Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p11
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 11/24)
Character Range: 970884–973893

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 (and where site-specific biodegradation data is not available). This approach may be applied in the context of the HSLs for petroleum hydrocarbons (see Schedule B1 Section 2.4.9). It is noted that the measurement of oxygen in the soil profile can be difficult and care should be taken when using this data to support biodegradation.

Approaches to assessing and modelling biodegradation of petroleum hydrocarbon vapours are developing rapidly. Useful methods are likely to be developed and will come into general use. For example the American Petroleum Institute (API) in 2009 released Biovapor, a model permitting simulation of biodegradation (including the consideration of the presence of a slab) on a site-specific basis. It can be accessed and downloaded at www.api.org/ehs/groundwater/vapor/index.cfm.

While currently it is not possible to recommend Biovapor as an appropriate method for use in Australia, it is not the intent of this guidance to restrict development of new approaches and techniques for improved assessment. Therefore, where new methods can be justifiably applied, their use may be considered.

    4.4.6           Vapours from non-aqueous phase liquids
Non-aqueous phase occurs when the sorbed phase, aqueous phase, and vapour phase of a chemical have reached saturation in soil. Concentrations above this saturation limit (Csat) for all of the specified chemicals of a mixture result in a non-aqueous phase liquid or solid (US EPA 2000).

At contaminant concentration less than Csat, the equilibrium vapour concentration at the contaminant source is proportional to the soil concentration, according to the vapour modelling equation presented by Johnson and Ettinger (US EPA 2004a). When a non-aqueous phase is present however, the vapour concentration at the contaminant source is independent of the soil concentration but proportional to the mole fraction of the individual component of the non-aqueous phase mixture, according to Raoult's Law (US EPA 2000). It is noted that the calculation of Csat relies on laboratory derived parameters and hence there may be some variability in calculated Csat values and site-specific values associated with the observed presence of non-aqueous phase liquids.

Raoult's Law states that 'the vapour pressure of each chemical component in an ideal solution is dependent on the vapour pressure of the individual component and the mole fraction of the component present in the solution'.

Therefore, as the number of components in a solution increases, the individual vapour pressures decrease as the mole fraction of each component decreases with each additional component.

In order to calculate the mole fraction for mixtures, or a solution of compounds, it is