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grained channel sand and sub-channel silt, respectively The concentration dropped to 5,030 mg/kg in the highly permeable coarse-grained sand at the base of the channel Three samples from GT-5 were selected for analysis Hydrocarbon odors were evident in one of these (GT-545'), and the laboratory reported a diesel concentration of 38 6 mg/kg in this sample Because there was no field evidence of contamination in GT-6, only the sample at 45 feet (base of the Modesto channel)was analyzed No contaminants were detected Although no samples from GT-7 exhibited evidence of contamination, four were selected for analysis one from the Modesto aquifer, two from the Modesto aqurtard, and one from the Riverbank aquifer Only the Riverbank sample contained detectable concentrations of hydrocarbons (40 7 mg/kg diesel at 60 feet) Because the PID detected organic vapors in most samples from GT-8 below a depth of 40 feet, five samples from this boring were analyzed Moderate diesel concentrations were detected in both water- bearing zones the laboratory reported 1640 mg/kg to the (moist) Modesto channel sand at 45 feet and 3620 mg/kg in the(saturated)Riverbank sand at 65 feet Xylene was also detected in this sample No samples from boring GT-9 exhubited field evidence of contamination, but four of the deepest samples were analyzed A concentration of 4 2 mg/kg diesel was reported at a depth of 40 feet In this well, the base of the Modesto channel is at approximately 42 feet 5.2.2 Groundwater Contamination Richesun & Associates collected groundwater samples from G 1 and GT-3 in November 1990, and Upgradient collected samples from GT-3 and from GT-1 and GT-2 in September 1994 and January and July 1995 (Table 5) Diesel and gasohne-range hydrocarbons were detected at fairly low concentrations in two or more wells in September and January After the groundwater table rose above the screened interval during the Spring of 1995, groundwater sampling became pointless and was discontinued after July 1995 During the present investigation, hydropunch samples were collected and analyzed from GT-4, GT-5, GT-7, GT-8, and GT-9 After GT-5, GT-6, and GT-9 were converted to monitoring wells and developed, water samples were collected from these wells in early October (see section 3 3 2) The results of these samples are shown in Table 5, and the laboratory reports are included in Appendix D Diesel was detected in most of the hydropunch samples at moderate to very high concentrations, ranging from 3,600 to 1,730,000µg11 The exception was GT-9, where no hydrocarbons were detected by the mobile laboratory Gasoline was also detected in most of these samples, at concentrations that ranged from 76 to 22,800 µg/1, but BTEX compounds were generally not detected Low to moderate diesel concentrations were detected in most well samples, ranging from 78 to 1,600 µg/1 The detection of diesel in GT-6 and GT-9 is surprising, because no hydrocarbon odors were evident during drilling or sampling of these wells and no hydrocarbons were detected by the mobile lab in the hydropunch 11