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 (SO₄^2-), which is the dominant strong acid anion causing acidification of surface waters and soils in the eastern United States (U.S.). 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 stream-water SO₄^2- concentrations have been observed in unglaciated watersheds. We calculated SO₄^2- 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 SO₄^2-, 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 SO₄^2- over the next two decades. The specific years when the watersheds crossover from retaining to releasing SO₄^2- correspond to a general geographical pattern of later net watershed release from north to south. The single most important variable that explained the crossover year was the runoff ratio, defined as the ratio of annual mean stream discharge to precipitation. Percent clay content and mean soil depth were secondary factors in predicting crossover year. The conversion of watersheds from net SO₄^2- retention to release anticipates more widespread reductions in stream-water SO₄^2- concentrations in this region.