Climate Change to Offset Improvements in Watershed Acid-Base Status Provided by Clean Air Act and Amendments: A Model Application in Shenandoah National Park, Virginia

The impact of climate change on watershed acid-base chemistry is currently understudied. Projecting the future condition of upland watersheds in the southern Appalachians of the United States, where high sulfate adsorption has prevented significant acid-base recovery, is complicated because of the unknown interaction with likely changes in temperature and precipitation. Here the biogeochemical model PnET-BGC and statistically downscaled general circulation models were used to evaluate potential effects of future climate on watershed biogeochemistry at White Oak Run in Shenandoah National Park, Virginia. Downscaled climate scenarios on average predict warmer temperatures and greater precipitation for this watershed by the end of the century. Model results suggest the combination of these effects on discharge drive the largest changes in acid-base chemistry, as increased runoff depletes the soil pool of sulfur and base cations resulting in lower stream concentrations. Superimposed on this abiotic effect is an increased biotic demand for base cations, resulting in an overall loss of alkalinity under projected climate conditions. The predicted loss of alkalinity resulting from climate change is comparable to what would be expected if acid deposition had remained elevated during this century. Specifically, model results indicate the predicted loss of alkalinity resulting from climate change would offset about 68 +/-16% of the modeled gains from the Clean Air Act and Amendments, where the 95% uncertainty bounds are associated with the ensemble of climate predictions applied to the calibrated biogeochemical model. Such an outcome would represent a considerable setback from one of the desired effects of clean air regulation. Even though acid deposition has greatly decreased in the United States in recent decades, streams in the upland portions of the southern Appalachians have largely failed to recover because the soils are slower in releasing adsorbed sulfate. Meanwhile, climate change continues to emerge as a global biogeochemical driver, impacting temperature and hydrological patterns. We applied a biogeochemical model to the White Oak Run watershed in Shenandoah National Park, VA, to evaluate how future changes in climate might affect the acid-base chemistry of this stream. Our results suggest that climate change could offset more than half of improvements in acid-base conditions that result from decreased acid deposition.