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SITE CLOSURE REQUEST REPORT Former Exxon Service Station No.7-3330 • 3128 West Benjamin Bolt Drive Stockton,California Delta Project No.D093-810 Page 15 estimated reduction in TPH as gasoline and benzene in ground water and soil has been greater than 97 percent during the project. The reduction percentage calculations are included in Appendix J. The total volume of liquid-phase petroleum hydrocarbons quantified as gasoline remaining in soil beneath the site is estimated to be approximately 2.5 gallons. A calculated 0.0019 gallons of benzene may remain in soil beneath the site. The estimated amount of dissolved TPH as gasoline in ground water that may remain is less than 0.008 gallons. The estimated amount of dissolved benzene potentially remaining in ground water is negligible (less than 0.00007 gallons). These calculations however, may be high, since confirmation soil samples were collected prior to the ground water pumping test and the soil vapor extraction test and ground water analytical results have been below laboratory detection for four consecutive quarters. If hydrocarbons are still present, they will continue to degrade over time due to the naturally occurring indigenous bacteria in soil. Natural biodegradation modeling is often required to evaluate sites with nearby sensitive receptors. It . is Delta's opinion that detailed modelling is not required for this site since the sensitive receptors discussed earlier in this report do not appear to be at risk due to this distance from the site and the negligable presence of petroleum hydrocarbons in soil is limited on-site as assessed by soil sample analytical results from confirmation borings. Residual concentrations of hydrocarbons quantified as total oil and grease were detected in two samples at the limits of the excavation of the used oil tank. These results reported concentrations of 60 and 77 mg/kg. Initial samples analyzed for TPH as gasoline, TPH as diesel, BTEX, polychlorinated biphenyls, and purgeable aromatics were below the laboratory's limits of detection for all analytes. Detectable metal concentrations were reported, however all values were below their respective total threshold limit concentrations. Ground water analytical results have been below the laboratory's limits of detection in all four existing wells for four consecutive quarters for TPH as gasoline and BTEX. It is anticipated that natural attenuation mechanisms will result in a net decrease in any petroleum hydrocarbon mass (if remaining) under aerobic (oxygen rich) conditions. Delta collected DO data from monitoring wells MW-3 through RPTW 1.810