Assessment of the environmental significance of heavy metal pollution in surficial sediments of the River Po.

The magnitude and ecological relevance of metal pollution of the middle Po river deriving from the River Lambro tributary was investigated by applying different (complementary) sediment quality assessment approaches: (1) comparisons of concentrations with regional reference data, and (2) comparisons with consensus-based sediment quality guidelines (SQGs), as well as by investigations of the partitioning patterns of target heavy metals (Cd, Cu, Ni, Pb, Zn). Total metal concentrations in the surficial sediments revealed significant pollution inputs on the whole river stretch investigated, with a distinct peak at the inlet of the River Lambro. Based on the geoaccumulation index of target heavy metals, the middle reach of River Po has to be considered as moderately polluted with Cd (1<mean I(geo)<2). At the Lambro inlet the surficial sediments have to be considered as moderately to strongly polluted with Cd (2<mean I(geo)<3) and moderately polluted with Cu, Pb and Zn (1<mean I(geo)<2 for each). Sediment-bound Cd, and Zn were found to be predominantly associated with the exchangeable phase of the sediment (33-58% and 25-43%, respectively), whereas Cu, Ni and Pb showed the strongest association with the Fe/Mn oxides (34-64%, 34-46% and 29-45%, respectively). Based on the consensus-based SQGs, most samples (50-63%) had contamination characteristics associated with moderate probabilities (30-52%) of acute toxicity for all investigated metals, while Cu and Zn in approximately 13% of samples revealed contamination characteristics associated with high probabilities (74-85%) of toxicity.