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1 <br />' CHAPTERFOUR Enuironmenta~ conseuuences <br />percent change in contrast, fora "black tazget" against a cleaz sky, at the most optically sensitive <br />distance from an observer. Inherent conservatism in the modeling analyses should be considered <br />when determining the significance of potential impacts. In addition, factors that affect the impact <br />' analysis include: <br />• magnitude of deciview change <br />• frequency <br />• time of the year <br />' • meteorological conditions during times when deciview thresholds are exceeded <br />Since the visibility screening level analysis did not predict "significant, adverse" visibility <br />impacts at the more restrictive 0.5 deciview level, project operations certainly would not result in <br />a perceptible (i.e., 1.0 deciview reduction) visibility impact at any of the PSD Class I wilderness <br />azeas. The analysis of potential visibility impacts below 1.0 deciview (`just noticeable change" <br />' threshold) would not be significant. <br />Visibility impacts can also include the impact of a visible plume. The EPA has established <br />screening procedures to address the issue of visible plumes using the VISCREEN model. <br />' VISCREEN assesses the likelihood of a visual plume being observed at a given location as well <br />as when looking from a given location to a specific landmazk. The VISCREEN model was nut to <br />predict visibility impacts at the Flat Tops Wilderness Area. For the Flat Tops Wilderness Area, <br />' these distances aze approximately 40 miles (67 km) and 68 miles (113 km). <br />The perceptibility of a plume is defined by two parameters: contrast and color difference, or <br />' Delta-E. A contrast of 0.02 (where 1.0 would be a black/white contrast) and aDelta-E of 1 are <br />generally assumed to be the threshold of human perceptibility. The screening criteria that <br />VISCREEN uses are a contrast of 0.05 and aDelta-E of 2.0. The Level I VISCREEN analysis <br />indicates that this project would cause a contrast less than 0.004, and Delta-E less than 0.45. <br />Therefore, no significant impact is expected. <br />' More detail regazding the VISCREEN results are presented in Table D-6 in Appendix D. <br />Class I Water Acid cation Analyses <br />' Piceance and Parachute Sites <br />The water quality impacts were evaluated as an AQRV. The potential impacts are associated <br />with the deposition of sulfates and nitrates which can contribute to the acidification process. The <br />' methodology for the assessment was based on "A Suggested Methodology for an Acid <br />Deposition Screening Technique Applicable Within 200 km of Isolated Sources" (Fox 1983). <br />The change in pH and ANC were calculated using the modeled annual average NOz <br />' concentrations for the Ned Wilson Lake located in the Flat Tops Wilderness Area. The <br />following assumptions provided additional conservatism to the analysis. <br />' • Both the Piceance facility and Pazachute facility were modeled together. <br />• The dispersion model, meteorological data, and receptor grid used were the same as for the <br />Class I increment analysis. <br />• It was assumed that all NOx was converted to NOz. <br />' Meteorology and Air Quality 4-29 <br />