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Ms. Linda Turkatte June 11, 1996 Page 5 Stratigraphy The Site stratigraphy is displayed on geologic cross section A-A' (Figure 4)and on boring logs included as Attachment A. The stratigraphy beneath the Site and vicinity can generally be divided into an upper silt and clay-dominated interval extending from the ground surface to approximately 38 to 44 feet bgs and an underlying sand-dominated interval extending to the maximum depth explored (50 feet bgs). The upper silt and clay-dominated interval is comprised of varying amounts of light yellowish brown to greenish gray clay, silty clay,sandy clay, gravelly clay,clayey silt,silt,and sandy silt. Lesser amounts of discontinuous lenses,of interbedded clayey sand, silty sand, and sand also occur within this interval. Additional lithologic characteristics observed within this interval include fine-grained sand and clay nodules, iron and,magnesium oxide staining, and root material. During drilling this interval was typically dry to moist. The lower, sand-dominated interval was observed at the MW-1 (approximately 43 feet bgs), MW-6 (approximately 38 feet bgs), and MW-8(approximately 44 feet bgs)locations with a thickness of at least 12 feet in well MW-6. This interval consists of loose to very dense,gray to dark bluish gray,fine-to medium- grained sand. Groundwater was encountered during drilling at or slightly above this interval at each of these three locations suggesting that it is the primary water-bearing horizon beneath the Site and vicinity. Hydrrogeology Groundwater, was first encountered during drilling at the MW, I Iocation at a depth of 42 feet bgs. Groundwater was not encountered during drilling at the VE-1, SB-1, and SB-2 locations which were advanced to a maximum depth of 41 feet bgs;however,after the augers were pulled up five'feet at the SH-I and S13-2 locations, groundwater entered the boreholes. Grab groundwater samples were subsequently collected fromm these two boreholes as previously described. On February 7, 1996,groundwater was measured at all Site investigation wells at depths ranging from 28.76 to 29.81 feet below top of PVC casing translating to groundwater elevations of 18.74 to 20.24 feet below msl (Table 1). A groundwater elevation contour map provided as Figure 5 indicates a groundwater flow direction to the southeast,with a hydraulic gradient of 0.045 feet per foot(ft/ft)between wells MW-1 and MW-6 and 0.003 ft/ft between wells MW-1 and MW-8. This flow direction is generally consistent with the easterly regional groundwater flow direction reported at the nearby properties. However,the most recent available groundwater flow direction data in SJCEHD files from September 1994 had indicated a groundwater flow direction to the northeast based on data from the Texaco and Former ARCO properties.' Based on the difference in first encountered groundwater during drilling(approximately 42 feet bgs)and stabilized groundwater levels (approximately 30 feet bgs), it appears that groundwater beneath the Site occurs under confined or semi-confined conditions. Based on the stratigraphy discussed in the preceding section, it appears that the sand-dominated interval first encountered between depths of approximately 38 j and 44 feet bgs is the primary uppermost water-bearing horizon. H:1UMDRILLLAPT 50090-011-02