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6.0 ENVIRONMENTAL MONITORING 6.1 LANDFILL GAS CONTROL AND MONITORING Decomposition of organic waste in a landfill produces various gases, principally methane and carbon dioxide. When organic waste is ini- tially placed in a landfill, it contains oxygen and decomposes aero- bically for a short time, producing only carbon dioxide. After the oxygen is depleted, anaerobic decomposition takes place, and both methane and carbon dioxide are produced. The concern over landfill gas production stems from methane's combustibility in concentrations of 5 to 15 percent in air. The potential for lateral gas migration beyond the landfill boundaries and dilution of the gas to the combustible range near on-site or off- site structures must be considered in the landfill design. Studies of gas flow through various soils and experience at landfill sites have determined that flow is less in lower -permeability soils (i.e., clays) and greater in granular soils (i.e., sands). Subsurface gas migration from landfills depends greatly, therefore, on the natural soils or lining systems beneath and surrounding the wastes. Since the landfill has a significant buffer area and is underlain with low -permeability soils, the potential for subsurface gas migration off site or to on- site structures is extremely low. To further minimize the potential for off-site gas migration, ga monitoring probes will be strategically placed along the property boundary and adjacent to on-site structures to detect any subsurface gas movement (see Figures 4 and 5). If the low -permeability soils and buffer area are ineffective in preventing lateral gas migration and an environmental problem develops, an active gas extraction/control system will be constructed. Gas collected in the system will be either flared on site or used as an energy source. The gas control system will only be constructed in those areas of the fill where gas PJ9 9390209A.00D 28