Page 346 - Materials Chemistry, Second Edition

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The Sanitary Landfill 317
soil moisture content near the surface, upward landfill gas flow is inhibited. A cell with a frozen sur-
face will similarly promote lateral migration. The effects of geology and surface conditions on gas
migration are shown in Figure 10.12. Lateral gas migration is more common in older facilities that
lack both liners and gas control systems.
In order to ensure safety to humans and structures, landfill gas must be regularly monitored.
Methane concentrations must not exceed 25% of the lower explosive limit (LEL) in facility struc-
tures, and must not exceed the LEL at the perimeter of the facility property. The LEL is defined as
the lowest percent by volume of a mixture of explosive gases in air that will propagate a flame at
25°C and atmospheric pressure (see Figure 10.13). Methane is explosive when present in the range
of 5 to 15% (by vol.) in air. At methane concentrations greater than 15%, the gas mixture will not

Clay or synthetic cap Clay soil, frozen or
(low permeability)
saturated soil, or pavement
(low) permeability

Refuse

Sand and gravel soil
(high permeability)
(a)

Clay or synthetic liner Sand and gravel cap
(low permeability) (high permeability)

Refuse
Clay soil
(low permeability)

(b)
FIGURE 10.12 Effects of surrounding geology and surface features on landfill gas migration (U.S. EPA, 1994).

FIGURE 10.13 The lower explosive limit, upper explosive limit and explosive range for a hypothetical gas.