Acidic lakes and streams in the United States: The role of acidic deposition

A statistically designed survey of lakes and streams conducted by the Environmental Protection Agency in acid-sensitive areas of the United States, the National Surface Water Survey (NSWS), was used to identify the role of acidic deposition relative to other factors in causing acidic conditions in an estimated 1180 lakes and 4670 streams. Atmospheric deposition was the dominant source of acid anions in 75 percent of the acidic lakes and 47 percent of the acidic streams. Organic anions were dominant in one-fourth of the acidic lakes and streams; acidic mine drainage was the dominant acid source in 26 percent of the acidic streams. Other causes of acidic conditions were relatively unimportant on a regional scale. Nearly all the deposition-dominated acidic systems were found in six well-delineated subpopulations that represent about one-fourth of the NSWS lake population and one-third of the NSWS stream population.