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Characterization of Selected Field Properties, Major-Ion Chemistry, Acid Neutralizing Capacity, and Nutrients

Appalachian National Scenic Trail showing HUC10 shell.
Figure 1. Appalachian National Scenic Trail showing HUC10 shell.
The following summary was excerpted from the Appalachian Trail water resource review by Argue et al. (2011). Download the full report.

Definitions

ANC—acid neutralizing capacity

Catchment—the incremental drainage area for a single NHD hydrologic feature such as a stream segment

NHD—National Hydrography Dataset

Ecoregional Section Abreviations and Names

WM—White Mountains (M211A)

VtNHU—Vermont-New Hampshire Upland (M211B)

GTBM—Green-Taconic-Berkshire Mountains (M211C)

LNE—Lower New England (221A)

HV—Hudson Valley (221B)

NRV—Northern Ridge and Valley (M221A)

AM—Alleghany Mountains (M221B)

BRM—Blue Ridge Mountains (M221D)

Introduction

The U.S. Geological Survey (USGS), at the request of the National Park Service (NPS), examined existing data from 1,817 sampling locations to provide a summary of water quality in headwater (first- and second-order) streams in close proximity to the Appalachian National Scenic Trail (APPA). Data were aggregated and summarized using U.S. Forest Service (USFS) ecoregions (Bailey et al. 1984) at the “section� level (eco-sections). In addition to summarizing their results by eco-section, the USGS relied on NHDPlus catchment delineations to evaluate data, with criteria for inclusion being based on physical attributes including stream elevation, percentage of developed land cover, and percentage of agricultural land cover. The NHDPlus web site (http://www.horizon-systems.com/nhdplus/) describes the dataset as follows: "NHDPlus is an integrated suite of application-ready geospatial data sets that incorporate many of the best features of the National Hydrography Dataset (NHD), the National Elevation Dataset (NED), and the Watershed Boundary Dataset (WBD)." Eco-section results referenced throughout this summary are based on catchment data.

Differences in selected physical attributes among eco-sections may help to identify factors that influence variations in water quality along the Appalachian Trail. Physical attributes of the catchments that contained selected sampling sites are summarized by eco-section in figures 2 through 16. The highest median elevations and steepest slopes (Table 1) were in the southern (Allegheny Mountains and Blue Ridge Mountains, 2,495 and 2,474 ft., respectively) and the northern (White Mountains and Green–Taconic–Berkshire Mountains, 2,064 and 1,394 ft., respectively) eco-sections, whereas the Lower New England eco-section had the most developed land (0.53 percent; Table 2). The Northern Ridge and Valley, Hudson Valley, Vermont–New Hampshire Upland, and Green–Taconic–Berkshire Mountains eco-sections contained the highest median percentages of agricultural land (7.47, 4.09, 3.92, and 2.98, respectively; Table 2).

Table 1. Elevation and slope from catchments within specified eco-sections.
Table 2. Percent (%) developed and agricultural land from catchments within specified eco-sections.

Focal Eco-Sections

USGS identified nine USFS eco-sections that had a sufficient number of sampling sites with relevant water quality data. One of these eco-sections, the Northern Glaciated Allegheny Plateau (211F) intersects the HUC10 shell but does not actually intersect lands administered by the APPA. Consequently, we have omitted the Northern Glaciated Allegheny Plateau from this summary. Although USGS and the NPS Northeast Temperate Network (NETN) both use USFS ecoregions to summarize data, the versions used by USGS and NETN differ. The most notable difference involves the Northern Ridge and Valley (NRV) eco-section (Figure 1), which is divided into two pieces in the USGS report with the northern portion retaining the Northern Ridge and Valley name, and the sourthern portion being called the Alleghany Mountains (AM). The two pieces are separated near the Pennsylvania-Maryland border, and while the Alleghany Mountains eco-section is present in the version used by NETN the boundary is further to the west and barely intersects the HUC10 shell. The remaining six eco-sections include the White Mountains (WM), Vermont–New Hampshire Upland (VtNHU), Green–Taconic–Berkshire Mountains (GTBM), Lower New England (LNE), Hudson Valley (HV), and Blue Ridge Mountains (BRM) as shown in figure 1.

Field Properties

Temperature, dissolved oxygen, pH, and specific conductance data were evaluated to assess the “field� condition of headwater streams.

