Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:2:p8
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 2 (pt 8/9)
Character Range: 915427–918337

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    * using fate and transport models (e.g. for vapour intrusion pathways) which provide estimates of the amount of contamination that reaches the exposed populations.
Models should only be used where they are suitable for the exposure scenario being evaluated, and the limitations of the model are adequately addressed and discussed in the assessment report. An overview of contaminant fate and transport modelling is presented in Schedule B2. Specific expertise and experience are required to carry out this type of modelling because of the highly complex nature of most contaminant fate and transport problems.

Some exposure settings and assumptions (as in Tier 1 assessments) may not be realistic for the site under consideration as they are based on generic assumptions and parameters that are not going to be realistic for all sites. A Tier 2 assessment may be used to derive more site-specific values by amendment of the assumptions to reflect actual site conditions.

Where available, data on biodegradation of contaminants and bioavailability of chemicals should be considered (by an appropriately qualified toxicologist), and exposure factors (and assumptions) should reflect the scenarios under consideration. It is not necessary to assume that the HIL assumptions detailed in Schedule B7 prevail under all circumstances; both sites and the exposed populations may be very different from the HIL scenarios and this can and should be accounted for in a Tier 2 or Tier 3 assessment.

    2.4.3          Risk assessment endpoints
There are two different approaches to risk assessment which can be referred to as 'forward' and 'backward' assessments.

In a forward assessment, site data is assessed to estimate whether the observed contaminant concentrations potentially pose a health risk to exposed populations. Risks are expressed as hazard indices or as increased lifetime cancer risks and these risk estimates are used to support a decision regarding the acceptability of the risk.

In the backward assessment, the starting point of the assessment is the level of risk or exposure that is deemed to be acceptable for the site. The endpoint is an HIL or site-specific risk-based level, which may be used for further assessment or to provide a basis for clean-up.

    2.4.4          Deterministic versus probabilistic estimates
A deterministic approach means that variables input to an exposure model are expressed as single values or point estimates, which are considered by the assessor to represent the best estimate of the value of the variable. The advantage of this approach is that it is simple and easily understood. Its potential disadvantage is that selection of many point estimates at the upper end of their likely ranges leads to compounding of the uncertainty. Sensitivity and uncertainty analysis are used