Appalachian Trail Air Pollution Effects

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, soils condition, forest community types, and atmospheric deposition of sulfur (S) and nitrogen (N). Vegetation, water chemistry, and wildlife depend upon the physico-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. E&S staff were responsible for substantial project elements, under contract to the NPS. Timothy Sullivan coordinated synthesis and integration of project findings and scenario and critical loads modeling. He also was responsible for preparation of the project report. Todd McDonnell was responsible for interpretation of site-specific modeling results and extrapolation to the broader AT corridor, generated estimates of flow conditions at stream sample sites, and managed the project database.

This report examines resource sensitivity and effects of deposition along the AT by considering an area 40 km wide that covers the full length of the AT. 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. Major objectives of this research included:

1. Model S and N deposition across the region
2. Measure soil and low-order stream acid-base chemistry
3. Determine vegetation species composition and health
4. Map soil and water acid-base condition
5. Analyze and map acidification critical loads (CL), target loads (TL), and associated exceedances
6. Characterize the AT landscape

Model estimates of future stream and soil chemistry and critical and target loads were generated using the Model of Acidification of Groundwater in Catchments (MAGIC) for 50 stream watersheds that had both soil and stream chemistry measurements. Modeling results suggested that streams in the Northern section of the AT corridor would generally exhibit increased ANC and reduced SO₄^2- by the year 2100 in response to reduced levels of acidic deposition. In contrast, streams located in the Central and Southern sections of the corridor were generally simulated to continue declining in ANC in the future despite lower levels of acidic deposition, and this was partly attributable to increased future SO₄^2- leaching as soil S adsorption continues to decrease on these older, unglaciated, and more highly weathered soils. A matrix of CL (long-term, represented by the year 3000) and TL (years 2050 and 2100) simulations was developed based on three receptors (stream, soil, soil solution). We developed models for predicting TL continuously across the entire landscape along the AT corridor and also at discrete locations where soil or water chemistry data exist. Model results suggested that each of the modeled watersheds was simulated to have experienced a decrease in soil percent base saturation (BS), soil exchangeable Ca, and stream water ANC and an increase in stream water NO₃^– concentration since pre-industrial (year 1860) times. Scenarios of emissions reductions suggested some future ANC recovery at some sites. Nevertheless, three-quarters of the sites were simulated to remain at least 20 µeq/L (on average about 13%) below pre-industrial ANC conditions, even with aggressive future emissions reductions (-90% S, N reduction). Target load exceedance at MAGIC modeling sites was calculated by overlaying TL with total ambient deposition of S or N to reflect the likelihood of ecological harm.

A number of landscape characteristics and aspects of exchangeable soil and stream water chemistry were significantly related to the MAGIC-calculated S TL needed to attain soil BS = 12% and stream ANC = 50 µeq/L by the year 2100. Regression models with the highest predictive ability were those that included aspects of either soil or water chemistry rather than those that relied on landscape characteristics alone. The principal model to extrapolate aquatic S CL results was based on stream ANC, percent mafic geology, and percent clay in soils (R²=0.80). The principal model to extrapolate terrestrial S CL results was based on soil BS (R²=0.75).

Sites that show acidified drainage water and acidified soils are scattered among other sites that have better buffering capacity. These differences were partly due to differences in geology, elevation, and climate and partly to factors that could not be differentiated. Model projections suggested that acid-base chemical conditions at the most sensitive and impacted sites are not likely to improve in the future without further reductions in atmospheric S deposition.