On a Monday morning, the operator of a non-hazardous waste storage facility, which had been closed over the weekend, noticed that the leachate basin had overflowed. The city hall had also reported the presence of leachate in the rainwater collection ditch outside the site. The pump used to transfer leachate between a first and second storage basin had not been shut off on Friday evening. As a result, the second basin had overflowed over the weekend and leachate had entered the rainwater network. It was then discharged through the stormwater basin to the rainwater settling basin. The leachate had become mixed with the water already present in this basin, and the resulting overflow flowed into the Auzon River. After inspecting the bends in the river over several hundred metres, no leachate residue, fish mortality or abnormal colour were observed in the river.

The operator plugged the stormwater overflow outlet on the stormwater basin to stop the flow of leachate. A tanker lorry was used to pump 20 m³ of water from the stormwater basin to prevent it from overflowing again. The rainwater drainage ditch was cleaned. A pump, powered by a portable generator set, was installed to pump the contents of the stormwater basin back into the leachate basin (300 m of pipe). Pumping operations continued for 10 days.

Following the event, the operator took the following actions:

• level sensors, equipped with alarms, were installed in the basins;
• an overflow was installed in the leachate basins to convey the leachate to a retention basin in the event of an overflow.