Detecting background values of potentially toxic metals by integrated sedimentological and geochemical studies: an example from the Cecina River Basin (Tuscany, Italy)

In order to assess unambiguously legislation limits and levels of concern for potentially toxic metals in sediments, natural concentrations (background values) should be established accurately. Through an example from the Cecina River basin, in central Tuscany (Italy), where natural Cr and Ni concentrations may locally exceed the threshold values designated for contaminated areas (150 mg/kg for Cr and 120 mg/kg for Ni) up to 20 times, we show to what extent background metal concentrations can be determined precisely by integrated sedimentological and geochemical studies. XRF analyses of 76 samples collected by hand-drilling at 1 m depth from fluvial-channel, overbank, backswamp and beach-ridge deposits reveal distinctive sediment compositions that enable multi-element fingerprinting of individual depositional facies based upon diagnostic metal contents. Sediment provenance is the major factor controlling spatial heavy-metal distribution across the study area. Specifically, sediment supplied by Cecina River and its tributaries is characterized by remarkably high natural Cr and Ni concentrations, reflecting erosion of ultramafic complexes cropping out patchily in the drainage basin. In contrast, significantly lower contents are recorded from ophiolite-free source areas. At the basin scale, Cr and Ni serve as valuable indicators of sediment pathways through distinct segments of the routing system. A stepwise increase in Cr concentration is recorded along the 75 km-long Cecina River, from source (100 mg/kg) to mouth (400 mg/kg). The major shifts in metal concentration are recorded close to the tributaries-trunk junctions, thus reflecting direct supply of ultramaphic detritus from the tributaries into the main river. Where headwater streams flow through large ophiolitic complexes, very high Cr (900-1600 mg/kg) and Ni (800-1100 mg/kg) concentrations are delivered to the upstream reaches of the tributaries. A remarkable decrease to 1/3-1/5 of total metal concentration is observed downstream, over very short distances (of just few kilometers), reflecting sediment dilution from non-ophiolitic sources. The highest Cr concentrations are recorded, however, close to the Cecina fluvial mouth, where huge natural metal accumulations in beach sands (commonly > 2000 mg/kg) were controlled by hydraulic selection and sorting processes. The complex spatial distribution of Cr and Ni values across the Cecina River basin documents the inadequacy of using a single threshold value of metal concentration for legislative purposes. Detecting unique geochemical signatures for a variety of facies associations may represent a powerful tool for environmental planning that can be used pragmatically by the regulatory bodies to assess the anthropogenic impact on alluvial and coastal plains