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Additional Site Assessment Report and Human Health Risk Screening Evaluation www.arcadis.com 10 hazards for each scenario evaluated were less than the threshold HI of 1, and therefore indicated an acceptable noncancer health hazard for exposures to on-site soils. SAIC (2012) concluded that “The results of the HHSE addendum indicate that exposure to chemicals present in soil at the Site should not result in unacceptable health risks for an adult or child on-site resident, or for construction workers. Therefore, no remedial action was necessary at the Site to protect human health, and a risk management decision of no further action could be made for on-site affected soils.” 6.3 2019 Leidos Human Health Risk Screening Evaluation Summary Leidos evaluated health risks to potential future residents, and to construction workers who may be exposed to soils during site construction activities. The Leidos evaluation showed that the estimated health risk related for both future resident or future construction worker were equal to or less than the DTSC target. The HI for TPH in vapor exceeded 1 for residential receptors within the neighboring rail ROW. Leidos stated that TPH vapor concentrations would be expected to attenuate away from the boring location, and residential land use within the rail ROW is unlikely. Leidos (2019) concluded that “Based on the results of this investigation and risk assessment, Leidos and CEMC are of the opinion that additional work is not required, and recommend preparing a No Further Action Required report for the Site as the next step toward closure.” 6.4 2022 Arcadis Human Health Risk Screening Evaluation The soil analytical results collected in 2019, 2020, and 2022 were compared to the LTCP (SWRCB 2012) residential screening levels and the SFBRWQCB residential soil ESLs (SFBRWQCB 2019). The residential soil ESLs were developed using human health risk assessment methodology. However, instead of solving for an estimated health risk with a known constituent of potential concern (COPC) concentration, the algebraic equation is rearranged to solve for a concentration with a known health risk. The methodology used to generate the ESLs is similar to the methodology used in the three previous HHREs, but as stated, the ESL equations solve for a concentration instead for a health risk. The selected cancer risk used in the ESL development is 1 x 10-6 and the selected HI is 1. Accordingly, ESLs represent concentrations equal to or less than the regulatory targets for health-based closures. The site-specific screening results are presented in Tables 1 and 2. Based on the LTCP (SWRCB 2012) residential screening levels and SFBRWQCB residential soil ESLs (SFBRWQCB 2019), the residual concentrations in soil are less than the applicable screening level for each COPC analyzed, except for TPH-d. However, the TPH-d results with the exceedances are in deeper soils (greater than 4 feet below grade) and are likely not available for direct contact. The depth of the soil with the exceedances is a mitigating factor. The soil evaluation is consistent with the three previous HHREs, both residential and construction worker receptors are not at unacceptable health risk associated with soil exposure. The COPC concentrations in soil vapor are less than their respective screening criteria, except for TPH-g in B-13 collected in January 2020. The soil vapor SFBRWQCB ESLs (SFBRWQCB 2019) were generated using the previously accepted default attenuation of 0.03. However, in March 2022, the DTSC Note 4 (DTSC 2022) was revised and removed the reference to the 0.03 attenuation factor. Now, the DTSC recommends using a site- specific attenuation factor and published a draft document recommending a 0.005 attenuation factor (DTSC