Metal Mining and the Natural Cycling of Mercury in Freshwater Lakes: How Legacy Mining Pollution is Affecting the Toxicity of Mercury

Abstract

Methylmercury (MeHg) is a global pollutant and potent neurotoxin that bioaccumulates in aquatic and terrestrial foodwebs. As such, predicting the fate of MeHg in the environment is important in addressing ecosystem and human health concerns. Pollution that results from mining activities (i.e. ore roasting and smelting) is one way in which geochemistry of lakes can be altered, in turn altering the cycling of mercury (Hg) overall and the kinetics of MeHg production and bioaccumulation. My thesis is focused on examining the effect of different pollution gradients on the cycling of Hg at two different sites impacted by legacy mining in Canada. In Yellowknife (Northwest Territories, Canada) I assessed the effect of sulfate and arsenic (As) emissions on the production of MeHg in lake sediments, while in Sudbury (Ontario, Canada) I examined the effect of selenium (Se) emissions on the bioaccumulation of Hg and MeHg in freshwater biota. In Yellowknife, an environment polluted with both sulfate and As from mining activities, lakes were sampled using a factorial design which controlled for environmental variables known to affect MeHg production (i.e. sulfate, iron, productivity, pH, and dissolved organic matter). We used stable Hg isotope tracers to quantify Hg methylation and demethylation rate constants in sediments. Results showed that %MeHg in the water is best correlated with sulfate concentrations, while the rate at which Hg is methylated (Km) in sediments is negatively correlated with total As, and positively correlated with dissolved organic carbon, total phosphorus, and %MeHg in the water. Furthermore, a detailed examination of a lake with representative limnological characteristics of the area showed that addition of sulfate and organic carbon does increase the production of MeHg in the sediments, while addition of arsenate (0 to 10 mM) showed significant decrease in MeHg production, regardless of sulfate concentrations. Next, Se emissions in Sudbury (Ontario, Canada) correlated with lower total Hg and MeHg in tissues of zooplankton, amphipods (Hyalella azteca), mayflies (Stenonema femoratum), and young-of-the-year perch (Perca flavescens). However, despite ten years of emission reductions, results show that total Se concentrations in the majority of lakes have increased, most likely due to the long residence time of Se in the watershed and the water column. Consequently, Se continues to exhibit a protective effect on total Hg and MeHg bioaccumulation in biota, even a decade after emissions have greatly decreased. Canada’s numerous mining operations have left a legacy of pollution and it is important to understand the effects of these pollutants on the biogeochemistry of surrounding lakes. The results from my thesis demonstrate how mining emissions can alter the kinetics and bioaccumulation of MeHg in freshwater lakes, highlighting the complexity of Hg cycling in response to mining activities. My thesis is an important step in identifying and modeling the controls of MeHg production and bioaccumulation in environments impacted by emissions from mining operations.