Eléments traces métalliques dans les sols. Contribution à l'étude de leur déterminisme spatial en Région limoneuse belge.

The natural background levels of metallic trace elements (MTEs) in cultiva-ted soils should be known in order (i) to understand the processes in which MTEs are involved and (ii) settle a reference system dedicated to the the ap-praisal of agricultural practices, to the detection of contaminations, or to the time-course follow-up of regional values. A methodological framework has thus been set up to evaluate the importance of natural driving factors, the pe-dogeochemical background, of the spatial distribution of MTEs in soils and also to compare the current levels of MTEs content in agricultural soils from the Loamy Region with natural values.
As main results from the study of parent materials, (i) lithological units are now characterized with element content signatures, (ii) the prominent part of clay, total iron, and total aluminium contents as driving factors of the content in MTEs has been measured, and (iii) the geographical homogeneity as well as the absence of spatial continuity of the cover loessic parent material have been showed up. The relations between MTEs and driving variables have been confirmed through the study of the influences of pedogenesis factors on the vertical and lateral differenciations of MTEs content in soils. It was also demonstrated that taking into account the natural volumes of soils when sam-pling facilitates the interpretation of the results and allows finer estimations of the MTEs content, even in homogeneous context. The reference values about natural and usual content in MTEs for various soil horizons that are proposed reveal a relatively weak level of man-made contamination in the Loamy Re-gion.
It has also been shown that a regional reference system dedicated to the detec-tion of local contaminations gains in relevance as it integers ancillary datas as well as the local specificities of small natural regions. The spatial variability of MTEs content has been evaluated through various cartographic approaches which led to the conclusion that the Soil Map of Belgium is an extremely worthy document for geochemical mapping.