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i Sratigraphic and hydrogeologic units beneath the site are discussed in the following sections The { relation between these units and the distribution of hydrocarbon contaminants in the soil and groundwater are considered m section 5 0 4.1.1 Modesto Formation t The Cnll2es property is surfaced with 2-4 feet of gravel and roadbase consisting of black gravely clay Beneath this fill material, the Modesto Formation is present to a depth of approximately 55 feet The formation is generally gray, fight brown, or light reddish brown m color $ Seven stratigraphnc units can be recognized within the Modesto Formation The uppermost unit is a silt bed that is brown to orange-brown m color, slightly sandy, and may be more than 15 feet thick Few discrete samples were collected from the upper part of thus bed, but white calcareous veils or soli fractures and root holes were observed in some samples from the lower part of the bed at a depth of 15 feet The silt is dry and somewhat sta requiring 1040 hammer blows to advance the sampler 6 inches Drill cuttmgs suggest that very-fine-grained sand may be locally interbedded with the silt(e g TB-2) The silt bed is underlain by light reddish brown, silty, fine-grained sand It forms a marker bed in the interval from about 10 to 25 feet below surface grade, although it has a planar geometry and tluckens fairly gradually from about 2 feet near the south property line to about 7 feet in the center of the Glhes property(Figure 6, panel E-F) It is moderately well sorted, loose, and dry to damp The planar sand bed overlies another silt bed that is present from less than 20 feet in some borings to 35 feet in others This bed is yellow-brown, orange-brown, or red-brown at the top but becomes grayer downward It contains more sand than the upper silt bed, and weathering features (root holes, cemented veinlets and nodules, and clay skins) are more common It is damp to moist and locally slightly compacted Along the southern edge of the Gillies property, silty sand was sampled in borings GT-6 and GT-9 at 20 and 25 feet, suggesting that the silt bed mterfingers southward with sand (Figure 6, panels A B, A-C, and E-F) Underlying this silt bed is a large channel deposit that was not obvious before the most recent phase of drdh ng The deposit ranges from a minimum of 9 feet in thickness in GT-5 to a maximum of 20 feet in thickness in GT-3, and the base of the channel ranges from a depth of 42 feet to a depth of 54 feet below surface grade in the 10 borings (Figure 7) Comparison of Figures 6 and 7 reveals that fence panels A B, A-C, and E-F in Figure 6 are oriented transverse to the channel, and panel A-D is oriented generally parallel to the channel It is evident that the channel is incised more than 10 feet into the underlying silt in the vicinity of borings GT-3 along the axis of the channel, which trends northeast- southwest directly beneath the former UST facility The channel apparently turns slightly eastward near GT-4 before continuing to the northeast, and probably passes between GT-6 and GT-8 (Figure 6, panel A-B) i Very-coarse-grained to gravely sand is present in the deepest portions of the channel, elsewhere, medium-to-coarse-grained sand is present at the base of the deposit (Figure 6) Above the base, the sand fines upward through well-sorted fine- and/or medium-grained sand to silty sand at the top The 5