ARCHIVED REPORTS_XR0012169 - Laserfiche WebLink

Home Browse Search

i Tosco Marketing Company 2000 Crow Canyon Place Ste 400 San Ramon Caldornia 94583 ' Telephone 510 277 230$ Facsimile 510-277 2361 IrI Environmental Compliance Wrosca Department I September 12, 1997 I Ms Mary Maeys San Joaquin Public Health Services Environmental Health Division 304 East Weber Avenue, Third Floor Stockton, CA 95202 Unocal Station;e `i QM ., .TRANSMITTAL 4707 Pacific Avenue - 6DeaStockton, California ✓ 4� 5- 6- Dear r Ms Maeys Tosco Marketing Company is in receipt of your letter dated August 13, I997 requesting a workplan for the referenced facilit} Attached please find a workplan prepared by our consultant, GeoStrategies, Inc Additionally, we hati e 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 potennometric surface maps shows that groundwater flows consistently towards the northeast beneath the site Based on this mfarmation, it is Tosco's technical opinion that replacement of groundv.ater monitoring wells MW-10 and MW-3,upgradient or crossgradient of the site,would be of no value, since existing wells are located in hath 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-I 1 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 Gfoundwater monitoring data indicate that the dissolved hydrocarbon plume is now restncted 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