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Well Installation and Site Investigation Report d. ARCO Facility No.5469 Stockton,California Page 7 SUMMARY The following summarizes the findings of the investigation: o Five ozone injection wells (IW-1 through IW-5) were installed onsite for the purpose of initiating interim remedial action. The injection well sparge points were installed to depths where sand and sandy silt strata were encountered between 15 and 25 feet below static groundwater levels. o Soils beneath the site consist primarily of silt, with clayey silt and sandy silt. In addition to the sand encountered between 53.5 and 63 feet bgs, sand strata were logged at boring CPT-1 between approximately 68 and 73 feet bgs and 90 to 92 feet bgs. Silt, clayey silt, and sandy silt, with interbedded sand lenses, were logged in the injection well borings between 40 and 63 feet bgs. 0 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. DISCUSSION Ground Zero Analysis, Inc. (Ground Zero), working for the Knowles Family 1993 Revocable Trust property north of the ARCO site, proposed a model for site conditions that hypothesized three distinct petroleum hydrocarbon leaks merging and commingling beneath the Knowles and ARCO sites (Workplan for Additional Site Activities, dated October 31, 2001). Ground Zero noted that the ARCO plume is characterized by presence of MTBE and the lack of TPHG and BTEX constituents. Stratus concurs with the interpretation that impact emanating from the ARCO UST pit is characterized by the presence of MTBE and the absence of TPHG and BTEX compounds. MTBE continues to be detected at relatively high concentrations in ARCO wells MW-5 and MW-6 (3,800 �tg/L and 85,000 gg/L during the fourth quarter 2002). Concentrations of TPHG were reported in these wells, and in well MW-7, during the third and fourth quarters 2002, but notes in the laboratory report indicate that the compounds detected and reported as TPHG appear to be due to the high MTBE concentrations, not to the presence of gasoline compounds. BTEX compounds are not reported for wells MW-5 or MW-6, and were only reported in well MW-7 at very low concentrations during the fourth quarter 2002. P:1Bp-Arco for URS154691Repom\5469report2Uo3.doe