Methane migration is the movement of methane through soil, rock, made ground and other subsurface pathways away from the place where the gas was generated. At landfill sites, methane forms as a major component of landfill gas and, unless adequately contained and controlled, can migrate laterally beyond the deposited waste.
The most serious consequences occur when migrating methane reaches buildings, service ducts, chambers, basements or other confined spaces where it can accumulate. Methane mixed with air becomes flammable at approximately 5% methane by volume. If an ignition source is then present, fire or explosion can result.
Landfill gas migration is therefore best understood as a source-pathway-receptor problem. The source may be a landfill producing methane. The pathway may be a permeable geological layer, fractured rock, drainage material or utility trench. The receptor may be a house, factory, underground chamber or other confined space.
Understanding all three is fundamental to assessing methane migration risk.
Key Takeaways
- Methane can migrate considerable distances underground when a suitable pressure gradient and permeable pathway exist.
- Distance from a landfill alone does not determine risk. Geology, utility trenches, drains, fissures and other preferential pathways may be more important.
- Falling atmospheric pressure can increase landfill gas migration by increasing the pressure differential between gas within the waste or ground and the atmosphere.
- Methane becomes flammable in air at approximately 5% by volume, its Lower Explosive Limit (LEL).
- Surface methane monitoring and subsurface migration monitoring are different activities and should not be confused.
- Landfill gas contains more than methane. Carbon dioxide and trace constituents may also migrate, although their behaviour and concentrations can differ during subsurface transport.
- Detecting methane does not automatically identify its source. Landfills, coal workings, natural geology, organic soils and leaking natural-gas infrastructure may all need consideration.
- Modern landfill-gas regulation is jurisdiction-specific, but the underlying engineering principles are international.
Image shows – Evidence of methane migration that causes die-back in grasses and the death of trees.
What Is Methane Migration?
Methane does not necessarily remain where it is generated.
Within a landfill, decomposition of biodegradable waste under anaerobic conditions produces landfill gas containing principally methane and carbon dioxide, together with smaller quantities of other gases.
Gas movement occurs in response to pressure and concentration gradients. If landfill gas pressure exceeds that in surrounding ground, gas may move outwards through materials capable of transmitting it.
This movement may be vertical, eventually resulting in emissions through the landfill surface, or lateral through the sides of the landfill and surrounding ground.
It is this latter process that is usually meant by subsurface landfill gas migration.
This distinction is important. Methane detected above the surface of a landfill is evidence of a surface emission. Methane detected within soil or rock outside the waste mass may indicate subsurface migration. These are related phenomena, but they require different monitoring and investigation techniques.
Why Does Landfill Gas Migrate?
A landfill containing biodegradable waste can be regarded as a large gas-generating body.
Methane and carbon dioxide produced within the waste occupy pore spaces and voids. Unless that gas is adequately collected, vented or otherwise controlled, pressure can develop.
Gas tends to move from regions of higher pressure towards regions of lower pressure. Where the surrounding strata provide a pathway, landfill gas can therefore move laterally away from its source.
Modern engineered landfills use containment, capping and gas collection systems intended to control this behaviour. However, landfill gas migration remains site-specific and cannot safely be assessed simply from the age or apparent engineering standard of a landfill.
Historic and closed landfills can be particularly important because many pre-date modern containment engineering. Former quarries, gravel pits and other excavations were commonly filled with waste without the engineered low-permeability barriers now associated with modern landfill construction.
At such sites, the surrounding geology may itself become the principal route through which landfill gas moves.
The Source-Pathway-Receptor Model
A useful conceptual model for methane migration divides the problem into three components: source, pathway and receptor.
The Source
Where is the methane being generated?
Potential sources include:
- biodegradable waste within active or closed landfills;
- made ground containing degradable organic material;
- abandoned coal workings;
- naturally methane-bearing geological strata;
- peat and other organic soils;
- leaking natural-gas mains and services; and
- other buried organic deposits.
The mere presence of methane close to a landfill does not prove that the landfill is responsible.
The Pathway
How could gas travel from the source towards a receptor?
Potential migration pathways include:
- permeable sands and gravels;
- fissured or fractured rock;
- thin permeable geological strata;
- coal seams and old mine workings;
- drainage blankets;
- service trenches;
- sewers and drains;
- ducts and culverts;
- poorly sealed boreholes; and
- interconnected voids within made ground.
