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As previously described, the uppermost sand body is an incised channel deposit in the lower part of . the Modesto Formation It fines upward from gravel to fine sand and is more than 25 feet thick in places The base of the channel is approximately 50 feet below surface grade along the axis of the channel, but it rises to 40 feet to the north (Figure 3) In some places, the upper contact is less than 30 feet below grade and is gradational to silt (borings GT-6 and GT-13, Figure 4) The channel deposit has excellent aquifer characteristics, but has been above the water table for the past five years Underlying the Modesto Channel are several feet of indurated silt and (locally) clay These deposits have low permeability and form an aquitard in comparison to overlying and underlying sand beds North of the former UST facility, where the Modesto channel is thin, the underlying aquitard reaches a thickness of 20 feet in boring CPT-2 (Figure 4) and 30 feet in boring CPT-6 Beneath the UST facility, where the channel is thickest and had scoured into the underlying silt bed, the aquitard is as little as 5 feet thick, but it thickens southward to more than 10 feet in CPT-3 and CPT-5 (Figures 5 and 3) The aquitard is brown to reddish brown in color, contains decomposed plant fragments, has a blocky (pediform) structure, is locally fractured, and the fractures and vugs aie commonly filled with a white calcareous precipitate These features all indicate that the aquitard is a relict soil (paleosol) that formed on fine-grained overbank deposits prior to incision of the Modesto Channel Underlying the Modesto aquitard is the 1" Riverbank aquifer, which is not as thick or laterally extensive as the Modesto aquifer It is thin or absent at the south edge of the site (borings CPT-3 and CPT-5 in Figures 5 and 3) and thickens to more than 10 feet northward (boring GT-6, Figure 5) The base of this channel is only slightly scoured into the aquitard that underlies it, and in most borings occurs at a depth of 65 to 70 feet The channel is relatively thin in the vicinity of the former UST cavity, but it thickens northward toward boring GT-12 and the base is incised to a depth of 75 feet (Figure 3) The I" Riverbank aquifer is fine to coarse grained and micaceous, and contains fewer pebbles and lithic sand grains than the Modesto aquifer The 1 S` Riverbank aquifer exhibits evidence of moderate to strong weathering, particularly in the lower part The normally gray sand is oxidized and the color is mottled red and brown The basal few feet we a poorly sorted mixture ranging from gravel to clay, and thin red silt lenses were observed in GT-12 i The underlying aquitard consists of reddish brown silt Fractures are commonly filled with white precipitate (caliche?) In several borings, this aquitard is between 5 and 10 feet thick and is present in the depth interval 70-80 feet South of the UST cavity, however, where the overlying 1st Riverbank aquifer is either thin or absent, the sub-Modesto and sub-l" Riverbank aquitards are indistinguishable and form a relatively continuous aquitard that is more than 25 feet thick (borings CPT-5 and CPT-3, Figures 3 and 5) Like the sub-Modesto aquitard, the sub-I"Riverbank aquitard is interpreted as a paleosol formed during a period of landscape stability between two episodes of channel incision The next aquifer is present at depths ranging from 80 to 100 feet The 2"a Riverbank aquifer is a strongly incised channel deposit that is very gravel-rich in its lower part, but it fines upward to 4