Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:2:p5
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 2 (pt 5/9)
Character Range: 907121–910209

the saturated zone and the potential presence of preferential migration pathways
    * physical characteristics above-ground: sizes and locations and structures of current or future buildings (if known); potential presence of preferential vapour pathways; nature, size and location of outdoor spaces
    * characteristics of the exposed populations: exposed populations may be people residing or working at the site or off-site areas, future occupiers of the site after redevelopment, or environmental populations such as ecosystems in receiving environments e.g. natural surface waters.
Consideration of preferential migration pathways is an essential part of the development of the CSM where volatile and/or dissolved contaminants are present in the subsurface (refer to Schedule B2 and US EPA (2012a) for further information).

A CSM is generally a written description of the site that is accompanied by a schematic, graphical interpretation that depicts what is known or has been inferred about the site. It can also be presented as a flow diagram, as shown by the example in Figure 2. It can be simple or complex, with a more complex example shown in Figure 3.

Primary  Secondary  Transport   Exposure  Exposed Population
Sources  Sources    Mechanisms  Pathways  Characterisation

 Figure 2. Example Conceptual Site Model flow diagram (modified from ASTM, 1995)

A conceptual site model can inform the development of and be incorporated into the detailed scope for a human health risk assessment such as is shown in Figure 3. This example deals with the scoping of a risk assessment for hazardous air pollutants (HAP) and persistent, bioaccumulative and toxic pollutants (PBT)
The schema identifies the sources, contaminants of concern (stressors), exposure pathways, potential receptors, and adverse human health effects that the risk assessment will address. The pathways presented are for illustrative purposes only and are not relevant to any specific scenario.

Sources

Stressors

Pathways/
Media

Routes

Populations

Endpoints
(Specific non-cancer target
organ endpoints shown;
for example purposes)

Metrics
(HAP-specific and cumulative (e.g. by cancer type, weight of evidence, by target organ-specific hazard index)  by
State

2.4              The tiered approach

    2.4.1          Fundamentals of the tiered approach

     2.4.1.1         Tier 1
The Tier 1 (or screening level) assessment is the first stage of assessment at the site. It includes a comparison of known site data with published risk-based guidance levels, such as the HILs. The assessment provides an initial screening of the data to determine whether further assessment is required. HILs are generic Tier 1 guidance values that are designed to be protective of most exposed populations under a variety of circumstances. 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