Within an oil depot, domestic fuel oil stains were detected on the floor of the retention basin for a tank, first placed in service in 1972, the day after its initial filling after the 10-year inspection. The 32,000 m³ of domestic fuel contained were transferred to another tank and then to another depot altogether. During the evening, heavy rains caused the domestic fuel to rise to the surface. The operator did not immediately drain the retention basin so as to have an easier time subsequently pumping the supernatant hydrocarbons on the stormwater. The day after, domestic fuel was found in two underground manholes and a collector pipe for channelling oily water from the retention basin, even though the drain bleed valve had been closed. The hydrocarbon detector on the oil separator was activated, causing its outfall to close as well.

The Inspection of Classified Installations observed that the leak was stemming from a bleed collection container punctured over a 1.5-mm diameter hole by corrosion. The hydraulic pressure exerted by the product subsequent to tank filling the day before had removed the sediments that served to plug this orifice, thus making it possible for the domestic fuel to seep and imbibe the surface layer of the basin, composed of debris, with a layer of impermeable clay lying beneath. On the next day, the supernatant domestic fuel on the stormwater infiltrated into the earthen dike along the path of the basin drain due to a seal defect around this pipe. The product then reached the manhole where this drain ends with a crack in the corner of its masonry.

The operator proceeded by having surveyors control tank stability and then intensified the piezometer measurement campaign downstream of the basin. Only the surface earth layer of the basin was contaminated, with no pollution actually being detected on the outside. Since the earthworks proved to be technically complicated and the random fill present underneath the tank impossible to treat, the retention floor was to be cleaned by means of bacteriological treatment (with the target of 500 mg HC / kg of dry material). Given that the deep water table was relatively invulnerable, analyses did not reveal very high concentrations of HAP, BTEX and the hydrocarbon index in groundwater. Their reinforced monitoring effort was maintained for 6 months, from the beginning of the pollution cleanup work (i.e. beginning 2008). This effort calls for repairing the retention drainage network and adopting guidelines so that operations personnel are monitoring the oil separator during weekly inspection visits. The Inspectorate commissioned a study on the set of techniques that enable guaranteeing the retention seal while authorising service start-up of the tank by end of 2007. The operator lodged a complaint against the subcontractor assigned to perform the 10-year inspection of the tank for not having noticed the corrosion hole during visual verification, since the scanner could not be employed at this point.