30 Sep 01
pressure gradients. Gas flow by diffusion is induced by concentration gradients resulting from gas
production. Vertical or lateral migration paths for gas movement out of the landfill are influenced by the
presence of migration corridors or barriers, or both. Migration corridors include sand and gravel lenses,
void spaces, cracks, fissures, utility conduits, and drainage culverts. Barriers to gas migration include
clay layers, geomembranes, GCLs, geological formations, and high or perched water tables. The
potential impacts of landfill gas are as follows:
Explosion Hazard. Methane can migrate through the subsurface and collect in adjacent structures
creating a potential explosion hazard.
Vegetation Distress. Landfill gases can distress the vegetation on a landfill cover.
Odor. Odor control becomes a design parameter if the landfill is located adjacent to existing or
Physical Disruption of Cover Components. Gas pressure build-up beneath the geomembrane
can force the geomembrane to protrude or "bubble out" from the cover.
Toxic Gases. Gases produced by landfills can be toxic or may not comply with regulatory
(2) Design of Gas Control Systems. Gas control systems consist of collection, conveyance,
and outlet components and are designed to be either passive or active. A passive system allows the
landfill gas to exit the collection system without mechanical assistance, whereas an active system does
the opposite, using mechanical assistance, such as blowers, to extract gases. Depending on the
potential impacts of the landfill gas and local regulatory criteria, gases are either dispersed into the
atmosphere or collected and treated. Options for gas control systems are described below.
(a) Continuous Blanket Systems. The EPA recommends that a continuous blanket system to
collect gas have a minimum of 300 mm (12 in.) of granular fill or an equivalent geosynthetic material
located below the impermeable barrier layer and above the random fill. A continuous blanket system -
will allow free movement of gases to either collection or outlet pipes. Vertical outlet pipes transport the
collected gases from beneath the landfill cover to the atmosphere. The number of vent pipes should be
minimized and are normally spaced about 60 m (200 ft) apart. This provides approximately one vent
per 4000 m2 (one per acre). Perforated horizontal collection pipes can also be incorporated into the
design of either passive or active blanket systems. A geotextile filter layer may also be required to
prevent clogging of the gas collection blanket material.
(b) Shallow Trench Systems. For landfills where the waste materials are relatively shallow
(less than 12 m [40 ft] in depth), collection trenches may be used. The trenches are usually excavated
about 0.5 to 1 m (2 to 3 ft) into the waste, are lined with a geotextile, have a perforated pipe installed,
and then are filled with coarse rock. The trenches should be spaced approximately 60 m (200 ft) apart