DETERMINING WATER SOURCES AND MIXING IN THE SANTA FE RIVER SINK-RISE SYSTEM, NORTH-CENTRAL FLORIDA

Chemical analyses and modeling were used to evaluate sources and mixing of waters that recharge the eogenetic Upper Floridan Aquifer (UFA) within the Santa Fe River Sink-Rise system of north-central Florida. Here, the upper Santa Fe River terminates at a swallet, called River Sink, where water enters the UFA and resurges ~5 km down gradient as a first magnitude spring, called River Rise. Chemical analysis of sampled water from the River Sink, monitoring wells, karst windows and River Rise, shows water in this portion of UFA is comprised of calcium bicarbonate type water (Ca-HCO₃), calcium sulfate type water (Ca-SO₄) and sodium chloride type water (Na-Cl). Sr isotope ratios show binary mixing, controlled by river stage, between sources from Hawthorn Group siliclastics that act as a confining unit and Sr released from Eocene-aged carbonates. The Ca-HCO₃ type water can be explained by dissolution of carbonate minerals in the UFA. The Ca-SO₄ type water has several possible origins including dissolution of sulfate minerals such gypsum and anhydrite, or from the oxidation of sulfide minerals such as pyrite. The Na-Cl type water, which has Sr isotopes distinct from the binary mixing, may derive from marine aerosols entrained in precipitation as storms move inland from the coast. Mixing of different water types can promote calcite dissolution, thereby forming secondary porosity. Speciation-solubility modeling of sampled water shows water in the Sink-Rise system ranges from water undersaturated to supersaturated with respect to carbonate minerals. We model the extent of secondary porosity formation by assuming equilibrium conditions for dissolution and ignoring calcite precipitation, which is believed to be a minimum in the UFA.