The Appalachian National Scenic Trail (AT) is one of the longest footpaths in the world, spanning ~2,180 miles (3,500 km) from northern Georgia to central Maine. The Trail includes large latitudinal and elevational gradients in climate, soil condition, forest community types, and atmospheric deposition of sulfur (S) and nitrogen (N). Vegetation, water chemistry, and wildlife depend upon the physio-chemical state of this AT environment and are susceptible to changes caused by air pollution. The Appalachian Trail (AT) MEGA-Transect Atmospheric Deposition Effects Study was funded by the National Park Service (NPS) and directed by the U.S. Geological Survey. Under contract to the NPS, E&S staff were responsible for substantial project elements, including coordination of synthesis and integration of project findings, scenario and critical loads modeling, and extrapolation of project results to the broader AT corridor. The overall goal of this study was to evaluate the condition and sensitivity of the environment along the AT corridor. This was accomplished by investigating current impacts and predicting ecosystem recovery under scenarios of reduced acidic deposition in the future. Project completion was April 2010. Products included an agency report and a peer reviewed scientific journal article.