Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:2:p13
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 2 (pt 13/21)
Character Range: 73900–77940

to the future or continuing use of the site. In high moisture conditions, however, such as occur within the capillary fringe or as a result of seasonal watertable fluctuations, it is not possible to obtain reliable soil vapour readings. In these conditions, consideration may be given to obtaining vapour headspace readings from appropriately constructed groundwater monitoring wells fitted with a soil vapour monitoring cap that seals the groundwater well from the atmosphere.

Soil vapour measurements are also preferred where contaminated groundwater is present at less than 2 m below the ground or basement foundation, though in fine-grained soils the ability to obtain soil vapour measurements may be constrained by moisture conditions, as the thickness of the capillary fringe increases as the soil texture decreases.

Where the watertable rises seasonally to intersect basements or building foundations, indoor air measurements will be required to assess vapour risk. The assessment approach may also include soil vapour measurements taken in the dry season as part of a multiple-lines-of-evidence approach.

Additional information on vapour assessment and the multiple-lines-of-evidence approach is provided in Section 9.2 of Schedule B2 and Friebel and Nadebaum (2011a, 2011b).

    2.4.13      Limitations of the HSLs
As with all generic screening levels, actual site-specific conditions may mean that the assumptions underpinning the derivation of the screening levels are not valid for the site and consequently a site-specific assessment will be required. The principal limitations applicable to the HSLs are listed in Table 3 below, together with suggested alternative assessment approaches.
Immediate action should be taken where potentially explosive or acutely toxic gas concentrations are present in buildings or in-ground services (e.g. utility trenches, sumps or drains) connecting a vapour source to a building. Emergency management actions, such as relocation of building occupants, should be implemented as necessary.

Table 3.  Site scenarios where the application of the HSLs is limited and possible alternative assessment approaches
Site scenario                                                                                                                                                                                                           Alternative assessment approach
The identified contamination has an atypical petroleum composition                                                                                                                                                      Site-specific risk assessment including assessment of cumulative effects of chemical constituents

Contaminated groundwater or LNAPL is entering or is in contact with a basement or building foundation                                                                                                                   Consider indoor air sampling
Depth to groundwater impact is less than 2 m                                                                                                                                                                            Consider soil vapour measurements for vapour intrusion
The impacted soil source thickness is significantly greater than 2 m                                                                                                                                                    HSLs may be conservative for thinner soil sources. For thicker soil sources, refer to Section 2.4.7 of the HSLs application document (Friebel & Nadebaum 2011b)

A preferential migration pathway is present that could connect a vapour source to a building interior                                                                                                                   Site-specific assessment

Hydrocarbon odour present in buildings or in-ground services (not attributable to an indoor or ambient source) which indicates an active preferential migration pathway and potentially