Origins and distribution of saline ground waters in the Floridan Aquifer in coastal southwest Florida

Twenty-three ground-water samples from the Floridan aquifer in coastal southwest Florida show that water quality deteriorates to the south and west. The waters grade from a fresh calcium magnesium bicarbonate sulfate type to a very saline sodium magnesium chloride type downgradient. Bromide-chloride and specific conductance ratios indicate that dilution of marine-like ground water is a signigicant mechanism in the evolution of the different water types found. Calcium, magnesium , and bicarbonate concentrations occur within a relatively narrow range and are primarily a function of mineral equilibria. Magnesium and strontium concentration distributions suggest several mineral-water interactions, including aragonite inversion, incongruent solution of magnesium calcite to a lower magnesian form, and dedolomitization. Sulfate concentrations increase downgradient and evince gypsum-anhydrite solution, particularly in the fresher waters. The extent to which each factor affects dissolved specie concentrations is a function of the location of the water in the flow system. (USGS)