Arsenic and heavy metals contamination by effluent dam rupture in a subtropical coastal lagoon

Dam accidents, often resulting from inadequate structural monitoring, pose significant environmental risks. In southern Brazil, the rupture of an evaporation-infiltration lagoon released over 500,000 m³ of treated domestic effluent into a coastal lagoon, raising concerns about potential contamination from nutrients and heavy metals. This study aimed to (1) assess the environment's self-purification capacity regarding dissolved nutrients, (2) determine total heavy metal concentrations in water and sediments throughout the coastal lagoon using inductively coupled plasma mass spectrometry, (3) correlate variables influencing heavy metal availability to identify potential sources, and (4) evaluate environmental risks by comparing concentrations to established water and sediment quality guidelines. Potential sources of contamination included natural origins, boat traffic associated with fuel leaks and antifouling paints, and the irregular discharge of domestic effluents into the lagoon. The results revealed nutrient self-purification and elevated arsenic levels in the water, likely from natural sources. However, manganese and zinc concentrations exceeded water quality limits, while zinc and copper levels were notably high in northern sediments, with no definitive association to the dam's sludge. These findings highlight significant toxicity risks to biota and emphasize the need for continuous monitoring. Mitigation strategies should be implemented, particularly in the most contaminated areas, given the lagoon's intense use for recreation and seafood harvesting. Overall, the results reinforce the threat of pollution to biodiversity, ecosystem services, the livelihoods of fishing communities, and the local economy, emphasizing the importance of this study in guiding management actions amidst significant challenges.