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Phase II Environmental Site Assessment Report Satellite Apartments 530 N.Stanislaus Street APN 139-224-090 Page 4 screening levels using an attenuation factor. Attenuation factors are the ratio between the respective VOC chemical of concern(COC)concentration in indoor air and soil gas based on the equation below: AF=CiA/Csv Where AF=Attenuation Factor; GA=COC concentrations in indoor air; and Csv=COC concentrations in soil vapor By reworking the default attenuation factor ratio, soil vapor screening levels for detected VOC concentrations can be calculated by dividing the default attenuation factor into the indoor air screening level (Csv=GA/AF). According to the Department of Toxic Substances Control and California State Water Resources Control Board, Supplemental Guidance: Screening and Evaluating Vapor Intrusion (Final Draft) dated February 2023,the recommended default attenuation factor is 0.03 for soil gas.However,attenuated screening levels for VOCs in soil gas are not formal cleanup goals; they are conservative guidelines intended to be used to determine if further assessment or cleanup may be warranted for a given property.Actual attenuation factors may be greater than the default depending on the local lithology, ambient temperatures, building design, and other factors. COCs detected at the Site above their calculated soil vapor screening levels include benzene(13 µg/m3 in ASG-1) and chloroform(7.2 µg/m3 in Sub-2). Benzene,toluene, ethylbenzene and xylene(BTEX)are VOCs associated with petroleum products. BTEX was detected in ASG-1 and ASG-2 within the parking lot,with the highest concentrations detected closest to Grant Street and the eastern adjoining automotive repair complex. There are no known point sources of petroleum and petroleum-based waste product releases at the Site. BTEX concentrations significantly decrease towards the west, away from the automotive repair complex; it is likely the that automotive complex is the source of BTEX contamination in soil gas. For this reason, Condor compared the benzene concentrations to the State Water Resources Control Board's (State Water Board's) Low-Threat Underground Storage Tank Case Closure Policy's(LTCP,2012)no bioattenuation zone soil gas criteria for future construction to evaluate the human health risk associated with benzene for future occupants at the Site. Assuming no bioattenuation is applicable to Site conditions (the more conservative assumption), the soil vapor screening level for benzene is 85 µg/m3 for residential use,which exceeds the maximum benzene concentration reported during this investigation(13 µg/m3). Chloroform was detected in sub-slab sample Sub-2 at 7.2 4g/m3,which is slightly above the calculated soil vapor screening level of 4.0 µg/m3. The most likely source of chloroform is disinfected tap water. Public water systems such as sewers and plumbing may leak disinfected city water, and the associated volatiles concentrate beneath dwellings and foundation slabs. It is Condor's opinion that it is unlikely that this chloroform concentration poses a significant risk to current Site occupants in indoor air. The default DTSC attenuation factor of 0.03 is likely overly conservative for risk evaluations in the State of California, since it is derived from studies conducted in areas where the majority of construction design includes basements and variable climate conditions. Previously published attenuation factors of 0.001 were superseded by the updated default attenuation factor but are likely more appropriate for Site conditions.The DTSC is currently conducting studies in the State of California to obtain a more appropriate attenuation factor,and as a result, it is likely the default attenuation factor of 0.03 will be updated in the future for California-specific risk evaluations.For a greater understanding of the Site-specific risk associated with chloroform,indoor air and ambient air sampling could be conducted to evaluate actual chloroform concentrations in indoor air. i,i CONDOR