Background levels of potentially toxic metals from soils of the Pisa coastal plain (Tuscany, Italy) as identified from sedimentological criteria

Identification of reliable background values of potentially toxic metals in sediments requires detailed integration of geochemical data with accurate sedimentological studies. Through analysis of 60 soil samples from the Pisa coastal plain, this study shows to what extent sediment provenance and facies characteristics may influence the natural distribution of potentially toxic metals (Cr, Ni, Cu, Zn, Pb) within alluvial and coastal sediments. Metals supplied to the alluvial plain are mostly concentrated within the finest sediment fraction (floodplain clays), while coarser crevasse and overbank deposits exhibit invariably lower metal contents. Beach-ridge sands display the lowest metal concentrations. Transport of ophiolitic detritus by the longshore drift may account for locally high Cr concentrations within beach deposits. Geochemical fingerprinting of individual facies associations in terms of natural metal contents results in the construction of a geologically-based geochemical map. This map offers a more reliable depiction of spatial distribution of background levels than interpolation techniques based uniquely upon statistical methods. Matching background values against metal concentrations from topsoil samples leads to the reliable assessment of the pollution status of Pisa coastal plain. Metal contents exceeding the threshold values designated for contaminated areas (Cr) simply reflect catchment geology, and are not the product of artificial contamination. On the other hand, anthropogenic disturbance may be detected even where metal contents (Pb, Cu) lie below the threshold values. The use of sedimentological criteria is presented here as a pragmatic tool to enhance predictability of natural metal contents in sediments, with obvious positive feedbacks for legislative purposes and environmental protection.