Methylmercury in Managed Wetlands

Abstract

Methylmercury (MeHg) is a bioaccumulative and neurotoxic species of mercury that is produced in anoxic environments, such as wetlands, via microbially-mediated processes. While the mercury cycle and MeHg production in natural wetlands has been relatively well studied, little attention has been paid to the MeHg dynamics of artificial wetlands. This chapter reviews the extent, drivers, and consequences of MeHg production in wetlands created for the management of stormwater runoff or the provision of aquatic habitat, as well as how MeHg production and concentrations vary in response to specific management interventions in these environments. Methylmercury in artificial wetlands likely derives mostly from internal production by sulfate reducing bacteria. Construction strategies have important implications for the MeHg dynamics of artificial wetlands because sites created by flooding upland areas experience increased net MeHg production early in their life, followed by subsequent declines. Artificial wetland created by excavation into underlying soil, however, usually have low net MeHg production and concentrations after floodup, which subsequently rises as organic matter and a typical mercury methylating microflora develop. Minor dryout-rewetting cycles appear to have little effect on MeHg production, but major dryouts can elevate MeHg concentrations or export for weeks or months. Dredging appears to result in only a temporary reduction in MeHg production, with net MeHg production and concentrations rapidly rebounding. Artificial wetlands may be important in the landscape-level MeHg cycle, but questions remain about the extent, seasonality, and drivers of aqueous and biotic export of MeHg from these systems. More research is also needed on the MeHg dynamics of different types of treatment wetlands, the influence of different climates and wastewater streams, and the extent and biogeochemical drivers of methylation and demethylation in these managed systems.