Characterization of dam sediments for ecotoxicological hazard assessment bound to draining
Sediment silting up in hydroelectric dams Implies regular draining operations in order to prevent disfunction of the dams. These operations often lead to drastic lethal affects on aquatic fauna and flora. A toxicological study has clearly shown two main factors responsible for acute toxicological effects : dissolved oxygen deficit and suspended solids increase. These effects have been quantified by experimental tests on Brown trout fry (Salmo trutta fario) the results of which are summarized. The second step consisted in the prediction of water quality evolution downstream during dam draining operation, regarding both factors suspected, in order to assess ecotoxicological hazard. The proposed methodology based on a sedimentological study made in Grangent dam, located on river Loire and immediately downstream of St. Etienne urban area, and in St. Hilaire dam situated on the river Isère downstream part It consists in sampling cored sediments and in measuring dissolved oxygen kinetic by mixing sediments with water In a reactor. The sample conservation as carried out in jar glasses kept at 4 °C. Results are reproducible as long as residual dissolved oxygen concentration is higher than 3 mg/l. If this condition is respected, oxygen consumption is strictly dependent on suspended sediment concentration. An increase in test water temperature makes the oxygen consumption rise. In accordance with these results, recommendations for dissolved oxygen measures are about 5 g/l suspended sediment concentration, with an experimental temperature reaching the temperature observed during dam draining. The variability of the results for different samples depends on the dam studied. Surface tore samples collected in St. Hilaire dam consume dissolved oxygen faster than the same bottom tore samples. However, in Grangent dam, only one surface core sample collected near the dam construction has clearly shown reducing properties. The difference between two dam sediments is very important : Grangent dam sediment have, on average, dissolved oxygen consumption three more times higher than St. Hilaire in the same duration. Taking into account dissolved oxygen kinetics by mixed sediments, and a simple aeration model using water and sediment deposits from downstream dam, may expert both suspended sediment and oxygen consumption evolution. Hazardous mortality of trout fario fry in downstream dam are quickly graphically visualized. Dam draining operator may so assess, for different suspended sediment land the harmful exposition to fish at any distance from it. This knowledge allows him to plan more strictly floodgate working : pointly dissolved oxygen continuous measurement on downstream dam is sufficient to give a mark related to model hazard assessment. The model reliability has been demonstrated by the examination of data collected during St. Hilaire dam draining operation.