Speciation and transport of newly deposited mercury in a boreal forest wetland: A stable mercury isotope approach

Abstract

As part of the Mercury Experiment to Assess Atmospheric Loadings in Canada and the United States (METAALICUS) the fate and transport of contemporary mercury (Hg) deposition in a boreal wetland was investigated using an experimentally applied stable mercury isotope. We applied high purity (99.2% ± 0.1) 202Hg(II) to a wetland plot to determine if (1) the 202Hg was detectable above the pool of native Hg, (2) the 202Hg migrated vertically and/or horizontally in peat and pore waters, and (3) the 202Hg was converted to methylmercury (MeHg) in situ. The 202Hg was easily detected by ICP/MS in both solid peat and pore waters. Over 3 months, the 202Hg migrated vertically downward in excess of 15 cm below the water table and traveled several meters horizontally beyond the experimental plot to the lake margin along the dominant vector of groundwater flow. Importantly, at one location, 6% of aqueous 202Hg was detected as Me202Hg after only 1 day. These results indicate that new inorganic Hg in atmospheric deposition can be readily methylated and transported lakeward by shallow groundwater flow, confirming the important role of wetlands as contributors of Hg to aquatic ecosystems.