaining condition. Examination of the graph 

shown on figure 31 indicates that net ground-water seepage 

was negative at pond stages greater than or equal to 84.9 ft 

and positive at stages less than 84.9 ft. Lowering pond stage by 

1.2 ft beneath the 84.9-ft level resulted in a gain in ground-water 

seepage of 6,000 ft3

/d (31 gal/min). Note that these estimates are 

only appropriate for climatic and ground-water-level conditions 

similar to those observed during the test period. Different conditions could cause significant differences in hydraulic 

gradients near the pond and affect seepage rates. 

Seepage Ponds as Sources of Irrigation Supply 29 

Seepage Ponds as Sources of 

Irrigation Supply 

Results at the two pond test sites indicate that the use of 

excavated seepage ponds as sources for irrigation supply is 

limited by pond-storage volume and low net ground-water seepage rates during periods of low precipitation. Although larger 

volumes of water initially could be available from deeper ponds 

encompassing larger areas, the rate of replenishment— dependent on net ground-water seepage—is low during dry periods 

and would not refill the pond quickly enough to provide a continuous reliable source. 

At the Glynn County pond site, the volume of water 

stored in the pond at minimum observed stage (during the dry 

season) was 13.5 Mgal and the maximum simulated rate of 

replenishment during the pond pumping test was 103 gal/min. 

If 1,000 gal/min were withdrawn from the pond for 10 hours 

per day, it would take about 30 days to drain the pond completely (fig. 32). If the same pumping conditions were 

imposed at the Bulloch County pond site, the rate of depletion of 

pond volume would be even greater because of its lower 

storage volume (1.97 Mgal during the dry season) and lower 

net ground-water seepage rate (15 gal/min during pond pumping test). Under these conditions, it would take about 3.5 days to 

drain the pond completely (fig. 32). Note that different 

climatic conditions and pumping rates would influence the 

rate of pond-storage depletion, and that reducing pond stage 

beyond the range of observed test conditions also would 

increase rates of net ground-water seepage into the ponds. 

Despite these limitations, the aforementioned analysis gives 

some indication of the sustainability of pumping from excavated 

seepage ponds in the 24-county coastal Georgia area. Because the 

two pond test sites are considered to represent the extremes of 

likely conditions to be encountered the coastal Georgia area, it 

is likely that other seepage ponds would have similar storagedepletion rates. Thus, seepage ponds have limited utility as 

sources of irrigation water supply during dry conditions in 

coastal Georgia. 
30 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

87.0 

88.5 85.5 

85.0 

85.5 

81.0 81.0 81.5 

91.5

86.0 

86.0

86.5

87.0 87.5 

88.5 

89.5 

89.0

90.0 90.5 91.0 

86.5 

84.5

84.0

83.5 

81.5 

81.5 

82.0 

81.0 

82.5 

83.0 

88.0 

A. 

N 

Base from U.S. Geological Survey digital orthophoto 0 500 1,000 FEET 

Lanier 1:24,000; 1993 

0 100 200 METERS 

EXPLANATION 

85.5	 SIMULATED POTENTIOMETRIC CONTOUR— 

Shows altitude at which water level would have stood 

in tightly cased wells. Contour interval 0.5 feet. 

Datum is NAVD 88 

DIRECTION OF GROUND-WATER FLOW 

Figure 29. Simulated potentiometric surface of the surficial aquifer at the Bulloch County pond site 

in Georgia (A) prior to pumping test on June 4, 2001; and (B) after pumping test on June 5, 2001. 
Seepage Ponds as Sources of Irrigation Supply 31 

84.0 

84.5 

85.5

86.0 

85.0 

84.5 

88.5 

88.0 

83.5 

83.0 

86.5 

87.0 

87.5 

88.0 

88.5 

B. 

85.0 

85.5 

86.0 

86.5 

87.0 

87.5 

N 

Base from U.S. Geological Survey digital orthophoto 0 500 1,000 FEET 

Lanier 1:24,000; 1993 

0 100 200 METERS 

EXPLANATION 

85.5	 SIMULATED POTENTIOMETRIC CONTOUR— 

Shows altitude at which water level would have stood 

in tightly cased wells. Hachures indicate depression. 

Contour interval 0.5 feet. Datum is NAVD 88 

DIRECTION OF GROUND-WATER FLOW 

Figure 29.	 Simulated potentiometric surface of the surficial aquifer at the Bulloch County pond site in Georgia 

(A) prior to pumping test on June 4, 2001; and (B) after pumping test on June 5, 2001.—Continued 
32	 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

A. 

8,000 

Estimated 

Simulated 

SIMULATED NET GROUND-WATER NET GROUND-WATER SEEPAGE, NET GROUND-WATER SEEPAGE, 

SEEPAGE, IN CUBIC FEET PER D

AY IN CUBIC FEET PER DAY IN CUBIC FEET PER DAY 

4,000 

0 

–4,000 

–8,000 

–12,000 

FEB 23

MAR 2MAR 9

MAR 16MAR 23MAR 30

APR 6

APR 13APR 20APR 27

MAY 4 

MAY 11MAY 18MAY 25

JUNE 1 

2001 

B. 

6,000 EXPLANATION 

90 percentile


3,000 75 percentile 

50 percentile 

0 Mean 

25 percentile 

–3,000 10 percentile


ESTIMATED SIMULATED


Figure 30. Estimated and simulated 7-day average net ground-water seepage at the Bulloch County pond site 

in Georgia, February–June 2001—(A) graph and (B) boxplot summarizing all values. 

+ 4,000 

+ 2,000 

Summary


Begin test 

End test 

water stored in the pond. To develop a better understanding of 

these controls, two pond sites were selected based on contrast- –2,000 ing hydrologic, physiographic, and soil conditions. The Glynn 

The availability of water from seepage ponds is controlled 

by the permeability of surficial deposits, the amount of precipitation recharging the ground-water system, and the volume of 0 

County pond site is located in the Coastal Lowlands physiographic division near the Atlantic Ocean in an area character- –4,000 

83.6	 84.0 84.4 84.8 85.2 85.6 ized by low relief and high-permeability sandy soil. The Bulloch 

County pond site is located in the Coastal Terraces physiographic division in an area also characterized by low relief, but 

having somewhat less-permeable clayey soils. During 1961–90, 

precipitation at the Glynn County site averaged about 49 in/yr 

and at the Bulloch County site averaged about 47 in/yr. The 

Glynn County site is typical of sites in the lower part of the 

coastal area, whereas the Bulloch County site is typical of sites 

in the upper part of the coastal area. Based on testing and analysis at the two sites, it appears that the viability of seepage 

ponds as supplies for irrigation is limited by low seepage rates 

and high dependence on climatic conditions. 

POND STAGE, IN FEET ABOVE NAVD 88 

Figure 31. Pond stage and simulated net groundwater seepage (+, water from aquifer to pond; 

–, water from pond to aquifer), Bulloch County 

pond site in Georgia, June 4 –5, 2001. 
AVAILABLE POND STORAGE, IN MILLION GALLONS 

14 

12 

10 

8 

6 

4 

2 

0 

2.0 

1.5 

1.0 

0.5 

0 

0 1 2 3 4 

0 5 10 15 20 

NUMBER OF DAYS 

25 30 

B. 

A. 

NUMBER OF DAYS 

Figure 32. Hypothetical rate of depletion of pond 

storage at a pumping rate of 1,000 gallons per 

minute for 10 hours per day for climatic and hydrologic conditions similar to those observed during 

pond pumping tests at the (A) Glynn County and 

(B) Bulloch County pond sites in Georgia. 

Although the Glynn County and Bulloch County seepage 

pond sites are located in areas of similar topography, some major 

differences exist in pond area and depth, permeability of the 

underlying sediments, and conditions under which ground water 

enters or exits each pond. The Glynn County pond encompasses 

about 5 acres and has a maximum depth of about 21 ft. Computed 

pond volume prior to a pond pumping test ranged from about 

13.5 Mgal at minimum stage to 18 Mgal at maximum stage. 

The Bulloch County pond also encompasses about 5 acres but 

has a maximum depth of only 6 ft. The volume of water is considerably less than the Glynn County pond, with computed pond 

volume prior to a pond pumping test ranging from 1.97 Mgal at 

minimum stage to 5.79 Mgal at maximum stage. 

Soil permeability is substantially different at the two pond 

sites. At the Glynn County site, the pond is excavated into a 

highly permeable 50- to 55-ft-thick layer of sand; whereas at the 

Bulloch County site, the pond is excavated into low-permeability clay throughout most of the pond’s extent and depth, and 

incises clayey sand of the surficial aquifer in limited areas. 

Model-calibrated horizontal hydraulic conductivity for the surficial aquifer ranged from 0.5 to 50 ft/d at the Glynn County 

pond site and ranged from 0.0005 to 70 ft/d at the Bulloch 

County pond site. Calibrated vertical hydraulic conductivity 

Summary 33 

ranged from 0.4 to 40 ft/d at the Glynn County pond site and 

from 0.05 to 10 ft/d at the Bulloch County pond site. 

Pond-aquifer flow has different characteristics at the two 

pond sites: the Glynn County pond is influenced by both vertical 

and horizontal flow from the surficial aquifer, and the Bulloch 

County pond is influenced mostly by vertical flow in areas where 

the pond incises the surficial aquifer. Because the Bulloch County 

pond is mostly in low-permeability clay layers, ground water 

enters the pond mostly through vertical flow where the clay layers breach the surficial aquifer. Ground-water levels at both sites 

responded markedly to precipitation, rising rapidly in response to 

precipitation, and slowly declining during dry periods. 

Ground-water seepage was estimated using a water-budget 

approach that utilized on-site climatic and hydrologic measurements, computing pond-volume changes during pond pumping 

tests, and by digital simulation using steady-state and transient 

ground-water flow models. During their respective study periods, the Glynn County pond was mostly losing water, whereas 

the Bulloch County pond was mostly gaining water. At both 

sites, most ground-water seepage entered the pond following 

major rainfall events that provided recharge to the surficial 

aquifer. This recharge raised water levels in the surficial aquifer 

and produced steepened hydraulic gradients toward the pond. 

