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migration (transverse to the channel's axis) was no more than 100 feet at any depth horizon This is . most easily seen by comparing Figure 4 (cross-channel) with Figure 5 (along-channel) Figures 6-13 illustrate the contaminant plume in map view within and below the Modesto Channel These maps were also used to determine the remaining mass of diesel fuel in the soil, as described in section 2 3 Like the cross sections discussed above,the sequence of maps shows the influence of the Modesto Channel on the migration of diesel fuel At 35 feet, at about the base of the former UST excavation and the top of the Modesto Channel, the contaminant plume is relatively symmetrical in shape (Figure 6) Progressing downward through 40 and 45 feet, however, it becomes increasingly elongate in an east-west direction along the channel's axis (Figures 7 and 8) Below 50 feet (the base of the channel) however, the plume shrinks and becomes smaller and smaller (Figures 9-13) This implies that below the base of the channel, migration was vertical rather than lateral Thus, the size of the plume decreases downward through the Riverbank Formation, and at the level of the 2nd Riverbank aquifer it is restricted to the immediate vicinity of the UST excavation At the level of the 3"d Riverbank aquifer, (100 feet below grade), diesel is almost undetectable (concentrations of a few parts per million below the UST excavation) 3 2 Depth to Groundwater and Groundwater Flow Directions As shown in Table 2, the depth to groundwater on October 6 was remarkably uniform, ranging between 64 5 and 65 5 feet This is a few feet deeper than in June, and the decline is a continuation of the seasonal decline that began in the latter part of the first quarter A groundwater elevation map for each aquifer was constructed from the data in Table 2 to illustrate the groundwater flow direction Figure 14 shows the elevation in the 15` Riverbank aquifer Seven of the eight wells in the I" Riverbank were used to construct the map, GT-9 had recently been repaired, but could not be used for mapping because the casing elevation has not yet been re- surveyed The elevation of the piezometric surface in this aquifer increases to the west from—38 20 feet in GT-21 to—38 10 feet in GT-13A The contours depict a fairly planar piezometric surface that is very slightly arched The axis of the arch trends from west to east The primary groundwater flow direction is eastward, as illustrated by the arrows in Figure 14 The hydraulic gradient averages about 0 0005 ft/ft, which is an exceptionally flat gradient In the previous two quarters, the gradient was approximately an order of magnitude steeper Differences in the piezometric, surface are `evident in the 2nd Riverbank aquifer although the groundwater flow direction is similar The contours are wavy, implying that the surface is less planar and more undulating (Figure 15) This could be due to the fact that the upper contact of this aquifer may be less planar(see Figure 4) The piezometnc surface is also slightly shallower than in the I" Riverbank aquifer, but the difference is nearly indistinguishable For example, the elevation in GT-17 is—38 13 feet in the 2"d Riverbank and—38 17 feet in the I"Riverbank In the first quarter of 2005, the static water levels in the 3�d Riverbank aquifer were higher by a few inches than those in the 2nd Riverbank aquifer, but the opposite was true in the second quarter This quarter, the elevation is nearly the same in all wells that are screened in both aquifers (GT-12, GT- 14, GT-15, and GT-18) (Figure 16) The piezometnc surface appears to be very planar in this . aquifer, and slopes eastward at a gradient of 0 0006 ft/ft Thus, there is general agreement on