USING STRONTIUM AND CALCIUM AS GEOCHEMICAL TRACERS TO DIFFERENTIATE FRESHWATER INPUTS TO AN ESTUARY, BISCAYNE BAY, SOUTH FLORIDA

This study focuses on differentiating the dominant freshwater sources that lead to brackish water conditions in Biscayne Bay by using a novel technique employing the dissolved strontium/calcium ratios in each source as a tracer. Sr²⁺ and Ca²⁺ exist in solution at different ratios in precipitation and groundwater, and are distinct from the ratios found in marine water. Groundwater in South Florida occurs within the Biscayne Aquifer, a limestone aquifer composed mostly of the minerals calcite and aragonite. Strontium ions commonly replace calcium ions in the crystalline structure of aragonite, and the groundwater flowing through the aquifer will have a Sr²⁺/Ca²⁺ ratio reflective of the limestone bedrock. Due to its limited contact time with the aquifer matrix, surface water may have a similar Sr²⁺/Ca²⁺ ratio, but at significantly lower concentrations. In rainfall, the concentrations of strontium and calcium are very low and their ratio is dissimilar to that of the limestone aquifer. Finally, seawater has the highest concentrations of strontium and calcium and a distinctive Sr²⁺/Ca²⁺ ratio. We developed a geochemical mixing model using salinity and the Sr²⁺/Ca²⁺ ratio for each of the freshwater sources mixing with marine water. Each freshwater source has a distinct mixing curve on a plot of salinity versus Sr²⁺/Ca²⁺. Surface water samples from spatially distributed sites in Biscayne Bay, plot between the curves allowing for determination of the percentage of each freshwater source mixing with marine water. This technique can be used to assess the effectiveness of future water management practices on Biscayne Bay and can be applied to coastal limestone aquifers in other regions.