Critical Loads for Protection of Aquatic Resources in the Blue Ridge, Ridge and Valley, and Appalachian Plateau Ecoregions of Virginia and West Virginia

T.J. Sullivan1, B.J. Cosby2 and T.C. McDonnell1

Streams found throughout the Blue Ridge, Ridge and Valley, and Appalachian Plateau ecoregions in Virginia and West Virginia are among the most acid-sensitive and acid-impacted aquatic ecosystems in the United States. Atmospheric sulfur (S) and nitrogen (N) deposition are relatively high; base cation supplies in soils, which could serve to buffer atmospherically deposited acidity, are limited in many small upland watersheds. Such acid-sensitive and acid-impacted aquatic resources in this region are especially numerous in Shenandoah National Park, Class I wilderness areas, and other National Forest lands. We modeled critical loads of atmospheric deposition to protect and restore stream resources to varying levels of protection, using a combination of the Model of Acidification of Groundwater in Catchments (MAGIC) and the Steady State Water Chemistry (SSWC) model. MAGIC was used to calculate effective weathering at 92 intensively studied watersheds. Estimates of weathering were then extrapolated to the regional landscape using empirical relationships with measured stream water chemistry at 533 sites together with spatial data on watershed lithology, soil characteristics, and watershed morphometry. Critical loads and exceedences were calculated with the SSWC model using extrapolated estimates of effective weathering from MAGIC, regional estimates of atmospheric deposition and runoff, and estimated base cation uptake by forest vegetation from USDA Forest Inventory and Analysis (FIA) data.

1 E&S Environmental Chemistry, Corvallis, OR 2 University of Virginia, Charlottesville, VA