Page 347 - Materials Chemistry, Second Edition

CAT3525_C10.qxd 1/31/2005 12:00 PM Page 318
318 Waste Management Practices: Municipal, Hazardous, and Industrial
explode, as the gaseous mixture is considered ‘rich.’ This 15% threshold is the Upper Explosive
Limit (UEL) defined as the maximum concentration of a gas, above which the substance will not
explode when exposed to a source of ignition. The explosive hazard range occurs between the LEL
and the UEL. It must be noted that at methane concentrations above the UEL, fire may still be pos-
sible and asphyxiation will occur. In addition, a sudden dilution of the methane in the local atmos-
phere can bring the mixture back within the explosive range.
Methane is generated from MSW only when the moisture content of the waste exceeds 40% under
anaerobic conditions. For example, if a landfill is holding wastes having 15% moisture, the waste will
be ‘fossilized’, i.e., it will not decay and therefore will produce very little methane (Vesilind et al., 2002).
The frequency of landfill gas monitoring is determined based on soil conditions, surface hydrol-
ogy, hydrogeology, and location of facility structures. If methane gas levels exceed established lim-
its, a remediation plan must be prepared within 60 days of detection. Air must be sampled within
facility structures where gas may accumulate and in soil at the property boundary. Other monitor-
ing methods may include sampling gases from probes within the landfill unit. A typical gas-moni-
toring probe installation is shown in Figure 10.14. The frequency of monitoring should be sufficient
to detect landfill gas migration based on subsurface conditions and changing landfill conditions.
Monitoring must be conducted at least quarterly (40 CFR 258.23). The number and location of gas
probes is site-specific and dependent on subsurface conditions, land use, and location and design of
facility structures. At the facility and in neighboring properties, structures with basements or crawl
spaces are more susceptible to landfill gas infiltration and must also be monitored.

Protective cover

Cap with
flexible
tubing

Compacted soil backfill
3'+
1/2"−1 1/4" PVC pipe

Grout or clay plug

1/2"−1 1/4" perforated
6"+
PVC pipe
Varies
Natural
ground

Natural
ground
Washed pea gravel

4" minimum bores

FIGURE 10.14 Typical gas monitoring probe (U.S. EPA, 1994).