A 1,850-kg leak of styrene-based resins occurred at the level of an outside retention basin within which two 30-m³ reservoirs had been located. A valve at the low point on one of the reservoirs was improperly raised following a transfer step, and the shutoff valve placed at the bottom of the retention basin (used to drain stormwater in the open position) was not operational. The resin poured into the plant’s stormwater network and then into the network serving the entire industrial park. Two critical deficiencies, one human the other material, caused the accident. The evacuation of stormwater into the retention basin was to proceed by pumping and no longer by gravity. The valve was to be eliminated or permanently set in the closed position. The plant operator was served a formal notice to fit all hazardous product reservoirs with retention basins, and a supplemental prefecture order imposed installing a site network isolation system.