Recovery from acidification in Nova Scotia: temporal trends and critical loads for 20 headwater lakes

The chemical response of 20 headwater lakes in Nova Scotia to reduced acid deposition was investigated using trend analysis, and the need for further reductions was assessed using two steady-state, critical load models. Significant decreases were observed in the concentration of nonmarine sulphate (SO42-) and hydrogen (H+) at four wet deposition monitoring stations across Atlantic Canada since 1984. Dominant trends in surface water were decreasing SO42- concentrations, with little improvement in alkalinity and H+. Based on the Steady State Water Chemistry (SSWC) and First-order Acidity Balance (FAB) models, and using a critical chemical limit for acid-neutralizing capacity of 20 mu mol(c)(center dot)L(-1), critical load is exceeded at 9 and 13 of the 20 study lakes, respectively. Application of the SSWC model suggests that sulphur (S) deposition must be reduced by 37.3 mmol(c)center dot m(-2 center dot)year(-1) from 1997 levels to prevent critical load exceedance at 95% of the study lakes. Using the FAB model, the minimum reductions in nitrogen and S deposition necessary to protect 95% of the study lakes are 32.7 and 42.1 mmol(c)center dot m(-2)center dot year(-1), respectively. Additional reductions beyond those proposed for 2030 are required to minimize critical load exceedance and promote recovery in alkalinity and pH of surface waters at the study catchments.