During a transfer of benzene at a petrochemical plant, the pumps for transferring the benzene from the storage tanks to the rail wagons stalled regularly over the course of the afternoon, requiring the operators to repeatedly purge them before re-priming. At 21:45, a VOC (volatile organic compound) detector triggered an alarm to the central control room post, and another hydrocarbon alarm was tripped at the entrance to the complex’s final effluent treatment plant 15 min later, followed by other alarms triggered by VOC sensors at the same station. The operator then inspected its premises and noted that benzene was overflowing at the outlet of one of the primary separation pits of the effluent collection network. Two other separation pits connected between themselves and with the above-mentioned, located near the storage areas used in the afternoon, also proved to be filled with benzene. At 23:00 the operator of the complex’s final effluent treatment plant channelled the effluents, which had been pre-skimmed in the settling tanks, into the overflow basin. The overflow ceased at 23:30.

At 03:45, the operator ordered the pits to be covered with foam in order to limit the evaporation of benzene, because the VOC detectors of the local monitoring network had measured for one hour and 15 min concentrations of benzene exceeding the threshold for informing the local municipalities (0.325 mg/m3 for two consecutive fifteen-minute periods), with spikes exceeding the threshold of 1 mg/m3 being measured at 02:00. However, the foam layer could not be put in place in the processing plant, whose basins have moving skimmers.

On the next morning, the operator started to empty the contaminated pits and spread sand on the connecting channel in order to absorb the benzene, which was then recovered and stored in drums. The hydrocarbon separation pits and the processing plant’s storm water basin were also emptied, recovering 50 m3 of contaminated water, containing about 10 tonnes of benzene. The pumping operations ended four days later. As soon as the warning threshold for atmospheric benzene was surpassed, faxes were sent to the nearby municipalities. The operator issued a press release on the morning of the second day and in the afternoon informed the municipalities that the accident had ended. The operator’s report showed that the volume of effluents emitted into the Merle stream on 10 April was 9 289 m3 over 10 hours, representing an average benzene concentration of 9.3 mg/l, which surpassed the authorised.

The operator’s investigations revealed that the volume of benzene purged from the pumps would normally have flowed into the collecting pits linked to a covered hydrocarbon-processing installation. However, two days before the accident, this pit’s outflow channel, which had previously been emptied, had been shut off and the volume purged that day exceeded the capacity of the pit, which caused it to overflow into the site’s effluent separation and treatment network. To correct the situation, the operator installed a high level detector in the collector pit and a hydrocarbon sensor in the overflow channel and reviewed its procedures.