Soil gas monitoring: Getting the best results

A common misunderstanding in soil gas monitoring is an appreciation of what is actually being measured - specialist skill is needed to correctly interpret the results, says Geoff Card

Soil gas monitoring is routinely carried out in a variety of situations, including contaminated land sites prior to redevelopment as part of a risk assessment. Gas measurements from these sites are taken from the headspace of standpipes, but the gas in the headspace is not always a true measure of the gas in the soil, which could be radically different.

When a borehole is initially constructed and the gas monitoring standpipe is sealed into the ground, for example, the air in the headspace is identical to atmospheric air. In this case, gases such as methane and carbon dioxide will reflect the normal background concentrations in the atmosphere.

Time plays significant role
With time, however, ground gas from the soil pores pass into the headspace either by advection or diffusion. A high concentration of ground gas in the soil pores will mix with air in the headspace and be diluted. For sandy or gravely soil, equilibrium with the headspace will be achieved relatively quickly. But for clay soils of low permeability, it will take longer.

Gas monitoring instruments record in terms of concentration and flow or pressure. Most instruments sample by pumping a volume of gas from the headspace to the instrument but the sampling action changes the gas concentration in the headspace. So if the headspace volume is small compared to the volume sampling rate, then the gas concentration will be quickly depleted.

Depending on how fast gas in the surrounding soil can replenish the headspace, the monitoring results can potentially significantly alter the gas regime, particularly in clayey soils of low permeability. For this reason it is good practice to measure both an instantaneous reading of concentration and flow (or pressure) as well as steady state readings and the time taken to reach steady state. These parameters are important when making any evaluation in terms of risk.

Soil pore structure is another important factor to consider when interpreting gas measurements. The groundwater table and the degree of saturation of the soil can alter the gas transmission characteristics through the soil pore structure. Gas can move more freely through ground that is partially saturated than in situations where voids are completely water filled.

The soil pore structure and the solid/water/gas phases can combine to produce porous systems with different gas transmission properties. In saturated ground, the ability of gas bubbles to move through the soil pores require the displacement of water. In low permeability clayey soils, such as alluvium deposits, high gas pressures may be required to move water held in soil pores.

In this way many organic estuarine alluvial soils can contain large quantities of methane which are only released when the soil structure is disturbed such as during the construction of a monitoring well.

Influence of permeability
Another factor that must be considered when interpreting results is soil permeability. As the compression of soil vertically is greater than that horizontally due to gravity, horizontal permeability can be up to several orders of magnitude higher than the vertical permeability. This is particularly true in made ground or landfill materials that are compacted to engineering standards.

It is for this reason that water in the soil will usually flow more easily in a horizontal direction than vertically. Gas bubbles will also tend to form and coalesce in a horizontal plane. As more bubbles accumulate in the horizontal plane they link up, forming cracks or fissures within the soil through which gas can flow with little resistance.

Biological reactions must be taken into consideration when interpreting gas readings. Aerobic and anaerobic changes that occur in the soil zone surrounding the borehole due to its construction can introduce atmospheric air. This can result in aerobic methane oxidation. Various types of methane oxidising bacteria have been identified known as type 1 or type 2 methylotrophs.

It is not uncommon that methane readings in the borehole headspace may reduce with time as the surrounding zone of methane in the soil is oxidised to carbon dioxide and water vapour. Once the oxygen-rich atmosphere is consumed by the methylotroph bacteria and returns to anaerobic conditions then methane producing soil bacteria will once again dominate and methane concentrations in the headspace will increase. It is vital that sufficient gas monitoring readings are taken over a period of time and at an appropriate frequency between readings.

In sandy or gravely soils of relatively high permeability, equilibrium may be achieved in a matter of days after each reading. In contrast, in clayey or silty soils of relatively low permeability this may take many weeks. Indeed it may never reach equilibrium being in a constant state of flux, due to continuous changes in atmospheric pressure and wind velocity at ground surface.

It might seem concerning that the true gas regime in the ground is not being recorded, but the borehole headspace measurement is a surrogate test of what happens in reality.

It is a realistic model of how ground gas is likely to migrate through cracks or fissures in the soil - including flow paths created by construction activities - and accumulate in a confined space within a building and give rise to a risk. There is good justification to adopt the headspace measurement rather than attempt to determine the true gas regime existing within the soil pores.

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