Heavy metal risk assessment after oxidation of dredged sediments through speciation and availability studies in the Reno river basin, Northern Italy

The distribution of heavy metals was investigated in sediments of both natural and artificial watercourses of the Reno river basin (Northern Italy) with the aim of assessing their pollution risk before and after dredging operations. The different solubility and availability of metals in wet and dry sediments were investigated in order to identify the main critical variables controlling metal adsorption into sediments, their speciation and, therefore, their potential environmental hazard. Twenty-four sampling stations were selected in the Reno basin network, and sediment sampling was seasonally carried out in 2012-2013. Pseudo-total metal content was determined through acid digestion with aqua regia, and the geoaccumulation index (Igeo) of metals was estimated using regional mean background values. Leaching tests were carried out through partial dissolution techniques (deionized water and diethylenetriaminepentaacetic acid (DTPA) extraction) on wet and dry samples, while the speciation of metals was investigated by a five-step sequential extraction. All analyses were performed by inductively coupled plasma optical emission spectrometry (ICP-OES). Artificial watercourses presented higher contamination levels than natural ones, and a different pollution level was found when Igeo was applied. The sequential extraction showed that metals in river sediments are mostly immobilized in the residual phase (e.g. Cr), while in canals, weak bonds were found (e.g. Cd). The dredging of sediments, and their consequent oxidation, enhances the availability of metals according to their affinity with organic matter (e.g. Cu and Pb) or carbonates (e.g. Zn). The different remobilization rate obtained by changing the oxidation status of sediments highlighted the importance of metal availability studies for assessing and predicting their environmental hazard. The effect of oxidation processes on the availability of heavy metals depends on the geogenic or anthropogenic nature of the element, on the redox status of the sediment and on the affinity of the metal with the different mineralogical phases of the sediment. In redox changing environments, the prediction of the environmental risk from metals before and after sediment land disposal gives more useful information than the knowledge of total metal concentration. The use of leaching techniques, combined with the calculation of background values, is strongly recommended for the assessment of metal hazard in sediments.