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3.0 SUBSURFACE CONDITIONS 3.1 Geologic Conditions The geology beneath the site consists predominately of dry, hard, silty soils to 202 feet bgs, the maximum depth explored beneath the site Weak to moderate caliche cement was encountered within much of the silty stratum Two laterally continuous intervals of sandy stratum are generally interbedded within the fine grained soils The upper sandy interval (sand zone 1) was observed with thicknesses ranging from approximately 2 to 6 5 feet in the continuously logged soil borings, at depths ranging from approximately 48 to 64 feet bgs The lower sandy interval (sand zone 2) was observed with thicknesses ranging from approximately 12 to 23 feet, at depths ranging from approximately 68 feet to 104 feet bgs Select strata within sand zone 2 were cemented Approximately 10 to 18 feet of fine grained soils (predominately silt) appear to separate sand zone 1 and sand zone 2 Interpreted geologic relationships are illustrated on cross sections A to A' (Figure 3), B to B' (Figure 4), and C to C' (Figure 5) The surface trace of each geologic cross section is included on Figure 2 Increased soil hardness was generally observed in the fine grained strata encountered below approximately 140 feet bgs Soil samples collected from 152 feet bgs and 202 feet bgs in boring B-32/MW-1 were previously submitted for physical analysis Relatively low permeability values of 6 5 x 10-7 centimeters per second (cm/s) and 2 4 x 10-6 cm/s, respectively, were reported for these samples Stratus previously reported in the SCM that strata beneath the site do not appear to be horizontal Strata beneath the site were likely tilted from horizontal due to orogenic uplift in the Coast Range Mountains immediately west of the site Stratus previously calculated a strike of N17°E, with a 5° dip towards the east-southeast, using correlative sand stratum encountered in borings B-33, B-35, and B-38 (the upper contact of sand zone 2) as a marker bed for the calculation The strike and dip at a sandstone outcrop located approximately 1 mile west of the site were measured at N15°E, and 19° east-southeast, respectively 3.2 Hydrogeology Historical groundwater elevation data is provided in Appendix G and groundwater elevations measured at the time of the second quarter 2004 groundwater monitoring event are provided in Table 3 Groundwater elevations for shallow-screened wells at the site are depicted on Figure 6 Groundwater elevations, and groundwater elevation contours, for wells screened within sand zone 2 are shown on Figure 7 Based on information collected during the drilling of boring B-32, the regional water table appears to be deeper than approximately 202 feet bgs at the site The on-site water supply well is screened between approximately 396 and 416 feet bgs and 496 to 536 feet bgs, with a gravel pack installed from approximately 35 to 540 feet bgs The static water level in the water supply well has been measured at approximately 45 feet bgs Based this information, the water supply well appears to be screened within a confined regional water supply aquifer P 1Bp-Arco for URS16100Txports%a=6100-assessment su[Lupdate doc Page 7 6 MA W5