TRACING SURFACE AND GROUND WATER INTERACTIONS THROUGH AN EOGENETIC KARST AQUIFER IN O’LENO STATE PARK, FLORIDA USING SR CONCENTRATIONS AND 87SR/86SR RATIOS

O’Leno State Park in north-central Florida preserves and protects a portion of the Santa Fe River where it flows into a sinkhole and reemerges from the Floridan Aquifer at a first magnitude spring (>2.8 m³/sec) approximately 5 km to the south. Karst features in the region form along the erosional edge of the Miocene Hawthorn Group, which confines the Floridan Aquifer. The area between the River Sink and Rise contains numerous karst windows, which have been shown to be connected to conduits through cave diving and chemical and thermal tracing. We have sampled surface water from the karst windows as it flows into and through the subsurface, and ground water from eight monitoring wells distributed throughout the park. These samples provide information on chemical evolution and mixing of ground water contained in matrix porosity of the Floridan Aquifer and surface water as it flows through a mapped conduit system.

Water from the River Sink, Rise, and karst windows shows a linear relationship between ⁸⁷Sr/⁸⁶Sr isotope ratios and 1/Sr concentrations (r²=0.98), indicating two end-member mixing. One end member has Sr concentrations > 1 mg/L and 87Sr/86Sr isotope ratios of between 0.7078 and 0.7079, reflecting a source of Sr from dissolution of the carbonate minerals in the Eocene Floridan Aquifer. The other end member has concentrations as low as 0.03 mg/L and ⁸⁷Sr/⁸⁶Sr isotope ratios up to 0.7095. Low concentrations originate from dilution during storm runoff, but a Sr isotope ratio greater than modern seawater value suggests that the Sr originates from the siliciclastic material in the Hawthorn Group. All 87Sr/86Sr ratios and concentrations of the ground water fall below the linear relationship of Sr concentrations and isotope ratios of water from the karst windows. Ground water from some wells have concentrations and isotope ratios that vary little through time, although the wells may be separated by as little as 100 m. Other wells show variations in concentrations and isotope ratios through time. These differences reflect variability in loss of surface water to matrix porosity probably from small-scale heterogeneity in aquifer permeability.