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and should not be interconnected. This will allow for individual lines to be valved independently for
future active system control and balancing.
(c) Well Systems. Well systems consist of a series of gas extraction wells (perforated or
slotted vertical collection pipes) that penetrate to near the bottom of the refuse. Well systems are
recommended for landfills or portions of landfills that exceed 12 m (40 ft) in depth. The vent borehole
diameter may range from 0.3 to 1 m (1 to 3 ft). The components of extraction wells are usually similar
to those of standard ground water monitoring wells (i.e., riser, screen, gravel pack). A well system,
either active or passive, is useful for layered landfills where vertical gas migration is impeded. The
design of a well system requires estimates of the rate of gas production and the radius of influence of the
wells. Because of the variability of landfill refuse, design procedures are difficult to apply to gas
collection systems. Gas collection wells are typically spaced at a frequency of one per acre.
(d) Wellheads and Header Piping. Wellheads for passive gas vents are typically configured
to prevent precipitation and wildlife from entering the well. Wellheads for active well systems may
include sampling ports, pressure gauges, control valves, and flexible connections. Header piping is used
for active systems to transport gas from the collection system to a flare. The header piping is typically
made of PVC or HDPE and should be sized to provide for minimal head losses and additional capacity,
should supplementary extraction wells be required at a later date. The piping can be placed on the
landfill surface or be buried. The advantage of placing the header pipe on the landfill surface is ease of
pipe maintenance. However, this type of installation will expose the piping to UV radiation and increase
the possibility for damage from maintenance equipment and vandalism, potentially blocking surface
runoff. The potential for blockage by condensate freezing in the pipes is also increased if the header
pipe is placed on the landfill surface. If the header pipe is buried within the landfill cover, it is typically
located above the geomembrane cover system but below frost depth. A minimum of 150 mm (6 in.) of
bedding material should be placed over the geomembrane prior to placement of the header pipe.
Condensate collection points should be located at low points in the header pipe system to prevent
blocking of the pipe with condensate. Depending on local regulations, condensate is sometimes allowed
to drip back into the waste either through the wellheads or a separate percolation drain.
(e) Monitoring Probes. Gas monitoring probes are used in conjunction with both active and
passive systems to detect landfill gases that may migrate off-site. Usually, the regulatory compliance
point is the property boundary, and the maximum concentration allowed is usually 10% of the LEL
(Lower Explosive Limit) for methane. Probes are typically placed around the perimeter of the landfill at
a minimum spacing of 150300 m (5001000 ft), although they may be closer, depending on site
conditions. At some sites, probes may be closely spaced (every 3060 m [100200 ft]) if there are
buildings near the landfill. A typical monitoring probe consists of a small-diameter slotted pipe in a
borehole that extends to an elevation corresponding with the bottom of the waste or to the water table,
whichever is shallower. Specific monitoring probe designs are given in EM 1110-1-4001, Soil Vapor
Extraction and Bioventing.
m. Random Fill and Regraded Waste.