Impact of Iron Amendment on Net Methylmercury Export from Tidal Wetland Microcosms

Abstract

Tidal wetlands can be important sources of methylmercury (MeHg) in aquatic ecosystems. As a result, wetland restoration could increase MeHg concentrations. Previous research has shown that addition of Fe[II] to wetland sediment can reduce MeHg production by decreasing concentrations of bioavailable Hg complexes with dissolved sulfur species. In this study, the potential for reducing MeHg production via an iron amendment was evaluated in laboratory microcosms that used intact sediments from a tidal marsh in San Francisco Bay. The microcosms were maintained under simulated tidal conditions and amended at four iron doses (0, 180, 360, and 720 g-Fe/m(2)). Two experiments were conducted: one with devegetated sediments and one with live wetland vegetation. Following iron addition to the devegetated sediments, porewater S[-II] concentrations decreased for each dose relative to the control with the average weekly export of MeHg in the surface water decreased by 82% and 89% for the two highest doses, respectively. Despite substantial variability within treatment groups, similar trends were observed for the vegetated microcosms. The results suggest that iron addition has the potential to provide a landscape-scale control on MeHg released by restored tidal wetlands; however, additional research is required to evaluate the efficacy of this approach under field conditions.