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that the high concentration at 65 feet was not due to simple vertical migration Perhaps it is due to lateral migration within groundwater, or to lateral nugration along the top of the Riverbank hardpan followed by a"spillover" effect north of the termination of the hardpan Lateral contaminant migration is examined further in the next section 5.4 Lateral Extent of Soil Contamination Figure 10 vividly illustrates that the margins of the diesel plume are nearly vertical within the silt bed above the Modesto channel, but below 35 feet the plume spreads out very abruptly within the upper part of the channel This lateral spreading with depth is best seen in a series of isocon maps at various depths (Figures 11-15) Figure 11 shows the extent of diesel contamination at a depth of 35 feet, dust below the base of the excavation At this depth, the shape of the plume bears no relation to the geometry of the Modesto channel A high concentration was detected in GT-1, and although the sample at 35 feet in GT4 was not analyzed, a concentration near 8,000 mg/kg is estimated to have been present, based on a PID reading of 200 ppm and the concentration of 14,600 mg/kg at 40 feet The concentrations reported by SEMCO in three samples at the base of the excavation are lower than expected, but have been incorporated into Figure 11 for completeness The extent of diesel contamination at 45 feet, in the lower part of the Modesto channel, is depicted in Figure 12 The areal extent of contamination is considerably greater than at 3 5 feet, and the plume has . become slightly elongated in a northeast-southwest direction With increasing depth, the diesel plume begins to narrow in a north-south direction and lengthen in an east-west direction, with the long axis subparallel to the axis of the Modesto channel At 50 feet, concentrations in excess of 20,000 mg/kg may be present along the was of the channel (Figure 13) i In contrast, gasoline concentrations between 45 and 50 feet define a plume that resembles the diesel plume at 35 feet (Figure 14) This probably indicates that a smaller volume of gasoline was released, and therefore the gasoline plume did not become ponded along the axis of the Modesto channel The diesel plume reaches its maximum elongation at a depth of 55 feet (Figures 10 and 15) At this depth, the contamination is contained in either the basal silt bed of the Modesto Formation or in the silty to clayey sand of the uppermost Riverbank Formation 5.5 Volume of Contaminated Soil Figures 11-15 can be used to estimate the volume of contaminated soil and other parameters The volume is calculated by summing the volume at successive depth intervals Due to the irregular shape of the plume,the area enclosed by the zero line on each map has been divided into smaller squares, and then the area of each square has been multiplied by the thickness of the interval to obtain the volume 13