A narrow, highly permeable pathway can sometimes be much more significant than the average permeability of the surrounding ground.
The Receptor
Where could migrating methane cause harm?
Important receptors include:
- houses and other occupied buildings;
- basements;
- suspended floor voids;
- utility chambers;
- manholes;
- pumping stations;
- tunnels;
- service ducts; and
- other confined or poorly ventilated spaces.
Finding methane in the ground is therefore only the beginning of an investigation. The critical issue is whether there is a credible connection between a methane source, a migration pathway and a receptor where harmful concentrations could develop.
How Methane Travels Underground
Subsurface methane movement is controlled by several interacting mechanisms.
Pressure-Driven Gas Movement
Where gas pressure at the source exceeds pressure in the surrounding ground, gas can be driven outwards through permeable materials. This pressure-driven, or advective, transport can be particularly important close to landfills and along preferential pathways.
Diffusion
Methane can also move because of concentration differences, migrating from areas of higher methane concentration towards areas of lower concentration.
In real landfill migration situations, pressure-driven movement and diffusion may both contribute.
Geological Controls
Geology can dominate both the rate and direction of migration.
A landfill surrounded by low-permeability clay presents a very different migration setting from one excavated into fractured sandstone, permeable gravel or other transmissive strata.
Even where much of the surrounding geology has low permeability, a relatively thin permeable stratum can provide a preferential migration pathway.
This is why placing gas-monitoring boreholes simply at regular intervals around a landfill without understanding the geology can provide a misleading impression of risk. Monitoring locations should be informed by a conceptual model of the site.
Man-Made Methane Migration Pathways
Human activity can create pathways considerably more permeable than the surrounding natural soil.
A common example is the utility trench.
A pipe or cable may have been installed in a trench excavated through relatively impermeable ground and then surrounded by sand, gravel or another granular backfill. The result may be an artificial gas pathway extending for tens or hundreds of metres and, in some cases, leading towards buildings.
Drains, sewers, culverts and other underground structures can have similar effects.
For this reason, a methane migration investigation should never stop with a geological map. Drainage drawings, utility plans, historic landfill plans and knowledge of previous excavation and construction may be equally important.
Atmospheric Pressure and Methane Migration
Atmospheric pressure can have an important influence on landfill gas behaviour.
When atmospheric pressure falls rapidly, gas pressure within a landfill and surrounding ground does not necessarily fall at the same rate.
This can temporarily increase the pressure differential between gas within the landfill or ground and the atmosphere. Where suitable pathways exist, the increased pressure differential can promote outward gas movement.
It is therefore misleading to think of falling atmospheric pressure simply as “sucking” methane from a landfill. What matters is the changing pressure differential and the ability of the gas-generating mass and surrounding ground to equilibrate with the atmosphere.
This is also why a methane concentration measured on a single monitoring visit should not automatically be regarded as representative of all conditions.
A measurement obtained during stable or rising atmospheric pressure may differ substantially from measurements during or following a rapid pressure fall.
Current Environment Agency guidance for landfills in England consequently includes atmospheric pressure and weather conditions among the factors that must be considered when monitoring landfill gas.
Loscoe: A Landmark Landfill Gas Migration Explosion
One of Britain's most important landfill gas migration incidents occurred at Loscoe, Derbyshire, in March 1986.
Landfill gas migrated from a nearby landfill through the surrounding ground and reached a bungalow, where methane accumulated before an explosion severely damaged the property.
Loscoe became an important case in the development of British understanding of landfill gas migration, particularly the relationships between landfill engineering, geology, gas pressure and changing atmospheric conditions.
The incident demonstrates why distance between a landfill and a building cannot, by itself, determine risk.
The important questions are:
- Was methane being generated?
- Was there a pathway capable of transmitting it?
- Did that pathway connect with a receptor where methane could accumulate?
- What site or atmospheric conditions increased the rate of migration?
The human history should also be recorded accurately. Three members of the Middleton family were present in the bungalow at the time of the explosion. Earlier accounts, including a previous version of an article on this website, have sometimes referred simply to a “couple” being present and thereby omitted their adult son, Trevor Middleton.
The separate history of the Loscoe incident on this website has now been corrected accordingly. Any further expansion of the family's first-hand account will be undertaken separately from this technical methane migration article.
