Trace elements in soils developed in sedimentary materials from Northern France

A pedo-geochemical survey was carried out in the Nord-Pas de Calais region (France) on soils developed in sedimentary materials to estimate their baseline geochemistry and the effect of selected factors on it. Total concentrations of Al, Fe and 18 trace elements, as well as common soil characteristics, were determined in samples from 271 surface and 487 deep horizons developed in 22 sedimentary rocks and superficial deposits. Compared with the upper continental crust, the parent materials are enriched with most of the trace elements, particularly with As, Cd, Sb and Se, and are impoverished in Cu. Calcareous rocks from the Carboniferous, Cretaceous and Jurassic eras are often enriched in Cd. Correlation analysis shows that the least mobile (i.e. ionic potential (Z/r) ranges from 3 to 7) geogenic elements (Bi, Co, Cr, Cu, In, Ni, Pb, Sn, Tl, V, Zn) are associated with the < 2 μm fraction (which we define as “lutum”). More mobile elements (As, Cd, Hg, Mn, Mo, Sb, Se) are less strongly associated with this fraction. Cadmium is particularly associated with Mn. Topsoils are enriched with all the trace elements examined, except Co, Cr and Ni, probably because of anthropogenic contamination. The amount of Pb introduced into the soils of the entire region (12 450 km2) by human diffuse contamination is estimated to be 45 000 metric tonnes. Organic matter seems to act as a sink for numerous exogenous trace elements. The distribution of [trace element]/([Al] or [Fe]) in the deep soil horizons from Nord-Pas de Calais gives the background contents for soils developed from most sedimentary materials from the Brussels–London and the Parisian basins.