Environmental Engineering Reference
In-Depth Information
A. LFG Movement and Migration
Gases generated in the landfill will move throughout the mass of waste in addition to
movement or migration out of the site. The mechanism of gas movement is via gaseous
diffusion and advection or pressure gradient. That is, the gas moves from high to low gas
concentration regions or from high to low gas pressure regions (Kjeldsen et al 2002).
Movement of gas within the mass of waste is governed by the permeability of the waste,
overlying daily or intermittent cover, and the degree of compaction of the waste. Lateral
movement of the gases is caused by overlying low permeability layers such as the daily cover
and surface and sub-surface accumulations of water. Vertical movement of gas may occur
through natural settlement of the waste, between bales of waste if a baling system is used to
compact and bale the waste, or through layers of low permeability inert wastes such as
construction waste rubble. Where landfill gas extraction is practised to recover the gas for
energy use, the gas is collected in gas wells, and piped to the surface (Waste Management
Paper 27, 1994).
Fully contained landfill sites where, after completion, the landfill is capped with an
impermeable synthetic and natural containment system to prevent migration of landfill gas
out of the site and which have gas recovery systems in place, have low gas emission levels
(Mosher et al 1999). The capping liner system is also designed to prevent ingress of
precipitation. For landfill sites, where landfilling is still in operation and where the waste is
only partially covered by an impermeable layer, there are higher emissions of landfill gas.
Waste landfills are a source of volatile organic hydrocarbons, both to the site workers and to
the surrounding neighbourhood. The contribution of a range of chemicals identified in landfill
gas has been shown to be significant contributors to the toxic air pollutants in local
neighborhoods adjacent to landfill sites (Scheff et al 2001). Certain chemicals, including
chlorinated hydrocarbons, have been identified as being derived from landfill as the major
source. Additionally, other work has shown that chlorinated hydrocarbons are found in
landfill gas at concentrations which exceed occupational exposure levels (Allen et al 1997).
However, it is unlikely that long exposure to such levels would be experienced by landfill site
workers and even less likely for members of the public (Allen et al 1997).
Sub-surface gas migration out of the mass of waste into the surrounding environment
may occur from older sites, where containment was not practised, or through containment
sites, where significant leakage has occurred. In addition, leachate movement out of such sites
may cause later degradation to landfill gas. Migration of gas outside the site requires
migration pathways such as high-permeability geological strata, through caves, cavities,
cracks in the overlying capping layer and through man-made shafts, such as mine shafts and
service ducts, etc. Gas may migrate considerable distances from the boundaries of the site
through these possible pathways. It has been reported that changes in the major and trace
components of landfill gas occur during subsurface migration (Ward et al 1996). For
example, reduction in methane concentration occurs due to oxidation, and some alteration of
trace landfill gases occurs due to adsorption onto soil particles, oxidation, degradation,
condensation and dissolution.
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