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Bitumen mining and upgrading in northeastern Alberta, Canada, releases toxic pollutants into the atmosphere, including mercury (Hg) and methylmercury (MeHg). This Hg and MeHg is then deposited to the surrounding landscape; however, the fate of these contaminants remains unknown. Here, we compare snowpack chemistry to high-frequency measurements of river water quality across six watersheds (five impacted by oil sands development and one unimpacted). Catchment scale snowpack Hg and MeHg loads normalized to watershed area were highest near oil sands operations. River water Hg concentrations and loads tracked discharge and tended to be higher downstream of mining operations, while MeHg concentrations and loads increased through the summer, reflecting peak summer MeHg production rates. Except in the reference watershed, snowpack Hg and MeHg loads equaled or exceeded the amount of Hg and MeHg exported during freshet and, in some cases, the entire hydrologic year. This suggests landscapes across the oil sands region, which are dominated by low-relief wetlands and other shallow-water systems, are accumulating Hg and MeHg. Importantly, during years of high discharge, these low-relief systems appear to become better connected and flush MeHg (and Hg) from the watershed. Thus, these watersheds may act as temporary, rather than as permanent, natural repositories of oil sands contaminants.

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We report observations of stable isotope ratios and ion concentrations from seasonal snowpack and summer bulk precipitation from remote alpine sites in the Southern Canadian Rocky Mountains. Spatial deposition patterns for sulfur (S) and δ34S‐SO42− values indicate dominantly distant sources with little impact from local to regional pollution. Comparable S loads and total snowpack δ34S‐SO42− values for glacier snowpack indicates S emissions were well mixed prior to dry deposition or incorporation into snowfall. A uniform S load and similar δ34S‐SO42− values in a detailed study of summer bulk precipitation implies well‐mixed distant emissions. We interpret the deposited 0.9 kg S ha−1yr−1 as atmospheric background deposition in midlatitude Western Canada. This study will improve calculations for sites impacted by point source emissions and provide a baseline for attributing changes associated with climate change, industrialization, and urban growth. Field evidence from this study supports theoretical and laboratory research on the relative importance of oxidation pathways on atmospheric δ34S‐SO42− values for long‐range transported sulfate. δ34S‐SO42− of the dominant S source in summer bulk precipitation (~ +2‰) versus snowpack (≥ +9‰) cannot be explained by seasonal emission sources, temperature effects on fractionation, or Rayleigh distillation. The study supports a seasonal difference in the relative importance of the different SO2 to SO42− oxidation pathways with homogeneous oxidation by OH and heterogeneous oxidation by H2O2 most important in summer, and O2 catalyzed by transition metal ions in a radical chain reaction pathway more significant in winter.