Flooding not only induces a risk which is related to direct loss of life for humans, animals and plants. During flooding, toxicants which are normally stored or otherwise unavailable might be released. The spreading of these pollutants during the flooding or, often more important, the persistent presence of toxicants in the soil or sediment after the flooding poses an environmental risk. Within Task 10 of FLOODsite, the effect of toxic stress induced by flooding was studied in a case study (Western Scheldt). For this case study, a relation between toxic concentration levels in water and sediment and acute and chronic toxic stress levels on organisms was needed. The scientific principle on which the EU Water Framework Directive (WFD) is based is that relationships between the biological state and physical and chemical properties of surface waters dictate the ecological state of the water system. Toxic stress is part of the chemical properties of the water system. Within the EU 6th framework programme REBECCA (Contract SSP1-CT-2003-502158) the relation between concentration levels of individual toxicants and their potential total toxic effect was researched. This knowledge can then be used within FLOODsite to calculate ecotoxicological stress during and after flooding. The aim of this research was to develop a model framework in which the ecotoxicological stress levels in the water phase can be predicted (OMEGA). The model includes insights in how to combine the toxic risk of different toxicants. This model was used in a case study for the Western Scheldt for an area with nature reserve areas which is regularly flooded to predict the toxic risk for different groups of organisms. Also, a case study for a simulated dike breach flooding in the Western Scheldt area (nearby the city of Middelburg) was carried out to establish the spreading of pollutants during a flood. The source and 'fingerprint' of the pollutants was diverse and based on land use (farming, industry, etc.), urban area and background concentrations in the water. By 'fingerprint' is meant that each source had a different mixture of toxic components. The results of this case study were maps with areas of high and low pollution after a spreading, which could thereafter be used for toxic risk prediction.
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