Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p9
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 9/24)
Character Range: 965134–968221

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 building structure extends into a saturated contaminated zone (that is, into the groundwater table) or where biodegradation is of significance (see Section 4.4.5). These issues should be addressed on a site-specific basis using more suitable (justified) techniques.

    4.4.3          Outdoor air concentrations
The direct measurement of outdoor air concentrations is appropriate where practical, taking care with any data collected to minimise background influences.

Alternatively, outdoor air concentrations can be estimated using models such as the Jury et al. (1983) model. The Jury et al. model calculates the maximum flux of contaminant vapours from an infinite soil contaminant source via vapour phase diffusion. Chemical movement to the atmosphere is modelled via volatilisation loss through a stagnant air boundary layer at the soil surface, making it appropriate for use in an outdoor exposure setting. The Jury et al. (1983) model is widely accepted as an appropriate methodology for vapour modelling into outdoor air and has been applied by regulatory agencies in the United Kingdom (EA 2009) and United States (US EPA 1996) in the development of Tier 1 soil investigation levels. The Jury et al.  model is also recommended for estimating outdoor vapour concentrations in the Standard guide for risk-based corrective action (ASTM E1739–95 (2010)).

The modelling of outdoor air exposures also needs to account for vapour dispersion, between the soil surface and breathing zone of potentially exposed populations. This can be done using an outdoor box model which may predict ambient vapour concentrations on the downwind edge of the area source at the breathing zone height, as described by ASTM (E1739–95 (2010)). Alternatively, vapour dispersion in a well-mixed box may be estimated using calculated air dispersion factors, as described in US EPA (1996). Either method is considered appropriate.

    4.4.4          Finite and infinite sources
The Johnson and Ettinger model is constructed as both a steady-state solution to vapour transport (infinite or non-diminishing source) and as a quasi-steady-state solution (finite or diminishing source) for soil contamination. A finite source model was not provided for groundwater in US EPA (2004) since groundwater migration reduces the certainty of concentration attenuation.

In situations where the soil or non-aqueous phase liquid source of dissolved phase volatile groundwater contamination is no longer present, dissolved phase concentrations should diminish or attenuate as the contaminants volatilise (or biodegrade). Dissolved phase contamination therefore becomes 'finite'.

In circumstances where relatively small amounts of contaminants are present in the source zone, the infinite source assumption can give rise to a physically impossible output when