Within a chemical plant producing sodium carbonate, 45 m³ of ammonia-laden water at a concentration of 22 g/l accidentally spilled into the site’s 23-ha settling basin. The ammonia cloud (NH3) that formed by means of vaporisation above the basin did not disperse as a result of unfavourable meteorological conditions (i.e. -4°C, fog, little wind), but instead drifted towards the city of Nancy between 7:30 and 10:00 pm. A pungent odour upset the neighbouring residents of Laneuville-les-Nancy, Jarville-Vandoeuvre and the southern part of the Nancy Metropolitan Area; the police and fire services recorded some 600 phone calls during this period.The prefecture set up a crisis response unit. Radio messages were broadcast informing the population to remain indoors. Some one hundred fire-fighters were deployed to perform toxicity measurements within the affected zone and inform the local population. The NH3 concentration, which reached levels of 50 to 60 ppm at the point of toxic release, was measured between 10 and 32 ppm outside the plant (NH3 Average and Maximum Exposure Limits of 20 and 50 ppm). The state of alert was in effect until 11:30 pm.The ammonia-laden water stemmed from the liming of ammonium chloride during production of the sodium carbonate. It is a manufacturing by-product typically distilled in order to separate the gaseous NH3 recycled as part of the process from the saline water transferred into a settling basin prior to discharge into the natural surroundings.The saline effluents were loaded with impurities (gypsum) that precipitated into the solution intended for distillation and encrusted the devices and piping. The day of the accident, a gypsum crust broke loose when starting up a distillation unit and plugged the suction chamber of the pump assigned to discharge the saline effluent to the settling basin. During the subsequent unscheduled maintenance visit, an operator shut off the distillation columns at their outlet while forgetting to stop the liquid supply line on one of them. After removing crust accumulation on the installation, the column that had gradually filled with undistilled ammonia liquid, due to a liquid load pressure in the column exceeding the vapour pressure, opened on the discharge pump at the basin and the undistilled liquid poured into the basin for 15 min.Several remedial actions were carried out following the accident, including: improvement of the operating procedure (greater detail added), training of onsite teams in implementing this new procedure, installation of NH3 sensors at the distillation column outlet, and lowering of the sensor alarm threshold at the level of the settling basin.