Acid Deposition

High-elevation forests in the Southern Appalachians have some of the highest levels of acid deposition in the United States. High-elevation streams there are particularly susceptible to acidification, which can negatively impact aquatic biota. The decline of some high-elevation forests in the Southern Appalachiansfor also been partly attributed to acidic deposition. Acid deposition can also contribute to nitrogen saturation, which can have long-term consequences on forest productivity.

When raindrops fall through air that contains gaseous oxides of sulphur and nitrogen, these gases can dissolve in the droplets and form dilute acids, or "acid rain." There are other forms of acid deposition besides rainfall. Total loading of pollutants responsible for acid deposition (hydrogen ions or pH, sulfates, nitrates, and ammonium) is a combination of wet, dry, and occult deposition. Wet deposition includes rain, snow, sleet, and hail. Dry deposition is acidic particulate matter and gases deposited to surfaces. Occult deposition includes fog and cloudwater, but technical problems make measurement of these sources of deposition difficult (SAMAB 1996c).

The National Acid Precipitation Assessment Program (NAPAP), a 10-yeareffort mandated by the U.S. Congress, examined the effects of acidic deposition on a variety of aquatic resources, forests, and human health. Thiswork revealed that some of the highest deposition loadings of sulfur, nitrogen, and acidity in the United States have been measured at high elevations in the Southeast. The annual average pH of wet precipitation in 1993 for this region was second only to areas of the Northeastern and North Central United States. The loading of sulfate and nitrate in wet deposition over the period of 1983-1990 is highest in upland areas, including many parks and wilderness areas.

Although it is difficult to quantify the contributions of dry deposition and cloudwater deposition to total loading in the mountainous areas, it is reasonable to expect that the loading estimates could be doubled in these sensitive areas. Unfortunately, portions of streams at high elevations are probably least able to neutralize or "buffer" incoming acidity, especially during storm-generated episodes. In some of these sensitive streams, aquatic biota (fish and invertebrates) are being affected by both chronic and episodic acidification (SAMAB 1996c).

Decreases in acid deposition are expected as the Clean Air Act Amendments of 1990 are fully implemented. Sulfate concentrations in precipitation seem to be decreasing in the Southern Appalachians. Therefore, it is unlikely that sulfur deposition will cause additional streams to become chronically acidified in the region. However, vehicle emissions are a second major source of nitrogen compounds, and the importance of that source is expected to grow as populations increase (SAMAB 1996c).

While decreased sulfur deposition could result in decreased cation leaching from forest soils, the concurrent decrease in particulate emissions has resulted in decreased atmospheric inputs of Ca and Mg to ecosystems. Consequently, acidity of rainfall has not improved. This decrease in the concentrations of buffering chemicals that can offset the acid effects of sulfates and nitrates may contribute to slower recovery from acidification (SAMAB 1996c).