Relative Importance of Anthropogenic versus Natural Sources of Acidity in Lakes and Streams of Central Ontario

Abstract

Anthropogenic acid deposition has been implicated in the acidification of surface waters of central Ontario and elsewhere. However, alternative sources of acidity have been proposed, including (i) the release of protons and their accompanying acid anions by soils (e.g. organic acids) and (ii) the exchange of internally generated protons for the cations of atmospherically deposited neutral salts (e.g. sea salts). We show that for a typical acidified catchment in central Ontario the first hypothesis is not important during periods of high discharge or on an annual basis. As most of the neutral salts deposited on inland areas are sulphate and nitrate of anthropogenic origin, any acidity generated via the second mechanism can be reduced by reducing the emission of the salts. A third and alternative hypothesis is that deposition of strong acid alters only the balance between humic anions and sulphate in runoff waters, but not pH; we have shown this to be incorrect using catchments from the Sudbury region of Ontario where rates of acid deposition have recently declined. We conclude that in central Ontario, acidic deposition derived from anthropogenic SO₂ sources is the primary causal factor in recent lake acidification, and that a reduction in the anthropogenic emissions affecting this area will result in a decrease in lake acidity.