Genèse et fonctionnement des sols en milieu équatorial

La genèse des sols en milieu équatorial présente une forte composante biologique. La structure générale des profils ferrallitiques s'explique par le recyclage biologique des principaux éléments intervenant dans les équilibres minéraux-solutions, et la plupart des minéraux secondaires des sols ferrallitiques sont en rééquilibrage constant avec les conditions du milieu. La genèse des podzols est liée à une exportation précoce des composés organo-métalliques formés dans les horizons de surface, dépendante de la dynamique de l'eau à l'échelle des systèmes. (Résumé d'auteur

Plutonium fluxes from the Rhône River to the Mediterranean Sea

Plutonium isotopes carried by the Rhône River mainly come from the catchment basin weathering affected by atmospheric fall-out and from the liquid effluent discharges from the fuel reprocessing plant located at Marcoule. Nowadays, while 238Pu industrial inputs are still at least ten times higher than terrestrial inputs, 239+240Pu inputs from industrial and terrestrial source terms are of similar importance, i.e. 1 GBq y-1 each. An empirical relation between the 239+240Pu activities in the river water downstream Marcoule and the Rhône flow rate is obtained from data collected over the 1987-1998 period. This relation underlines that the river bed sediments act as a delayed source of plutonium depending on the hydrological conditions of the river. During flood events this delayed source may contribute up to 40 % of the plutonium activity at the lower course of the Rhône River. Furthermore over the 1987-1998 period the accumulated 239+240Pu and 238Pu activities within the whole sedimentary compartment of the river or deposited on the terrestrial surroundings during flood events arc 15 and 4 GBq respectively, thus representing about 10 % of inputs. At present, this stock that can potentially be re-suspended, is an important 239+240Pu source term at the scale of the Rhône River catchment basin

Radioactivity levels in major French rivers: Summary of monitoring chronicles acquired over the past thirty years and current status

Since the beginning of the 1990s, liquid releases of gamma-emitting radionuclides from French nuclear facilities have generally fallen by almost 85%. Almost 65% of gamma-emitting liquid effluents released into freshwater rivers concerned the River Rhône (Southeast France), with around 85% of this originating from the Marcoule spent fuel reprocessing plant. Upstream of French nuclear plants, artificial radionuclides still detected by gamma spectrometry in 2006, include 137Cs, 131I as well as 60Co, 58Co and 54Mn in the case of the Rhine (Switzerland nuclear industries). In the wake of the fallout from the Chernobyl accident, 103Ru, 106Rh-Ru, 110mAg, 141Ce and 129Te were detected in rivers in the east of France. Some of these radionuclides were found in aquatic plants until 1989. In eastern France, 137Cs activity in river sediments and mosses is still today two to three times greater than that observed in similar environments in western France. No 134Cs has been detected upstream of nuclear plants in French rivers since 2001. Downstream of nuclear plants, the gamma emitters still detected regularly in rivers in 2006 are 137Cs, 134Cs, 60Co, 58Co, 110mAg, 54Mn, 131I, together with 241Am downstream of the Marcoule spent fuel reprocessing plant. Alpha and beta emitters such as plutonium isotopes and 90Sr first entered freshwaters at the early 1950s due to the leaching of soils contaminated by atmospheric fallout from nuclear testing. These elements were also introduced, in the case of the Rhône River, via effluent from the Marcoule reprocessing plant. Until the mid 1990s, plutonium isotope levels observed in the lower reaches of the Rhône were 10 to 1000 times higher than those observed in other French freshwaters. Data gathered over a period of almost thirty years of radioecological studies reveal that the only radionuclides detected in fish muscles are 137Cs, 90Sr, plutonium isotopes and 241Am. At the scale of the French territory, there is no significant difference since the mid 1990s between 137Cs activity observed downstream of nuclear facilities and that observed upstream, whether in sediments, mosses and fish. Finally, this study highlights that the natural radioactivity of surface freshwaters are around 25 times greater than artificial radioactivity from gamma emitters. However, non gamma emitters released by nuclear industries, such as 3H, may lead to artificial activity levels 2 to 20 times higher than natural levels. © The Royal Society of Chemistry