Although higher ground-water seepage rates at the Glynn 

County pond might be expected due to more permeable soils 

and greater pond depth, net seepage rates were actually higher 

at the Bulloch site. During the period prior to a pond pumping 

test at the Glynn County pond, net ground-water seepage estimated using water-budget analysis and simulation ranged from 

–2,220 ft3/d (–11.5 gal/min) to 2,950 ft3/d (15 gal/min). Rates 

at the Bulloch County pond prior to a pond pumping test estimated using water-budget analysis and simulation ranged from 

–10,670 ft3

/d (-55 gal/min) to 6,000 ft3/d (31 gal/min). This 

apparent discrepancy results because hydraulic gradients in the 

vicinity of the Bulloch County pond generally are steeper than 

at the Glynn County pond because of the relatively high relief 

along the southern part of the area near Interstate Highway 16 

(see maps for each site, figs 8 and 23). 

Net ground-water seepage rates vary in response to 

changing pond stage. Simulated values during pond pumping 

tests conducted at the two sites indicate that ground-water seepage to the pond increases with decreased pond stage. At the 

Glynn County pond site, simulated net ground-water seepage 

varied from 1,500 ft3/d (7.8 gal/min) at the beginning of the test 

(high pond stage and low hydraulic gradient) to 19,850 ft3/d 

(103 gal/min) at the end of the test (low pond stage and high 

hydraulic gradient). At the Bulloch County pond site, values 

ranged from -3,400 ft3

/d (–17.7 gal/min) at the beginning of the 

test to 2,880 ft3/d (15 gal/min) at the end of the test. Pumping 

from the Glynn County pond produced steepened hydraulic 

gradients, which when combined with its generally higher permeability and deeper incision into the surficial aquifer, results 

in a higher net seepage rate than at the Bulloch County pond. 

The availability of water to seepage ponds is highly dependent on climatic conditions—ponds will not refill unless there is 

adequate precipitation to recharge the surficial aquifer, which 
34 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

subsequently drains (seeps) into the pond. At the Glynn County 

pond, following a 32-hour pumping test in which pond stage was 

lowered by 2 ft, water levels in the pond and monitoring wells 

showed little recovery in the weeks following the test because 

there was no appreciable rainfall in the area. Water levels in 

many of the wells continued to decline until precipitation events 

during July 2000, almost 2 months after the pumping test. At the 

Bulloch County pond, following a 28-hour pumping test in which 

pond stage was lowered by 1.74 ft, water levels in the monitoring wells recovered to prepumping levels within 7–9 days in 

response to recharge from several storms. These storms provided more than 3.4 inches of rainfall, which quickly recharged 

the surficial aquifer. Despite this rainfall, pond stage only 

recovered slightly during this period. By the end of July 2001, 

the pond surface remained 0.6 ft below pretest stage. Lower permeability of surficial deposits at the Bulloch County pond site 

probably enabled less precipitation to infiltrate into the surficial 

aquifer than at the higher permeability Glynn County pond site. 

Results at the two pond test sites indicate that the usage of 

excavated seepage ponds as sources for irrigation supply is 

limited by pond-storage volume and low net ground-water 

seepage rates during periods of low precipitation. Although 

larger volumes of water initially could be available from deeper 

ponds encompassing larger areas, the rate of replenishment— 

dependent on net ground-water seepage— is low during dry 

periods and would not refill the pond quickly enough to provide 

a continuous reliable source. Pumps withdrawing 1,000 gal/min 

for 10 hours per day at each site would eventually drain each 

pond under dry conditions similar to those observed during 

pond pumping tests—at the Glynn County site, the pond would 

drain within 30 days; at the Bulloch County site, the pond would 

drain within 3.5 days. Greater storage volume and net groundwater seepage rates at the Glynn County pond resulted in a longer 

pumping period than at the Bulloch County pond. Because the 

two pond test sites are considered to represent the extremes of 

likely conditions to be encountered in coastal Georgia, it is likely 

that other seepage ponds would have similar storage-depletion 

rates. Thus, seepage ponds have limited utility as sources of irrigation water supply during dry conditions in coastal Georgia. 

References Cited 

Beck, B.F., 1978, The feasibility of using ponds as shallow 

wells in the coastal area of Georgia, in Investigations of alternative sources of ground water in the coastal area of Georgia: 

Georgia Department of Natural Resources, Geologic and 

Water Resources Division Open-File Report 80-3, p. B1– B25. 

Grubbs, J.W., 1995, Evaluation of ground-water flow and 

hydrologic budget for Lake Five-O, a seepage lake in northwestern Florida: U.S. Geological Survey Water-Resources 

Investigations Report 94-4145, 41 p. 

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

subregional water budgets using results from the U.S. 

Geological Survey modular three- dimensional finite-difference ground-water flow model: U.S. Geological Survey 

Open-File Report 90-392, 46 p. 

Krabbenhoft, D.P., Anderson, M.P., and Bowser, C.J., 1990, 

Estimating groundwater exchange with lakes—2. Calibration of a three-dimensional, solute transport model to a 

stable isotope plume: Water Resources Research, v. 26, 

no. 10, p. 2455–2462. 

LaForge, Lawrence, Cooke, C.W., Keith, Arthur, and 

Campbell, M.R., 1925, Physical geography of Georgia: 

Georgia Geological Survey Bulletin 42, 189 p. 

Lee, T.M., 1996, Hydrogeologic controls on the groundwater 

interactions with an acidic lake in karst terrain, Lake Barco, 

Florida: Water Resources Research, v. 32, no. 4, p. 831–844. 

Lee, T.M., and Swancar, Amy, 1997, Influence of evaporation, 

ground water, and uncertainty in the hydrologic budget of 

Lake Lucerne, a seepage lake in Polk County, Florida: 

U.S. Geological Survey Water-Supply Paper 2439, 61 p. 

McDonald, M.G., and Harbaugh, A.W., 1988, A modular threedimensional finite-difference ground-water flow model: 

U.S. Geological Survey Techniques of Water-Resources 

Investigations, book 6, chap. A1, 586 p. 

Metz, P.A., and Sacks, L.A., 2002, Comparison of the hydrogeology and water quality of a ground-water augmented lake 

with two non-augmented lakes in northwest Hillsborough 

County, Florida: U.S. Geological Survey Water-Resources 

Investigations Report 02-4032, 74 p. 

Pollman, C.D., Lee, T.M., Andrews, W.J., Sacks, L.A., 

Gherini, S.A., and Munson, R.K., 1991, Preliminary analysis 

of the hydrologic and geochemical controls on acid-neutralizing capacity in two acidic seepage lakes in Florida: Water 

Resources Research, v. 27, no. 9, p. 2321–2335. 

Sacks, L.A., Lee, T.M, and Radell, M.J., 1994, Comparison of 

energy budget evaporation losses from two morpholmetrically different Florida seepage lakes: Journal of Hydrology, 

v. 156, p. 311–334. 

Sacks, L.A., Lee, T.M., and Tihansky, A.B., 1992, Hydrogeologic setting and preliminary data analysis for the hydrologic-budget assessment of Lake Barco, an acidic seepage 

lake in Putnam County, Florida: U.S. Geological Survey 

Water-Resources Investigations Report 91-4180, 28 p. 

Stewart, R.B., and Rouse, W.R., 1976, A simple method for 

determining the evaporation from shallow lakes and ponds: 

Water Resources Research, v. 12, no. 4, p. 623–628. 

Winter, T.C., 1976, Numerical simulation analysis of the interaction of lakes and ground water: U.S. Geological Survey 

Professional Paper 1001, 45 p. 

Winter, T.C., 1978, Numerical simulation of steady-state threedimensional groundwater flow near lakes: Water Resources 

Research, v. 14, no. 2, p. 245–254. 

Winter, T.C., 1981, Uncertainties in estimating the water balance 

of lakes: Water Resources Bulletin, v. 17, no. 1, p. 82–115. 

Winter, T.C., and Rosenberry, D.O., 1995, Evaluation of 

11 equations for determining evaporation in a small lake in 

the north central United States: Water Resources Research, 

v. 31, no. 4, p. 983–993. 
Appendix A 35


Appendix A. Ground-Water Model for the 

Glynn County Pond Site in Georgia 

Appendix A 37 

Contents 

Model Development. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39


Steady-State Simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42


Transient Simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43


Calibrated Hydraulic Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46


Sensitivity Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47


Limitations of Digital Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53


References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54


Figures 

A1. Maps showing model area, model grid and boundaries, and well locations, Glynn County 


pond site in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40


A2. Diagram showing hydrogeologic framework and pond-aquifer flow and model layers and 


boundary conditions for the Glynn County pond site model in Georgia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41


A3. Maps showing recharge zones (shown in magenta) for the Glynn County pond site model


in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42


A4–A5. Graphs Showing—


A4. Difference between observed and simulated water levels (residual) for model layer A1,


steady-state simulation, Glynn County pond site in Georgia, November 6, 1999 . . . . . . . . . . . . . . 43


A5. Comparison of estimated average net precipitation and simulated recharge by stress 


period for the transient model, Glynn County pond site in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . 44


A6. Boxplots showing difference between observed and simulated heads (residuals), transient 


model for Glynn County pond site in Georgia, November 6, 1999–May 3, 2000. . . . . . . . . . . . . . . . . . . . . . 45


A7. Graphs showing observed and simulated water levels in selected wells at the Glynn County 


pond site in Georgia, November 6, 1999–May 3, 2000. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46


A8–A12. Maps Showing—


A8. Simulated horizontal hydraulic conductivity in feet per day in model layers A1 through A5 


at the Glynn County pond site in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48


A9. Simulated horizontal hydraulic conductivity in feet per day in model layers A6 through A8 


at the Glynn County pond site in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49


A10. Simulated vertical hydraulic conductivity in feet per day in model layers A1 through A3 


at the Glynn County pond site in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50


A11. Simulated vertical hydraulic conductivity in feet per day in model layers A4 and A5 at the 


Glynn County pond site in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51


A12. Simulated vertical hydraulic conductivity in feet per day in model layers A6 through A8 


at the Glynn County pond site in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52


A13. Bar graphs showing sensitivity of simulated ground-water levels and net ground-water seepage 


to changes in selected model parameters for the model, Glynn County pond site in Georgia. . . . . . . 53


A14–A15. Graphs Showing—


A14. Sensitivity of simulated ground-water levels to selected parameters, Glynn County 


pond site in Georgia for the model—steady-state model and transient model . . . . . . . . . . . . . . . 54


A15. Sensitivity of simulated ground-water levels to changes in pond stage for the model, 


Glynn County pond site in Georgia. Based on steady-state simulation . . . . . . . . . . . . . . . . . . . . . . . 54

38	 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

Tables 

A1.	 Simulated steady-state water budget, Glynn County pond site in Georgia, 

November 6, 1999 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43


A2. Comparison of estimated and simulated net ground-water seepage, Glynn County 


pond site in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47


A3. Simulated transient water budget immediately prior to and after pond pumping test,


May 1–3, 2000, Glynn County pond site in Georgia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Appendix A 39 

Appendix A. Ground-Water Model for the Glynn County Pond Site in Georgia


Model Development 

Pond-aquifer flow at the Glynn County pond site was 

simulated using the U.S. Geological Survey (USGS) digital, 

three-dimensional, finite-difference ground-water flow model, 

MODFLOW (McDonald and Harbaugh, 1988). Steady-state 

and transient simulations were used to evaluate changes in 

ground-water level and seepage to and from the pond prior to 

and during a 32-hour pond pumping test. Initial conditions on 

November 6, 1999, were simulated as steady state, followed by 

simulation of transient changes in recharge, pond stage, groundwater levels, and seepage from November 6, 1999, through the 

pond pumping test, May 1–3, 2000. ZONEBUDGET 

(Harbaugh, 1990) was used to estimate rates of ground-water 

seepage to the pond. 

