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Commingled Plume#35 S E C ® R Chevron 9-4275/ARCO#2133 Site Conceptual Model September 1, 2006 Page 12 6.22 Lateral Distribution of Benzene in Groundwater The lateral extent of benzene in the shallow water zone is delineated by the existing monitoring well network (Figure 11) The historical maximum dissolved benzene concentration at the Chevron 9-4275 facility was observed in well MW-1 at 9,400 pg/L The historical maximum dissolved benzene concentration at ARCO 2133 was observed in well E-3 at 9,860 pg/L During the second quarter of 2006, benzene was detected slightly above the reporting limit in two ARCO monitoring wells E-10 (0 59 pg/L) and BHD-1 (0 75 pg/L) Concentration graphs for well with historically high benzene concentrations are shown in Appendix 1 6.23 Lateral Distribution of MtBE in Groundwater The lateral extent of MtBE in the shallow water bearing zone is not delineated by the existing monitoring well network (Figure 12) Historical maximum dissolved concentrations of MtBE at the Chevron 9-4275 facility have been observed in well MW-2 at 79,000 pg/L During the second quarter of 2006, the maximum dissolved MtBE concentration was observed in well MW-3R at 68 pg/L, indicating no lateral definition of MtBE to the north SECOR continues to communicate with offsite parcel owners to install proposed well MW-10 to evaluate dissolved MtBE north of the site Groundwater samples collected from MW-6 and MW-8 northeast of the site continue to exhibit concentrations of MtBE at levels below the State of California Primary Maximum Cleanup Level (MCL) of 13 pg/L Concentration graphs for well with historically high MtBE concentrations are shown in Appendix I • 6.24 Vertical Delineation of Hydrocarbons in Groundwater In February and March 2006, six deep soil borings (AS-8, AS-9, AS-10, MW-1 R, MW-2R, and MW-3R) were completed to depths ranging from 60 to 70 and feet bgs Hydropunch groundwater samples were collected from the six borings at depths between 45 feet bgs and 72 feet bgs in an attempt to complete vertical delineation of hydrocarbons in groundwater Based on the results of analysis of depth-discrete groundwater samples collected in March 2006 from the six deep borings, vertical delineation of TPHg and benzene in groundwater has been completed to low to non-detectable levels (Table 9) Low concentrations of TPHg (300 pg/L) and benzene (1 pg/L) were reported in the 65-foot groundwater sample collected from MW-2R (Chevron) TPHg was also reported at 320 pg/L in the 60-foot groundwater sample collected from AS-10 (ARCO) The deepest groundwater samples collected from AS-9 and AS-10, both at 60 feet bgs, contained MtBE at concentrations of 36 pg/L and 067 pg/L, respectively MtBE was not detected in AS-8 above laboratory reporting limits MtBE was also reported in the deepest groundwater samples collected from MWA R and MW-3R at concentrations of 44 pg/L (63 feet bgs) and 24 pg/L (61 feet bgs), respectively MtBE was not detected above laboratory reporting limits in the groundwater sampled collected from MW-2R at 65 feet legs 94275-ARCO 2133 SCM 9-2006 12