Changes in the chemical composition of stream water in two catchments in the Shenandoah National Park, Virginia, in response to atmospheric deposition of sulfur

Abstract

Rates of atmospheric deposition of sulfur are as high in areas of the southeastern United States as they are in the northeastern United States and southeastern Canada. Sulfate concentrations and acidity of surface waters in the southeastern United States, however, remain low because of sulfate adsorption by soils. Because the capacity of soils to adsorb sulfate is finite, the questions of when and how fast the surface waters in the southeast might respond to the high rates of acidic deposition are of some importance. Modeling studies have suggested that time constants for sulfate breakthrough may be of the order of decades. Given that acidic deposition levels have been high in the southeastern United States since the 1940s, it might be expected that trends of increasing sulfate concentration and acidity would be apparent in data from monitored catchments. An analysis of 8 years of discharge chemistry data for two catchments in Shenandoah National Park, Virginia, indicates that sulfate concentrations and acidity are increasing in these catchments. The discharge‐weighted mean annual sulfate concentrations increased at a rate of 2 μeq L−1 yr−1 for both streams over the 8 years of the study. Both catchments are currently retaining approximately 65% of deposited sulfate. The ratios of the rate of decrease of alkalinity to the rate of increase of sulfate concentration are 34% and 25% for the two streams. If deposition rates remain constant, the observed rates of increase indicate that steady state conditions will be approached in 100 years, at which time the stream alkalinities may be as low as −65 μeq L−1.