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volume of groundwater in immediate contact with longer chain, immobile residual petroleum <br /> constituents adsorbed to soils to meet the commonly accepted 5 ppb taste and odor threshold. <br /> Nonetheless, during this time these residual concentrations in excess of Basin Plan <br /> objectives will not pose a threat to current or future beneficial uses. It is highly unlikely that <br /> petroleum hydrocarbon constituents detected in localized areas in the immediate area of the <br /> release will migrate substantially beyond current limited spatial extent. Though the longer chain <br /> hydrocarbons comprising weathered TPH-g biodegrade more slowly than certain petroleum <br /> constituents, such as benzene, they are also more recalcitrant (i.e., less volatile, less soluble and <br /> highly absorbent) and much less mobile. It is also highly unlikely that this particular very <br /> limited pocket of shallow groundwater will be used directly as a source of drinking water. Thus, <br /> the significant period of time that it will take for water quality in this limited area to meet all <br /> Basin Plan objectives is a reasonable time frame. Closure of the site, given the facts in this <br /> particular case, is appropriate. Nevertheless, both the County and CVRWQCB do not agree that <br /> residual diminishing concentrations of petroleum hydrocarbons remaining at the site do not <br /> threaten the beneficial use of underlying groundwater. It would therefore be appropriate to <br /> monitor site groundwater for dissolved phase BTEX, TPH-g, and oxygen four additional quarters <br /> to validate more convincingly the above findings that are based on the currently available <br /> information. <br /> III. SUMMARY AND CONCLUSION <br /> 1. There is no evidence of MTBE in groundwater as a consequence of the release at this <br /> site. Corrective actions including soil vapor extraction and removal of an estimated 850 cubic <br /> yards of contaminated soil have removed a significant mass of residual petroleum hydrocarbons <br /> at Petitioner's site. <br /> 2. The plume of dissolved phase hydrocarbons is stable, concentrations of constituents <br /> are declining, and naturally occurring biodegradation of residual petroleum hydrocarbons <br /> adsorbed to soil and present in groundwater is occurring. <br /> 3. Ten years after the release was stopped, groundwater meets Basin Plan objectives with <br /> respect to BTEX constituents within about 70 feet of the site; with respect to TPH-g, <br /> groundwater meets Basin Plan objectives within 150 feet of the site. <br /> 13 <br />