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i <br />L <br />CRAPTERTHREE Affected Environment <br />` Table 3.5-3 presents the annual mean and second-highest short-term concentrations measured at <br />Occidental Oil Shale Tract Cb monitoring station (CDPHE 1996). The NAAQS and CAAQS aze <br />shown in Table 3.5-2 for comparison. The short-term (24-hour or less) NAAQS and CAAQS are <br />not to be exceeded more than once a year. Representative quarterly air quality monitoring data <br />from the Unocal Parachute Creek shale oil program for November 1990 through October 1991 <br />are presented in Table 3.5-4. <br />Current visibility is excellent, and the visual range is estimated to exceed 50 to 100 miles <br />(IMPROVE 1996). <br />3.6 NOISE <br />3.6.1 Regional Overview <br />The National Academy of Sciences (NAS) and the EPA have developed expected noise levels <br />' based upon population density as shown in Table 3.6-1 and Figure 3.6-1. The populations of Rio <br />Blanco and Garfield counties, in 1996, were 6,964 and 36,832 people, respectively (Colorado <br />Department of Local Affairs 1997). Based on these population levels, ambient noise in most <br />areas is expected to range from 35 to 40 decibels (dB). However, noise levels in the more <br />urbanized areas, such as towns, would be higher. <br />t Decibels are the unit of measure used to represent sound pressure levels, and dB(A) is the unit of <br />measure used to represent sound pressure levels using the A-Weighted Scale. The A-Weighted <br />decibel measure is used to evaluate ambient noise levels and common noise sources. It is a <br />measure designed to emphasize frequencies perceived by the human ear. <br />Ambient noise levels can also be expressed using the Ldn scale (NAS ] 977). The Ldn scale is a <br />logarithmic average of daytime and nighttime decibel levels with a ten dB penalty applied to <br />nighttime levels. This penalty reflects the fact that nighttime noise levels aze more noticeable <br />than daytime sounds. <br />3.6.2 Noise in the Project Area <br />The Project Area is located in an undeveloped, rural area that is relatively undisturbed by human <br />activities. At present, the Project Area is used primarily by livestock and wildlife. Typically, <br />noise level values for rural areas aze 30 to 50 A-weighted decibels [dB(A)] with an Lso value of <br />37 (Cuniff 1977). The Lso value is the median noise level that is exceeded 50 percent of the <br />time. Noise monitoring conducted neaz the Piceance Site revealed that noise levels vary between <br />35 and 45 dB(A), depending on wind speed (BLM 1982). For the sake of comparison, the noise <br />levels required to adequately protect human health and welfaze are shown in Table 3.6-2. <br />Based on noise monitoring conducted by the USGS and BLM, noise levels may vary from the <br />Piceance Site, along the pipeline, and the Parachute Site. The man-made noise source closest to <br />the Piceance Site is Piceance Creek Road (Rio Blanco County Road 5), located about one-half <br />mile from the eastern edge of Piceance Site. The proposed pipeline corridor also crosses <br />Piceance Creek Road twice between the Piceance Site and the Pazachute Site. Noise levels <br />` associated with traffic on this road have been measured at or below 53 dB(A) 90 percent of the <br />Meteorology and Air Quality 3-25 <br />