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 (BAFs) 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. R elationships among metal distributions with Fe/Mn oxides/hydroxides an d sulfides were determined in both environments, using statistical met hods. The central lagoon area shows low pollution and a balanced influ ence of all the sediment components in the metal complexing processes, mostly due to the manganese and iron matrices. The inner canals are v ery polluted with heavy metals strongly bonded to H2S that form highly insoluble compounds, as demonstrated through the correlation analysis . The authors suggest different procedures for lagoon restoration due to the different sediment behaviours, i.e., to avoid sediment turbatio n by dredging or by artificial bottom aeration which could result in a n accelerated formation of more soluble oxygenated metal compounds mak ing 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 heav y metal concentrations in bottom sediments. While Fe, Mn, Zn and Cu sh ow a good correlation with the percentages of mud (grain size < 62 mic ron) Cr, Cd, Pb and Ni are quite independent from the grain size of th e sediment.