Chemical characteristics and acid sensitivity of boreal headwater lakes in northwest Saskatchewan

Boreal ecosystems in northwest Saskatchewan may be threatened by acidification as this area is downwind of atmospheric emissions sources from regional oil sands mining operations. To evaluate the status of lakes in this region, a survey of 259 headwater lakes was conducted during 2007–2008 within ~300 km of Fort McMurray, Alberta. Acid sensitivity by ecoregion increased from Mid-Boreal Upland to Churchill River Upland to Athabasca Plain, with 60% of lakes classified as sensitive (50–200 μeq L–1 acid neutralizing capacity (ANC)), and 8% as very sensitive (<50 μeq L–1 ANC) to acid deposition. Organic anions dominated the acidity balance in most lakes, but non-marine sulphate varied positively with lake elevation and % upland cover (r2 = 0.24). Base cation concentrations (Ca, Mg, K, Na) were correlated with % deciduous forest in the catchment area (r2 = 0.33), while dissolved organic carbon (DOC) was related most strongly to % bog and lake flushing variables (r2 = 0.53). Variation in runoff coefficients derived by isotope mass balance corresponded with catchment area attributes that proxy controls on evaporation, infiltration and storage, and showed some ecoregional differences. The findings have implications for assignment of runoff values required to calculate critical loads of acidity. Although acidification appears not to be significantly advanced, many dilute oligotrophic lakes with pH 6.0 to pH 6.5 are vulnerable to acid deposition.