Heavy metal pollution in central Venice lagoon bottom sediments: evaluation of the metal bioavailability by geochemical speciation procedure.

Bottom sediments of the central area of the Venice Lagoon (Italy) and some lagoon canals, chosen as an anaerobic reference environment, were analyzed by a sequential extraction procedure for Cr, Cu, Ni, Pb, Zn, Mn, and Fe. Bio-Available Phases (BAPs) and Non-Bio-Available Phases (NBAPs) have been determined in the Malamocco-Marghera Canal, to define the significant level of heavy metal contamination of the sediment. Relationships among metal distributions with Fe/Mn oxides/hydroxides and sulfides were determined in both environments, using statistical methods. The central lagoon area shows low pollution and a balanced influence of all the sediment components in the metal complexing processes, mostly due to the manganese and iron matrices. The inner canals are very polluted with heavy metals strongly bonded to H2S that form highly insoluble compounds, as demostrated through the correlation analysis. The authors suggest different procedures for lagoon restoration due to the different sediment behaviours, i.e., to avoid sediment turbation by dredging or by artificial bottom aeration which could result in an accelerated formation of more soluble oxygenated metal compounds making the toxic metals readily available to the aquatic environment. The sedimentological study of the central lagoon given here evidenced the correlation between grain size and mineralogical composition and heavy metal concentrations in bottom sediments. While Fe, Mn, Zn and Cu show a good correlation with the percentages of mud (grain size < 62 micron) Cr, Cd, Pb and Ni are quite independent from the grain size of the sediment.