The different steps in the development of the floodplain or overbank sediment sampling procedure for continental-scale geochemical mapping are described with examples from the Greek pilot project of the Western European Geological Surveys’ Working Group on ‘Regional Geochemical Mapping’. The Greek overbank and stream sediment results were compared statistically with those of the national regional stream sediment survey. The purpose was (1) to compare the geochemistry of overbank sediment, collected from a single site, with that of several tens or hundreds of regional stream sediment samples taken from the upstream drainage basin, and (2) to develop a cost-effective procedure to assess the geochemistry of a drainage basin. ‘Composite’ samples taken over the whole thickness of the overbank sediment sequence showed that element contents were close to the median, or within the interquartile range, of the same elements in the regional stream sediment samples. Overbank sediment samples collected from pre-defined intervals were not considered satisfactory, for they were taken across overbank sediment layers. Therefore, the method was subsequently refined by collecting samples from different overbank sediment layers, with the recommendation to collect samples at 25-cm intervals when thick layers of over 1 m were encountered.
In selected river basins, surficial overbank and active stream sediments reflect contamination, whereas overbank sediment layers at depth show natural element contents, or pristine conditions. Such data are required for a realistic assessment of the degree of contamination of individual drainage basins. After considering results from Greece and other countries, it is recommended to sample both the surface overbank sediment (0–10 cm), and the deepest possible layer in order to measure pristine or pre-industrial conditions. The Greek results demonstrate that overbank sediments are useful for mapping natural geochemical conditions and baseline concentrations, as well as assessing anthropogenic contamination. The cost-effectiveness of overbank sediment sampling for the ‘Global Geochemical Baselines’ project of the International Union of Geological Sciences (IUGS) and International Association of GeoChemistry (IAGC) is verified.