Detecting background concentrations of potentially toxic metals in soils of the Po Plain (Italy)

Integrated geochemical, sedimentological and soil studies from the Po River Plain (northern Italy) provide the opportunity to examine distribution of background concentrations of potentially toxic metals within a mixed geological and pedological framework. Overbank sediments supplied to the Po Plain by Po River and by NW-Apenninic tributaries are characterized by remarkably high natural concentrations of Cr and Ni, reflecting erosion of ultramafic complexes in the drainage basin. Cr and Ni contents within these soils invariably exceed the Italian maximum permissible concentrations for residential and recreational sites. In contrast, significantly lower contents, below the threshold limits, are recorded from ophiolite-free source areas. Spatial distribution of other metals does not reflect important differences in sediment provenance, but is instead primarily a function of changes in grain size (Zn) and degree of soil weathering (Pb and Cu). Identification of a geochemical signature for individual sedimentary and soil units leads to the construction of a conceptually new type of (geologically- and soil-oriented) geochemical map. This map may represent an efficient alternative approach to conventional geochemical mapping based upon geostatistical methods alone, and constitutes a powerful tool to assess the anthropogenic impact of metals on soils.