Impact of Acid Rain on Lake Water Quality in Eastern Canada

Abstract

Forty-six percent of Canada is classified as having aquatic ecosystems that are highly sensitive to acidic deposition. The existing effect of acidic SO₄ deposition on surface waters has been evaluated for eastern Canada (east of the Ontario-Manitoba provincial border) by using a lake chemistry data base that contains chemical information for 7403 lakes.Distribution statistics for alkalinity (ANC), sulphate, and base cation (Ca+Mg) concentrations are presented for 8 subregions which show that the Maritime provinces contain the highest proportion of acidic lakes even though the highest deposition (and lakewater sulphate concentrations) occur in Ontario and Quebec. Consideration of ion ratios (ANC:Ca+Mg, SO₄:Ca+Mg, and ANC:SO₄) show that the geographical pattern of acidification is a function of both deposition and terrain sensitivity (defined in terms of the terrain’s ability to supply alkalinity and/or base cations to its water systems). The ratios provide evidence that a large area of south-central Ontario and southern Quebec has been acidified. The most dramatic evidence of acidification occurs in southern Nova Scotia and a small part of New Brunswick. Northwestern Ontario and Labrador show little evidence of the acidification effect. Some of the data bases are now temporally long enough to demonstrate measured long term changes in lake chemistry. Depending on location, both continued lake acidification and “recovery” have been observed. Differences in chemical behavior are primarily related to whether or not the base cation export from a lake’s terrestrial basin changed significantly in response to elevate SO₄ deposition.