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ERI 205503 ROI Unocal Service Station 5886 Stockton California June 3, 1997 <br /> �, • Residual gasoline hydrocarbons (total petroleum hydrocarbons as gasoline [TPHg]) were not <br /> detected at or above stated laboratory method detection limits in soil samples collected from <br /> hand-angered borings HAI through HA3 except for 3 3 milligrams per kilogram (mg/Kg) in <br /> HA2 Concentrations of total extractable petroleum hydrocarbons as diesel (TEPHd) were <br /> detected up to 8 4 mg/Kg and MTBE up to 12 mg/Kg <br /> • Residual gasoline and diesel hydrocarbons were not detected above stated laboratory method <br /> detection limits in soil samples collected from borings B4 through B7 except for 1 0 mg/Kg <br /> TEPHd detected at 5 5 feet below ground surface (bgs) in boring B5 <br /> • Residual hydrocarbons in soil appear to have been delineated beneath the site <br /> • Groundwater was encountered at approximately 9 to 14 feet bgs During May 1997, static <br /> groundwater elevations ranged from approximately 6 to 9 5 feet bgs and the groundwater <br /> gradient was towards the southwest at a hydraulic gradient of 0 002 Groundwater elevations <br /> were essentially the same in the shallow and deep wells, indicating hydraulic communication <br /> between the two depths <br /> • Analyses of groundwater samples did not detect TPHg and BTEX above stated laboratory <br /> method detection limits except for up to 4 3 micrograms per liter (uglL) benzene detected in <br /> wells MW2 and MW3 <br /> • TEPHd was not detected in groundwater samples collected from wells, MW1, MW4S though <br /> MW7S, MW6D, and MW713 Diesel range hydrocarbons were detected in wells MW2, <br /> MW3, MW4D, and MW5D up to 170 uglL However, Sequoia Analytical Laboratory <br /> (Sequoia) notes reported diesel range hydrocarbons were unidentified and do not appear to be <br /> diesel Additionally, Sequoia reported groundwater samples from wells MW2, MW3, <br /> MW4D, and MW5D contained sodium sulfate laboratory contamination <br /> • MTBE was not detected in groundwater samples collected from wells MW4, MW5, and <br /> MW7 (both shallow and deep casings) above stated laboratory method detection limits <br /> MTBE was detected in wells MW I through MW3 and MW6S and MW6D up to 8,000 <br /> ug/L <br /> • The lateral and vertical extent of TPHg and BTEX appear to be delineated beneath the site <br /> Concentrations of MTBE have been delineated in each direction except towards the south and <br /> southwest <br /> Based on results of the investigation, ERI recommends <br /> • Continuing quarterly groundwater monitoring and sampling to evaluate the extent of <br /> dissolved hydrocarbons in groundwater and the direction and gradient of groundwater <br /> flow and verify results of the initial round of sampling of new wells <br /> • Identifying the dissolved diesel range hydrocarbons detected in groundwater beneath the site <br /> tI <br />