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Mr. Robert McClellon March 7, 2011 Page 4 areas below the historical groundwater table, there will likely also be some kind of "aquitard" layer (perhaps a 40 -mil HDPE membrane) placed between the underdrain and original ground to limit the amount of groundwater collected by the underdrain system (Geologic Associates, 2008). It should be noted that the underdrain in the expansion area is designed in the event that the groundwater table rises to historic levels however, the current groundwater level is far below historic high water table levels and it is unlikely that the historic high groundwater levels will reoccur in the future. Materials will be installed by qualified installation contractors, will be inspected by construction quality assurance monitors, and will be certified by a California -registered engineer. The installation of the composite liner system will create a barrier to LFG migration. In addition, within five (5) years of placing waste in the new cell, the existing LFG collection and control system (GCCS) will be expanded into the new cell to capture LFG generated in the landfill expansion and prevent LFG pressure fronts that could lead to migration. The composite liner system in conjunction with a properly operated and maintained GCCS will provide control of LFG and limit the potential risk of LFG migration from the new cell. Forward Expansion Cross Section The cross section shown in Figure 1 represents the proposed landfill cell expansion, proposed liner system, typical GCCS components, adjacent geology, and a typical proposed LFG monitoring probe. Figure 1 illustrates that the geology in which the LFG monitoring probes are to be installed consists of inter -bedded sandy silt, clayey silt, silt, and clay. If LFG were to migrate from the lined expansion, it would follow the preferential pathways of permeable soils (such as sand, sandy silt, and silt) to the probes, regardless of the set -back distance from the landfill. The migration of LFG in site soils has been confirmed by existing probes on site. In addition, the cross section depicts the construction details of the proposed landfill cell including the geomembrane liner system and the GCCS components. The GCCS is designed to impart a negative pressure (vacuum) to the LCRS and to extraction wells that are to be constructed within the waste mass. Each extraction well is expected to have a negative pressure radius of influence (ROI) up to approximately 150 feet (depending upon the applied vacuum and waste characteristics). This design is intended to create a negative pressure zone of influence through the waste mass and LCRS drainage layer, drawing LFG toward the extraction wells and LCRS system. The spacing of LFG collection wells and operation thereof will be designed to extend the ROI to the edge of waste. Since the permeabilities of the waste mass and LCRS drainage layer are much greater than that of the surrounding liner system and native soils, the preferential pathways for LFG within the cell is through the LCRS drainage layer and the waste mass toward the GCCS. By imparting this negative pressure zone with the waste mass, LFG will be contained within the landfill offsetting any tendency to migrate from the landfill to the probes. The key component in the