Increase in the bioavailability of trace metals after sediment resuspension

This study evaluated the potential mobility of metallic contaminants (Cd, Cr, Cu, Ni, Pb and Zn) present in sediments at the mouth of São Francisco Channel (Sepetiba Bay, RJ), using resuspension tests. Fourteen surface sediment and superficial water samples were collected along a salinity gradient from the river mouth into the bay. The resuspension tests consisted in the mechanical agitation of humid sediment aliquots in local unfiltered water, in a ratio of 7.5 g:100 mL, during 1 h and 24 h. The determination of the metals in the bioavailable fraction was performed in ICP-OES, after extraction in 1 mol L−1 HCl. Significant differences were observed between the central and external sectors for Cd (p = 0.009), Cr (p = 0.017), Ni (p = 0.047) and Zn (p = 0.007) concentrations (Kruskal–Wallis). The water’s temperature (p < 0.001), pH (p < 0.05) and dissolved oxygen (p < 0.001) decreased in both time intervals in comparison to the initial measures before resuspension. The concentrations of Zn in the sediment increased from 222 to 316.2 mg kg−1 after 24 h agitation (Kruskal–Wallis; p < 0.05) at the external sector, possibly because this region has stronger oxidizing conditions. In general, a significant increase in the metals’ concentrations was observed after resuspension, especially for the samples from the central sector that showed more than 90% of increase in metals concentrations after 24 h of agitation. These increases could lead to higher ecological risks to the local biota, especially during dredging activities.