Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:4:p2
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 4 (pt 2/24)
Character Range: 946050–948885

The data evaluation stage should provide a good understanding of the source. Consideration needs to be given as to how the contaminant concentration will be applied in a Tier 2 human health risk assessment; that is, what values will be used as the input concentrations representative of site conditions. Note that, depending on the complexity of the site, there may be more than one 'source' requiring further assessment. Note also that the source may be in soil, water, non-aqueous phase liquid or vapour. Sources in different physical forms generally should be assessed separately and require separate input values. The depth of different sources also requires consideration. For example, contamination may be present in soil at depth during site investigations, but be moved to the surface of the site during construction.
There are a number of options for choosing the value to use as an input concentration. The most appropriate method will depend on the data set, and different methods may be required for different source areas or contaminants, since these may show very different distributions. If a series of data over time is available, consideration of trends will be needed; this is particularly important for groundwater sources. Some commonly used approaches are described herein; however, more sophisticated statistical methods may be used if the data set is suitable. Whatever approach is used to define the source input value, it should be clearly explained and justified.

Maximum observed contaminant concentration. This generally provides a conservative assessment because if estimated risks from the maximum concentrations are not of concern, then the site should be suitable for use under the CSM considered. Maximum concentration is often suitable for groundwater sources where trends are poorly defined. However, a maximum concentration may not be representative of the source as a whole and may result in an overestimation or underestimation of risk if the data is extremely limited.

Mean concentration. The mean contaminant concentration can be a suitable input concentration provided that it can be shown that it adequately represents the source being considered. It is important that small areas of high concentrations or hotspots are not ignored by averaging with lower values from other parts of the site. The mean value may be more representative of the source as a whole than the maximum, and may provide a better estimation of the actual concentration that a population would be exposed to over a period of time.

95% upper confidence limit (UCL) of the arithmetic mean contaminant concentration. This provides a 95% confidence level that the true population mean will be less than, or equal to, this value. The 95% UCL is a useful mechanism to account for uncertainty in whether the data set is large