Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:2:p6
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 2 (pt 6/9)
Character Range: 909976–912892

The assumptions on which the HILs are based (including site conditions and the exposure scenarios) should be understood in order to determine whether the HILs are applicable for a given site. HSLs are similar to Tier 1 values and can be applied in the same manner provided that the associated guidance on suitability and application of the values is considered.

Exceedence of Tier 1 criteria is generally used to define the contaminants that require more detailed assessment at Tier 2. However, chemicals that act via the same toxicological mechanism should be considered carefully before being excluded from Tier 2 assessment. An assessment of the significance of exceedences may be necessary where they are marginal or present over a limited area. Under some circumstances further assessment of contaminants exceeding Tier 1 criteria may not be conducted (e.g. where the extent of the exceedence and cost of remediation is small and further assessment is not cost-effective). Where no further assessment is proposed, a clear and transparent explanation should be provided.

A Tier 1, screening level, assessment is often bypassed where there are no appropriate risk-based guidance levels (including HILs and HSLs), for example, if:
    * there are no risk-based guidance levels for a particular contaminant identified at the site or
    * the land use applicable to the site is not covered by the risk-based guidance levels or
    * the physical characteristics (e.g. geology, hydrogeology) of the site are such that the risk-based guidance levels may not be appropriate, for example, the presence of preferential migration pathways where there is a potential vapour risk.

     2.4.1.2         Tier 2
A Tier 2 assessment is typically required when one or more contaminants are present at the site at levels that exceed Tier 1 guidance criteria, if there are no appropriate Tier 1 criteria, or if there are unresolved and significant uncertainties (limiting the reliability of the assessment conducted) identified in the Tier 1 assessment. Tier 2 assessment includes a site-specific risk assessment and the development of site-specific risk-based criteria for comparison with site data. Site-specific risk-based criteria are derived to be adequately protective of human health, but also to take into account site-specific conditions such as relevant exposure pathway linkages to avoid being unnecessarily conservative. Exceedence of Tier 2 criteria may result in a need for a Tier 3 assessment. As with Tier 1 exceedences, an assessment of the significance of exceedences may be necessary where they are marginal or present over a limited area. If Tier 2 criteria are exceeded, but further assessment (or action) is not proposed, the information and logic used to inform the decision should be documented clearly and transparently.

     2.4.1.3         Tier 3
A Tier 3 assessment may be required where exceedence