During a surveillance patrol, the operator of a sugar refinery noticed that the level of muddy water in a settling basin was abnormally low compared to the inflow of 24,000 m³/day. He ordered the basin’s filling to be stopped. The next day, he saw that the basin was completely empty. An estimated 280,000 m³ of water had infiltrated into the ground underneath. The operator conducted weekly piezometric monitoring. The analyses revealed:

• a slight decrease in pH;
• a significant increase in COD;
• a sharp increase in iron concentration, which rose to 21 mg/l for a limit set at 0.2 mg/l;
• nickel and copper values above the detection limit.

The basin, which had not been used for several years, had been returned to service the previous month (22/12/2017) to store wastewater from washing of beets. According to the inspection authorities for classified facilities, there were two possible causes:

• a defect when laying the bottom of the basin after dredging resulted in an overly thin or insufficiently compacted layer of silt that led to infiltration over a large area;
• infiltration through natural or man-made underground cavities (military sap: a network of covered trenches just below the basin).

A hydrogeological survey from 1990 had found that water was leaking from the basin. The operator confirmed that it had emptied the first time it was filled.

The inspection authorities for classified facilities asked the operator to continue the weekly piezometric monitoring and to provide it with the corresponding analyses in order to assess the potential impact of surface water seepage on groundwater. They also asked to received specialist firms’ findings on the measures to adopt to ensure sealing of the basins, the follow-up measures to be implemented and the work that may be necessary. The operator must also submit the 2017 annual review report of the dykes in the basins and integrate experience feedback into the procedures for operating hydraulic structures.

They also asked the operator to provide an annual activity report on the actions taken on the basins and associated pipelines.