Ground Gas Risk Assessment

Investigate ground gas risk: get a rapid condition discharge

Ground gas risk assessment is a crucial process in evaluating the potential hazards posed by gases present underground. These gases, if not properly managed, can present significant risks to human health, property, and the environment. In the UK, various naturally occurring and anthropogenic gases can be found under the ground, necessitating thorough investigation, consideration of remedial options and mitigation measures where necessary.

Ground gases can arise from a variety of natural sources or from human activities. On many contaminated land sites, gases such as carbon dioxide and carbon monoxide may accumulate due to the breakdown of organic materials, particularly in landfill sites or areas of made ground. Even former infilled ponds and carbonate rich strata can generate or trap gases that migrate through the soil, posing potential hazards to future developments.

Examples of ground gas found in the UK

Several types of ground gases can be encountered, including:

1. Methane (CH₄). A highly flammable and potentially explosive gas, methane is often associated with landfill sites, peat bogs, and former coal mining areas.
2. Carbon Dioxide (CO₂). A colourless and odourless gas that, in high concentrations, can cause asphyxiation. It is commonly found in areas of natural organic decay and former industrial sites.
3. Radon (Rn). A naturally occurring radioactive gas derived from the decay of uranium in rocks and soils. Radon exposure is a leading cause of lung cancer and is particularly prevalent in regions with granite bedrock.
4. Hydrogen Sulphide (H₂S). A toxic gas with a distinctive rotten egg smell, hydrogen sulphide is typically found in areas with decaying organic material, wetlands, and landfill sites.
5. Volatile Organic Compounds (VOCs). These compounds originate from industrial contamination and petroleum spills. They can pose serious health risks, including cancer and respiratory issues.

Why are these gases a problem?

Ground gases present several risks, including:

• Explosion and fire hazards: methane and certain VOCs can accumulate in confined spaces, creating an explosive atmosphere.
• Health risks: gases like radon and carbon dioxide can cause severe health issues through prolonged exposure.
• Environmental damage: the migration of harmful gases can lead to contamination of air and water sources, posing risks to ecosystems.
• Structural damage: some gases, particularly methane, can impact the integrity of buildings by accumulating in foundations and confined spaces.

Where might ground gas be found?

There are various sources of ground gas across the UK, including:

• Landfills and waste disposal sites: decomposing organic matter in landfills produces methane and carbon dioxide.
• Former industrial and brownfield sites: previous chemical, petroleum, or manufacturing activities can leave behind VOCs and other hazardous types of ground gas.
• Coal mining areas: abandoned coal mines may release methane and carbon dioxide due to residual organic material.
• Peatlands and wetlands: natural sources of ground gas are created by decomposition of organic material in these environments, including methane.
• Radon-affected areas: regions with high uranium content in the bedrock, such as parts of Cornwall and Derbyshire, have elevated radon levels.

Why and when is a ground gas risk assessment needed?

A thorough ground gas assessment is essential to identify the type, concentration, and source of gases present beneath a site. This process often includes a programme of gas monitoring and continuous monitoring to determine variations over time and under different conditions. The data collected helps to establish acceptable levels of ground gases that will not pose a risk to human health, property, or the environment.

A ground gas risk assessment is required to:

• Ensure public safety: identifying and mitigating potential gas hazards prevents health risks and accidents.
• Comply with planning conditions and regulations: many local planning authorities mandate ground gas assessment reports for developments in areas susceptible to ground gas to ensure risk is at an acceptable level.
• Protect property and infrastructure: proper assessment helps avoid structural damage caused by ground gas accumulation.
• Fulfil environmental responsibilities:  ensuring compliance with environmental legislation helps prevent pollution and ecosystem damage.

Ground gas risk assessments are typically required:

• During the planning process, especially for any proposed development site on or near landfill sites, industrial or mining areas.
• For new housing and commercial developments, ensuring that homes and workplaces are safe from harmful gas exposure.
• For infrastructure projects. Large-scale projects such as tunnels and underground facilities require thorough ground gas assessment.
• When suspected contamination is present. If historical land use suggests potential contamination, a ground gas assessment is necessary.

How is a ground gas risk assessment carried out?

A ground gas risk assessment follows a systematic approach:

1. Preliminary desk study: examining historical land use, geological data, and previous reports to identify potential gas sources and specified receptors.
2. Site investigation: field studies include inspections of infilled ponds, borehole drilling and gas monitoring to assess all sources of ground gas on a development site.
3. Gas monitoring and sampling: installing gas monitoring wells to measure gas concentrations and flow rates over time.
4. Risk assessment analysis: evaluating the data using a conceptual site model and guidelines, such as CIRIA C665. Depending on the findings, further assessment may be required.
5. Mitigation strategy development: remedial options are considered before recommended measures are made which might include gas membranes, ventilation systems and soil remediation.
6. Regulatory compliance and reporting: ensuring findings are documented and submitted to relevant authorities for approval.

Who can conduct a ground gas risk assessment?

Qualified professionals carry out these assessments, including:

• Geo-environmental consultants: specialising in land contamination and risk assessment.
• Geotechnical engineers: experts in soil and ground conditions.
• Geologists and hydrogeologists: providing expert advice on natural gas emissions and subsurface processes.
• Regulatory bodies: local authorities and environmental agencies review assessments for compliance.

The role of ground gas risk assessment in the planning system

Ground gas risk assessments play a crucial role in the UK’s planning framework. Under the National Planning Policy Framework, developers must demonstrate that their proposals account for land contamination and gas risks. Local authorities require these assessments as part of planning applications, particularly for:

• New housing developments: ensuring safe living conditions for residents.
• Commercial and industrial sites: protecting workers from exposure to hazardous gases.
• Infrastructure projects: safeguarding tunnels, basements, and underground structures from gas infiltration.
• Brownfield redevelopment: managing risks associated with former industrial land.

Failure to conduct a proper ground gas assessment can lead to a worst case scenario of a planning refusal, or planning conditions being imposed, costly delays, or remedial work after construction.

Make informed decisions about ground gas risks

Ground gas risk assessment is an essential component of safe and sustainable development in the UK. With various hazardous gases present in the ground, having a thorough understanding of their risks and implementing mitigation measures is crucial. These may include gas protection membranes, venting systems, or modifications to development design to ensure safe occupancy.

By carrying out robust assessment and applying the right controls, developers can safely bring contaminated or previously used land back into beneficial use. These assessments help protect public health, the environment, and infrastructure while ensuring compliance with planning conditions and environmental regulations.

Developers, environmental consultants, and regulatory bodies must work together to manage ground gas risks effectively, ensuring that a development site is safe for future use. Gas protection measures may be achievable, or a development design might have to be altered to reduce any threat to an acceptable level while keeping a scheme cost effective.

Geo-environmental consultants from Arbtech have extensive experience in ground gas assessment work. For further information, fill in the contact form at the top of this page.