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Mr.Nuel Henderson, P.G., SJCEHD September 21,2005 ARCO Station 6100,Tracy,California Project No.U6100-15-1 Additional Site Assessment Work Plan Page 4 mass of 4,338 pounds of TPHG has been removed from the subsurface using the 500 cfm thermal oxider. A water supply well was installed in the southwestern comer of the property in 1969 and currently supplies water to the service station. The well was constructed of 8-inch diameter casing to 540 feet bgs, screened from 396 to 416 and 496 to 536 feet bgs, and gravel packed from 35 to 540 feet bgs (The IT Group, September 10, 1999). Reportedly, a sanitary seal was not installed. Water samples have been collected from this well on a periodic basis since August 1999. Petroleum hydrocarbon and fuel additive concentrations have consistently been reported below laboratory detection limits. Site Geology 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-321MW-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 has previously reported 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. K:\Bp-Arco for URS\6100\Work Plans\arco6100-workplan 09-2005.doc