Pertinence of pedological stratification of regional-scale reference systems about metallic trace elements content in soils.Walloon case studies. The natural background levels of metallic trace elements (ETM) in soils should be known in order (i) to understand the processes in which they are involved and (ii) to settle reference systems dedicated to the appraisal of agricultural or forest management 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, also-called the pedogeochemical background, of the spatial distribution of ETM in soils. The methodology also aims at defining indicator values of the current degree of ETM contamination in agricultural soils. The spatial distributions of both ETM content and soil forming factors constitute the basement parameters of regional reference soil systems (RSS) about ETM content. The stratification of these regional reference soil systems on a pedological base, which means that soil volumes are clearly identified, appears to be valuable to refine predictions. This is shown through some case studies in the Walloon Region from Belgium. The more the regional reference systems will consider ancillary soil information as well as the geographic specificities of small natural regions, the more they will be relevant to detect local contaminations. For that reason, the digital soil map of Belgium can be considered as a potential powerfull tool for geochemical mapping, after an adaptation of its legend.