On a Saturday, within a wholesale business specialised in chemical products and awarded the ‘SEVESO’ classification for handling hazardous substances (as per the SEVESO directive), the personnel of a supervisory company noticed the leak of a 70% hydrofluoric acid (HF) on a 1,000-litre container. The liquid had been stored for 5 days on a facility laid out for this purpose. The team on-call diluted 2 kg of drippings found in the retention basin and sealed the container. The HF liquid supplier and processor performed analyses to determine the cause of this leak. On Monday morning, the operator stated that rainwater from the previous day’s storm had drained out of the retention basin, with the seal being breached at the junction between the slab and the basin edge. The operator requested the basin builder to install an impermeable lining and then to identify the causes of this breach, given that the basin had been installed on the site for less than 2 years.