Groundwater polluted by chloroform was detected when the piezometers of a chemical plant were analysed in May 2017. Following various issues in sending the report to the lab and in the reports contents, the operator did not find out the results until three months later. He requested new samples on 22 August. These samples confirmed the presence of chloroform in the groundwater. The inspection authorities went to the site on 31 August. An emergency-measures decree imposing the following steps was subsequently issued on 4 September:

• determination of the downstream uses;
• establishment of heightened groundwater monitoring;
• investigations into the pollution source and the characterisation of its extent;
• definition of remediation work.

A hydrogeological survey conducted in September by an engineering services firm identified an incident that had occurred one year earlier as the source of the incident. On 12 August 2016, a 1 m³ container leaked 500 l of wastewater and chloroform onto the concrete pad of the storage area. Following a leak test during the survey, the operator realised that the low point of the concrete bund was defective and not watertight. After the bund was sealed, the operator had to test the soil around it to confirm this theory. An engineering services firm was contacted to conduct further investigations in order to draw up a management plan and determine the necessary remediation work.

The operator discovered that site had been polluted by chloroform in 1999. The site had subsequently been remediated and the groundwater monitored. Because chloroform had no longer been detected since 2009, this monitoring had been progressively cut back.