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3.5.4 Third Riverbank Channel <br /> The laboratory analyzed at least one soil sample from the 3`d Riverbank Channel in GT-13, GT-14, <br /> and GT-15 A TPH-d concentration of 7 mg/kg was detected in GT-15 at a depth of 120 feet <br /> (Figure 4) No other samples were contaminated (Table 1) These results almost certainly imply that <br /> the downward-leaching diesel fuel did not reach the 3`d aquifer, and strongly support the conclusion <br /> that any contamination detected in this aquifer is dile to later vertical oscillation of the water table <br /> causing a"smear zone" in the capillary fringe <br /> The only other samples from this aquifer are consistent with that interpretation In GT-11, the <br /> sample at 110 feet had a concentration of 2 mg/kg, but the sample at 120 feet was below the 1 <br /> mg/kg detection limit <br /> Water samples GT-13B, GT-1413, and GT-15B were collected from this aquifer, and they also <br /> support this interpretation All three had detectable concentrations of TPH-d, ranging from dist over <br /> the 50 µg/l detection limit for groundwater (56 lig/l in GT-1413) to 390 µg/i in GT-15B The diesel <br /> concentration detected in GT-15B is in much better accord with soil and other data than is the 1400 <br /> 4g/1 reported by the laboratory from sample CPT-1 at the same depth a year ago These two <br /> samples are about 25 feet apart Lighter hydrocarbons within the gasoline range were also detected <br /> in the sample from GT-15B (Table 3), but because neither gasoline oxygenates nor volatile <br /> aromatics were detected above the much lower detection limit of 0 5 µg11, we infer that the lighter <br /> hydrocarbons represent the light fraction of diesel fuel rather than gasoline <br /> Figure 15 is a contour map of the three water samples from this aquifer With only three data points, <br /> the interpretation is tentative The plume appears to be located in the same general area as the <br /> plume in the overlying (2nd Riverbank) aquifer It is depicted as a northeast-southwest trending <br /> ellipse that is centered between GT-15 and the UST cavity GT-14 may be near the southern <br /> (apparently downgradient) margin of the plume, but other margins are not well delineated <br /> Although we originally planned to screen GT-12 within this aquifer, flowing sand beneath the 2nd <br /> Riverbank aquitard prevented the driller from achieving the intended depth Adding a clustered <br /> well, either at this location or at GT-16 would likely be possible and would aid in defining the <br /> apparent northeast(upgradient?) margin of the plume <br /> 4.4 SUMMARY AND CONCLUSIONS <br /> The vertical and lateral extent of fuel contamination in soil and groundwater of the Riverbank <br /> Formation was investigated by drilling four hollow-stem auger borings and installing seven new <br /> monitoring wells The borings ranged in depth from 105 to 127 feet and fully penetrated a fluvial <br /> channel aquifer in the Modesto Formation and three channel aquifers in the underlying Riverbank <br /> Formation Twenty soil samples were analyzed for gasoline and diesel-range hydrocarbons Diesel <br /> fuel was detected in all four aquifers in boring GT-15, which is the closest to the former <br /> underground storage tank facility Concentrations of 16 parts per million in the Modesto aquifer, <br /> 440 parts per million in the I" Riverbank aquifer, 26 parts per million in the 2nd Riverbank aquifer, <br /> 12 <br />