Methane Migration Into Buildings
Methane becomes particularly dangerous when migration brings it into an enclosed space.
Potential entry routes include:
- cracks in floors and foundations;
- gaps around service pipes;
- drainage connections;
- construction joints;
- sumps;
- ducts;
- wall and floor cavities; and
- permeable ground beneath suspended floors.
Once methane enters a building, the resulting concentration depends upon the rate of gas entry, the volume of the receiving space and the amount of ventilation.
An apparently modest flow of gas can therefore become important when it enters a small or poorly ventilated void continuously.
Methane is colourless and, in its pure form, odourless. The characteristic smell associated with distributed natural gas is produced by deliberately added odorants. Landfill methane must not therefore be assumed to provide an olfactory warning before hazardous concentrations develop.
Understanding Methane's Lower Explosive Limit
Methane's Lower Explosive Limit (LEL) in air is approximately 5% methane by volume.
Its approximate flammable range in air is 5% to 15% methane by volume.
Below the LEL, the methane-air mixture is normally too lean to sustain flame propagation. Above the Upper Explosive Limit (UEL), the mixture is too rich, although a methane-rich atmosphere can become flammable as additional air is mixed into it.
The LEL should therefore never be interpreted as a desirable operating or safety target.
If methane is entering a confined space and its concentration is increasing, waiting until the concentration reaches 5% before treating the situation seriously leaves little safety margin.
Landfill Gas Regulation: UK, European and International Approaches
The physical principles governing landfill gas migration are international. The legislation, permit conditions and regulatory authorities applying those principles are jurisdiction-specific.
This distinction is increasingly important for professionals using technical information published online, because a good engineering method may be applicable internationally even though a particular legal requirement applies only in one country.
The European Regulatory Tradition
A major foundation of modern European landfill regulation is Council Directive 1999/31/EC on the landfill of waste, usually called the EU Landfill Directive.
Among its landfill-gas requirements are the principles that appropriate measures must be taken to control the accumulation and migration of landfill gas, and that landfill gas must be collected from landfills receiving biodegradable waste and treated and used where practicable. Where collected gas cannot be used to produce energy, it is to be flared.
The Directive therefore established a common European framework based upon landfill gas containment, collection, treatment, monitoring and control.
EU member states implement these requirements through their own national legislation, permits and regulatory systems. The Directive establishes the framework; it does not mean that every landfill in Europe operates under identical national regulations or permit wording.
Landfill Gas Regulation in the United Kingdom
The UK position is more complicated than simply referring to “Environment Agency regulations”.
Environmental regulation is devolved. Landfill regulation and permitting are administered separately by:
- the Environment Agency in England;
- Natural Resources Wales in Wales;
- the Scottish Environment Protection Agency (SEPA) in Scotland; and
- the Northern Ireland Environment Agency (NIEA) in Northern Ireland.
The legislation, permitting procedures and current regulatory guidance are not identical in the four nations.
However, modern UK landfill regulation developed from a closely related European regulatory framework, and the fundamental landfill-gas engineering principles remain strongly aligned: assess gas generation, prevent uncontrolled accumulation and migration, collect and treat gas where required, monitor its behaviour and maintain appropriate management throughout the operational and aftercare phases.
For example, Scottish landfill requirements include collection and treatment of landfill gas at biodegradable landfills, while Welsh landfill guidance likewise retains the principles developed under the Landfill Directive.
Environment Agency material is therefore cited frequently on landfill-gas.com because it provides extensive and useful technical guidance. Where it is cited, readers should understand that it is principally regulatory guidance for England, not legislation applicable automatically throughout the UK.
Professionals working in Scotland, Wales or Northern Ireland should consult the requirements and guidance of the appropriate national regulator.
The Wider Influence of the European Approach
The influence of the European landfill model extends beyond EU member states.
European environmental standards, engineering practices and regulatory concepts have influenced landfill design and regulation in other jurisdictions, including countries developing or modernising their waste-management systems and projects designed to European or international financing standards.
It would be inaccurate, however, to say that all such countries have “the same” landfill gas regulations.
What is transferable is the underlying engineering approach:
identify gas generation; control accumulation; prevent uncontrolled migration; collect and treat gas where appropriate; monitor its behaviour; protect sensitive receptors; and maintain controls for as long as the landfill continues to present a gas risk.
