Early in the afternoon, passers-by noticed dead fish in the MERDERET River and alerted the municipal public works office. The pH of river water, highly basic (pH 10), was used to locate the source of this pollution: a cheese factory located in the town.

In the morning, a technician from an external company was performing maintenance on the refrigeration installation in the plant and degassed the pipes requiring maintenance. To prevent the discharge of ammonia (NH3) into the atmosphere, the refrigerant used in the installation was placed into two 200-litre drums filled with water to dissolve the gas. A specialised company then removed the drums. Unfortunately, this company emptied the drum contents into the soil (200-250 litres at around 1:30 pm, then another 200 litres at around 3:30 pm), on a section of drained land. Through the plant’s stormwater drainage system, the degassing water was poured into the GRAVIER, a tributary of the MERDERET. Procedures for selecting subcontractors and working methods were subsequently reviewed.