The model domain consists of a variably spaced grid having 

75 rows and 106 columns, encompassing an area of 0.35 square 

mile (fig. A1). Cell size ranges from 20 by 20 feet (ft) near the 

pond to 100 by 120 ft at the outer margins of the model grid. 

Smaller cell sizes were used near the pond to better simulate 

steeper hydraulic gradients. In the model, the surficial aquifer is 

divided into eight layers—layer A1 is simulated as a watertable layer, whereas layers A2–A8 are simulated as confined 

layers (fig. A2). The pond incises the top five layers of the 

model in order to more accurately represent pond geometry and 

simulated fluxes into and out of the pond. Approximate altitudes of the top of each layer are shown on figure A2. Layer 

thicknesses are as follows: A1, from 1 to 14 ft; A2 through A6, 

4.5 ft; A7, 16.5 ft, and A8, 5 ft. 

Initial estimates of horizontal hydraulic conductivity were 

uniformly distributed by layer and are within estimated ranges 

for silty sand (Heath, 1983), ranging from 30 to 60 feet per day 

(ft/d). Vertical hydraulic conductivity was assigned an initial 

value of 20 ft/d, which is from about 1.5 to 3 times less than horizontal values. Pond bed sediments occur mostly in layer A6, 

and were assigned an initial vertical hydraulic conductivity of 

30 ft/d, or a vertical to horizontal ratio of 1:1. Hydraulic property values were adjusted as part of the calibration process, 

which resulted in zonation of properties by layer and generally 

lower values (see “Calibrated Hydraulic Properties” section). 

Lateral boundary conditions for the model were selected to 

coincide as closely as possible with natural no-flow boundaries 

(fig. A1). No-flow boundaries are assigned to the northern and 

southern sides of the model and correspond to flow lines in the 

surficial aquifer inferred from topographic maps. The eastern 

and western boundaries are simulated as specified head layers 

located at least 0.3 mile from the pond site to minimize influence on simulation results. Head at the western boundary is 

based on reported water levels at a gas station monitoring well 

located in the western part of the simulated area (Ron Wallace, 

Georgia Environmental Protection Division, written commun., 

November 2000). Head at the eastern boundary was assigned 

based on average tidal conditions in Cyprus Mill Creek, based 

on the USGS Brunswick East 71⁄2-minute topographic quadrangle map. Water levels in the surficial aquifer were lower than 

normal during the period of simulation due to effects of a prolonged drought in the area. 

The uppermost layer (A1) was simulated as (1) a free 

surface boundary in which recharge was applied to simulate the 

water table and (2) as a zone of specified head in the area of the 

pond. The base of the model (layer A8) is represented by a noflow boundary at the top of the basal clay layer. 

The recharge package of MODFLOW was used to simulate 

both recharge to the aquifer and water discharging from the 

aquifer as evapotranspiration. Net precipitation, representing 

the difference between precipitation and evapotranspiration, 

was estimated using continuous data from the weather station, 

averaged during 10-day periods—when positive, values represent net recharge; when negative, values represent net evapotranspiration. To reflect variations in soil infiltration capacity 

and uptake by plants (transpiration), the model was divided into 

four recharge zones (fig. A3): 

•	 Zone 1: No recharge zone reflecting (buildings and surfaced parking lots and streets), 

•	 Zone 2: Unlined drainage ditches that collect runoff from 

parking lots and fields (recharge applied as a line source), 

• Zone 3: Wooded areas, and 

• Zone 4: Field areas with grass or dirt (little vegetation). 

The pond is simulated as a constant-head boundary in the 

first five layers of the model, with no recharge applied (fig. A2). 

The depth and geometry of the pond bottom were determined 

from a bathymetric survey conducted during August 1999. Pondstage changes recorded by a continuous gage were applied to 

each stress period of the transient model. A second pond, located 

about 750 ft east of the study pond, is simulated as a constanthead boundary in the first three layers of the model. Stage for 

this pond was assigned based altitudes derived from the USGS 

Brunswick East 71⁄2-minute topographic quadrangle map. 
40 Pond-Aquifer Flow and Water Availability in the Vicinity of Two

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia

Figure A1. (A) Model area, (B) model grid and boundaries, and (C) well locations, Glynn County pond site in Georgia.

EXPLANATION

CELL TYPE

WELL AND SITE NAME

GEORGIA

Model

area

0 1,000 FEET

0 100 200 METERS

500

0 1,000 FEET

1000 200 METERS

500

A.

B.

N

Model area

Specified head

Actively simulated

No-flow boundary

Inactive

C.

34H484

34H499

34H480 34H493

34H498

34H481

34H482

34H483

34H494

34H490

34H484

34H488

34H487

34H485

34H478 34H479

34H486

34H489

34H477

34H476

34H496

34H506

34H491 34H492

34H497

Base from U.S. Geological Survey

digital orthophoto 

Brunswick East 1:24,000; 1983 

1

1

5

5

10

10

20

20

30

40

50

60

70

75

ROW

30 40 50 60 70 80 90 100 106

COLUMN

Weather

station

Pond

temperature

pofile

Stage

recorder

EXPLANATION

CELL TYPE

WELL AND SITE NAME

GEORGIA

Model

area

0 1,000 FEET

0 100 200 METERS

500

0 1,000 FEET

1000 200 METERS

500

A.

B.

N

Model area

Specified head

Actively simulated

No-flow boundary

Inactive

C.

34H484

34H499

34H480 34H493

34H498

34H481

34H482

34H483

34H494

34H490

34H484

34H488

34H487

34H485

34H478 34H479

34H486

34H489

34H477

34H476

34H496

34H506

34H491 34H492

34H497

Base from U.S. Geological Survey

digital orthophoto 

Brunswick East 1:24,000; 1983 

1

1

5

5

10

10

20

20

30

40

50

60

70

75

ROW

30 40 50 60 70 80 90 100 106

COLUMN

Weather

station

Pond

temperature

pofile

Stage

recorder
Appendix A 41 

A1 

A2 

A3 

A4 

A5 

A6 

A7 

A8 

Water table 

Recharge area 

SURFICIAL AQUIFER 

Study pond 

Off-site 

pond 

CLAY CONFINING UNIT 

Cyprus Mill Creek 

– 33.0 

–16.5 

–12.0 

–7.5 

– 3.0 

1.5 

– 38.0 

6.0 

7.0 to 20.0 

B. 

A. 

Pond evaporation 

Altitude, in 

feet above 

or below (–) 

NAVD 88 

Direction of ground-water flow 

NOT TO SCALE 

EXPLANATION 

RECHARGE CELL—Inputs recharge during wet periods, receives 

ground-water discharge through evapotranspiration during dry periods 

SPECIFIED HEAD BOUNDARY 

NO-FLOW BOUNDARY 

ZONE OF REDUCED HORIZONTAL HYDROLOGIC CONDUCTIVITY 

USED TO SIMULATE POND BED 

A2 MODEL LAYER 

Figure A2. Diagram showing (A) hydrogeologic framework and pond-aquifer flow; and (B) model layers and 

boundary conditions for the Glynn County pond site model in Georgia. 
42	 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

Zone 1: Buildings and surfaced parking lots and streets Zone 2: Drainage ditch 

Zone 4:

Model area 

N 

Zone 3: Wooded areas Field areas with grass or dirt 

Base from U.S. Geological Survey digital orthophoto 0 1,000 2,000 FEET 

Brunswick East 1:24,000; 1983 

0 200 400 METERS 

Figure A3. Recharge zones (shown in magenta) for the Glynn County pond site model in Georgia. 

Steady-State Simulation 

The steady-state model was used to (1) provide initial heads 

for transient simulations and (2) refine the initial estimates of 

hydraulic properties and their spatial distribution. The steadystate simulation was based on observed conditions during the 

10-day period prior to November 6, 1999, a period when pond 

stage and ground-water levels were relatively stable, and net precipitation was near zero. Net precipitation rates of –0.0006 ft3

/d 

to zone 4, –0.001 ft3

/d to zone 3, and 0 ft3/d to zones 1 and 2 

were applied to the MODFLOW recharge package for the 

steady-state simulation. 

Calibration was achieved by adjusting values of horizontal 

and vertical hydraulic conductivity and recharge (net precipitation) in order to match observed heads. Simulated head values 

from the calibrated steady-state model compared well to 

observed water levels, with residuals ranging from –1.0 to 0.6 ft, a 

root mean square of 0.38 ft, and a standard deviation of 0.38 ft in 

the 15 wells completed in layer A1. Water-level residuals are 

shown for each of the 15 wells in figure A4; well locations are 

plotted on figure A1. 

The simulated steady-state water budget for the prepumping 

period indicates that inflows are derived mostly from specified 

head cells along the western boundary of the model (95 percent) 

and from the pond (5 percent) (table A1). Outflows from the 

model are mostly through recharge cells that simulate evapotranspiration (73 percent), and to specified head cells at the pond 

and along the eastern boundary of the model (27 percent). 
Appendix A 43 

WA TER LEVEL RESIDUAL, IN FEET 

0.8 

0.4 

0 

–0.4 

–0.8 

–0.12 

34H476 

34H477 

34H478 

34H479 

34H480 

34H481 

34H482 

34H483 

34H484 

34H485 

34H486 

34H487 

34H488 

34H489 

34H490 

SITE NAME 

Figure A4. Difference between observed and simulated water levels (residual) for model layer A1, steady-state 

simulation, Glynn County pond site in Georgia, November 6, 1999. 

