IS MEHGHS AN IMPORTANT METHYLMERCURY TRANSPORT AGENT IN H2S-BEARING WATERS?

Inlets and outlets to five wetlands and a lake that receive sulfate from mining regions in NE Minnesota were sampled ten times from May to October in 2010. Waters were analyzed for numerous cations, sulfate and other anions, sulfur and oxygen isotope ratios in dissolved sulfate, dissolved organic carbon, methyl- and total- mercury (MeHg and THg), and a variety of nutrients. Changes in isotopic ratios between inlet and outlet samples were used to quantify sulfate reduction processes while differences in MeHg were used to help identify and quantify methylmercury release and transport processes. Little correlation was found between the mass of sulfate reduced and the amount of methylmercury generated. However, two clear instances were found where MeHg and THg appeared to increase together in equal amounts at water body outlets, suggesting the increase was related to enhanced transport specifically of MeHg. Geochemical speciation calculations using full water chemistry and available equilibrium constants for dissolved MeHg species suggest that the enhanced MeHg transport coincided with the presence of gaseous H₂S and conversion of organically bound MeHg to MeHgHS. MeHgHS is considered a volatile species so its formation would be expected to greatly enhance MeHg transport from reduced sediments to the water column. If accurate, then MeHg transport rates during sulfate reduction processes depend on the presence or absence of reduced iron. When reduced iron is present in excess, then H₂S (and HS^-) is limited and little MeHgHS should be produced. If absent, then sulfate reduction at neutral to low pH generates H₂S, HS-, and MeHgHS.