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L ' CRAPTERFOUR Enuironmenta~ Consequences ' 4.5.3 Mitigation Additional measures that can be undertaken to mitigate impacts include the following: • Use electric-powered compressors where possible (along pipeline, etc.). • Selection of low NOx (clean burning) combustion burners for the boilers. ' 4.5.4 Unavoidable Adverse Impacts The Proposed Action and each of [he alternatives would lead to short-term increases of ' particulate matter (PM~o) and gaseous pollutants (NO„ CO, SOz and VOCs) during construction. These emissions would temporarily elevate pollutant concentrations in the immediate vicinity of the construction activities. During operation, emissions of NO„ SO,, PM~o, CO and VOCs ' would increase ambient concentrations in the Project Area. ' 4.6 NOISE 4.6.1 Introduction ' The noise impact assessment estimates ambient noise levels resulting from construction activities, drilling, and operation of processing facilities. The assessment is performed by adding ' expected noise levels from these activities to existing background levels. Ambient noise levels can be expressed using the La„ scale (National Academy of Sciences 1977). The Ld„ scale is a logarithmic average of daytime and nighttime decibel levels with a 10 decibel penalty applied to nighttime levels. This penalty reflects the fact that nighttime noise levels aze more noticeable than daytime sounds. ' The EPA (1974) has identified an Ld„ level of 55 decibels [dB(A)] as the maximum sound level that would not adversely impact public health and welfaze (Table 4.6-1). For the purposes of this assessment, a level of 5~ dB(A) is used as the criteria for a significant adverse impact. ' Noise Sources ' The equipment and process facilities used for the proposed Yankee Gulch Project would generate some noise above the background levels at both the Piceance Site and the Parachute Site. Equipment associated with the operation of the Yankee Gulch Project that would be the primary ' noise sources includes: • project-related vehicle trafric (cazs, pickups, and heavy-duty trucks) ' boilers • electricity generators ' • fans • pumps ' Meteorology and Air Quality 4-33