The risk presented by high rates of hazardous ground gas emissions into structures is well documented and means of monitoring and assessing this risk are set out in both BS 8576:2013¹ and CIRIA C665². The number of properties across the UK that are affected by such hazardous gas emissions is however low and since the Luscoe³ explosion in 1986, which resulted from a fairly unique set of circumstances, there have been only a few instances of hazardous ground gas emissions resulting in property damage or injury and almost all of these have been related to recently capped landfill sites in fractured rock or to shallow mining. The vast majority of new properties are not located in such high risk areas and are not affected by hazardous ground gases and so do not require ground gas mitigation measures. However, despite the lack of any meaningful ground gas risk, a significant number of development sites are being unnecessarily monitored and new buildings are being constructed with the inclusion of superfluous ground gas exclusion measures, such as membranes and sub-slab venting.

It would be possible to significantly reduce if not eliminate ground gas risk through the compulsory inclusion of ground gas exclusion measures. Whilst this may be appropriate in areas of high risk, such as where shallow gas-bearing mine workings are known to exist, it would not be necessary on the vast majority of sites. Such unnecessary inclusion of gas venting and membranes would not only increase building costs and construction programmes, but would also have a notable impact on the sustainability of such developments. All ground gas exclusion membranes currently available in the UK are formed from virgin polymers and these are highly unlikely to be recycled when the property is demolished. Furthermore, the greenhouse gas footprint resulting from the production of these polymers would not be insignificant were they to be installed by default in all new homes. HDPE has a carbon footprint of 1.60 kg CO2/kg polyethylene (⁴ ICE, 2008), so a typical 2000 gauge HDPE membrane with a density of 0.45 kg/m2 over the footprint of a typical UK house⁵ of 50m2 will result in the release of an equivalent of approximately 36kg of CO2 per house. At the 2019 figure of 188,000 completed new houses⁶ that equates to an equivalent of approximately 6,768,000 kg of CO2 or around 40,000,000 km of emissions from petrol vehicles averaging 40mpg each year⁷.

Ground gas monitoring represents a moderate cost in terms of the overall ground investigation budget, but can significantly extend the period that it will take to complete the investigation and, on most sites, it will prove to have been unnecessary. Reducing this time period by only carrying out one or two rounds of gas monitoring is unjustified, as either the risk is negligible or very low - such that no monitoring is required- or it is sufficiently high to justify monitoring, at which point the programme of monitoring needs to be sufficient to reduce the uncertainty and allow a valid ground gas risk assessment to be carried out.

In 1985 Pecksen⁸ stated that 'in order to determine whether an investigation for methane is necessary, it is essential to obtain as much information as possible on previous site use and site geology. This can save considerable time and possible expenditure on unnecessary investigations, but more importantly assists in determining the extent and form of any investigation required'. Both BS 8576 and CIRIA C665 reports reiterate the sentiment that in order to complete a ground gas assessment, it is necessary to understand the potential sources of gas in and around a site as well as to understand the geology and likely preferential pathways on the site and in the surrounding ground as well as the hydrogeological regime. There is therefore a requirement for an assessment of the potential for there to be a ground gas risk on all sites; however, on the majority of sites it is likely that this assessment can be based solely on a thorough desk study and understanding of the on and off-site sources, the motive forces that could drive on to site migration and the pathways through which such migration could occur. This desk study-based information should be drawn together in the formulation of a detailed conceptual site model (CSM) which should ideally be based upon a cross-section, such as the examples provided in BS 8576. An assessment of the viability of a gas risk can then be carried out and in the light of this the decision can be made as to whether or not gas monitoring is necessary. Should the assessment indicate that there are no viable gas sources or pathways linking them to the site then the ground gas risk can be regarded as being negligible or very low and as such ground gas monitoring would not be required at the site.

