THE EFFECTS OF ACIDIC DEPOSITION ON STREAMS IN THE APPALACHIAN MOUNTAIN AND PIEDMONT REGION OF THE MID-ATLANTIC UNITED-STATES

Streams in the Appalachian Mountain area of the mid-Atlantic receive s ome of the largest acidic deposition loadings of any region of the Uni ted States. A synthesis of the survey data from the mid-Appalachians y ields a consistent picture of the acid base status of streams. Acidic streams, and streams with very low acid neutralizing capacity (ANC), a re almost all located in small (<20 km2), upland, forested catchments in areas of base-poor bedrock. In the subpopulation of upland forested systems, which comprises about half the total stream population in th e mid-Appalachian area, data from various local surveys show that 6-27 % of the streams are acidic, and about 25-50% have ANC less than 50 mu eq L-1. After excluding streams with acid mine drainage, National Stre am Survey estimates for the whole region show that there are 2330 km o f acidic streams and 7500 km of streams with ANC less than 50 mueq L-1 . Many of the streams with base flow ANC less than 50 mueq L-1 become acidic during storm or snowmelt episodes. Sulfate from atmospheric dep osition is the dominant source of strong acid anions in acidic mid-App alachian streams. Their low pH (median, 4.9) and high levels of inorga nic monomeric aluminum (median, 129 mug L-1) leached through soils by acidic deposition are causing damage to aquatic biota. Quantification of the extent of biological effects, however, is not possible with ava ilable data. Localized studies have shown that stream water ANC is clo sely related to bedrock mineralogy. Attempts to quantify this relation ship across the mid-Appalachians, however, were frustrated by the lack of adequate scale geologic mapping throughout the region. Sulfate mas s balance analyses indicate that soils and surface waters of the regio n have not yet realized the full effects of elevated sulfur deposition due to watershed sulfate retention. Sulfur retention is likely to dec rease in the future, resulting in further losses of stream ANC.