Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:1850:p13
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 1850 (pt 13/117)
Character Range: 381829–384918

be approximately normal. Also, if data sets are sufficiently large, statistical inferences can still be made, since in that case the sample mean is approximately normally distributed (Gilbert, 1987).

Where a reliable and full site history is available, judgemental sampling is generally preferred, however, grid sampling may be appropriate where there is an inadequate site history and there is reason to suspect contamination may be present or, there is a large area of contamination that requires characterisation. An example of the latter would be heterogeneous fill suspected or known to contain contaminated materials.

As grid spacing must be small to have a high probability of finding small hotspots, in practice professional judgement is used to locate areas of smaller grid size in areas most likely to contain hotspots and over areas where a higher degree of confidence is desirable. Information on implementing a systematic or grid sampling approach, including applications for soil and groundwater, can be found in US EPA (2006b).

Determining grid size/sampling density from mathematical formulae (for example, Appendix D of Standard AS 4482.1-2005) is not an acceptable approach without consideration of likely contaminant distribution and acceptable hotspot size.

    6.2.4          Stratified sampling
In stratified sampling, the assessment area (generally the potentially contaminated area) is separated into non-overlapping sub-areas (or strata) which are known or expected to be more homogeneous than the whole assessment area. Different sampling patterns and densities may be used in the different sub-areas.

The strata may be chosen on the basis of spatial or temporal proximity, or on the basis of pre-existing knowledge (e.g. site history, soil type), or professional judgement. The main advantages of this design are:
    * potential for achieving greater precision in estimates of the mean and variance where the measurement of interest is strongly correlated with the variable used to define the strata
    * calculation of reliable estimates for subgroups of special interest.
Information on implementing stratified sampling approaches can be found in US EPA (2006b).

    6.2.5          Transect sampling
In transect sampling, the samples are collected along a vector (a line of specified bearing, commonly 90˚) across an assessment area. Transect sampling may be appropriate when specific spatial characteristics of the contamination are to be targeted, for example, where there is a predictable contaminant distribution downwind/downgradient from a point source of contamination.

    6.2.6          Composite sampling
In contrast to a discrete sample taken from a single location and analysed individually, a composite sample is taken by physically combining a number of subsamples, usually a maximum of four, into a single well-mixed sample for analysis. The subsamples should be preferably composited in the analytical laboratory.

Compositing can be cost-effective where the analysis costs are large relative to sampling costs. However, its use should