The engineering principles can therefore be applied internationally while the precise legal duties, monitoring frequencies, action levels and reporting requirements must always be checked for the jurisdiction concerned.
The United States Uses a Different Regulatory Structure
The United States reaches many similar engineering objectives through a different regulatory framework.
Federal requirements include the Resource Conservation and Recovery Act (RCRA) Subtitle D criteria for municipal solid waste landfills, including requirements concerned specifically with explosive-gas migration.
Separate Clean Air Act requirements address landfill air emissions and gas collection and control at qualifying facilities, while states may establish additional requirements.
Despite these differences in legal structure and terminology, there is considerable common ground between US and European landfill gas engineering.
Both regulatory traditions recognise that landfill gas can create risks to people and the environment, that uncontrolled methane migration must be prevented or addressed, that monitoring is necessary and that gas collection or other engineering controls may be required.
For engineers, environmental professionals and landfill operators, the useful distinction is:
The physics of landfill gas migration is universal.
The law governing how that migration must be monitored, reported and controlled is jurisdiction-specific.
US RCRA Methane Limits: An Important Distinction
The US RCRA Subtitle D requirements provide a useful example of the distinction between a physical explosion limit and a regulatory action level.
Under 40 CFR Section 258.23, owners or operators of municipal solid waste landfill units must ensure that methane generated by the facility does not exceed 25% of the Lower Explosive Limit within facility structures, excluding gas-control or recovery-system components.
With methane's LEL at approximately 5% by volume, 25% of the LEL corresponds to approximately 1.25% methane by volume.
At the facility property boundary, methane must not exceed the LEL itself, approximately 5% methane by volume.
These figures are sometimes incorrectly reversed.
If an exceedance occurs, the federal regulation requires immediate steps to protect human health, placement of the relevant information in the operating record and notification of the State Director within seven days, followed by implementation of a remediation plan within 60 days unless an alternative schedule is established through the regulatory process.
The 1.25% limit inside facility structures is deliberately below methane's physical LEL. It therefore provides a margin before the atmosphere reaches an ignitable methane concentration.
Regulatory limits from one jurisdiction should never simply be transferred to another. Applicable legislation, permits and regulator guidance must be checked for the site concerned.
Landfill Gas Is More Than Methane
Methane receives most attention in migration investigations because of its flammability, but landfill gas is a mixture.
It normally contains substantial quantities of both methane and carbon dioxide and may also contain much smaller quantities of non-methane organic compounds and sulphur compounds.
The US Environmental Protection Agency reports that numerous organic hazardous air pollutants have been identified in uncontrolled landfill gas, including compounds such as benzene, toluene, ethylbenzene and vinyl chloride.
This does not mean that hazardous concentrations of these compounds will necessarily occur at an off-site receptor whenever methane migration is detected.
Different components behave differently during subsurface transport. Concentrations can change because of dilution, biodegradation, partitioning into soil moisture and other attenuation mechanisms.
It does mean, however, that methane concentration alone cannot provide a complete chemical characterisation of migrating landfill gas.
Where a conceptual site model identifies a credible pathway to occupied buildings or another sensitive receptor, the need for additional gas or vapour analysis should be considered as part of the site-specific risk assessment.
Carbon Dioxide Also Matters
Carbon dioxide deserves particular attention in any landfill gas investigation.
Like methane, it is produced in large quantities as biodegradable waste decomposes and it can migrate through the ground.
Unlike methane, carbon dioxide is not flammable. At elevated concentrations, however, it can create a serious hazard in enclosed spaces and can contribute to an oxygen-deficient atmosphere.
The relative concentrations of methane, carbon dioxide and oxygen can also provide useful evidence when interpreting ground gas conditions and possible gas sources.
Professional landfill and ground-gas monitoring therefore commonly involves measurement of more than methane alone.
Detecting Subsurface Methane Migration
Suspected methane migration is commonly investigated using monitoring boreholes or gas probes installed within the ground between the suspected source and potential receptors.
However, the value of a monitoring point depends heavily upon its position, depth, screened interval and construction.
A borehole screened within the wrong geological horizon may fail to detect a migration plume passing above or below it.
Similarly, a line of probes positioned without considering service trenches, drainage systems or geological discontinuities may provide false reassurance.
An effective investigation therefore starts with the conceptual site model rather than with the monitoring instrument.
