THE COMBINED USE OF SR-87 SR-86 AND CARBON AND WATER ISOTOPES TO STUDY THE HYDROCHEMICAL INTERACTION BETWEEN GROUNDWATER AND LAKEWATER IN MANTLED KARST/

The hydrochemical interaction between groundwater and lakewater influe nces the composition of water that percolates downward from the surfic ial aquifer system through the underlying intermediate confining unit and recharges the Upper Floridan aquifer along highlands in Florida. T he Sr-87/Sr-86 ratio along with the stable isotopes, D, O-18, and C-13 were used as tracers to study the interaction between groundwater, la kewater, and aquifer minerals near Lake Barco, a seepage lake in the m antled karst terrane of northern Florida. Upgradient from the lake, th e Sr-87/Sr-86 ratio of groundwater decreases with depth (mean values o f 0.71004, 0.70890, and 0.70852 for water from the surficial aquifer s ystem, intermediate confining unit, and Upper Floridan aquifer, respec tively), resulting from the interaction of dilute oxygenated recharge water with aquifer minerals that are less radiogenic with depth. The c oncentrations of Sr2+ generally increase with depth, and higher concen trations of Sr2+ in water from the Upper Floridan aquifer (20-35 mu g/ L), relative to water from the surficial aquifer system and the interm ediate confining unit, result from the dissolution of Sr-bearing calci te and dolomite in the Eocene limestone. Dissolution of calcite [delta (13)C = -1.6 permil (parts per thousand)] is also indicated by an enri ched delta(13)C(DIC) (-8.8 to -11.4 parts per thousand) in water from the Upper Floridan aquifer, relative to the overlying hydrogeologic un its (delta(13)C(DIC) < -16 parts per thousand). Groundwater downgradie nt from Lake Barco was enriched in O-18 and D relative to groundwater upgradient from the lake, indicating mixing of lakewater leakage and g roundwater. Downgradient from the lake, the Sr-87/Sr-86 ratio of groun dwater and aquifer material become less radiogenic and the Sr2+ concen trations generally increase with depth. However, Sr2+ concentrations a re substantially less than in upgradient groundwaters at similar depth s. The lower Sr2+ concentrations result from the influence of anoxic l akewater leakage on the mobility of Sr2+ from clays. Based on results from mass-balance modeling, it is probable that cation exchange plays the dominant role in controlling the Sr-87/Sr-86 ratio of groundwater, both upgradient and downgradient from Lake Barco. Even though groundw ater from the three distinct hydrogeologic units displays considerable variability in Sr concentration and isotopic composition, the dominan t processes associated with the mixing of lakewater leakage with groun dwater, as well as the effects of mineral-water interaction, can be as certained by integrating the use of stable and radiogenic isotopic mea surements of groundwater, lakewater, and aquifer minerals.