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The shallow perched water-bearing zone is located in the upper half of the Corcoran Clay. The existence of a shallow perched zone was suspected in October 1987, while drilling deep monitoring well MW-1. This shallow perched zone was confirmed by the installation of monitoring well MW-4 in May 1988. The shallow perched water-bearing zone does not appear to be extensive; rather, it is confined to the eastern half of the landfill (see Drawing 2). Groundwater monitoring wells MW-5 and SB-1 are also installed in the shallow perched water-bearing zone. This water-bearing zone is not present at SB-2. Groundwater in the shallow perched water-bearing zone appears to be mounded near monitoring well SB-1; the flow occurs both northwest and southeast with a gradient of 0.01. Groundwater in the shallow perched water-bearing zone appears to be restricted to thin (0.01-inch) sandy interlayers that occur in the upper portions of the Corcoran Clay. Recharge to this perched zone probably occurs by infiltration from the overlying sediments. The absence of the interbedded sand lenses in the lower half of the Corcoran restricts further vertical movement of groundwater and causes the perched groundwater condition. Groundwater in the deep regional aquifer on site generally flows southeast. The deep regional water-bearing zone is overlain by approximately 200 feet of unsaturated low- permeability sediments.These sediments should restrict vertical infiltration of groundwater from the shallow perched water-bearing zone in the Corcoran Clay. No springs occur at the disposal site or within 1 mile of the site perimeter. 2.3 Groundwater Gradient and Velocity According to the Solid Waste Assessment Test Report ([SWAT] EMCON, 1988), groundwater moving beneath the site within the deep regional water-bearing zone flows to the southeast toward Corral Hollow Creek. Groundwater in the shallow perched water- bearing zone appears to be mounded near monitoring well SB-1. Based on recently measured groundwater elevations (December 8, 1994), groundwater flow in the shallow water-bearing zone is inferred to be bi-directional, with flows diverging toward the northwest and toward the southeast in the vicinity of monitoring well SB-1. Figure 2 shows the groundwater contours of the shallow water-bearing zone. The average calculated groundwater velocities for the northwesterly and southeasterly flows are 3.12 and 2.90 feet per year, respectively. The three wells (MW-1, MW-2, and MW-3) monitoring the deep water-bearing zone were dry. Therefore, groundwater contour maps could not be constructed for this water-bearing zone. SAC\N.PJOM3M390031.9BG-94\cd :1 Rev.0,4/20/95 0939-003.19 2-4