A massive number of fish were killed by pollution that affected a 10-km stretch of the River Cuisance. Cyanide discharged by a surface treatment plant was suspected to be the cause. According to the press, the local fishing society reported that the river is polluted once or twice a year, but a link has yet to be established. The French Biodiversity Agency (AFB) tested the water in the river to determine the pollution’s cause. The initial results pointed to the operator. Samples taken a month later by the AFB from a manhole at the plant’s outlet showed a cyanide concentration of 22061 µg/l while the authorised maximum is 50 µg/l. The plant’s cyanide baths are supposed to operate in a closed circuit that is fitted with an ion-exchange resin filter system and then be discharged for treatment off-site. Discharges to the natural environment are prohibited by law. The inspection authorities for classified facilities found multiple issues at the plant, in particular lacks of:

• operating instructions on using cyanide;
• staff training on operating cyanide facilities;
• procedures on how to respond to failures of the ion-exchange resin system.

The local government authority issued a decree requiring the operator to:

• search for the cause of the discharge;
• inspect the condition of the site’s water collection networks;
• monitor the water quality of the Cuisance both upstream and downstream of the plant’s discharge point;
• test how well cyanide safety and operating instructions are followed;
• check the condition of the facilities before shutdown and restart and record the results of these checks.

To reduce the risk of cyanide being discharged in the event of a flushing failure, the operator plans to pass all baths that may contain cyanide through a cyanide removal system.