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emissions estimates do not include additional dust control application/measures. Using these <br /> potential annual PM emissions rates, a screening level visibility analysis was completed for this <br /> EA using the EPA's VISCREEN model <br /> (https://www3.epa.gov/ttn/scram/dispersion_screening.htm#viscreen). The VISCREEN model <br /> was ran in screening-level "default" mode for all parameters with the exception of atmospheric <br /> stability (stability class index 5 was used for this screening level analysis as opposed to the <br /> default level 6) and the primary particulate matter average diameter index for the plume was set <br /> to 7 instead of the VISCREEN default value of 6. The following describes the rational for <br /> running VISCREEN in the Level-2 analysis mode: <br /> • The location of the closest Class I Area (USFS Rawah Wilderness) is approximately 13 <br /> kilometers from the project location and has base elevations of at least 500 meters or <br /> more above the base elevation for the project area (Pit). EPA Guidance for VISCREEN <br /> describes that the worst-case atmospheric stability class should be shifted one category <br /> less stable (i.e. from 6 to 5)when the observer/Class I area receptor is located on terrain <br /> at least 500 meters above the project surface. <br /> • The particulate matter size distribution based on total mass for the project-specific <br /> emissions is such that approximately 90% of the PM emissions are of course particles <br /> (greater than Ium and less than/equal to 10um) and the other 10% of the mass is made up <br /> of fine particles (— Ium or less). The VISCREEN default diameter sizes for course and <br /> fine PM were used with the project-specific PM emissions distributions to calculate an <br /> average weighted plume median diameter size index of 7 for input instead of the 6 size <br /> index default value for primary particulate matter. <br /> In summary, for the project-level visibility screening analysis, the VISCREEN model for the <br /> estimated annual project-specific particulate matter emissions shows that the applicable visibility <br /> screening criteria would not be exceeded inside the nearest Class I area Rawah Wilderness. <br /> As previously described, the bulk of the activities for the proposed action will primarily have <br /> temporary negative air quality impacts for criteria pollutants (primarily PM) and HAPs related <br /> air quality during surface disturbing and material processing events when persistent dry weather <br /> conditions exist, and are not predicted to cause a significant impact to visibility at the nearest <br /> Class I area. To ensure additional protection from project-related dust emissions, the following <br /> good management practices/mitigation measures should be implemented for the project: <br /> • Reduce vehicle speed on unpaved surfaces (minimizes dust emissions—PM emissions <br /> can be a direct linear relationship with vehicle speed; reduce speed by 10% to reduce PM <br /> emissions by 10%); <br /> • Minimize surface disturbing activities when windy episodes are forecasted and dry <br /> weather conditions exist(minimizes dust emissions—operating on days when the wind <br /> gusts [not sustained wind speed] do not exceed critical threshold velocity [—43 mph] <br /> almost eliminates wind erosion PM emissions for that day); <br /> • Apply water or other dust control measures to unpaved surfaces that will experience <br /> multiple daily traffic trips or surface disturbance associated with extraction/mining <br /> during dry weather conditions; especially at areas on unpaved roadways near residences, <br /> places of business or similar (minimizes dust emissions—routine application of water can <br /> reduce PM emissions by 50%; greater dust control efficiency can be achieved for other <br /> DOI-BLM-CO-N020-2017-0003 21 <br />