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Appalachian National Scenic Trail Atmospheric Deposition Summary

Figure 1. NADP atmospheric deposition observation locations in close proximity to the Appalachian National Scenic Trail used for analysis (Blue = Active; Red = Inactive).
Figure 1. NADP atmospheric deposition observation locations in close proximity to the Appalachian National Scenic Trail used for analysis (Blue = Active; Red = Inactive).


Deposition of acids, nutrients, and base cations in precipitation have been tracked by the National Atmospheric Deposition Program (NADP) since 1978. NADP is a nationwide collective of more than 200 monitoring sites that individually target specific focal areas, and 21 of these stations are located on or near the APPA (Figure 1). NADP includes the National Trends Network (NTN), Mercury Deposition Network (MDN), the Atmospheric Integrated Research Monitoring Network (AIRMoN), the Atmospheric Mercury Network (AMNet), and the Ammonia Monitoring Network (AMoN). Atmospheric deposition was selected as a vital sign (Dieffenbach 2011) for the the Appalachian National Scenic Trail (APPA), with acid deposition being the primary focus. Acid deposition was initially studied by the National Acid Precipitation Assessment Program (NAPAP), which was subsequently renamed the National Trends Network.

Rain, snow, or dry deposition containing sulfur and nitrogen compounds can acidify soils and surface waters, negatively affecting fish, plants, and other biota. Small ponds and streams at high elevations, such as those found along the APPA, are particularly susceptible because the soils in relatively high elevation watersheds often have limited ability to buffer acid deposition.

Estimates of atmospheric deposition are critical for understanding water chemistry and stress (Likens and Bormann 1974) because the atmosphere is generally acknowledged to be the source of problematic materials such as sulphur and nitrogen. In support of this, Swain et al. (1992) estimated that 90% of the mercury entering remote lakes in Voyageurs National Park (Minnesota) is derived from atmospheric deposition.

Figure 2. Change in S concentration between 1994-1998 and 1999-2003.
Figure 2. Change in S concentration between 1994-1998 and 1999-2003.

In 2005, the NPS compared interpolated NADP/NTN data from two time periods, 1994-1998 and 1999-2003, to derive nitrogen and sulfur concentration change isopleth maps for the eastern U.S. (Figures 2 and 3) and presented these maps in Shriver et al. (2005). S and N concentrations were either unchanged or reduced along the entire length of the APPA between 1995-1999 and 1999-2003.

Deposition Effects Study

In 2010, the U.S. Geologic Survey (USGS) and the National Park Service Air Resources Division jointly launched a study to evaluate the recovery rates of high elevation, historically acidified soils (Lawrence 2010). Before initiating this study, known as the Appalachian Trail Deposition Effects Study, a comprehensive and coordinated survey had not been conducted to determine the extent of acid-sensitive soils and surface waters on the APPA.

Figure 3. Change in N concentration between 1994-1998 and 1999-2003.
Figure 3. Change in N concentration between 1994-1998 and 1999-2003.

The project combined three levels of sampling intensity. The 12 level I (most intensive) sites measured 6 elements: atmospheric deposition, soil chemistry, water chemistry, wood tissue chemistry, tree ring cores to assess growth, and composition of understory vegetation. The 50 level II sites focused on soil and water chemistry. And, at the 200 level III sites (least intensive) only water chemistry was sampled.

Key project objectives included development and refinement of thresholds for ecosystem effects from acid deposition, and the development of critical loads for acid deposition. Identifying ecological threshold values that are specific to APPA soils, forests, and streams will result in more accurate estimation of critical loads, allowing for improved assessments of current and future ecosystem health and will make it possible for NPS to set meaningful air quality management goals.


