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Draft Supplemental Environmental Impact Report Page IV.E-12 Forward Inc. Landfill 2018 Expansion Project methane gas buildup that could be ignited by machinery or onsite workers,however,the site includes a landfill gas collection system that reduces the chance of a dangerous on-site landfill gas build-up except in the waste mass itself. Perimeter wells have low concentrations of methane indicating that some off-site migration is occurring. Despite the fact that methane is lighter than air (vapor density of 0.55 versus 1), it is concurrently produced with carbon dioxide and will not separate. Instead,both gases will remain mixed and follow pressure and density gradients during transport based on the properties of the mixture, rather than the properties of the individual components (EPA 1993)11. 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 workers were exposed to low lying areas within the landfill where methane accumulated (Everett, Wilson and Hoyhnan, 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 (SSO)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 SSO. 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. SSO 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 Solid Waste Disposal Facility Criteria Technical Manual,USEPA 530-R-93-017, 1993