Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p58
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 58/117)
Character Range: 507935–510935

or considered a primary vapour intrusion assessment method due to a number of limitations and disadvantages, which are discussed in Davis et al. (2009a) and Baker et al. (2009). However, flux chamber methods may be applicable when a direct measurement of vapour flux is required and as an additional line of evidence in combination with other methods.

The technique enables direct measurement of vapour flux from the surface of the ground or building foundation, thus providing a direct estimate of the parameter of interest (rather than calculating it from sub-surface vapour distributions). Flux methods effectively integrate all sub-surface processes (for example, phase partitioning, biodegradation, preferential pathways, advective and diffusive transport), often close to the point of potential exposure.

There are two primary types of flux chamber methods: a static (closed) chamber method, and a dynamic chamber method.

9.4.4.2         Static chamber
The static chamber method requires the placement of the flux chamber on the surface of the ground or building foundation, excluding passage of air through the chamber. This allows vapours to be trapped and the stagnant chamber vapour concentration to build up over time. Active samples can be collected at discrete intervals through a time period and at the end of a time period.

9.4.4.3         Dynamic chamber
The dynamic chamber method involves the use of an inert sweep gas which is continually introduced into the chamber with an equivalent amount of gas allowed to escape. The system is allowed to reach steady-state, (assumed to be four or five chamber volumes) before the chamber is sampled. The sample can be a discrete sample or monitored continuously.

9.4.4.4         Factors for consideration when using flux methods
When designing a flux chamber sampling program the following should be considered:
    * Coverage of the area of concern —adequate coverage of possible vapour conduits, areas of maximum source concentrations and consideration of other site-specific building features as required
    * Deployment period — this should be adequate to address the issues of concern and, where possible, enable temporal variability to be assessed.
    * Basements — flux chambers may not be suitable for dwellings with basements because of additional potential fluxes from the basement walls to the interior of the dwelling.
    * Sub-surface conditions — flux monitoring provides little information about the processes that may be occurring within the vadose zone such as oxygen penetration and hydrocarbon degradation. Longer-term controls on emissions and hence potential changes in sub-surface conditions may not be detected with such a device, unless long-term near-continuous emission monitoring is undertaken.
    * Buildings — because of the usually limited surface area of coverage, flux chambers may not measure the actual flux into a built structure, especially if there is preferential access to the structure. Also,