The investigator should consider:
- Where is methane most likely to originate?
- Which geological units are capable of transmitting gas?
- Are artificial preferential pathways present?
- At what depths are the likely pathways?
- Where are the nearest sensitive receptors?
- Under what weather and pressure conditions may migration increase?
- Where should monitoring points be located to intercept the most credible pathways?
Only after those questions have been considered can an appropriate monitoring network be designed.
Surface Methane Monitoring Is Not Migration Monitoring
This distinction is particularly important at landfill sites.
A surface methane survey is designed principally to identify landfill gas escaping through the landfill cap or other surface features.
Subsurface migration monitoring asks a different question:
Is landfill gas moving laterally through the ground towards or beyond the landfill boundary?
A surface survey may provide valuable information about landfill-gas containment and gas-collection performance, but it cannot substitute for correctly designed subsurface monitoring where lateral migration is the concern.
Likewise, a low methane concentration measured in outdoor air near a landfill does not demonstrate that methane is absent from soil gas beneath the same location.
The environmental medium and migration pathway being measured matter.
Why Monitoring Conditions Matter
Ground-gas concentrations can vary substantially with time.
Relevant variables can include:
- atmospheric pressure;
- rainfall;
- groundwater levels;
- soil moisture;
- temperature;
- changes to gas extraction;
- landfill capping operations; and
- seasonal conditions.
Monitoring data should therefore be interpreted alongside the conditions under which they were obtained.
For example, current Environment Agency guidance for England requires landfill monitoring and reporting throughout the operational and aftercare periods and requires atmospheric and weather conditions to be considered as part of landfill gas monitoring.
The objective of a good monitoring programme is not simply to accumulate readings. It is to collect sufficient evidence to test and refine the conceptual site model and demonstrate whether the identified risks are adequately controlled.
Landfill Gas Monitoring Is a Separate Technical Discipline
Monitoring methane migration is only one component of the much broader subject of landfill gas monitoring.
A comprehensive landfill gas monitoring programme may include:
- monitoring gas within the waste mass;
- gas extraction well monitoring;
- perimeter soil-gas monitoring;
- surface methane emissions monitoring;
- flare and utilisation plant monitoring;
- trace-gas analysis;
- monitoring buildings and structures;
- atmospheric pressure recording;
- gas flow and extraction-system performance measurements; and
- monitoring throughout landfill aftercare.
Each has a different purpose, method and interpretation.
Trying to cover the entire discipline within an article about methane migration would obscure the central source-pathway-receptor problem considered here.
For more on that wider subject, see our Landfill Gas Monitoring Procedures article. Landfill-gas.com is also developing this important subject as a separate technical resource for landfill operators, consultants and environmental professionals.
Distinguishing Landfill Methane From Other Sources
The presence of methane near a landfill does not automatically prove landfill gas migration.
Other potential sources must sometimes be investigated.
Coal and Mine Workings
Coal-bearing strata and abandoned mine workings can contain and transmit methane. Old shafts, workings and fractured strata may create complex migration pathways bearing little relationship to present-day surface topography.
Natural Geological Methane
Methane can occur naturally as a result of geological and biological processes.
This becomes particularly important where methane-bearing strata are present locally or where background methane existed before landfill development.
Natural-Gas Infrastructure
Leaks from buried natural-gas mains and services can also introduce methane into the ground.
In a disputed migration case, source identification may therefore become a forensic exercise involving gas composition, geology, infrastructure records, historic information and, where justified, specialised analytical techniques such as stable-isotope analysis.
Abbeystead: Why Natural Methane Sources Matter
The 1984 Abbeystead disaster in Lancashire is a powerful reminder that a serious subsurface methane hazard does not have to originate from landfill waste.
An explosion in an underground valve house at the Abbeystead waterworks killed 16 people.
The incident remains relevant to ground-gas professionals because it demonstrates the importance of considering geological methane and unexpected migration pathways rather than automatically attributing methane to the most obvious nearby industrial source.
Source identification should be evidence-led.
Investigating Suspected Off-Site Methane Migration
Where methane has been detected away from a known landfill source, an investigation should progressively answer four questions.
1. Is the Methane Genuinely Anomalous?
Background ground-gas conditions need to be understood before anomalous results can be interpreted properly.
2. What Is the Likely Source?
Landfill gas, geological methane, mine workings and natural-gas infrastructure may all need to be considered depending upon the location.
