Sources of acidity in lakes and streams of the United States

Acidic (acid neutralizing capacity [ANC]≦0) surface waters in the United States sampled in the National Surface Water Survey (NSWS) were classified into three groups according to their probable sources of acidity: (1) organic-dominated waters (organic anions >SO₄^* + NO₃^-); (2) watershed sulphate-dominated waters (watershed sulphate sources > deposition sulphate sources); and (3) deposition-dominated waters (anion chemistry dominated by inputs of sulphate and nitrate derived from deposition). The classification approach is highly robust; therefore, it is a useful tool in segregating surface waters into chemical categories. An estimated 75% (881) of acidic lakes and 47% (2190) of acidic streams are dominated by acid anions from deposition and are probably acidic due to acidic deposition. In about a quarter of the acidic lakes and streams, organic acids were the dominant source of acidity. In the remaining 26% of the acidic streams, watershed sources of sulphate, mainly from acid mine drainage, were the dominant source of acidity.