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Church, M. Robbins. Acidic deposition: acidification of surface waters. In R.W. Herschy and R.W. Fairbridge, editors, Encyclopedia of Hydrology and Water Resources, Encyclopedia of Earth Sciences Series, Kluwer Academic Publishers, Boston. (pp. 34-36)

Acidic deposition originates as emissions of oxides of sulfur and nitrogen from fossil fuel combustion. The composition of the resulting deposition can become dominated by sulfate, nitrate, ammonium and hydrogen ions -- acid rain. Watersheds with highly weatherable minerals or well buffered soils may not be adversely affected by acidic deposition. Other watersheds with thin and acidic soils, however, may be markedly affected. In such watersheds acidic deposition can cause acidification (loss of acid neutralizing capacity -- ANC) of surface waters through a combination of uptake and exchange reactions involving watershed biota (vegetation and microbial communities) and soils. Depending on prior watershed conditions and depositional loading, these reactions can proceed to the point that lakes and streams become episodically or chronically acidic (i.e., ANC<0 µeq/1. Decreases in pH and increases in concentrations of monomeric aluminum compounds (leached from watershed soils) occur during severe surface water acidification and can kill aquatic organisms.

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