ARCHIVED REPORTS XR0006610 - Laserfiche WebLink

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P� Corrective Action Plan—Tosco (76) Service Station No 11192, Stockton, California March 13, 2002 Soils encountered during GR's subsurface investigation in April of 2001 (borings B-1 through B-8 and CPT-1) consisted primarily 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 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) 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 14024508 4