Table A1. Simulated steady-state water budget, Glynn County 

pond site in Georgia, November 6, 1999. 

[<, less than] 

Inflow Outflow 

Component 

of budget Cubic feet 

per day 

Percent 

of flow 

Cubic feet 

per day 

Percent 

of flow 

Specified head 

Lateral boundaries 6,778 95 27 <1 

Pond 391 5 1,906 27 

Recharge1 0 5,146 73 

Total 7,169 7,079 

1

Simulates recharge in inflow cells; evapotranspiration in outflow cells 

Transient Simulation 

Following the initial steady-state simulation, the model was 

used to simulate transient conditions from November 6, 1999, 

through the pond pumping test, May 1– 3, 2000. Pond-aquifer 

flow was simulated using the model and rates of net groundwater seepage (flux) into or out of the pond were determined 

using the MODFLOW postprocessor ZONEBUDGET (Harbaugh, 1990) for selected periods prior to and during the pond 

pumping test. 

The transient simulation consisted of 41 stress periods, 

based on 10-day precipitation patterns monitored at the on-site 

weather station (fig. A5). Before the pond pumping test, each 

stress period consisted of daily time steps, with the longest 

stress period before the pond pumping test having 20 time steps 

(days) and the shortest having one time step (day). During the 

pond pumping test, the model was discretized into one stress 

period divided into 32 one-hour time steps. 

Each stress period was initially assigned a recharge rate 

(fig. A5) that reflected the net precipitation (precipitation 

minus evapotranspiration) based on observations at the 

weather station. In some instances, evapotranspiration 

exceeded precipitation; therefore, recharge was negative, 

representing discharge from the pond-aquifer system. 

Recharge rates were applied based on the four recharge zones 

described previously and were adjusted during model calibration when appropriate. No recharge was applied to the pond or 

to zone 1 because it represents paved areas with no inflow or 

outflow. With the exception of zone 2, which receives runoff 

from paved areas, calibrated values of recharge were at or 

below the estimated average net precipitation for each stress 

period (fig. A5). During rainfall events, values in zone 2 

exceeded rates of net precipitation because it received runoff 

from an adjacent paved parking lot. Otherwise, recharge values 

assigned to zone 2 were similar to the other two areas. 

Calibrated hydraulic conductivity values assigned to the 

transient model were identical to those assigned to the steadystate model, with the additional properties of specific yield and 

specific storage incorporated into the transient model. The 

uppermost layer (A1), simulated under water-table conditions, 

was assigned a specific yield of 0.15. Layers A2–A8, simulated 

as confined layers, were assigned a specific storage of 0.0003. 
44 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia ESTIMATED AVERAGE NET PRECIPITATION OR SIMULATED RECHARGE, IN INCHES PER DAY

11 

9 

7 

5 

3 

1 

0 

–1 

Zone 2 

Zones 3 and 4 

Simulated recharge Estimated net precipitation

Net precipitation = precipitation minus evapotranspiration 

123456789 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 

STRESS PERIOD 

Figure A5. Comparison of estimated average net precipitation and simulated recharge by stress period for 

the transient model, Glynn County pond site in Georgia. When positive, values represent net recharge, when 

negative, values represent net evapotranspiration. 

The transient model was calibrated by adjusting values of 

recharge and hydraulic conductivity in an effort to minimize the 

residuals between observed and simulated head and between the 

simulated and calculated net ground-water budget. All changes 

to hydraulic properties made during the transient calibration 

were also made in the steady-state model, so that both models 

contained identical hydraulic-property values. 

Eighty percent of the simulated head values during the transient simulations were within –1.19 and 0.88 ft of observed values for layers A1 and A3 (fig. A6). Simulated water levels in the 

deeper layers (A5–A8) were generally higher than observed 

water levels; however, residuals were low with 80 percent of the 

values within –0.74 and 1.74 ft of observed values. There were 

no monitoring wells installed in layers A2 or A4. Graphs of simulated and observed water levels in selected wells indicate the 

model reasonably simulates water-level changes in the upper 

and lower parts of the surficial aquifer (fig. A7). 

In addition to ground-water levels, the transient model 

was calibrated by adjusting aquifer parameters to minimize 

differences between simulated net ground-water seepage to the 

pond (as determined by summing fluxes into the pond using 

ZONEBUDGET) and estimated values from the water-budget 

approach (table A2). Simulated results showed a good 

comparison to estimated results—one-half of the values were 

within –39 and +34 percent of estimated values, and 80 percent 

of the values were within –65 and +128 percent of estimated 

values. The largest differences generally occurred when seepage estimates were near zero. 

Simulated transient water budgets were computed for the 

stress period immediately prior to the pond pumping test (3-day 

period) and for the period immediately after the test (32-hour 

period). The simulated budgets indicate that ground-water storage provided most of the water flowing into the model, with 

71 percent contributed prior to pumping and 67 percent contributed immediately after the cessation of pumping (table A3). 

Specified-head cells at the pond and along the western boundary 

provided the balance of inflow, with 29 percent contributed 

prior to the test and 33 percent contributed after the test. 

Ground-water discharge from the model was mostly to recharge 

cells representing evapotranspiration prior to the test (98 percent) and to specified-head cells at the pond after the test 

(62 percent). Lowering pond stage by pumping produced a 

steepened hydraulic gradient toward the pond, resulting in 

greater ground-water seepage. 
Appendix A 45 

NOV 6 

NOV 15NOV 22NOV 29

DEC 7

JAN 11 JAN 19

FEB 3

FEB 17FEB 22

MAR 2MAR 9

MAR 24MAR 31

MAY 1MAY 2MAY 3 

–1.0 

– 0.5 

1.5 

1.0 

2.0 

RESIDUALS, IN FEET 

Pumping test 

0 

0.5 

– 0.5 

–1.0 

1.0 

0.5 

0 

Layers A1 (17 wells) and A3 (1 well) 

Layers A5– A8 (4 wells) 

1999 2000


EXPLANATION 

90

Well 34H493 

75 PERCENTILE— Percentage Well 34H492 

of wells having equal or 50
 Well 34H494 less than indicated values 


25 Well 34H491 


10

Figure A6. Difference between observed and simulated heads (residuals), transient model for Glynn County 

pond site in Georgia, November 6, 1999–May 3, 2000. 
46	 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

10


9


8


7


Well 34H476 (model layer A1) 

Simulated 

Observed 

WATER LEVEL, IN FEET ABOVE NAVD 88 

6 

10 

9 

8 

7 

6 

10 

9 

8 

7 

6 

10 

9 

8 

7 

6 

DEC 7

FEB 3

FEB 17FEB 22

MAR 2MAR 9

MAR 24MAR 31

NOV 6 

NOV 15NOV 22NOV 29

JAN 11 JAN 19 

MAY 1MAY 2MAY 3 

Well 34H492 (model layer A8) 

Well 34H481 (model layer A1) 

Well 34H494 (model layer A8) 

1999	 2000 

Figure A7. Observed and simulated water levels in selected wells at the Glynn County pond site in Georgia, 

November 6, 1999–May 3, 2000. 

Calibrated Hydraulic Properties 

Vertical and horizontal hydraulic conductivity was 

adjusted during calibration to provide a closer match of simulated values to observed ground-water levels and estimated 

ground-water seepage to the pond. Calibrated hydraulic conductivity for the Glynn County pond site is plotted in figures 

A8–A12. In general, calibrated values fall within the reported 

ranges for silty or fine sand (Heath, 1983) and are consistent 

with site observations of lithology and estimated permeability 

values for Glynn County reported by Beck (1978). 

Values of hydraulic conductivity were zoned in layers A1 

through A7 to reflect variations in hydraulic conductivity that 

might result from different types of vegetation and related 

depths of root penetration. The pond is surrounded by a 

wooded area consisting largely of pine trees and by open fields 

consisting largely of grassed areas (see figure A1). For layers 

A1 through A5, representing aquifer depths from 0 to about 

32 ft, the influence of root penetration was believed to be more 

pronounced in the wooded area and less pronounced in the 

open fields. Three zones were delineated to reflect this variation (figs. A8, A10, and A11)—a western zone and eastern 

zone consisting largely of open fields, and a central zone consisting of wooded area surrounding the pond. The western 

zone includes the open-field area immediately surrounding the 

pond. In layers A6 and A7, representing depths from about 

19 to 53 ft, two zones were delineated to the west and east of 

the pond. In layer A8, representing depths from 40 to 58 ft, 

a single zone was designated. 

Simulated horizontal and vertical hydraulic conductivity 

was mostly higher in the western half of the simulated area 

and lower in the eastern half. In layers A1–A4, simulated 

horizontal hydraulic conductivity was 50 ft/d in the western 

zone, 5 ft/d in the central zone, and 0.5 ft/d in the eastern zone 

(fig. A8). In layer A5, horizontal conductivity was similarly 

distributed, with the exception that the central zone had a 

higher value of 30 ft/d (fig. A8). In layer A6, horizontal 

hydraulic conductivity was 40 ft/d in the western zone and 

4 ft/d in the eastern zone; and in layer A8, a single value of 

4 ft/d was simulated (fig. A9). 
Simulated vertical hydraulic conductivity in layers A1 

through A4 was from 30 to 40 ft/d in the western zone, 5 ft/d in 

the central zone, and 0.5 ft/d in the eastern zone (figs. A10 and 

A11). In layer A5, vertical hydraulic conductivity was similarly 

distributed, with the exception that the central zone had a higher 

value of 30 ft/d. In layer A6, vertical hydraulic conductivity was 

40 ft/d in the western zone and 4 ft/d in the eastern zone; and in 

layer A8, a single value of 1 ft/d was simulated (fig. A12). 

Table A2. Comparison of estimated and simulated net groundwater seepage, Glynn County pond site in Georgia. 