Most temperature values (56.7 percent; Figure 2) were less than 12ºC, which may be indicative of cool headwater streams. Water temperatures greater than 20ºC, which are generally considered an instream upper limit for the protection of aquatic health (U.S. Environmental Protection Agency 2006), were recorded in 170 catchments. These warm temperatures were primarily observed between the months of July and September and were measured in all regions along the Appalachian Trail (including catchments with elevations greater than 2,500 ft). Water temperatures were related to season, elevation, and latitude with catchments in the northern section generally having the cooler water temperatures in all seasons than catchments in the central and southern sections.

Dissolved oxygen (DO; mg/L; Figure 3) is necessary for the survival of many aquatic organisms, and is affected by many factors including ambient temperatures, atmospheric pressures, and ionic or biological activity. Concentrations of dissolved oxygen ranged from 3.8 to 12.9 mg/L, with a median value of 9.5 mg/L (Figure 3). Warm water holds less dissolved oxygen than cold, and this effect probably explains som of the seasonal variability in figure 3.

 
Figure 2. Seasonal average water temperature values for select eco-sections intersecting the APPA.
 
Figure 3. Seasonal average dissolved oxygen values for select eco-sections intersecting the APPA.

pH is a measure of the Hydrogen ion (H+) concentration. pH helps to characterize the acid-base status of the water and has relevance to many water chemistry measures. Natural waters generally have pH values in the range of 6.5 to 8.5 (Hem 1992). pH values ranged from 4.0 to 8.9 with a median value of 6.7 (Figure 4), with approximately 38 percent of aggregate values (302) having pH values less than 6. A pH value of 6 is generally considered an instream lower limit for the protection of aquatic health (U.S. Environmental Protection Agency 2006).

Specific conductance (μS/cm) is a measure of the ability of water to conduct an electric current, and is related to the concentration of dissolved solids (Hem 1992). Median catchment specific conductance values ranged from 4.0 to 761 μS/cm with a overall median value of 23.8 μS/cm (Figure 5). Approximately 75 percent of the catchments had specific conductance values less than 50.0 μS/cm.

 
Figure 4. Minumum, maximum, and median pH values for select eco-sections intersecting the APPA.
 
Figure 5. Minumum, maximum, and median specific conductance values for select eco-sections intersecting the APPA.

Major Ions

Major ions summarized in this review include calcium, chloride, magnesium, nitrate, potassium, silica, sodium, and sulfate. Median concentrations of cations (calcium, magnesium, sodium, and potassium; Figures 6 through 9, respectively) were less than 1.5 mg/L and ranged from 0.47 to 1.46 mg/L, whereas median concentrations of anions (chloride, sulfate, and nitrate; Figures 10, 11, and 12, respectively) ranged more widely with medians below 10 mg/L and ranges from less than 0.08 to 9.64 mg/L.

Major ion concentrations were used to “characterize� water types based on the percentage of milliequivalents of the major ions present within each sample. In most samples compiled for this study, no one cation composed more than 50 percent of the total cations, and sulfate was the most frequently predominant anion. Consequently, calcium-magnesium sulfate was the most frequently determined water type. In the Lower New England and Hudson Valley eco-sections however, calcium-bicarbonate was the most frequently calculated water type.

Median concentrations of calcium, magnesium, and sodium (cations) from the Hudson Valley and Lower New England eco-sections were among the highest compared to the other eco-sections (Figures 6 through 8), with concentrations of magnesium in the Vermont–New Hampshire Upland and Green–Taconic–Berkshire Mountains eco-sections (8.30 and 5.75 mg/L) being an exception. Median concentrations of calcium, magnesium, and sodium in the White Mountains, Blue Ridge Mountains, and Allegheny Mountains are among the lowest compared to the other eco-sections (Figures 6 through 8). The White Mountains eco-section also had the lowest median concentration of potassium (Figure 9).

 
Figure 6. Minumum, maximum, and median calcium values for select eco-sections intersecting the APPA.
 
Figure 7. Minumum, maximum, and median magnesium values for select eco-sections intersecting the APPA.
 
Figure 8. Minumum, maximum, and median sodium values for select eco-sections intersecting the APPA.
 
Figure 9. Minumum, maximum, and median potassium values for select eco-sections intersecting the APPA.

