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4.1.4 Physical Barriers A physical barrier controls migration of the leachate or leachate-impacted groundwater by placing a physical barrier around the downgradient edge of the plume. The technology consists of excavating a trench to a continuous, low-permeability aquitard under the property. Due to equipment limitations, maximum practical and cost-effective barrier depth is between 60 to 70 feet. The trench is typically 2.5 to 4 feet wide. After completion, the trench is backfilled with low permeable material such as a soil-bentonite slurry. If the excavated soil is not impacted then the excavated soil can be mixed with bentonite to form the slurry. The soil-bentonite slurry forms a low-permeability wall inhibiting the flow of leachate-impacted groundwater. In some applications,particularly where the solid waste has been placed below the level of the groundwater table,an upgradient barrier may be used to prevent the groundwater from contacting the waste. Alternately,either in place of, or in addition to an upgradient barrier, a downgradient barrier may be used to restrict off-site migration of impacted groundwater. Although properly designed and installed physical barriers are capable of inhibiting leachate migration, they have limited effect on groundwater movement. Additionally, unless the physical barrier completely surrounds the area of concern, it is difficult to completely intercept and contain all impacted groundwater. Therefore, physical barriers are typically used in concert with other technologies such as capping or groundwater extraction (to maintain a negative hydraulic gradient),or both. As described in Section 4.1.3, the Corral Hollow SanitaryLandfill is underlain b two Y water-bearing zones; a shallow perched zone within the Corcoran Clay and a deep regional water-bearing zone overlain by approximately 200 feet of unsaturated low permeability sediments (see Drawings 1 and 2). These sediments would restrict vertical infiltration of groundwater from the shallow perched zone. The shallow perched zone is located in the upper half of the Corcoran Clay,which is approximately 50 feet thick. A slurry wall for the purpose of containment would have to penetrate that shallow perched zone and be anchored deep in the Corcoran Clay. It is estimated that a bentonite slurry wall approximately 600 feet long and 100 feet deep would have to be constructed downgradient of MW-5 and perpendicular to the direction of groundwater flow (see Figure 4.) 4.2 Alternative Evaluation This section presents an evaluation of the alternatives. Each alternative was evaluated based on implementability,effectiveness,regulatory agency acceptance, and life cycle cost. SAC\N\PJM3M390031.9BG-94\c :1 Rev. 4 0 � 2 /9S 0939.003.19 4-4