Gas and vapour migration

Gas and vapours migrate by pressure driven flow and/or diffusive flow through the soil pore spaces. Migration can also be driven by the buoyancy of gases. Migration of a vapour source in groundwater (i.e. the dissolved contamination) can be a significant pathway for vapours to reach off-site receptors on many sites. 

The bulk gases (methane and carbon dioxide) can also migrate in groundwater. Methane is slightly soluble in water (25 ml methane/litre water at STP). Carbon dioxide is more soluble in water, ionising to form bicarbonate and carbonate ions. This difference in solubility is one of the factors responsible for observed variations in bulk gas composition in monitoring wells. 

Taking methane as an example the volume dissolved in water depends on the partial pressure. The mass of methane dissolved in groundwater at 10°C is given by  

An example calculation of the volume of gas that can migrate in this way is given in Box 4. 3. 

3 Estimate of methane migration in groundwater (Hooker and Bannon, 1993) 

---

Pick s law is also a component of the model used by Johnson and Ettinger to model gas or vapour migration into buildings. According to Pick s law the rate of mass transfer of a gas or vapour by diffusion can be estimated as follows (USEPA, 2003) ... 

Sewers and oil separators are often put forward as a somce of ground gas and vapours. However, modem prefabricated oil separators are well vented and sealed and the risk of migration from these through the ground is negligible. 

An example of how the equation is applied is given in Box 4.4. Darcy s law is also used in the equations quoted by Johnson and Ettinger (1991) to estimate vapour migration from the groxmd into a building. An example showing the application of Darcy s law to estimate vertical emissions of gas from the ground is provided later in this chapter (see Box 4.7). 

To model the surface emission rates of a gas or vapour an analysis of the vertical flow of gas in the ground is required. The most commonly used method to estimate surface emission rates is based on the simple assumption that a 50 mm borehole has a radius of influence of 1.78 m which is equivalent to a surface area of 10 m (Pecksen, 1991). This radius of influence was an arbitrary value chosen to ensure that surface emissions were not underestimated. It is important to understand that the area of 10 m applies to the surface area surrounding the borehole at ground level and is an estimate of the area of emission of gas from a single borehole. It should not be confused with the area of influence over the depth of the borehole in which gas is assumed to migrate and enter the headspace. The area of emission and the area of influence are not necessarily the same but are often misinterpreted by designers of gas protection systems. 

The depth of gas wells is also based on a characterisation of geological and hydrogeological conditions at the site and on the perceived level of risk associated with grormd gas or vapours. The depth of wells should be sufficient to intercept any gas or vapour sources or migration pathways. The present authors have seen several examples where gas has not been encoimtered but where the wells have not been installed deep enough to intercept the source. Examples include ... 

Flux chambers can be an important tool in assessing the risk of surface gas or vapour emission and migration into buildings. The technique involves placing a box over the ground surface and measuring the accumulation of gas inside the chamber over time (see Figure 5.6). 

Of particular interest in the usage of polymers is the permeability of a gas, vapour or liquid through a film. Permeation is a three-part process and involves solution of small molecules in polymer, migration or diffusion through the polymer according to the concentration gradient, and emergence of the small particle at the outer surface. Hence permeability is the product of solubility and diffusion and it is possible to write, where the solubility obeys Henry s law,... 

Another mass transport process about which more needs to be known is the migration of condensable vapours that occurs ahead of the carbonisation front. Both water and other volatile substances are known to vapourise as temperatures rise and to be carried by gas flows into cooler regions where they condense. As tenqieratures at their new position increase these compounds may again evaporate and move on to cooler regions. This cycle of vapourisation and condensation is important since, as is widely appreciated in briquette-making, it can enhance heat transfer rates quite significantly. 

Fig. 29. Haltenbanken fluids are phase separated reflecting the vertical migration, and molecular patterns of triaromatic steroids, for example, are influenced by phase fractionation, hence the C20 triaromatic steroid will be enriched relatively to the C28 compound in the gas phase, whilst the C28 compounds will be partitioned with relative enrichment into the oil phase (oil leg). These effects often pnzzle interpreters, but are readily rationalized in relation to vapour pressure of the compounds (cf. Karlsen et al. 1995). Figure modified after England Mackenzie (1989).

---