ARCHIVED REPORTS_REPORT ON SOIL VAPOR SAMPLING AND HUMAN HEALTH RISK ASSESSMENT DEC 2010

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AECOM Report on Soil Vapor Sampling and Human Health Risk Assessment 32 now available. As previously discussed, the October 2010 data were determined to be of sufficient quality for use in the risk assessment, while the May and September 2010 data were not. Therefore,the evaluation of soil vapor data from the October 2010 sampling event is presented in this section. Soil vapor data from all three sampling events at the site (May, September, and October 2010)were also used to evaluate the worst-case potential risk associated with the vapor intrusion inhalation pathway as part of an uncertainty assessment. The methods and results of the revised vapor intrusion risk evaluation are discussed below. 3.1 Step 1: Screening Level / COPC Selection 3.1.1 Methods In the first step of the evaluation, the maximum detected soil vapor concentration of each of the analyzed compounds from the October 2010 sampling event is compared to the CHHSLs for shallow soil vapor(CaIEPA, 2005)and the San Francisco Bay Regional Water Quality Control Board (SFBRWQCB)ESLs (SFBRWQCB, 2008). For compounds that were not detected,the maximum reporting detection limit for each compound was also compared to the CHHSLs and ESI-s.The evaluation of a compound using its maximum detection limit represents a worst-case evaluation.The comparison of soil vapor results to CHHSLs and ESLs is presented above in Section 2.0. CHHSLs and ESLs are oonservative screening levels,for residential and commercial/industrial exposure scenarios, based on conservative modeling inputs. They are designed to be protective of human health, based on a target potential ELCR of 1x10 and a target hazard quotient(HQ) of 1, and may be used to assess the need for further risk evaluation. Soil vapor concentrations below these screening levels do not pose a human health risk of concern(CaIEPA, 2005, SFBRWQCB, 2008)and chemicals present at these levels do not require further risk evaluation. Compounds detected in soil vapor at concentrations greater than these screening values are identified as COPCs for further evaluation. CHHSLs are not currently available for TPHg. AECOM used the methodology described by California EPA's Office of Health Hazard Assessment(OEHHA)(CaIEPA, 2005; DTSC, 2009)to derive a shallow soil gas CHHSL for TPHg. There is no available toxicity data for TPHg, therefore, C9-C16 aromatics were used as a surrogate for evaluating TPH-g, consistent with Massachusetts Department of Environmental Protection (MADEP) guidance (MADEP, 2002). California's DTSC Human and Ecological Risk Division(HERD)toxicity criteria for TPH are based on criteria published by MADEP. Therefore,this approach is consistent with DTSC/HERD'S TPH guidance (DTSC, 2009). The derivation of the CHHSL for TPHg is shown in Table 2. 3.1.2 Results The only detection of compounds in soil vapor during the October event was TPHg in sample SV-1. The detected TPHg concentration was below the derived CHHSLs based on both a commercial/ industrial and residential exposure scenario. The detected TPHg concentration was also below the commercial/industrial ESL. TPHg was detected above the residential ESL. However, the commercial/ industrial screening levels are considered to be more appropriate for this site because it is not currently used for residential purposes, nor is it likely to be in the future. Therefore, TPHg was not selected as a COPC in soil vapor for further evaluation in a risk assessment. December 15,2010 60146576M10