In a chemical wholesaler, contaminated water was stored in a retention tank following a leak on a valve on the network separating the stormwater drainage system from the chemical water. On 31st October, the joint municipal water treatment plant activated the pumping of water contained in the retention basin after being informed that the operator’s holding reservoir was probably polluted. This resulted in the pollution of the MARSANGE River, which was detected by the appearance of foam (the discharged water was composed of acids, alkalis and active surface agents). As a result of this pollution, the operator retreated 15,000 m³ of polluted water at his own expense (cost of treatment was estimated at €100,000); however, the operator was unable to precisely indicate for how long the site had been operated with a leaky valve and how much contaminated water had been discharged. The operator also checked the status of the other valves on the site, blocked the leaky valve, shared information about the accident with its other sites, and ensured that its emergency procedures would take into account this type of situation.