Modelling acidification, recovery and target loads for headwater catchments in Nova Scotia, Canada

摘要

The response of twenty acid-sensitive headwater catchments in Nova Scotia toacidic deposition was investigated for the period 1850–2100 using a dynamichydrochemical model (MAGIC: Model of Acidification of Groundwater inCatchments). To ensure robust model simulation, MAGIC was calibrated to thelong-term chemical trend in annual lake observations (13–20 years). Modelsimulations indicated that the surface waters of all twenty catchmentsacidified to the 1970s but showed subsequent recovery (increases in acidneutralising capacity (ANC) and pH) as sulphate deposition decreased.However, under proposed future emissions reductions (approximately 50% ofcurrent deposition) simulated ANC and pH will not return to estimatedpre-industrial levels by 2100. An ANC of 20 μmol_c L−1 and pHof 5.4 were defined as acceptable chemical thresholds (or critical chemicallimits) for aquatic organisms in the current study. Under the proposedemissions reductions only one catchment is predicted to remain below thecritical limit for ANC by 2100; three additional catchments are predicted toremain below the critical limit for pH. Dynamic models may be used toestimate target loads, i.e., the required deposition reductions to achieverecovery within a given time. Setting target loads at approximately 30%of current depositions would allow three of the four lakes to reach thechemical criteria by 2030. In contrast to the generally good prognosis forsurface waters, soils lost an average of 32% of estimated initial basesaturation and recovery is estimated to be very slow, averaging 23% lowerthan pre-acidification levels in 2100.