Around 10:00 pm inside a chemical plant, hydrochloric acid (HCl) was released within the acid vat room. The leak occurred at the level of a discharge joint on a pump transferring the acid to the shop areas. Leaking continued for 30 min before the operator actually closed the tank bottom valves and stopped the pump. A gas cloud formed; however, nearly all of the HCl remained confined within the basin. Firefighters responding to the emergency call suppressed the cloud using water, which nonetheless flowed towards an adjacent canal that empties into the SORGUE River. Personnel was evacuated from the site, and the site owner built a dam with sludge in order to contain runoff inside the plant; 1 to 2 m³ of acid did spill from the basin and to the outside. At 11:00 pm, the pH reading was at 5.5 on the main arm of the SORGUE and then returned to normal around midnight, without any apparent fish mortality. The joint was replaced 2 days hence; it was intact, not punctured, but offset with respect to its standard housing. Follow-up verifications were performed on tank flanges and joints, in conjunction with the change in joint assembly (use of holed joints affixed to the flange). The operator introduced flange protections at the level of pump discharges to prevent acid projections outside the basin; moreover, an earthen embankment was placed along the storage zone to confine the water. The operator also improved onsite lighting and modified work guidelines (revision to the racking procedure, making it necessary to bleed the system should even a slight flow be detected, prohibiting repeat reloading). The report issued by the operator indeed noted the presence of repeat reloading on the circuit in association with a low level in the tank, which allowed for air intake. This repetition in reloading activated a water hammer that caused the joint to slip out of its housing. The origin therefore lies in a level sensor malfunction inside the tank.