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• - tt • Tosco Marketing Company 2000 Crow Canyon place.Ste.400 PRO - San Ramon,California 94593 ' Telephone:510-277-2305 Facsimile:510-277-2351 /� Environmental Compliance TO S C o Department September 12, 1997 Ms.Mary Maeys San Joaquin Public Health Services Environmental Health Division 304 East Weber Avenue, Third Floor Stockton,CA 95202 Unocal Station#6981 4707 Pacific Avenue Stockton,California Dear Ms.Maeys: Tosco Marketing Company is in receipt of your letter dated August 13, 1997 requesting a workplan for the referenced facility. Attached please fmd a workplan prepared by our consultant,GeoStrategies,Inc. Additionally,we have attempted to address the concerns brought forth in your letter. As you are aware,the City of Stockton had required the destruction of four groundwater monitoring wells,designated MW-3,MW-10,MW-12 and MW-13, to facilitate the March Lane/Pacific Avenue Interchange Project. PHS/EHD has requested on several occasions that the monitoring wells be replaced. Unocal has also responded that consideration for replacement wells should be deferred until the interchange project is completed, so that new wells would not be damaged by ongoing construction. Unfortunately,an existing groundwater monitoring well (MW-11) has recently been paved over during the interchange project. A review of the potentiometric surface maps shows that groundwater flows consistently towards the northeast beneath the site. Based on this information, it is Tosco's technical opinion that replacement of groundwater monitoring wells MW-10 and MW-3,upgradient or crossgradient of the site,would be of no value, since existing wells are located in both directions. Tosco also believes that a replacement monitoring well, located midway between MW-12 and MW-13 would be sufficient to maintain downgradient delineation of the hydrocarbon plume,should MW-11 be found. Tosco is aware that the screens of the monitoring wells installed at the site have been submerged by the rising groundwater. However,the bottoms of the vapor extraction wells,originally installed in the unsaturated zone,now extend below the water table and groundwater samples are also collected quarterly from these wells. This suggests that the submerged groundwater monitoring wells continue to afford a realistic representation of conditions within the dissolved hydrocarbon plume and additional wells with higher screen intervals are not warranted. The remediation system has been operating since September 1994,and will continue to operate. Influent concentrations indicate the system has effectively removed residual hydrocarbons in the unsaturated zone. Groundwater monitoring data indicate that the dissolved hydrocarbon plume is now restricted to the area north of the dispenser islands. Because the lateral extent of the dissolved hydrocarbon plume appears to have stabilized and is apparently shrinking,sparge wells do not appear warranted at this time.