Within a surface treatment plant that had been closed for the summer holiday period, a leak of a basic degreasing agent (pH = 10) seeped into the stormwater drainage network and then polluted a river subsequent to a thunderstorm event. The spill stemmed from a retention basin connected to this network (yet the operator was unaware of any such connection); this oily liquid substance was deliberately stored in the basin at the time of maintenance work in the plant. In order to contain the pollution, emergency service personnel used an inflatable plug to block the discharge pipes leading from a stormwater retention basin. Notified at the end of the day, the Hazardous Installations Inspectorate requested the operator to pump and dispose of 2 m³ of polluted water. On the next day, a site manager informed the inspectorate of a rise in basin water level and of the difficulties encountered in identifying a disposal service; pumping operations (on a volume of 80 m³) were to ultimately begin at the end of the day and be completed by the following morning. The increased volume of polluted water was due to the fact that the stormwater network also collects water from a small brook. The administrative investigation would reveal that the use of retention basins as intermediate storage tanks was a typical practice within the unit during maintenance work on installations or even during normal operations. The inspectorate also noted the following: the absence of both guidelines for maintenance tasks and an emergency response plan in the case of accident, unavailability of product safety data sheets, inadequate network drawings, and the inoperability of liquid detection alarms within retention areas. Lastly, the inspectorate noted the facts and proposed that the Prefect issue an official injunction imposing all necessary installation compliance work.