Inside a water treatment plant, an exothermic reaction caused an explosion on a retention basin and the ensuing release of acid vapours. A 50-m safety perimeter was established around the plant, with the closest residences situated 300 m away. Some fifty firefighters identified the leak source; the gases formed could escape via the ventilation system connected to the premises where the explosion first happened. The products contained in the destroyed tank were recovered in the surrounding retention basin. They were then pumped and discharged to an external treatment facility. The accident exerted no impact on water production, which could proceed using the two intact remaining trenches and moreover no discharge was released into the natural environment. The explosion occurred within a steel tank located in the basement of the site’s storage building; this tank served as temporary offset retention for the storage of sulphuric acid (H2SO4) during restoration of the site’s normal retention capacity. The accident might have been due either to the sudden arrival of acid into the tank, thereby triggering a violent reaction with the steel followed by hydrogen production, or to a violent reaction caused by a mix of H2SO4 with a product contained in the basin or with a product discarded into the drain hole by a third party.