Geochemical fractionation of trace metals and ecological risk assessment of surface sediments for a simulated dredging area in Sepetiba Bay - Brazil

Abstract The Sepetiba Bay (Southeast Brazil) is a known Cd and Zn contaminated site that received spills of a large slag pile leachate from a Zn smelter. With important harbors, Sepetiba Bay demands periodic dredging operations which affect directly on the mobility of the metals. The main goal of this work was to assess metal mobility in sediments of a simulated dredging area in this tropical bay. To achieve this goal 18 superficial sediment samples were collected within this simulated area. pH and Eh of the interstitial water were measured in situ, while the sediments were analyzed for grain size, organic carbon and total nitrogen, total metals. Metal mobility assessment was evaluated with a sequential extraction procedure, proposed by the European Community Bureau of Reference (BCR). The results demonstrate that Cd and Zn are mainly associated with the exchangeable fraction (mean concentrations 1.4 mg kg− 1 and 149.4 mg kg− 1 respectively) and reducible fractions (mean concentrations 0.27 mg kg− 1 and 65.54 mg kg− 1 respectively), while Fe, Cr, Cu, Ni, and Al are associated with the residual fraction. Metals that are associated with the residual fraction are probably related to the mineral assembly of the sediment and should not represent a considerable environmental risk for the biota. The two metals associated with the exchangeable and reducible fraction respond to strongly reducing characteristics on surface sediments, which are affected by reduced hydrodynamics and high organic matter input. However, Cd and Zn could constitute a risk, mainly during resuspension events, when alterations of pH, and Eh, promoted by oxygenation of sediments can release them to the water column and result in increased association to the exchangeable fraction.