Steady-state critical loads of acidity for forest soils in the Georgia Basin, British Columbia

There has been growing interest in acid rain research in western Canada where sulphur (S) and nitrogen (N) emissions are expected to increase during the next two decades. One region of concern is southern British Columbia, specifically the Georgia Basin, where emissions are expected to increase owing to the expansion of industry and urban centres (Vancouver and Victoria). In the current study, weathering rates and critical loads of acidity (S and N) for forest soils were estimated at nineteen sites located within the Georgia Basin. A base cation to aluminium ratio of 10 was selected as the critical chemical criterion associated with ecosystem damage. The majority of the sites (58%) had low base cation weathering rates (≤50 meq m–2 y–1) based on the PROFILE model. Accordingly, mean critical load for the study sites, estimated using the steady-state mass balance model, ranged between 129–168 meq m–2 y–1. Annual average total (wet and dry) S and N deposition during the period 2005–2006 (estimated by the Community Multiscale Air Quality model), exceeded critical load at five–nine of the study sites (mean exceedance = 32–46 meq m–2 y–1). The high-elevation (>1000 m) study sites had shallow, acid sensitive, soils with low weathering rates; however, critical loads were predominantly exceeded at sites close to Vancouver under higher modelled deposition loads. The extent of exceedance is similar to other industrial regions in western and eastern Canada.