A 147 t leak of adiponitrile (ADN), used in the fabrication of nylon, occurred over the weekend at a chemical site. The installations include: a tank car unloading station, a main storage tank with a capacity of 8,900 m³, a 2,500-m³ buffer tank S2 which continuously supplies the production unit, piping with pumps and valves (manual V7 and motorised V8 pumps at the base of S1, manual V2 on the main pipe connecting S1 and S2, manual V3 on the unloading line and located between the main pipe and the small pressure surge tank S3) used to transfer the ADN to one of the 2 tanks or to perform transfer operations between these 2 tanks located in 2 separate catchpits. The tank car stations and the piping are equipped with hermetic zones and trenches connected to a remote pit. On Friday, an operator had performed a transfer operation between the two tanks. The enquiry revealed that V3 was not completely closed, that the mechanical key control system between V2 and V3 was inoperative (V2 remained open), as did the limit switch contact control on V3, controlling the closure of V8 whose position indicator light was out of service. The ADN flowed from tank S1 through V7 (normally open), V8, V2 and V3 by gravity and slowly filled up (2.6 m³/h) tank S3 open to the atmosphere and finally spilling out via the latter’s vent at a height of 4 m. Owing to the continuous wind throughout the weekend, 120 t of ADN did not flow into the trench under the tank, but onto the gravely soil nearby. This spillage was not detected by multiple rounds conducted by the weekend security team, certainly due to the low flow rate and the abundant rainfall that had partially filled up the catchpits, wetted down the installations and created numerous puddles on the ground. The leak was discovered the following Monday when the tank levels were recorded. The operators closed the valves and transfer pipes, pumped the substances still present on the ground and drained the catchpits into empty tanks that were available. The unloading instructions were modified (valve V7 systematically closed), the vent on tank S3 is connected to the trench leading to the remote pit. Hydrogeological studies were conducted. The polluted zone was evaluated at 1,600 m², and 4 piezometres were used to pump and draw down the water table level for several months.