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. Problem Assessment Report (Figures 7-11). These maps and sections form the bass for the following discussion. As documented in the Problem Assessment Report, diesel fuel leaked vertically downwai d through the low-penneability silt bed in the upper Modesto Formation and then spread laterally Nithin the Modesto channel deposit (Figure 4) The size of the impacted area increased with increa.,,ing depth until the base of the channel was reached, and diesel migration was generally along the channel's axis (compare Figure 7 with Figure 11) As shown in Figure 5, concentrations approachi ng 20,000 ppm diesel have been measured at the Modesto Aquifer-Modesto Aquitard contact and contouring suggests that concentrations may locally reach 25,000 ppm between GT-4 and GT-11 However, at least in the vicinity of the UST excavation, the lower part of the Modesto Formation and the upper part of the Riverbank Formation formed a significant barrier to further downward flow, W indicated by the rapid decline in concentrations through the aquitard measured concentrations in the upper Riverbank Formation range from 1 to 740 ppm East and north of the UST excavation, however, the diesel fuel apparently entered a lugher permeability pathway in the aquitard and migrated downward and eastward into the riverbank Aquifer (Figure 4) A concentration of 19,000 ppm was detected at the base of the aquiU rd in GT- 10, high concentrations probably extend into the Riverbank near this well, as shown by the dashed contours in Figure 4 Diesel migration within the Riverbank was not controlled by the geometry of the Modesto channel, but by zones of higher permeability in the Riverbank Lateral 3nigration, particularly to the northeast, appears to have exceeded downward migration, as mclicai ed by the • concentration of 3,620 ppm at 65 feet in GT-8 (Figure 5) Confirming the results of previous drilling, concentrations in GT-11 decline rapidly below this depth, and diesel was reporod in only one sample below 85 feet(GT-11-110 5-111' at 1 8 ppm) TPH-g and total BTEX concentrations are also plotted on Figures 4 and 5, but the data are not contoured As in many of the previous samples, TPH-g concentrations in samples from GT-10 and GT-11 were consistently below 500 ppm, except for one sample at the base of the Modest) Channel in GT-11 This was also the only sample that contained benzene, at a concentration of 0 14 ppm The magnitude of the gasoline leak and the degree to which it spread, both laterally and vertically, were obviously less than that of the diesel release A TPH-g concentration of 34 ppm was detected in GT-11 at 71 feet, below this, no samples contained detectable concentrations of gasoline It is worth noting, however, that this is 10 feet deeper than any previous sample in which ga,oline was detected(Table 2) 3.5 Laboratory Evidence of Groundwater Contamination Diesel was detected in five of seven water samples, but gasoline was detected in only two samples One of these (GT-10) also contained BTEX compounds and MTBE (Table 3) This is rather surprising, because the UST's were removed before MTBE was in widespread use and N tTBE had not been detected previously in the other wells The fact that the highest concentrations were detected in GT-10 is probably due to the combination of two factors the well is screenec properly, 7