ARCHIVED REPORTS_1995 - Laserfiche WebLink

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12. 0 LANDFILL GAS CONTROL AND MONITORING Decomposition of organic waste in a landfill produces various gases, principally methane and carbon dioxide. When organic waste is initially placed in a landfill, it contains oxygen and decomposes aerobically for a short time, producing carbon dioxide, methane, nitrates, and minor amounts of other gases. 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 and greater in granular soils. Subsurface gas migration from landfills therefore depends greatly on the natural soils or lining systems beneath and surrounding the wastes. A Calderon Air Quality Solid Waste Assessment Test (ASWAT) , dated July 1988, has been completed for the site to comply with the Calderon amendments to the Health and Safety Code, Section 41805. The ASWAT included installation of four gas monitoring probes around the perimeter of the active fill area and near the on-site structures (Figure 13) . Figure 14 shows a typical cross section of the gas probes that were installed at the site for the ASWAT. The ASWAT was performed to determine (1) the effect of landfill gases on ambient air at the active site boundary (2) the composition of the gas stream within the landfill, and (3) whether gas is migrating off-site. Results of the ASWAT are summarized below. Volatile Organic Compound (VOC) concentrations at the Foothill Sanitary Landfill are relatively similar in upwind and downwind ambient air. In particular, downwind VOC concentrations do not appear to be higher than upwind VOC concentrations except for methyl chloride and trichloroethylene, which were detected at only 0.9 ppb and 0.2 ppb, respectively. The concentrations of benzene, perch lorethylene, carbon tetrachloride, 1, 1, 1-trichloroethane, and trichloroethylene fall at or below their respective detection limits. In areas of buried municipal refuse, the major gas components detected in the gas probes were methane and carbon dioxide, which are refuse decomposition by-products. Methane concentrations were 63 to 76 percent by volume. The results of the integrated surface walks indicated a methane concentration of 3 ppm in the active landfill area on the west side of the landfill site and 23 ppb in the active landfill area on the northwest side. 32