SITE INFORMATION AND CORRESPONDENCE

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i 4) the site will attain Basin Plan water quality objectives with no further action. With regard to the occurrence of MTBE in site groundwater, MTBE and other oxygenates have been analyzed for via EPA Method 8260 on a quarterly basis since March of 1998 (five sampling events through February of 1999). MTBE has been detected in site P $ wells with screen intervals at or near the water table; once in each of six monitor wells and i twice in one monitor well. Five of the eight"hits," with concentrations ranging from 2.3 i hin the existing, of 1999 t g to 10 ppb, were reported to February , and occurred in wells w defined plume, as well as in up-gradient, cross-gradient, and down-gradient wells. CONTENTIONS AND FINDINGS Contention: The UNOCAL contends its case should be closed because soil and groundwater assessment is complete and corrective actions conducted have diminished the presence of residual petroleum hydrocarbon constituents to a point where they pose a " public health and safety, the environment, and to current or probable future low risk" to ub c y, P beneficial uses of water. Findings: UNOCAL's contention has merit. As explained below, the facts in the record support the finding that additional soil and groundwater investigation or additional remediation is not necessary and that residual petroleum hydrocarbon.constituents at UNOCAL's site do not pose a threat to human health and safety, or the environment, and do not adversely affect, or threaten to affect, current or anticipated future beneficial uses of water. In addition, the level of site cleanup is consistent with the maximum benefit to the people of the state and will meet the applicable objectives in the Central Valley RWQCB Basin Plan within a reasonable time frame. The primary sources of the release (the USTs, dispenser, and associated piping) have been removed as well as a substantial volume of affected soil from the immediate vicinity of the former USTs. Free product has never been observed in any site monitor wells and SVE has removed a substantial mass of residual petroleum hydrocarbons in the vicinity of the former pump island. With regard to the detection of MTBE in shallow'groundwater at the site, its widespread distribution (up-gradient, cross-gradient, and down-gradient), the low concentrations detected, and the timing (winter when infiltration of rainfall runoff is greatest) of five of the eight reported detections, suggest that the occurrences are likely representative of background water quality conditions and not necessarily related to the pre-1988 release at the site. With regard to affected groundwater and its potential to adversely impact current or anticipated future beneficial uses, substantial evidence in the record indicates that concentrations of dissolved phase hydrocarbons are decreasing, the plume is stable, MTBE is not present in significant concentrations regardless of source, and attenuation and biologic degradation of residual petroleum hydrocarbons in site soil and groundwater is occurring. Further, available data indicate that the affected groundwater monitored by site wells, in addition to being of inferior quality, is not hydraulically connected to the deeper groundwater aquifers that are used locally for municipal, industrial or agricultural supply. 4 k I� i i