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• Groundwater Assessment Report ARCO Facility No 2186 Stockton,California Page 5 Morrow Surveying, Inc of West Sacramento, California, surveyed well casing elevations and the locations of all wells and soil bonngs on August 26, 2001 Well elevations were established to the nearest 0 01 vertical feet, based on City of Stockton Benchmark 19, elevation 1178 feet mean sea level Onsite vapor extraction wells (VW-IA,B through VW-3A,B) were also surveyed at this time The survey was tied into the previous survey of the site performed by Morrow Surveying in September 1995 Waste Disposal Drill cuttings generated during the field activities were placed on, and covered with, plastic sheeting and stored onsite Wastewater generated was placed in DOT-approved 55-gallon steel drums and stored onsite pending disposal Dillard Environmental Trucking Company of Byron, California, transported the soil and wastewater to an appropriate disposal facility FINDINGS Site Geology and H_ydropeolo%!y A silty zone composed predominantly of silt with some interbedded clay was encountered from approximately 0 to 27 feet bgs This silty zone is underlain by a sandy zone composed of sand and silty sand The sandy zone appears to be laterally continuous beneath the site, ranging in thickness from approximately 10 to 15 feet The sandy zone extends to approximately 32 to 43 feet bgs, where a second, lower silty zone was encountered Vapor wells installed in this sandy zone appear to be dry The lower silty zone vanes in thickness from approximately 12 to 35 feet, extending to depths of approximately 47 to 68 feet bgs A second, lower sandy zone is encountered beneath the lower silty zone, extending to depths of at least 76 feet bgs The contact between the lower silty zone and lower sandy zone appears to be interfingered Silts and clays were encountered at the base of borings MW-2, MW-6, and MW-7, but there is not enough data from this basal fine-grained stratum to determine if it is a laterally continuous aquitard beneath the lower sandy zone, or a layer within the lower sandy zone Interpreted geologic relationships are illustrated on cross sections A-A' (Figure 3) and B-B' (Figure 4) The surface trace of each cross section is shown on Figure 2 Historical and third quarter 2001 groundwater elevations are presented in Table 1 Depth to groundwater at the site has ranged from approximately 35 to 54 feet bgs since groundwater monitoring was initiated in 1995 Depth to groundwater was measured at approximately 46 feet bgs in September 2001 Groundwater flow has historically been toward the northeast, . east, and southeast The groundwater flow direction on September 13, 2001 was calculated P\ARCO\2186\REPORTS\2186report doc