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 3-4 specific information was used in the JE Model. Default input parameters, provided by DTSC (2005) and USEPA(2004),were used where site-specific information was not available.The model inputs are summarized in Table 4 (attached). Some of the key input parameters are also discussed below. The soil vapor exposure point concentrations(EPCS)used in the model are equal to the maximum detected soil vapor concentration of identified COPCs (i.e., benzene, C5-C8 aliphatics, C8-C10 aliphatics, and the TICS). The TICS were grouped into one of the carbon ranges for which CaIEPA(2009) has recommended toxicity values (Le., C5-C8 aliphatics and C9-C18 aliphatics)and then summed by sample date. The summed concentrations for each carbon range are shown in the table below. Tentatively Identified Constituents(TICs) Date C5-C8 Aliphaticsl C9-C18 Aliphaticsl (ppbv) (Ppb°) 5/202010 106,400 11,500 9/92010 126,500 12,400 10/20/2010 4,687 524 rTmnge. he sun of the detected results for TICS within the associated carbon fraction The site soil is comprised of clayey and sandy sift from the surface to approximately 40 to 45 feet bgs with the screens of the vapor wells located in silt to sandy silt at 5 to 6 feet bgs. Therefore, loam was used as the U.S. Soil Classification System (USCS)soil category that best described site soils, based on guidance in Table 11 of the JE Model User's Guide (USEPA, 2004). Soil geotechnical data, including soil dry bulk density, soil total porosity, and soil water-filled porosity were collected in May 2010 for the SV-1 location at 5 to 5.5 feet bgs and 5.5 to 6 feet bgs, and analyzed by Sierra Testing Laboratories. These data are provided in Attachment B of the Report of Soil Vapor Well Installation and Soil Vapor Sampling Activities, dated July 15, 2010 (AECOM,2010a). The depth below grade to bottom of enclosed space floor was set equal to 15 centimeters (cm),the JE Model default representative of a slab-on-grade building construction. The property owner oonfirmed that the building in question is slab-on-grade construction. The most current toxicity information available, as recommended by California EPA's OEHHA Toxicity Criteria Database(CaIEPA, 2009b)and chronic Reference Exposure Levels(RELs) (CaIEPA, 2008),were used to evaluate benzene.To evaluate the carbon ranges, including the TICS, toxicity values were obtained from CaIEPA's TPH Guidance (CaIEPA, 2009). It should be noted that the CaIEPA TPH Guidance has been withdrawn by the agency, however, it is being used in this evaluation for consistency with the previous evaluation. The JE Model spreadsheets are presented in Attachment C. December 15,2010 6014657&M10