Linking Mining Wastewater Discharge to Methylmercury Production in a Sub-Arctic Peatland

It is well established that the addition of sulphate (SO42-) to peatlands increases methylmercury (MeHg) concentrations in pore waters via microbial methylation. Less information exists about the effects of different concentrations and sources of SO42- loading on MeHg production in remote, non- SO42- impacted regions like Canada’s north, where increased SO42- loadings come not from the atmosphere, but often from mining waste water and rock tailings. A three year field study (two years of loading; one year of recovery) examined the effects of simulated wastewater (containing 27.2 mg/L SO42-) on MeHg production. Methylmercury concentrations increased to concentrations \u3e 4.0 ng/L (background average = 0.09 ng/L) by the end of each field season but during the recovery year decreased to \u3c 0.80 ng/L - still above background. Changes in partitioning between pore waters and peat were observed in the experimental fen, suggesting that the SO42- additions significantly impacted MeHg production in pore waters, and down-gradient movement. To evaluate different SO42- loadings and sources, laboratory column experiments were conducted at a range of SO42- concentrations in solution, as well as using mine tailings rock that leached SO42-. All additions increased MeHg concentrations; the highest MeHg concentrations were seen in the intermediate 5 mg/L additions suggesting limits for SO42- utilization by microbes. Results from this work indicate that even very small additions of SO42- to these pristine peats will increase MeHg in pore waters. Potential downstream impacts of MeHg on biota will require careful consideration of both wastewater and waste rock management for SO42-