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Hydrology and Water Chemistry in Weeks Bay, Alabama: Implications for Mercury Bioaccumulation




Monrreal, Robert

Type of Degree



Geology and Geography


Recent studies within Weeks Bay, an estuary of Mobile Bay, have revealed that both water and fish are contaminated by mercury (Hg), an element known to be extremely toxic to wildlife and humans. Seasonal variations of total mercury in precipitation were analyzed using data collected at two Mercury Deposition Network stations. The results showed that the most likely source for mercury is from atmospheric deposition. Once in an aqueous environment, inorganic mercury can methylate to toxic methylmercury (CH3Hg). To understand the water chemistry in which mercury methylates, seasonal measurements of temperature, pH, dissolved oxygen (DO), oxidation-reduction potential (ORP), specific conductance (SpC), and dissolved organic carbon (DOC) were conducted in water samples taken from Weeks Bay, from groundwater wells, and from a major surface tributary. Correlations of these constituents indicate that high salinity and pH seawater invade below acidic, low salinity water in the bay 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 activities that may initialize Hg methylation. Geochemical analyses show that most major ions exhibit conservative behavior while sulfate shows slight depletion during water mixing. River and bay water are enriched in 18O and 2H relative to groundwater, indicating they have undergone greater evaporation or mixing with isotopically heavier seawater. In summary, water chemistries can vary both spatially and seasonally in the bay based on environmental conditions. Storms, stream discharge, and seasonal climate changes can affect the conditions within the bay. Seasonal data indicate that high temperature, low pH, low conductivity, low DO, conditions ideal for mercury methylation are found in the bay especially during summer months.