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1 <br />12.0 LANDFILL GAS CONTROL AND MONITORING <br />' Decomposition of organic waste in a landfill produces various gases, principally <br />methane and carbon dioxide. When organic waste is initially placed in a landfill, <br />it contains oxygen and decomposes aerobically for a short time, producing only <br />carbon dioxide. After the oxygen is depleted, anaerobic decomposition takes <br />place, and both methane and carbon dioxide are produced. The concern over <br />landfill gas production stems from methane's combustibility in concentrations of 5 <br />to 15 percent in air. <br />The potential for lateral gas migration beyond the landfill boundaries and dilution <br />' of the gas to the combustible range near on-site or off-site structures must be <br />considered in the landfill design. Studies of gas flow through various soils and <br />experience at landfill sites have determined that flow is less in lower -permeability <br />soils (i.e., clays) and greater in granular soils (i.e., sands). Subsurface gas mi- <br />gration from landfills therefore depends greatly on the natural soils or lining sys- <br />tems beneath and surrounding the wastes. <br />' To minimize the potential for off-site gas migration, gas monitoring probes will be <br />strategically placed along the property boundary and adjacent to on-site struc- <br />tures to detect any subsurface gas movement (see Figures 12-1 and 12-2). If the <br />low -permeability soils and buffer area are ineffective in preventing lateral gas mi- <br />gration, which may pose a hazard to public health or adjacent land uses, an ac- <br />tive gas extraction/control system will be constructed to mitigate those conditions. <br />' Gas collected in the system will be either flared on site or used as an energy <br />source. The gas control system will only be constructed in those areas of the fill <br />where gas control is necessary, energy recovery is economically feasible, or <br />' both. <br />On-site cover soils will be placed daily to adequately control odors. If odors cre- <br />ate a nuisance, the cover soil thickness will be increased or the gas will be ex- <br />tracted and either flared on site or used for energy recovery. <br />Gas probes installed adjacent to site building or along the property boundary will <br />be routinely monitored for methane gas during landfill operations. The probe lo- <br />' PJ9 9390219A.00W 51 <br />Rev. 0 July 19, 1989 <br />