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Irwestone Store x/3573 400 North El Dorado Street, Stockton, California October 25, 1995 VES Quarterly Report and Request for Closure-4589 Page 12 of 14 r The calculated remaining concentrations of TPH-G and benzene in soil are 1 .3-mg/kg and 0.12-ug/kg, respectively These levels are far below the most stringent California LOFT Maximum Allowable Levels for TPH-G and benzene in soil of 10-mg/kg and 300-ug/kg, respectively. It appears extremely unlikely that the reduced level of hydrocarbons remaining after SVES operation will migrate This is supported by the fact that observable hydrocarbon impacts did not migrate downward beyond the depth of 28-feet BG during the fifteen years (estimated) following the release In addition, relatively impermeable asphalt, concrete, and/or budding structures cover the entire surrounding surface area. This surface cover will maximize surface water runoff and therefore minimize the potential for hydrocarbon transport based on surface water infiltration. It is also reasonable to expect that the minor hydrocarbon levels remaining in the soil will be reduced by natural biodegradation over time. Based on the slight decrease of TPH-G levels following the one month shutdown period, this may be occurring already. Groundwater has not been encountered at this site to a total investigation depth of 41 5-feet BG 5.0 RECOMMENDATIONS In consideration of the diminished TPH-G levels and the consistently low to nondetect benzene levels observed in the soil vapor, the asymptotic state of system operation, the percentage of hydrocarbon mass removed, and the low remaining concentrations of both TPH-G and benzene with respect to LUFT guidelines, RMI firmly believes that there is sufficient evidence of satisfactory subsurface remediation, and recommends that no further operation of the SVES be required. RMI believes that additional verification drilling investigation is unnecessary based upon a drilling investigation performed prior to SVES operation that yielded entirely nondetect TPH-G results and trace to nondetect BTEX results (The findings of that investigation can be reviewed in the Site Assessment Report submitted to San Joaquin County on February 2, 1994.) In consideration of that drilling investigation, followed by six months of SVES operation, RMI believes that no further action is necessary. 11LYAN4WIfR1W, INCORPORAIM Demwer 1WVijnameatal Ceastsaction Mamgemeat ImAagda