3. What Pathway Connects the Source With the Monitoring Location?
Concentration data become much more informative when interpreted alongside geology, groundwater conditions, underground utilities, landfill engineering and pressure information.
4. Is There a Receptor at Risk?
Ultimately, migration becomes important because gas may reach a location where it can accumulate or otherwise create harm.
A high methane concentration in an isolated subsurface monitoring point and a lower but persistent concentration beneath an occupied building can represent very different risk situations.
Risk assessment therefore requires considerably more than ranking boreholes according to methane percentage.
Testing Methane Migration Pathways
Techniques used during detailed migration investigations may include:
- permanent soil-gas probes;
- multi-depth monitoring installations;
- borehole pressure measurements;
- gas composition analysis;
- permeability testing;
- pumping or gas-extraction trials;
- groundwater and dissolved-gas investigation;
- inspection of drains and underground services; and
- monitoring within or beneath potentially affected buildings.
The appropriate combination depends upon the conceptual site model and the questions the investigation needs to answer.
For further technical discussion see Testing Methods for Landfill Gas Migration Control.
Controlling Landfill Gas Migration
Once a migration pathway has been identified, the appropriate remedial measure depends upon the source, pathway and receptor.
At landfill sites, active gas extraction is often central to migration control because reducing gas pressure within the waste reduces the driving force for outward movement.
Other measures may include:
- additional landfill gas extraction wells;
- perimeter extraction systems;
- passive or active vent trenches;
- low-permeability barriers;
- improved landfill capping;
- sealing preferential pathways;
- building gas-protection systems;
- sub-floor ventilation; and
- combinations of these measures.
No single method is appropriate for every site.
A barrier, for example, may interrupt one migration route while potentially diverting gas towards another. An extraction system that lowers methane concentrations near the source still requires monitoring where necessary to demonstrate that the relevant migration pathway and receptor are adequately controlled.
Gas control and monitoring should therefore be designed as parts of the same risk-management system.
Methane Migration After Landfill Closure
Closure does not immediately stop methane generation.
Biodegradable waste can continue generating landfill gas for many years after disposal ceases, although generation rates generally decline over time.
Landfill gas management and monitoring may consequently continue throughout a lengthy aftercare period.
This can become particularly important when changes are proposed to an active gas extraction system.
Reducing or terminating gas extraction alters the pressure regime within the waste. Decisions about changing the gas-management system should therefore be supported by site-specific monitoring and risk assessment rather than being based solely on the age of the landfill.
What Should You Do If Methane Migration Is Suspected?
The first priority is to establish whether there is an immediate risk to people.
Methane detected within a building, confined space or utility chamber requires a very different response from methane detected in an isolated external monitoring borehole.
Potential ignition sources, ventilation, oxygen concentration, occupancy and the possibility of continuing gas entry all need consideration.
Where there is no immediate emergency, the next step should be development or review of the conceptual site model.
Avoid jumping directly from “methane has been detected” to “the landfill is responsible”.
Equally, do not assume that one low monitoring result demonstrates that no migration pathway exists.
A sound methane migration assessment depends upon assembling and interpreting multiple lines of evidence.
Professional Guide to Subsurface Methane Migration
A more detailed professional guide is now being prepared by landfill-gas.com:
Subsurface Methane Migration: Investigation, Monitoring and Explosion Risk Assessment
The guide is intended for landfill operators, environmental consultants, developers, engineers, regulators and others who need to investigate or understand an actual or suspected methane migration problem.
It will go beyond this introductory article to cover:
- developing the conceptual site model;
- identifying methane sources;
- geological and man-made migration pathways;
- design of investigation and monitoring networks;
- barometric pressure and temporal effects;
- interpretation of methane, carbon dioxide and oxygen data;
- source attribution;
- migration into buildings and confined spaces;
- LEL and explosion-risk assessment;
- UK, European and US regulatory approaches;
- historic case studies;
- selection of remediation techniques; and
- practical investigation and report-review checklists.
This page will be updated with a link to the guide when it is published.
Frequently Asked Questions About Methane Migration
What is methane migration?
Methane migration is the movement of methane away from its source through soil, rock, made ground, utilities or other subsurface pathways. At landfills, methane produced by decomposing biodegradable waste can migrate laterally where suitable pressure gradients and pathways exist.