Concentrations of common anions were similar to those of cations—generally lowest in the White Mountains, Blue Ridge Mountains, and Allegheny Mountains (Figures 10 through 12). Concentrations of sulfate in these three eco-sections were lower than in the other six eco-sections and lowest in the Blue Ridge Mountain eco-section. Concentrations of chloride in these three eco-sections also were lower than in the other six eco-sections and statistically lowest in the White Mountains eco-section. Concentrations of chloride and sulfate in the Hudson Valley and Lower New England eco-sections were generally higher than in the other eco-sections (Figures 10 and 11). Concentrations of nitrate were generally lower than concentrations of sulfate and chloride (Figure 12) and were highest in the Northern Ridge and Valley, Green–Taconic–Berkshire Mountains, and Vermont–New Hampshire Upland eco-sections: 0.416, 0.330 and 0.261 mg/L, respectively (Figure 12).

Overall, major ion concentrations were lower in the White Mountains, Blue Ridge Mountains, and Allegheny Mountains eco-sections and higher in the Lower New England and Hudson Valley eco-sections. The Vermont–New Hampshire Upland, Green–Taconic–Berkshire Mountains, and Northern Ridge and Valley eco-sections represent the transition zones along the Appalachian Trail between the regions of highest and lowest elevation, and ions in these four transitional eco-sections were generally lower than or similar to concentrations from the Hudson Valley and Lower New England eco-sections and greater than concentrations from the White Mountains, Blue Ridge Mountains, and Allegheny Mountains eco-sections.

 
Figure 10. Minumum, maximum, and median chloride values for select eco-sections intersecting the APPA.
 
Figure 11. Minumum, maximum, and median sulfate values for select eco-sections intersecting the APPA.

Figure 12. Minumum, maximum, and median nitrate values for select eco-sections intersecting the APPA.
 

Acid Neutralizing Capacity

Acid neutralizing capacity (ANC) is the capacity of a solution to react with and neutralize acid (Rounds 2006). ANC values, measured in microequivalents per liter (μeq/L), help to characterize the acid-base chemistry of water and indicate a net strong base in solution if ANC is positive or a net strong acid if the ANC value is negative. Higher ANC values indicate greater ability to resist acidic inputs from sources such as acid deposition. Values for ANC ranged from -11.3 to 4,400 μeq/L, with a median ANC value for the entire Appalachian Trail of 98.7 μeq/L (Figure 13).

Figure 13. Minumum, maximum, and median acid neutralizing capacity (ANC) values for select eco-sections intersecting the APPA.

As with pH, the median ANC value for the White Mountains eco-section was the lowest of all the eco-sections, with a median value of 45.0 μeq/L. Median ANC values from the Blue Ridge Mountains and the Allegheny Mountains were statistically similar and among the lowest: 62.4 and 66.9 μeq/L, respectively (Figure 13). The Vermont–New Hampshire Upland eco-section had the highest median ANC value (820 μeq/L) (Figure 13). The Green–Taconic–Berkshire Mountains, Lower New England, and Hudson Valley eco-sections also had relatively high median ANC values (350, 394, and 575 μeq/L, respectively; Figure 13). The three eco-sections (White Mountains, Blue Ridge Mountains, and Allegheny Mountains) that had the lowest ANC values also had the lowest concentrations of most major ions (Figures 6 through 12).

For comparison purposes, ANC values were divided into three categories: ANC values less than zero were defined as acidic, ANC values between 0 and 50 μeq/L were classified as sensitive, and ANC values greater than 50 μeq/L were classified as insensitive (Kahl and Scott 1988; Driscoll et al. 2001; Sullivan et al. 2007). Classification as sensitive and insensitive generally refers to the extent that the stream is susceptible to becoming acidic either seasonally or through event-driven episodic acidification. The percentages of catchments that had ANC values classified as acidic, sensitive, or insensitive are shown in Table 3.

Table 3. Number of catchments per eco-section, expressed as a percent, considered acidic, sensitive, or insensitive.

The eco-sections with the most dilute waters—White Mountains, Blue Ridge Mountains, and Allegheny Mountains—had the highest percentages of catchments classified as either acidic or sensitive compared to all other eco-sections (Table 3). The Allegheny Mountains had the highest percentage of catchments classified as acidic (17.6 percent), while the White Mountains eco-section had the highest percentage of catchments classified as sensitive (56.7). These results contradict previous investigations in the northeastern United States that have shown that a substantial number of lakes are chronically acidic (Kahl and Scott 1988; Charles and Christy 2001). This apparent discrepancy may be a result of ANC data compiled for this inventory under–representing areas near or along the Appalachian Trail where noncarbonate alkalinity is a substantial contributor to ANC and where the ANC of streamwaters could be classified as acidic.

