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F Remedial Action Plan Frontier Transportation �i June 17, 2004 Page 3 N In January 2000, BEII advanced two borings (B-5 and B-6), installed four additional groundwater monitoring wells (MW-8 through MW- 11 ), and collected nine soil vapor samples for a soil vapor survey as part of an additional site assessment (See j Figure 2, Site Layout Map for well locations) . Soil sample analysis identified no j detectable concentrations of petroleum hydrocarbons with the exception of boring B-6 . Maximum contaminant levels in B-6 were identified 100 mg/kg as TPH-g, 2,343 µg/kg as BTEX, and 10,600 µg/kg as MTBE at 10 ' bgs . Maximum soil vapor concentrations were identified at sample point V5 at 4 ' bgs. Groundwater samples were subsequently collected from all monitoring wells. Results are tabulated in Table 1 . In May 2002, BEH installed two additional groundwater monitoring wells (MW- 12 and MW- 13) screened from 30 ' bgs to 40 ' bgs . Soil sample results identified lmaximum TPH-g, BTEX, and MTBE concentrations of 700 mg/kg, 115,000 µg/kg, and 25,000 µg/kg in soil collected from MW- 13 at 10 ' bgs . Soil concentrations attenuate by 99% at 15 ' bgs . No detectable TPH-g, BTEX, and MTBE were identified in MW- 12. Also, monitoring well MW-3 was abandoned prior to excavation and removal of 325 tons of impacted soil and replaced by MW-3R. V In June 2003 , BEH installed an additional groundwater monitoring well (MW- 14) screened from 30 ' bgs to 40 ' bgs . Soil sample results identified a maximum MTBE concentration of 1 ,020 µglkg at 10 ' bgs using EPA Method 8260B . No detectable TPH-g �YrwLAI and BTEX were identified in MW44. kti t. , rd 1 In January 2004, BEII installed four additional groundwater monitoring wells. Deep monitoring wells MW- 15, MW- 16 and MW- 18 were advanced to 40 ' bgs and screened from 30 ' bgs to 40 ' bgs . Shallow monitoring well MW- 17 was advanced to 20 ' bgs and screened from 5' bgs to 20 ' bgs. Soil sample results identified a maximum MTBE concentration of 1 ,3 10 µg/kg at 10 ' bgs in MW- 15 using EPA Method 8260B . No other oxygenates or lead scavengers were detected. The maximum TPH-gas detected was 1 .7 mg/kg at 10 ' bgs in MW- 15 using EPA Method 8015M. No detectable BTEX 1 was identified in the soil column for MW- 15 through MW- 18 . Groundwater sample results identified a maximum MTBE concentration of 99 µg/kg in MW-17 using EPA Method 8260B . See Figure 2 for well locations. 3.0) SITE GEOLOGY/HYDROGEOLOGY The site is located on the boundary of the Sacramento Valley and the San Joaquin Valley within the west-central portion of the Great Valley Geomorphic province of California. The Great Valley has been filled with a sequence of older to younger alluvium of Pliocene to Holocene age which overlay sedimentary rocks of Cretaceous to Tertiary age. These sedimentary units, in turn, overlay a crystalline basement of Paleozoic and Mesozoic metamorphic and igneous rocks. The shallow subsurface geology in the site vicinity is a heterogeneous mix of gravel, sand, silt, and clay (Reference : Site Characterization Report, Twining Laboratories, October 16, 1996). BEII