DETERMINING THE RELATIVE PROPORTIONS OF SPRING WATER INPUTS USING NATURALLY OCCURRING TRACE ELEMENTS AND OXYGEN AND HYDROGEN ISOTOPES OF THE ST. JOHNS RIVER MIDDLE BASIN, DELAND, FL

The geochemical and isotopic composition of surface waters and spring water was investigated to determine the relative contributions of source waters to the St. Johns River. Stable isotopes of oxygen (d¹⁸O) and hydrogen (dD) as well as ion concentrations of Ba, Ca²⁺, K⁺, Mg²⁺, Na⁺, SO₄^-, and Sr²⁺ were used as tracers to quantify groundwater from first- and second-magnitude springs discharging into the St. Johns River. This method uses natural variation in source waters from isotopic precipitation/evaporation processes, and dissolution of carbonate ions in groundwater. Previous data demonstrates that groundwater discharges in the karst areas of central Florida may contribute proportionally more water to the St Johns River than historically predicted. Groundwater from springs in this section of the river is distinguished by elevated concentrations of Ca²⁺, Mg²⁺, Na+, and SO₄^-, and lower concentrations of Sr²⁺ compared to river surface waters. Additionally, the d¹⁸O and dD of spring water show depleted values of heavy isotopes, while inflowing surface water show enriched ratios of heavy isotopes. Together these tracers are used to differentiate relative contributions of surface water and spring water over a six-month period. This elevated groundwater input from springs could in part explain the occurrence of higher nutrients for this portion of the St. Johns River.