Influences on Mercury Bioaccumulation Factors for the Savannah River

Mercury TMDLs (Total Maximum Daily Loads) are a regulatory instrument designed to reduce the amount of mercury entering a water body and ultimately to control the bioaccumulation of mercury in fish. TMDLs are based on a BAF (bioaccumulation factor), which is the ratio of methyl mercury in fish to dissolved methyl mercury in water. Analysis of fish tissue and aqueous methyl mercury samples collected at a number of locations and over several seasons in a 118-km reach of the Savannah River demonstrated that species-specific BAFs varied by factors of three to eight. Factors contributing to BAF variability were location, habitat, and season-related differences in fish muscle tissue mercury levels and seasonal differences in dissolved methyl mercury levels. Overall (all locations, habitats, and seasons) average BAFs were 3.7 × 10⁶ for largemouth bass, 1.4 × 10⁶ for sunfishes, and 2.5 × 10⁶ for white catfish. Determination of representative BAFs for mercury in fish from large rivers necessitates collecting large and approximately equal numbers of fish and aqueous methyl mercury samples over a seasonal cycle from the entire area and all habitats to be represented by the TMDL.