A mixture of 124 m³ of heavy hydrocarbons (HC) and water originating from a refinery was discharged into the RHONE canal which was polluted over 1 km. The part of the station in question primarily recovered effluents from the tank basins, deballasting pits, bitumen pits and unit floor slabs. At around 1:30 pm, the analysers located on the treated effluent outfall, after oil removal, indicated the presence of hydrocarbons. In this situation, a procedure provides for the diversion of these effluents into a stormwater basin for subsequent treatment. The “alignment” operation, performed manually, requires a sump pump. On the day of the incident, as soon as HCs were detected at the outfall, the sump pump started running (1:34 pm) until 1:53 pm. The analysers then gave no further indications. A maintenance team was called out and reported the presence of HCs at the outfall at 2:20 pm; 7 minutes later, the pump restarted. After 3 pm, the site’s safety team deployed a floating boom and absorbent pads at the outfall, and then placed mobile pumps close to the site’s outfall and the canal. One side of the river banks was polluted (a strip 1 m wide and 300 to 400 m long, followed by an area of less visible pollution). On the next day, no more HCs were arriving, but 4 additional booms and absorbent pads were installed. Two days later, high-pressure cleaning was started and continued until 01/06 when the water level rose and covered the pollution. The operations were stopped. The mobile site was removed on 09/06 as the cleaning operation was therefore considered ineffective given the residual pollution.

Following analysis, it appeared that when the sump pump was stopped, the discharge valve had not been closed, and the associated check valve – found to be stuck in the open position – had not prevented the gravity backflow of the heavy hydrocarbons originating from the bottom of the stormwater tank and the discharge into the canal. In addition, the analysers had malfunctioned as they were not suitable for detecting heavy hydrocarbons. The operator is considering the installation of vacuum breakers to prevent return flows from the stormwater basin to the sump, and is revising the management of the stormwater tank. He is looking into slaving the discharge valve to the pump status and replacing the analysers.