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Draft Supplemental Environmental Impact Report Page IV.E-10 Forward Inc. Landfill 2014 Expansion Project -" the individual components (EPA 1993)6. The mixture of methane and carbon dioxide in landfill gas is comparable to that of air. The flammability and lack of odor make methane a dangerous gas at landfills if not collected over time. Explosions and effects on worker health have occurred at unmitigated landfills, most often when worker were exposed to low lying areas within the landfill where methane accumulated (Everett, Wilson and Hoylman, 1984). Along the northern landfill boundary there is a perimeter collection and migration monitoring system where methane and other gas concentrations are monitored and controlled. Throughout the site, there is a comprehensive landfill gas collection system that routes most methane to either a flare or engine for controlled combustion. Further augmentation of the gas collection system is planned as part of the proposed project and required for continued compliance with regulation. In addition to reducing the impact of a methane gas-related hazards, the expansion of the gas collection will remove some VOCs currently impacting groundwater quality (Geologic,2008). A subsurface oxidization event can be caused by a variety of factors, including spontaneous combustion or by placing too much vacuum on a landfill gas collection system. In spontaneous combustion, waste material buried in a landfill is heated by chemical oxidation and biological decomposition. The resulting heat can cause the material to reach the point of ignition, causing rapid oxidization. Landfill gas collection systems are designed to operate under a vacuum, so that methane and other gases generated by the decomposition of municipal solid waste in a landfill can be captured by the landfill gas collection system and conveyed in pipes to either a landfill gas flare that destroys the gas or a co-generation plant that converts landfill gas into renewable electric energy. Placing too much vacuum on a landfill gas collection system can cause oxygen to be drawn into the landfill waste mass, which in turn can cause an increase in the temperature of the waste and lead to subsurface oxidization. Placing too little vacuum on the landfill gas collection systems can cause landfill gas to escape through the cover of the landfill, which would violate federal and state air pollution regulations that limit that amount of landfill gas emitted into the atmosphere. Subsurface oxidization events are more likely to oxidize or burn slowly without visible flame or large quantities of smoke and are characterized by rapid oxidation of organic waste in the landfill. The waste mass may oxidize around a gas extraction well,in the influence zone of the extraction well,or near a surface feature that allows oxygen to enter the waste mass. Subsurface fires in gas collection systems are detected by elevated temperature at the gas extraction well head or by the detection of soot in the gas collection system. At times, underground combustion/oxidation will go undetected until a sinkhole or smoke appears. Normally flames are not visible during this type of fire unless the subsurface fire is excavated and exposed to the atmosphere. With the correct conditions present, spontaneous combustion can occur in household trash or at construction debris facilities. This type of combustion will produce excessive amounts of CO and other trace gases due to incomplete oxidation. To confirm subsurface oxidation by using CO measurements, the results must be acquired through quantitative laboratory analysis. The _ CalRecycle staff considers levels of CO in excess of 1,000 ppm to be a positive indication of an active underground landfill subsurface oxidation event. Levels of CO between 100 and 1,000 ppm are viewed as suspicious and require further air and temperature monitoring. Levels between 10 and 100 ppm may be an indication of subsurface oxidation but not active combustion. e Solid Waste Disposal Facility Criteria Technical Manual,USEPA 530-R-93-017, 1993