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Relatively low concentrations of dissolved petroleum hydrocarbons have been intermittently <br /> reported in samples collected from off-site wells MW-2 and MW-13 Petroleum hydrocarbons <br /> were not detected in samples collected during the first quarter 2006 groundwater sampling event <br /> Given the available groundwater analytical data, the current on-site and off-site shallow screened <br /> well network appears adequate for monitoring petroleum hydrocarbon impact at these depths <br /> Deeper Screened Wells <br /> Free product has historically been measured in well MW-1 A maximum free product thickness <br /> of 1 89 feet was measured in December 2000 Following initiation of free product removal by <br /> Stratus personnel, the measured product thickness in well MW-1 has generally decreased <br /> During the first quarter 2006, a product sheen (immeasurable thickness) was observed in well <br /> MW-1 Free product has also been measured in well MW-15 since December 2004, at <br /> thicknesses ranging from 0 02 to 0 56 feet Free product was not observed in recently completed <br /> well MW-20, or any of the wells situated south of Patterson Pass Road The lateral extent of free <br /> product thickness appears to be adequately assessed <br /> Samples collected from off-site wells MW-10 and MW-12 have consistently been impacted with <br /> relatively high concentrations of GRO and BTEX At the time of the first quarter 2006 <br /> groundwater sampling event, GRO and benzene were reported at concentrations of 14,000 µg/L <br /> and 2,600 µg/L, respectively (MW-10), and 42,000 µg/L and 11,000 µg/L, respectively <br /> (MW-12) <br /> Samples collected from recently installed wells MW-16 and MW-18 were impacted with <br /> relatively low concentrations of benzene (2 4 µg/l, and 8 8 µg/L, respectively) GRO and MTBE <br /> were not detected in the samples collected from wells MW-16 through MW-18 Given this <br /> information, the current well network appears adequate for monitoring off-site petroleum <br /> hydrocarbon impact to groundwater GRO, benzene, and MTBE concentrations for the deeper <br /> screened monitoring well samples collected during the first quarter 2006, and from wells MW-16 <br /> through MW-18 in May 2006, are included on Figure 9 <br /> On-site Water Supply Well <br /> Petroleum hydrocarbon and fuel additive concentrations have consistently been reported below <br /> laboratory detection limits for the quarterly samples collected from the ARCO water supply well <br /> i <br />• <br /> K 1Bp Arco For URS161001Reportsl6 t00 assessment rpt&scm update doc Page 1 I OTRATU5 <br />