Acidic precipitation: Considerations for an air quality standard

Acidic precipitation, wet or frozen precipitation with a H⁺ concentration greater than 2.5 μeq l^?1, is a significant air pollution problem in the United States. The chief anions accounting for the H⁺ in rainfall are nitrate and sulfate. Agricultural systems may derive greater net nutritional benefits from increasing inputs of acidic rain than do forest systems when soils alone are considered. Agricultural soils may benefit because of the high N and S requirements of agricultural plants. Detrimental effects to forest soils may result if atmospheric H⁺ inputs significantly add to or exceed H⁺ production by soils. Acidification of fresh waters of southern Scandinavia, southwestern Scotland, southeastern Canada, and northeastern United States is caused by acid deposition. Areas of these regions in which this acidification occurs have in common, highly acidic precipitation with volume weighted mean annual H⁺ concentrations of 25 μeq l^?1 or higher and slow weathering of granitic or precambrian bedrock with thin soils deficient in minerals which would provide buffer capacity. Biological effects of acidification of fresh waters are detectable below pH 6.0. As lake and stream pH levels decrease below pH 6.0, many species of plants, invertebrates, and vertebrates are progressively eliminated. Generally, fisheries are severely impacted below pH 5.0 and are completely destroyed below pH 4.8. At the present time studies documenting effects of acidic precipitation on terrestrial vegetation are insufficient to establish an air quality standard. It must be demonstrated that current levels of precipitation acidity alone significantly injure terrestrial vegetation. For aquatic ecosystems, current research indicates that establishing a maximum permissible value for the volume weighted annual H⁺ concentration of precipitation at 25 μeq l^?1 may protect the most sensitive areas from permanent lake acidification. Such a standard would probably protect other systems as well.