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Draft Supplemental Environmental Impact Report Page IV.0-8 Forward Inc. Landfill 2014 Expansion Project Impacts Impact significance under CEQA is evaluated relative to the existing environment. For the purposes of this analysis,the"existing noise environment"is the current noise environment,which includes the 6w existing operational conditions at the landfill(i.e.,212 trucks per day). As discussed above, this section includes a summary of impacts and mitigations considered in the 2013 EIR, and has been updated to include impacts and mitigations that are new or have been substantially altered by changes in the proposed project. To facilitate review of the section and comparison of analyses between the 2013 EIR and this document, the heading for each impact or mitigation measure lr reflects whether that impact is the same, revised, replaced, or new. Standards of Significance CEQA Guidelines identify significant impacts as those that cause standards to be exceeded where they are currently met. An impact is also considered significant if it"measurably" worsens an existing unacceptable noise environment. Impacts may also be significant if they "substantially"worsen noise exposures even if adopted standards are not exceeded. Measurable is not defined in any guidelines. Under ambient conditions, most people cannot distinguish a change in the noise environment that differs by less than 3 dBA between the pre-and post-project exposure. A clearly perceptible increase in noise level differences occurs around +5 dBA. The operational noise impact studies prepared for the 1993, 1994, 2000, and 2002 EIRs for the Forward and Austin Road landfills adopted a+5 dBA increase as a significant noise impact. Subsequent to those Eft studies, ambient levels in the area have increased. For the purposes of this analysis, the noise impact from any new landfill traffic would be potentially significant if it would increase exterior noise levels by at least 5 dBA or more at any of the nearby residential land uses that are currently at or below the County's maximum allowable Ldn noise exposure level of 65 dBA for transportation sources (see Table IV.0-2). As discussed in the 2013 EIR, the existing elevated traffic noise levels related to landfill and other �- existing truck traffic in the area already exceed the County's maximum allowable exposure level of 65 dBA Ldn along several road segments in the project areas. Like the 2013 EIR, this SEIR applies an adjustable threshold based on absolute noise level. When existing exterior noise levels are more than ,. 65 dBA Ldn at a residential receptor, the threshold would be an increase of 3 dBA. This adjustable threshold is a methodology based on a federal noise study of neighborhood annoyance related to airports that has been used extensively in California in City and County Noise Elements for assessing the effects of highway transportation noise levels (FICON, 1992). Impact C.1. Construction Noise Impacts (Same as 2013 EIR Impact C.1.) .. Temporary construction noise would result during site preparation activities for the project, such as the proposed creek relocation and preparation of the proposed expansion areas. Noise levels associated with such activities would be short-term and would be similar to the equipment noise levels that are ` already occurring at the landfill. Such activities would not substantially affect existing noise levels in the vicinity of the project and would have no effect on the long-term noise exposure in the project vicinity. The revised project includes the development of expansion areas within the permitted landfill boundary,but does not include the previously proposed horizontal expansion onto the adjoining 184- acre Brocchini property. Eliminating the large 184-acre expansion area would reduce construction .. noise impacts by eliminating construction activities on this property compared with the 2013 project. Construction noise impacts of the revised project would remain less than significant.