A fuel oil leak originating in the boiler room at a paint production plant polluted the AIRAINES River. The employee assigned boiler operations started the equipment up around 7:30 am. Around 4:00 pm, the site manager noticed brackish water in the river along with a strong odour of fuel oil stemming from the “Fontaine aux Malades” stream. He met with the employee responsible for the boiler, who had just reported the domestic fuel oil tank overflow supplying the boiler (which had been shut off). At 4:10 pm, firefighters installed dams at the plant exit and at the level of the inflow to a fish farm located 3 km downstream. At 4:30 pm, the operator requested a specialised firm to provide assistance in 2 pumping units of 10 m³ each. Pumping proceeded between 5:30 and 8:30 pm at the plant site, then from 8:30 to 10:30 pm at the fish farm. According to initial findings, 500 litres of a heavy fuel oil / domestic fuel oil mix had spilled onto the ground and 25 to 30 litres actually flowed into the river. This overflow would have been caused by a poorly closed valve (the fuel oil return valve was not burned in the circuit), inducing a return of heavy fuel oil into the domestic fuel tank. This tank is solely used to start up the boiler, since the heavy fuel is consumed during normal operations. In principle, fuel oil (whether domestic or heavy) that goes unused during combustion returns via a shared circuit at first and then 2 separate circuits within each of their respective tanks. In remaining accidentally open at the time of switching to the heavy fuel, the domestic return valve enabled filling the domestic fuel tank with heavy fuel, thus leading to its overflow. The unsealed retention, which was insufficient to receive such a quantity of heavy fuel, also overflowed. The fish farm breeding basins were polluted, as were AIRAINES River flora and fauna; 3 km of river banks needed to be cleaned. The operator planned to refurbish and enlarge the 2 retention basins of the two individual fuel oil tanks into a single volume to yield a total capacity of 30 m³ (completion of these works slated for January 16). Moreover, within 2 months, 4 solenoid valves were scheduled for installation to replace the manual valves used on return circuits of both types of tanks, making it possible to manage the boiler feed circuits independently.