Characterisation of the submarine disposal of a Bayer effluent (Gardanne alumina plant, southern France): III: Trace metals in the seawater column in the vicinity of the discharge area in the Mediterranean Sea.

In the context of evaluating the environmental impact of deep-sea tailing practices, we conducted a case study on the Bayer effluent released into the Mediterranean Sea by the French Gardanne alumina plant. This effluent results from the filtration of red mud, which has previously been discharged into the Cassidaigne canyon for 55 years. In 2015, regulatory changes permitted the released of a filtered effluent instead of the slurry. This paper investigated the chemical impact on seawater composition a few months after this change. High metal concentrations were found in the upper 10 m above the outfall, i.e. the plume where the effluent mixes with seawater and forms hydrotalcite particles and concretions. Elevated Al concentrations at station L1 above the outfall indicated the plume presence and potential hydrotalcite dissolution up to 200 m depth, while a more distant station (L2) showed less impact, suggesting further Al remobilization from red muds previously deposited on the seafloor. Generally, metals and metalloids concentrations were within Mediterranean background levels, with some anomalies possibly linked to local submarine groundwater discharge.