[negative seepage values indicate net outflow from pond; positive numbers 

indicate net inflow] 

Date 

Net ground-water seepage 

in cubic feet per day Difference Percent 

difference 

Estimated Simulated 

11/16/1999 –411 137 548 133 

11/26/1999 – 772 – 1,071 – 299 – 39 

12/6/1999 –1,536 –1,244 292 19 

12/16/1999 – 882 – 765 117 13 

12/26/1999 – 1,567 – 1,331 236 15 

1/5/2000 –1,518 –2,224 –706 –46 

1/15/2000 –1,359 –860 499 – 37 

1/25/2000 – 1,578 384 1,962 124 

2/4/2000 863 1,254 391 45 

2/14/2000 – 106 314 420 395 

2/24/2000 716 764 48 7 

3/5/2000 –10 –360 –350 – 3,608 

3/15/2000 –1,489 –1,640 –151 –10 

3/25/2000 –694 –461 233 34 

4/4/2000 2,950 2,622 – 328 – 11 

4/14/2000 662 355 – 307 – 46 

4/24/2000 714 42 – 672 – 94 

Mean –354 –240 114 –183 

Tenth percentile – 1,548 – 1,455 – 479 –65 

Twenty-fifth percentile – 1,489 – 1,071 – 307 –39 

Median –694 –360 117 7 

Seventy-fifth percentile 662 355 391 34 

Ninetieth percentile 775 960 519 128 

Appendix A 47 

Neither specific yield nor specific storage had any appreciable impact on simulated water levels or net ground-water 

seepage and required only minor adjustments during the transient simulation. Specific yield was 0.15 in layer A1 reflecting 

water-table conditions. Specific storage in layers A2–A8 was 

0.0003 reflecting confined conditions. 

Sensitivity Analysis 

The sensitivity of the model to changes in model inputs was 

examined through a series of simulations whereby a single calibrated model parameter was varied while other parameters 

were held constant, and resulting simulated values for head and 

net ground-water seepage were compared to that of the calibrated model. Changes in the root mean square of residuals 

between observed and simulated hydraulic head are summarized in figures A13–A15. Changes in simulated net groundwater seepage are summarized in figure A13. Input parameters 

evaluated during the sensitivity analysis were: 

•	 Steady-state model: horizontal hydraulic conductivity, 

vertical hydraulic conductivity, pond-bed hydraulic 

conductivity, and pond stage. 

•	 Transient model: specific yield in the water-table zone 

(layer A1) and specific storage in the confined zone 

(layers A2–A8) was based on stress period 41, time 

step 16; net precipitation (recharge-evapotranspiration) 

was based on stress period 5, time step 1. 

Stress period 41 was selected for evaluation of storage properties because lowering pond stage during the pond pumping test 

was considered to have the greatest impact on ground-water 

levels which are dependent, in part, on storage properties. 

Sensitivity of net precipitation could not be evaluated during 

this period, because it was near zero during the pumping test. 

Stress period 5 was a period of negative net precipitation 

(evapotranspiration exceeded precipitation). 

Table A3. Simulated transient water budget immediately prior to and after pond pumping test, 

May 1–3, 2000, Glynn County pond site in Georgia. 

Before pumping test, May 1, 2000 After pumping test, May 3, 2000 

Component Inflow Outflow Inflow Outflow 

of budget Cubic feet Percentage Cubic feet Percentage Cubic feet Percentage Cubic feet Percentage 

per day of flow per day of flow per day of flow per day of flow 

Storage 49,700 71 440 1 22,200 67 9,500 29 

Specified head 19,900 29 1,000 1 10,800 33 20,500 62 

Recharge 0 0 68,000 98 0 0 3,000 9 

Total 69,600 69,440 33,000 33,000 
48 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

Layers A1– A3 

Layer A4 

Layer A5 

0.5 

5 

5 

50 

50 

50 

30 

0.5 

0.5 

Not 

simulated 

Not 

simulated 

Not 

simulated 

Western zone Central Eastern zone 

zone 

Figure A8. Simulated horizontal hydraulic conductivity in feet per day in model layers A1 

through A5 at the Glynn County pond site in Georgia. 
Appendix A 49


Layer A6 

40 4

4 

Western zone Eastern zone 

Layers A7 – A8 

Figure A9. Simulated horizontal hydraulic conductivity in feet per day in model layers A6 

through A8 at the Glynn County pond site in Georgia. 
50 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

0.5 

0.5 

0.5 

1 

5 

5 

5 

30 

30 

40 

40 

40 

1 

Layer A1 

Layer A2 

Layer A3 

Not 

simulated 

Not 

simulated 

Not 

simulated 

Western zone Central Eastern zone 

zone 

Figure A10. Simulated vertical hydraulic conductivity in feet per day in model layers A1 

through A3 at the Glynn County pond site in Georgia. 
Appendix A 51 

Layer A4 

Layer A5 

0.5 

5 

Not 

simulated 

40 

30 

30 

40 

40 

40 

0.5 

Not 

simulated 

Western zone Central Eastern zone


zone


Figure A11. Simulated vertical hydraulic conductivity in feet per day in model layers A4 and A5 

at the Glynn County pond site in Georgia. 
52 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

Layer A6 

40 4 

Western zone Eastern zone 

Layers A7 and A8 

1 

Figure A12. Simulated vertical hydraulic conductivity in feet per day in model layers A6 

through A8 at the Glynn County pond site in Georgia. 
Appendix A 53 

Simulated ground-water levels are most sensitive to 

changes in horizontal, vertical, and pond-bed hydraulic conductivity (fig. A13A); simulated net ground-water seepage is 

most sensitive to changes in horizontal hydraulic conductivity, 

specific storage, and specific yield (fig. A13B). Decreasing 

horizontal, vertical, and pond-bed hydraulic conductivity had 

a similar impact on simulated ground-water levels (fig. A14). 

Increased horizontal hydraulic conductivity had a pronounced 

affect on simulated ground-water levels, while increased vertical and pond-bed hydraulic conductivity showed little effect. 

Simulated ground-water levels were highly sensitive to 

increases and decreases in pond stage (fig. A15). Changes in 

pond stage directly affect hydraulic gradients near the pond 

and impact head in the surficial aquifer, resulting in model 

sensitivity to this parameter. 

Limitations of Digital Simulation 

The digital ground-water flow model developed for the 

study area addresses questions related to ground-water flow 

through the surficial aquifer in the vicinity of Glynn County, 

Georgia. The conceptual model—supported by field measurement of ground-water levels, pond stage, and estimated water 

budget—is believed to represent flow-system dynamics accurately. Additional measurement of the vertical distribution of 

aquifer head near and beneath the pond would improve characterization of the conceptual model for the site. 

The model is further limited by difficulty in obtaining sufficient measurements to account for all of the spatial variation 

in hydraulic properties and boundary conditions throughout the 

model area. The relative importance of hydraulic properties 

and selected boundary conditions on the calibrated model 

results are presented in the “Sensitivity Analysis” section. Simulated ground-water levels were sensitive to changes in pond 

stage and horizontal, vertical, and pond-bed hydraulic conductivity. Simulated net ground-water seepage was sensitive to 

horizontal hydraulic conductivity, specific storage, and specific yield. 

Pond stage was adequately represented by field measurements; however, the model was unable to account for all of the 

spatial variation in hydraulic properties due to sparse field data 

on hydraulic properties, particularly vertical and horizontal 

hydraulic conductivity and pond-bed hydraulic conductivity. 

Additionally, because hydraulic conductivity and recharge are 

correlated in the mathematics of the ground-water flow equation on which MODFLOW is based, an overestimation of 

recharge would also result in an overestimation of hydraulic 

conductivity. Direct measurement of hydraulic conductivity 

through field or laboratory testing would decrease the level of 

uncertainty of the calibrated model. 

DIFFERENCE IN SIMULATED NET GROUND-WATER DIFFERENCE IN ROOT MEAN SQUARE

SEEPAGE, IN CUBIC FEET PER DAY OF RESIDUALS, IN FEET 

A. 

0.20 

0.15 

0.10 

0.05 

0 

–0.05

Horizontal Vertical Pond-bed Specific Net Specific 

hydraulic hydraulic hydraulic yield precipi- storage 

conductivity conductivity conductivity tation 

0.08 

0.01 

– 0.01 

0.19 0.19 

0.14 

SIMULATED PARAMETER 

B. 

500 

0 

–500 

–1,000 

–1,500 

–2,000 

Horizontal Specific Specific Vertical Pond-bed Net 

hydraulic storage yield hydraulic hydraulic precipiconductivity conductivity conductivity tation

– 1,574 –1,485 

–908 

–358 

– 230 

399 

SIMULATED PARAMETER 

Figure A13. Sensitivity of simulated (A) ground-water 

levels; and (B) net ground-water seepage to changes in 

selected model parameters for the model, Glynn County 

pond site in Georgia. Calibrated parameters were 

modified by multiplying by 0.1 and rerunning the model to 

determine impact on simulated net ground-water 

seepage. Hydraulic conductivity based on steady-state 

model; specific storage and specific yield based on 

stress period 41, time step 16, transient model; net 

precipitation (precipitation minus evapotranspiration) 

based on stress period 5, time step 1, transient model. 
54	 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia ROOT MEAN SQUARE OF RESIDUALS, IN FEET 

References Cited 1.1 

1.0 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

0.3 

0.2 

1.1 

1.0 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

0.3 

Pond bed hydraulic

 conductivity 

Specific yield 

B. 

A. 

Vertical 

hydraulic 

conductivity 

Specific storage 

Net precipitation 

(precipitation minus 

evapotranspiration) 

Horizontal 

hydraulic 

conductivity 

Beck, B.F., 1978, The feasibility of using ponds as shallow 

wells in the coastal area of Georgia, in Investigations of 

alternative sources of ground water in the coastal area 

of Georgia: Georgia Department of Natural Resources, 

Geologic and Water Resources Division Open-File 

Report 80-3, p. B1–B25. 

Harbaugh, A.W., 1990, A computer program for calculating subregional water budgets using results from the 

U.S. Geological Survey modular three-dimensional 

finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 90-392, 46 p. 

Heath, R.C., 1983, Basic ground-water hydrology: U.S. 

Geological Survey Water-Supply Paper 2220, 84 p. 

McDonald, M.G., and Harbaugh, A.W., 1988, A modular 

three-dimensional finite-difference ground-water flow 

model: U.S. Geological Survey Techniques of WaterResources Investigations, book 6, chap. A1, 586 p. 

