An assessment of weathering rates and steady-state critical loads of acidity in the Athabasca Oil Sands Region, Alberta

In order to assess the long-term acid sensitivity of mineral soils in the Athabasca Oil Sands Region (AOSR), reliable estimates of mineral weathering rates are required. In this study weathering rates were estimated for seven acid sensitive study-sites in the AOSR using the PROFILE model, an empirical clay content model and the zirconium (Zr)-depletion method. Regardless of method or study site, average base cation weathering rates were low (< 250 eq ha-1 yr-1). Mean weathering rates estimated with three different approaches: PROFILE, an empirical clay content model, and Zr-depletion were very similar at each study-site with no single method consistently higher or lower than the other methods. Overall average weathering rates reported for the seven study-sites ranged from 24 to 206 eq ha-1 yr-1.;The Bc:Al ratio had the greatest impact on critical loads and exceedances, with study-sites under exceedance ranging from 0--33% (ECMAP) and 6--17% (CMAQ) depending on the value of the Bc:Al ratio (1, 2 or 10). Assumptions regarding the gibbsite coefficient had little impact on critical loads and exceedance, although assumptions regarding nitrogen dynamics are more important. To evaluate the relative importance of nitrogen deposition, CLs and exceedances were calculated for sulphur alone. The removal of nitrogen deposition reduced percent exceedance levels from 12% to no exceedance under the ECMAP deposition scenario and from 11% to 9% under the CMAQ deposition scenario.;This study demonstrates that current levels of acid deposition arising from activities in the AOSR have the potential to adversely impact acid-sensitive upland sites in the region.;The simplest and least data-intensive clay content method was used to calculate weathering rates in order to estimate critical loads of acid deposition at 314 sites in the AOSR using the steady-state Simple Mass Balance model (SMB). Weathering rates for these predominantly acid sensitive soil sites ranged from 26 to 2774 eq ha-1 yr-1, with a mean of 512 eq ha-1 yr-1. Using the currently adopted critical chemical criterion for the region (a Bc:Al ratio of 2), critical loads (S + N) in the AOSR averaged 613 eq ha -1 yr-1, ranged from 259 to 4522 eq ha -1 yr-1, and were lower than CLs calculated at the provincial scale. Employing two acid deposition scenarios based on measured (ECMAP) and modeled (CMAQ) data, 11--12% of study-sites received acid deposition in excess of the critical load. The majority of these exceeded sites are located within 50 km northeast of Fort MacKay where modeled acid deposition is highest.