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Well Installation and Site Investigation Report ARCO Facility No.5469 Stockton,California Page 8 High concentrations of TPHG, benzene, and MTBE also continue to be detected in ARCO wells MW-4 and MW-8. However, these wells are located along the northern edge of the ARCO property, and appear to be situated within a plume emanating from another source. In the CPT boring, situated downgradient of the UST pit, only very low concentrations of BTEX compounds and fuel oxygenates were detected in the soil and groundwater samples. In soil samples, MTBE was only detected at 42 and 52 feet bgs, and toluene and xylenes were only detected at 93 feet. In groundwater samples, DIPE was detected at 62 feet bgs, BTEX compounds were detected at 91 feet bgs, and MTBE was detected at 103 feet bgs. The absence of oxygenate compounds between 62 and 103 feet bgs suggest that the MTBE plume emanating from the UST pit is not migrating vertically. Data from boring CPT-1 indicates the lack of downward vertical migration of MTBE immediately downgradient of the UST pit. Water table wells MW-4, MW-7, and MW-8 appear to be situated within a dissolved contaminant plume that originates off-site. The impact detected in deep-screened wells MW-9 and MW-10 may be due to the plume migrating onto the site from another source. Except for wells MW-4 and MW-8 discussed above, historical analytical data from groundwater wells at the ARCO site indicate that BTEX compounds have only been detected in fairly low concentrations, and only on a sporadic basis. Detectable concentrations of BTEX compounds were not detected in either soil or water samples collected from the CPT boring above 90 feet bgs. The CPT boring encountered a sandy stratum from 90 to 92 feet bgs, and BTEX compounds were detected in both the soil and water samples collected this depth. In light of the conclusion above that the dissolved plume from the ARCO UST pit does not appear to be migrating vertically, the presence of BTEX compounds in the sandy stratum at 90 feet bgs suggests this sandy stratum could be functioning as a conduit or preferential pathway for the migration of contaminants from another source. ARCO's MTBE plume appears to be adequately characterized to the south (by well MW-3), to the east (by well MW-2), and to the north-northwest (by well MW-1) of the UST pit. Vertical delineation of the plume appears to be complete. Additional lateral characterization of the MTBE plume east of the station building remains to be completed. Stratus continues to request access to this property to install the off-site groundwater monitoring wells proposed in the March 2002 Work Plan. LIMITATIONS This report was prepared in general accordance with accepted standards of care that existed at the time this work was performed. No other warranty, expressed or implied is made. P:1Bp-Arco for URS154691Reports15469report2003.doc