Physical and Hydrochemical Evidence of Lake Leakage and Assessment of Karst Features in the Vicinity of Lake Seminole, Southwestern Georgia and Northwestern Florida

Lake Seminole is the surface manifestation of an interconnected aquifer-stream-reservoir system characterized by a constructed impoundment emplaced in the karst terrain of the lower Apalachicola-Chattahoochee-Flint River Basin. Physical, chemical, and isotopic constituents of water samples indicate surface water mixing with ground water of the underlying Upper Floridan aquifer, lake leakage beneath Jim Woodruff Dam, and karstic dissolution of the limestone aquifer matrix. Major ions, nutrients, radon 222, and stable isotopes of hydrogen and oxygen collected in 2000 from 30 wells, 7 lake locations, and 5 springs indicate that in-lake springflow evolves chemically and isotopically along ground-water and surface-water pathways. Although mixing ratios of ground water to surface water in springs varied with location and season, springflow from May to October exhibited more ground-water-like qualities (higher specific conductance, dissolved oxygen, and lower temperature) than surface water. Ratios from November to April were difficult to quantify due to rapid mixing of spring and lake water during sampling and reduced flow from springs. Lake leakage is evidenced in the bottom of the Apalachicola River about 300 yards downstream of the dam, where lake water "boils" up from a limestone ledge at rates of about 140-220 cubic feet per second. Dye tracing by the U.S. Army Corps of Engineers indicates that the river boil also receives water from a similar boil on land, which discharges to a sinkhole adjacent to the river. Isotopic data indicate about a 13 to 1 mixing ratio of lake water to ground water. Calcite saturation indices of ground-water samples indicate a higher potential for karstic dissolution from late fall through early spring than in the summer. The relatively short residence time (5-7 hours) and rapid flow velocity (nearly 500 feet per hour) for lake water to leak into the aquifer and exit at the boil suggests that this water would not reach equilibrium while in transit.