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wHydrocarbon odor was present at the base of the aquifer at 65 feet and in the underlying aquitard at 70 and 75 feet (see boring log in Appendix A), but the single aquitard sample from 70 feet was below the laboratory detection limit of i mg/kg(Appendix B) We interpret the slight diesel odor as indicating that diesel fuel did not leach through the 151 Riverbank aquifer directly into the underlying aquitard, but when the water table dropped below 70 feet in the early 1990's, diesel may have been carried downward in the dissolved phase into the aquitard Two samples of the aquifer were collected from GT-8 in 1996, and the results were similar to those from GT-15 in 2004 The sample at 60 feet was below the diesel detection limit, but the sample at 65 feet had a concentration of 3620 mg/kg (Figure 4) In the 1996 Problem Assessment Report we concluded that this concentration indicated that diesel had leached to the base of the aquifer, at least locally The new data from GT-15 support that conclusion In retrospect, if GT-8 had been drilled �cs � into the underlying aquitard, it would probably have demonstrated that the underlying aquitard was 7 not impacted above the detection limit for soil , The same may be true for GT-4 and GT-10, neither of which reached the 151 Riverbank aquifer or the underlying aquitard The borings nearest to GT-15 that sampled the aquitard and/or the aquifer are TB-2 (drilled in 199 1) and GT-I I (drilled in 1999) (Figure 2) Hydrocarbon odors were not reported in TB-2, but concentrations of 24 and 25 mg/kg were detected in the lower portion of the aquifer between 65 and 70 feet (Table 1) These concentrations could represent the base of the leached diesel plume (in the unsaturated zone) or the top of the dissolved-phase plume (the depth to groundwater at the time was 74 feet) Moderate hydrocarbon odor was detected several years later in the aquifer samples from GT-11, but no samples were analyzed However, a concentration of 740 mg/kg was detected in the underlying agwtard at 71 feet (Figure 4) This concentration is probably above what would be expected as the result of downward movement of the water table, and therefore likely represents leaching within the unsaturated zone when the water table was below 70 feet Boring GT-11 was drilled through the UST cavity, which implies that diesel leached 5-10 feet deeper there than it did farther away at GT-15 One additional boring merits consideration Soil samples cannot be collected in CPT borings, but a groundwater sample was collected from the aquifer and analyzed in 2003 from boring CPT-1 (located between GT-1 I and GT-15) The laboratory reported a concentration of 670,000 000 Pg/1 (parts per billion—this is equivalent to 670 parts per million in soil) If correct, this result would imply that 15-20% of the diesel detected in the aquifer or 90% of that detected in the aquitard in GT-I i and GT-8 has dissolved in groundwater since 1999 in order to reach such an elevated concentration Due to diesel's low solubility, we view this as rather unlikely and therefore treat the groundwater result from CPT-1 with great skepticism (as discussed in the Fourth Quarter 2003 Report) The map illustrating diesel concentrations in groundwater disregards that result and utilizes only the newest data from GT-5, GT-6, and GT-9 (Figure 13) The map implies that the plume is centered east of the UST cavity in this aquifer This is consistent with earlier data from GT-10, where very high concentrations were detected in both the soil and groundwater at the top of this aquifer 10