|This guidance document is intended to describe several considerations expected in a landfill permit application and suggest methods of preventing or overcoming existing or future gas generation hazards and complications at a municipal solid waste landfill.
Landfill gas is created by the natural decomposition of organic materials by microorganisms in the landfill. The rate and quantity of landfill gas generation depends on many factors; the key ingredients are the type and quantity of organic materials and the moisture content. Landfill gas is typically composed of methane (50-55%), carbon dioxide (45-50%), and trace amounts of other gases such as oxygen, nitrogen, and hydrogen sulfide. At each landfill in the State, deposition methods, covering, and content of the waste will vary from other landfills in the State. There is no single gas monitoring strategy that will work at all landfills in the State. Each landfill should have a site-specific gas monitoring plan for landfill gas. The gas monitoring plan should also be specific to the gas measuring device, and the measuring device operating manual should be incorporated by reference into the site gas monitoring plan. Methane should be controlled because it can migrate away from a landfill and become an environmental and safety hazard.
Methane gas is a by-product of the decomposition of waste, but methane is not produced until the waste decomposition process has completely used up all the oxygen in the deposited waste. Methane by itself is dangerous when the concentration in air is between the lower and upper explosive limits and when ignited has caused burn injuries and death where it has accumulated in close quarters. Once landfill gases are produced under the landfill surface, they can move away from the landfill.
Landfill gases tend to expand and fill any available space, so that they move, or “migrate,” through the limited pore spaces within the refuse and soils covering the landfill. The natural tendency of landfill gases that are lighter than air, such as methane, is to move upward, usually through the landfill surface. Upward movement of landfill gas is restrained by densely compacted waste or landfill cover material (daily soil cover and caps). When upward movement is inhibited, the gas tends to migrate horizontally to other areas within the landfill or to areas outside the landfill, where it can resume its upward path. Basically, gas follows the path of least resistance. For these reasons, gas monitoring probes need only penetrate to the lowest level of waste or an impermeable strata.
It is difficult to predict the distance that landfill gas will travel because so many factors affect its ability to migrate underground, but travel distances greater than 1,500 feet (1/4 mile or more) have been verified. Barometric pressure changes caused by moving weather front systems can also have an effect on the confining pressure at a landfill. Barometric pressure changes can produce large variations in methane gas measurements during a given day as well as between the regulated monitoring periods.
Confining pressure variations and the randomness accompanying the variations give rise to one defensive design characteristic that must be a part of every landfill gas monitoring probe design. The probe must be sealed to prevent external air from entering the probe while the gas measurements in the probe are being taken. Gases migrating from a landfill may eventually reach buildings and homes. Foundation cracks and gaps, pressure differences between the inside and outside of the building or home, mechanical ventilation systems and leakage areas (utility entry points, construction joints, or floor drain systems) provide entry points for gases.
Buildings with basements generally provide the easiest access for gases migrating in the soil. The amount of gases let into a building depends on a number of factors, including construction and maintenance practices.
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