Benthic transfer and speciation of mercury in wetland sediments downstream from a sewage outfall

In order to evaluate the role of hypoxic conditions of overlying water in the benthic flux and speciation of Hg, we analyzed sediment cores from hypoxic or oxic sites downstream from a sewage outfall in the Damyang Riverine Wetland, Korea. Each core was analyzed for total Hg (THg), monomethylmercury (MMHg), and elemental Hg (Hg0) from sediment, and for THg and MMHg from pore water. Hypoxic conditions of the overlying water near the sewage outfall were associated with a peak production of Hg0, but the lowest production of MMHg, in the upper 2 cm sediments. The benthic fluxes of THg and MMHg were estimated at 130-2109 ng m−2 day−1 and −12 to 260 ng m−2 day−1, respectively. The order of MMHg flux from sediment to overlying water at each site did not follow the order of MMHg concentration in sediment, but was highest in hypoxic water conditions. The results suggest that maintaining oxic conditions in wetland water is important for decreasing the transfer of MMHg from sediment into overlying water.