Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p4
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 4/117)
Character Range: 356014–359063

stage of an assessment are reviewed in an integrated manner (using a multiple-lines-of-evidence approach where appropriate) to refine the CSM and used to inform subsequent decisions on whether further investigation or management is necessary. Note changes to the CSM may also involve revision of the data quality objectives (DQOs)—see Section 5.

The initial CSM is constructed from the results of the PSI and is used to identify data gaps and inform a decision on whether detailed investigation is required. The CSM should be continually challenged and updated throughout the assessment process.

The sub-optimal performance of many remediation systems can be traced back to the failure to undertake adequate site characterisation and to fully integrate the information gained into the CSM. For large and complex sites, 3-D imaging (visualisation) software may be useful for displaying and interpreting the results of the investigations and to refine the CSM.

4.3              Essential elements of conceptual site models
The CSM should identify complete and potential pathways between the known or potential source(s) and the receptor(s). Where the pathway between a source and a receptor is incomplete, the exposure to chemical substances via that pathway cannot occur but the potential for that pathway to be completed (for example, by abstraction of groundwater or a change in land use) should be considered in the assessment.

The essential elements of an initial CSM are:
    * known and potential sources of contamination and contaminants of concern including the mechanism(s) of contamination (e.g. 'top down' spill or sub-surface release from corroded tank or pipe)
    * potentially affected media (soil, sediment, groundwater, surface water, indoor and ambient air)
    * human and ecological receptors
    * potential and complete exposure pathways.
For the assessment of vapours, (refer Section 9.2.3) additional detail will be needed about preferential pathways for vapour migration and the design of buildings or planned buildings at the site — including the location of sub-surface utilities, foundation construction and condition, and ventilation and heating (Davis et al. 2009a).

4.4              Assessing data gaps and uncertainties in conceptual site models
Data gap identification and uncertainty assessment are key activities in developing and refining a CSM during site assessment. It is, therefore, important that the CSM addresses:
    * how representative the available data is likely to be
    * what the potential sources of variability and uncertainty are
    * how important the identified gaps are to the objectives and reliability of the site assessment.
In developing the CSM, the assessor needs to distinguish between variability and uncertainty. Variability arises from true heterogeneity in the environment such as lateral variations in soil properties or lithology or changes in contaminant levels over time and space. Uncertainty represents lack of knowledge about factors, such as contaminant levels (which