Is a coffee cup in a field statistically representative?

coffee cup

This is what 240g of soil looks like. As you can see, it fits comfortably in a coffee mug, with room to spare. This is how much soil is routinely tested per analyte from a site measuring 60 x 90 metres, or, to put it another way 400-500 lorry loads of soil. The accepted sampling density for a site this size would be around 24 samples, taken from depths of 0.5m to 1m, meaning that 0.000037% is tested. Appendix D of the 2015 WM3 guidance clearly states that a statistically representative amount of sampling is required to effectively classify the waste. Can a rate of 0.000037% be considered even remotely representative?

Clearly, increasing sampling density would go a long way towards increasing confidence in the overall risk assessment or budget proposal, but in the past, it has been prohibitively expensive to achieve. Recent developments in on site technology mean that a much higher sampling density can easily be obtained at no extra cost . The QED Hydrocarbon Analyser from QROS, together with an XRF analyser for heavy metals can easily analyse 50 samples each a day. The QED and XRF gives you results in real time, with excellent proven correlation with laboratory analysis.

A combination of the QED and XRF will give results for total BTEX, Gasoline Range Organics, Diesel Range Organics, Total Petroleum Hydrocarbon, Total C10+ aromatics, sum 16 PAHs, Benzo Pyrene, hydrocarbon identification and all the required heavy metals. This is sufficient to assess the overall condition of a site and classify the waste category of the soil.

On average it should take a day to dig 12 sampling locations. Analysis by the QED and XRF will also be easily achieved in this time. The cost of analysing 24 samples with a QED and XRF over the day is £544. The cost of sending the samples to the lab for a 2 – 3 week turnaround is approximately £1680. It takes almost as long to package, label and document samples going to the lab as it takes to analyse the samples with the on site analysers. Volatile compounds are often lost during packaging and transport. On site analysis removes these issues, making the results gained in this way a much truer representation of the contamination present. WM3 states: “the time between sampling and analysis should be minimised to avoid deterioration or contamination of the sample.”

The price indication above demonstrates that the use of on site analysis can achieve significantly better sampling density and more reliable results, but at a cost that is significantly lower than using laboratory analysis.

The Environment Agency supports On Site Analysis, and the latest 2016  MCERTS policy document states:

The EA will accept the results of insitu testing / RMT provided that the results are complemented by suitable MCERTS accredited laboratory analyses and evidence is provided of appropriate quality control and calibration procedures. The entire dataset will be taken into account using a lines of evidence approach when reaching any regulatory conclusions. The use of in situ testing/RMTs, alongside laboratory analyses, can significantly improve the quality of site investigation, and can reduce the cost of both site investigation and remediation. They can be used for improved targeting of conventional sampling, better spatial delineation of contaminated areas and the development of conceptual site models.

The QED and XRF both have rigorous quality control and calibration procedures. Typically, around 2% of samples analysed by on site methods are sent to the lab for confirmatory analysis.

On site methods now produce analytical data that is as accurate and reliable as conventional laboratory analysis. In the US, the QED is approved as an alternative to the laboratory method for TPH and requires no confirmatory testing.

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