28    SUBSURFACE LANDFILL GAS MIGRATION:
A CASE HISTORY OF AN AIR INJECTION SYSTEM
FOR MIGRATION CONTROL
Eric R. Peterson, Project Engineer
Peter J. Carrico, Project Scientist
SCS ENGINEERS
Reston, Virginia 22090
Jeffrey M. Smithberger, Engineer III
Fairfax County Division of Solid Waste
Fairfax, Virginia 22030
INTRODUCTION
Subsurface landfill gas (LFG) migration may be controlled by a number of methods including air
injection. This type of system utilizes a line of interconnected wells through which air is injected to
develop a positive pressure in the soil to inhibit LFG migration beyond the air curtain. The following
case history includes a description of an LFG migration investigation leading to the selection, design,
construction and start up of such a system. Since LFG monitoring following startup of air injection
systems has revealed that LFG behind an air curtain can be pushed towards the area to be protected,
such monitoring is an important component of start-up operations.
Monitoring results for the subject project found that under certain soil conditions, LFG trapped
behind the air curtain travelled great distances and was separated into its major components of carbon
dioxide and methane. The methane travelled ahead of the carbon dioxide and attained concentrations
greater than 80% as a result of this separation.
LANDFILL GAS MIGRATION
General
LFG is a product of the natural biological decomposition of organic material contained in wastes
deposited in landfills. In a landfill environment, buried organic wastes (such as newspaper, lawn
clippings, food wastes, agricultural residue, etc.) initially undergo aerobic decomposition. As oxygen
contained in the refuse is depleted, anaerobic decomposition processes commence. The production of
methane gas (a principal component of LFG) by methanogenic bacteria usually begins shortly thereafter, and continues for many years. The LFG which results from anaerobic decompostion is predominantly methane and carbon dioxide.
Production of LFG creates positive pressure within the landfill. This pressure acts as a driving force
(convection), causing LFG migration into surrounding soils and through surface soils. In addition, a
concentration gradient causes diffusive flow of LFG away from the landfill to off-site areas. LFG
flows from areas of high pressure or concentration into areas of low pressure or concentration. Most
of the LFG vents to the atmosphere through the surface of the fill; however, under certain soil
conditions and landfill configurations, the aforementioned driving forces can result in off-site subsurface LFG migration of more than 1,000 feet from the refuse mass.
LFG migration can create a hazard, since the methane present in LFG is combustible. Methane can
cause an explosion when allowed to accumulate in concentrations between 5 and 15 percent by volume
in air (the explosive limits for methane gas) and when in the presence of oxygen and a source of
ignition.
43rd Purdue Industrial Waste Conference Proceedings, © 1989 Lewis Publishers, Inc., Chelsea, Michigan 48118.
Printed in U.S.A.
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