Heavy Metal Behaviour During Resuspension of the Contaminated Anoxic Sludge of the Venice Canals

A field study on the water column in a canal of Venice was perfomed to investigate heavy metals behaviour during dredging interventions on the contaminated anoxic sludge. Concentrations in the dissolved and particulate phases were analysed following a specifically designed filtration scheme. The variations of physico-chemical parameters, monitored during the sampling, allowed the comparison between the trend of the variables characterising sludge resuspension and pollutant concentrations in the water column. Dissolved nutrient species and hydrogen sulphide were also analysed. Due to the presence of a large amount of reducing species, the sludge resuspension induces rapid variations in the oxidation-reduction conditions in the overlying water. In response to these changes, two distinct behaviours were observed for dissolved species. Some elements such as Zn, Cu, Pb and As are scavenged by resuspended amorphous sulphides, while others (Mn and Ni) undergo a mobilisation by dilution of pore-water concentrations.