Environmental factors – Rapid changes in atmospheric pressure influence the risk of gas entry into properties. As atmospheric pressure decreases the likelihood of gas entry into the properties on the site increases. This relationship was evident from early monitoring in a vacated Gorgebridge property (see figure 1).
The highest concentration of CO2 represented by the blue line, were recorded when the atmospheric pressure, represented by the dotted red line, was lowest and had fallen rapidly.
However, there are several other factors that can come into play that influence migration in addition to changes in barometric pressure namely:
- porosity and permeability of soil and bedrock
- temperature differentials
- rain or snow
It is known that groundwater levels influence the migration of gasses through abandoned coal mines for example and that if water levels rise, seepage can occur at the surface through natural fault-lines or artificial pathways connecting to the surface e.g. tunnels, adits, shafts etc.
Figure 1: CO2 versus atmospheric pressure in an affected house (data source Fairhurst)
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