Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p8
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 8/24)
Character Range: 962386–965394

in Schedule B2.

    4.4.2          Indoor air concentrations:
The direct measurement of indoor air concentrations may be appropriate where practical and where the data can be adequately interpreted. When assessing impacts from contaminated land it is important that background influences on indoor air concentrations are identified and characterised.

There are many circumstances where the measurement of indoor air concentrations is not the preferred method of assessing exposure from a subsurface contamination source. The information should be used in conjunction with other measurements such as sub-slab soil vapour, soil vapour at depth and outdoor air.

If direct measurements of indoor air (soil vapour and outdoor air) are conducted, sufficient numbers of samples to address temporal and spatial variability need to be collected.

Alternatively, indoor air concentrations can be modelled (estimated) using an attenuation factor (refer to US EPA 2012b for a range of values that could be considered), a model such as the Johnson and Ettinger (1991) model, or another appropriate (justified) model. The Johnson and Ettinger (1991) model is a one-dimensional 'heuristic' analytical solution to model advective and diffusive vapour transport into indoor spaces. It provides an estimated attenuation coefficient that relates the vapour concentration in the indoor space to the soil vapour concentration at the source of contamination (US EPA 2004a). A vapour attenuation factor, '', is calculated, which is the ratio of the concentration of a chemical vapour in an indoor scenario relative to that measured in the soil. This model has been updated and modified since 1991 (Abreu & Johnson 2005, 2006) and is also described in Davis et al. (2004, 2009a). Inputs to the model include chemical properties of the contaminant, saturated and unsaturated zone soil properties, and structural properties of the building (US EPA 2004a).

The Johnson and Ettinger model as described by US EPA (2004a) is the most commonly used model for estimating vapour concentrations in indoor air and has been used in the derivation of the Health Screening Levels (HSLs) for petroleum hydrocarbons. The US EPA model provides additional functionality permitting the estimation of soil vapour concentrations from soil, groundwater and phase separated liquid. It is provided at: http://epa.gov/oswer/riskassessment/airmodel/johnson_ettinger.htm

There are a number of ways in which this vapour model can be manipulated to improve the confidence in the outcomes from the model; however, confidence in any model output without corresponding data for the purpose of validation is not high. Additional guidance is provided in Davis et al. (2004, 2009a).

It is noted that the Johnson and Ettinger model (as described by US EPA (2004a) and other similar vapour intrusion models do not adequately address vapour risk issues where there are preferential vapour migration pathways connecting a vapour source with the building, where the