ABSTRACT: In inland areas of northwest central Florida, sulfate concentrations in the Upper Floridan aquifer are extremely variable and sometimes exceed drinking water standards (250 milligrams per liter). This is unusual because the aquifer is unconfined and near the surface, allowing for active recharge. The sources of sulfate and geochemical processes controlling ground-water composition were evaluated in this area. Water was sampled from thirty-three wells in parts of Marion, Sumter, and Citrus Counties, within the Southwest Florida Water Management District; these included at least a shallow and a deep well at fifteen separate locations. Ground water was analyzed for major ions, selected trace constituents, dissolved organic carbon, and stable isotopes (sulfur-34 of sulfate and sulfide, carbon-13 of inorganic carbon, deuterium, and oxygen-18).
Sulfate concentrations ranged from less than 0.2 to 1,400 milligrams per liter, with higher sulfate concentrations usually in water from deeper wells. The samples can be categorized into a low sulfate group (less than 30 milligrams per liter) and a high sulfate group (greater than 30 milligrams per liter). For the high sulfate waters, concentrations of calcium and magnesium increased concurrently with sulfate. Chemical and isotopic data and mass-balance modeling indicate that the composition of high sulfate waters is controlled by dedolomitization reactions (dolomite dissolution and calcite precipitation, driven by dissolution of gypsum). Gypsum occurs deeper in the aquifer than open intervals of sampled wells. Upward flow has been documented in deeper parts of the aquifer in the study area, which may be driven by localized discharge areas or rapid flow in shallow parts of the aquifer. Mixing between shallow ground water and sulfate-rich water that dissolved gypsum at the base of the aquifer is probably responsible for the range of concentrations observed in the study area. Other solutes that increased with sulfate apparently originate from the gypsum itself, from other mineral assemblages found deeper in the aquifer in association with gypsum, and from residual seawater from less-flushed, deeper parts of the aquifer. These ions are subsequently transported with sulfate to shallower parts of the aquifer where gypsum is not present.
The composition of low sulfate ground water is controlled by differences in the extent of microbially mediated reactions, which produce carbon dioxide. This, in turn, influences the extent of calcite dissolution. Ground waters which underwent limited microbial reactions contained dissolved oxygen and were usually in ridge areas where recharge typically is rapid. Anaerobic waters were in lower lying areas of Sumter County, where soils are poorly drained and aquifer recharge is slow. Anaerobic waters had higher concentrations of calcium, bicarbonate, sulfide, dissolved organic carbon, iron, manganese, and silica, and had lower concentrations of nitrate than aerobic ground waters. For low sulfate waters, sulfate generally originates from meteoric sources (atmospheric precipitation), with variable amounts of oxidation of reduced sulfur and sulfate reduction. Sulfide is sometimes removed from solution, probably by precipitation of a sulfide minerals such as pyrite. In areas where deep ground water has low sulfate concentrations, the shallow flow system is apparently deeper than where high sulfate concentrations occur, and upwelling of sulfate-rich water is negligible. The range of sulfate concentrations observed in the study area and differences in sulfate concentrations with depth indicate a complex interaction between shallow and deep ground-water flow systems.