Nutrients

Nutrients (nitrate, ammonia, total nitrogen, and total phosphorus; Figures 12, and 14 through 16, respectively) in surface water are essential to aquatic life in appropriate concentrations. Excessively high concentrations of nitrogen can be toxic to fish and harmful to humans. High concentrations of phosphorus can produce an overgrowth of algae, which can degrade aquatic habitats. Sources of excess nutrients are commonly associated with developed and agricultural land uses including urban run-off, pesticide use, effluent, and atmospheric deposition. In first- and second-order streams along the Appalachian Trail, concentrations of nutrients are typically not high enough to be harmful to aquatic life. Concentrations of nutrients in streams along some regions of the Appalachian Trail may be evaluated through comparisons to estimated background concentrations and to regional nutrient criteria (Table 4).

Figure 14. Minumum, maximum, and median ammonia values for select eco-sections intersecting the APPA.
Figure 15. Minumum, maximum, and median total nitrogen values for select eco-sections intersecting the APPA.
Figure 16. Minumum, maximum, and median total phosphorus values for select eco-sections intersecting the APPA.

Concentrations of ammonia were the lowest among the three nitrogen species with an overall median concentration of 0.016 mg/L. The highest median concentrations of ammonia were in catchments in the Green–Taconic–Berkshire Mountains eco-section (0.065 mg/L; Figure 14). Median concentrations of total nitrogen were similar along most of the Appalachian Trail and ranged from 0.355 to 0.530 mg/L for most eco-sections (Figure 15), with the Blue Ridge Mountains and Allegheny Mountains being the exceptions; these two eco-sections had lower median concentrations of total nitrogen (0.216 and 0.119 mg/L, respectively; Figure 15).

Concentrations of total phosphorus ranged from less than 0.004 to 4.20 mg/L (note: the max value for the Hudson Valley (HV) eco-section (4.20) was removed from Figure 16 because it was 20 times larger than the next value), with an estimated median concentration of 0.018 mg/L. Median concentrations of phosphorus were higher in the middle and southern eco-sections than in the northern eco-sections (Figure 16). The region of the Appalachian Trail where concentrations of total phosphorus appeared to change is near the southernmost extent of the most recent glacial advance between the Green–Taconic–Berkshire Mountains and the Lower New England eco-sections. Median concentrations of phosphorus may be higher in the nonglaciated regions of the Appalachian Trail because more abundant deposits of weathered sediments may provide a source of suspended sediment, which typically entrains and transports phosphorus in the streams.

Concentrations of nutrients from selected sampling sites in headwater streams along the Appalachian Trail were compared to estimated national background concentrations (Dubrovsky et al. 2010; Table 4). Estimated national background concentrations were based on data from streams across the nation in areas with minimal or no development and included sites in semiarid and arid regions (Dubrovsky et al. 2010). Overall concentrations of ammonia, nitrate, total nitrogen, and total phosphorus were lower at sites along the Appalachian Trail as compared to estimated national background concentrations (Table 4). However, on a smaller scale, the Vermont–New Hampshire Upland, Green–Taconic–Berkshire Mountains, Northern Glaciated Allegheny Plateau, and the Northern Ridge and Valley eco-sections had median concentrations of ammonia that were higher than estimated national background concentrations. Additionally, the Vermont–New Hampshire Upland, Green–Taconic–Berkshire Mountains, and the Northern Ridge and Valley eco-sections had median concentrations of nitrate that were higher than estimated national background concentrations.

The USEPA has developed recommended ecoregional criteria for total nitrogen and total phosphorus that are intended to approximate reference or background concentrations and serve as guidelines to protect against nutrient enrichment (U.S. Environmental Protection Agency 2000; 2002). The nutrient criteria reflect the regional influences of natural variations in geology, land cover, and climate that affect background concentrations of nutrients. Most of the selected catchments in this study were in one of two nutrient ecoregions, the Nutrient-Poor Largely Glaciated Upper Midwest and Northeast (New York to Maine) and the Central and Eastern Forested Upland (New Jersey to Georgia). Approximately 63.4 percent of concentrations of total nitrogen in the Nutrient-Poor Largely Glaciated Upper Midwest and Northeast nutrient ecoregion were greater than the recommended USEPA nutrient criterion. Approximately 70.5 percent of the concentrations of total phosphorus in the Central and Eastern Forested Upland nutrient ecoregion were greater than the recommended USEPA nutrient criterion.

Table 4. Comparison of median catchment concentrations of nutrients and the number, expressed as percent (%), that exceed estimated national background concentrations.

