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sandy silt at the top The color ranges from grayish brown to reddish brown, but is most commonly medium to dark brown Feldspar and Iithic grains are common, but mica is not as abundant as in the 1St Riverbank aquifer Judging from Figures 3-5, the 2nd Riverbank aquifer is continuous and relatively uniform in thickness in a north-south direction (Figure 3) but is much more variable in thickness and extent in a more east-west direction (Figures 4 and 5) This implies that the aquifer is a somewhat linear channel deposit that trends in a general north-south direction and pinches out to the east and west Boring GT-3 lies on the axis of the channel, and borings GT-11 and GT-15 are also close (Figure 4) The channel deposit is slightly thinner in boring CPT-1, suggesting that the axis of the channel curves around this boring Underlying the 2nd Riverbank aquifer is another aquitard that is more than 10 feet thick in all four of the new borings This aquitard ranges from brown to red-brown in color and black blotchy staining and white caliche-cemented fractures are common The grain size ranges from clay to sandy silt The clay ranges from very firm when dry to sticky when wet In the CPT borings,this aquitard had very high sleeve fraction (8-15 tons/ftp) and extremely low permeability (10"i5 cm/sec) This results in high pore pressure, which explains why the basal portion of the 2nd Riverbank aquifer exhibited flowing sand behavior in GT-12 All of these characteristics taken together imply that this aquitard is more clayey and more strongly weathered than the overlying aquitards, suggesting a longer period of soil formation between the 2nd and 3rd Riverbank aquifers or that the aquitard was deposited in a somewhat different environment, perhaps a shallow lake The 3rd Riverbank aquifer has now been penetrated in GT-1 1, GT-13, GT-14 and GT-15, providing some additional data on this aquifer In GT-11, GT-13, and GT-15, the aquifer consists of fine-to- medium-grained brown sand from 110 to 125 feet It is coarser grained in GT-14, pebbles and cobbles are present, and the boring was still in gravel at the total depth of 120 feet Thin sand stringers in borings CPT-I and CPT-4 between 115 and 120 feet are laterally correlative to the thicker channel deposit in GT-I1 and GT-14 and are interpreted as channel-margin deposits They appear to grade laterally to overbank mud (Figures 4 and 5) The base of the 3`d Riverbank aquifer is biown to reddish-brown silt,penetrated in GT-13 and GT-15 (Appendix A) 3.2 Thickness Patterns, Channel Trends, and Hydraulic Conductivity Figures 6-9 summarize the previous information on the geometry of the various aquifers and illustrate the apparent trends of the four paleochannels All 22 borings drilled to date penetrated the Modesto Channel, so its thickness and geometry are well known (Figure 6) East of the UST cavity the channel trends S50°W and is more than 20 feet thick along its axis Boring GT-4 was drilled on the axis in this area, and a permeability test conducted on a sample of gravely sand at the base of the channel had a permeability of 1 2 x 10-2 cm/sec, which is indicative of high hydraulic conductivity This value is within the range of results yielded by cone penetrometer data from CPT-1 (also on the axis) In CPT-I, the hydraulic conductivity ranged from 5 x 100 to 5 x 10-4 cm/sec in the aquifer The average permeability of this aquifer is probably about 10"2 cm/sec 5