SITE INFORMATION AND CORRESPONDENCE

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Ms. Mary F. Ways January 18, 1995 The Southland Corporation-9110 Thornton Rd., Stockton, CA 3 Adsorbed hydrocarbons Adsorbed hydrocarbons in the vadose zone typically represent the largest mass of hydrocarbons in the subsurface at sites with relatively deep groundwater. Southland conducted an extensive soil remediation by vapor extraction in the (predominantly sandy) soil at the site, using four separate extraction blowers, with extraction rates of about 80 cfm per well. Although PHS/EHD has questioned the reported radius of influence (100 feet), the pressure response data clearly indicate that during extraction from central well MW-1, all monitored wells within 80 feet responded with more than 0.1 inch of water vacuum, which reasonably agrees with the recorded lithology of the site. After termination of the vadose zone remediation operations, the extent of the remaining adsorbed hydrocarbon phase at the site was assessed under a County PHS/EHD-approved work plan, conducted in the field under PHS/EHD staff supervision. During drilling of confirmation borings, field observations indicated possible presence of hydrocarbons in borings C,D,E,F, and J, however the reported field measurements (by PID) were low, with maximum 71 ppmv recorded in the field, and analysis of samples from the zones of suspected hydrocarbons indicated non-detectable hydrocarbon concentrations. The samples collected at different horizons throughout the site, contained no detectable benzene or TPH-G concentrations, indicating that adsorbed hydrocarbons at the site have been remediated. Vapor phase hydrocarbons During the Initial phase of vadose zone remediation, vapor hydrocarbon concentrations in the extraction wells ranged up to 4,800 mg/m3. After vapor extraction remediation, 18 vapor samples were collected from 16 wells at the site. None of the samples analyzed contained benzene, toluene or ethylbenzene. Three of the 18 samples contained xylenes (at under 3 mg/m3), and all samples contained under 75 mg/m3 TPH-G. The County PHS/EHD has expressed concerns over these TPH-G concentrations, as possible evidence of significant remaining hydrocarbons. The review of the data (attached as Table 1) indicate that most air samples from the wells contain approximately 30 mg/m3 of TPH-G. As this value is relatively uniformly distributed across the site, it is unlikely that this value is due to a localized zone of high hydrocarbon concentrations, and more likely represents residual hydrocarbons. The estimated mass of these non-BTEX hydrocarbons, if contained in a 30% pore space soil conservative volume of 100 ft x 100 ft x 40 ft thick, at a concentration of TPH of 75 mg/m3, is about 0.5 lbs. As this mass is small, and benzene was not measured in the wells tested, the risks associated with this mass are low, and these hydrocarbons should not be regarded as significant. STHL5TO.T%T(5THL-6) ❑❑� GROUNDWATER EILILIJ TECHNOLOGY .