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. Groundwater Monitoring Well,CPT,and Ozone Microsparge System Installation Report Tosco(76)Service Station No 11192,Stockton,California MaY 9 2003 A former Shell service station was located across Pershing Avenue to the east of the subject site, and is currently occupied by a closed bank Six groundwater monitoring wells were installed by others at and in the vicinity of the former Shell station Pertinent site features are shown on Drawing G2 Geology and Hydrogeolog The site is situated on generally flat topography at an elevation of approximately six feet above sea level Smith Canal is located approximately 1,300 feet south of the site and the Calaveras River is approximately 3,500 feet north of the site Both of these waterways are channeled, flow generally southwest, and drain into the Stockton Deep Water Channel Boring logs from wells MW-1 through MW-7 indicate that the site is predominantly underlain by clay and clayey sand with lesser amounts of silt, except in monitoring well MW-6, in which poorly graded sand with gravel was encountered from 25 to 40 feet bgs Soils encountered during GR's subsurface investigation in April of 2001 (bonngs B-1 through B-8 and CPT-1) consisted prmanly of a layer of clay encountered from approximately 6 inches bgs to depths ranging from 3 feet to 11 feet bgs The clay is underlain by silty sand containing a high percentage of fine sand In most of the Geoprobes, this layer was observed to the total explored depth of 17 feet bgs 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 (GR, 2002) The results of the April 2001 CPT boring confirm the subsurface conditions observed in the Geoprobes The silty sand observed in Geoprobes B-1 through B-8 was also observed in CPT-1 from approximately 13 to 27 feet bgs From approximately 27 to 55 feet bgs an interval of fine- gramed 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 felt a change in hthology at 68 feet bgs, which he interpreted as consisting of sand According to the CPT operator, the sand interval extended to the total depth explored of 75 feet bgs (GR, 2002) 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 Apn126, 2001, located within 5 feet of MW-11 1402458 2