Inside a phytosanitary product plant, 13 kg of dimethachlor, i.e. resulting in 448cu.m of water polluted by 30 mg/litre of dimethachlor, were discharged into the Razil stream. On 3rd May, a leak, noticed on a flange at the entrance to a workshop, caused the overflow of some tens of litres of dimethachlor onto the roof of the K4 building platform. Despite being retained onsite, the substance discharged could not be appropriately and fully recovered by the operator. The first rainfall therefore drove the residual phytosanitary product into the drainage network and then to the 2 stormwater tanks. Heavy rains during the night of Friday 13th May caused one of the tanks to overflow: spilled polluted water reached the local stream. Upon return from an extended weekend, confirmation was provided by means of analytical verifications of stormwater tank contents, combined with an analysis of the average automatically extracted sample, that the watercourse had been polluted. Following this delayed announcement, the operator closed the stormwater tank valve in order to halt additional flow into the Razil. A visual inspection of this watercourse downstream of the facility did not reveal any impact on aquatic flora even though the highly toxic effect on aquatic organisms from this substance (classified N) was still feared. On 17th May around 3 pm, the operator extracted 2 samples in the stream: one downstream of the plant, the other just ahead of the section running under the Lower Rhone Canal. The dimethachlor results read 0.029 mg/litre and 0.026 mg/litre, respectively. The operator planned on implementing a cleaning protocol subsequent to a pollution incident that included the verification of protocol efficiency, provision of an alarm system in the event of a thundershower, and revision of both the bulk dimethachlor circuit design and the stormwater tank valve management procedure.