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4,3.1. Modesto Formation Aquifer Borings drilled in previous (dry) years encountered unsaturated conditions in the Modesto Formation, including dry sand at the base of the Modesto channel Borings drilled during the current phase of investigation encountered moist conditions at the base of the channel along its margins, and saturated conditions in the center of the channel in GT4 (Figure 6) Hence, this bed is referred to in Figure 5 as the Modesto aquifer The Modesto channel is very coarse grained, moderately sorted, unweathered, and contains no silt or clay One sample from the lower part of the channel yielded a hydraulic conductivity of 12 x 10 = cm/sec, which is typical of permeable aquifer sand(Table 2) 4.3.2 Modesto Formation Aquitard When GT4 was drilled, saturated sand was penetrated at a depth of 46 feet in the Modesto aquifer, but water entered the boring very slowly (see Section 4 1 1 above) After the boring was deepened to 54 feet in the silt bed, the water level apparently dropped to 48 feet and stabilized, implying that water in the Modesto aquifer is ponded (perched) above the basal Modesto silt bed Further, unsaturated conditions are present in the uppermost Riverbank Formation from 55 to 65 feet (Figure 6) Hence, the silt bed is a confining layer that, where present, separates water in the Modesto Formation from water in the Riverbank Formation Therefore, it is termed the Modesto aquitard in Figure 5 Samples of the aquitard were collected just below the base of the Modesto aquifer(GT4-50' and GT- 7-45') and in the clay bed in the middle of the aquitard (GT-7-50') and were analyzed for hydraulic conductivity Conductivities range from 13 x 10"5 to 4 4 x 10$, which are typical of low-permeability Wt and clay beds(Table 2) This strong contrast in permeability with the overlying channel sand creates a significant barrier to downward groundwater percolation 4.3.3 Riverbank Formation Aquifer The principal water-bearing zone in the site area is the Riverbank Formation This aquifer has not been completely penetrated at the site, but the monitoring wells are completed within it and indicate that it is at least 40 feet thick, extending from at Ieast 65 to more than 100 feet in depth As described in sections 4 12 and 4 13, the aquifer is a downward-coarsening channel deposit that appears to increase in grain size and permeability northward across the site Samples of this aquifer at a depth of 65 feet in GT-6, GT-8 and GT-9 were submitted for permeability analysis and confirmed this impression The uppermost Riverbank sand is intermediate in hydraulic conductivity between the Modesto aquifer and aquitard Values range from just over 10"3 to just under 104 (Table 2) No samples of the lower part of the aquifer were collected during this phase of investigation, but the downward increase in gram size is presumably associated with an increase in hydraulic conductivity as well 8