Crystal Clear: Appalachian Trail MEGA-Transect Atmospheric Deposition Effects Study

a bright green field with mountains in the background
Field along the Appalachian Trail.

NPS Photo

The Appalachian National Scenic Trail is a 2,184 mile (3,515 km) long public footpath that traverses the scenic, wooded, pastoral, wild, and culturally rich lands of the Appalachian Mountains. Much of the trail follows ridge tops, and these high elevation and ridge-top ecosystems are very sensitive to acidic deposition. Potential consequences of increased acidification include forest die-back and streams that are no longer able to support certain sensitive fish species. The goal of this project is to establish the status and susceptibility of this publicly managed land with respect to acidic deposition and collect data that will enable future changes to be identified.


The Appalachian National Scenic Trail MEGA-Transect Deposition Effects Study assesses the condition and sensitivity of the trail region to acidic deposition by investigating current impacts and identifying deposition “critical loads” (the amount of acidic deposition below which deleterious effects are not expected). The study is exploring the time needed for ecosystem recovery under future deposition scenarios. The project began in spring 2010 and is led by scientists from US Geological Survey (USGS), USDA Forest Service (USFS), universities, and private companies. The MEGA-Transect Deposition Effects study is supported by funds provided to the National Park Service (NPS) by American Electric Power (AEP), an operator of coalfired power plants in the Ohio Valley. In 2008 the NPS was awarded damages from an enforcement action settlement arising from Clean Air Act violations. These funds are used for the restoration of land, watersheds, vegetation, and forests affected by air pollution emitted by AEP power plants.


The researchers used a three-tiered sampling approach with various levels of sampling intensity. At level I sites (12) soil, water, and tree tissue samples were combined with measurements of understory vegetation. At level II sites (50) soil and water chemistry samples were obtained, and at level III sites (200) only water chemistry was sampled. More than 250 headwater streams along the Appalachian Trail (AT) corridor were sampled twice during 2010–12 with a goal of targeting high and low flow at each stream. Sixty-nine percent of the stream samples had an acidneutralizing capacity (ANC) less than 100 meq/L, a relatively low value. These relatively low values reflect several factors, including high levels of acid deposition, steep slopes that produce rapid runoff, and slow mineral weathering rates that provide limited neutralization of acid deposition. However, there was a wide range of ANC values among the streams (-54 to 1717 meq/L), a reflection of wide variability in ecosystems along the trail. Streams north of the Maryland-Pennsylvania border (median ANC = 21 meq/L) were more acidic than those to the south (median ANC = 70 meq/L). Furthermore, 20percent of the northern AT streams had inorganic monomeric aluminum concentrations greater than 2.5 micromoles per liter (mmol/L), an indicator of stress in sensitive aquatic biota, whereas only 2percent of southern AT streams exceeded this value. Next steps include development of a statistical model of acid sensitivity for the entire trail corridor and the production of a final report and one or more peerreviewed papers

Last updated: November 6, 2018