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09/02 '00 NED 15 46 FAX 10258409968 ERMWEST WC 019 site-specific screening levels to determine the significance of residual constituent concentrations. Consistent with the ASTM(methodology,development of site-specific, risk-based screezumg levels for the subject properties entailed three malar steps,as follows: Identification of constituents of potential concern in each medium(i e., soil and ground grater) Identification of realistic exposure pathways associated with each medium. Development of risk-based screening levels for each medium,using appropriate exposure assumptions and current toxicity information from California EPA and USEPA; development of these screening levels was based.on the identification of realistic exposure pathways defined in the preceding step. Each of these steps is described in the following sections IF.E TdFI ATION OF CONSMUENTS OF PO'TEN77AL CONCERN Underground storage tanks (USTs) formerly used for gasoline storage were removed from the subject properties Following removal of the UST's,impacted soils were excavated and remediated, and shallow ground water was extracted and treated Details of these activilaes are described in the Closure ,R rf, 1325 and 1515 West Weber-Avenue, Stockton, Cal7fvrma (herein after Closure Report;ERK 1999) In spite of the rernedial activities conducted at the subject properties,some residual constituents remain in subsurface so;Is and ground water Specific constituents found in post-remediation samples are listed below by medium. * Subsurface soils benzene, toluene,efhylbenzene,and xylenes.' * Ground water: benzene,toluene, ethylbenzene, xylenes,methyl-tert- butyl ether(MFBE),2-methyl-2-propanol(TBA),I,2-dichloroethane ' Identification of these compounds was based on the results of the excavabun confirmation sampling and post-excavation sampling described in the CI au= 10 Report(ERM,1999)