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3 5 3 Second Riverbank Channel Seven soil samples were collected from the 2"d Riverbank channel or underlying mudstone in the new bonngs No hydrocarbons were detected in the 80 or 90-foot samples from GT-12, GT-13, or GT-14 but TPH-d was again detected in GT-15 at a concentration of 26 mg/kg at a depth of 90 feet (Figure 4) As we suggested in section 3 5 2, concentrations of that magnitude could represent either the base of the diesel soil plume or dissolved diesel carried downward when the water table dropped in the early 1990's Hydrocarbon odors in the aquitard between the I" and 2"d Riverbank aquifers could be evidence of downward transport in groundwater, but they could also support the idea of capillary-fringe biodegradation of diesel to concentrations below the soil detection limit, because the water table has been between 65 and 70 feet for over a year and between 60 and 75 feet for 5 of the past 10 years As the historical low-water depth has not exceeded 77 feet during the 10 years of the present investigation, the aquitard at 90 feet has remained within the saturated zone which could explain why the fuel has not degraded below the soil detection limit in sample GT-15- 90' As explained in section 3 3, the samples collected from GT-1, GT-3, GT-12, GT-14A, GT-15A, and perhaps GT-13A are representative of groundwater cofiditions in the 2nd aquifer The sample from G-1 may be either from the 2"d or 3 d aquifer or both, depending on the screened interval which remains uncertain The results are shown in Table 3, and the laboratory report is in Appendix D All of the above-listed samples were used to prepare a map of the dissolved-phase plume for the 2nd Riverbank aquifer (Figure 14) This map shows a rather circular plume centered north of the UST cavity As Table 3 shows, the diesel concentration in two wells has risen above the detection limit since the July 2004 sampling event Whether the detection of diesel in GT-3 and GT-5 is significant and is due to the 1-foot drop in the water table since July is uncertain, but it seems unlikely Comparison of Figures 11 and 14 shows that the location of the plume in the 2"d Riverbank aquifer is fairly consistent with the (apparent)present groundwater flow direction The same cannot be said for the plume in the 1st Riverbank aquifer However, as discussed in section 3 4 1, groundwater flow in this aquifer has been mostly eastward or southward for several previous quarters, and that is consistent with the present location of the diesel plume I With the present data the eastern limit of diesel in the 2"d Riverbank aquifer is undefined It is likely that this limit will be delineated by GT-16 and GT-17 in the next phase of drilling (Figure 14), but if contamination is present in either of these, it may be necessary to screen either GT-20 of GT-21 in this aquifer in order to locate the eastern limit In the July 2004 work plan, we proposed installing screened casing only within the I"Riverbank aquifer in GT-20 and GT-21 As Figure 14 shows, the western limit of contamination in this aquifer is not defined by GT-13A It appears that GT-14 is located too far south to add any useful information regarding the extent of contamination in this aquifer so we recommend moving it approximately 100 feet to the west in order to define the limit of contamination west of GT-13 11