On the hydrocarbon storage site at a refinery, a 50-m³ leak of aromatic hydrocarbons was identified on a 10,000-m³ vertical tank with a floating roof. The discovery was made by a technician during a sampling step conducted when transferring the product to a manufacturing unit. The technician stopped this flow by closing the drain valve. A portion of the hydrocarbons was held in the retention ring, while the remainder spilled out into the retention basin (presence of two stains 10 m² in surface area on the ground) and infiltrated. The hydrocarbons contained inside the ring were then pumped. The operator shut down the specific tank in the aim of completely draining and degassing it to enable additional investigations (to determine origin of the failure and the type of repairs needed). Moreover, he installed a pump operating at 40 m³/hr in a pre-existing shaft, located approx. 30 m upstream of the pollution in order to contain potential groundwater pollution. This measure was accompanied by tracking the evolution in pumped water quality. Two days later, the industrial owner noticed the first traces of hydrocarbons arriving in the control well. He called upon the services of a hydrogeologist to assist in taking additional measurements. According to the site operator, the leak stemmed from a failure of the stormwater drain system installed on the floating roof located inside the tank; hydrocarbons were discharged via the drain valve at the bottom of the tank in the surrounding retention ring. This valve, in the normal position, had to stay open to allow for rainwater to flow from the floating roof. Furthermore, the operator was responsible for identifying those tanks on the site whose equipment had been identically configured: a single tank fit this condition and was shut down while awaiting verification inspection.