Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p21
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 21/24)
Character Range: 998683–1001595

in soil that is soluble in the relevant physiological milieu (usually the gastrointestinal tract) which is potentially available for absorption. This can be assessed by validated in vitro test systems.

TRVs used in risk assessment have been derived from animal experiments or from epidemiological data (e.g. arsenic). In most cases, TRVs are derived from the administered dose (i.e. intake of the pure substance), rather than the absorbed dose or the absorption of a substance from soil (note that this should be reviewed for all chemicals of concern to determine relevance) and hence account for the bioavailability of the pure substance. However, because they rarely, if ever, intrinsically account for the soil matrix, relative bioavailability is not accounted for.

It is usually not necessary to account for absolute bioavailability (as defined above) in the oral and inhalation pathways, because it may already be incorporated in the TRV. The derivation of the TRV should be understood, with reference to how the experimental dose was administered. In cases where doses have been injected, for example, there may be a case for introducing a factor to represent the bioavailability of the substance when administered orally or by inhalation.

In the oral and inhalation pathways, it should in theory always be reasonable to introduce a factor to allow for relative bioavailability, since this is almost never intrinsically part of the reference dose. Unfortunately, data is limited and so it is not appropriate to introduce this factor except where more generic relative bioavailability values are available (currently limited to arsenic and lead, however site-specific assessments are often required in addition to the use of generic values).

Relative bioavailability of contaminants in soil is complicated, highly variable and difficult to predict. This is because it depends strongly on the nature of the soil matrix (for example, soil type, age of soil, organic carbon, potential particle size, etc.) and on environmental conditions, particularly redox potential. HILs are derived using 100% relative bioavailability assumptions with the exception of lead and arsenic (refer to Schedule B7, Appendix A1). In site-specific assessments, however, it may be appropriate to refine this assumption.

A detailed review was carried out by Ng et al. (2009) as part of the review of the NEPM. It concluded that in vitro assays were appropriate as a surrogate method for estimating relative bioavailability for some contaminants (e.g. lead and arsenic), and could be used for Tier 2 assessments. Currently it is considered that there are no reliable in vitro methods for other contaminants though further research may provide adequate validation in the future.

In vivo methods are available for the resolution of contaminated land issues though these methods are not likely to be practical due to their expense.