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III. Environmental Setting, P g Impacts, and Mitigations G. Air Quality Screening-level health risk assessments for landfill gas emissions have been calculated for several recent new landfill sitings in California. They generally employ standard conservative methods that are designed to overestimate the potential for health risk from a proposed facility. Often,worst-case meteorological conditions and standard models are used to simplify the screening-level analysis,which is generally considered to present an upper bound to the level of health risk posed by the project. The screening-level health risk assessment result determines whether further analysis is required. These risk assessments,done under the AB 2588 guidelines, use a health risk standard of ten additional cancer cases in one million(10 x 10-6). A screening-level risk assessment prepared for the proposed Keller Canyon Landfill in Contra Costa County indicated a lifetime cancer risk of eight in one million(8 x 10-6)as being the highest risk for any residence within 2,000 feet of the landfill. This estimate was based on: the existing topography and meteorology of the area around the proposed landfill,50 million cubic yards of refuse capacity for the landfill,the nearest residence being located about 2,000 feet from the waste placement area, and a landfill gas collection system that was 75 percent efficient in collection and 90 percent efficient in destruction of toxic air contaminants (Contra Costa County, 1990a). A screening-level risk assessment was also prepared for the proposed Marsh Canyon Sanitary Landfill in Contra Costa County. The calculated lifetime cancer risk of this proposed landfill was 1.5 in one million(1.5 x 10-6). This estimate was based on: the existing topography and meteorology of the area around the proposed landfill, 100 million cubic yards of refuse capacity for the landfill,the nearest residence being located 3,000 feet from the waste placement area and 180 residential units located within 3,600 feet,and a landfill gas collection system that was 75 percent efficient in collection and 90 percent efficient in destruction of toxic air contaminants (Contra Costa County, 1990b). The Bay Area Air Quality Management District determined that the gas emissions of the Keller Canyon Landfill and the Marsh Canyon Landfill would not represent a significant impact to air quality or public health. Both of these landfill are Class II landfills with a higher waste disposal volume than the project(15 million cubic yards). The SWAT results did not indicate the presence of any chemical above the recommended level. On the basis of these examples of screening-level health risk assessments(Class II landfills not having,g significant health effect),and the comparative size of the proposed project,toxic air IH.G.15 i