Geochemical cycle of mercury associated with wet deposition and inflows in a subalpine wetland

Abstract

Subalpine wetland is a mercury (Hg) sensitive ecosystem, but there is poor understanding of Hg behavior in this typical wetland. Here, distribution and speciation of Hg in waters of a subalpine wetland (Dajiuhu) in China were investigated, and an initial model of the Hg geochemical cycle in the wetland was established based on Hg mass balance calculations. Concentrations of both total Hg (THg, 9.52 ± 6.61 ng L−1) and total methyl mercury (TMeHg, 0.34 ± 0.44 ng L−1) in the waters during the wet season were higher than in the dry season. The majority of THg was in dissolved form whereas most TMeHg was in particle form. The geochemical models suggested that, due to the wet deposition and surface runoff, the input of THg and TMeHg into the wetland in the wet season (222 and 2.74 g year−1, respectively) was higher than that in the dry season (57.9 and 1.15 g year−1, respectively). The output of THg and TMeHg from the wetland underground runoff in the wet season was estimated to be 154 and 2.51 g year−1, respectively, and in the dry season was 15.9 and 0.43 g year−1, respectively. Other losses of Hg were due to volatilization of Hg0 from the sediment water (30.5 and 12.5 g year−1 in the wet and dry seasons, respectively). The flux of the settling of particulate Hg in the wet season was higher than that in the dry season. The fluxes of Hg diffusion from the porewater were relatively low in comparison to the fluxes of inflows and wet deposition. The flux of oxidation was higher than reduction, while the flux of methylation was higher than demethylation. These results indicated that the elevated levels of THg and MeHg in the Dajiuhu wetland are a consequence of rainfall and surface runoff inputs.