Les berges sources secondaires de radionucléides artificiels à vie longue (Fleuve Rhône)

Titre du congrès :11th International Symposium on the Interactions between Sediments and Water
Ville du congrès :Esperance
Date du congrès :17/02/2008

In recent years, great progress has been made in reducing the direct emission of contaminants into the environment from industrial sources. However the transport and fate of previously-released pollutants is still of great concern. In particular, it is now recognized that certain natural sinks, such as deposits of sediment in streams, rivers and oceans, can entrain the contaminants for long periods of time, only to serve as sources of contamination at some later date. In rivers, one of the primary mechanisms for the remobilization of contaminated deposits is thought to be major flood events. As a result of climate change, floods have become more abundant and more destructive in many regions of the globe including Europe. Then substantial changes in risk of translocation of sediments and associated contaminants onto flooded areas or towards the marine environment is expected. The Rhône Valley was the centre of the French nuclear industry development from the 50s and low level radioactive liquid effluents have been released over years mostly until 1990 when environmental protection directives led to radical decreases. The Rhône River is characterised by a major annual and inter-annual variability of the liquid flow due to the heterogeneity of the rainfall, which affect its large catchment area (97800 km²) and atypically large floods have reappeared over the last 15 years. Sedimentary storages of artificial radionuclides (137Cs, 241Am, 238Pu, 239+240Pu) within an alluvial embankment at the lower course of the River were observed and vertical accretion described based on a morphological approach combining chronological and sedimentary indicators. Our results show that deposits of the middle of the last century still contain activity levels one to two order of magnitude higher than suspended particles today transferred by the River. Some of these embankment types are currently moving back at a rapid space (approximately 3 metres a year since 1997) leading to reactivation of contaminated sedimentary storages.