Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p10
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 10/24)
Character Range: 967925–971165

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 a long period of time is modelled. Assessments should consider whether sufficient source (considered on a site-specific basis and may address aspects that include mass, extent and potential for biodegradation) exists to support the volatilisation modelled for the time period under consideration.

The finite source model can be used for site-specific risk assessment, provided that field evidence for the finite nature of the source is presented.

    4.4.5          Biodegradation
In common with experience in the United States (US EPA 2012c), there are two classes of volatile substances that together account for a large percentage of sites affected by soil and groundwater contamination in Australia:
    * petroleum hydrocarbons compounds such as from diesel and petrol
    * chlorinated hydrocarbons such as dry cleaning solvents (tetrachloroethene or PCE) and degreasing agents (trichloroethene (TCE), 1,1,1-trichloroethane (TCA) and PCE).
The principal behaviour difference between petroleum hydrocarbons and chlorinated solvent vapours in the subsurface is that petroleum hydrocarbons readily biodegrade under aerobic (oxygenated) conditions whereas chlorinated solvents typically biodegrade much more slowly and under anaerobic conditions (Davis et al. 2009b, US EPA 2012a, 2012b and 2012c).

Important factors that influence aerobic biodegradation in the unsaturated zone include source concentration, total oxygen demand (the oxygen required to biodegrade the organic contaminants as well as any soil organic matter), the distance between the source and the building and soil type (US EPA 2012a). For some petroleum vapour concentrations there may be sufficient separation distance between a persistent vapour source and a building foundation, referred to in some published literature as 'exclusion distance', that attenuation of biodegrading petroleum chemicals is essentially complete, as the supply of oxygen is non-limiting. Consideration of exclusion distances may be useful in the assessment of the significance of the vapour pathway for petroleum hydrocarbons. Further information is provided in Schedule B2 (Section 9).

However, aerobic biodegradation may be limited by the supply of oxygen such as may occur under large areas of hardstand or where soil organic matter consumes the available oxygen. Aerobic biodegradation is also not applicable to chlorinated solvents (except vinyl chloride where evidence suggests good biodegradation in the presence of oxygen) and other volatile contaminants. Chlorinated hydrocarbons, however, are known to biodegrade in highly reducing environments. Incomplete degradation of chlorinated solvents can produce toxic degradation products such as vinyl chloride and this should be considered where relevant.

Davis et al. (2009b) recommend a process for incorporating biodegradation into vapour assessment of petroleum hydrocarbons where there is sufficient evidence to justify its use