Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p48
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 48/117)
Character Range: 480920–484066

into the subsurface
    * conditions are present that could promote lateral migration (e.g. landfill gas production, highly layered soils).
As an investigation progresses, soil vapour sampling results should be used to inform and establish the site-specific boundaries for the area of potential vapour intrusion concern.

    9.2.3          Conceptual site model
A well-developed CSM incorporating vapour risk is essential for understanding current site conditions, determining potential vapour behaviour (including possible variation in soil vapour concentration) and, as part of the DQO process, identifying data gaps and uncertainties and priorities for investigation. The general requirements for the development of CSMs and DQOs are discussed in Sections 4 and 5.

Site-specific data which may be needed for vapour intrusion pathway risk assessment includes measurement of:
    * VOCs in soil vapour within the fill and/or native soils below/adjacent to existing buildings
    * VOCs in groundwater beneath or adjacent to potentially affected buildings or future buildings
    * VOCs in indoor air, outdoor (ambient) air, or soil
    * ambient VOCs that may contribute to VOCs measured at the site
    * VOCs in preferential migration pathways such as service trenches for utilities
    * physical properties, such as soil moisture content, saturation porosity and grain size distribution, relevant to vapour intrusion.
Consideration of preferential vapour migration pathways is an essential part of the development of the CSM. These may intersect vapour sources or soil vapour migration routes, for example, building sumps, drains, or utility and service connections to any buildings. Natural preferential pathways may also occur, for example, tree roots or fractured bedrock where the fractures are interconnected and in direct contact (including connection by permeable fill) with the building foundation and vapour source.

    9.2.4          Multiple-lines-of-evidence approach
For the assessor to conclude that the vapour intrusion/emission pathways are unlikely to be active or to present a significant risk, multiple lines of evidence are required. This requires the assessor to present several reasoned lines of evidence as to why the pathway is considered inactive/unlikely to present a significant risk.

The following are some possible lines of evidence which may be considered (listed in no particular order):
    * soil vapour spatial concentrations  sub-slab,  near-slab (or crawl space) with some level of vertical profiling if appropriate
    * groundwater spatial data with vertical soil vapour profiling if appropriate
    * information on background outdoor and indoor sources
    * building construction and operating conditions
    * indoor air data and concurrent ambient air data
    * comparison of vapour constituent ratios in soil vapour with crawl space/indoor air
    * biodegradability of vapours and availability of oxygen.

Measurement of indoor or ambient air is the most direct approach to assessment of vapour exposure. However, indoor air sampling can be expensive if many samples over a reasonably long