CHEMICAL EVOLUTION OF GROUNDWATER NEAR A SINKHOLE LAKE, NORTHERN FLORIDA .1. FLOW PATTERNS, AGE OF GROUNDWATER, AND INFLUENCE OF LAKE WATERLEAKAGE

Leakage from sinkhole lakes significantly influences recharge to the U pper Floridan aquifer in poorly confined sediments in northern Florida . Environmental isotopes (oxygen 18, deuterium, and tritium), chlorofl uorocarbons (CFCs: CFC-11, CCl3F; CFC-12, CCl2F2; and CFC-113, C2Cl3F3 ), and solute tracers were used to investigate groundwater flow patter ns near Lake Barco, a seepage lake in a mantled karst setting in north ern Florida. Stable isotope data indicated that the groundwater downgr adient from the lake contained 11-67% lake water leakage, with a limit of detection of lake water in groundwater of 4.3%. The mixing fractio ns of lake water leakage, which passed through organic-rich sediments in the lake bottom, were directly proportional to the observed methane concentrations and increased with depth in the groundwater flow syste m. In aerobic groundwater upgradient from Lake Barco, CFC-modeled rech arge dates ranged from 1987 near the water table to the mid 1970s for water collected at a depth of 30 m below the water table. CFC-modeled recharge dates (based on CFC-12) for anaerobic groundwater downgradien t from the lake ranged from the late 1950s to the mid 1970s and were c onsistent with tritium data. CFC-modeled recharge dates based on CFC-1 1 indicated preferential microbial degradation in anoxic waters. Verti cal hydraulic conductivities, calculated using CFC-12 modeled recharge dates and Darcy's law, were 0.17, 0.033, and 0.019 mid for the surfic ial aquifer, intermediate confining unit, and lake sediments, respecti vely. These conductivities agreed closely with those used in the calib ration of a three-dimensional groundwater flow model for transient and steady state flow conditions.