Modelling the impacts of sulphur and nitrogen deposition of forest soils in the Georgia Basin, British Columbia

Acid deposition has been a major environmental concern in eastern North America and Europe for decades, owing to its negative impacts on surface waters and forest soils. Initially acid rain research was focused in eastern Canada due to high levels of acid deposition; however, recently there has been growing interest 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 due to industry and expansion of the urban centres (Vancouver and Victoria).; In this thesis the potential acidifying impacts of S and N deposition were modelled at nineteen study sites located across the Georgia Basin. Sulphur and nitrogen deposition at the study sites, supplied by the AURAMS model at a resolution of 42 km by 42 km grid, was positively correlated with S and N concentrations in moss (Isothecium stoloniferum) sampled across the Georgia Basin. However, large uncertainties remain concerning S and N deposition, particularly at high elevation sites. Base cations (Ca, Mg, K, Na) and chloride deposition was estimated using a polynomial regression method based on location and elevation derived from historic wet deposition data and snow samples within and around the Georgia Basin. The regression models accounted for >60% of the spatial variation in base cations and chloride concentration.; Base cation weathering rates at the nineteen study sites were estimated with the PROFILE model. Weathering rates at sixteen of the nineteen sites were low (<1.00 keq/ha/yr) with the most sensitive soils (lowest weathering rates) located at the high elevation sites. Critical loads (CLs) were calculated with the steady-state mass balance model under four different scenarios based on varying gibbsite equilibrium (Kgibb) and critical chemical limits (Bc/Al ratios). The lowest values were obtained with the approach developed in eastern Canada (default log Kgibb of 9 and Bc/Al of 10); values ranged from 0.64 to 4.55 keq/ha/yr, with the majority of sites having low CL values (<1.00 keq/ha/yr). Currently, six sites receive acid deposition in excess of the CL.; To predict the changes in soil chemistry from 1900 to 2100 the MAGIC model was applied to the nineteen study sites. Soil base saturation (BS), soil solution pH, Bc/Al ratio and ANC varied considerably among sites. With the exception of ANC, soil solution chemistry exhibited little change between 1900 and 2006 although lower pH, Bc/Al and ANC values were predicted to have occurred during the period of peak emissions in 1970. In contrast soil BS reached minimum levels during the 1990's. At present, thirteen sites have a Bc/Al ratio below 10, based on likely future emission scenarios little change in soil chemistry is predicted. Notably eleven of the thirteen sites had predicted Bc/Al ratios below 10 in 1900, suggesting that the Bc/Al ratio of 10 may not be an appropriate measure of acidity for forest plots in the Georgia Basin.; Keywords. Georgia Basin, soil acidification, critical loads, AURAMS, moss, regression models, PROFILE, SSMB, MAGIC.