Characterization of the submarine disposal of a Bayer effluent (Gardanne alumina plant, southern France): V: Evolution of metal and metalloid concentrations in the seawater column of the discharge area (Mediterranean Sea) from 2016 to 2024.

This study examined the evolution of trace metal and metalloid concentrations in the Cassidaigne canyon water column (Mediterranean Sea), at the Gardanne alumina plant discharge area, between 2016 and 2024. Following the 2015 ban on solid waste discharge, the plant progressively switched to the discharge at sea of a regulatory clarified effluent, resulting from filter-presses, CO2 treatment and biological unit implementations. Several anomalies in element concentrations at discrete depths in the water column above the outfall (station L1) reflected local environmental influences, such as submarine groundwater discharges and lithogenic inputs, including mistral-induced upwelling, remobilizing sediment. From 2016 to 2021, Al concentrations below 200 m depth resulted from the effluent-seawater mixing (the plume) and subsequent metals and metalloids removal and/or release during hydrotalcite particles formation and dissolution. Concretions at the outfall of the effluent (330 m depth) stopped forming after introduction of the CO2 treatment of the effluent in 2018. Despite occasional biological treatment disruptions leading to increased Al, V, and Cu concentrations at sea in 2020 and 2021, the effluent's chemical footprint remained globally confined in the first hundred meters above the outfall, without broader upper water column impact. Deep Al anomalies at a distant site (station K) in the canyon axis also indicated potential local remobilization from legacy bauxite deposits. Overall, both the plume and the historical residues appeared as localized but distinct sources of contamination. Particular renewed attention emerged from monitoring committees regarding ancient deposits, the spatial extent of which in the canyon far exceeds the chemical footprint of the effluent.