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l P A G E 2 •R . { site-specific screening levels to determine the significance of residual constituent concentrations. Consistent with the ASTM methodology, development of,site-specific, risk-based screening levels for the subject properties entailed three major steps, as follows: j • Identification of constituents of potential concern in each medium (i.e., soil and ground water). • 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. ;s Each of these steps is described in the following sections. 11 Ij IDENTIFICATION OF CONSTITUENTS OF POTENTIAL CONCERN Underground storage tanks (USTs) formerly used for gasoline storage were removed from the subject properties. Following removal}of the USTs, impacted soils were excavated and remediated, and shallow ground water was extracted and treated. Details of these activities are described in the Closure Report, 1325 and 1515 West Weber Avenue, Stockton; California (herein after Closure Report; ERM, 1999). In spite of the remedial activities conducted at the subject.;properties, some residual constituents remain in subsurface soils and ground water. 11 Specific constituents found in post-remediation samples are listed below by medium: • Subsurface soils: benzene, toluene, ethylbenzene, and�xylenes.1 • Ground water: benzene, toluene, ethylbenzene, xylenes; methyl-tert- butyl ether (MTBE); 2-methyl-2-propanol (TBA); 1,2-diehloroethane i! I i Identification of these compounds was based on the results of1 the excavation confirmation sampling and post-excavation sampling described in the CIosure Report(ERM,1999). I