Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p52
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 52/117)
Character Range: 492109–495003

of importance for soil vapour where sampling is shallow (less than 12 m). Wind can also enhance vapour intrusion rates by depressurising a building relative to the underlying soil, causing more vapours to enter the building from the sub-surface. Similarly, high volume air conditioning systems in buildings may affect vapour intrusion.

To account for wind and barometric pressure effects, consideration should be given to sampling during stable weather conditions and recording local barometric pressure and wind-speed data over the three days before and during the sampling event.

Variations in soil temperature result in the expansion and contraction of soil air, leading to partial exchange with the atmosphere. Hence vapour measurements may change daily and from season to season. However, temperature effects decrease with depth below ground and typically show minimal variation much below 1 m below ground. Temperature variations are not expected to have a large influence on soil vapour or indoor air concentrations unless the source is very close to the surface.

Soil moisture increases due to rainfall infiltration may inhibit gas exchange processes and, in particular, vapour movement towards the ground surface, and oxygen ingress from the atmosphere. An increase in moisture content decreases the air-filled porosity and results in lower vapour and gas diffusion rates in the vadose zone. This is likely to be particularly the case for heavier textured (clay) soils (Davis et al. 2009a).

Sampling of soil vapour (particularly from depths shallower than 1 m to 1.5 m) directly after significant rainfall events (greater than 25 mm) should generally be avoided, unless the rainfall is representative of normal conditions. Soil vapour samples collected from depths greater than 1.5 m are unlikely to be significantly affected by rainfall events. No specific guidance on how long to wait before sampling shallow soil vapour (shallower than 1 m to 1.5 m) after a rainfall event is given as it is dependent on the soil type and other climatic conditions.

If uncertainty remains as to the potential for a rainfall event to affect the outcome of a vapour assessment, then consideration should be given to repeat sampling and measurement of soil moisture at the time of vapour sampling.

9.4              Soil vapour sampling

    9.4.1          Introduction
This section provides a summary of commonly used methods for sampling and characterising soil vapour at a site and largely has been adapted from information provided in Davis et al. (2009a) and API (2005). The decision on which methods to use will be informed by consideration of the investigation objectives and analytical requirements as documented in the SAQP.

More detailed information can be found in Davis et al. (2009a), Baker et al. (2009), API (2005), NJDEP (2005a and 2005b), NYSDOH (2006), and ITRC (2007b, 2007c).