Changes in sulfate deposition in eastern USA following implementation of Phase I of Title IV of the Clean Air Act Amendments of 1990

Phase I of Title IV of the Clean Air Act Amendments of 1990 (CAAA) was impl emented on 1 January 1995. On that date limitations were imposed on sulfur dioxide (SO2) emissions from 110 coal-fired electric utility plants (263 un its) in 21 states of which 17 are east of the Mississippi River. Sulfur dio xide emissions at these plants dropped an average 2.86 million tons (38%) f rom 1995 through 1997, when compared to mean 1993-1994 emissions from these plants. As a result of these reductions, wet sulfate (SO42-) deposition de creased an average of 2.4-4.0 kg ha(-1) (10-25%) over a large area of the e astern USA. Based on more than 15 years of data, the decrease in wet deposi tion was unprecedented in magnitude and spatial extent. The largest decreas e occurred across the Mid-Appalachian and Northeast regions of the country, the regions specifically targeted by Phase I for reductions in acidic depo sition. Although the reductions in wet SO42- deposition were generally cons istent with observed reductions in SO42- concentration, deviations in preci pitation volumes between the pre- and post-Phase I periods did result in so me sites recording significantly (p < 0.05) lower post-Phase I concentratio ns but higher wet depositions. Deviations in precipitation within and betwe en regions also influenced the magnitude and spatial distribution of the ob served changes in wet SO42- deposition. Clearly, deviations in the amount a nd distribution of precipitation need to be considered along with concentra tion changes in evaluating the CAAA effectiveness in reducing acidic deposi tion. This is particularly important in the eastern USA, since many of the most acid-sensitive ecosystems are located in regions, mostly mountainous a nd forested, that are subject to considerable fluctuations in precipitation volumes and subsequently acidic wet deposition. (C) 2000 Elsevier Science Ltd. All rights reserved.