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Updated Corrective Action Plan—Tosco(76)Service Station No 11192^ Stockton, California - June 26,2002 1,000 feet north-northwest of the site The depths of all three municipal wells were reported to exceed 550 feet bgs and contain screened intervals beginning at a minimum of 350 feet bgs The SRS was conducted within a 500-foot radius centered around the subject site The SRS involved distributing questionnaires to all residences and businesses within the study area, requesting information about potential groundwater receptors GR distributed 141 questionnaires via the U S Post Office and a total of 49 questionnaires (35%) were returned to GR None of the returned questionnaires identified the presence of a well within the SRS study area (GR, 2001) On August 8, 9 and 10 2001, GR installed three offsite and one deep onsite groundwater monitoring wells(MW-8 through MW-11) The three shallow wells were installed to depths ranging from 28 to 29 feet bgs The deep well, MW-11 was installed to a total depth of 79 feet bgs Soil samples collected from well borings MW-8, MW-9 and MW-10 were reported as ND for all analytes Soil sample MW- 11-14, collected at 14 feet bgs, contained 17 ppm of MtBE Groundwater samples collected from wells MW-8, MW-9 and MW-10, were reported as ND for all analytes The only analyte detected in groundwater from well MW-11 was MtBE at a concentration of 110 ppb (EPA Method 8260) On December 3 through December 7,2001,SECOR International Incorporated(SECOR)performed a dual-phase extraction(DPE)event at the site The DPE was performed on wells MW-3 and MW-7,and operated for a total of 110 hours Flow rates during the test varied from 10 90 to 33 41 cubic feet per minute (cfm) and vacuum rates varied from 24 to 25 inches of mercury (in Hg) Over the course of the five day event,SECOR reportedly removed a total of 0 pounds(lbs)ofTPHg,0 Of lbs of benzene, and 6 64 lbs of MtBE The amount of hydrocarbons removed is based on laboratory samples collected during the test SECOR determined that the vacuum radius of influence was between 36 and 60 feet Approximately 20,830 gallons of groundwater was extracted during the DPE event Based on the low mass removal achieved with this test, SECOR concluded that DPE is not an effective remedial alternative for this site - r,, o�,,,,�,iJS 4 y ,r M i 9 E" w11AC-L -----v TeA W), MCL 23 Geology and Hydrogeology 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 MWA 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 (IT, 1991) 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 toll 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 14024108 3