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SCS ENGINEERS- These three sections are as follows: • The Upper Section consisting of coarse grained alluvial deposits. We presume the original landfill excavation and vertical extent of refuse are generally within the limits of the Upper Section • The Corcoran Clay Member consisting of sandy silts and clays. • The Lower Section consisting of coarse grained alluvial deposits. _ Shallow soils around the landfill perimeter are predominantly within the Upper Section and are considered relatively permeable with respect to LFG migration. _ The Corcoran Clay Member is approximately 50 feet thick beneath the eastern margin of the landfill, and is first encountered at a depth of approximately 10 to 15 ft below _ ground surface (bgs) in this area. The Corcoran Clay layer thins to the west pinching out into the coarse grained alluvial deposits of the Upper and Lower Sections of the Tulare Formation near the eastern margin of the landfill. The Corcoran Clay units are considered relatively impermeable with respect to subsurface LFG migration. Groundwater below the site occurs in two water bearing zones: (1) the upper zone which consists of perched unconfined water above the Corcoran Clay Member; and (2) — a lower water bearing zone below the Corcoran Clay Member. Ground water in the upper zone is present at a depth of approximately 15 ft bgs in the vicinity of GW-7 (at approximate elevation 225 ft MSL), and is expected to act as a barrier to LFG migration — in this area. Ground water has been encountered in the lower zone at depths of approximately 300 ft below grade in other parts of the site and is not expected to affect LFG migration patterns. — The proposed well depths take the above factors into account. For example, well GW-7 will extend to a depth of approximately 15 ft bgs, or just above the shallow water table. It is anticipated that the depth of wells GW-1 and GW-6 will be similarly limited; the borings will be advanced to just above the water table (or Corcoran Clay layer). All other monitoring wells will extend to the depths of adjacent refuse. Ground — water is not anticipated at those locations. We anticipate that all monitoring well borings will be advanced through the alluvial _ deposits of the Upper Section. Therefore, proposed probe depths are primarily a function of the total boring depth. Probe depths will be adjusted in the field, based on subsurface conditions. All probes will be set above the Corcoran Clay units. Locations of On-Site Structures Relative to the Waste Disposal Area-- Existing on-site structures include an equipment shed and gate house. Both are considered temporary and will be removed at the end of closure activities in late 1995. For this reason, permanent LFG monitoring wells are not proposed for installation immediately adjacent to these structures. Proposed well GW-2 will provide information on subsurface LFG conditions near the temporary structures, however. The interior of 9 —