G-BASE project

Figure 4.2 An example from the G-BASE project of a gridded image for arsenic in stream sediments (top) and stream waters (bottom) from eastern England. Note The maps show how stream water and stream sediment maps can be used in combination to explain the distribution and behaviour of elements in the surface environment. The Mesozoic sedimentary ironstone referred to in the text is shown by the white outline.

Table 4.4 List of sampling equipment for collecting stream sediments, stream waters and panned concentrates as used by the G-BASE project...

One sample in every hundred samples collected by the G-BASE project is a duplicate sample. A predefined sample and duplicate site number is allocated to samplers who must collect a duplicate stream sediment and stream water sample from a single site. A duplicated panned sample is not generally collected because such samples are not routinely submitted for chemical analysis. It is stipulated that a stream sediment and water duplicate should be collected within 25 m of the original sample. 

The G-BASE project collects samples in random number order (Plant, 1973), as this helps identify any correctable systematic errors introduced during sample preparation and analysis, processes in which the samples are handled in numeric order. For every block of one hundred numbers, five numbers are reserved for control samples so when they are submitted within a batch of samples they are blind to the analyst. The control samples inserted are one duplicate sample, two replicate samples, two blanks, and two secondary reference materials (SRM) used to monitor accuracy and precision as well as to level data between different field campaigns (see Johnson et al, 2008). Along with the original sample ofthe duplicate pair, this means 8% of samples submitted are control samples, a point not to be overlooked in setting the budget for analyses. 

These blind control samples are in addition to any primary reference materials (PRM) that the laboratory may also analyse. For the G-BASE project, the BGS laboratories usually insert a PRM at the beginning and end of each batch of 500 samples. As G-BASE generally collects and analyses 2000—3000 samples each field campaign 8% of the samples is more than adequate to carry out quality control procedures. However, if sample numbers are <500, then it is recommended that the number of duplicates and replicates per hundred samples should be doubled. 

Table 4.5 A table summarising the main health and safety issues for the G-BASE project in the UK and suggested mitigating actions...

This account has been specifically concerned with stream sediments, stream waters and panned heavy mineral concentrates, all collected from the same site. For the G-BASE project, the stream sediments and stream waters are submitted for inorganic chemical analyses, water samples benefiting by improvements to detection Emits in the past decade enabling ultra-low element concentrations to now be reported. 

Historically, the G-BASE project has replaced values recorded as detection by a value one half the detection limit for soils and stream sediments and for consistency with older data. This practice continues. There is no sound basis for such a remedy... 

An effective but simple way of graphically illustrating the variability associated with the analytical data is to plot x—y plots of the duplicate and replicate pairs. Most statistical packages will have an option for plotting simple x—y plots. The G-BASE project uses MS Excel running a macro that will automatically plot duplicate-replicate and duplicate-duplicate results. Figure 5.8 shows three examples from the G-BASE East Midlands atlas area duplicate-replicate data for soils. This method gives an immediate visual appreciation of any errors present in an analytical batch and an indication of within site variability, as shown by the duplicate pairs, or the within sample variability, as indicated by the replicate pairs that demonstrate... 

Thompson (1983) and Thompson and Howarth (1978) describe a method of estimating analytical precision using duplicate pairs. This is not a procedure routinely used by the G-BASE project but is a particularly useful way of estimating the analytical precision when no truly representative reference materials are available. 

Figure 5.8 Examples of duplicate-replicate plots from soil analyses from the Geochemical Baseline Survey of the Environment (G-BASE) project generated by an MS Excel macro (for the relationship of control samples, see Fig. 5.4). Note that the cited detection limits for Cu, U and I are 1.3, 0.5 and 0.5 mg/kg, respectively.

The G-BASE project has used several statistical packages to perform this nested ANOVA analysis (e.g., Minitab and SAS). It currently uses an MS Excel procedure with a macro based on the equations described by Sinclair (1983) in which the ANOVA is performed on results converted to logio (Johnson, 2002). Ramsey et al. (1992) suggest that the combined analytical and sampling variance should not exceed 20% of the total variance with the analytical variance ideally being <4%. 

The G-BASE project predominantly uses parametric levelling as described in the following section. When comparing geochemical results analysed by different... 

Figure 5.13 Aluminium in stream waters from England and Wales from the Geochemical Baseline Survey of the Environment (G-BASE) project illustrating how data sets with different lower detection limits can be combined.

Finally, the effort to assess the quality of the data is wasted if the information is not passed on to the data user. Such users may not be a scientist, so effort must be made to present the data quality information in a manner that they can understand. The G-BASE project when distributing data to users makes use of font formatting, colour coding and highlighting in Excel spreadsheets to pass on information on quality, and Fig. 5.14 is a key to such an Excel document. This gives the user data in a completely numeric format with simple and straightforward advice about how it should be used. It is then up to the data user to choose whether to ignore the advice. 

This chapter is published with the permission of the director of the British Geological Survey (NERC). The authors also wish to acknowledge the efforts of several generations of geology students who have worked as voluntary workers collecting samples for the G-BASE project. Their efforts working to strict procedures have ensured that the G-BASE project output is based on data of high quality and reliability. We appreciate the constructive reviews from C. A. Palmer and an anonymous referee. 

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