Ground Gases (CH4, Rn, etc.)

Unique identifier / Notation

GH0027

Synonyms

Volcanic gases,

Magmatic gases,

Landfill gas,

Gas-contaminated land.

Ground gases that result from material decay (natural or anthropogenic) typically include radon, methane, carbon dioxide, hydrogen sulphide, but may also include the break down products of other compounds, such as nitrogen, alcohols, alkanes, cycloalkanes and alkenes, aromatic hydrocarbons (monocyclic or polycyclic); esters and ethers, as well as halogenated compounds and organosulphur. Ground gases derived from magma (molten or semi-molten natural material derived from the melting of land or oceanic crust) include carbon dioxide, sulphur dioxide, hydrogen sulphide and hydrogen halides. Ground gases are gases released in combination with water vapour and particulate matter during volcanogenic events, or via fumaroles, and hydrothermal systems (adapted from NHBC (UK), 2007; IVHHN, 2020; US EPA, no date; and USGS, no date).

Primary reference(s)

IVHHN, 2020. Health impacts of volcanic gases. International Volcanic Health Hazard Network (IVHHN). Accessed 15 October 2020.

NHBC, 2007. Guidance on evaluation of development proposals on sites where methane and carbon dioxide are present. National House Building Council (UK). Accessed 29 September 2020.

US EPA, no date. What is radon gas? Is it dangerous? United States Environmental Protection
Agency (US EPA). Accessed 29 September 2020.

USGS, no date. Volcano hazards programme. United States Geological Survey (USGS). Accessed 14 October 2020.

Additional scientific description

Volcanogenic gases escape from magma as a consequence of the pressure relief that occurs as the magma rises to the surface. These gases are also released via geothermal systems.

Chemical or biological processes generate ground gases, for example, the breakdown of uranium-bearing minerals releasing radon from granite or by oxidation and or biogenic reduction (releasing hydrogen sulphide). In addition, naturally occurring ground gases are generated by the biogenic decay of organic matter, for example methane, carbon dioxide and phosphine gas.

Landfill gas is a product of the largely biogenic decomposition of anthropogenic waste. Its composition reflects that of the waste, but is dominated by methane and carbon dioxide, becoming more carbon dioxide rich as the waste ages, and with a small amount of non-methane organic compounds. Methane is a potent greenhouse gas (US EPA, no date).

Ground gases comprise a hazard because of the risk to human health and or their flammability. As an example, the UK limits for the following gases are summarised below from sources other than earthquake triggered gases:

Methane is a colourless, odourless flammable gas. When the concentration of methane in air (oxygen 20.9% by volume [% v/v]) is between the limits of 5% v/v and 15% v/v, an explosive mixture is formed. The Lower Explosive Limit (LEL) of methane is 5% v/v, which is equivalent to 100% LEL. The 15% v/v limit is known as the Upper Explosive Limit (UEL), but concentrations above this level cannot be assumed to represent safe concentrations, because of the potential for dilution to the UEL (NHBC, 2007).
Carbon dioxide is a colourless, odourless gas, which, although non-flammable, is both a toxic gas and an asphyxiant. As carbon dioxide is denser than air, it will collect in low points and depressions, which can be an extreme hazard during foundation construction and earth movements on development sites. The Long-Term Exposure Limit (LTEL, 8-hour period) and the Short-Term Exposure Limit (STEL, 15-minute period), are 0.5% v/v and 1.5% v/v carbon dioxide, respectively (HSE, no date).
Radon is a colourless, odourless radioactive gas derived from the radioactive decay of radium, itself from radioactive decay of uranium. The UK target level for homes is 100 Bq m3 (PHE, no date).
Levels of hydrogen sulphide of 100 ppm and higher are considered immediately dangerous to life and health (NHBC, 2007).

Another source of ground gas associated with continental margins is methane hydrates (Geology.com, 2005-2020). Similarly, ground gases and vapours are emitted from volcanogenic sources.

Metrics and numeric limits

No globally agreed limits for ground gases (earthquake trigger).

Key relevant UN convention / multilateral treaty

Not identified.

Examples of drivers, outcomes and risk management

Ground gases are a hazard in terms of risk to human health, flammability and climate change (greenhouse gases). For these reasons, where possible, ground gas is monitored and controlled. Where buildings may come into contact with ground gases, specialist construction techniques are deployed to protect human health (e.g., NHBC, 2007). In the case of earthquake-triggered gases, consideration should also be given to the associated particulate matter.

Landfill gas management has been a focal point for national scale reductions in carbon dioxide emissions. For example, in 2018 waste management-related carbon dioxide formed 4.6% of UK carbon dioxide emissions (BEIS, 2020).

Ground gases occur in mining environments, for example in mining for coal (carbon dioxide, methane), potash (methane, nitrogen) and shale gas (BGS, no date). In the UK, in these environments, control measures are guided by the Health and Safety Executive.