At a fungicide and pesticide factory, an insecticide leak plus an unsealed retention basin led to a 40-kg groundwater discharge of oxamyl. The factory operator detected this groundwater contamination in 2 months later adjacent to the hydraulic barrier downstream of the site. To contain the pollution, the water table pumping rate was increased (known as hydraulic confinement), and the pumped water, cleaned by means of a double filtration process on activated charcoal, was discharged at the surface. Groundwater monitoring both on and off the site was intensified, notably upstream of a drinking water supply well. A hydrological study was undertaken to model progress of the pollution plume. The operator emptied all necessary storage cells until the implicated retention basin could be repaired and its seal verified. The seal had been inspected in September 2012. One month later, a pipe leak had been quickly detected in the vicinity of a pressure sensor and stopped, thus limiting the quantity of product released into the retention basin to just a few litres. The basin had been cleaned using a high-pressure jet that would have deteriorated its resin lining.

The leak occurred on the threaded coupling of the tank bottom valve. Made of carbon steel (even though the operator’s specifications indicated the use of stainless steel for oxamyl-based products), this tank had become highly corroded; 400 litres of product (i.e. 40 kg of oxamyl) spread into the retention basin, which contained water (winter climate conditions). A technician discovered the leak after the fact by smelling product in the retention basin.

The operator inspected all of the site’s pipe fittings to verify compliance with specifications. The cleaning and control procedure was revised once these works had been completed; a seal test was to be systematically conducted subsequent to each high-pressure cleaning operation. The operating protocol for seal tests was also revised in order to improve small leak detection (use of an indicator barrel to measure evaporation losses). Moreover, the operator examined the possibility of covering some outdoor storage zones, provided no explosive atmosphere risk would be generated.