One of the principal influences on elemental fluxes from forested catchments in south-central Ontario is the atmospheric deposition rate of strong acids. While sulphate deposition has decreased by ∼40% in the past two decades, nitrate deposition has remained unchanged and is now equivalent to sulphate deposition. Sulphate concentrations in headwater lakes and their inflows have decreased, but much less than expected based on the anticipated direct response of the catchments. Reduction-oxidation (redox) processes occurring in wetlands have been identified as the reason for delayed recovery, and climate events as controlling these redox processes. A new version of the biogeochemical model MAGIC (model of acidification of groundwater in catchments) with a wetland compartment that incorporates redox processes driven by climate events has been generated. The application of MAGIC to a subcatchment of Plastic Lake in south-central Ontario indicates that the basic structure of the model appears to be consistent with the observed data. Moreover, the wetland component was essential in reproducing the observed trends, which include sulphate retention in non-drought years and re-oxidation of previously stored (reduced) sulphur in drought years.