For a 5-week period, a flow of unidentified fatty liquid was spilling into the RHONE River, along a course following a trench cut. A more massive inflow of the liquid then triggered setting up floating dams and having firefighters employ absorbent products. Municipal public works teams dug a 3-m deep trench and discovered a layer of oil stemming from one of the tanks from a nearby oil storage facility. This methyl ester of vegetable oil (also called EMHV, or diester) turns out to be an environmentally-nontoxic additive for diesel and was being stored in retention basins within a former reservoir made of riveted and heat-insulated sheet metal (capacity: 1,000 m³). This reservoir had been installed on a support block (composed of a concrete ring enclosing compacted fill material) and had once contained heavy fuels. The reservoir was not impermeable to EMHV (due to the liquid’s low viscosity and ability to solubilise heavy hydrocarbons), giving rise to an 86-m³ leak and ensuing infiltration into the ground through the support block. Emergency measures had to be taken. The seal on all tanks was to be verified and for several of them, especially the oldest ones, replacement was imposed.