Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:3:p2
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 3 (pt 2/8)
Character Range: 100461–103190

relevant investigation or screening level, and
    * no single value should exceed 250% of the relevant investigation or screening level.
The maximum observed contaminant concentration generally provides a conservative assessment of exposure because if estimated risks from the maximum concentrations are not of concern, then the site should be suitable for use under the CSM considered. 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.

The mean contaminant concentration can be a suitable metric provided that it can be shown that it adequately represents the source being considered. It is important that small areas of high concentrations or hot-spots 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.

The 95% UCL of the arithmetic mean 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 enough for the mean to provide a reliable measure of central tendency. Note that small data sets result in higher values for the 95% UCL. Further guidance on the use of 95% UCLs can be found in NSW DECC (2006), US EPA (2006b) and US EPA (2007a).

Groundwater data being used to assess exposure should consider a relevant average at the site or off-site (as appropriate based on the CSM) together with a reasonable maximum based on understanding of seasonal and other trends in groundwater quality. Where trends are poorly defined in the early stages of an investigation, greater weight should be placed on the maximum concentration.

If air data or soil vapour data is available for the site, then the use of that data needs to be considered within the context of the CSM and the activities at the site or adjacent to the site that may affect the presence of substances in the air, including confounding substances. Consideration of both a reasonable maximum and a relevant average case should be considered where possible.

The effects of applying a multiplication factor to account for biodegradation to soil, soil vapour and groundwater HSLs where relevant should be considered in the data analysis. The data should be evaluated for trends and the presence of hot spots prior to the application of any biodegradation factors.

    3.2.2          Exceedence of Tier 1 investigation and screening levels
The magnitude