Dredged river sediments: Potential chemical time bombs? A case study

The influence of dredging and land-disposal of a severely polluted river sediment on the release of heavy metals was studied. During the first 20 days of sediment oxidation and ripening, the pH of the sediment decreased from 8.0 to 3.5 and considerable Zn-(g/L level), Ni-, Cd-, Co-, Cu-, Pb- and Cr-(mg/L level) concentrations were released into the porewater. In order to be able to make a faster prediction of the evolution of sediment properties upon ripening, the oxidation rate was accelerated by resuspending the sediment in water. During resuspension, a sharp pH-drop (to pH 5.3) after 48 h was accompanied by an important release of Zn and Ni, whereas Cd and Cr were only released after 168 h, when pH decreased below 4 Resuspension of the air-dried sediment also resulted in a considerable release of Zn, Ni, Cd, Co, Cu, Pb and Cr. The results of porewater analysis, resuspension- and leaching tests indicate that this river sediment represents a "chemical time bomb'' and that the removal of the contaminated sediment is urgent. Additionally, the resuspension experiment applied in this study can be used as a tool for the fast evaluation of the environmental risk of polluted sediments upon oxidation.