Ground gas

The term ‘ground gas’ refers to gases such as carbon dioxide and methane, which are generated within the ground and/or within landfills, commonly from the breakdown of vegetative matter. Ref The HS2 London-West Midlands Environmental Statement, Glossary of terms and list of abbreviations, DETR 2013.

Approved document C, Site preparation and resistance to contaminants and moisture, suggests that ‘methane and other gases’ includes hazardous soil gases (other than radon, which is dealt with separately), which originate from waste deposited in landfill or are generated naturally.

They may include gases such as methane, carbon dioxide, volatile organic compounds (VOCs) and nitrogen which can migrate through the subsoil and through cracks and fissures into buildings, where if they build up to hazardous levels, cause harm to health or compromise safety:

• Methane is an explosive and asphyxiating gas.
• Carbon dioxide is toxic.
• VOCs are flammable and toxic and can have an unpleasant odour.

They may be apparent as:

• Landfill gas, produced by the action of micro-organisms on biodegradable waste materials, including; methane, carbon dioxide and small quantities of VOCs.
• Elevated levels of carbon dioxide and nitrogen, occurring naturally in coal-mining areas
• Methane and carbon dioxide occurring in organic rich soils and sediments such as peat and river silts.
• VOCs occurring as a result of petrol, oil or solvent spillages.

The assessment of the risk from ground gases should adopt a tiered approach. A preliminary risk assessment should be undertaken, and depending on the outcome, either a generic quantitative risk assessment (GQRA) or detailed quantitative risk assessment (DQRA), or both, may be necessary.

Where risks are unacceptable then these need to be managed through appropriate building remedial measures or site-wide gas control measures, such as the removal of the gas generating material or 'covering' (placing one or more layers of materials over the site) together and the use of gas extraction systems.

Control measures for dwellings may include gas resistant barriers above an extraction or ventilation layer from which gases can be passively dispersed and vented to the atmosphere.

Control measures for non-domestic buildings, which may have a larger footprint, may require mechanical dispersal and ventilation systems, monitoring and alarms. These systems will require ongoing maintenance and calibration.

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• Landfill tax.
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