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Extended Site Characterization Report; 7500 West Eleventh Street, Tracy, CA. Page 46 Despite the limitation stated above, the estimates that have been made, when taken together with the calculated mean hydraulic conductivity of the formations through which components of hydrocarbon fuels have migrated, support the interpretation of the extent and geometry of the primary plume that is shown on Figures 13 through 16. 7.3.3 Secondary Plume Exploratory Boring PP-7, which was drilled during the first phase of site characterization in April of 2000, was Iocated, as shown on Figure 2, close to the southeast corner of the 4: former truck service bay, which is to the rear of the Casa Mendoza restaurant. PP-7 was also close to the end of Pipe Trench No. 2, which had contained a pipeline running from there in the direction of Tank Pits Nos. 1 and 2, the purpose of which could not be determined. However, Dietz Irrigation did report the presence of apparently-disturbed ground in that area when the pipeline was removed from Pipe Trench No. 2 in December 1998 (Dietz Irrigation 1999b). Boring PP-7 was also Iocated a short distance down gradient from the site of the two 1,000-gal, underground fuel storage tanks formerly located at 24195 Chrisman Road, 'y which, as shown on Figure 13, is the adjoining property to the south. Diesel, at a concentration of 25,200 µg/L and gasoline, at a concentration of 11,000 µg/L were detected in a sample of groundwater recovered from PP-7, which penetrated to a maximum depth of 16 ft BGS. The concentrations of gasoline and diesel detected in soil samples from that boring were also high. They were initially encountered near the surface and persisted to a depth of at least 16 ft. As is shown on Table 3, 3,800 mg/Kg of gasoline was detected in a sample of soil recovered from that boring at a depth of 4 ft _A beneath the ground surface. At 12 ft, the concentration of that analyte was Iower, at 280 : + mg/Kg. Although no gasoline was detected at 16 ft, diesel, at a concentration of 2.50 mg/Kg, was detected at that depth, with the concentrations of diesel at higher elevations =+ reaching 2,690 mg/Kg. Given the presence of those fuel hydrocarbons in samples recovered from near the ground surface as well as from greater depths in PP-7, it would appear that, at some time in the past, there was a primary source of fuel hydrocarbons located in that vicinity. While it is true that fuel hydrocarbons may have migrated to that area from the leaking underground fuel storage tanks that were located at 24195 Chrisman Road, given the fact that PP-7 is separated from that property by a retaining wall, to the south of which the ground surface is 4-ft. lower than the surface elevation at PP-7, the presence of moderate- to-high concentrations of fuel hydrocarbons in the shallow soils in PP-7 indicates that -� there was separate source of leakage in the vicinity of that boring. Consideration of the overall distribution of components of fuel hydrocarbons in samples of soil and groundwater recovered from other borings in the general area around PP-7 indicates that there is a secondary plume of diesel and gasoline emanating from that vicinity. It affects the subsurface to a depth of some 20 ft beneath that area and has = migrated down the groundwater gradient to commingle with the primary plume of "1 sic