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1 6eo/ogtcalTeckncrslitc Page 8 Site Conceptual Model Report(2006 Revision) Valentine(Castle Motors) Project No 723 2 June 16,2006 ' 4 feet bgs These soils are underlain by silty sand to sandy silt, and sand units to 29 feet bgs Clays are present from 29 — 32 feet bgs The soils from 32—46 feet are interbedded fine grained units with some sands, silty or clayey sand layers that generally do not exceed 1 feet in thickness The remainder of the borehole encountered more fine grained units from 46 to 68 feet bgs A soil sample from 27 feet bgs was identified as a fine to ' coarse grained, slightly moist sand by the field geologist This layer transitioned downward to a brown to grey, slightly moist, firm, clayey silt with some iron oxide staining at 28 feet This data correlates fairly to the CPT log that shows the interval as a ' silty sand to sandy silt unit transitioning to a sand to silty sand and then clayey silt to silty clay units CPT-3 The CPT log shows primarily fine grained units extending to the total depth of ' 70 5 feet bgs Fine grained units are present from the surface to 23 feet bgs Five feet of sand to silty sands are noted at 23 to 28 feet bgs and these are underlain by fine grained units from 28 to 69 feet bgs A sand to silty sand unit is present from 69 feet bgs to the ' CPT boring terminus at 70 5 feet A soil sample from 30 feet bgs was identified as a brown to grey, moist, sandy clayey silt by the field geologist This unit transitioned downward to a very fine grained, brown, moist, silty sand This correlates fairly to the ' CPT log that shows these depths to be undifferentiated fine grained units Figures 3 and 4 provide a geologic interpretation of the units observed in the borehole logs- see Figure 2 for cross section locations Note The cross sections were developed using data gathered by different individuals utilizing different methodologies Therefore, they need to be looked at as one of several possible interpretations of actual site conditions ) ' The continuous sampling of MW-7 revealed that the subsurface has a diverse litholo The p g gY site exhibits little correlation between boreholes and this situation is exacerbated by the fact ' that different geologists logged the boreholes and at different sampling intervals (i e- continuous versus 5 feet) For this reason the cross sections have coarse units combined (gravels, sands, clayey sands, silty sands) and fine units combined (clays, silts, sandy clays) ' This serves to identify the preferential pathways for contaminant migration through units of greater hydraulic conductivity The borehole logs are included in Appendix C ' Figure 3 illustrates the geology trending from northwest of to southeast side of the site The diagram indicates that more sands are present in the off-site location of MW-7 with on-site soils containing a greater portion of clays and silts The residual contamination bound in the ' on-site fine grained soil units continues to source the groundwater plume GTI's interpretation of the geology does not identify any sands units as laterally continuous The ' silt/clay units do appear to be laterally continuous, however Figure 4 illustrates the geology trending from southwest to northeast of the site In this ' interpretation, both silt/clay and sand units appear to be laterally continuous across the site As with the location of MW-7, more sands are present in the off-site locations of MW-8 and MW-9 The sand unit at 60 — 75 feet bgs in the vicinity of well MW-201 could be a preferential pathway for off-site migration