Switching from representative to appropriate sampling, and from deterministic to probabilistic interpretations

It is argued that the objective of sampling should be modified from the pursuit of representative samples, to the taking of appropriate samples that have acceptable levels of uncertainty for a particular purpose. Extreme cases of sampling very heterogeneous materials, such as contaminated land and waste materials, are used to illustrate the practical impossibility of taking representative samples within a realistic budget. This tutorial review explains the terminology of uncertainty and four new methods that are available for the estimation of measurement uncertainties that arises from the processes of both primary sampling and chemical analysis. Two of these methods allow the estimation of sampling bias, which is often overlooked as a cause of uncertainty. Recent application of the methods on contaminated land have shown their practicability and have revealed very large discrepancies between the estimated levels. Sampling protocols that provide such realistic estimates of measurement uncertainty, enable a probabilistic approach to be taken to the interpretation of the measurements, rather the current deterministic approach. The existing criteria for the classification of waste material or contaminated land generally depend on a deterministic comparison between measured concentration of a contaminate and a threshold level. With large values of measurement uncertainty in the estimated concentration of the contaminant due to primary sampling, there is a large probability of the erroneous mis-classification of land. This can have legal, financial and possible health implications from both the unnecessary special treatment of 'uncontaminated' waste or land and the erroneous acceptance of uncontaminated waste or land, which is in fact contaminated. A new probabilistic classification scheme for waste is discussed that allows for the overall measurement uncertainty as well as the estimated concentration of the contaminant. A term ‘possibly contaminated’ is given to waste where the measured concentration is below a threshold, but the uncertainty interval exceeds the threshold. The acceptable probability of misclassification can be selected to reflect the risks at that particular site