At around 1:10 a.m., an opening formed in the upper portion of a 628 m³ vertical tank at a chemical plant. The lined steel tank was 96% full of rainwater containing mercury (concentration of 0.49 mg/l of mercury). It had been disused for some time and had been returned to service to collect excess rainwater following the shutdown of the site’s detoxification station. Workers noticed the incident at around 4:30 a.m. during a monitoring inspection. The on-call managers diverted the tank’s supply and recovered the water collected on the available base slabs and bund walls. In the end:

• 327 m³ was confined in the tank;
• 142 m³ was recovered;
• 134 m³ flowed into the River Saône (a flow of 65 g of mercury).

An inspection conducted in May 2017 had found that the tank was damaged. The tank was subsequently scheduled to be removed from service by late April 2018. This date was intended to provide enough time to replace it by another existing tank. Pending this final removal from service, two tanks were being used on a priority basis. However, following an unexpected shutdown of the wastewater treatment plant due to delivery issues since 14 January with one of the reagents used at the plant as well as to heavy rain, the tank that was to be removed from service had to be used as-is to store large quantities of effluent.

The operator implemented the following corrective actions since the incident:

• the tank was immediately and permanently removed from service;
• the two other tanks were inspected;
• the reagent supply strategy was revised (safety threshold for triggering orders, diversification of suppliers).

In May 2012, a mercury brine tank used by the electrolysis department had ruptured, sending 2.5 kg of mercury into the River Saône (ARIA 42346).