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Spatial Distribution of Acid-Sensitive and Acid-Impacted Streams in Relation to Watershed Features in the Southern Appalachian Mountains

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Spatial Distribution of Acid-Sensitive and Acid-Impacted Streams in Relation to Watershed Features in the Southern Appalachian Mountains Water Air Soil Pollut (2007) 182:57–71 DOI 10.1007/s11270-006-9320-x Spatial Distribution of Acid-sensitive and Acid-impacted Streams in Relation to Watershed Features in the Southern Appalachian Mountains T. J. Sullivan & J. R. Webb & K. U. Snyder & A. T. Herlihy & B. J. Cosby Received: 26 July 2006 /Accepted: 18 November 2006 / Published online: 19 December 2006 # Springer Science + Business Media B.V. 2006 Abstract A geologic classification scheme was com- sandstone and quartzite. Streamwater acid-base chem- bined with elevation to test hypotheses regarding istry throughout the region was also found to be watershed sensitivity to acidic deposition using associated with a number of watershed features that available regional spatial data and to delimit a high- were mapped for the entire region, in addition to interest area for streamwater acidification sensitivity lithology and elevation, including ecoregion, physio- within the Southern Appalachian Mountains region. It graphic province, soils type, forest type and watershed covered only 28% of the region, and yet included area. Logistic regression was used to model the almost all known streams that have low acid presence/absence of acid-sensitive streams throughout neutralizing capacity (ANC ≤20 μeq l−1) or that are the region. acidic (ANC ≤0). The five-class geologic classifica- tion scheme was developed based on recent lithologic Keywords acid neutralizing capacity . acidification . maps and streamwater chemistry data for 909 sites. Appalachian Mountains . geology. The vast majority of the sampled streams that had streamwater . watershed ANC ≤20 μeq l−1 and that were totally underlainby a single geologic sensitivity class occurred in the siliceous class, which is represented by such lithologies as 1 Introduction Ongoing efforts to control the atmospheric emissions and deposition of sulfur (S) and nitrogen (N) near and upwind of the Southern Appalachian Mountains (SA) * : T. J. Sullivan ( ) K. U. Snyder region are aimed, in part, at reversing past acidifica- E&S Environmental Chemistry, Inc., P.O. Box 609, Corvallis, OR, USA tion and preventing future acidification of stream- e-mail: [email protected] waters (Brewer, Sullivan, Cosby, & Munson, 2000). : Sullivan et al. (2004) provided a model-based analysis J. R. Webb B. J. Cosby of future changes in streamwater acid-base chemistry Department of Environmental Sciences, University of Virginia, as part of an assessment by the Southern Appalachian Charlottesville, VA, USA Mountains Initiative (SAMI). The results of that analysis pertain primarily to numbers, percentages, A. T. Herlihy and extent of streams within the SAMI domain Department of Fisheries and Wildlife, Oregon State University, affected now or projected to be affected in the future Corvallis, OR, USA by acidic deposition. 58 Water Air Soil Pollut (2007) 182:57–71 Additional questions that might be deemed to be water acid-base status. For example, Lynch and Dise important, but that cannot be addressed using a (1985) found that watersheds in Shenandoah National quantitative, population-based analysis, could include: Park, VA that had a larger proportion of their area above 732 m elevation had lower streamwater ANC. & Where within the SAMI domain are the most acid- Such a gradient may have been due to the general sensitive streams located? observation that well-developed soils are less common & Are there features of the landscape that correlate with at higher elevation, where slopes are often steeper and current streamwater acid-base chemistry and/or sen- temperatures lower. Furthermore, slower weathering, sitivity to change in streamwater acid-base chemis- greater precipitation and base cation leaching, and try? If so, what are those features and how are they higher acidic deposition at higher elevation likely all distributed across the landscape? play significant roles in such observed relationships. The regional aquatic resource classification scheme Similar results have been found for Great Smoky presented here was developed to address questions Mountains National Park in North Carolina and such as these. Tennessee (Silsbee & Larson, 1982). The importance Acid neutralizing capacity (ANC) is an effective of elevation, elevational gradients, and landscape stratifying variable for evaluation of streamwater acid- position as controllers of drainage water acid-base base chemistry. ANC is both a measure of current acid- chemistry are well known (Johnson, Driscoll, Eaton, base status and a product of watershed processes that Likens, & McDowell, 1981). determine the presence of acidic and