SITE INFORMATION AND CORRESPONDENCE

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GEOIVIATRIX any of the surrounding monitoring wells (upgradient and downgradient), which are located within an 80-foot radius of well MW-1 and are screened at depths coincident with the separate- phase petroleum (Figure 6). Data indicate that the Iow levels of TPHd detected in groundwater samples collected by Radain are due to soluble biogenic material in the groundwater samples. Groundwater samples that were treated with silica gel to remove this material did not contain detectable concentrations of TPHd. These data indicate that the residual petroleum in the subsurface, including the separate-phase petroleum and residual petroleum beneath the water table, is depleted of soluble constituents and is not providing a source of dissolved petroleum constituents to groundwater. Analysis of the separate-phase petroleum by CRTC indicate that the material is a highly weathered San Joaquin Valley Crude; the type of crude oil that was transported through the OVPL. 5.0 SCREENING-LEVEL HEALTH RISK ASSESSMENT Pursuant to the Consistent Technical Approach,the potential risks to human health posed by residual petroleum in site soil were assessed using appropriate land use scenarios. A screening- level HRA was used, as opposed to a more detailed, site-specific quantitative risk assessment, because only a few petroleum constituents have been detected in soil, no petroleum constituents have been detected in groundwater, and the areal extent of residual petroleum is limited. The screening-level HRA for soil consists of a comparing existing analytical data, which include historical data and data presented in this report, to existing risk-based criteria to assess whether the petroleum constituents detected at the site present a potential risk to human health. The screening-level HRA performed for the separate-phase petroleum consists of a qualitative evaluation of potential exposure. 5.1 SCREENING-LEVEL HEALTH RISK ASSESSMENT FOR SOIL As previously mentioned,the site is a former pump station located in a primarily light industrial/agricultural area of Vernalis, California. The potential future use of this property, which is currently vacant, is likely to be industrial. Therefore, this screening-level HRA is based on the assumption that potential exposure to petroleum constituents will occur under an industrial land use. The most likely receptors under such a land use scenario are industrial and/or construction workers. The risk-based criteria selected for the evaluation of soil are the U.S. EPA Region 9 Preliminary Remediation Goals (PRGs; USEPA Region 9, 1998). These risk-based eriteria establish concentrations of chemicals in soil that are unlikely to result in unacceptable risk to %1sf31deptdatalDoc Safe15000s1501 PST HRA Rpt.doc 10