Ground Gas Assessment

The main sources of ground gases can be divided into two categories: natural and man-made.  

Natural sources include: 

• Organic-rich soils, such as peat and alluvial deposits, can generate methane and carbon dioxide through microbial decomposition 
• Carbonate-rich strata, which can produce carbon dioxide through chemical reactions with acidic groundwater 
• Geothermal activity, can release a range of gases, including hydrogen sulfide and carbon dioxide 

Man-made sources of ground gases include: 

• Landfill sites, which generate methane and carbon dioxide through the decomposition of organic waste 
• Made ground and infilled land, which can contain a variety of organic and inorganic materials that generate gases as they degrade 
• Coal mines and other underground workings, can accumulate methane, carbon dioxide, and other gases 
• Industrial sites and processes, which can release a range of gases through spills, leaks, and discharges 

Ground gases can pose a range of risks to human health, property, and the environment, depending on their composition, concentration, and behaviour.  

The main risks associated with ground gases include: 

• Explosion and fire, if flammable gases like methane accumulate in confined spaces and reach their lower explosive limit 
• Asphyxiation and toxic effects, if gases like carbon dioxide, carbon monoxide, or hydrogen sulfide build up to dangerous levels and displace oxygen or cause poisoning 
• Odour nuisance and public health concerns, if gases with strong or unpleasant odours migrate into buildings or public spaces 
• Damage to vegetation and ecosystems, if gases migrate to the surface and affect soil or air quality 
• Corrosion and degradation of building materials and infrastructure, if gases react with moisture or other substances in the subsurface or the built environment 

A ground gas risk assessment is typically required in the following situations: 

• When a Phase 1 Desk Study or preliminary risk assessment identifies potential ground gas sources or pathways that could affect a proposed development or existing site 
• When a site is located within a certain distance of a known or suspected ground gas source, such as a landfill, mine, or industrial facility, as defined by regulatory guidance or planning policy 
• When a site has a history of land use or development that could have generated or accumulated ground gases, such as infilled quarries, gasworks, or underground storage tanks 
• When a site investigation or monitoring program detects the presence of ground gases at concentrations or flow rates that exceed certain thresholds or risk criteria 
• When a regulatory authority or planning department requires a ground gas risk assessment as part of a permit application, environmental impact assessment, or remediation project 

A ground gas risk assessment typically involves the following steps: 

• Desk study and preliminary risk assessment to identify potential gas sources, pathways, and receptors, and to develop an initial conceptual site model 
• Site investigation and gas monitoring to characterise the subsurface conditions, install monitoring wells, and collect data on gas concentrations, flow rates, and other parameters 
• Data analysis and risk assessment to evaluate the potential risks to specified receptors, taking into account factors such as gas composition, concentration, pressure, and migration pathways 
• Reporting and recommendations to document the findings, conclusions, and recommendations of the assessment, including any necessary mitigation or remediation measures 
• Stakeholder consultation and regulatory liaison to communicate the results of the assessment and agree on the appropriate course of action with interested parties and decision-makers 

There are several remedial options available for managing ground gas risks, depending on the specific site conditions, project objectives, and regulatory requirements. These options can be grouped into three main categories: 

• Source removal or modification, which involves excavating, treating, or containing the gas-generating materials to reduce or eliminate the source of the risk 
• Pathway interruption, which involves installing physical or hydraulic barriers, such as membranes, venting layers, or cut-off walls, to prevent or control gas migration from the source to the receptor 
• Receptor protection, which involves modifying the design or operation of the building or infrastructure to prevent gas ingress, accumulation, or ignition, using measures such as gas-resistant membranes, positive pressurisation, or enhanced ventilation 

The selection and implementation of the appropriate remedial option will depend on a range of factors, including the nature and extent of the gas source, the sensitivity and vulnerability of the receptors, the technical feasibility and cost-effectiveness of the options, and the regulatory and stakeholder acceptability of the proposed solution. 

Our ground gas assessment services can benefit your project in several ways: 

• Providing a comprehensive and scientifically robust assessment of the ground gas risks and liabilities associated with your site, enabling informed decision-making and risk management 
• Helping you to comply with planning conditions, regulatory requirements, and industry best practices, avoiding delays, penalties, or reputational damage 
• Identifying and prioritising the most appropriate and cost-effective remedial options for your site, taking into account the specific technical, financial, and regulatory constraints and opportunities 
• Supporting you in communicating and justifying your approach to ground gas risk management to regulators, stakeholders, and the public, using clear and defensible data, analysis, and reporting 
• Providing expert advice and guidance throughout the entire process, from initial assessment to final verification, drawing on our extensive experience and knowledge of ground gas behaviour and best practices 
• Delivering reliable, efficient, and value-added services that protect your interests and reputation as a responsible and proactive developer or site owner