Hazardous ground gases largely originate from the biodegradation of organic matter in the ground, be it recently placed made ground or a landfill. However, methanogenesis is a highly energy inefficient process and will only occur once all of the more energetically favourable degradation pathways have been exhausted. If oxygen is present or if electron acceptors such as sulphate, nitrate or ferric iron are present, aerobic degradation will always occur in preference to methanogenesis. Methanogenesis can only degrade simple organic molecules and compounds that can be hydrolysed down to these simple compounds. This is evident from the preservation of bog oaks, leather and clothed bronze age bodies in peat deposits. Therefore, for a fill deposit to be a source of methane generation, it must be anaerobic and of a relatively recent origin and it must contain a moderate to high organic content. Typically soils within 1m to 3m of a permeable surface remain aerobic through the diffusion of oxygen from the air, meaning that such limited depths of made ground are unlikely to be a source of methane unless they are saturated or sealed from the surface. The CL:AIRE Research Bulletin RB17⁹ provides a framework for assessing the scale of the ground gas risk based on the organic content of the soil following a forensic description of its content.

Other potential gas sources include areas of hydrocarbon contamination that can become anaerobic and degrade to produce methane and carbon dioxide or the migration of leachate with a high organic content. Volatile organic compounds (VOCs) can also represent a ground gas risk upon degradation or a toxic, flammable or explosive risk in themselves and the significance of their presence should always be investigated where identified. Fortunately, most VOCs are highly odorous and can be detected by the human nose at very low concentrations (1.5ppm to 5ppm for benzene¹⁰, 28 for TCE¹¹) or onsite through the use of portable equipment such as a photo-ionisation detector (PID).

Older landfills and areas of ground filled prior to around 1950 are unlikely to be actively generating landfill gas, although they may still contain pockets of trapped methane and carbon dioxide. Natural deposits of alluvium and peat may similarly contain trapped methane and carbon dioxide, but as such old deposits are not actively generating gas, there will be no motive force to drive migration and the volumes that may be released following disturbance are unlikely to be sufficient to pose a ground gas risk in all but the short or immediate term.

Geological sources of ground gas such as coal or organic mudstones are less likely to represent a ground gas risk, except where a man-made pathway such as shallow mine workings are present. Chalk will liberate carbon dioxide through the reaction with acidic rainwater, but the volumes of carbon dioxide produced are highly unlikely to be sufficient to pose a ground gas risk.

Perhaps the most common hazardous ground gas source for urban developments are leaking sewers and gas mains. These sources are often localised and can produce significant volumes of ground gas but are often overlooked. The source of the gases can be readily identified through microbial and chemical analysis of the groundwater or gas chromatography of the gas. As the risks associated with such leaks are best addressed through fixing the leak, as opposed to installing gas exclusion measures, an investigation of the origin of unexplained gas, particularly within urban areas should always be considered.

Leachate of a high organic content can represent a source of methane in itself, but methane in solution is unlikely to represent a significant source of ground gas, except under highly unusual circumstances, such as occurred at Abbeystead in 1984¹² where the gas has been dissolved to high concentrations under pressure and dissolution occurred as a result of pressure relief in the immediate vicinity of the site. Under normal conditions, the maximum solubility of methane in water is in the order of 20mg/l to 30mg/l and BGS¹³ research indicates in groundwater it is typically less than 0.5mg/l. Therefore, at the regular hydraulic conductivity values found in granular soils and under typical hydraulic gradients, groundwater flows would be less than 1m3/day which would not be sufficient to deliver a critical volume of methane below a property. A hazardous concentration could therefore not develop even if total dissolution of maximum solubility values occurred below a property, prior to it being vented by the standard number of air changes per hour required under the Building Regulations. In fact it can be demonstrated that even where groundwater saturated with methane, flowing through a sandy gravel, releases its entire methane load below a 1m by 2m by 2.4m high sealed room without any ventilation, that it would take over 40 days for an explosive concentration to develop. Methane dissolution risk is thus unlikely to be significant at most development sites in the UK.