Decreased Atmospheric Sulfur Deposition Across the Southeastern U.S.: When Will Watersheds Release Stored Sulfate?

Emissions of sulfur dioxide (SO2) to the atmosphere lead to atmospheric deposition of sulfate (SO42–), which is the dominant strong acid anion causing acidification of surface waters and soils in the eastern United States. Since passage of the Clean Air Act and its Amendments, atmospheric deposition of SO2 in this region has declined by over 80%, but few corresponding decreases in streamwater SO42– concentrations have been observed in unglaciated watersheds. We calculated SO42– mass balances for 27 forested, unglaciated watersheds from Pennsylvania to Georgia, by using total atmospheric deposition (wet plus dry) as input. Many of these watersheds still retain SO42–, unlike their counterparts in the northeastern U.S. and southern Canada. Our analysis showed that many of these watersheds should convert from retaining to releasing SO42– over the next two decades. The specific years when the watersheds crossover from retaining to releasing SO42– correspond to a general geographical pattern of later net waters...