HYDROLOGY AND WATER CHEMISTRY CONTROLS ON MERCURY BIOACCUMULATION IN ALABAMA COASTAL WATERSHEDS

Estuaries and coastal watersheds of Alabama Gulf Coast are highly susceptible to contamination by mercury (Hg), an element known to be extremely toxic to wildlife and humans. Largemouth Bass in Weeks Bay contain mercury concentrations > 1 mg/kg, the limit set by the Food and Drug Administration. Water quality measurements of temperature, pH, oxidation reduction potential (ORP), dissolved oxygen (DO), turbidity, and electrical conductivity were collected at more than 60 locations in Weeks Bay and Wolf Bay in an effort to understand the hydrochemical conditions in which mercury methylates. In Weeks Bay high-salinity seawater invades below acidic, low-salinity water to form a wedge interface. The mixing of warm, acidic, and low-salinity waters in the upper bay (near the mouth of the Fish River) provide a favorable conditions for Hg methylation. Low DO and ORP values observed in this mixing zone indicate high microbial activity that may initialize Hg methylation. In Wolf Bay, by contrast, less freshwater inflow produces high-salinity water, which in turns prevents key microbial processes that initialize Hg methylation and bioaccumulation. Oxygen and hydrogen isotope analysis provides information on the degree of evaporation and water mixing in bays. Wolf Bay water is enriched in 18O and 2H relative to Weeks Bay water, river water, and shallow groundwater, indicating that its has received less freshwater input, or undergone greater evaporation and mixing with isotopically heavier seawater. Our results imply that Hg biotransformation is strongly influenced by hydrochemical conditions in coastal watersheds. Rapid development of Alabama coastal zones and the risks associated with Hg consumption underscore the need to understand the hydrology, water chemistry, and fate and biotransformation of Hg.