Of the necessity of knowledge of the natural pedo-geochemical background content in the evaluation of the contamination of soils by trace elements

In order to evaluate the contamination of the Dornach (Switzerland) site within the framework of the CEEM-Soil project, each participating team was allowed to take a maximum of 15 samples. The French team's sampling was organized in such a way as to answer the following questions: (i) what is the natural concentration of the soils at this site (local pedo-geochemical background content)?; (ii) what are the levels of Cd, Cu, Pb and Zn contamination of the soil?; (iii) what is the depth reached by the surface contamination that is derived from atmospheric fallout?; (iv) how is the contamination spread along the longest axis of the area under study? The relationships between total Fe and the trace metals have allowed local variations in the natural pedo-geochemical background content to be detected and thus permitted the anthropogenic contamination to be estimated. There would appear to be a low level of Pb contamination over all the site investigated (an increase of the order of 5–10 mg kg −1 on the background level), limited to the surface humus-bearing layers. There is also a significant contamination by Cu over all of the site (an increase of the order of 30–40 mg kg −1). This contamination has remained in the surface horizons (0–20 cm). Very high Zn and Cd concentrations have been found in the four surface (0–4 cm) and deep horizons (15–70 cm) taken under the forest and very much lower values in the samples taken from cultivated soils. The most likely explanation is an unequal inheritance between the upper part of the site (wooded with thinner very clayey soils) and the lower cultivated part of the site (with thicker less clayey soils developed in a loamy material). For various reasons, it seems unlikely that a contamination of the wooded part should be so much higher than the cultivated part due to the interception of atmospheric dust by the trees. The local pedo-geochemical background Cd and Zn content of the upper wooded part proved to be clearly higher than that which would be encountered in most soils of Switzerland and France. Given this evaluation of the background content, it seems that only the surface horizons have been affected by Zn contamination (an addition of approx. 60–100 mg kg −1). In the case of Cd, the increase in concentrations is only 0.5–1 mg kg −1 for the ploughed horizons, as well as the for the A horizons.