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. thickness from north to south across the site. This bed dips eastward and ranges in depth i rom 12 to 19 feet, but it was not present in GT-10 at the expected depth of 15-17 feet Underlying the silt bed is a sand bed that grades downward from orange-brown or lig it grayish brown at the top to dark gray or reddish-brown at the base Rock fragments (schist, chert) are common in this bed The grain size coarsens downward from fine grained to very coarse grained, and pebbles are present near the base The bed ranges in thickness from a minimum of 9 f"-et in GT- 5 to a maximum of 20 feet in GT-3 It is 19 feet thick in GT-10 and GT-11 (Appendix A) Cross sections oriented east-west illustrate that the sand fills a large channel that is eroded several feet into the underlying silt bed(Figure 4) Cross sections oriented north-south show that the I ase of the channel is flatter in this direction but dips slightly to the south (Figure 5) The base of tI a channel ranges from a depth of 42 feet to a depth of 54 feet below surface grade, mapping the bast indicates that the channel is rather linear and trends northeast-southwest directly beneath the foirrier UST facility(Figure 6) The beds that underlie the channel deposit form the lower unit of the Modesto Form,ition The upper part of this unit consists of silty clay in boring GT-6, but the clay is apparently erodod in most of the other borings and the channel deposit rests on the underlying silt bed (Figure 4) The silt bed varies in thickness and the base does not appear to be flat, suggesting that the bed lies of an older erosion surface that formed prior to deposition of the silt When dnlled in 1996, with the water table at a depth of approximately 48 feet, this bed was dry and consolidated, and it appear ed to be a significant aquitard In bongs GT-10 and GT-11, however, with the water table at 45 feet, this bed • was moist to saturated and was unconsolidated in the upper foot or two It was blux,.h-gray to f reddish-gray in color 3.1.2 Riverbank Formation Three sand beds separated by silt beds have been penetrated in the Riverbank Formation .tt the site The sand beds range from fine-grained to coarse-grained, and the color ranges from gray to brown or reddish brown Rock fragments are less common in the Riverbank sand beds than in tht Modesto Formation beds, and mica is present Riverbank sand beds also contain more silt and clay Differences in the thickness of the underlying silt beds and in the depth to the base of the ipper two sand beds suggests that the sand beds fill erosional channels (Figure 5) The lowest bed has been penetrated only in GT-11, so its geometry is unknown In GT-1, 2, and 3, the upper sand bed of the Riverbank Formation was consolidated or slightly cemented, and up to 80 blows of the drill hammer were needed to collect a 6-inch sample This weathered and oxidized interval was present in GT-11 from 70 to 120 feet.-but no e-Nidence of cementation was observed The weathered interval formed during a period of landscap-_ stability following deposition of the Riverbank Formation, and is referredto as the San Joaquin or Snelling Soil (Figure 3) 3.2 Hydrogeologic Units `-VLA; ' "13P btu, j 5