Water Column Methylmercury in the Wabigoon/English River–Lake System: Factors Controlling Concentrations, Speciation, and Net Production

Abstract

Ten years after major releases of inorganic mercury from a chloralkalai plant were sharply reduced, methylmercury (MeHg) levels remain markedly elevated in waters in most of the Wabigoon River system downstream of Dryden, Ontario, Canada. The water column appears to be a very important site for methylation and demethylation. In water, MeHg and inorganic Hg appear to be in quasi equilibrium, as the MeHg: total mercury ratio is independent of contact time with sediments, the atmosphere, and the theoretical residence time of waters. The MeHg: total Hg ratio increased with distance downstream of Dryden, but MeHg always averaged less than 10% of the total Hg concentration. MeHg was mostly soluble, as defined by field centrifugation. Net production of MeHg appeared to be directly related to the amount of MeHg required to establish and maintain equilibrium conditions with inorganic Hg. A model is developed which adequately describes water column MeHg concentrations. Considerations of the model suggests different restoration strategies for different parts of the Wabigoon/English River–Lake system.