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GEOMATRiX PRG of 4.64 mg/kg). Sample SQLs from shallower depths at this location were all less than the industrial PRGs. Based on these observations, existing data indicate that concentrations of PAHs are not sufficient to pose an unacceptable risk under an industrial land use scenario. It is also worth noting that the concentrations of all chemicals detected in soil from 0 to 10 feet bgs were less than the Region 9 PBGs for residential land use. The 0- to 10-foot depth usually is considered by Cal-EPA to represent hypothetical surface soil under a residential land use scenario. For chemicals not detected in soil,only benzo(a)pyrene and dibenz(a,h)arithracene had SQLs that exceeded residential PRGs. For benzo(a)pyrene, all of the SQLs(which ranged from 0.08 to 4.56 mg/kg) exceeded the residential PRG of 0.034 mg/kg. For dibenz(a,h)anthracene, 8 of 12 SQLs(which ranged from 0.09 to 0.65 mg/kg) exceeded the residential PRG of 0.099 mg/kg. Given the existing data these PAHs may not be present at sufficient concentrations to pose an unacceptable risk under a residential scenario. However, additional sail samples (0 to 10 feet bgs)would have to be collected and analyzed using detection limits (or SQLs)below residential PRGs for PAHs to verify this possibility. 5.2 RISK EVALUATION OF SEPARATE-PHASE PETROLEUM It is unlikely that the small area of separate-phase petroleum at the site poses an unacceptable risk to human health under an industrial land use scenario. As discussed in Section 4.4, the extent of separate-phase petroleum at the site is quite limited. Therefore, the potential exposure area is relatively small(Figure 5). Because the depth of the separate-phase petroleum is greater than 15 feet bgs, direct contact is improbable. The only mechanism by which people could be exposed to chemicals in this material is for the chemicals to volatilize, migrate vertically upward through the vadose zone, and then be released to the air at the ground surface. Results of the analyses of the separate- phase petroleum by CRTC indicate that the material is composed of a highly weathered crude oil; a material that would not contain significant amounts of volatile compounds. A sample of the separate-phase petroleum collected in 1995 did not contain the volatile chemicals benzene, toluene, ethylbenzene, or xylenes at levels of potential concern (Table 1). Benzene,toluene, and ethylbenzene were not detected, and xylenes were detected at very low concentrations. These are the most volatile components of petroleum hydrocarbons evaluated in risk assessments and, therefore, are the chemicals of greatest potential concern for the volatilization pathway. 11sMdepcdwa\Dac_Safe1500M5QI I\SI HRA Rpt.dot 12