HYDROLOGY, WATER CHEMISTRY, AND MERCURY METHYLATION IN WEEKS BAY, ALABAMA

Recent studies within Weeks Bay, an estuary of Mobile Bay, have revealed that both water and fish are highly susceptible to contamination by mercury (Hg), an element known to be extremely toxic to wildlife and humans. We analyze seasonal variations of total mercury in precipitation using data collected at two Mercury Deposition Network stations. The results show that the most likely source for mercury is atmospheric deposition from power plants and other industrial processes. Once in an aqueous environment, inorganic mercury can methylate to toxic methylmercury (CH3Hg⁺). Organisms, such as plankton, can consume methylmercury and allow mercury concentrations to increase through the food chain, with the top predators ingesting the largest amount of mercury. The initial methylation of mercury may occur with the assistance of sulfate-reducing bacteria under certain biogeochemical conditions. To understand the water chemistry in which mercury methylates, we conducted seasonal measurements of temperature, pH, dissolved O₂ (DO), oxidation-reduction potential (ORP), specific conductance (SpC), and dissolved organic carbon (DOC) in water samples taken from the Bay, from groundwater wells and from a major surface tributary. Correlations of these constituents along with laboratory analysis of oxygen and hydrogen isotopes suggest that water chemistry in the Bay is controlled mainly by mixing of warm, saline bay water with cold, acidic surface river water. The data also show the contributions of seawater, freshwater, and groundwater into the different parts of Weeks Bay. Our preliminary results suggest that Hg methylation may be enhanced by water chemistry changes in acidity, salinity, redox potential, and DOC. Biologic methylation of mercury occurs in areas with high DOC, low pH, and low SpC. These conditions exist primarily in the tributaries of Weeks Bay where seawater intrusion has a smaller effect on the chemistry of the waters. Knowledge of the water chemistry associated with mercury methylation is essential for developing efficient, field-based water quality protocols that can be used to indicate Hg contamination, or it's potential