Methane, like carbon dioxide, is odorless. However, unlike carbon
dioxide, methane is relatively insoluble in water. When methane is prese
in air in concentrations between 5 and 15 percent, it may be explosive.
Methane can move both vertically and laterally from the landfill under a
pressure gradient or a concentration gradient (diffusion). Generally, it
vents into the atmosphere, where it is harmlessly dissipated. However,
migrating landfill gas can seep into buildings, utilities, or other enclo
spaces, and create a hazardous condition. Migrating gas can also damage
vegetation on the surface of a surrounding landfill by displacing oxygen
from the root zone. The final landfill cover (e.g. soil, pavement,
vegetation) can influence the extent of vertical and lateral landfill gas
migration. Landfill gas is known to have migrated laterally several hund
feet from landfill sites.
In order to determine whether a potential hazard exists due to
subsurface migration of LFG, a monitoring system is generally installed.
typical monitoring system consists of a series of monitoring wells
constructed between the landfill and the area of concern (e.g., an inhabi
structure). The wells are generally equipped with one to three monitorin
probes for detecting LFG concentrations. Figure 19 shows a typical
monitoring well configuration.
When monitoring wells indicate migration of potentially hazardous lev
of methane gas (5 percent or greater), gas migration controls are require
(RCRA requires that methane levels be maintained at 5 percent or 1 less a
the property boundary of the landfill, and less than 1.25 percent in any
onsite structures.) Gas control techniques can generally be classified i
two categories: passive or active methods. These methods are outlined in
Table 23.
The various passive type LFG control systems commonly used are
illustrated on Figures 20 and 21, Figure 22 illustrates a typical landfil
gas extraction well configuration, and Figure 23 illustrates a forced air
injection and landfill gas extraction system.
As shown on Figure 20, LFG interceptor trenches, barrier trenches, an
combination interceptor and barrier trenches are installed between the
landfill and the structure(s) to be protected, or between the landfill an
the property line if the structure is located offsite. Generally, trench
are installed as close to the structure(s) property line as practical to
protected. Sufficient space should be left between the structure(s) or
property line to allow for installation of monitoring wells to monitor
trench effectiveness.
The spacing (D) of the air injection and gas extraction wells shown o
Figure 23 is dependent upon several factors including:
a) Landfill depth.
b) Horizontal versus vertical soil permeabilities in which wells
are placed.
c) Distance between the landfill and structure(s) property line to
be protected.
Figure 24 provides an example calculation for determining injection a
extraction well spacing in soil.
5.10-86
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