Metal geochemistry in bottom sediments from an urbanized tropical estuary: implications for land disposal of dredged materials

Abstract The Fundão canal (RJ/Brazil) is an urbanized estuary impacted by eutrophication, metal contamination and silting, and was subjected to dredging operations to fulfill the demands of the 2016 Olympic Games. Given the scarcity of studies focused on the degradation of tropical estuaries and the need to evaluate the environmental Olympic legacy, this study assesses metal biogeochemistry in bottom sediments of the Fundão canal ( n = 18) and examines ecological implications for land disposal of dredged materials. Three lines of evidence were adopted: sediment and soil characterization; metal determination; and acute toxicity, reproduction and avoidance bioassays with Eisenia andrei using mixtures of local soils (Spodosol and Ferralsol) amended with a composite dredged sediment sample prepared by equally mixing all sediment samples. The sediments are predominantly muddy and organically enriched. These characteristics control metal distribution and are influenced by coastal morphology, mangrove vegetation, and marine and fluvial inputs. A sandy and silted sector was identified in the middle of the canal, exhibiting lower metal concentrations. Overall, metal concentrations exceeded Brazilian sediment quality guidelines and were associated with domestic and industrial waste inputs, particularly Zn (684.0 ± 318.8 mg/kg), Cu (161.1 ± 64.2 mg/kg), and Hg (1.2 ± 0.8 mg/kg). The acute toxicity of the dredged sediment sample was higher in sandy soils (Spodosol LC 50 = 2.50%) compared to clayey ones (Ferralsol LC 50 = 4.54%). Earthworm reproduction was the most sensitive endpoint (Spodosol EC 50 = 0.68%), followed by avoidance responses (Spodosol EC 50 = 1.73%). These results suggest that dredging without accompanying sanitation and source-control measures may offer only short-term benefits.