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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 ethanol, tert-butyl alcohol (TBA), di-isopropyl ether (DIPE), ethyl tert-butyl ether (ETBE), 1,2-dichloroethane (1,2 DCA) tert-arnyl methyl ether (TAME) and ethylene dibromide (EDB) Groundwater samples from the Geoprobes and were reported as ND for all analytes in samples B-3, B-5, B-6, B-7 and B-8 Sample B-1 contained MtBE at 90 parts per billion (ppb) and DIPE at 2 8 ppb Sample B-2 contained only MtBE at 15 ppb Sample B-4 contained MtBE at 11 ppb Sample CPT-1, collected at 75 feet below ground surface (bgs), contained MtBE at 120 ppb and 1,2-DCA at 23 ppb Also in April of 2001, GR conducted a well search and Sensitive Receptor Survey (SRS) in the vicinity of the subject site The well search was conducted within a i/z-mile radius around the site Based on the available data supplied by the California Department of Water Resources (DWR), three municipal wells were identified within the i/z-mile radius of the study area, the closest well is located approximately 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 four groundwater monitoring wells, three offsite and one deep onsite (MW-8 through MW-11) The three shallow offsite 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 parts per million (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 (cfin) 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) of TPHg, 0 01 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 were 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 Groundwater has been momtored and sampled on a quarterly basis from 1991 to the present During the year of 1997, groundwater was not monitored or sampled Groundwater analytical data 1402458 4