CL:AIRE, the consultancy supporting sustainable land use has published a new technical bulletin on continuous ground gas monitoring.
Ground gas rose to prominence in 1986, when an explosion in the village of Loscoe in Derbyshire damaged properties and injured three people. Caused by landfill gas leakage, it led to a public inquiry and new guidance. The guidance included a new source-pathway-receptor model and identified migration drivers, such as falling atmospheric pressure.
In 2013, in the Gorebridge in Midlothian, Scotland, carbon dioxide from old mine workings affected residents in a new housing estate, leading to the demolition of 64 properties. With continuous monitoring now in use on thousands of sites, the CL:AIRE technical bulletin evaluates over ten years’ worth of experience, for example in the design and construction of monitoring wells.
The greatest landfill gas hazards are associated with landfills created after the 1956 Clean Air Act, when the putrescible content of waste increased because of restrictions on what could be burned and before the 1990 Environmental Protection Act which required containment systems.
The bulletin notes that "ground-gases are fluids that expand and contract in response to changes in temperature and pressure and can flow in all directions", unlike solid contaminants, like asbestos, which stay put, and liquids, which flow downwards.
Gases may be below the water table and travel any direction. Landfill gas, the report states, (typically 60% methane, 40% carbon dioxide) can be ‘stripped out’ by going into solution when it passes through wet soils, enriching the highly flammable methane content.
If groundwater rises the ground-gases will be compressed and pressurised, while if when it falls the gases are under suction. This is known as the ‘piston effect’. The bulletin points out: "The UK’s temperate maritime climate is dominated by weather systems blowing in from the Atlantic and a fall in atmospheric pressure is an important ground-gas driver on many sites." Factors such as climate change or rising groundwater levels also affect gassing levels.
The bulletin provides technical and scientific data which it hopes will lead to improved risk assessment and better design and specification for gas protection measures for new and existing building.