Environmental Engineering Reference
In-Depth Information
the landfill due to a reduction in available air pathways. This reduction will also reduce the
rate of gas flow and decrease gas recovery rates.
Geologic Conditions.
Geologic conditions must be determined to estimate the potential
for off-site migration of gas. Permeable strata such as sands, gravels, and weathered bedrock
provide a potential pathway for off-site migration, especially if these layers are overlain by a
layer of low permeability soil. Geologic investigations must be performed to determine the
potential for off-site migration. Additional attention must be given to areas where houses and
other structures are present to ensure off-site migration will not impact these structures.
Depth to Ground Water.
The water table surface acts as a no-flow boundary for gas. As a
result, it is generally used to help estimate the thickness of the zone through which gas can
travel. A consistently high ground water table will significantly reduce the potential for off-
site migration of gas. The depth to groundwater (as well as seasonal variations) also needs to
be evaluated during the design process to evaluate well construction requirements and the
potential for water table upwelling (i.e., the upward rise of the water table toward a vacuum
well screened in the unsaturated zone).
Man-Made Features.
In some instances, underground utilities such as storm and sanitary
sewers or the backfill that surrounds these features may produce short-circuiting of airflow
associated with an active landfill gas collection system. As a result, airflow may be
concentrated along these features rather than within the landfill. Man-made features also
provide a potential pathway for the off-site migration of landfill gas.
Landfill Cover and Liner Systems.
The components of many hazardous and solid waste
landfill cover systems consist of a vegetated surface component, a drainage layer, and a low
permeability layer composed of one or more of the following: geomembrane, geosynthetic
clay liner (GCL), or compacted clay. A geomembrane in the cover system will prevent the
intrusion of air into the waste. Therefore, a higher operating vacuum can be applied to the gas
collection system without the danger of overdrawing. Thus, the effective radius of influence
of each well is increased. Overdrawing occurs when oxygen from the atmosphere is pulled
into the landfills interior during the anaerobic phase.
Landfill liner systems consist of various combinations of low permeability layers and
leachate collection layers. The low permeability layers are created using natural low
permeability geologic formations, compacted clay, geomembranes, and geosynthetic clay
liners. Liner systems prevent the migration of LFG to the surrounding areas. Liner systems
also prevent gases in the surrounding geologic formations from being pulled into the LFG
collection system.
Barometric Pressure.
The amount of gas escaping from a landfill's surface changes as
barometric pressure changes. Gas generation within a landfill will result in a positive pressure
gradient from the inside to the outside of the landfill. For a passive LFG collection system,
increases in atmospheric pressure will cause a decrease in gas flow from a landfill because the
pressure differential between the inside and the outside has decreased. For an active gas
collection system, there is a higher probability of atmospheric air intrusion through the
landfill cover during periods when the barometric pressure is rising. The amount of air
