The importance of atmospheric base cation deposition for preventing soil acidification in the Athabasca Oil Sands Region of Canada

Industrial activities in the oil sands region of Alberta, Canada have resulted in greatly elevated emissions of SO2 and N (NOx and NH3) and there are concerns over possible widespread ecosystem acidification. Acid sensitive soils in the region are common and have very low base cation weathering rates: the median base cation weathering rate estimated for 63 sites using PROFILE was just 17 mmol(c) m(-2) yr(-1). Deposition of S and N in throughfall was approximately twice as high as deposition measured with open collectors and could be as high as 360 mmol(c) m(-2) yr(-1) within 20 km of the main industrial center, although deposition declined logarithmically with distance from the industrial activities. Base cation deposition however, mostly exceeded the combined inputs of S and N in bulk deposition and throughfall, particularly during the summer months. The potential for soil acidification at a site close (<3 km) to the largest mine was assessed using the dynamic ecosystem acidification model, MAGIC (Model of Acidification of Groundwater in Catchments). Despite very low base cation weathering rates (similar to 6 mmol(c) m(-2) yr(-1)) and high (similar to 250 mmol(c) m(-2) yr(-1)) acid (S + N) deposition at the site, soil base saturation and soil solution pH and molar Ca:Al ratio were predicted to increase in the future assuming acid and base cation deposition constant at current rates. This work shows that despite extremely low soil base cation weathering rates in the region, the risk of soil acidification is mitigated to a large extent by high base cation deposition, which in contrast to S emissions is derived from fugitive dust sources in the mines, and is poorly quantified for regional modeling studies. (C) 2014 Elsevier B.V. All rights reserved.