A chemical pollution incident occurred in a company’s stormwater retention basin. One of the firm’s lorries was stolen and prior to that had unloaded the drums it was transporting. These 1,000-l capacity drums contained either 35% soda or 33% hydrochloric acid. Of the 2 drums containing acid, one was ripped open and emptied, as were both of the soda drums (one torn open and emptied). Another drum containing an unidentified product was also opened and emptied. First responders set up a safety perimeter and closed the stormwater retention basin by means of a system of valves in order to confine the pollution. The impact of this pollution on the water and natural environment was limited thanks to both the basin closure and hydrochloric acid neutralisation by the soda. Since the basin had been closed shortly after the polluting event, the vast majority of this pollution did in fact remain confined within the basin. The SEINE River Navigation Authority observed no impact on the river’s level. The acid-base reaction (hydrochloric acid + soda), which took place inside the basin, very quickly had the effect of neutralising the pollution. A pH of 5 was recorded around the pollution, while a pH 7 value was measured further into the basin. Gendarme officers removed samples in order to analyse: hardness, conductivity, pH, and Na+ and Cl- ion concentrations. Following this incident, the company was required to clean up the polluted basin, and an official Prefecture correspondence would be sent to the company as a reminder of its obligations; the company was given the opportunity to consult the metropolitan public works office and SEINE Navigation Authority regarding the eventual basin drainage, or at the very least a partial or total basin cleaning.