Data summaries presented in Figure 4 are obtained directly, in real-time, from NADP for each of the 21 monitoring stations located within the HUC10 shell. The HUC10 shell, or the general frame of reference used to establish an area of interest around the APPA, is the "outer" boundary of all HUC10 hydrologic units that are within 5 miles of the APPA land base. The HUC10 shell is based on watersheds defined by the USGS at the fifth level of the Hydrologic Unit Code system, with each being given a discrete 10-digit code (HUC10). The hydrologic unit system was developed by the USGS and subsequently modified by the Natural Resource Conservation Service (NRCS). There are 177 individual HUC10 hydrologic units within this shell. Though they are termed watersheds, Omernik (2003) explains that hydrologic units are not always true watersheds and that some hydrologic elements contained within the HUC10 shell may not include all upstream components of a true watershed. However, for the purpose of defining an area of interest around the APPA we believe the hydrologic unit system is satisfactory.

The NADP annual charts describe each point as either meeting or not meeting the "annual criteria." It is important to understand that the annual criteria is a data "completeness" criteria and should not be interpreted to mean that the annual value is above or below an established "ecological" threshold. Failing to meet a data completeness criteria means that some portion of the annual dataset that point represents may be missing or there may be issues with some portion of the dataset behind that point. A point that fails to meet the annual criteria does not mean that the point in question necessarily represents any level of ecological concern. Values that do not meet the completeness criteria are also excluded from the trend line to ensure that potentially problematic data do not influence the trend line.


Earlier data reviews for the APPA (Shriver et al. 2005) found that in general S concentrations had declined whereas N concentrations declined in some segments and remained the same or increased slightly in other segments. Overall, however, the conclusions regarding deposition offered in Shriver et al. (2005) were positive. A review of the data available from each of the NADP sites is equally positive and offers further indications that S and N concentrations continue to decline and that the pH of atmospheric deposition is increasing. These results are almost certainly the result of more restrictive emission regulations. Even segments of the APPA where Shriver et al. (2005) found that N concentrations remained the same or increased slightly now show declines in N concentration. For example, Figure 3 indicates that Shenandoah National Park is located in the zone where N concentrations remained stable, but looking at the NADP summaries for Shenandoah National Park (Figure 4) NO3 and Total N concentrations have declined while NH4 fluctuates throughout the period. Other stations located in the same zone as Shenandoah display similar results -- decrease in NO2 and Total N, fluctuations in NH4, and increasing pH.

The information presented by NADP indicate that acid deposition has declined, but these indications cannot answer whether improvements in deposition correlate with improvements in the resources that were previously impacted by adic deposition. So, do reductions in acid deposition result in improved stream habitat and better soil quality, or not? That is the question that Lawrence (2010) set out to answer in the Appalachian Trail Deposition Effects study and we do not yet have that answer. Intuitively, the answer should be yes, that resources will improve as a result of reductions in adid deposition, but it isn't clear how quickly changes will become apparent or whether there are other reasons that resource recovery might be delayed.

Sources cited

Dieffenbach, F, 2011. Appalachian National Scenic Trail vital signs monitoring plan. Natural Resource Technical Report NPS/NETN/NRR2011/XXX. National Park Service, Northeast Temperate Network, Woodstock, VT.

Lawrence, G. 2010. Appalachian Trail Atmospheric Deposition Effects Study: Implementation Plan (a proposal)

Likens, G. E., and F. H. Bormann. 1974. Acid rain – A serious regional environmental problem. Science 184:1176-1179.

Omernik, J.M. 2003. The Misuse of Hydrologic Unit Maps for Extrapolation, Reporting, and Ecosystem management. Journal of the American Water Resources Association. (JAWRA) 39(3):563-573.

Shriver, G., T. Maniero, K. Schwarzkopf, D. Lambert, F. Dieffenbach, D. Owen, Y. Q. Wang, J. Nugranad-Marzilli, G. Tierney, C. Reese, T. Moore. November 2005. Appalachian Trail Vital Signs. Technical Report NPS/NER/NRTR--2005/026. National Park Service. Boston, MA.

Swain, E.B., D.R. Engstrom, M.E. Brighan, T.A. Henning, and P.L. Brezonik. 1992. Increasing Rates of Atmospheric Mercury Deposition. Science 257: 748-787.

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