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N <br /> June 9, 1993 <br /> ARCO Service Station 2168, Stockton, California Working To Restore Nature <br /> Analytical results indicated concentrations of 1,2-dichloroethane in the wells ranged from <br /> 0 5 to 160 ppb, and concentrations of 1,2-dichloropropane ranged from below detection <br /> limits to 18 ppb The highest concentrations were detected in well MW-5, which is <br /> upgradient of the waste-oil tank Concentrations of TPHg and benzene are shown on Plate <br /> 22 <br /> WORK IN PROGRESS <br /> RESNA is preparing to do an offsite subsurface investigation that will include the <br /> M installation of two offsite ground water monitoring wells <br /> CONCLUSIONS AND RECOMMENDATIONS <br /> Results of our investigations indicate residual gasoline hydrocarbons are not delineated in <br /> soil beneath the site It is our opinion that gasoline hydrocarbons on the ground-water <br /> surface have migrated onsite from an offsite source This conclusion is evidenced by the <br /> distribution of hydrocarbon mass in soil between 26 and 41 feet bsg beneath the site, the <br /> detection of dissolved hydrocarbons in ground water, and data that shows that only low <br /> . concentrations of hydrocarbons were detected in soil collected above 26 feet bsg in the <br /> i borings drilled We conclude that soil between 26 and 41 feet represents a zone of previous <br /> ground-water elevation fluctuations that has been impacted by offsite to onsite hydrocarbon <br /> transport <br /> ADDENDUM TO WORK PLAN <br /> At the request of ARCO, RESNA will conduct an additional investigation that will include <br /> installing two ground-water monitoring wells offsite Work will be conducted according to <br /> our previous Work Plan No 31009-1W (Applied GeoSystems, May 17, 1991) We will <br /> conduct the following work <br /> Task 1: Pre-Field Activities <br /> o Obtain permits from San Joaquin County to drill two borings and install vapor- <br /> extraction wells onsite <br /> o Obtain permits from the City of Stockton to drill two borings and install two ground- <br /> water monitoring wells in Lincoln Avenue (Plate 3) <br /> a Subcontract an underground line locating service to mark underground lines with <br /> respect to the proposed boring/well locations and contact Underground Service Alert <br /> at least 48 hours before drilling to delineate public utility lines at the site <br /> 0209ccar <br /> 31009-7 5 <br />