Evaluating zinc bioavailability in dredged sediments of Guanabara bay (BRAZIL): implications for coastal management standards

Abstract Guanabara Bay (GB) is an environment contaminated by metals, and it is the bay that receives the most dredging in Brazil and South America. Previous studies have shown the impact of dredging on planktonic and benthic species, which were not correctly measured by the Brazilian dredging standard CONAMA Resolution 454/12 (2012). Given this, the present study compared two acid extractions targeting metal bioavailability (1 M HCl and acetic acid) in sediments from four areas of GB sampled in summer and winter, before and after resuspension. The results showed that Zn concentrations exceeded the standard in all samples in extractions methods and in two seasons after resuspension. In winter, three of the four areas showed concentrations exceeding levels 1 and 2 of CONAMA Resolution 454/12 (2012), both before and after resuspension. The bioavailability change index (BC) was applied to show the remobilization of metals in the sediment after dredging. Results evidenced that Zn remobilization occurred after resuspension for all samples and in most resuspension intervals, indicating an environmental risk related to the resuspension activity. This demonstrates the efficiency of using bioavailable extractions for the monitoring of dredging activities. In addition to being cheaper and easier to perform than the pseudo-total extractions recommended in most global standards, it provides valuable information for improving dredging management policies. According to the results, a revision of the dredging standard is recommended, considering the inclusion of bioavailable extractions.