GEOCHEMISTRY OF AN INTERCONNECTED AQUIFER-STREAM-RESERVOIR SYSTEM IN KARST TERRAIN: SINKHOLE DEVELOPMENT POTENTIAL IN AND NEAR LAKE SEMINOLE, SOUTHWESTERN GEORGIA AND NORTHWESTERN FLORIDA

Lake Seminole is a surface-water impoundment constructed in the karst terrain of the lower Apalachicola-Chattahoochee-Flint River Basin, southwestern Georgia and northwestern Florida. Analyses of major ions, nutrients, radon-222, and stable isotopes of hydrogen and oxygen in water samples collected from 29 wells, 7 lake locations, and 5 springs in the Lake Seminole area during 2000 and 2001 were used to investigate lake-aquifer interaction including surface-water mixing with ground-water from the underlying Upper Floridan aquifer, lake leakage beneath Jim Woodruff Lock and Dam, and karstic dissolution of the limestone-aquifer matrix.

Geochemical data analyses indicate that ground water discharges to Lake Seminole along upstream reaches of the lake's impoundment arms, and that the lake leaks into the underlying aquifer near the dam. In-lake springflow from May to October 2000 exhibited more ground-water-like qualities than surface water. Dye-tracing experiments conducted at sinkholes in the lake bottom along the western lakeshore and in front of the dam indicate that lake water is transported through the aquifer at velocities of about 500 feet per hour to locations about 300 yards downstream of the dam. Water "boils up" from a limestone ledge in the channel bottom of the Apalachicola River at rates of about 140-220 cubic feet per second, which represents about 1 to 3 percent of the average daily flow in the river. Isotopic data from the River Boil indicate about a 13-to-1 mixing ratio of lake water to ground water.

Saturation index calculations indicate that while surface water is predominately undersaturated with respect to calcite year-round, a higher potential for limestone dissolution exists from late fall through early spring than during summer. The relatively short residence time (5–7 hours) and rapid flow (nearly 500 feet per hour) of lake water leaking into the aquifer and exiting at the River Boil implies that although calcite-undersaturated water does not travel a substantial vertical distance between recharge and discharge areas, there is an increased potential for dissolution and enlargement of flow pathways by erosion. A relatively low potential exists, however, for dissolution to cause sinkhole collapse followed by catastrophic lake drainage because of low vertical and lateral hydraulic gradients.