Modelling a severe transient anoxia of continental freshwaters due to a Scheldt accidental release (sugar industry)

Most anthropogenic pollution can be controlled, such as domestic and industrial releases, loads from agriculture, etc. But some of them, which are associated to illegal discharges, industrial accidents, etc. are more difficult to forecast. The Tereos sugar industry accident occurred during the night of 9th April, 2020. Eighty-eight thousand cubic meters of effluents loaded with organic matter discharged in the Scheldt river (a 350 km-long transnational river which flows through northern France and western Belgium). The accident had dramatic consequences on the receiving watercourse, over 120 km downstream. Fish mortalities have been observed and severe deoxygenation, reaching zero concentration in dissolved oxygen, have impacted river chemical quality. The objective was to understand and describe the dynamics of the chemical pollution and its propagation along the transboundary hydrographic network of the Scheldt. A method based on the processes of organic matter degradation in river ecosystem was enhanced. It is demonstrated that the accident is doubtless the cause of the water column deoxygenation. This paper shows how the water quality modelling can help to understand and therefore to prevent the consequences of an accidental pollution on a watershed.