Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:2:p7
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 2 (pt 7/21)
Character Range: 57725–60618

parameters) and overall limitations are provided in Friebel and Nadebaum (2011a). A sensitivity analysis was used to evaluate the effect that these parameters have on the derived HSLs (Friebel & Nadebaum 2011c).

A review of vapour models was undertaken by CSIRO as a precursor project to the development of the HSLs (Davis et al. 2009c). As a result of this review, a modified Johnson and Ettinger vapour exposure model (US EPA 2004) was selected to derive HSLs for the vapour inhalation pathway. The model has been used assuming a finite source for soils equivalent to a source thickness of 2 m which avoids the extreme conservatism associated with assuming an infinite source and reflects empirical field observations. For groundwater and soil vapour, an infinite source (i.e. steady state model) has been assumed as replenishment of vapours may occur by contaminated groundwater flowing beneath the site.

It is noted that the Johnson and Ettinger model and other similar vapour intrusion models do not adequately address vapour risk issues where there are preferential vapour migration pathways, where the building structure extends into a saturated contaminated zone (i.e. into the groundwater table) or where biodegradation is of significance (see section 2.4.10 for further information).

The soil and groundwater HSLs are based on three-phase equilibrium theory and soil vapour is limited by the maximum solubility limit of the chemical in the soil pore water phase or the groundwater. The soil saturation concentration of a particular contaminant is the condition where pore water is at its solubility limit and soil vapour is at the maximum vapour concentration. When a calculated HSL in soil or groundwater exceeds this limit, the vapour in the soil or above groundwater cannot result in an unacceptable vapour risk and is denoted as NL (not limiting) in the HSL tables (Tables 1 A(3)  1A(5)). Soil vapour HSLs are based on the vapour pressures of individual chemicals. Calculated soil vapour HSLs that exceed the possible maximums are similarly denoted as NL.

The HSLs have been derived using accepted approaches to assessment for non threshold (cancer) risk and threshold (non-cancer) risk. Exposure factors for the individual carcinogenic and non-carcinogenic compounds of concern were derived from a near-final draft of enHealth (2012).

    2.4.3          Sub-slab to indoor air attenuation factor
Unlike the derivation of the soil vapour interim HILs, the attenuation factor adopted for petroleum hydrocarbon compounds is not used directly to calculate indoor air concentrations from soil gas concentrations (or vice versa); rather it is used to calculate one of the many input parameters (advective air flow) in the Johnson & Ettinger model. For further information refer to section 7.3.2 of Friebel and Nadebaum (2011a).

As for other input parameters, the selected value