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Soil and Groundwater Investigation Report ARCO Facility No 2168 Stockton, California Page 6 RESULTS OF INVESTIGATION Site Geology and Hydrogeology_ The subsurface materials encountered within the upper 55 feet beneath the site are relatively consistent with materials logged during previous investigations Silt and clayey silt are present beneath the site to a depth of approximately 30 feet bgs Variable layers of silty sand, sandy silt, and silt are present from approximately 30 to 55 feet bgs Subsurface materials encountered from 55 feet bgs to the total depth explored (120 feet bgs) in boring/well MW-8C are generally coarser grained, containing a higher degree of sand, than materials encountered in boring/well MW-1C A cross section presenting an interpretation of the subsurface conditions beneath the site is shown on Figure 3 Figure 2 presents the surface trace of the cross section In bonng/well MW-8C, beginning at approximately 55 feet bgs, fine to medium-grained sand is present and becomes coarser with depth, grading to gravelly sand at approximately 75 feet bgs, which continues to approximately 85 feet bgs This ten-foot thick gravelly sand layer is present in boring/well MW-1C, however, the overlying material is primarily silt with various layers • of clayey silt, silt, silty sand, and sand From 85 to approximately 100 feet bgs in MW- 8C, silty sand and sandy silt is encountered, and grades to a silt and clayey silt extending to approximately 108 feet bgs Again, material encountered in boring/well MW-1C within 85 and 108 feet bgs is finer grained From 85 to 90 feet bgs, mixtures of sand and silt are present This overlays a silt layer that is nearly 20 feet thick (90 to 108 feet bgs) A layer consisting of silty sand to gravelly sand, and approximately 7 feet thick in MW- 8C and 4 feet thick in MW-1C, is present from approximately 110 feet to 116 feet bgs Both of the borings/wells were terminated in a clayey silt that is present beginning at 116 feet bgs Bonng logs detailing site stratigraphy are presented in Appendix A A Delta representative measured groundwater levels on August 30, 2000 Groundwater was encountered from 18 85 to 22 99 feet bgs in the monitoring wells Third quarter 2000 groundwater elevation data is presented in Table 2 Figure 4 shows groundwater elevation contours (corrected to mean sea level) for the site on August 30, 2000 Groundwater elevations from monitoring wells MW-1B, MW-1C, MW-8B, and MW-8C were not used in constructing the groundwater elevation contour map Groundwater flow direction was calculated to be towards the northeast at a gradient of 0 011 ft/ft Delta submitted four soil samples to Sierra Testing Laboratories, Inc of El Dorado Hills, California for permeability testing Samples were analyzed using American Society of Testing and Materials (ASTM) Method D5084-91 Samples collected from 50, 71, and 96 feet bgs in boring/well MW-1C and from 40 feet bgs in boring/well MW-8C were tested Sample MW1C50, classified onsite as clayey silt with fine-grained sand, was tested to have a permeability value of 1 3 x 10`5 cm/sec (4 1 m/yr) Samples MW1C71