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Monitoring Well Installation and VET Report,and IRAP—Former Unocal Station#0187,Stockton CA December 22 2000 were saturated, suggesting that none of these units would act as a complete barrier to the downward migration of hydrocarbons 2 The lateral extent of the dissolved hydrocarbon plume is delineated in the A Zone and B Zone wells In the A Zone, impact is limited to the vicinity of Wells U-3, NP-1-22 and NP-2-22 Concentrations in the A Zone drop dramatically from the source area(wells U-3 and NP-1-22)to downgradient well NP-2-22 3 In the B-zone wells, dissolved hydrocarbons were detected in wells NP-1-65,NP-2-65, U-13 and U-20 Again, there is a noticeable drop in concentrations from the source area (wells NP-1-65 and NP-2-60)to well U-13 Benzene was detected in the furthest downgradient well (U-20), but at a concentration below the current Maximum Concentration Level for benzene of 10 ppb 4 Dissolved hydrocarbons remain undelineated in Zone C Hydrocarbons are detected in wells NP-1-111, NP-2-116, U-12, U 19, U-21 and U-23 Highest TPHg and benzene concentrations detected in C Zone wells were in downgradient wells U-19 and U-21 5 Reconnaissance observations indicate there are several sites in the vicinity that have also had underground storage tanks • The building occupying the northeast corner of California Street and Lindsay Avenue appears to be a former service station building • USTs were removed from the parking area on the northwest corner of East Miner Avenue and Amencan Street • The parking area on the southwest corner of East Miner avenue and California Street was previously used as a service station This site is reportedly under investigation(PHSEHD personnel) Releases from these sites could have contributed to the dissolved hydrocarbons detected in wells U-12/U-13 and U-20/U-21 In addition, it is reported (PHSEHD personnel) that USTs were removed from a site in the vicinity of the southeast corner of American Street and Lindsay Avenue Historical monitoring data gathered since 1994 suggest that this site is downgradient of the dissolved plume emanating from the subject site However, if groundwater flow has vaned in the past, it is possible that an release from this site might also have contributed to the dissolved hydrocarbons currently detected in wells U-21/U-21 or U-22/U-23 6 Based on the VE test results, it is estimated that initially 30 to 40 pounds per day of hydrocarbons could be extracted at test conditions using well UV-I This would be expected to decrease over time The expected radius of influence would be less than 20 feet Vapor extraction would have very limited effectiveness at this site, possibly treating a small source area RECOMMENDATIONS The newly installed groundwater monitoring wells have been integrated into the ongoing quarterly monitoring program for the subject site GR recommends that the wells be monitored and sampled for two additional quarters(first and second quarters of 2001) During the first and second quarter 2001, GR will attempt to obtain information regarding the other UST sites in the vicinity GR will evaluate, to the 24004208-1 10