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3.0 INVESTIGATIVE FINDINGS 3_1 Site Geology The alluvial deposits encountered during the site investigations generally consisted of clayey silts, silty clays, silty sands, and poorly graded sands. Fine grained soil layers consisting of stiff, brown to olive- brown, silty clays and clayey silts with interbedded dense, tan-brown, silty sands extend to depths of approximately 60 feet bgs. These fine grained soil layers overly coarser grained soils comprised of medium dense,cohesionless,fine to medium grained sands to the maximum depth explored of 81.5 feet bgs. Saturated soils were encountered within this sand unit.A generalized east-west trending geological cross section is presented as Figure 3. Copies of the boring logs and well completion diagrams are included in Appendix B. 3_2 Site H�ogeology On October 23, 1996, the groundwater depths within the investigation area ranged from approximately 59.18 to 60.89 feet below the top of well casings, or approximately -38.75 to -38.7 feet with respect to Mean Sea Level. The direction of groundwater flow was generally to the east with a gradient of approximately 0.0008 ft/ft. A cumulative summary of the groundwater depth and elevation measurements is presented on Table 3. Groundwater elevation contours and the groundwater flow direction and gradient measured on October 23, 1996 are depicted on the Groundwater Elevation Map- October 1996,Figure 4. 3_3 Soil Analytical Results The cumulative soil analytical results are presented on Tables 1 and 2. Soil analytical results for soil samples obtained from borings VW-1, VW-2, SB-3, MW-1 and MW-6 are further depicted on the Cross Section A-A',Figure 3. 3_4 Groundwater Analytical Results The concentrations of petroleum hydrocarbons in groundwater for the October 1996 sampling event are summarized on Table 3 and are depicted on Figure 5. 3_5 Vapor Extraction Pilot Test Field and laboratory analytical data from the VET is summarized on Table 4. During the VET, an extraction flowrate of 50 scfm and a vacuum of 48 inches H2O were maintained at extraction well VW-2. While extracting from well VW-2 at 50 scfm, vacuum responses ranging from 0.2 to 0.7 inches H2O were observed approximately 28 feet away at well MW-1. Vacuum responses ranged from 0.3 to 0.4 inches H2O at well VW-1 located approximately 40 feet from extraction well VW-2. No response was recorded during the VET in well MW-3 located approximately 54 feet from extraction well VW-2. A TPHg vapor concentration of 101,000 mg/m3 was extracted from well VW-2. Copies of the laboratory reports and chain-of-custody documentation for the vapor samples obtained during the VET are presented in Appendix C. Project No.58100-06-11C -4- December 17,1996