By Michael F. Peck (1), John S. Clarke (2), Malek Abu-Ruman (3), and Michael T. Laitta (4)
AUTHORS: (1) Hydrologic Technician, (2) Hydrologist, U.S. Geological Survey, 3039 Amwiler Road, Suite 130, Peachtree Business Center, Atlanta, GA 30360-2824; (3) PhD Student, Georgia Institute of Technology, Atlanta, GA; and (4) Student (Hydrology), U.S. Geological Survey 3039 Amwiler Road, Suite 130, Peachtree Business Center, Atlanta, GA 30360-2824.
REFERENCE: Proceedings of the 2001 Georgia Water Resources Conference, held March 26-27, 2001, at The University of Georgia, Kathryn J. Hatcher, editor, Institute of Ecology, The University of Georgia, Athens, Georgia, p. 768-771.

Introduction

The Upper Floridan aquifer is the principal source of water in the coastal area of Georgia, but declining water levels and localized saltwater contamination have resulted in 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 at golf courses, farms, or communities by digging through sandy surface soils until the water table is reached. Because these ponds are largely cut off from surface-water runoff, water is largely 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, is evaluating hydrogeologic conditions and pond-aquifer relations at two pond test sites in coastal Georgia to determine their potential as supplemental sources of water for irrigation. This paper describes and contrasts hydrogeologic conditions at seepage pond sites at Brunswick, Glynn County, and in southern Bulloch County, Georgia (fig. 1).

Study Areas

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 maximum potential yield (fig. 1). The Glynn County site is located in the Coastal Lowlands physiographic division near the Atlantic Ocean in an area characterized by flat topography and high-permeability sandy soil. The Bulloch County site is located in the Coastal Terraces physiographic division in an area characterized by flat topography and low-permeability clayey soils. Rainfall at the Glynn County site averages 54 inches per year and at the Bulloch County site averages about 47 inches per year (Krause and Randolph, 1989).

Approach

Each pond test site is being characterized by constructing test wells, conducting pond bathymetric surveys, and monitoring ground-water levels, pond stage, and climatic conditions. Long-term aquifer tests are being conducted at each pond to estimate ground-water seepage and to evaluate effects on ground-water levels.

Water budgets are being developed at each site to evaluate the annual exchange of water between the surficial aquifer, pond, and atmosphere. Automated weather stations were 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. From these data, rates of recharge and evaporation are computed using the Penman equation (Winter and Rosenberry, 1995).

Ground-Water Seepage

Seepage represents ground water either entering or leaving the pond. When positive, more ground water enters than leaves the pond; when negative, more water leaves than enters the pond. Ground-water seepage can be estimated using the following volumetric relation:

Seepage = Change in stage + Pumping - Precipitation + Evaporation + Transpiration

In a preliminary investigation, Beck (1979) used hydraulic conductivity derived from grain-size analyses of sediments in coastal Georgia to estimate rates of ground-water seepage. Beck (1979) concluded that a yield of 100,000 gallons per day or 69.4 gallons per minute (gal/min) could be obtained from a 15-foot-(ft) deep pond having a 100-ft radius.

Glynn County Study Site

The first study site is a 3-acre pond located on the campus of Coastal Georgia Community College at Brunswick, Ga. (fig. 1). The pond was excavated to about 15 ft below sea level into the upper part of a 50- to 55-ft-thick sequence of quartz sand that is part of the surficial aquifer (fig. 2). A dense, low-permeability clay layer was penetrated at about 40 ft below sea level during drilling of several deep wells. Natural gamma logs from wells located within 0.75 mile of the pond site, indicate that the clay layer occurs at similar altitudes throughout the area.

The sandy surficial aquifer is recharged by rainfall near the pond. Data from 14 test wells completed in the uppermost part of the surficial aquifer indicate that ground-water flow varies seasonally, but generally is eastward toward Cyprus Mill Creek, part of a major estuary system about 2,500 ft east of the pond (fig. 3). Locally, ground water seeps from the pond along the northern, southern, and eastern shores. A bottom-temperature survey of the pond water in August 1999 indicates that cooler ground water seeps into the pond along parts of the western shore. Ground water seeps from the pond along the eastern-southeastern shore, toward the estuary.

Pond stage and ground-water levels respond rapidly to precipitation events and decline during dry periods when evaporation and transpiration increase. Continuous-recorder data indicate that during October 1999 to May 2000 (immediately prior to a pumping test), pond stage declined about 0.5 ft, and ground-water levels declined about 2 ft.

A bathymetric survey of the pond indicates that the pond depth is about 21 ft at the deepest point, and the bottom of the pond ranges from about 8 ft above sea level to 13 ft below sea level. The volume of water in the pond was calculated using the bathymetric data and the observed range of pond stage. The volume of water in the pond at the highest stage in October 1999 (8.52 ft above sea level) was 17.2 million gallons and decreased to 12.9 million gallons when pond stage was lowest (5.34 ft above sea level) in June 2000.

