A strong acrid smell emanating from the sewer of a petrochemical platform generated many complaints, especially from local schools. The facility activated its internal emergency plan and fire-fighters dispatched to the site surveyed the vicinity. The emergency plan was lifted 2 hours 40 min following activation.

The overflow of water/hydrocarbon settling tanks into the platform’s final treatment plant, and from the stormwater basin into the natural environment, subsequent to heavy rainfall (episode with a 4-year return period), caused this incident.

Hydrocarbons evaporation caused Benzene emissions outside the site. The local air quality monitoring network revealed values above 100 µg/m³ for 3½ hours (with a peak at 506 µg/m³). The direct spilling of untreated effluent into the environment during a 2½-hour period produced an excessive discharge of COD, hydrocarbons and benzene into the Merle River, despite the installation of floating booms and hydrocarbon skimming/pumping systems.

The final treatment plant involved in the incident, which had been receiving stormwater effluent from all platform industries as well as from a nearby light industrial zone, comprised a stormwater basin designed to accommodate effluent from 26 ha of land area. The volume of effluent discharged during this overflow episode was evaluated at 6,000 m³, while the benzene quantity was estimated at less than 5 tonnes. A comparable event 3 months earlier had led to an overflow of 14,000 m³ of effluent during 3 hours. To reduce the probability of a repeat occurrence of this type of accident, the surface areas of platform zones equipped with sprinklers were quantified, in addition to analysing the compatibility between existing treatment resources and previously identified needs.