Collaborative trial in sampling for the spatial delineation of contamination and the estimation of uncertainty
The fitness-for-purpose of a sampling protocol to spatially delineate a region of contamination has been assessed for the first time by use of a collaborative trial in sampling, conducted on a synthetic reference sampling target (RST). This trial employed the RST to show the agreement between one participant’s estimate of the extent and intensity of contamination with that of the ‘true’ value and those of other participants, when they were all using the same nominal protocol. The collaborative trial showed the performance of the protocol when it was applied in any of its four, equally probable orientations. Nine samplers each independently collected soil samples using a herringbone sampling protocol, applied in two randomly selected orientations. Test portions of the samples were then chemically analysed using a single analytical system and the resulting ‘hot spot’ of contamination spatially delineated using two independent methodologies. This spatial extent of contamination was compared with the dimensions of the true hot spot to score the participants, based on a novel adaptation of the International Harmonised Protocol. The value of the score was derived from a weighted sum of the false negative and false positive areas designated as contaminated by the participants. Within- and between-sampler variations were used to assess the performance of the sampling protocol both for the spatial delineation and for the estimation of contaminant concentration at particular sampling locations. The sampling protocol investigated in this CTS was found to be fit-for purpose on this, relatively simple, RST. For a single sampling location situated on a hot spot, sampling repeatability was estimated as 60.08%, and sampling reproducibility 85.79%. This uncertainty contrasts with the sampling reproducibility of 3.77% for a single sampling location situated on the background population of uncontaminated soil. This difference is partially due to a variation in the soil heterogeneity between the contaminated and uncontaminated sample populations. Sampling bias was not significant for either samplers or the sampling protocol, although such a bias may have been masked by the heterogeneity of the sampling target.