SITE INFORMATION AND CORRESPONDENCE

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r contact with longer chain, immobile residual petroleum constituents adsorbed to soils to meet the commonly accepted 5 ppb taste and odor threshold. Nonetheless, during this time these residual concentrations in excess of Basin Plan objectives will not pose a threat to current or future beneficial uses. It is highly unlikely that petroleum hydrocarbon constituents detected in localized areas in the immediate area i of the release will migrate substantially beyond current limited spatial extent. Though the longer chain hydrocarbons comprising weathered TPH-g biodegrade more slowly than certain petroleum constituents, such as benzene, they are also more recalcitrant (i.e., less volatile, Iess soluble and highly absorbent) and much less mobile. It is also highly unlikely that this particular very limited pocket of shallow groundwater will be used directly as a i source of drinking water. Thus, the significant period of time that it will take for water quality in this limited area to meet all Basin Plan objectives is a reasonable time frame. Closure of the site, given the facts in this particular case, is appropriate. 1 SUMMARY AND CONCLUSION I 1. There is no evidence of a significant release of MTBE at this site. Corrective j actions including soil vapor extraction and removal of an estimated 850 cubic yards of i contaminated soil have removed a significant mass of residual petroleum hydrocarbons at ' UNOCAL's site. ; 2. The plume of dissolved phase hydrocarbons is stable, concentrations of constituents are declining, and naturally occurring biodegradation of residual petroleum hydrocarbons adsorbed to soil and present in groundwater is occurring. 3. Ten years after the release was stopped, groundwater meets Basin Plan objectives with respect to BTEX constituents within about 70 feet of the site; with respect to TPH-g, groundwater meets Basin Plan objectives within 150 feet of the site. f 4. UNOCAL's site is located in a commercial/residential area. No water supply wells are located within 800 feet and the nearest surface water is about 2,000 feet from the site. 5. Current water supply wells in the vicinity of the site are constructed with sanitary seals which extend to depths of about 100 to 200 feet below ground surface for the purpose of excluding poor quality shallow groundwater. Any future supply wells which may be constructed in the vicinity of the site will in all likelihood be similarly constructed. 6. Additional soil and water remediation at UNOCAL's site is not necessary. 7. The level of site cleanup is consistent with the maximum benefit to the people of the state. 8. Detectable concentrations of BTEX in shallow groundwater in contact with the limited residual petroleum hydrocarbons adsorbed to soil particles may remain above MCLs and thus violate the Basin Plan objectives in a very localized, small volume of I surrounding groundwater for decades. 9. Detectable concentrations of TPH-g in shallow groundwater in contact with the + limited residual petroleum hydrocarbons adsorbed to soil particles will likely remain above 5 ppb (the commonly accepted odor threshold for drinking water) and thus violate the Basin Plan's narrative odor objective in a very localized, small volume of surrounding groundwater for anywhere from decades to hundreds of years.