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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 shown that flow is less in lower -permeability soils <br />(i.e., clays) and greater in granular soils (i.e., sands). Subsurface gas migration <br />from landfills therefore depends greatly on the natural soils or lining systems be- <br />neath and surrounding the wastes. <br />To detect potential 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 6-1 and 6-2). If the <br />low -permeability soils and buffer area are ineffective in preventing lateral gas mi- <br />gration, which may pose a threat to public health or adjacent land uses, an active <br />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 />cations and monitoring frequency will be coordinated with the RWQCB. Detailed <br />records of all monitoring results will be maintained on site to provide continued <br />documentation that methane gas is controlled and not posing a potential hazard. <br />Monitoring results will be submitted to the RWQCB at regular intervals. When fill <br />operations are completed, the monitoring program's results will be reviewed and <br />future activities adjusted based on these data. <br />9390218A.00W 62 Rev. 0 July 20, 19815 <br />