Influence of sediment provenance on background values of potentially toxic metals from near-surface sediments of Po coastal plain (Italy)

Systematic variability in metal concentrations is recorded in near-surface (1 m deep) Late Quaternary sediments of Po coastal plain, across the boundary between alluvial deposits of Apenninic provenance and a formerly active, now abandoned, delta lobe of the Po River. In the latter area, high Cr and Ni concentrations, exceeding the national standard limits, are attributed to sediment provenance from ultramafic source rocks of Po catchment basin, and not to anthropogenic impact. Spatial distribution of carbonate-related elements, such as Ca and Sr, displays consistent differences between the two areas, confirming a primary control of provenance domains on sediment composition. Accurate facies characterization of near-surface sediments, coupled with detailed information on sediment dispersal patterns, enable a reliable interpretation of the spatial variability of major elements and trace metals in the study area. Particularly, detailed reconstructions of source area composition and changes in flow directions through historical times allow precise correlation of apparently anomalous geochemical patterns with distinctive depositional events, such as shifts in channel course, crevasse splay formation and beach-ridge evolution. The findings of unusually high, natural (provenance-controlled) heavy metal concentrations in pre-industrial near-surface levels highlight the contribution of a sedimentological approach to a reliable interpretation of geochemical data. This should be taken into account when determining background values versus anthropogenic impact.