ROOT MEAN SQUARE OF 

RESIDUALS, IN FEET 

0.1	 1.0 10 

MULTIPLE OF CHANGE FROM CALIBRATED VALUE 

Figure A14. Sensitivity of simulated ground-water 

levels to selected parameters, Glynn County pond site 

in Georgia for the model—(A) steady-state model; and 

(B) transient model. 

1.6 

1.4 

1.2 

1.0 

0.8 

0.6 

0.4 

0.2 

–2.0 –1.5 –1.0 –0.5 0 0.5 1.0 1.5 2.0 

CHANGE FROM CALIBRATED VALUE, IN FEET 

Figure A15. Sensitivity of simulated ground-water levels to 

changes in pond stage for the model, Glynn County pond site 

in Georgia. Based on steady-state simulation. 
Appendix B 55


Appendix B. Ground-Water Model for the 

Bulloch County Pond Site in Georgia 

Appendix B 57 

Contents 

Model Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59


Steady-State Simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62


Transient Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63


Calibrated Hydraulic Properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66


Sensitivity Analysis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69


Limitations of Digital Simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69


Reference Cited. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70


Figures 

B1. Maps showing model area and well locations and model grid and boundaries, Bulloch County 


pond site in Georgia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .60


B2. Diagram showing hydrogeologic framework and pond-aquifer flow, and model layers and 


boundary conditions for the Bulloch County pond site model in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . .61


B3. Map showing recharge zones for the Bulloch County pond site model in Georgia. . . . . . . . . . . . . . . . . .62


B4–B5. Graphs Showing—


B4. Difference between observed and simulated water levels (residual) for steady-state 


simulation, Bulloch County pond site in Georgia, February 22, 2001. . . . . . . . . . . . . . . . . . . . . . . . . . .63


B5.	 Comparison of estimated average net precipitation and simulated recharge by stress 


period for the transient model, Bulloch County pond site in Georgia. When positive, values 


represent net recharge, when negative, values represent net evapotranspiration . . . . . . . . . . .64


B6. Boxplots showing difference between observed and simulated heads (residuals) for the transient


model, Bulloch County pond site in Georgia. 64

B7. Graphs showing observed and simulated water levels in selected wells at the Bulloch County 


pond site in Georgia, April 5–June 13, 2001. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65


B8–B9. Graphs Showing—


B8. Simulated horizontal hydraulic conductivity in feet per day in model layers A1 through A4 


at the Bulloch County pond site in Georgia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .67


B9. Simulated vertical hydraulic conductivity in feet per day in model layers A1 through A4 


at the Bulloch County pond site in Georgia. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .68


B10. Bar graphs showing sensitivity of simulated ground-water levels; and net ground-water seepage 


to changes in selected model parameters for the model, Bullock County pond site in Georgia. . . . . .69


B11–B12. Graphs Showing—


B11. Sensitivity of simulated ground-water levels to selected parameters, Bulloch County pond site 


in Georgia (A) steady-state model; and (B) transient model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70


B12. Sensitivity of simulated ground-water levels to changes in pond stage for the model, 


Bulloch County pond site in Georgia. Based on steady-state simulation. . . . . . . . . . . . . . . . . . . . . . . . . . . .70

58	 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

Tables 

B1.	 Simulated steady-state water budget, Bulloch County pond site in Georgia, 


February 22, 2001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63


B2.	 Comparison of estimated and simulated net ground-water seepage, Bulloch County pond 

site in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66


B3. Simulated transient water budget immediately before and after pond pumping test,


June 4–5, 2001, Bulloch County pond site in Georgia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Appendix B 59 

Appendix B. Ground-Water Model for the Bulloch County Pond Site in Georgia


Model Development 

Pond-aquifer flow at the Bulloch County pond site was 

simulated using the U.S. Geological Survey (USGS) digital, 

three-dimensional, finite-difference ground-water flow model, 

MODFLOW (McDonald and Harbaugh, 1988). Steady-state 

and transient simulations were used to evaluate changes in 

ground-water level and seepage to and from the pond prior to 

and during the 28-hour pond pumping test. Initial conditions on 

February 22, 2001, were simulated as steady state, followed by 

simulation of transient changes in recharge, pond stage, groundwater levels, and seepage from February 22, 2001, through 

July 4, 2001, including a pond pumping test, June 4–5, 2001. 

ZONEBUDGET (Harbaugh, 1990) was used to estimate rates 

of ground-water seepage to the pond. 

The model domain consists of a variably spaced grid having 

80 rows and 95 columns, encompassing an area of 0.36 square 

mile (fig. B1). Cell size ranges from 20 by 20 feet (ft) near the 

pond to 85 by 109 ft at the outer margins of the model grid. 

Smaller cell sizes were used near the pond to better simulate 

steeper hydraulic gradients and pond bathymetry. In the 

model, the surficial aquifer is divided into four confined layers 

(fig. B2). Layer A1 is the uppermost layer and was used to 

maintain specified heads in the pond and to provide recharge to, 

and evapotranspiration from, the surficial aquifer. Layer A2 is 

a thin layer that includes pond bed sediments and was used to 

control the flow of water to and from layer A1 and the pond. 

Flow in the surficial aquifer was simulated using two layers— 

layers A3 and A4, representing the top and bottom of the unit, 

respectively. Approximate altitudes of the top of each layer are 

shown on figure B2. Layer thicknesses are as follows: A1, 

from 6 to 13 ft; A2, 1 ft; A3, 4 ft, and A5, 5 ft. 

Initial estimates of horizontal hydraulic conductivity, 

expressed in feet per day (ft/d) were assigned based on estimated 

ranges for various lithologies derived from the literature (Heath, 

1983). In layer A1, initial horizontal hydraulic conductivity was 

subdivided into two zones—clayey soils encountered at depths 

from 0 to about 13 ft across most of the area were assigned a 

value of 0.0005 ft/d, and an intermittent stream valley in the 

southwestern part of the simulated area was assigned a value of 

100 ft/d, representing coarse alluvial sand (see location, fig. B3). 

Initial horizontal hydraulic conductivity values for layer A2 were 

representative of coarse sand (75 ft/d). In layers A3 and A4, initial horizontal hydraulic conductivity was 50 ft/d (coarse sand). 

Initial vertical hydraulic conductivity was also assigned on 

the basis of estimated ranges for various lithologies derived 

from the literature (Heath, 1983). For layer A1, an initial value 

of 1 ft/d, reflecting silty sand, was assigned in the vicinity of the 

intermittent stream valley. Layer A2 is a thin layer that was used 

to control the flow of water to and from layer A1. Initial vertical 

hydraulic conductivity for layer A2 was 0.1 ft/d. For layers A3 

and A4, initial vertical hydraulic conductivity was 5 ft/d. Layers 

A1–A4, simulated as confined layers, were assigned a specific 

storage of 1 x 10-5. Vertical and horizontal hydraulic conductivity values were adjusted as part of the calibration process (see 

“Calibrated Hydraulic Properties” section). 

Lateral boundary conditions for the model were selected to 

coincide as closely as possible with natural no-flow boundaries 

(fig. B1). No-flow boundaries assigned to the western and eastern sides of the model correspond to flow lines in the surficial 

aquifer. An additional no-flow boundary along the southernmost part of the model area was located in a topographically 

high area along the Interstate Highway 16 median where flow 

conditions are poorly defined. The northern and part of the 

southern boundaries are simulated as specified-head layers 

located at least 0.1 mile from the pond site to minimize influence on simulation results. Head at the lateral boundaries were 

varied based on monitoring well observations (see boundary 

and well locations, fig. B1). Water levels in the surficial aquifer 

were lower than normal during the period of simulation due to 

effects of a prolonged drought in the area. 

The uppermost layer (A1) was initially simulated as a 

largely inactive layer, with recharge cells designated in the 

vicinity of an intermittent stream valley in the southwestern part 

of the area, and specified-head cells designated in the vicinity 

of the pond. Boundary conditions in layer A1 were subsequently modified to include (1) a free-surface boundary in 

which recharge was applied to simulate the water table and 

(2) as a zone of specified head in the area of the pond (fig. B1). 

The base of the model is a no-flow boundary at the top of the 

basal clay layer, at the bottom of layer A4. 

The recharge package of MODFLOW was used to 

simulate both recharge to the aquifer and water discharging 

from the aquifer as evapotranspiration. Net precipitation, 

representing the difference between precipitation and evapotranspiration, was estimated using continuous data from the 

weather station, averaged during 10-day periods—when 

positive, values represent net recharge; when negative, values 

represent net evapotranspiration. To reflect variations in soil 

infiltration capacity and uptake by plants (transpiration), the 

model was divided into two recharge zones (fig. B3): 

• Zone 1: Wooded areas; and 

• Zone 2: Field areas with grass or dirt (little vegetation). 

The pond is simulated as a constant-head boundary in 

layer A1. The depth and geometry of the pond bottom were 

determined from a bathymetric survey conducted during fall 

2000. Pond-stage changes recorded by a continuous gage were 

applied to each stress period of the transient model. A second 

pond, located about 0.13 mi north of the study pond, was 

deemed too small to impact the pond-aquifer flow system and 

was not included in the model simulation. 
60 Pond-Aquifer Flow and Water Availability in the Vicinity of Two

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia

Figure B1. (A) Model area and well locations; and (B) model grid and boundaries, Bulloch County 

pond site in Georgia.

1

1

5

5

10

10

20

20

30

40

50

60

70

80

ROW

30 40 50 60 70 80 90 95

COLUMN

0 500 1,000 FEET

0 100 200 METERS

0 500 1,000 FEET

1000 200 METERS

Base from U.S. Geological Survey 

digital orthophoto

Lanier 1:24,000; 1993

GEORGIA

Model

area

A.

B.

N

Model area

33R042

EXPLANATION

Specified head

Actively simulated

No-flow boundary

Inactive

CELL TYPE

TEST WELL AND 

 SITE NAME

EXISTING WELL 

 AND SITE NAME

STAGE RECORDER

33R042 WEATHER STATION

33R030

33R035

33R035

33R033

33R034

33R031

33R041

33R029

33R028

33R040

33R025

33R036 

33R037

33R027 

33R043 33R039

33R044

33R026

33R024

33R023

33R038
Appendix B 61 

SURFICIAL AQUIFER 

B. 

A. 

