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3 <br /> Human Health Risk Assessment <br /> Former Mobil Oil Bulk Plant 04-343 <br /> 500 East Grant Line Road <br /> - Tracy, California <br /> classified both as diesel fuel and gasoline, and BTEX have been detem.(_1 between 9.5 to 12 feet <br /> below grade in soil borings AW-9 (B-10), AW-10, AW-11, and ASN-12. The depth of this <br />' area mainly represents the capillary fringe zone. Plausible routes of human exposure include <br /> direct human contact with the hydrocarbon-affected soil or ground ,nater, the ingestion of <br /> drinking water from the near-surface aquifer, and inhalation of vapors from the subsurface. <br />' However, since the site has been excavated to 10 feet below grade and replaced with fill <br /> material, dermal exposure to hydrocarbon-affected soil in highly unlikely. <br /> In addition the round water has been demonstrated to not meet drinking level quality for <br /> g g q Y <br /> inorganic chemicals and, therefore, is unlikely to be Used for domestic purposes. The most <br /> plausible route of exposure involves the volatilization and upward trd.nsport of gasoline and <br /> diesel fuel vapors from the residual hydrocarbon-affected soil within *he capillary fringe zone <br />' into a hypothetical commercial building located on top of the site. Hydrocarbon vapors could <br /> potentially be present in such a building. Because this exposun: pathway is potentially <br />' significant, it is evaluated in this risk assessment. Potential current an ature human exposures <br /> to vapors from petroleum hydrocarbons in subsurface soil are described in the following sections <br /> and quantified in Appendix D. A summary of exposure pathways is presented in Table 5-1. <br /> 5.3 SELECTION OF EXPOSURE MODELS AND PARA'L'IETER VALUES <br /> Several sources of information have been consulted to aid in the selec,ion of exposure models <br />' and parameter values. These include, but are not limited to 1) Risk Assessment Guidance for <br /> Superfund(EPA, 1989b), 2) EPA Exposure Factors Handbook (EPA, 1989a), 3) California Site <br />' Mitigation Decision Tree Manual (DHS, 1986), and where appropriate, 4) the peer-reviewed <br /> scientific literature. <br /> 1 <br /> I <br /> 16 30-0136-11 <br /> F 5-3 <br /> 1 <br />