Long-term Observations of the Recovery from Acid Deposition in Central Ontario

English Abstract:

Long-term (mid-1970s – present) measurements of bulk deposition and stream chemistry by the Ontario Ministry of Environment have enhanced our understanding of the impacts of acid-rain and subsequent recovery in acid-sensitive forested catchments in central Ontario. In response to large emission reductions, sulphate concentrations in precipitation and streams have declined markedly over the past 40 years. However, sulphate levels in streams are also greatly affected by climate, with high acid and metal pulses occurring in stream water following summer droughts. A decrease in atmospheric nitrate deposition has occurred more recently (from mid-1990s) and although most N is retained in catchments, streams draining predominately upland catchments have exhibited a decline in nitrate concentration as well. Despite past decreases in acid deposition, chemical recovery (increase in pH) of surface waters has been less than expected, primarily due to losses of base cations from the shallow, base-poor soils. The combined influence of soil acidification and climate-mediated biogeochemical changes is causing acid deposition related issues to persist long after emission reductions have occurred.