Gas production in a sanitary landfill results from waste decomposition. Decomposition, in turn, contributes to settlement. Initial· decomposition is aerobic, requiring the presence
of oxygen; carbon dioxide, water, and heat are the principal products. As oxygen is depleted, methanogenic bacteria become dominant producing gas containing 50 to 60 percent methane (CH 4). The remainder of the gas is principally carbon dioxide (C0 2). LFG disperses in all directions from the landfill mass. Two mechanisms govern gas migration: convection and diffusion. Convection is movement in response to a pressure gradient; movement is ·in the direction of decreasing pressure. Diffusion is movement in response to a concentration gradient. Gas concentrations (CH4 and co 2) will be highest in the landfill. Theref~re, flow
resulting from diffusion will be in all directions away from the decomposing refuse. Normally, both concentration and pressure gradients are present, resulting in a combination of diffusional and pressure gas flow.
Since methane is potentially explosive in concentrations of between 5 to 15 perceht in air, it is important to control its movement. The Environmental Protection Agency (EPA), through the
Resource Conservation and Recovery Act (RCRA) of 1976, established guidelines for gas control at operating sanitary landfills. Local enforcement agencies (LEA's} have adopted or expanded
on the RCRA guidelines, and have applied requirements to closed landfills as well. According to RCRA, methane concentrations must not exceed 1.25 percent by volume in facility structures
and 5 percent by volume at the landfill property boundary. In response to growing concerns regarding potentially toxic emissions· or air quality degradation resulting from LFG production at landfill sites, regulations are also being developed to limit surficial gas emissions from both active and completed landfill sites.