A new sediment sampling campaign was recently undertaken along the banks of the Rhone, downstream, with a view to reconstructing the history of how these massive sediment depositions formed. The ultimate goal of this is to assess the consequences of highly intense flood events on the deposition of radionuclides or other contaminants on the river banks. This study is part of Project EXTREME, the aim of which is to analyse the effect of extreme climate events on the deposition of radionuclides in various biosphere compartments.
Carried out by the LERCM of the IRSN in conjunction with the Geomorphology team from the CEREGE (Centre for research and teaching in Environmental Geosciences, University of Aix Marseille III), these samples were taken for that part of Project EXTREME focusing on rivers. The subject of study came about as the result of the discovery, made during research for a thesis, of significant variations in activity levels in sediment on the banks of the Rhone. Samples were taken from a stretch of eroding bank measuring over 6 metres in height and which presented a stratigraphic record that made it particularly suitable for the subsequent description of mass transfers and related contaminant transfers during flooding. The stratigraphic reports, analysis of textures, measurements of natural and artificial gamma and/or alpha emitter activity levels should make it possible, when combined with records of the output and discharge of liquids from nuclear power plants located on the Rhone, and to reconstruct the history of these deposits. In particular, the results will be used to highlight the importance of flood events in the transfer of matter and to quantify stocks of radioactive substances already present or likely to be deposited in such environments.
So far, the samples have been processed (drying, screening, grinding, ashing, counting geometry conditioning) and will be analysed using gamma and alpha spectrometers at another IRSN laboratory, the LMRE (Environmental Radioactivity Measurements Laboratory). At the same time, granulometric analyses are being performed. Reporting on the results is scheduled for the end of summer 2006.
Within the framework of Project EXTREME, other climate events have been studied, in particular the exceptional flooding in December 2003 in Arles, and the radioecological impact of dust fallout from the Sahara.