Ground gas is often cited as a key aspect in any contaminated land assessment, and with good reason. On the 24th March 1986 in Loscoe, Derbyshire, a house exploded. This was because of a nearby landfill was generating explosive landfill gas which was migrating to the house. When the boiler fired up at 6.30am, the explosive ground gas that had accumulated in the house, ignited. Fortunately the residents survived, but in other cases, people have succumbed to the dangers of ground gas.
What is Ground Gas?
In the majority of cases, it is gas generated from the decomposition of organic matter in the ground. The two main gasses of concern are usually carbon dioxide (produced through decomposition in aerobic conditions – where there is plenty of oxygen) and methane (produced in anaerobic conditions). Therefore, to produce ground gas, there has to be organic material and conditions sufficient to facilitate decomposition.
How Can Ground Gas Become a Problem?
As in Loscoe, gas can migrate through the ground and into buildings. Within buildings, it can collect and accumulate to explosive concentrations (in the case of methane). It can also create a risk of suffocation (carbon dioxide). Under certain conditions, ground gas may migrate some distance from the source of production to the receptor (such as a building).
Investigating Ground Gas
Ground gas investigations have to consider the following
- Potential sources of the ground gas, which may be hundreds of metres away from the receptor.
- The environmental conditions at the source.
- The environmental conditions and geology between the source and the receptor.
- The sensitivity of the potential receptor, i.e. who / what may be harmed by the ground gasses.
- The design features of the structure that may be affected.
A ground investigation will typically require drilling boreholes along with a period of gas monitoring in order to understand the ground gas regime and develop the conceptual site model. The number of boreholes, and type / length of monitoring will depend on the potential ground gas source and the sensitivity of receptors, though in general anything from three weeks and up is common. In many cases, it can be the ground gas monitoring period that adds significant time to the ground investigation programme.
However, in some cases, it can be possible to rule out any likelihood of ground gas from the outset, simply be understanding the environmental and geological conditions. This can reduce project risks, including costs and time delays.
agb Environmental has significant experience of assessing and remediation get the risks posed by ground gasses. To understand more about ground gasses, or to discuss a specific project, call Simon Pike, Principal Consultant on 01638 663226 or email firstname.lastname@example.org.