In a pharmaceutical plant, 2178 litres of a dipropyl acetonitrile (DPAN) solution containing toluene overflowed a tank. A technician had been transferring 15 m³ of DPAN around a vessel that was already nearly filled (above 88,000 litres). Upon completion of the operation, he noticed product spilling into the retention basin and duly informed his supervisor. Both men returned 20 min later with pumping equipment, but the basin was empty; the solution had infiltrated into the ground through several cracks in the retaining structure. Piezometers were installed to track the evolution of this pollution; 60 m³ of polluted earth were excavated around the basin. No impact on the water table could be observed. The soil, composed of a 30-cm deep clay layer, had formed an impermeable barrier protecting the water table flowing underneath the polluted zone. Inspection Authorities recorded the facts. Two Prefectural orders and a causal tree analysis led to taking various preventive and corrective measures: rehabilitation of the damaged basin; execution of permeability tests on the other basins with repairs as needed; modification of manufacturing instructions coupled with a transfer ban whenever TA 809 tank volume exceeds 85,000 litres; installation of high-level alarms servo-controlled to the transfer pump on all tanks; inspection and repairs of all on-site retention basins; and implementation of a basin seal monitoring procedure after rainfall events and fire protection tests.