basic constituents The forest per se is not a major controlling variable in solution. We can expect future response to acidic for streamwater ANC. It can be, however, a useful deposition to differ among streams with different ANC indicator of the watersheds most likely to be sensitive levels. ANC has thus served as a primary variable for to, and/or impacted by, acidic deposition in the SAMI stratifying streams to be modeled for the SAMI aquatic region (Herlihy et al., 1993). Based on data from the effects assessment (Sullivan et al., 2002, 2004). Streams National Stream Survey (NSS), acidic streams are having ANC e 20 2eq l −1 are classified as sensitive to found only in forested watersheds in the northern chronic and/or episodic acidification with probable portion of the SAMI region, and streams with ANC adverse impacts on native brook trout (Salvelinus <50 2eq l−1 are more common in forested than fontinalis; Bulger, Cosby, & Webb, 2000). The unforested watersheds. In particular, there is a clear relationship between bedrock geology and the ANC of distinction in the Valley and Ridge Province between streams in the SA region has been well-recognized the more acid-sensitive streams of the forested ridges (Bricker & Rice, 1989; Cosby, Ryan, Webb, Hornberger, and the less acid-sensitive streams of the valleys, &Galloway,1991; DeWalle, Dinicola, & Sharpe, 1987; which contain mixed land use (Herlihy et al., 1993). Herlihy et al., 1993;Lynch&Dise,1985; Puckett & Acidic streams also tended to be located in smaller Bricker, 1992;Webbetal.,1994). Bedrock geology and, watersheds at higher elevation with steeper gradients. in glaciated terrain, surficial geology have been shown Acidic streams in the NSS were found almost to be important elsewhere (c.f., Norton, 1980;Melack, exclusively at elevations >300 m and in watersheds Stoddard, & Ochs, 1985; Peters & Driscoll, 1987;Clow less than 20 km2 (Herlihy et al., 1993). & Sueker, 2000). Bulger et al. (2000)demonstrated Ecoregions are depictions of ecosystem patterns that that classification of landscape by bedrock type can are created through a classification process that provide a basis for regionalizing the results of wa- captures the spatial distribution of relatively homoge- tershed acidification modeling in western Virginia. neous landscape areas at a specific scale (Bailey, 1995; These observations, plus the availability of recently- Bryce, Omernik, & Larsen, 1999; Omernik, 1995). developed geologic map coverages for most of the Ecoregions can be redefined in different ways at SAMI region, suggested that a useful regional different scales. The scale of application is important landscape classification scheme could be developed because the influence of specific landscape features based in part on the relationship between lithology (e.g., geologic class, vegetation type) on resource and streamwater ANC. integrity (e.g., forest health, streamwater acid-base In addition to geology, other landscape character- chemistry) is highly scale-dependent. For example, a istics have also been found to correlate with stream- stream located in a watershed that contains limestone, Water Air Soil Pollut (2007) 182:57–71 59 but that occurs in a region of largely granitic bedrock, Figure 1 presents the general scheme used to may have high ANC even though the region as a whole classify the non-carbonate lithologic units according may be best represented by low-ANC streams. Eco- to the properties expected to influence the ANC of regions were designed to provide a spatial framework associated streamwaters. Table 1 provides a list for ecological assessments, research, inventory, mon- indicating the assignment of individual lithologic itoring, and management. Watersheds within a given units to each of the five sensitivity classes. Note that ecoregion will tend to be somewhat similar to one lithologic map units with primary rock types defined another and different from those in other ecoregions by structure rather than composition were classified (Omernik & Bailey, 1997; Omernik & Griffith, 1991). based on secondary rock type or state-by-state Thus, a variety of watershed features are known or formation descriptions. Examples of lithologic map suspected to be associated with watershed and units in this category include: conglomerate, meta- drainage water acid-sensitivity. The objective of the sedimentary rock, breccias, and schist. research reported here was to test hypotheses suggest- ing that streamwater acid-base chemistry is controlled 2.2 Development of Landscape Classification System by geologic, edaphic, and topographic variables, using spatial data available at the regional scale. A regional spatial analysis was conducted to deter- mine the extent to which streamwater ANC in the SAMI region correlated with landscape features that were (1) known or suspected to be important contrib- 2 Materials and Methods utors to surface water acidification
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