Hydrology and Earth System Sciences www.hydrol-earth-syst-sci.net 1027-5606 1607-7938 11 2 2007 10.5194/hess-11-951-2007 http://www.hydrol-earth-syst-sci.net/11/951/2007/ http://www.hydrol-earth-syst-sci.net/11/951/2007/hess-11-951-2007.html http://www.hydrol-earth-syst-sci.net/11/951/2007/hess-11-951-2007.pdf 951 963 2007-03-02 Modelling acidification, recovery and target loads for headwater catchments in Nova Scotia, Canada C. J. Whitfield cwhitfield@trentu.ca J. Aherne P. J. Dillon S. A. Watmough Watershed Ecosystems, Trent University, Peterborough, Ontario, Canada Environment and Resource Studies, Trent University, Peterborough, Ontario, Canada The response of twenty acid-sensitive headwater catchments in Nova Scotia to acidic deposition was investigated for the period 1850–2100 using a dynamic hydrochemical model (MAGIC: Model of Acidification of Groundwater in Catchments). To ensure robust model simulation, MAGIC was calibrated to the long-term chemical trend in annual lake observations (13–20 years). Model simulations indicated that the surface waters of all twenty catchments acidified to the 1970s but showed subsequent recovery (increases in acid neutralising capacity (ANC) and pH) as sulphate deposition decreased. However, under proposed future emissions reductions (approximately 50% of current deposition) simulated ANC and pH will not return to estimated pre-industrial levels by 2100. An ANC of 20 μmol_c L^−1 and pH of 5.4 were defined as acceptable chemical thresholds (or critical chemical limits) for aquatic organisms in the current study. Under the proposed emissions reductions only one catchment is predicted to remain below the critical limit for ANC by 2100; three additional catchments are predicted to remain below the critical limit for pH. 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