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and 30 feet in GT-15 (Table 1) Diesel-range hydrocarbons (TPH-d)were detected in the sand at the base of the Modesto Channel (GT-15-40') at 16 mg/kg (parts per million) and in the sample of the underlying aquitard (GT-15-50') at 440 mg/kg (Figures 3 and 4) Gasoline-range hydrocarbons (TPH-g) were also detected in this sample, at I1 mg/kg, but no BTEX compounds were present (Table 1) The hydrocarbon ranges of gasoline and diesel fuel overlap in the C9-C 14 range, and due to the absence of BTEX, we interpret the low TPH-g concentration as the lightweight fraction of diesel fuel rather than as the heavyweight fraction of gasoline, and conclude that this sample is not impacted by gasoline Comparison of the data from this phase of drilling to the data collected during drilling in 1991 1994, 1996, and 1998 suggests that the vast majority of the initial in-place diesel fuel in the Modesto aquifer has attenuated naturally since this investigation began For example the nearest boring to GT-15 is GT-8, which was drilled in 1996 TPH-d was detected in this boring at 18 6 and 1640 mg/kg in the basal two samples of the Modesto Channel, but a sample from the underlying aquitard at 50 feet was below the detection limit (Figure 4 and Table 1) Two other nearby borings that were drilled in 1996 and 1998 had higher diesel concentrations in both the aquifer and in the underlying aquitard Specifically, a TPH-d concentration of 363 mg/kg was detected in the aquitard in GT-4 and concentrations of 5030 mg/kg to 14,800 mg/kg were detected in the aquifer(Figure 4) In GT-10, TPH-d was detected at 19,000 mg/kg in the aquitard and ranged from 37 to 9300 mg/kg in the aquifer (Figure 3) When all seven borings that are relatively close to the UST cavity and to GT-15 are considered, (i e TB-2, GT-1, GT-3, GT-4, GT-8, GT-10, and GT-11), the evidence that attenuation has occurred is even more striking Considering only the 26 aquifer samples from these borings in which TPH-d was detected and omitting the samples that were not contaminated, the mean concentration in the aquifer was 1217 mg/kg The only sample that contained detectable TPH-d in GT-15 had a concentration of 1 6 mg/kg The ratio between these values is 16 mg/kg — 1217 mg/kg = 0 0013, meaning that the concentration in GT-15 today is only 0 13% of the mean concentration in the other seven borings that were drilled between 1991 and 1998 In the underlying aquitard, the concentration in the one sample from GT-15 is 440 mg/kg, and the mean of the six samples of the aquitard from the earlier borings is 6737 mg/kg Thus, the concentration in GT-15 in 2004 is 440 mg/kg — 6737 mg/kg = 0 065 = 6 5% of its concentration in the other six borings drilled between 1991 and 1998 The base of the Modesto aquifer ranges from about 40 to 55 feet across the site The present depth to groundwater approaches 70 feet Hence, the Modesto channel is in the unsaturated zone, and no water samples could be collected 3 5.2 First Riverbank Channel One soil sample was collected from the I" Riverbank aquifer in the four new wells Samples from the underlying aquitard were collected only in GT-15 No hydrocarbons were detected in the samples from GT-12, GT-13, or GT-14, but gasoline-range hydrocarbons were detected at 270 mg/kg and diesel-range hydrocarbons were detected at 4400 mg/kg in the basal sample of the aquifer in GT-15 (Figure 4) We interpret these concentrations as indicating that diesel fuel leached downward through the aquitard at the base of the Modesto Formation and lodged in the l" Riverbank aquifer 9