Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:2:p4
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 2 (pt 4/7)
Character Range: 2921857–2924691

most relevant approach to the assessment of this pathway is through the collection of soil vapour data. On this basis, interim HILs have been developed for soil vapour.

It is noted that for short-duration exposures such as during intrusive works where direct contact with the source may occur (not addressed in the derivation of HILs) other exposure pathways (in addition to vapour inhalation) may be more significant and require evaluation on a site-specific basis.

Interim HILs are relevant to soil vapour that may have been derived from a soil or groundwater source, or a combination of both.

The values have been derived assuming a slab-on-ground building is present directly above the contaminant source. Groundwater, if present, is assumed to be deeper than the building foundations (i.e. no contact between the building foundations and groundwater at any time).

In circumstances where the building type differs (for example, inclusion of a crawl space or basement), or where there is the potential for preferential vapour pathways to be present, a site-specific assessment should be undertaken.

Further information on the assessment of volatile substances, including VOCCs, can be found in Schedule B2.

    2.2.7         Free cyanide
Cyanide-impacted soils are often dominated by stable cyanide-metal complexes that are of low inherent toxicity and are non-volatile. No HIL for complexed cyanide is presented because of the low toxicity. Free cyanide (defined as the cyanide ion (CN-) or hydrogen cyanide (HCN)) is only formed in environments that are dominated by weak cyanidemetal complexes (for example, silver cyanide) and dissolved cyanide complexes.

The HIL has been derived on the basis of free cyanide and it is recognised that the measurement of free cyanide in soil is difficult. Measurement is difficult due to instability of not only free cyanide but also cyanide metal complexes that can produce free cyanide. A cautious approach (Department of Resources, Energy and Tourism 2008;  ICMI 2009), is to measure free cyanide and other dissociable cyanide species that could produce free cyanide either by dilution or by other natural processes (refer to US EPA method 9016). The US EPA Weak Acid Dissociable Cyanide (WAD) method measures free cyanide plus the cyanide associated with most unstable metal cyanide complexes. The WAD cyanide refers to any species where cyanide is liberated at pH 4.5. Such species include HCN (aq) and CN-, and the majority of Cu, Cd, Ni, Zn and Ag complexes. If the WAD result conforms to the HIL, then the free cyanide level is also in compliance with the HIL.

The presence of free cyanide in soil and the potential for formation of HCN is complex and depends on the soil pH, ionic strength and complexation. The ability of standard vapour models to estimate the concentration