Background values of potential toxic metals in the Pisa coastal plain: an example of integrated sedimentological and geochemical study

Detailed geochemical characterization of Quaternary deposits between Serchio and Arno river mouths enables the construction of a reliable geochemical map for selected potentially toxic metals in the Pisa coastal plain. Following the procedures carri ed out for the compilation of the Pedogeochemical Ma p of Emilia-Romagna (1:250,000 scale), 60 samples, col lected at 30 selected sites within depths of 20-30 cm and 100-120 cm, respectively, were sampled by hand-drill ing and form the geochemical dataset. Geomorpholog ica l and sedimentological investigations allowed the characterization of the study sites in terms of facies associations. Zr, Fe203, Al203, Ca O and Sr appear to represent reliable indicators of soil alteration, allowing an obvious distinction between i) late Pleistocene units, corresponding to the Coltano sands; ii) Holocene (pre-Roman) deposits, including the channel-levee systems of Arno and Serchio rivers, and iii) recent sediments of alluvial and coastal origin. Subtle geochemical indicators also enable the differentiation of so il alteration at top of major beach-ridge systems aligned parai lei to the present shoreline, thus providing an obvious indication of coasta l progradation during the late Holocene. Arno and Serchio fluvial deposits exhibit similar geochemical compositions. Ni/Y and MgO, however, appear as valuable indicators for their distinction in terms of sediment provenance. Systematic variability in chromium and nickel concentrations is observed in the study area across the three major facies associations (beach-ridge sands, levee and crevasse-splay silty sands, floodplain silts and clays), with increasing metal contents within the finer-grained deposits. This reflects a primary contro! on sediment composit ion by selection processes during t ransport. Beach-ridge sands, in particular, display a high compositional variability that could reflect an influence of transport pathways on the spatial distribution of metals. A similar contro! of grain size o n meta l distribution is shown by zinc and, subordinately, by copper and lead. This study shows that a geochemical ma p constructed on a geologica! basis may furnish a real istic depiction of spatial metal distribution within superficial sediments, providing a powerful tool for planning environmental strategies. Similarly to what recently documented from wide areas of Romagna coastal pia in (Amorosi & Sammartino, 2005; 2007) and Parma alluvial plain (Sammartino et al. , 2007), chromium (and subordinately Ni) in the Pisa coastal plain may display very high natura! concentrations, locally exceeding the pollution thresholds defined by law. As a consequence, evidence for contamination should not be established before detailed geochemical characterization of fa ci es is undertaken. Similarly, limiting values for meta l concentrations should not ignore the actual background values, which are primarily a function of provenance and grain size.