Geochemical fractionation of trace metals and ecological risk assessment of surface sediments in Sepetiba Bay, Brazil

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 the mobility of the metals. The main goal of this work was to assess metal mobility in sediments and its associated toxicity in a fictive dredging area, to evaluate the risks of the operation. To achieve this goal, 18 superficial sediment samples were collected and characterized for pH and Eh. Sediments were analyzed for grain size, organic carbon, and total nitrogen, and metal mobility 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 0.3 mg kg-1 and 65.5 mg kg-1, respectively), while Fe, Cr, Cu, Ni, and Al were associated with the residual fraction. Metals in the residual fraction are probably associated with the mineral lattice of the sediment and should not represent an environmental risk for the biota. The application of the enrichment factor and three risk assessment indexes (Risk Assessment Code, Risky Pollution Index, and Bioavailability Risk Assessment Index) show that the sediments are considerably enriched in metals that constitute a relevant risk for the sediment biota. In the case of dredging operations, Cd and Zn should be released to the overlying waters and be available to organisms, threatening the whole ecosystem. The proposed approach was shown to be much more precise than what is frequently presented in the Environmental Impact Assessments that only consider the threshold limits of the legislation.