A mineral oil (without PCB) leak coming from a chemical site that was being dismantled polluted the lower branch of the River GARONNE. This pollution was detected the day after dismantling two electrical transformers at the plant. The facility was being dismantled in 2 stages: drainage of the oil from the bottom via a flange provided for this purpose, followed by the cutting of the transformers using shears to facilitate their removal. At around midday on 28/06, when cutting up the first transformer, a significant amount of oil poured out when the technician cut into the expansion tank in the upper part of the device; the closed connection valves between the expansion tank and the core of the device prevented the transformer from being completely drained. The bottom of the retention tank was pumped out the next morning. The same operation was carried out on the 2nd transformer after opening the connection valves. However, this action did not prevent another oil leak into the retention basin. In total, 1,100 l of mineral oil were discharged into the retention basin. At around 1 pm on 29/09, the pollution of the River GARONNE by hydrocarbons was reported. The polluting discharge was from the chemical plant’s stormwater network: the oil leaks from the transformers occurred in a retention basin that was peripherally connected to the stormwater network. The operator estimated that 200 l of oil had been discharged into the natural environment via this network. Absorbent booms were placed over the polluted area, and the retention basins and all of the pipes affected by the flow of oil were cleaned. Demolition of the transformers has been postponed. A complete diagnosis of the condition of the adjacent soils is being carried out. To reduce the likelihood of such an accident recurring, cement plugs are being installed to isolate the retention tank from any other network.