SULFUR CONTAMINATION IN THE EVERGLADES AND LINKS TO MERCURY METHYLATION

Sulfur is a significant contaminant for the freshwater Everglades. About 60% of the Everglades have levels of surface water dissolved sulfate exceeding the background threshold of 1 mg/L. Ten year average sulfate concentrations at heavily impacted sites range up to 60 mg/L. Spatially, sulfate exhibits a north to south concentration gradient, reflecting discharge of high sulfate canal water in the north. Canal water receives much of its sulfate load as runoff from agricultural fields within the Everglades Agricultural Area. Isotopic data are consistent with sulfur used in agriculture as a primary source of the sulfate contamination in canals and discharged to the Everglades.

Sulfate entering Everglades’ wetlands stimulates microbial sulfate reduction which elevates the rate of methylmercury (MeHg) production in the anoxic soils. A combination of external sulfate loading, high atmospheric deposition of mercury, and a favorable wetland environment gives rise to elevated levels of MeHg in the Everglades. Current restoration plans to deliver more water to the Everglades from canals may increase overall sulfur loads, extend the sulfate contamination zone, and increase MeHg production and bioaccumulation in parts of the ecosystem.

Monitoring data suggest that ecosystem response to changes in sulfate loading is rapid. A decline in sulfate concentration in surface water in the central Everglades during the late 1990s coincided with a decline in MeHg production and bioaccumulation there within 3-7 years. Because of the serious impacts of sulfate on the Everglades and the rapid response of the ecosystem to reductions in sulfate loading, a comprehensive Everglades restoration strategy for sulfur is needed. Strategies may include best management practices for sulfur use in agriculture, and improvements to stormwater treatment areas that increase sulfate removal efficiency. These approaches will help achieve lower overall sulfate levels in the Everglades, and resulting benefits.