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SCS ENGINEERS- Probes will generally be set as follows: • Shallow probe at 5 to 10 ft below grade. • Intermediate probe at or near half the depth of the wellbore. • Deep probe at the depth of the wellbore. Well and probe depths are subject to change pending conditions encountered in the field. Note that wells GW-6 and GW-7 will be fitted with two probes as they will be advanced to a depth of less than 30 ft below grade. Perforated pipe sections will generally be placed at depths where underlying high- permeability soils are encountered. All boreholes will be backfilled with "pea" gravel around the perforated pipe sections and native soil throughout the remainder. A minimum 5-ft soil/bentonite seal will be placed in each well just above each perforated pipe section and at the surface to preclude "cross-contamination" of sampling zones. At the top of each probe a 1/4-inch labcock valve will be installed for monitoring purposes. A locking, protective steel casing will be set in concrete around the probe clusters. Proposed monitoring well construction details are shown in Figures 3 and 4. Design Criteria Potential for LFG Generation-- The active Corral Hollow Landfill has been used for disposal of MSW since the mid- _ 1970s. There are no precise records of the quantities of waste in place and we are unaware of any studies that provide an estimate of the LFG generation rate at the site. Results of the 1988 Air SWAT program confirmed that LFG was being generated within the refuse mass (EMCON Associates, Air Quality Solid Waste Assessment Test Report Corral Hollow Sanitary Landfill, Tracy, California, San Jose, September, 1988). However, the methane levels detected (7 to 40% v/v) were somewhat lower than we would typically expect to find in undiluted LFG from the landfill interior. These results may be indicative of air intrusion into the sample probes, or aerobic decomposition in the shallow refuse zone (LFG wells were installed in the refuse mass to a depth of about 8 ft below grade). Nonetheless, we assume that LFG is actively being generated within the refuse mass. The rate of gas generation will likely peak within one to two years of site closure, and then decline with time throughout the post-closure period. Placement of a low- permeability final cover system will enhance the potential for off-site subsurface LFG migration. Soil and Hydrogeologic Conditions-- _ The Corral Hollow Sanitary Landfill lies within the low alluvial plains and fan unit of California's Central Valley. Basement rock in this area consists of both christiline and marine consolidated rock capped by the unconsolidated Tulare Formation which consists of clays and silts with inter-bedded and discontinuous lenses of sand and gravel. The landfill is directly underlain by the Tulare Formation which has been divided into three sections (EMCON Associates, Solid Waste Assessment Test (SWAT) Report, Corral Hollow Sanitary Landfill, San Joaquin County, California, San Jose, June 1988). 6