Sections of the Appalachian Trail pass through urban areas and are adjacent to agricultural land (Table 2). Intermittent influences from the regions of the Appalachian Trail where the percentages of developed or agricultural land were moderate may have increased the number of catchments that had concentrations of total nitrogen and total phosphorus greater than the USEPA’s nutrient ecoregion criteria. Dubrovsky and others (2010) compared simulated modeled estimates of background concentrations from Smith et al. (2003) to the 14 USEPA nutrient ecoregion criteria. This comparison demonstrated that, for the Nutrient-Poor Largely Glaciated Upper Midwest and Northeast and the Central and Eastern Forested Upland nutrient ecoregions, the criteria would be difficult to meet even with only a small percentage of development. Overall concentration of total nitrogen and total phosphorus within the study area are lower than estimated national background concentrations but generally similar or higher than USEPA ecoregion nutrient criteria.

Effects of Environmental Attributes on Water Quality

Variations in water quality along the Appalachian Trail were evaluated with respect to selected environmental attributes such as elevation, slope, precipitation, and temperature to determine how natural or anthropogenic factors may be influencing the water quality of the headwater streams along and near the Appalachian Trail. Generalized major geologic units were used to group water quality data along the Appalachian Trail to determine geologic effects on water quality, and anthropogenic factors such as developed and agricultural land use in the drainage area of headwater streams and atmospheric deposition of sulfate and nitrate in the catchments of headwater streams were evaluated with respect to possible influence on water quality.

Elevation

Catchment pH, ANC, and concentrations of major ions were found to be significantly correlated with elevation along the entire length of the Appalachian Trail (except in the Lower New England eco-section). Stream water chemistry appears to be affected by the synergistic influences of environmental attributes. Generally, in the Eastern U.S. as elevation increases, drainage areas tend to become smaller and steeper, and geology at elevation tends to be increasingly resistant to weathering. The short residence times typical of small steep drainages coupled with relatively inert geology yields comparatively more dilute waters compared to surface waters at lower elevations.

Geology

Ten major geologic units, combined into six geologic groups (sedimentary, metasedimentary, metasedimentary that contain carbonate, gneissic, granitic, and volcanic), underlie the catchments evaluated for this summary, with over 78 percent of the study area being underlain by sedimentary rocks.

Catchment values of pH, specific conductance, and ANC were compared among the six geologic groups. Generally, sedimentary rocks and metasedimentary rocks had similar median catchment values for pH, specific conductance, and ANC. For the sedimentary rocks, the median pH was 6.6, specific conductance was 23.7, and ANC was 88.7 μeq/L. Similarly, catchments in the metasedimentary rocks had a median pH of 6.5, median specific conductance of 20.3, and median ANC of 86.3 μeq/L. However, catchment values for pH and ANC were statistically higher in the metasedimentary rocks that were classified as containing carbonate minerals compared to the other two groups of sedimentary rocks (median pH of 7.3; median ANC value of 590 μeq/L), leading to the conclusion that the presence of carbonate minerals appear to strongly influence geologic attributes with respect to pH, specific conductance, and ANC in the headwaters along the Appalachian Trail.

Land Cover

The percentages of developed or agricultural land cover in the catchments associated with this inventory were generally low, although sections of the Appalachian Trail do pass through or near major urban areas or agricultural land. Along the entire Appalachian Trail, the percentages of developed and agricultural land were significantly and positively correlated with pH, ANC, concentrations of major ions, and concentrations of nitrate, total nitrogen, and total phosphorus.

Atmospheric Deposition

Previous studies of areas near the Appalachian Trail have demonstrated the negative effects of atmospheric deposition on water quality. Along the Appalachian Trail, atmospheric deposition is estimated to be high (greater than 20 kg/ha) at both the highest and lowest elevations. The Green–Taconic–Berkshire Mountains, Lower New England, Hudson Valley, and Northern Ridge and Valley eco-sections have median deposition estimates greater than 24 kg/ha for sulfate (Figure 11) and greater than 19 kg/ha for nitrate (Figure 14). Generally, higher elevations are more likely to receive high depositions, however for the five eco-sections in the central region of the Appalachian Trail, climatic patterns and possibly sources of sulfate and nitrate, to some extent, outweigh the elevation-induced effects of precipitation on estimates of atmospheric deposition.

Concentrations of sulfate in streamwater were statistically greater in the eco-sections that received the highest atmospheric deposition estimates of sulfate. However, concentrations of nitrate and total nitrogen in streamwater were not statistically greater in the eco-sections that received the highest atmospheric deposition of nitrate.

