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 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 dayunder climatic and hydrologic conditions similar to those observed during pond pumping tests at each sitewould 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.
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USGS Numbered Series
Pond-aquifer flow and water availability in the vicinity of two coastal area seepage ponds, Glynn and Bulloch Counties, Georgia