| Abstrakt: |
St. Catherines Island is a 10 mile (16 km) long barrier island located on the Georgia coast between the Savannah and Altamaha Rivers. Periodic measurements of hydraulic head and water chemistry in the Upper Floridan aquifer were obtained over 21 months from a 5 mile (8 km), north-south transect of four water-supply wells on the island. Head data show that the artesian groundwater follows a general south to north groundwater flowpath into the cone of depression centered near Savannah, Georgia. Previous studies have shown that the loss of artesian head within this system has led to both lateral and vertical saltwater intrusion along the Georgia coast. Saltwater intrusion is of considerable interest because the Upper Floridan aquifer serves as the principal water supply for the southeastern coastal region. Water chemistry data from the Upper Floridan aquifer in this study reveal a consistent mixing pattern along the south to north groundwater flowpath. Total dissolved solids average 358 mg/L in the most up-gradient well and progressively decline to 309 mg/L in the down-gradient well. Similarly, average chloride concentrations along the flowpath decline from 13.4 to 9.1 mg/L and sulfate from 113 to 73 mg/L. The artesian groundwater also has low dissolved oxygen content (4 to 8%) and is under reducing conditions (- 0.30 to -0.34 relative V) and slightly alkaline (pH 7.6 to 7.8). Most significant is the decrease in sulfate and chloride concentrations. The decline in sulfate, which represents 82% of the observed decline in total dissolved solids, can be explained by biotic sulfate reduction. However, because chloride is a conservative tracer, its decreasing concentration within the Upper Floridan is due to abiotic mixing and dilution with either surficial freshwater moving downward along the flowpath, or by saline water entering at a discrete point up-gradient of the transect. Samples collected from shallow wells on the island show average chloride concentrations in the surficial aquifer to be 2 to 4 times greater than what is found in the Upper Floridan, thereby eliminating dilution by surficial freshwater as the cause of the decreasing chloride trend in the artesian aquifer. In regards to the intrusion of more saline water, modern seawater can be ruled out because it does not fall along the observed Upper Floridan mixing line as plotted on a trilinear diagram. However, chemical data from nearby Lower Floridan wells do plot up-gradient of the mixing line. Based on these data, it is concluded that saltwater intrusion is taking place within the Upper Floridan aquifer on St. Catherines Island by the discrete upward movement of more saline water from the Lower Floridan along near-vertical joints, faults, and or solution collapse features. These pathways are believed to ultimately be related to recurrent movement of known basement structures located beneath the sedimentary sequence of the Coastal Plain. [ABSTRACT FROM AUTHOR] |
