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Mr Harlm Knoll May 6, 1999 Page 2 of 3 Groundwater samples were submitted to Argon Laboratories (FLAP #1873) for analysis of TPHg and BTEX utilizing EPA methods 8015 modified and 8020, respectively Groundwater samples collected from wells MW 1 and MW5 contained elevated levels of petroleum hydrocarbons TPHg and BTEX were detected in well NMI at concentrations of 63 parts per billion (ppb), 0 8 ppb, <0 5 ppb, 1 0 ppb, and 1 5 ppb, respectively MW5 contained TPHg and BTEX at concentrations of 65,000 ppb, 10,000 ppb, 5,000 ppb, 1,500 ppb, and 9,500 ppb, respectively No petroleum hydrocarbon constituents were detected in wells MW3, MW4 or MW8 Figure 3 shows the estimated extent of dissolved benzene in groundwater Analytical results are summarized in Table 3 Laboratory data sheets and chain of custody documentation are included in Attachment B SUMMARY AND CONCLUSIONS • MTBE has not been detected above 7 ppb at the site, which is below the Department of Health Services advisory level of 13 ppb A summary of MTBE levels is presented in Table 4 • Concentrations of dissolved gasoline constituents beneath the site have decreased or remained stable, with exception of well MW5, since the USTs were removed in 1987 and secondary soil contamination was excavated in 1990 In well MW5, the trend since December 1996 is one of decreasing concentrations • Downgradient wells MW8 and MW4, located approximately 105 and 85 feet, respectively, from well MW5 do not contain petroleum hydrocarbons above laboratory detection limits • Well MW1, located immediately downgradient of the former tanks and approximately 85 feet from well MW5, contains benzene (0 8 ppb), toluene (<0 5 ppb), ethylbenzene (1 0 ppb), and toluene (1 5 ppb) at concentrations below Maximum Contaminant Levels (MCLS) • The areal extent of the dissolved contaminant plume has been largely defined and has generally decreased in dimension since 1992 • Twelve years after the USTs were removed (1987) and nine years after contaminated soil was excavated (1990), groundwater meets Basin Plan objectives approximately 80-100 feet downgradient of the original release • Additional soil and groundwater remediation at the site is considered technically and/or ecomonically infeasible • The shallow aquifer in the Stockton area is generally not used as a drinking water source due to saltwater intrusion from the delta and general poor water quality • Although the nearest known water supply well is located approximately 300 feet in the downgradient direction from the site, this well does not appear to be active and would not be expected to be impacted by the dissolved contaminant plume at the site, which appears to be shrinking and does not appear to extend off-site Although detectable concentrations of BTEX in the vicinity of monitoring well MW5 are likely to remain above MCLS for many years, the current or future beneficial uses of surface or drinking 1knowles hamheporW1 qm 1999 doc