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