WATER CHEMISTRY OF THE UPPER FLORIDAN AQUIFER ALONG A GROUNDWATER FLOWPATH ON ST. CATHERINES ISLAND, GEORGIA

St. Catherines Island is a 20 km by 2 to 4 km barrier island located on the Georgia coast between the Savannah and Altamaha Rivers. Hydraulic head and general water chemistry are being monitored on the island’s Pleistocene core along a north-south transect of four wells completed in the upper Floridan Aquifer, the principal artesian aquifer for the region. Head data show that the transect lies completely within a major drawdown cone centered near Savannah, Georgia, located 50 km to the north. Previous studies have documented both lateral and vertical saltwater intrusion in coastal Georgia due to the post-industrialization loss of artesian pressure within the upper Floridan.

The 8 km long transect in this study lies along a south to north groundwater flowpath within the drawdown cone of the upper Floridan aquifer. The hydraulic head averages 3.4 and 9.1 meters below sea level at the southern and northern ends of the transect, respectively. The groundwater is slightly alkaline (pH 7.6 to 7.9), has a low dissolved oxygen content (10 to 30%), and is under reducing conditions (-0.28 to -0.32 relative V). Trends in water chemistry along the flowpath consistently show a decrease in total dissolved solids; ranging from an average of 367 mg/l in the south to 310 mg/l in the north. For individual chemical species, south to north decreases were found in average chloride concentrations (13.8 to 9.2 mg/l) and in sulfate (124 to 78 mg/l). Decreases in sulfate concentration along the flowpath, which make up 81% of the decline in total dissolved solids, can largely be explained by sulfate reduction. However, since chloride is a conservative tracer, decreasing chloride concentrations along the flowpath are difficult to account for through chemical reactions. The chloride decline can most easily be explained by saltwater intrusion, with the source being located up-gradient (south) of the transect. Possible mechanisms include the downward movement of seawater through confining beds in the adjacent sound, and by the upwards flow of saline water from deep aquifers along vertical conduits. Additional sampling will be performed in an attempt to determine the source of the saline water.