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Site Conceptual Model ry SFrontier Transportation June 30, 2003 ,y max° o ' Page 2 were collected every five feet Laboratory analysis of soil samples identified no detectable ' concentrations of TPH-g, BTEX, or MTBE Groundwater samples were subsequently collected and results are tabulated in Appendix A ��o: � aE�i' to� ' In January 2000, BEE 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 Soil sample analysis identified no ' 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 bg Maximum soil vapor concentrations wereE--- �n identified at sample point V5 at 4' bg Groundwater samples were subsequently collected from all monitoring wells Results of soil and groundwater samples are tabulated in Appendix A ' In May 2002, BEII installed two additional groundwater monitoring wells (MW-12 and MW-13) screened from 30' bg to 40'bg Soil sample results identified maximum 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' bg ' Soil concentrations attenuate by 99% at 15' bg 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 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 I 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) BEH encountered predominantly clays with some silts and sands from 0' to 40' bg during well advancement Cross sections were generated showing the subsurface geologic framework of the site from west to east (A-A') and north to south (B-B') A discontinuous interbedded sandy clay layer was identified within clay from approximately 10' to 20'bg along A-A' in monitoring wells MW-1, MW-13, and MW-3 'This stratigraphic unit was not observed in the western most monitoring well MW-9 or the easternmost MW-6 A deeper sandy clay unit was identified in monitoring well MW-13 from 25' to-0' bg along A-A' This was the only well on the cross section to extend past 25' bg Three interbedded discontinuous sandy clay layers were identified 1