To estimate rates of ground-water seepage, a long-term pumping test was conducted in the pond during May 1–3, 2000. The water level in the pond was lowered 2 ft during a 33-hour period, pumping at an average rate of 1,000 gal/min. During the same period, there was no precipitation, estimated evaporation was about 10 gal/min, and transpiration was unknown. Thus, changes in pond stage during the pumping test are mainly due to the volume of water removed by pumping and contributed by ground-water seepage. Seepage estimates are limited by the accuracy of evaporation and transpiration estimates, and to pond-volume estimates determined using pond-stage and bathymetric data. Because transpiration is unknown, seepage estimates derived for the pond are lower than actual rates.

Rates of ground-water seepage vary depending on pond stage and related changes in hydraulic gradient and cross-sectional area. Decreasing pond stage results in an increased hydraulic gradient toward the pond and increased rates of seepage to the pond. During the pumping test; however, estimated seepage was about -280 gal/min, indicating water loss. This discrepancy results from errors in pond-volume and evaporation estimates, and from a lack of transpiration data. Following the pumping test, pond stage recovered about 0.1 ft in 25.5 hours, corresponding to a rate of about 90 gal/min, which when combined with the estimated evaporation rate of 10 gal/min, equals a seepage rate of about 100 gal/min. Although this rate compares favorably with the estimated seepage rate reported by Beck (1979) for a 15-ft deep, 100-ft radius pond (69 gal/min), it underestimates the actual seepage because of errors in pond-volume and evaporation estimates and a lack of transpiration data.

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 0.75 ft below pre-test conditions. Conversely, ground-water levels had recovered to pre-test conditions by early September 2000.

Bulloch County Study Site

The second study site is a 4-acre pond located in southern Bulloch County about 20 miles southeast of Statesboro (fig. 1). The pond was excavated as a borrow pit for road fill material during construction of nearby Interstate 16. The pond was excavated into layers of clay and clayey sand that are underlain by a clayey sand layer at an altitude of about 90 ft above sea level; this clayey sand layer forms the uppermost part of the surficial aquifer (fig. 4). The surficial aquifer is underlain by layers of clay and sandy clay at an altitude of about 60 ft above sea level; these layers act as a semiconfining unit. A bathymetric survey indicates pond depth ranges from about 2 to 6 ft (80 to 86 ft above sea level.

To characterize geologic and hydrologic conditions, nine shallow wells (5 to 16 ft deep) and 7 deep wells (28 to 30 ft deep) were installed. Two of the shallow wells are equipped with recorders to continuously monitor ground-water levels.

The volume of water in the pond was about 4.6 million gallons in October 2000, a period of low precipitation. A continuous weather station was installed to provide climatic data needed to calculate a hydrologic budget. A long-term pumping test is planned for Spring 2001 to provide data needed to estimate rates of ground-water seepage at the pond.

The sandy surficial aquifer is recharged by rainfall near the pond. Data from eight shallow test wells indicate that ground-water flow generally is southwest-ward toward an unnamed intermittent tributary to Ash Branch Creek (fig. 5). Preliminary data indicate that the pond is hydraulically separated from the surficial aquifer over most of its area due to 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 cut through the sandy clay layer and into an underlying clayey sand layer of higher permeability resulting in the flooding of, and eventual abandonment of the borrow pit. It appears that most of the water enters the pond in the vicinity of this breach. During the construction of shallow well P-6 (fig. 4), a sand layer was penetrated at a depth of about 9 ft and the water level in the well rose above land surface possibly indicating artesian conditions beneath 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.

Comparison of Hydrogeologic Conditions

The Glynn County and Bulloch County seepage pond sites are located in areas of similar topography. Some major differences however, 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 is about 3 acres and has a maximum depth of about 21 ft. Computed pond volume ranges from 17.2 million gallons at maximum stage to 12.9 million gallons at minimum stage. The Bulloch County pond is about 4 acres, but has a maximum depth of only 6 ft. The volume of water in the Bulloch County pond is considerably less than the Glynn County pond, with about 4.6 million gallons in storage during October 2000.

Soil permeability is substantially different at the two 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 its extent. Ground water at the two sites occurs under different conditions. At the Glynn County site, ground water occurs under water-table (unconfined conditions), and thus is highly affected by climatic conditions. Ground water at the Bulloch County site may occur under confined or semi-confined conditions, which may reduce the effects of climatic change on water availability. A long-term pumping test planned for spring of 2001 will provide data needed to further evaluate ground-water conditions at the Bulloch County site.

References Cited

Beck, B.F., 1979, 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.

Krause, R.E., and Randolph, R.B., 1989, Hydrogeology of the Floridan Aquifer System in southeast Georgia and adjacent parts of south Carolina and Florida: U.S. Geological Survey Professional Paper 1403-D, 65 p.

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, vol. 31, no. 4, p. 983-993.