Radiological consequences of the extreme flooding on the lower course of the Rhone valley (December 2003, South East France)

In early December 2003 unusual weather conditions led to major flooding of the lower Rhone valley. When it floods, the Rhone carries large masses of solid matter in suspension, which potentially includes associated artificial (anthropogenic) radioactive contaminants from soil drainage in the catchment area and from re-uptake of sedimentary matter that has been contaminated with low-level radioactive liquid effluents from almost twenty nuclear facilities situated along the Rhone valley. A sampling campaign was carried out to investigate the level and spread of both sediment mass and associated radioactive contamination across the flooded areas. An attempt was made to assess the radiological consequences of such an extreme event on contamination of the food chain. Our results show that almost 700,000 tons of sediment was transported onto the floodplain, of which 80% were coarse and fine sands. These materials transferred 6660 MBq of 137Cs, 93 MBq of 239 + 240Pu, 13 MBq of 238Pu and 204 MBq of 60Co over a surface area of 60 km2. More than 90% of deposited sediments are concentrated in a 10 km2 area of agricultural soils, and we estimated that 18% were plowed into the soil. Nevertheless, the level of activity measured in the vegetable crops and milk was not significantly different from the level measured in similar samples from regions that were not affected by the December 2003 floods. © 2005 Elsevier B.V. All rights reserved

Origins and levels of artificial radionuclides within the Rhône river waters (France) for the last forty years: Towards an evaluation of the radioecological sensitivy of river systems

The Rhône watershed extends over almost one hundred thousands square kilometers, i.e. one fifth of the metropolitan French territory. The Rhône is the major River entering the Western Mediterranean Sea as it generates the main source of sediments and freshwater (50%) to the sea, with an average water discharge of 1700 m3 s-1. The River input affects primary production significantly in the north-western Mediterranean area and plays a leading role on the marine ecosystem functioning in the whole Gulf of Lion. Almost twenty nuclear reactors are situated along the Rhône valley, representing Europe’s biggest concentration of nuclear power plants. Down flow all these installations the spent fuel reprocessing plant of Marcoule released over many years most of the liquid radioactive wastes including plutonium isotopes in the Rhône river. Artificial radionuclides also originate from the weathering of the catchment basin labeled by both the global and the Chernobyl accident atmospheric fallout. These radioactive inputs have led to a permanent contamination of the Rhône river waters. Long-term chronological series of artificial radionuclide activities within the waters at the lower reaches of the Rhône river were acquired from 1979 to 2002. These data give evidence for various radioecological responses of the aquatic system over years. Based on these observations several major radioecological sensitivity factors for river waters are identified

Suspended sediment and 137Cs fluxes during the exceptional December 2003 flood in the Rhone River, southeast France

At the beginning of December 2003, one of the biggest floods for at least 150 yr was recorded on the Rhone River. In the lower part of the river, the peak flood reached 11,000 m3 s-1. The geomorphological and radioecological consequences of such an event were investigated downstream all the nuclear installations by using measured and calculated fluxes and the total export of suspended sediment and associated 137Cs. Results pointed out the major role played by large floods in the annual suspended sediment load, as 3.70 × 106 tons of silts, 0.85 × 106 tons of sands, and 0.84 × 106 tons of clays were transferred towards the coastal environment. Nevertheless, these solid loads were found to be lower than those expected as regards the liquid discharge reached during this event and suggested that previous floods that occurred on the river and on its main tributaries during the last decade have probably led to the removal of available sediment from the channels and their banks. Besides, the 137Cs activity measured within the suspended load was estimated at 14.9 ± 0.4 Bq kg-1, which is a level characteristic of the suspended sediments from the Rhone catchment area and demonstrated that nuclear installations located along the Rhone valley did not significantly contribute to any increase in 137Cs activity in the water during the flood. The total 137Cs particulate export amounted to 77 ± 17 GBq and was mainly associated with the silt fraction that contributes to around 70% of the total 137Cs export. © 2007 Elsevier B.V. All rights reserved