Patterns and rates of chemical evolution of groundwater near a sinkhole lake, northern Florida

To better understand the hydrochemical interaction between groundwater and lakewater in a mantled karst setting, a comprehensive study was conducted to determine the processes that control the patterns and rates of chemical evolution of groundwater near Lake Barco, a sinkhole lake in northern Florida. Environmental isotopes, chlorofluorocarbons (CFCs), solute tracers, and geochemical modeling techniques were used to determine flow patterns, origin and age of waters, and to quantify the net sources and sinks of major ions, dissolved silica, and dissolved carbon species. The stable isotopic composition {dollar}(deltasp2{dollar}H and {dollar}deltasp{lcub}18{rcub}{dollar}O) of rainfall and groundwater upgradient from the lake (sampled near the water table and at several depths below the water table) plot together along the global meteoric water line. In contrast, the {dollar}deltasp2{dollar}H and {dollar}deltasp{lcub}18{rcub}{dollar}O composition of lakewater and groundwater downgradient from the lake were enriched relative to meteoric water as a result of evaporation and mixing. The relation between {dollar}deltasp2{dollar}H and {dollar}deltasp{lcub}18{rcub}{dollar}O in groundwater downgradient from the lake can be described by the expression {dollar}deltasp2{dollar}H(per mil) = 4.76{dollar}deltasp{lcub}18{rcub}{dollar}O(per mil) {dollar}-{dollar} 0.41 (r{dollar}sp2{dollar} = 0.992). A two end-member mixing model, developed to account for the enriched isotopic composition of groundwater, indicated that the amount of lakewater leakage that mixed with infiltrating meteoric water ranged from 11 to 67%, with a limit of detection of lakewater in groundwater of 4.3%.; Groundwater downgradient of Lake Barco was anoxic, with elevated concentrations of hydrogen sulfide, ferrous iron, and methane, resulting from leakage of lakewater through reducing, organic-rich sediments at the bottom of the lake. Based on the {dollar}deltasp2{dollar}H and {dollar}deltasp{lcub}13{rcub}{dollar}C content of methane, the dominant process for methane generation was by the carbon dioxide reduction pathway. Dissolved inorganic carbon species are being produced by reactions involving microbially mediated oxidation of organic matter along with three possible terminal electron accepting processes: ferric iron reduction, sulfate reduction, and methanogenesis. Rates of carbon dioxide production, determined from computed mass transfer for microbially mediated oxidation of organic matter and ages based on CFC-12 modeled recharge dates, were consistent with rates of microbial activity in other deeper aquifer systems.; This research study has provided a framework for a better understanding of recharge processes and the importance of both abiotic (mineral dissolution and precipitation) and biotic (microbially mediated degradation of organic matter) processes in controlling the cycling of solutes in a dilute groundwater-lakewater system. These results indicate that in areas where leakage of lakewater recharges shallow aquifer systems, flow patterns can be understood using changes in the concentration of major and minor chemical species as well as stable isotopes.