Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:5:p15
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 5 (pt 15/18)
Character Range: 3010842–3013734

are a number of limitations and uncertainties associated with the use of any model in the estimation of exposure concentrations. In particular, the methodology and uncertainties associated with vapour modelling from a soil source are not fully resolved.

Hence, at this stage for VOCCs, investigation levels have only been derived for soil vapour concentrations (where the soil vapour is the most appropriate data (direct measurement) for the assessment of exposure) and are considered interim HILs. The further development of HILs for these compounds will rely on improvements in understanding of the behaviour of chlorinated hydrocarbons in transferring from soil and soil vapour to indoor and outdoor air.

With respect to the measurement of volatile compounds in soil vapour, readers are referred to Schedule B2 and Davis et al. (2009) for field assessment methods for vapours.

All equations relevant to the derivation of the interim HILs for soil vapour are presented in Appendix B and in the HILs spreadsheet available from the ASC NEPM Toolbox.

    5.5.2         Indoor exposures
The interim soil vapour HILs for VOCCs are dominated by the vapour migration and intrusion (to indoor air) pathway. The quantification of vapour migration from the source (or point of measurement) to the point of exposure (indoors) requires an assessment of migration (via diffusion and/or advection) through overlying soil and into a building where it mixes within the building (including mixing as the building air is exchanged with ambient air). Other processes that limit/retard the migration of vapours (such as sorption, transformation and degradation) or enhance vapour migration (such as via preferential pathways) also occur, though these have not been considered in the HILs. Consequently the interim soil vapour HILs are conservative.

The movement of soil vapour into a building can be described on the basis of an attenuation factor (α), which is the ratio of the indoor air vapour concentration to the soil vapour concentration.

The approach adopted for the derivation of interim HILs has involved the use of an indoor air to soil vapour attenuation factor (or ratio). US EPA (2012a) has summarised measured attenuation factors (based on data from a range of residential sites) between indoor air and groundwater, external soil vapour, sub-slab vapour and crawl-space vapour concentrations.

The attenuation factors have been collated by US EPA (2012a) across a range of soil types and building types that include both slab and basement construction. The evaluation conducted by US EPA has considered the influence of background sources. Hence they are 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