Pond evaporation 

Study pond 

INTERBEDDED CLAY AND SAND CONFINING UNIT 

Clayey sand 

Direction of ground-water flow 

Study pond

Altitude, 

in feet 

above 

NAVD 88 

71 

76 

80 

81 

87–94 

SANDY CLAY 

CONFINING UNIT 

Recharge area 

Recharge area 

A3 

A4 

A2 

A1 

Intermittent 

stream 

Drainage 

ditch 

Pond stage 

NOT TO SCALE 

EXPLANATION 

RECHARGE CELL—Inputs recharge during wet periods, receives groundwater discharge through evapotranspiration during dry periods 

SPECIFIED HEAD BOUNDARY 

NO-FLOW BOUNDARY 

A2 MODEL LAYER 

Figure B2. Diagram showing (A) hydrogeologic framework and pond-aquifer flow, and (B) model layers and boundary 

conditions for the Bulloch County pond site model in Georgia. 
62 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

MODEL AREA 

Recharge 

Zone 1 

(wooded area) 

Recharge

Recharge

Zone 2 

(field area with (field area with 

grass or dirt) grass or dirt) 

Recharge

Zone 2

(field area with

Intermittent unnamed tributary to Ash Branch Creek 

N 

Base from U.S. Geological Survey digital orthophoto 

Lanier 1:24,000; 1993 

0 500 1,000 FEET 

0 100 200 METERS 

Figure B3. Recharge zones for the Bulloch County pond site model in Georgia. 

Steady-State Simulation 

The steady-state model was used to (1) provide initial heads 

for transient simulations and (2) refine the initial estimates of 

hydraulic properties and their spatial distribution. The steadystate simulation was based on observed conditions during February 22, 2001, a period of relatively stable ground-water levels. 

Because the period preceding February 22 was mostly dry, no 

recharge was applied for the steady-state simulation. In addition, because February is a winter month in which there is less 

active vegetation, losses by evapotranspiration were not 

included in the simulation. 

The steady-state model was calibrated by adjusting values 

of horizontal and vertical hydraulic conductivity in order to 

match observed heads. The calibrated steady-state model 

showed a good comparison to observed heads, with residuals 

ranging from –0.6 to 0.59 ft, a root mean square error of 0.37 ft, 

and standard deviation of 0.38 ft, in the 14 wells completed in 

layers A3 and A4 at the time of the simulation. Water-level 

residuals are shown for the 14 wells on figure B4. 

The simulated steady-state water budget for the prepumping 

period indicates that inflows are derived entirely from specified-head cells along the northern and southern boundaries of 

the model (table B1). Outflows are to specified-head cells at the 

pond (64 percent) and along the northern boundary of the model 

(36 percent). Net precipitation (precipitation minus evapotranspiration) during the prepumping period was near zero and was 

therefore not incorporated into the steady-state simulation. 
Appendix B 63 

WATER LEVEL RESIDUAL, IN FEET 

0.8 

0.4 

0 

–0.4 

–0.8 

Model layer A4 

Model layer A3 

33R023 

33R024 

33R025 

33R026 

33R027 

33R028 

33R029 

33R030 

33R034 

33R038 

33R039 

33R040 

33R041 

33R037 

SITE NAME 

Figure B4. Difference between observed and simulated water levels (residual) for steady-state simulation, 

Bulloch County pond site in Georgia, February 22, 2001. 

Table B1. Simulated steady-state water budget, Bulloch County 

pond site in Georgia, February 22, 2001. 

Component 

of budget

Inflow Outflow 

Cubic feet 

per day 

Percent 

of flow 

Cubic feet 

per day 

Percent 

of flow 

Specified head 

Lateral boundaries 5,818 100 2,093 36 

Pond 0 0 3,727 64 

Recharge1 0 0 0 0

Total 5,818 5,820 

1

Simulates recharge in inflow cells; evapotranspiration in outflow cells 

Transient Simulation 

Following the initial steady-state simulation, the model 

was used to simulate transient conditions from February 22, 

2001, through the pond pumping test, June 4–5, 2001, and 

subsequent recovery through July 4, 2001. Pond-aquifer flow 

was simulated using the model and rates of net ground-water 

seepage were determined using the MODFLOW postprocessor 

ZONEBUDGET (Harbaugh, 1990) for selected periods prior 

to and during the pond pumping test. 

The transient simulation consisted of 33 stress periods, 

based on precipitation patterns monitored at the on-site 

weather station. Before the pond pumping test, each stress 

period consisted of daily time steps, with the longest stress 

period before the pumping test having 25 time steps (days) and 

the shortest having one time step (day). During the 28-hour 

pumping test, the model consisted of one stress period divided 

into 32 time steps of 52.5 minutes each. 

Each stress period was assigned an initial recharge rate that 

reflected the net precipitation (precipitation minus evapotranspiration) observed at the weather station. In some instances, 

evapotranspiration exceeded precipitation; therefore, recharge 

was negative, representing discharge from the pond-aquifer 

system. Recharge rates were applied based on the two recharge 

zones described previously. No recharge was applied to the 

pond because it is represented by specified-head model cells. 

Calibrated values of recharge were at or below the estimated 

average net precipitation for each stress period (fig. B5). 

Initial hydraulic conductivity values used by the transient 

model were identical to those used by the steady-state model, 

with specific storage also incorporated into the transient 

model. Layers A1–A4, simulated as confined layers, were 

assigned a specific storage of 1 x 10-5. 

The transient model was calibrated by adjusting values of 

recharge and hydraulic conductivity in an effort to minimize 

the residuals between observed and simulated head and 

between the simulated and calculated net ground-water 

budget. All changes to hydraulic properties made during the 

transient calibration were also made in the steady-state model, 

so that both models contained identical hydraulic-property 

values. Eighty percent of simulated head values were within 

–3.4 and 1.5 ft of observed values during six transient stress 

periods (fig. B6). Graphs of observed water levels in selected 

wells are plotted together with simulated values in figure B7. 

The graphs indicate the model reasonably simulates waterlevel changes in the upper and lower parts of the surficial 

aquifer throughout the transient period. 
1.2 

64 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

Zone 1 

Zone 2 

Simulated recharge Estimated net precipitation

Net precipitation = precipitation minus evapotranspiration

ESTIMATED AVERAGE NET PRECIPITATION OR

SIMULATED RECHARGE, IN INCHES PER DAY

1 

0.8 

0.6 

0.4 

0.2 

0 

–0.2 

–0.4 

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 

STRESS PERIOD 

Figure B5. Comparison of estimated average net precipitation and simulated recharge by stress period for the 

transient model, Bulloch County pond site in Georgia. When positive, values represent net recharge, when 

negative, values represent net evapotranspiration. 

RESIDUALS, IN FEET 

4 

3 

2 

1 

0 

–1 

–2 

–3 

–4 

APRIL 5 APRIL 27 MAY 9 JUNE 9 JUNE 6 JUNE 13 

2001 

EXPLANATION 

In addition to ground-water levels, the transient model 

was calibrated by adjusting aquifer parameters to minimize 

differences between simulated net ground-water seepage 

(as determined by summing fluxes to the pond using 

ZONEBUDGET) and estimated values (table B2). Simulated 

results showed a good comparison to estimated results, with 

some exceptions—one-half of the values were within –42 to 

+38 percent of estimated values, and 80 percent of the values 

were within –75 to +515 percent of estimated values. The 

largest differences generally occurred when seepage estimates 

were near zero. 

Simulated transient water budgets were computed for the 

stress period immediately before the pumping test (1-day 

period) and for the period immediately after the pumping test 

(28-hour period). Comparison of the pre- and postpumping test 

simulated transient water budget indicates that specified head 

cells at the pond and along the southern boundary provided most 

of the inflow to the model, with nearly 100 percent contributed 

90 PERCENTILE—Percentage 

of wells having equal or 

75 less than indicated values 

50


25

10


Figure B6. Difference between observed and simulated 

heads (residuals) for the transient model, Bulloch County 

pond site in Georgia. 

prior to the test and 97 percent contributed immediately following the test (table B3). Outflow from the model was provided 

entirely from storage before the test and from specified-head 

cells at the pond and northern boundary (78 percent) and storage 

(22 percent) at the end of the test. Lowering head in the pond by 

pumping produced a steepened hydraulic gradient toward the 

pond, resulting in greater flow rates. 
Appendix B 65 

WATER LEVEL, IN FEET ABOVE NAVD 88 

90


88


86


84


82 


80


90


88


86


84


82 


80


90


88


86


84


82 


80


90


88


86


84


82 


80


90


88


86


84


82 


80


90


88


86


84


82 


80


Well 33R039 (model layer A4) 

Well 33R027 (model layer A3) 

Well 33R029 (model layer A3) 

Well 33R041 (model layer A3) 

Well 33R041 (model layer A3) 

Well 33R028 (model layer A3) 

Simulated 

Observed 

April 5 April 27 May 9 June 4 June 6 June 13 

2001 

Figure B7. Observed and simulated water levels in selected wells at the Bulloch County pond site in Georgia, 

April 5–June 13, 2001. 
66	 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

Table B2. Comparison of estimated and simulated net ground- Calibrated Hydraulic Properties water seepage, Bulloch County pond site in Georgia. 

[negative seepage values indicate net outflow from pond; positive numbers Vertical and horizontal hydraulic conductivity was adjusted 

indicate net inflow] during calibration to provide a closer match of simulated values 

to observed ground-water levels and estimated ground-water 

Net ground-water seepage seepage to the pond. Calibrated hydraulic properties at the 

in cubic feet per day Percent Bulloch County pond site are plotted in figures B8 and B9. Date Difference difference Calibrated horizontal hydraulic conductivity (fig. B8) was 

Estimated Simulated lowest in layer A1, representing the clayey soil zone (mostly 

03/02/01 2,222 1,336 –886 –40 0.0005 ft/d), and greatest in layers A3 and A4, representing the 

surficial aquifer (mostly 70 ft/d). Horizontal hydraulic conduc- 03/09/01 994 202 –792 –80 

tivity was also high in layers A1 and A2 (70 ft/d) in the vicinity 03/16/01 1,618 1,034 –584 –36 

of an intermittent stream channel in the southwestern part of the 03/23/01 5,690 5,889 199 4 simulated area that breached the surficial aquifer (fig. B8). 03/30/01 4,047 3,628 – 419 –10


04/06/01 4,048 6,015 1,967 49


04/13/01 3,415 917 –2,498 –73


04/20/01 39 720 681 1,746


04/27/01 –802 –1,205 – 403 50


05/04/01 –1,046 –1,777 –731 70


05/11/01 –3,089 –2,397 692 –22


05/18/01 4,464 2,581 –1,883 –42


05/25/01 –19 –314 –295 1,553


06/01/01 –10,666 –10,424 242 –2


06/15/01 2,987 –459 –3,446 –115


06/22/01 2,623 1,025 –1,598 –61


06/29/01 1,823 1,960 137 8


Calibrated vertical hydraulic conductivity in layers A1 and 

A2 (fig. B9) was highest in the vicinity of the intermittent 

stream channel in the southwestern part of the simulated area 

(15 ft/d). In layers A3 and A4, calibrated vertical hydraulic conductivity was 10 ft/d across most of the simulated area. Beneath 

the pond, vertical hydraulic conductivity of layer A2 (pond-bed 

hydraulic conductivity) had a pronounced affect on simulated 

net ground-water seepage and was adjusted numerous times 

during the calibration process. Calibrated values were from 0.05 

to 0.16 ft/d across much of the western half of the pond and were 

from 1 to 5 ft/d in isolated parts of the eastern half of the pond. 

