SALTWATER INTRUSION IN THE UPPER FLORIDAN AQUIFER ON ST. CATHERINES ISLAND, GEORGIA

St. Catherines Island is a 20 km long barrier island located between the Savannah and Altamaha Rivers on the Georgia coast. Hydraulic head and water chemistry were measured on a monthly basis in the Upper Floridan aquifer from four water-supply wells on the island along an 8 km, north-south transect. Head data show that the wells lie within the major drawdown cone of the Upper Floridan centered near Savannah, Georgia, with the transect following a south to north groundwater flowpath. It has previously been documented that the loss of artesian head within the Upper Floridan aquifer has led to both lateral and vertical saltwater intrusion at different sites along the Georgia Coast.

Data from this study show consistent trends in water chemistry along the south to north flowpath. Total dissolved solids in the most upgradient well average 358 mg/l and progressively decreases to 310 mg/l in the downgradient well. Average chloride concentrations along the flowpath range from 14.5 to 9.0 mg/l and sulfate ranges from 141 to 73 mg/l. The groundwater consistently has a low dissolved oxygen content (8 to 13%), is under reducing conditions (-0.27 to -0.36 relative V), and is slightly alkaline (pH 7.5 to 7.9). The decrease in sulfate concentration along the flowpath, which represents 81% of the decline in total dissolved solids, can be explained primarily by sulfate reduction. Because chloride is a conservative tracer, the decreasing chloride concentration along the flowpath can be explained by dilution from the surficial aquifer, or by saltwater intrusion taking place upgradient of the transect.

Samples collected from the surficial aquifer on the island have a chloride concentration 2 to 4 times greater than the Upper Floridan, thereby eliminating it as a possible source. Piper diagram analysis also rules out modern seawater as it does not plot along the observed mixing line of the Upper Floridan aquifer. However, chemical data from Lower Floridan aquifer, obtained from wells located on the nearby mainland, plot on the upgradient side of the observed mixing line. Therefore, it is concluded that the mostly likely mechanism for saltwater intrusion in the Upper Floridan aquifer on St. Catherines Island is by the upward movement of more saline water from the Lower Floridan aquifer along vertical fractures or faults.