Conditions that can cause increasing mercury methylation are of particular concern because methylmercury (MeHg) is the most toxic mercury species, and it is most rapidly bio-accumulated. The NECB NAWQA study has evaluated relations between concentrations of total mercury (HgT) and MeHg in stream water and streambed sediment over a range of urban land use. Fifty-five stream sites from Rhode Island to Maine were sampled during 1998-2000. Sediment and water samples were collected during summer low-flow conditions to show patterns of HgT and MeHg accumulation at various sites in watersheds. Results of the study suggest that urban areas have higher HgT but lower methylation rates than rural areas. Methylation efficiency was high at sampling sites with low urbanization and high wetland density. Concentrations of MeHg in water and sediment were correlated with concentrations of organic carbon. These results indicate that MeHg decreases in ecosystems that are urbanized. The findings from this study were presented at a U.S. Environmental Protection Agency sponsored workshop on "Fate, transport, and transportation of mercury in aquatic and terrestrial environments" in May 2001.

--Ann Chalmers, (802) 828 4511 or chalmers@usgs.gov

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