When clearing the brush from a trench, public works department employees contacted the local fire station after discovering the presence of drained motor oil 40 cm thick and extending 80 m in length. The garage of a road transport company, where oil changes and maintenance on lorries were being performed, had caused this discharge. A double-shell tank was being used to store oils drained from lorry transmissions, axles and engines: it was full with a content of 200 litres of water. A test conducted on the double shell confirmed its seal. The inspection hatches were saturated with oil. The drained oil tank overflow had occurred via the air vent outside the manhole subsequent to poor handling of the drained tank contents (erroneous calculation of the volume of used oils following a temporary activity surge). The hydrocarbon had spilled up through the manhole and flowed along the existing drains located at the periphery of the 2 trenches. This hypothesis was confirmed by pouring a tracer dye into the manhole. This substance was indeed found in the sealed well of the lift station, where the peripheral drains emptied. The pump was then lifting the liquid effluent and disposing it into the hydrocarbon separator. This installation, which featured a shutter valve but no alarm, wound up becoming saturated. The drained oils spilled into the ditch channelled via the overflow system. The operator estimated that almost 1,900 litres of oil were spilled in this manner. On 17th March, 5 tonnes were pumped into the ditch, with ditch cleaning being initiated by disposing of the fouled soil. The next day, the separator and tank were both drained. The appointed consultant asserted the absence of any local pollution of groundwater and surrounding parcels, yet indicated the possibility of precipitation leading to drain immersion and ground leaching that could contain oil residues. The operator agreed to undertake semi-annual monitoring of the site’s boreholes. Measures were also introduced, including: oil storage in barrels, raising of tank vents, adoption of a daily tank level recording procedure, servo-control of the level detector to a sound alarm, and manual control of the lift pump by the workshop manager.