Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p47
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 47/117)
Character Range: 478281–481201

be implemented.

9.2              Vapour  intrusion assessment framework
Source: API (2005), ITRC (2007a), Davis et al. (2009a) and ODEQ (2010)

    9.2.1          Introduction
For the vapour intrusion pathway to be complete, there must be three components present – a source of sub-surface vapours (in soil and/or groundwater), occupied buildings or the potential for occupied buildings, and a migration route to connect them. Once the pathway is identified as being complete or potentially complete, a staged approach to assessment informed by the iterative development of a CSM is recommended—refer Section 4. For smaller sites, a single phase of work may be adequate to determine vapour intrusion potential, while larger sites can require multiple phases of vapour sampling to fully define the area of concern and accurately characterise the risks.

    9.2.2          Preliminary screening
As a preliminary screening measure, the potential for a vapour intrusion risk should be considered where the Henry's law constant for a substance is greater than 10-5 atm/m3/mol and its vapour pressure is > 1 mm Hg at room temperature. In addition to these measures, a substance should be assessed as volatile if its saturated vapour concentration results in exposure concentrations that are a risk to the exposed population. Some chemicals with low Henry's law constants, or low vapour pressures, are so toxic that even a small amount that moves into the vapour phase could be enough to contribute to a risk. Hence both measures of volatility and toxicity need to be considered (Refer Schedule B4). This includes substances such as petrol, diesel, solvents and certain pesticides and PAHs.

In addition, some sites may be screened out of the assessment by the use of a lateral exclusion distance of 30 m from the sub-surface extent of the vapour source̶further information on the rationale for this criterion may be found in Davis et al. (2009a). A shorter exclusion distance may be considered for petroleum hydrocarbons where there are no other volatile contaminants of concern.

To apply this criterion to a groundwater source, there should be a high degree of confidence based on field data that the dissolved phase plume is stable or shrinking in lateral extent and is not continuing to expand.
This exclusion distance is not applicable to soil or groundwater sources where:
    * the source is intersected by utilities or other potential preferential pathways
    * continuous low permeability cover (for example concrete) is present between the source and the nearest buildings or enclosed spaces which impedes the diffusion of oxygen 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