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i Updated Corrective Action Plan—Tosco(76)Service Station No 11192, Stockton, California M June 26,2002 In Geoprobes B-3 and B-4, a layer of well-graded sand was encountered beneath the silty sand and extended to the total explored depth of 17 feet bgs Groundwater was encountered during probing at approximately 15 5 to 16 5 feet bgs The results of the CPT boring confirm the subsurface conditions observed in the Geoprobes The silty sand observed Geoprobes B-1 through B-8 locations was also observed in CPT-1 from approximately 13 to 27 feet bgs From approximately 27 to 55 feet bgs an interval of fine-grained clay with several thin interbeds of silty clay and clayey silt was recorded At approximately 55 5 feet bgs the CPT detected an approximately one foot thick layer of sandy silt to clayey silt At 57 feet bgs the CPT probe experienced refusal and the test was terminated While advancing the groundwater sampling probe,the CPT operator, Mr David Fisch of Fisch Environmental Exploration Services, felt a change in lithology at 68 feet bgs, which he interpreted as consisting of sand According to Mr Fisch, the sand interval extended to the total depth explored of 75 feet bgs (GR, 2001) Ll�-6.69y 61 �ec{;ss'4o(es nal reju,reA_ Soil types encountered during installation of wells MW-8 through MW-10 consisted of clay, silt, and silty sand to the total explored depth of 28 feet bgs, similar to soils encountered during the Geoprobe event The subsurface conditions observed during the installation of well MW-11 generally coincide with the results of the CPT conducted on April 26, 2001, located within 5 feet of MW-11 The static depth to groundwater in the ten existing monitoring wells was measured by GR at 1186 to • 15 92 feet below top of casing(TOC)on February 18,2002 Historical depth to water has ranged from 1140 to 32 33 feet below TOC The historical groundwater flow direction generally has ranged from north to northeast at a relatively flat gradient of 0 001 to 0 009 ft/ft A cross-section of the subsurface geology is presented on Figure 4 The cross-section details the geological conditions observed during previous subsurface investigation events, recent dissolved concentrations of petroleum hydrocarbons, and the approximate extent of the dissolved hydrocarbon plume Review of the cross-section indicates the presence of an approximately 30-foot thick section of clay separating the shallow and deeper saturated zones It is expected that the clay acts as an aquitard which prevents the vertical movement of groundwater between the two saturated zones However, the presence of MtBE in the deeper saturated zone may indicate hydraulic communication between the two zones MtBE itself in the deeper zone may also be an artifact resulting from the subsurface investigation process, as the CPT and well drilling technology have the potential to introduce cross- contamination between aquifers • 140245 08 4