At around 11.40 p.m., the pH meter at the rainwater outfall outlet in a chemical plant detected an anomaly. At around 1.20 a.m., the operator identified a leak from a tank containing 800t of ferric chloride. The corrosive liquid spilled out of the tank’s retention system and into the rainwater system first, then the AUGUETTE cove, a safety basin before discharge into the sea. The workshop was shut down and switched to safety mode. At 2.10 a.m., technicians diverted the rainwater system, using a sluice gate, to a retention basin. The residual water between the sluice gate and the cove was discharged into the cove. At 2.15 a.m., the operator noticed a brown/red colouring in the cove and set up a crisis unit at 3.50 a.m. At 4 a.m., the site’s firefighters put in place 2 absorbent dams, in addition to the existing ones in the cove, and pumped off the foam and supernatant liquid. At 6.50 a.m., the pH dropped back to 3, meaning that the leak was still live. The operator encountered issues shutting off the discharge from the retention system to the rainwater system.

As the ferric chloride was highly soluble, it seeped through the dams in the cove, forming a 400m-long, 200m-wide brown plume on the sea visible from the coast. As a precautionary measure, the mayor and maritime prefect banned bathing, sailing, mooring, fishing, and underwater diving in the affected area for 48h.

The leak from the retention system was plugged at 10 a.m. with sandbags and by pumping off the product. At 2 p.m., the plume was visible only over 4ha.

The quantity of ferric chloric discharged into the AUGUETTE cove was estimated to be 470t. On the day of the incident, firefighters observed low aquatic mortality (a few kilograms), due to the brown acidic solution that was generated but dissipated. Analyses concluded that the pollution had only had a low impact on water quality. Ferric chloride is a flocculant that precipitates in the form of orange solid complexes. Deposits on the seabed were observable over a combined surface area of 62m² spread over a 208ha area in early September.

The leak from the tank was located along the joint between the shell and base. It would appear that subsidence due to the presence of an underground cavity caused a sudden breach of the tank’s lining, then an acid attack on the metal sheet base until the tank breached locally. The lack of a tank level issue alarm delayed identification of the source of the discharge.

As regards the retention system, the operator identified a section not covered with acid-proof protection, which caused part of the discharge into the rainwater system. This retention system lining/product incompatibility was due to incomplete risk identification. This failure was not detected even though the operator conducted visual inspections and the retention system underwent repairs in 2018.

The operator observed that the sluice gate lost its tightness over time, which added to the discharge. The materials it was made out of were incompatible with prolonged exposure to a concentrated ferric chloride solution.

The internal emergency plan was not triggered, because the maritime pollution scenario was not one of the major accident scenarios included in the hazard analysis and taken into account to draw up the plan. These management principles were nonetheless implemented by the crisis unit set up by the operator.

The inspection authorities for classified facilities issued an order setting out emergency measures including:

• securing of the “ferric chloride” workshop facilities plus a pre-restart inspection of the affected facilities;
• monitoring the impact of the discharge on ground- and seawater via a monitoring plan, plus taking retention samples and assessing health risks;
• drawing up and implementing a management plan to eliminate health and environmental impacts, in the event of impacts identified in the monitoring plan;
• treating waste generated by the incident.

Environmental impact monitoring demonstrated a clear reduction in deposits on the seabed. Based on laboratory tests and in conjunction with a specialist company, the operator developed a procedure to recover flocs from the seabed. According to the latest monitoring results, the balance of benefits and risks nonetheless appeared not to be in favour of carrying out pumping work.

To prevent this kind of event, the operator took the following corrective measures:

• checking the compatibility of the retention systems with the products stored;
• checking the integrity of the rainwater diversion system;
• improving emergency response procedures in the event of environmental accidents, by extending the internal emergency plan to cover this issue.