In a company manufacturing crude oils and fats, a tank of “paste” overflowed around 9 a.m. in the zone dedicated to breaking up pastes (viscous oil-containing phospholipids). A technician rapidly washed up the 2 m³ of product on the floors, which were hermetic but without retention. The product, heavily laden with paste, entered the area’s grated storm drain; it then made its way into the plant’s southern rainwater system, then to the southern oil separator, not designed to receive charged fluid. The polluted water entered the network via the site’s last shutoff valve which had remained open, before reaching the municipal sewer running next to the plant to the east on the site of a neighbouring plant. At around 9:30 a.m., a technician emptying the rainwater from the storage tanks on the banks of the Seine River, raised the alert as soon as he saw the oily and foamy emulsion on the river. The facility operator closed the site’s shutoff valve and the storm drain valve. An object was used to mechanically block the passage, then the pollutants were pumped into the sewer system of the neighbouring plant. The Rouen Port Authorities noted the situation and took samples. According to the operator, the storm drain’s rainwater valve was open when it should have been closed. The polluted water should have been channeled toward the internal purification plant. At the time, the slick on the Seine measured around 50 m² (10×5 m). The Inspection authorities for classified facilities, alerted of the situation, did not find any visible residual pollution on the day of the visit. The facility operator was required to submit an incident report and include this event in the future hazard study associated with the upcoming request to extend the site’s activities. Measures have been taken to ensure the site’s shutoff valve is closed.