Abstract
Although decreased acid deposition over recent decades has led to widespread recovery of surface waters across the U.S. and Europe, high sulfate adsorption in the southern Appalachian uplands has prevented significant acid-base recovery in this region. To update the regional acid-base status and characterize the controls on watershed response to reduced acid deposition, streamwater samples collected at 64 sites in western Virginia on a quarterly basis from 1987 to 2011 were analyzed for chemical properties. Individual watershed response was strongly influenced by the dominant underlying bedrock, which affected sulfate adsorption and base cation supply. Although pH increased at a majority of sites across all bedrock types, acid neutralizing capacity decreased at most sites underlain by base-poor bedrock, suggesting the susceptibility to episodic acidification remains a serious threat to these streams. Results of a sulfur mass balance analysis indicate that sustained declines in sulfur deposition may lead to general decreases in the pool of stored sulfur in base-poor siliciclastic watersheds, which until recently have been experiencing net storage of sulfur.
At the same time, as anthropogenic climate change continues to emerge as a global phenomenon affecting temperature regimes and hydrological cycling among many other variables, new concerns are being posed to these watershed ecosystems. The PnET-BGC model was applied to the White Oak Run (WOR1) watershed in Shenandoah National Park, VA to evaluate the potential influence of future changes in climate on acid-base chemistry. While the streamwater concentration of sulfate is projected to decline as WOR1 recovers from peak deposition in the 1970’s, base cations are expected to decline even more in part as a result of increased plant demand, leading to an overall loss of alkalinity in the stream. The chemical forecast for WOR1 is dependent on the exact magnitude of change in climate, with precipitation and temperature having interactive effect on several relevant watershed processes, including drainage, mineralization, and vegetative demand. These projected impacts of climate change will likely diminish some of the gains in stream alkalinity as a result of the Clean Air Act and subsequent amendments.
The results of this project are valuable not only in deepening the understanding of these biogeochemical cycles independently and interactively, but also in assessing and informing policy with regard to the projected impacts of climate change and acid deposition on these sensitive aquatic systems. There is significant opportunity to continue and enhance the research presented here, including expanding the analyses to new watersheds and investigating trends on smaller timescales.
