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Request for Site Closure Page 5 Chevron L.S.A. Products Company,3246 East Fremont Street, Stockton, CA January 3, 1995 F hydrocarbon-impacted groundwater is not included in the estimate of hydrocarbon-impacted material to be left in place. Lack of Significant Leachate Potential As mentioned in the previous section, hydrocarbon-impacted groundwater is currently not present beneath the site. Hydrocarbon-impacted groundwater identified in a perched zone sampled from MWs-1, 2 and 3, and EA-1 through 4 has not impacted groundwater sampled from wells screened in the deeper water bearing zone. Hydrocarbons dissolved in the perched water zone have been sorbed and/or biologically degraded prior to reaching the deeper water bearing zone. These data indicate that the mass of hydrocarbons remaining at the site will not impact groundwater at its current elevation. Samples collected from domestic wells nearby the site support the lack of leaching potential of hydrocarbons remaining in place as does the lack of hydrocarbons in deeper monitoring wells currently on site. General models used to predict the leachability of hydrocarbons from leaking USTs exist in the LUFT manual (CRWQCB, 1990), however model results are rarely accepted by the CRWQCBS as sound rationale for setting acceptable concentration levels for hydrocarbons left in place. More current methods used to estimate acceptable concentrations of remaining hydrocarbons include the Designated Level Methodology (CRWQCB-CVR, 1989). General methodology based on "average" site conditions suggests the use of a 100-fold environmental attenuation factor to set appropriate concentration levels for hydrocarbons to be left in place. Based on site lithologic and hydrologic data it appears that the site meets the conditions of "average", considering the character and thickness of the vadose zone separating hydrocarbon- impacted soil from current groundwater levels. Using the State MCL for benzene in drinking water (1 ppb) as the water quality goal, total designated levels for benzene at the site would be 100 ppb or 0.1 ppm. That is, hydrocarbons remaining in place must be below 0.1 ppm for benzene. Based on this analysis, only one sample (pit sample #218) of hydrocarbons to remain in place exceeded the 0.1 ppm limit. However, samples collected from SB-1 drilled in 1991 after the pit was backfilled, indicate that hydrocarbons in the vicinity of sample #218 have degraded and/or volatilized to concentrations below laboratory detection limits from being exposed to the atmosphere for nearly a year in 1987, during which time the former UST cavity remained open. Therefore, concentrations above the calculated total designated level near sample #218 are currently lower than those observed during the original sampling event in 1987. - 0072dos.rpt ❑❑® GROUNDWATER ❑❑❑ TECHNOLOGY