The effect of SO2 emission controls on critical load exceedances for lakesin southeastern Canada

The UN ECE definition of critical load (CL) involving protection of aquatic ecosystem structure and function was adopted by using pH 6.0 as a damage t hreshold. Critical loads were determined for 4 lake clusters in SE Canada. An Integrated Assessment Model (IAM) was used to estimate steady-state lake pH distributions for each cluster for steps of wet sulphate (SO42-) deposi tion in the range 6 to 30 kg.ha(-1).yr(-1) The CLs were interpolated from t he damage vs deposition relationships and are, if anything, over-estimated for a number of reasons. Critical load values were <6, 6.9, 8.0 and 13.2 kg wet SO42-.ha(-1).yr(-1) for the Kejimkujik (Nova Scotia), Montmorency (Que bec), Algoma (Ontario), and Sudbury (Ontario) clusters, respectively. Wet S O42- deposition presently exceeds the CLs for all Canadian clusters by simi lar to 7 to 12 kg.ha(-1).yr(-1). Moreover, it is also expected to exceed th em by similar to 6 to 10 kg wet SO42-.ha(-1).yr(-1) even after all SO2 emis sion controls required by the Canada/US Air Quality Agreement are finally i mplemented. Further control of both Canadian and US SO2 emissions to achiev e lower SO42- deposition will be needed to reduce the magnitude of the CL e xceedances. (C) 1999 Published by Elsevier Science Ltd. on behalf of the IA WQ. All rights reserved.