Balancing measurement uncertainty against financial benefits: a comparison of in situ and ex situ analysis of contaminated land

The uncertainty of measurements taken for the purpose of characterizing contaminated land can subsequently cause decision errors, which can produce significant, financial consequences. Given the site-specific costs, such as those associated with the measurements or with site misclassification, the important question addressed is 'Are the measurements of acceptable quality for that given objective or fit-for-purposeb'? It is often considered by investigators that using a standard operating procedure (SOP) with an approved analytical method will give an acceptable level of uncertainty. This is despite evidence that sampling is usually the predominant source of uncertainty, not the chemical analyses, mainly as a result of the contaminant heterogeneity within sampling locations at a site. One in situ and one ex situ measurement technique were used to represent these two contrasting approaches to characterizing a site contaminated with lead in topsoil. The measurement uncertainty, from both sampling and analyses, was estimated for the two techniques, and its fitness-for-purpose was assessed using the innovative optimized contaminated land investigation (OCLI) method. It is objectively demonstrated by the OCLI method that the in situ method (portable X-ray fluorescence) was three times more cost-effective than the ex situ (AAS) method at characterizing a contaminated site, despite generating higher uncertainty on individual measurements.