Ground gas can be generated by a variety of natural and man-made sources. Natural sources include organic soils like peat and alluvial deposits, whereas landfill sites and made ground are the most common source of man-made ground gases.
The ground gases typically associated with both natural and anthropogenic gas sources include methane, carbon dioxide, carbon monoxide, and hydrogen sulphide.
Natural ground gas and landfill gas can be odorous, toxic, asphyxiating, flammable and explosive. Due to these potential hazards, sites that could be affected should be investigated to assess the risks and if necessary, remedial or counter measures implemented to manage the risk.
If a risk from ground gas is identified, the design of the investigation will consider the installation of monitoring wells. A suitable number of monitoring visits, dependent on the identified level of risk and the proposed development, should be undertaken.
Ground gas levels can be affected by prevailing weather. Monitoring should be carried during periods of rising and falling barometric pressures to allow for assessment of the worst-case scenario.