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EA Engineering,Science,and Technology 41 We<tero Regiolaiayette CCircle 3 Qperatiens 41 rEle lalayette.CA 84549 telephone:415-283.707 i Fax:415-283-3694 18 September 1990 E IC' E I tv?Ev ir) Clint Rogers SEP 2 5 1M Chevron U.S.A.Inc. '_NVIRONMENTAL HEALTH P.O.Box 5004 PERMIT/SERVICES 2410 Camino Ramon San Raman,CA 94583 RE: Soil and Groundwater Investigation at Former Chevron SS 97780, Stockton,California Dear Clint: At the request of Chevron U.S.A.Inc. (contract 400 5480),EA has conducted Soil and ground- water investigations at the above-referenced site. TWO boreholes were drilled and completed as monitoring wells to replace wells that had gone dry at the locations shown'on-Figure 1. These locations were chosen to monitor potential migration of hydrocarbons toward active water supply wells nearby. During drilling,soil samples were collected,and 11 of therm(5 Irom EAS,6 from EA6)were selected and analyzed for"1''H as gasolide and for vola}ile aromatics: benzene,tol- uene,ethylbenzene,and xylenes(BTM. After development the new wells were purged and sampled,and the groundwater samples wm, also analyzed for TPH and BTEX. No concentra- tions of petroleum hydrocarbons greater than method detection limits were measured in samples of either soil or groundwater. It appears that a four-year drought and a drop in groundwater elevation of nearly 30 feet may have enha-iced natural biodegradation and evaporation in the vadose.zone,limiting the dispersion of low levels of petroleum hydrocarbons(0.7 gg/L benzene, 2.5 gg/L toluene,and 2 gg/L xylenes)detected in groundwater samples froth EA2 in February 1988. EA2,which is located down:the prevailing local and regional gradient from tine tank field (the indicated source),was sampled only once before groundwater dropped below the bottom of the casing. The San Joaquin County Local Health District might require a deeper well to replace EA2,which would also permit determination of groundwater flow direction each time the wells are gauged. EA proposes to gauge the existing wells and sample all monitoring and nearby domestic wells every six months for another year. EA also proposes to install one monitoring wall northwest of the tank field to assess the condition of groundwater in the regionally downgradient direction (Figure 1). If,during the next year,no hyexocarbons are found in groundwater,the site should _ be reviewed and submitted for closure.