Snowmelt, glacial and atmospheric sources of mercury to a subarctic mountain lake catchment, Yukon, Canada

In montane regions, ongoing and future shrinkage of glacier cover, coupled with a shortening snow cover period, can profoundly alter river hydrology but also lead to the release of long-range contaminants, such as mercury (Hg), deposited and stored in snow and ice. In this study, field data coupled with hydrological and atmospheric models were used to estimate the contributions of atmospherically-deposited Hg released by snow or glacier ice melt, and from direct atmospheric deposition, to Kusawa Lake, in subarctic Yukon, Canada. The estimated net Hg accumulation rate in supraglacial snow obtained from field samples is 0.55 μg m−2 a−1. The direct annual atmospheric Hg flux to Kusawa Lake, obtained from model simulations, and which includes summertime wet deposition, is ∼6 times larger, averaging 3.04 μg m−2 a−1. The estimated mass of Hg from snow/ice meltwater entering the lake annually is 0.6 kg, while direct atmospheric deposition to the lake may contribute a further 0.4 kg, totaling 1.0 kg a−1. Levels of Hg in cores taken from glaciers in the catchment's headwaters are mostly above expected pre-industrial values, which suggests that some Hg now being released from glaciers is legacy anthropogenic Hg that accumulated in the past 150 years. At present, the delivery of Hg from melting glacial ice is the largest known source to Kusawa Lake, followed by contemporary atmospheric inputs (direct or via runoff). Efforts should be made to quantify other potential sources, such as subglacial meltwater, runoff from wetlands/forest, or melting permafrost, to better constrain the Hg balance in montane lake catchments of this region.