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Phase II Environmental Site Assessment Report Yosemite Senior Housing 309 W.Yosemite Avenue and 105 Sycamore Avenue APNs 217-210-100 and 217-210-110 Page 4 4.0 LABORATORY ANALYTICAL RESULTS AND DISCUSSION 4.1 SOIL GAS 4.1.1 Risk Assessment The USEPA and DTSC do not have published risk-based screening levels assigned to soil gas media; therefore, detected soil gas concentrations were compared with attenuated DTSC-SL or USEPA RSLs indoor air screening levels.DTSC has provided guidance on converting DTSC-SLs for VOCs in indoor air to soil vapor screening levels by application of a default attenuation factor.Attenuation factors are the ratio between the respective VOC chemical of concern(COC) concentration in indoor air and soil vapor based on the equation below: AF=CIA/Csv Where AF=Attenuation Factor; CIA=COC concentrations in indoor air; and Csv=COC concentrations in soil vapor By reworking the default attenuation factor ratio, screening levels for detected VOC concentrations can be calculated by dividing the default attenuation factor into the indoor air screening level(Csv=CIA/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 vapor. 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 clean up 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. 4.1.2 Analytical Results Soil gas analytical results compared to their respective attenuated indoor air screening levels are included on Table A, Appendix E. None of the VOC COCs in soil gas detected above their respective reporting limits exceeded their respective attenuated indoor air screening levels, except for PCE in three (3) of the collected samples and in two locations.PCE was detected in all collected samples.PCE was detected at 45 micrograms per cubic meter (µg/m3) at ASG-1-5.5; 6.5 µg/m3 at ASG-1-15; 6.5 µg/m3 at ASG-2-5.5; 11 µg/m3 at ASG-2-15; 41 µg/m3 at ASG-3-5.5; and 16 µg/m3 at ASG-3-15. It should be noted that the leak detection compound was detected at relatively low levels in three of the four soil gas samples collected, which could indicate minor dilution of the collected soil gas samples;the reported results of VOCs in those samples could be biased low.The DTSC-SL for indoor air for PCE with the applied default 0.03 attenuation factor is 15 µg/m3. Detected PCE concentrations compared to the attenuated indoor air screening level are included in Table 1,below. Nkiw CO CONDOR