The first stage of a gas risk assessment is to determine whether a potential risk from ground gases exists and therefore whether any gas monitoring is justified. This should be established by the Phase I Desk Study and / or the Phase II Site Investigation. Undertaking gas monitoring when no viable source and / or migration pathway has been identified can be costly, time-consuming and create issues with the misinterpretation of ‘normal’ ground gas concentrations, which may result in the installation of gas protection measures where none are necessary. Ground gases generally only present a risk where conditions are conducive to the generation of significant volumes of hazardous gases (principally methane and carbon dioxide), usually as a result of the (relatively) rapid degradation of organic matter, or where a pathway and flow mechanism exists between a significant gas 'reservoir' in the ground and a receptor.

​​

​​​The most common sources of ground gases are buried waste (i.e. landfills), significant thicknesses of Made Ground with a high organic content, natural soils with a high organic content (such as peat and alluvium) and old mine-workings. Recent household waste landfills tend to present the greatest risk as they can contain a high proportion of easily degradable organic material which continues to generate large volumes of ground gases. Older landfills may present less of a risk as, while they may contain high gas concentrations, they are less likely to be continuing source of gas generation. Similarly, old mine-workings and soils with a high organic content may be sources of high gas concentrations, but are unlikely to be generating gases in significant quantities. 

 

​

​

The presence of elevated ground gas concentrations does not necessarily indicate a risk on its own. The greatest risks occur where a pressure gradient allows the flow of significant volumes of ground gases via a pathway and into buildings / enclosures where gas can accumulate. Pressure gradients may result from the ongoing generation of significant volumes of gases within a source area, a change in atmospheric pressure, a rise or fall in groundwater level, or a pressure differential within a building. Gases may migrate between the source and a building (receptor) via various pathways including high permeability deposits, fractures or faults within the ground, old shafts and boreholes, and service corridors.

It is important to recognize that there are also processes that can result in the generation of ground gases other than methane and carbon dioxide. For example, underground fires and chemical reactions can be sources of gases such as hydrogen sulphide and carbon monoxide, chemical contamination may be a source of volatile organic compound vapours, while radon can occur naturally in certain geological formations.

The Conceptual Site Model is again important in determining if a potential risk exists, giving consideration to the possible sources, pathways and receptors as described above. The aim of gas monitoring is generally to provide a reasonable characterization of the gas regime and identify the likely ‘worst case’ conditions, which may, for example, correspond with periods of falling atmospheric pressure or rising groundwater levels.

​

​

Where there is gas risk potential, G&J can provide monitoring services, including spot monitoring, where gas concentrations and flow rates are measured from standpipes at discrete times over a defined period (e.g. fortnightly for three months), or continuous monitoring whereby monitoring equipment is left in-situ to allow real-time recording of gas concentrations and flow rates.  Continuous monitoring is often used on higher risk sites with a known gas issue, or where a shorter monitoring period is required.