Persistent metal and metalloid fluxes to the seafloor from an underlying mine tailings deposit 25 years after disposal off Portmán Bay (SE Spain)

Coastal disposal of mine wastes has been recognized as a critical source of metals and metalloids to the water column and seabed sediments, leading to ecotoxicological impacts and benthic habitat loss. Portmán Bay, SE Spain, is one of the most severe cases of coastal impacts due to mine waste dumping. From 1957 to 1990, 57 million tons of metal (Fe, Mn, Zn and Pb) and metalloid-rich (As) artificial soils were discharged into the shoreline, completely filling the bay and extending it seaward. Decades after mining ceased, concerns remain about the stability of the submarine deposit and potential remobilization that could be induced by a potential coastline restoration project or other natural processes. This study aims to provide a complete physical and geochemical characterization of the uppermost layers of the mine tailings deposit and the pre and post-dumping marine sediment off Portmán Bay, focusing on the dispersal patterns and pollution levels of metal(loid)s throughout the inner continental shelf. Post-dumping sediment metal(loid) concentrations are lower than those in the tailings but still significantly higher than pre-dumping levels. Enrichment factors in the seafloor sediments (post-dumping) have been identified for Pb (5 times higher than background natural values), Mn (8×), As (9×), Zn (22×) and Fe (56×). Additionally, 8.6 km² of the seabed could be considered hazardous based on the Action Level B threshold of the Sediment Quality Guidelines for Zn, As and Pb. These findings indicate ongoing metal(loid) flux into shelf sediments, providing insights for the environmental restoration of Portmán Bay and its seafloor.