Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:5:p16
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 5 (pt 16/18)
Character Range: 3013499–3016443

relevant for consideration in the derivation of generic HILs relevant for a wide range of sites. The use of these attenuation factors requires no further modelling of the vapour from the source (or point of measurement) to the point of exposure in indoor air.

The evaluation of the data sets referenced by US EPA only relates to the assessment of chlorinated hydrocarbon vapours, which is relevant to the derivation of the interim HILs. The potential for vapour intrusion risks for chlorinated hydrocarbons differs from that for petroleum hydrocarbons (US EPA 2012b) and hence the approach adopted in deriving interim HILs for chlorinated compounds should not be applied to other volatile compounds (in particular petroleum hydrocarbons) without consideration of chemical-specific issues such as the potential for biodegradation. Chlorinated hydrocarbons are recalcitrant (i.e. do not degrade) in the presence of oxygen, unlike petroleum hydrocarbons, which are readily biodegradable in the presence of oxygen.

The following figure is an extract from US EPA (2012a), which presents these measured attenuation factors.

Figure 5. Summary of attenuation factor distributions for groundwater, external soil vapour, sub-slab, and crawlspace for chlorinated hydrocarbon vapours  (ref: US EPA 2012a, Figure 34)
For sub-slab soil vapour attenuation factors for all residential premises, the measured values ranged from 0.00003 to 1, with a median value of 0.003 and a 95th percentile of 0.03. For soil vapour data collected external to an existing residential building (i.e. not directly beneath the building) there is a wider variation in attenuation factors, ranging from 0.00005 to 1.3, with a median value of 0.004 and 95th percentile of 0.3.

A conservative attenuation factor of 0.1 (which is towards the 95th percentile of the US EPA database) has been adopted for the derivation of soil vapour HILs where the pathway of vapour migration is from subsurface (sub-slab or shallow soil vapour) to indoor air for residential and commercial buildings. This decision was based on the available attenuation factors for chlorinated compounds presented by US EPA (2012a), the recalcitrant nature of most chlorinated hydrocarbons in aerobic environments and consideration of the underlying principles for deriving HILs outlined in Section 1.4 (in particular to embody a margin of safety for most exposure scenarios).

Inhalation indoors is the only significant pathway of concern considered for residential A, B and commercial D scenarios. The soil vapour interim HIL has been calculated on the basis of the equations presented in Appendix B, which uses exposure parameters relevant for the quantification of exposures by residents (HILs A and B) and workers (HIL D).

It is recognised that adopting 0.1 as the set attenuation factor will be very conservative for a number of sites where additional attenuation may occur, for example, sites with deeper