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Extended Site Characterization Report: 7500 West Eleventh Street, Tracy, CA. Page 47 gasoline and diesel that was generated by leafage from the fuel pipelines beneath the dispensing pumps of the former fueling station. 71 To further explore subsurface conditions in the vicinity of the secondary plume of ` affected groundwater, SJC's plan for the second phase of site characterization called for installation of Monitoring Well MW-12 at the location shown on Figure 13. Well MW- 12, installed on March 28, 2002, is 22 ft deep and is screened from above the water table to the bottom of the wells. At the direction of the SJCEHD, a second monitoring well, MW-12A, with a total depth of 36 ft, and a screened interval extending from 30 to 35 ft BGS,was installed on the same day, close to MW-12 to form a two-well cluster. -, Soil in the upper b ft. of the boring drilled to install Monitoring Well MW-12 was found >a to contain 1,900 mg/Kg of diesel-range petroleum hydrocarbons and 3,300 mg/Kg in the gasoline range. None of the BTEX compounds were detected. By a depth of 10 ft., the diesel-range hydrocarbons had fallen to 240 mg/Kg and the gasoline-range hydrocarbons had dropped to 1,200 mg/Kg. No concentrations of any analytes of concern were detected in samples from greater depth in that boring. A low concentration of diesel at a concentration of 150 mg/Kg, with no gasoline being present, was detected in a soil sample recovered from a depth of 5 ft. in the boring drilled to install Monitoring Well MW-12A. No other analytes of concern were detected in soil samples recovered from that' boring except for a trace of diesel-range hydrocarbons at a concentration of 1.2 i mg/Kg in a sample recovered from a depth of 31.5 ft., which result is suspected to be spurious. As was predictable, based on the presence of an aquitard at a depth of approximately 25 ft BGS that had been established from analysis the logs and other data obtained from the large number of borings drilled in the area during the first phase of site characterization conducted in April 2000, no detectable concentrations of any of the analytes of concern were found in the sample of groundwater recovered from Monitoring Well MW-12A on April 11 2002. Concentrations of diesel- and gasoline-range hydrocarbons detected in -= the sample recovered on the same day from Well MW-12, which monitors groundwater in the sands above the aquitard, were 280 t.g/L and 570 µg/L respectively. = ' The interpretations of the lateral and vertical extent of the secondary plume that were derived from the observations described above are shown on Figures 13 and 14. �r I I 1 ` 1 SJC