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---------------- Site Characterization Report: 7500 West Eleventh Street, Tracy, CA. Page 30 whether fuel hydrocarbons that leaked from those tanks migrated northward to affect soil beneath the 7500 West Eleventh Street property. This uncertainty is due, in part, to the presence of what appears to be a secondary plume of diesel and gasoline that originated on the 7500 West Eleventh Street site in the vicinity of boring PP-7. (See Figure 13 for _1 location.) 5.5.2 Secondary Plume of Diesel and Gasoline As noted previously, push-probe boring PP-7 was located close to the southeast corner of the 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 ji 1998. Boring PP-7 was also located a short distance down gradient from the previously- described site of the two 1,000-gal. underground fuel storage tanks formerly located at 24195 Chrisman Road, which is, as shown on Figure 13, the adjoining property to the south. Diesel, at a concentration of 25,200 pg/L and gasoline, at a concentration of 11,000 µg/L were detected in a sample of groundwater recovered from PP-7. The concentrations of t = gasoline and diesel detected in soil samples from this 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 1, 3,800 mg/Kg of gasoline was detected in a sample of soil recovered from that boring at a depth of 4 ft beneath the ground surface. At 12 ft, the concentration of that analyte was lower, at 280 mg/Kg. Although no gasoline was detected at 16 ft, r diesel, at a concentration of 2.50 mg/Kg, was detected at that depth, with the .1� concentrations of diesel at higher elevations reaching 2,690 mg/Kg. Given the persistence of those fuels 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. ' 1 ' 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 supports the interpretation shown on Section A-A' 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 sic