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August 11 1992 <br /> Working To Restore Nature <br /> ARCO Service Station 2168, Stockton, California <br /> i <br /> With the exception of trace levels of aromatic compounds,TPHg and HVO were not <br /> detected in vadose zone soil in the vicinity of the waste-oil tank; however the <br /> presence of 1,2 dichloroethane and TEH (carbon range C8 - C20 hydrocarbons) <br /> detected in MW-2(located downgradient of the waste oil tank) indicates soil beneath <br /> the waste-oil tank may contain hydrocarbons. Trace concentrations of gasoline <br /> hydrocarbons detected in soil at 40 feet bsg are probably due to gasoline <br /> hydrocarbons that exist in the ground water. <br /> ' Ground water beneath the site is affected by gasoline hydrocarbons (MW-1 through <br /> MW-3), hydrocarbons with a carbon range C8 and greater (MW-2) and 1,2 <br /> dichloroethane (MW-2) The lateral extent of gasoline hydrocarbons in ground water <br /> has not been completed The source for gasoline hydrocarbons and 1,2 <br /> dichloroethane in ground water has not been identified; TPHg does not appear to <br /> be from the underground gasoline storage tanks and may be from the dispenser <br /> islands, the 1,2 dichloroethane in ground water may be from the waste oil tank <br /> Additional assessment of soil and ground water sampling is needed to verify the <br /> source for gasoline hydrocarbons and 1,2 dichloroethane in ground water If an <br /> onsite source for these compounds is not identified then an offsite source should be <br /> investigated. Two operating service stations are located south and southwest of the <br /> ARCO station <br /> RECOMMENDATIONS <br /> We recommend additional assessment of soil beneath the dispenser islands and beneath the <br /> waste-oil tank. To delineate gasoline hydrocarbons at the site, we recommend evaluating <br /> soil further west of the dispenser islands and station canopy, beneath the station canopy, and <br /> south of the existing tanks <br /> Additional assessment of soil through the drilling of a slant boring beneath the waste oil <br /> tank is recommended to further evaluate soil for the presence of 1,2 dichloroethane <br /> We recommend additional assessment of ground water. To verify the extent of dissolved <br /> hydrocarbons beneath the site we recommend that additional wells be installed near the <br /> northwest and southeast corners of the site, and east of well MW-1. We recommend <br />` continued quarterly evaluation of ground water beneath the site. We recommend analyzing <br /> water samples from all monitoring wells for halogenated volatile organics in addition to <br /> TPHg and BTEX. <br /> 1 <br /> osa.6=u <br /> 31OW4 6 <br /> 1 <br />