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DRAFT <br /> alternative level of water quality will not result in water quality less than that prescribed <br /> in the relevant Basin Plan. Pursuant to SWRCB Resolution No. 92-49, a site may be <br /> closed if the Basin Plan requirements will be met within a reasonable time frame. <br /> In this particular case, as discussed above, TPH-g and BTEX in the shallow <br /> groundwater in immediate contact with the limited residual petroleum hydrocarbon <br /> constituents adsorbed to soils will likely remain above, and thus violate, the Basin Plan's <br /> objectives in a localized volume of surrounding groundwater for a significant period of <br /> time. This time period could be anywhere from a few decades for BTEX to degrade <br /> below MCLs to hundreds of years for that limited volume of groundwater in immediate <br /> contact with longer chain, immobile residual petroleum constituents adsorbed to soils to <br /> meet the commonly accepted 5 ppb taste and odor threshold. <br /> Nonetheless, during this time these residual concentrations in excess of Basin <br /> Plan objectives will not pose a threat to current or future beneficial uses. It is highly <br /> unlikely that petroleum hydrocarbon constituents detected in localized areas in the <br /> immediate area of the release will migrate substantially beyond current limited spatial <br /> extent. Though the longer chain hydrocarbons comprising weathered TPH-g biodegrade <br /> more slowly than certain petroleum constituents, such as benzene, they are also more <br /> recalcitrant (i.e., less volatile, less soluble and highly absorbent) and much less mobile. It <br /> is also highly unlikely that this particular very limited pocket of shallow groundwater will <br /> be used directly as a source of drinking water. Thus, the significant period of time that it <br /> will take for water quality in this limited area to meet all Basin Plan objectives is a <br /> reasonable time frame. Closure of the site, given the facts in this particular case, is <br /> appropriate. <br /> III. SUMMARY AND CONCLUSION <br /> 1. There is no evidence of MTBE at this site. Corrective actions including soil <br /> vapor extraction and removal of an estimated 850 cubic yards of contaminated soil have <br /> removed a significant mass of residual petroleum hydrocarbons at petitioner's site. <br /> ,7 <br />