Ground gases and vapours
Bubbling up from below
A variety of gases can occur in ground from a number of sources, for example:
- Potentially explosive methane and toxic carbon dioxide are generated by natural processes when organic matter is buried. This could be from natural peat layers within the ground or from waste disposal in landfills.
- Radon is a naturally occurring radioactive gas that occurs in high concentrations in some areas of the UK.
- Vapours can arise from the spillages of industrial chemicals or fuels.
The potential for a risk from ground gases is assessed at desk stage and, where necessary, the ground investigation will include the installation of monitoring wells in boreholes. These wells or standpipes comprise a vertical plastic pipe with slots cut across the depth range of interest, surrounded by a special clay (bentonite) seal near surface, fitted with an upper gas tap and finished with either a flush or raised metal cover. These wells are essential as it is important to monitor ground gases on several occasions as gas generation can be impacted by prevailing weather conditions. If a gas risk is identified at desk study stage, then a minimum of three wells and six monitoring visits are required although opions can obviously be expressed on less data.
Gas monitoring involes attaching a specialist detector to the well which then pumps out the gas held within the standpipe and provides analysis. Such gas detector’s commonly provide readings of methane, carbon dioxide, oxygen, hydrogen sulphide and carbon monoxide. Other important information, such as ground water levels, are also recorded at each borehole location.
In respect of vapours, there are hundreds of potential chemicals and so a Photo Ionisation Detector (PID) is employed by BRD to gain a semi-qualitative reading either direct from soil samples or from monitoring wells. The assessment of risk from vapours is also driven by compound specific laboratory analysis either of soils or gas samples obtained.
Radon gas is not something that is regularly tested for, but instead the risk from radon gas is assessed at the desk study stage.