Abstract
Stream acidification, caused by atmospheric deposition of acids derived from anthropogenic emissions ofSO2 and NOX, has major consequences for aquatic ecosystems. In response to growing environmental concerns, the United States federal government enacted a set ofpolicies including the Clean Air Act Amendments (CAAA) of 1990 aimed at curbing emissions, which have resulted in a significant decrease in rates of acidic atmospheric deposition. This project analyzed the water chemistry of 345 of Virginia's mountain streams sampled in 1987, 2000, and 2010 to assess the long-term response of stream chemistry to decreases in atmospheric acidic deposition that resulted from emissions reductions. Between 1987 and 2010, median stream SO42‘ concentration declined 18% (12.9 ueq/L) and median acid neutralizing capacity (ANC) increased 76% (44.4 ueq/L), indicating at least a partial recovery from acidification. A number of spatial, geographic, and geological characteristics were analyzed to identify factors responsible for variation in stream recovery from acidification. Stream elevation exhibited a weak but significant association with rate of recovery from acidification, with lower elevation sites showing slightly faster recovery. Although bedrock geology was significantly associated with both stream S042‘ and ANC concentrations, there was no relationship between bedrock geology and rates of recovery from acidification. Watershed area was not found to have any relationship with stream chemistry. The gypsy moth defoliation of the 1990s did not appear to affect recovery from acidification. While these geological, biological and geographical variables appear to have had little or no effect on stream chemistry, patterns of atmospheric wet iii deposition suggest spatial variability in stream recovery may be driven more by differences in extrinsic input. Overall, these results suggest the mountains of Virginia are recovering from acidification and that the previously reported lagged response of streams in this region, resulting from sulfate adsorption in the soils, is now diminishing.
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