Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p4
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 4/5)
Character Range: 2964328–2967262

However the dermal dose represents an absorbed dose rather than applied dose (as is commonly the case in establishing ingestion TRVs). Hence it may be necessary to modify the ingestion TRV. This is commonly done by applying a gastrointestinal absorption factor (GAF) to the ingestion TRV, which modifies the TRV by a factor that addresses absorption of the chemical across the gastrointestinal tract in the critical toxicity study. For soil-bound contaminants, there is little data on the influence of matrix on dermal absorption. A common approach to address this issue is to apply a dermal absorption factor (DAF) to modify the applied dose in soil to calculate the dermally absorbed dose. It represents the proportion of the contaminant in soil that is considered to be absorbed into the bloodstream through the skin.

TRVs rarely intrinsically account for the relative bioavailability of contaminants in the soil matrix. Established generic values for relative bioavailability in soil are available only for lead (US EPA 2007a). Further discussion on the relative bioavailability of lead considered in the derivation of the HIL is presented in Section 5.4.3. In addition, a relative bioavailability value for arsenic has been considered in the derivation of the HIL. For other contaminants, a relative bioavailability of 100% has been assumed in the derivation of the HILs.

The assumptions noted above with respect to relative bioavailability are relevant to the derivation of HILs only. The conduct of any site-specific risk assessment should further consider site-specific relative bioavailability where relevant (refer to Schedule B4 for further discussion).

4.6              Speciation
A chemical 'species' is the specific form of an element defined by its oxidation (valency) state and/or complex or molecular structure. Some of these chemical species are more important for risk assessment than others. In particular, valency state and speciation are of great importance in determining the toxicity of metals and metalloids (WHO 2006).

Cr (VI) and inorganic and organic Hg were considered as separate species in deriving the HILs, but the remainder of the HILs do not account for differences in the toxicity or bioaccessibility/bioavailability of the species of contaminants. Derivation of the HIL required assumptions to be made regarding the form of each metal in soil, and the assumptions made are detailed in the summaries in Appendix A.

4.7              Toxicity of groups of substances
A number of HILs represent groups of substances (including carcinogenic PAHs, DDT+DDE+DDD, aldrin and dieldrin, PCBs and PBDEs). Two approaches have been applied to generate a single HIL that represents several contaminants. Where this has been done, directions for application of the HIL are given. The toxicity profile for the group of substances provides details of the assumptions that are inherent in the HIL for the group.