The higher values may reflect areas where the pond bed was 

breached during construction of the pond and were infilled with 

more permeable sediments. Specific storage in the confined 07/06/01 4,214 2,634 –1,580 –37 

surficial aquifer (layers A3 and A4) had little impact on simu- Mean 1,253 631 –622 164 

lated water levels or net ground-water seepage and was held Tenth percentile –1,659 –1,963 –2,068 –75 constant at 0.00001. 

Twenty-fifth percentile – 4 –423 –1,406 –42

Median 2,023 971 –502 –16 

Seventy-fifth percentile 3,889 2,426 184 38 

Ninetieth percentile 4,289 4,306 684 515 

Table B3. Simulated transient water budget immediately before and after pond pumping test, June 4–5, 2001, 

Bulloch County pond site in Georgia. 

Before pumping test, June 4, 2001 After pumping test, June 5, 2001 

Component of Inflow Outflow Inflow Outflow 

budget Cubic feet Percentage Cubic feet Percentage Cubic feet Percentage Cubic feet Percentage 

per day of flow per day of flow per day of flow per day of flow 

Storage 15 0.1 13,100 100 4,600 3.4 29,900 22.0 

Specified head 13,100 99.9 0 0 13,1100 96.6 105,800 78 

Recharge 0 0 0 0 0 0 0 0 

Total 13,115 13,100 135,700 135,700 
Appendix B 67 

Layer A1 

Layer A2 

Layers A3–A4 

1 

1 

0.0005 

3 

10 

70 

70 

70 

100 

Not 

simulated 

Figure B8. Simulated horizontal hydraulic conductivity in feet per day in model 

layers A1 through A4 at the Bulloch County pond site in Georgia. 
1

68 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

Layer A1 

0.5 

15 

Not 

simulated 

Layer A2 

15 

.05 to .1 

.1 to .16 

EXPLANATION 

 SIMULATED VERTICAL HYDRAULIC 

 CONDUCTIVITY, IN FEET PER DAY 

16 to .23 

Layers A3–A4 

10 

1 .23 to .46 

Greater than 1 to 5 

1 1

Figure B9. Simulated vertical hydraulic conductivity in feet per day in model layers A1 through A4 

at the Bulloch County pond site in Georgia.
69 Pond-Aquifer Flow and Water Availability in the Vicinity of Two 

Coastal Area Seepage Ponds, Glynn and Bulloch Counties, Georgia 

Sensitivity Analysis A. 

0.7 

0.632 

0.349 

0.161 0.149 

0.069 

The sensitivity of the model to changes in model inputs was 

examined by varying a single calibrated model parameter, running the model, and comparing the resulting simulated 

values for head and net ground-water seepage to that of the 

calibrated model. For each simulation, the value of a single 

input parameter was varied while other inputs were held constant. Changes in the root mean square of residuals between 

observed and simulated hydraulic head are summarized in 

figures B10–B12. Changes in simulated net ground-water 

seepage are summarized in figure B10. Input parameters 

evaluated during the sensitivity analysis were: 

• Steady-state model: horizontal hydraulic conductivity, 

DIFFERENCE IN SIMULATED NET GROUND-WATER DIFFERENCE IN ROOT MEAN SQUARE 

SEEPAGE, IN CUBIC FEET PER DAY OF RESIDUALS, IN FEET 

0.6 

0.5 

0.4 

0.3 

0.2 

0.1 

0 

Vertical Pond-bed Net Horizontal Specific 

hydraulic hydraulic precipitation hydraulic storage 

conductivity conductivity conductivity

vertical hydraulic conductivity, pond-bed hydraulic SIMULATED PARAMETER conductivity, and pond stage. 

• Transient model: recharge and specific storage (based 

on stress period 24, time step 1). 

Simulated ground-water levels are most sensitive to 

changes in vertical and pond-bed hydraulic conductivity 

(fig. B10A); simulated net ground-water seepage is most 

sensitive to changes in horizontal hydraulic conductivity and 

net precipitation (fig. B10B). Decreases in vertical and pondbed hydraulic conductivity have a pronounced affect on simulated ground-water levels, whereas increases show little effect 

(fig. B11). Increases in horizontal hydraulic conductivity had a 

more pronounced affect on simulated ground-water levels than 

decreases. Simulated ground-water levels were highly sensitive 

to increases and decreases in pond stage (fig. B12). Changes 

in pond stage directly affect hydraulic gradients near the pond 

and impact head in the surficial aquifer, resulting in model 

sensitivity to this parameter. 

B. 

2,000 

– 3,298 

– 2,700 

1,011 

–1,448 

–917 

1,000 

0 

–1,000 

–2,000 

–3,000 

–4 ,000 Horizontal Net Vertical Pond-bed Specific 

hydraulic precipitation hydraulic hydraulic storage 

The sensitivity analysis for net precipitation was conducted 

during model stress period 24, time step 1, when simulated net 

precipitation was positive (fig. B11). Multiplying calibrated net 

precipitation by 0.1 resulted in decreased net ground-water 

seepage to the pond (fig. B10). Simulated ground-water levels 

are also sensitive to increases in net precipitation. Although 

increasing net precipitation by a factor of two had little effect, 

increasing by a factor of five had a pronounced affect on simulated ground-water levels (fig. B11). 

Limitations of Digital Simulation 

The digital ground-water flow model developed for the 

study area addresses questions related to ground-water flow 

through the surficial aquifer system in the vicinity of Bulloch 

County, Ga. The conceptual model—supported by field measurement of ground-water levels, pond stage, and estimated 

water budget—is believed to represent flow-system dynamics 

accurately. Additional measurement of the vertical distribution 

of aquifer head near and beneath the pond would improve characterization of the conceptual model for the site. 

conductivity conductivity conductivity 

SIMULATED PARAMETER 

Figure B10. Sensitivity of simulated (A) ground-water levels; and 

(B) net ground-water seepage to changes in selected model 

parameters for the model, Bulloch County pond site in Georgia. 

Calibrated parameters were modified by multiplying by 0.1 and 

rerunning the model to determine impact on simulated net 

ground-water seepage. Hydraulic conductivity based on 

steady-state model; specific storage and net precipitation 

(precipitation minus evapotranspiration) based on stress 

period 24, time step 1 transient model. 
70 

ROOT MEAN SQUARE OF ROOT MEAN SQUARE OF RESIDUALS, IN FEET RESIDUALS, IN FEET 

1.1 

1.0 

0.9 

0.8 

0.7 

0.6 

0.5 

0.4 

0.3 

0.2 

7.2 

6.2 

5.2 

4.2 

3.2 

2.2 

1.2 

0.2 

B. 

A. 

Horizontal 

hydraulic 

conductivity 

Vertical 

hydraulic 

conductivity 

Pond bed 

hydraulic 

conductivity 

Specific storage 

Net precipitation 

(precipitation minus 

evapotranspiration) 

0.1 1.0 10 

MULTIPLE OF CHANGE FROM CALIBRATED VALUE 

Figure B11. Sensitivity of simulated ground-water levels 

to selected parameters, Bulloch County pond site in 

Georgia—(A) steady-state model; and (B ) transient model. 

35 

30 

25 

20 

15 

10 

5 

0 

–45 –30 –15 0 15 30 45 

CHANGE FROM CALIBRATED VALUE, IN FEET 

Figure B12. Sensitivity of simulated ground-water levels 

to changes in pond stage for the model, Bulloch County 

pond site in Georgia. Based on steady-state simulation. 

Appendix B 

The model is further limited by difficulty in obtaining sufficient measurements to account for all of the spatial variation in 

hydraulic properties and boundary conditions throughout the 

model area. The relative importance of hydraulic properties and 

boundary conditions on the calibrated model results are presented in the “Sensitivity Analysis” section. Simulated groundwater levels were sensitive to changes in pond stage, decreases 

in vertical and pond-bed hydraulic conductivity, and increases 

in horizontal hydraulic conductivity and net precipitation. Simulated net ground-water seepage was most sensitive to changes 

in horizontal hydraulic conductivity and net precipitation. 

Pond stage was adequately represented by field measurements; however, the model was unable to account for all of the 

spatial variation in hydraulic properties due to sparse field data 

on hydraulic properties, particularly vertical and horizontal 

hydraulic conductivity and pond-bed hydraulic conductivity. 

Additionally, because hydraulic conductivity and recharge are 

correlated in the mathematics of the ground-water flow equation on which MODFLOW is based, an overestimation of 

recharge would also result in an overestimation of hydraulic 

conductivity. Direct measurement of hydraulic conductivity 

through field or laboratory testing would decrease the level of 

uncertainty of the calibrated model. 

Reference Cited 

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

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

ground-water flow model: U.S. Geological Survey Open-File 

Report 90-392, 46 p.

Heath, R.C., 1983, Basic ground-water hydrology: U.S. 

Geological Survey Water-Supply Paper 2220, 84 p. 

McDonald, M.G., and Harbaugh, A.W., 1988, A modular 

three-dimensional finite-difference ground-water flow 

model: U.S. Geological Survey Techniques of WaterResources Investigations, book 6, chap. A1, 586 p. 
Prepared by U.S. Geological Survey, Water Resources 

Discipline, Georgia District, Atlanta, Georgia. 

Figure design and production by Bonnie J. Turcott and 

Caryl J. Wipperfurth. 

Editing and page layout by Patricia L. Nobles.