Fifty litres of sodium hydroxide (NaOH) leaked from the supply of a boiler demineralisation unit of a flue production plant. The damaged floor beneath the demineralisation columns made it easier for the washing water rich in sodium hydroxide to seep into an old rain water network that empties into the SORGUE river. The rise in the pH led to the precipitation of calcium carbonate creating a major whitish turbidity in the river that disappeared in an hour. Following this accident, the company has planned to refloor and leakproof the unit, repair the piping, modify the automate program to avoid water hammer when closing the valves and reducing the discordance temperature.