Reductions in concentrations of sulfate and nitrate in atmospheric deposition related to the Clean Air Act Ammendments of 1990 (CAAA) have been observed throughout the Midwest and Eastern United States, specifically in the central Appalachians (U.S. Environmental Protection Agency 2010) that includes the Green–Taconic–Berkshire Mountains, Lower New England, Hudson Valley, Northern Glaciated Allegheny Plateau, and Northern Ridge and Valley eco-sections. Reductions in sulfate have been more significant than reductions in nitrogen—approximately 40 and 20 percent, respectively (U.S. Environmental Protection Agency 2010).

Literature Cited

Argue, D.M., Pope, J.P., and Dieffenbach, Fred, 2011, Characterization of the water quality of headwater streams along the Appalachian National Scenic Trail and within adjacent watersheds, Maine to Georgia: U.S. Geological Survey Scientific Investigations Report 2011–5151, 63 p., plus CD-ROM. (Also available at https://pubs.usgs.gov/sir/2011/5151.)

Bailey, R.G.; Avers, P.E.; King, T.; McNab, W.H, eds. 1994. Ecoregions and subregions of the United States (map). Washington, DC: U.S. Geological Survey. Scale 1:7,500,000. Colored. Accompanied by a supplementary table of map unit descriptions compiled and edited by McNab, W.H. and Bailey, R.G. Prepared for the USDA Forest Service.

Driscoll, C.T., Lawrence, G.B., Bulger, A.J., Butler, T.J., Cronan, C.S., Eagar, Christopher, Lambert, K.F., Likens, G.E., Stoddard, J.L., and Weathers, K.C., 2001, Acidic deposition in the northeastern United States—Sources and inputs, ecosystem effects, and management strategies: Bioscience, v. 51, no. 3, p. 180–198.

Dubrovsky, N.M., Burow, K.R., Clark, G.M., Gronberg, J.M., Hamilton, P.A., Hitt, K.J., Mueller, D.K., Munn, M.D., Nolan, B.T., Puckett, L.J., Rupert, M.G., Short, T.M., Spahr, N.E., Sprague, L.A., and Wilber, W.G., 2010, The quality of our Nation’s waters—Nutrients in the Nations streams and groundwater, 1992–2004: U.S. Geological Survey Circular 1350, 174 p., accessed August 28, 2010, at http://water.usgs.gov/nawqa/nutrients/pubs/circ1350/.

Hem, J.D., 1992, Study and interpretation of the chemical characteristics of natural water (3d ed.): U.S. Geological Survey Water-Supply Paper 2254, 263 p.

Kahl, J.S., and Scott, Matthew, 1988, Chemistry of Maine’s high elevation lakes—Results from the HELM project: Lake and Reservoir Management, v. 4, no. 1, p. 33–39.

Rounds, S.A., 2006, Alkalinity and acid neutralizing capacity (version 3.0): U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chap. A6, section 6.6, July, accessed August 2008, from http://pubs.water.usgs.gov/twri9A6/.

Smith, R.A., Alexander, R.B., and Schwarz, G.E., 2003, Natural background concentrations of nutrients in streams and rivers of the conterminous United States: Environment Science & Technology, v. 37, no. 14, p. 3039–3047.

Sullivan, T.J., Webb, J.R., Snyder, K.U., Herlihy, A.T., and Cosby, B.J., 2007, Spatial distribution of acid-sensitive and acid-impacted streams in relation to watershed features in the Southern Appalachian Mountains: Water, Air, and Soil Pollution, v. 182, p. 57–71.

U.S. Environmental Protection Agency, 2000, Nutrient Criteria: Technical guidance manual; rivers and streams: U.S. Environmental Protection Agency, EPA 822B-00-002, 253 p., accessed June 2, 2011, at http://water.epa.gov/scitech/swguidance/standards/criteria/nutrients/upload/2009_04_22_criteria_nutrient_guidance_rivers_rivers-streams-full.pdf.

U.S. Environmental Protection Agency, 2002, Summary table for the nutrient criteria documents: U.S. Environmental Protection Agency, Office of Water, 3 p., accessed August 30, 2010, at http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/files/sumtable.pdf.

U.S. Environmental Protection Agency, 2010a, Acid Rain Program progress—2009 environmental results, accessed March 3, 2011, at http://www.epa.gov/airmarkets/progress/ARP09_3.html.

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