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3.4.9 Gas samples

Contents Air Ground gas surveying Ground gas sampling from permanent monitoring points

Gas sampling may be necessary at some sites depending on the local geology and the radio-nuclides of potential concern. This may include collecting gas samples or filtering the gas to collect re-suspended particulates. Air

Air sampling is often restricted to monitoring activities for occupational and public health and safety and is not required to demonstrate compliance with risk- or dose-based regulations. Section describes examples of sites where air sampling may provide information useful to designing a final status survey. At some sites, radon measurements may be used to indicate the presence of radium, thorium, or uranium in the soil. Section 3.4.10 and Appendix B provide information on this type of sampling. Ground gas surveying

Where spills or leaks of volatile organic compounds have occurred, ground gas surveying is recommended. Areas of waste disposal may be identified by ground gas surveying for landfill gas or for volatile compounds if these were known to be deposited. Extensive guidance on ground gas monitoring is available [Burton-1994], [Digby].

Radium decays to radon, a short-lived radioactive gas. Detection of radon in ground gas may therefore provide information on the presence of buried radium-contaminated materials. Ground gas surveying for radon is already widely used in the mineral exploration industry to detect uranium ore bodies. Detection of radon in air may also be required to evaluate radiological dose arising from the inhalation of radon. An action level of 2,000 Bq/m3 was set by the former National Radiological Protection Board [NRPB-1998].

However, sole reliance on ground gas spike surveys is not recommended, and further investigation from permanent installations is recommended. Ground gas surveying from shallow permanent monitoring points, with confirmatory laboratory analyses, provides information on volatile or gaseous contaminants within the near-surface soils. Such monitoring techniques are used to identify the source of volatile or gaseous contaminants (or their parents, in the case of 222Rn), such as those that may be associated with areas of contaminated land.

Although ground gas surveying appears to be straightforward, there may be significant uncertainties in interpreting the data, principally due to variations in the permeability and moisture content of the ground, which affect the ability of ground gas to migrate. In addition, results are commonly influenced by meteorological factors, such as the extent of recent rainfall, barometric pressure and windspeed.

Ground gas surveys may be used as an indicator of the presence of a number of contaminants, including:

  • Tritium (possibly as water vapour).
  • 14C and other volatile organic compounds (VOCs) such as petroleum hydrocarbons or organic solvents [CIRIA-2006].
  • Radon (an indicator of the presence of radionuclides in the uranium and thorium decay chains). The presence of above background concentrations of radon in air directly indicates that there is a source nearby of radium or its parent isotopes.
  • Organic compounds that are not VOCs, but that produce CO2 gas during biological or chemical breakdown.
  • Mercury [CIRIA-2006].

The largest potential use of ground gas surveying on nuclear-licensed sites and defence sites will be the identification of sources of VOC contamination. Radon gas surveying may also have some potential use on these sites as the presence of radon indicates that radionuclides in the uranium or thorium decay chains are present.

Limitations of ground gas surveying are that migration of ground gas may be significantly affected by the near surface geological and man-made structures. Because of this the gas concentration may not be proportional to the concentration of contaminant in the source area. Interpretation of results may be difficult and a negative result does not necessarily indicate that there are no contaminants present. Ground gas sampling from permanent monitoring points

The sampling of permanently installed gas monitoring points is generally used for monitoring methane production from landfilled putrescible wastes. It is unlikely that such monitoring will be required on nuclear-licensed sites or defense sites. Extensive guidance on the identification of landfill gas already exists [Burton-1994], [EA-2003].

Installation of permanent gas monitoring points
See Section, alinea ‘Installation of permanent liquid monitoring points’.

Airborne particulates
Information available in literature about this topic will be included in a later edition of EURSSEM.