CHEMICAL EVOLUTION OF GROUNDWATER NEAR A SINKHOLE LAKE, NORTHERN FLORIDA .2. CHEMICAL-PATTERNS, MASS-TRANSFER MODELING, AND RATES OF MASS-TRANSFER REACTIONS

Chemical patterns along evolutionary groundwater flow paths in silicat e and carbonate aquifers were interpreted using solute tracers, carbon and sulfur isotopes, and mass balance reaction modeling for a complex hydrologic system;involving groundwater inflow to and outflow from a sinkhole lake in northern Florida. Rates of dominant reactions along d efined flow paths were estimated from modeled mass transfer and ages o btained from CFC-modeled recharge dates. Groundwater upgradient from L ake Barco remains ode as it moves downward, reacting with silicate min erals in a system open to carbon dioxide (CO2), producing only small i ncreases in dissolved species. Beneath and downgradient of Lake Barco the oxic groundwater mixes with lake water leakage in a highly reducin g, silicate-carbonate mineral environment. A mixing model, developed f or anoxic groundwater downgradient from the lake, accounted for the ob served chemical and isotopic composition by combining different propor tions of lake water leakage and infiltrating meteoric water. The evolu tion of major ion chemistry and the C-13 isotopic composition of disso lved carbon species in groundwater downgradient from the lake can be e xplained by the aerobic oxidation of organic matter in the lake, anaer obic microbial oxidation of organic carbon, and incongruent dissolutio n of smectite minerals to kaolinite. The dominant process for the gene ration of methane was by the CO2 reduction pathway based on the isotop ic composition of hydrogen (delta(2)H(CH4) = -186 to -234 parts per th ousand) and carbon (delta(13)C(CH4) = -65.7 to -72.3 parts per thousan d). Rates of microbial metabolism of organic matter, estimated from th e mass transfer reaction models, ranged from 0.0047 to 0.039 mmol L(-1 ) yr(-1) for groundwater downgradient from the lake.