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09/12/97 16:40 GR/GSI—SACTO 12094640138 -—- 002 lnaco Crew C ting CwyrmY PPMP Sano Crow , allfbr lac6 583 San nagton,Casforn�a 948ea Telephone;510.277-2305 Facannte:'.1P277�181 ttnrkonmentelC nnptlagee nepartmMt Septewwr 12, 1997 Ms-Mary Maeys San Joagtrirt public Health Services Environmental Health Division 303 8118t Weber Avenue, 'pry p'loor Stockron, CA 95202 Uiial Station it 6981 4707 Pacific Avenue Stockcon, California Deaf Md. Marys- Tosuo Marketing Company is in receipt of your letter dated.A.uguat 13, 1997 requesting a workplaft fat the referenced facility. Attached phase find a watlgtlan prepared by our eons ultam C3eoStrategioe,Inc- Additionally,we have attempted to address the conoarns brought forth,is your letter, AN you aze aware,the City of Stockton had requited the dostruction of four groundwater monitoring wells,designated MW-3,MW-10,MW-12 Uld MW-13, 10 facilitate the Much Latte/Paeific Avenue InterebeVe project pHS/Ti}1D has requested an several occastione that the monitoring wells be replaced- Unocal has also responded that consideration for replaacm itgt wells should he defatted until the intemhenge project is camPleted, $a that new wells would not be damaged by ongoing covs,truadoa, Unfortunately,an existing groundwater monitoring well (MW.11) has recently been paved over during the interchange projact. A review of the Potentiorrubaic surface maps shows that groundwater flows consistently towards the northeast beneath tile site, BASS on this information, it is T'osco'a teebnical oPbtiott that replacoment of groundwater Wu rmut rmul g wells IvEW-19 and MW-3,upgradient or crossgradient of the site,would be of no value,since existina wells are located in both directions. Tosco also believes that a replacement monitoring well,located midway between MW.12 and MW-13 would be snilkient to xnaintain dowrvadient delineation of die hydmearbon plume,should MW-l1 be(bund. Tosco is aware that the screens of the monitoring wells installed w the site have been submerged by the rising groundwater- klowever,the bottoms of the vapor extraction wells,originally installed in the uxlsautrated znne now C. tend below die water table a groundwater s as are alas ae to quarter y Vol Mose wells, TW suggests t at the ettbrnerg0 groun water rnonitoring wells emanue to afford a realistic yepre5aftrarlono conditions within the dissolved hydrocarboA plume and additional wells with higher screen intervals are not warranted. '1110 re,nediation system has been operating aince September 1994,and will codtinuo to operate. Influent concentrations iudioate the system has effectively removed residual kydrocarbons in the xassatwawd zone. Ground eater monitoring data indicate that the dissolved hydrocarbon plume Is now rest6Lsd to the area north of the dispenser islands. Because the lateral extent of the dissolved hydroca bib plume appears to have stabilized std is apparently shrWrlag,sparge wells do not appear warranted at ibis brae.