Can methane migrate from a landfill into houses?
Yes. If a credible pathway connects a methane source to a building, gas can migrate towards the property and enter through cracks, service penetrations, drains, utility trenches or sub-floor areas. Geology and preferential pathways can be more important than distance alone.
At what concentration does methane become explosive?
Methane's Lower Explosive Limit in air is approximately 5% by volume and its Upper Explosive Limit approximately 15% by volume. Within this range an appropriate ignition source can ignite the methane-air mixture.
Does finding methane prove that a nearby landfill is leaking gas?
No. Methane may originate from landfills, coal and mine workings, natural geological sources, organic soils or natural-gas infrastructure. Source attribution should form part of the investigation where more than one credible source exists.
Does methane migrate more readily when atmospheric pressure falls?
A rapid fall in atmospheric pressure can increase the pressure differential between subsurface gas and the atmosphere and thereby increase gas movement where suitable pathways exist. The effect depends upon landfill conditions, geology, the gas-control system and the rate of pressure change.
Is landfill gas only methane?
No. Landfill gas contains methane and carbon dioxide principally, together with smaller quantities of other gases and trace compounds. Its composition varies with the waste and landfill conditions.
How is subsurface landfill gas migration detected?
Monitoring boreholes or soil-gas probes are commonly installed between the suspected source and potential receptors. Results should be interpreted in conjunction with geology, utilities, atmospheric pressure, groundwater and the conceptual site model.
Is surface methane monitoring sufficient to detect subsurface migration?
No. Surface-emissions monitoring and subsurface migration monitoring answer different questions. Surface surveys investigate gas escaping through the landfill surface; appropriately positioned subsurface monitoring points are needed to investigate lateral migration through the surrounding ground.
Can methane migration continue after a landfill closes?
Yes. Landfills containing biodegradable waste can continue producing methane after waste deposition ceases. Gas management and monitoring may consequently remain necessary during landfill aftercare.
Are landfill gas regulations the same in the UK, EU and United States?
No. The regulatory structures and legal requirements differ. The UK nations have separate regulatory regimes; EU member states implement the EU landfill framework through national legislation; and the United States operates under federal and state requirements including RCRA Subtitle D. However, many of the underlying landfill-gas engineering principles are similar.
What is the most important principle when assessing methane migration?
There is no single methane measurement that determines migration risk. A defensible assessment considers the complete source-pathway-receptor relationship, the conditions controlling gas movement and the way those conditions change with time.
Conclusion: Methane Migration Is a Pathway Problem, Not Just a Methane Reading
Methane migration cannot be understood from concentration data alone. The investigator needs to know where the gas originated, what is driving its movement, which geological or artificial pathways it can follow, how conditions vary with time and whether the pathway ultimately reaches a receptor where gas can accumulate.
Historic incidents such as Loscoe demonstrate why this matters. Methane does not need to move uniformly through the ground, and the nearest monitoring point is not necessarily the point that best describes risk. A preferential geological layer, drain, service trench or other pathway may control where the gas actually travels.
Modern landfill engineering, gas extraction, monitoring and ground-gas investigation provide considerably better tools for controlling these hazards than were available when many historic landfills were created. Nevertheless, those tools are effective only when the monitoring data are interpreted in the context of the site geology, landfill engineering and physical mechanisms controlling gas movement.
The legal framework varies between the UK nations, the European Union, the United States and other jurisdictions, but the underlying engineering remains the same: understand the source, identify the pathway, protect the receptor and demonstrate that the gas is being adequately controlled.
Authoritative Sources and Further Reading
- Environment Agency – Management of Landfill Gas: LFTGN 03
- Environment Agency – Manage Landfill Gas
- Environment Agency – Monitor and Report Landfill Performance
- European Union – Council Directive 1999/31/EC on the Landfill of Waste
- Scottish Environment Protection Agency (SEPA)
- Natural Resources Wales
- Northern Ireland Environment Agency
- US Electronic Code of Federal Regulations – 40 CFR §258.23 Explosive Gases Control
- US EPA – Frequent Questions About Landfill Gas
Regulations, permit requirements and regulatory guidance change over time. Where this article discusses legal or regulatory requirements, readers should verify the current requirements applying in the jurisdiction and to the specific landfill concerned.
[Published May 2019. Updated and fully rewritten August 2026.]
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