Has the Rio Doce "time bomb" been defused? Using a weight-of-evidence approach to determine sediment quality.

The Fundão mine tailings dam rupture of 2015, in the Rio Doce basin, Brazil, resulted in the deposition of tailings downstream of the dam. It has yet to be determined if metals associated with the tailings have contributed toxicity to organisms, burying a time bomb that could be ticking. Currently the data on toxicity to benthic and aquatic organisms have not been assessed sufficiently to allow an informed assessment using an approach based on weight-of-evidence. This study was conducted to ascertain if sediments at "hot spots" that received Fundão tailings reflected elevated concentrations of metals and if these concentrations were sufficient to result in toxicity to freshwater organisms. The lines-of-evidence considered included assessing metals concentrations in relation to sediment quality criteria, establishing biogeochemical characterizations, completing an evaluation of potential metal release upon resuspension to provide information on bioavailability, and identifying acute and chronic toxicity effects using sensitive native species for waters (water flea, Daphnia similis) and sediments (burrowing midge larvae, Chironomus sancticaroli). Only porewater concentrations of iron and manganese exceeded Brazilian surface water criteria, whereas most trace elements exhibited no enrichment or elevated environmental indexes. The concentrations of bioavailable metals were assessed to be low, and metal concentrations did not increase in the overlying water upon resuspension; rather, they decreased through time. Toxicity testing in resuspended waters and bulk sediments resulted in no acute or chronic toxicity to either benthic or aquatic species. The low metal bioavailability and absence of toxicity of the tailings-enriched sediments was attributed to the strong binding and rapid removal of potentially toxic metal ions caused by oxyhydroxides and particles in the presence of iron-rich particulates. The findings of these sediment hot-spot studies indicate the Fundão dam release of tailings more than six years ago is not causing the current release of toxic concentrations of metals into the freshwaters of the Rio Doce. Integr Environ Assess Manag 2024;20:148-158. © 2023 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).