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

Area of Science:

• Environmental Science
• Ecotoxicology
• Geochemistry

Background:

• The 2015 Fundão mine tailings dam rupture released significant amounts of tailings into the Rio Doce basin.
• The potential long-term toxicity of these tailings to aquatic and benthic organisms remains inadequately assessed.
• A weight-of-evidence approach is needed to evaluate the environmental impact of the tailings.

Purpose of the Study:

• To determine if metal concentrations in "hot spot" sediments impacted by Fundão tailings are elevated.
• To assess if these metal concentrations pose a toxicity risk to freshwater organisms.
• To evaluate the bioavailability and potential toxicity of metals in Fundão tailings-enriched sediments.

Main Methods:

• Assessed metal concentrations against sediment quality criteria.
• Characterized sediment biogeochemistry.
• Evaluated potential metal release and bioavailability upon resuspension.
• Conducted acute and chronic toxicity tests using Daphnia similis and Chironomus sancticaroli.

Main Results:

• Only porewater iron and manganese exceeded Brazilian surface water criteria; most trace elements showed no enrichment.
• Bioavailable metal concentrations were low, and metal levels decreased in overlying water upon resuspension.
• No acute or chronic toxicity was observed in aquatic or benthic species exposed to tailings-enriched sediments or resuspended waters.

Conclusions:

• Fundão tailings-enriched sediments in hot spots do not currently pose a toxic risk to freshwater organisms in the Rio Doce.
• Low metal bioavailability, attributed to strong binding by iron-rich particulates, prevents toxic metal release.
• The Fundão dam tailings release is not causing current toxic metal concentrations in the Rio Doce freshwaters.