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Human Health Risk Assessment Former Mobil Oil Bulk Plant 04-343 500 East Grant Line Road Tracy, California i 7.0 CONCLUSIONS AND REC0MNMNDATIONS ' Alton Geoscience was contracted by Mobil Oil Corporation to conduct a human health risk assessment for residual petroleum hydrocarbons at former Mobil Oil Bulk Plant 04-343, located at 500 East Grant Line Road, Tracy, California. In August 1988, Mobil Oil Corporation abandoned the former bulk plant. All structures were removed from th,? property including one underground gasoline storage tank and four aboveground diesel sa..-age tanks. Following tremoval of the gasoline tank, analysis of soil and ground water samples indicated the presence of TPH and BTEX (Alton Geoscience, 1993a). The results of site characterization activities conducted prior to July 1990 were used to determine the lateral and vertical limits of hydrocarbon-affected soil beneath th. site. To the maximum extent possible, the excavation removed as much of the impacted soil as A,as accessible or known to exist. Activities conducted at the site subsequent to the soil excava:c,)n suggest the presence ' of adsorbed-phase hydrocarbons within the capillary fringe. The adso,bed-phase hydrocarbons appear to be comprised of a mixture of gasoline- and diesel-range h}rVocarbons. A health risk assessment involving the quantitative evaluation of -irks posed by residual petroleum hydrocarbons (specifically benzene, toluene, ethylbenz-"e, and xylenes) was ' conducted. The purpose of the health risk assessment was to evaluarc , based on the available empirical data, the potential risk to human health posed by residual p�iroleum hydrocarbons in ' the subsurface. The objective of the risk assessment was to provide � uidance on whether the presence of residual petroleum hydrocarbons pose a significant health risk in consideration of ' current and potential future site uses. The characterization of potential human health impacts is intended to provide risk management ' information necessary to determine if mitigation of potential human heath impacts is warranted. The risk assessment conforms with the methodologies set forth by the National Research ' Council, the United States Environmental Protection Agency, and the Caiifornia Environmental Protection Agency (NRC, 1983; EPA, 1986; DHS, 1986). Guidance 6icuments utilized for this tevaluation included, but were not limited to: the Califomia Site 14itigation Decision Tree Manual (DHS, 1.986); the Exposure Factors Handbook (EPA, 1989a) and the Risk Assessment Guidance for Simerfimrl (FPA 14Rom The n, vn +;+�+;.>�, + c u y_ r--- � /. Ka+untauaLir� rL.,ju1W v1 l+lG 11siC aa$eStiillCllt WCI-e by 30-OI36-f 1 ' 7-1 1