SITE INFORMATION AND CORRESPONDENCE

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Sent Sy,: BASELINE; 510 420 1707; Nov-9-99 16:31 ; Page 24129 *4.0 %MW groundwater may be used for domestic: purposes was 76 feet bgs, with the majority of the wells screened at much deeper intervals (Appendix F). Residual concentration of volatile organics present in sail and groundwater underlying the site and potentially from off-site sources migrating onto the site (e.g., MTBE and other organic and chlorinated compounds at the former ARCO station to the west) could intermittently volatilize into ambient air through cracks in the pavement and unpaved areas of the site, a potentially complete exposure pathway. Exposure to particulates in air is also assumed to be a potentially complete exposure pathway for these areus of the site that are not capped. Intermittent volatilization of volatile organics into indoor spaces is another potentially complete exposure pathway with possible construction of a restaurant on the northeastern portion of the site. A conceptual site model is presented in Figure 8A for current conditions, where the site is completely capped and fenced to prevent entry, and Figure 8B, for future development of the site as a "water square"or public meeting place. The conceptual site model includes contaminant sources, transport mechanisms, exposure pathways, and potentially exposed receptors. Potential exposure of site contaminants to both human and ecological receptors were evaluated in the conceptual site model, CHEMICALS OF CONCERN AND EXPOSURE POINT CONCENTRATIONS Soil and groundwater quality data have been collected at the Weber Block property over an approximately six-year period. See the citations referenced above for a listing of all sample dates and analyses requested for thcsc samples. No distinction between sampling data collected by different investigators and at different times was made in the screening assessment. All soil and groundwater data collected at the site were considered in the selection of chemicals of potential concern(COPCs) for the human health screening. However,the results from equipment blanks,as collected during the Phase 11 investigation, were not included in the consideration of selection of chemicals of concern since these samples were collected to establish whether decontamination measures were successful for sampling equipment. Groundwater data from the off-site upgradient location (i.e., WBGW4 and wells from the former ARCO station)were also evaluated in the selection of chemicals of potential concern,at the request of DTSC personnel (Cambridge and Lile, 1999). In addition, compounds where there are uncertainties in the reported concentration, but not in the identification of the chemical were also evaluated for inclusion in the PEA(i.e.,N, L,NJ, LJ, and E-qualified data). The following selection criteria were used in selecting chemicals of potential concern(COPCs) for the Weber Block property: Chemicals were included as COPCs in the PEA if they were positively identified in at least one sample in a given medium above the laboratory reporting limit. Chemicals that were not detected in any medium above laboratory reporting limits were eliminated from further consideration. Laboratory reporting limits were evaluated to ensure that these limits were not elevated due to matrix interference or other laboratory anomalies, and were consistent with 97 364s*.pea.wpd-!