Water-supply potential of major streams and the upper Floridan Aquifer in the vicinity of Savannah, Georgia

Abstract

Long-term pumping from the Upper Floridan aquifer in the Savannah, Georgia, area has lowered ground-water levels, resulting in increased salinity of ground water by seawater encroachment at Hilton Head Island, S.C., and by saltwater intrusion at Brunswick, Ga. Increased pumpage could cause further salinization of the ground-water resources. The Savannah and Ogeechee Rivers can be considered potential water-supply sources for the Savannah, Ga., area, on the basis of historic stream flow records and water-quality constituents and properties examined. Analyses of stream-discharge data indicate that the minimum average discharges for 7 consecutive days for a 10-year recurrence interval (7010), was 5,460 cubic feet per second (ft3/s) at Savannah River near Clyo, Ga., and 192 ft3/s at Ogeechee River near Eden, Ga. For example, 90 percent of the time, flows in excess of the 7010 discharges are about 900 and 200 ft3/s, respectively. However, Georgia Department of Natural Resources, Environmental Protection Division, imposes a non-depletable flow criterion; thus, the actual quantity of water available for withdrawal probably would be less than flows in excess of minimum flow criteria, such as the 7Q10. A ground-water flow model was developed and used in conjunction with other previously calibrated models in the coastal area to simulate the effects of additional pumping on water levels near sites of seawater encroachment at Hilton Head Island and saltwater intrusion at Brunswick. Based on model simulations and the constraint of preventing water-level declines at locations of encroachment and intrusion, the potential of the Upper Floridan aquifer to supply additional water in the Savannah area is limited under present (1985) hydrologic conditions. The water-supply potential ranges from less than 1 million gallons per day (Mgal/d) in Liberty, McIntosh, most of Bryan, and southern Chatham Counties, Ga., and in southern Beaufort County, S.C., to more than 5 Mgal/d in northern Jasper and northern Beaufort Counties, S.C. Because of the limited water- supply potential, hypothetical alternatives involving redistributions, redistributions and small increases, and decreases in pumpage were simulated to determine the effects on water levels. These simulations indicate that reductions and redistributions of pumping would not adversely affect water levels at locations of encroachment and intrusion. Increased pumping would cause water-level declines, which might increase salinization of the fresh-water aquifer. .