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• Request for Site Closure 4 Chevron U S A Products Company,2400 Airport Way,Stockton,CA January 15,1996 sample collection) The lack of detectable petroleum hydrocarbons in the soil sample collected at 50 feet(near the capillary zone) in boring MW-5 suggests that the former UST area located northwest of the former station building (Immediately upgradient of the well) is not a hydrocarbon source affecting groundwater In addition,samples from near the capillary zone In borings MW-2, MW-3 and MW-4 also show no detectable levels of petroleum hydrocarbons and no detectable volatile organic vapors from PID readings collected during drilling (Attachment 3-Boring Logs) Because the two soil samples showing detectable levels of petroleum hydrocarbons are bracketed by borings not Impacted with hydrocarbons(Figure 3),the lateral and vertical extent of hydrocarbon- impacted soil Is clearly minimal and of very low hydrocarbon concentration, resulting in a very low residual mass of hydrocarbons remaining In place Using assumptions of average concentration and thickness of the hydrocarbon-impacted zone,the residual mass of hydrocarbons in soil is calculated to be one pound (Figure 4) Lack of Significant Leachate Potential Hydrocarbons released from the former USTs and found in low concentrations in soil have not significantly impacted groundwater Four consecutive quarters (one full hydrologic season) of groundwater samples collected from MW-1 and MW-3 through MW-5 have been below MDL for BTEX and TPH-G Samples from MW-2 show decreasing concentrations of benzene over the last four quarterly sampling events,dropping from 1 9 ppb on October 7, 1994 to 0 97 ppb [below the Maximum Contaminant Level (MCL) for benzene in drinking water] In July 27, 1995 The presence of benzene, toluene and xylenes in samples collected from MW-2 on October 7, 1994 is anomalous given the lack of detectable petroleum hydrocarbons in the preceding five quarterly sampling events and in the two subsequent events Two possible explanations are an off-site source, or contamination of groundwater monitoring and sampling equipment Low concentrations of hydrocarbons have been detected in only one soil sample (MW1-45) collected in 1989 from near the soil/groundwater interface, suggesting that the mass of hydrocarbons is small l enough to be completely retarded (sorbed, biodegraded) or prevented from dissolving in groundwater Based on the observation of no dissolved hydrocarbons in groundwater over one full hydrologic season in four of the five monitoring wells on site, and a benzene level below the MCL in MW-2, it can be concluded that mass of hydrocarbons in soil in place on site does not threaten the present and future beneficial uses of the shallow groundwater OlNelo8 rpgChv-56) ❑® GROLINDWAATER ❑❑❑TECHNOLOGY