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4.0 SUBSURFACE CONDITIONS Stratus has developed this interpretation of subsurface stratigraphic conditions based on available historical soil boring logs (Appendix F) and the results of the recent CPT investigation (Appendix D) Historical groundwater data (Appendix G) and historical soil data (Appendix H) were utilized to evaluate the extent of hydrocarbon impact Interpreted geologic relationships are illustrated in geologic cross sections A-A' (Figure 3), B-B' (Figure 4), and C-C' (Figure 5) Surface traces of the cross sections are shown on Figure 2 41 Geologic Conditions Fine-grained material (silt and clay) comprises the predominant vadose zone soils (from surface grade to approximately 50 bgs) A laterally continuous sandy stratum approximately 5 to 15 feet thick was encountered between approximately 20 and 30 feet bgs Some sandy strata of limited lateral extent were also encountered within the silt and clay of the vadose zone Saturated soils appear to consist of interbedded fine-grained and sandy soil Both the fine- grained and sandy strata appear to be laterally continuous across the site Groundwater is first encountered at approximately 50 feet bgs, within a sandy stratum approximately 25 to 45 feet thick This unit appears to thin significantly and acquire a greater proportion of fine-grained material to the southeast Based on the limited number of borings extending deeper than 100 feet bgs, the interbedded relationship between the sandy and fine-grained strata appears to continue with depth, although the thickness of the strata appears to decrease with depth The stratigraphic interpretation presented in Figures 3, 4, and 5 do not suggest sandy strata are acting as preferential pathways for vertical migration of dissolved hydrocarbons 4.2 Hydrogeology Depth to groundwater beneath the site has been measured at depths ranging from approximately 37 feet bgs (1987) to 81 feet bgs (1992) Based on discussions with SJCEHD personnel, depth to groundwater in the site vicinity may have been as low as approximately 105 feet bgs in 1979 ' Groundwater was measured at approximately 51 feet bgs at the time of the fourth quarter 2005 monitoring event Prior to 2000, when the majority of the site monitoring wells were destroyed, the groundwater flow direction was variable, but predominately towards the east, southeast, and ' northeast, at low gradients A rose diagram summarizing historical groundwater flow directions calculated during historical monitoring events is included on Figure 2 ' 43 Extent of Petroleum Hydrocarbon Impact to Soil Available historical soil analytical data is provided in Appendix H As described in section 5 1 below, extensive efforts were undertaken by Teichert to excavate and remove petroleum ' hydrocarbon impacted soil from the site, including an excavation to the water table interface (38 feet bgs at the time of excavation in 1987) When the water table dropped in the early to mid 1990's, SVE was utilized to remove additional hydrocarbon mass (refer to Section 5 2) Limited soil analytical data is available following the SVE remedial actions Four soil samples were collected during the advancement of boring B-17 in September 1999 TPHG was reported K 1Te,clo,11SIocklon-10RReponslscm 04-2006 doc Page 1 STRATUS