Données sur les transferts du 137Cs et du 60Co dans un écosystème fluvial : le Rhône

L'étude radioécologique du Rhône permet d'évaluer qualitativement et quantitativement les radionucléides présents dans le fleuve. Les études menées in situ posent des questions concernant les modalités de transfert des radionucléides. Dans ce travail des expériences sont mises au point, afin d'analyser les mécanismes de bioconcentration dans les écosystèmes aquatiques. Pour le césium-137 les échanges entre l'eau, le sédiment et divers organismes aquatiques ont permis d'élaborer un modèle mathématique que l'on peut confronter aux valeurs mesurées sur le terrain. En ce qui concerne le Cobalt-60 les auteurs décrivent des expériences permettant l'évaluation de la contribution relative de l'eau et de la nourriture dans l'accumulation du radionucléide par un poisson.The radioecology of the Rhone Basin has been studied for the last 15 years. This has been an opportunity to make a quantitative and qualitative evaluation of radionuclides as a function of their different sources. Special attention is given to 137Cs (present both in fallout and liquid wastes) and 60Co, which characterize the liquid wastes of pressurised water reactors. In order to assess the transfer and bioconcentration of these two radionuclides in freshwater ecosystems, several experimental studies were undertaken.The 137Cs transfer studies were carried out with a 5-component experimental ecosystem and the data were included in a mathematical model. For 60Co, the experimental study concerns the relative contribution of water and food in the accumulation of the radionuclide by Cyprinus carpio.Water, sediment, plants and fishes were taken from 60 sampling stations set up along the river (figure 1). Water was filtrated, then percolated on resin columns. Sediment, plants and fishes were dried and burnt to ashes in an oven at 500° C. Radioactivity was measured by gamma spectrometry and radiochemistry.137Cs experimental transfers were studied between water, sediment, midge larvae, daphnid and carp. These components were taken in pairs in order to estimate the radionuclide transfer from one to the other. Thus ten experiments were carried out (figure 2). In order to study the relative importance of food and water as 60Co sources for the carp, an experiment was carried out simultaneously on three homogeneous groups of ten juvenile fishes. The individuals of the first group were maintained in separate aquaria and offered 45 daily rations of labelled food over a 63-day period. Bach carp of the other two groups was placed in a compartment of a large tank with contaminated water. One group was fed with radioactive food, the other with non-radioactive food (table 1).Natural radioactivity remained steady all along the river. It ranged around 1 Bq.l-1 in water, 2250 Bq.kg-1 DW in sediment, 1700 Bq.kg-1 DW in aquatic plants, 110 Bq.kg-1 WW in fish. The fallout impact was characterized by 137Cs presence. PWR liquid wastes contained mainly, 58Co, 60Co, 134 Cs, 137Cs. The Chernobyl fallout gave an increase of Cs and the presence of 103Ru and 106RU+Rh specially during May and June 1986 which later decreased (tables 2, 3 and 4).137Cs transfer between water and sediment was very fast and important. Less than 2 % of the radionuclide was released from sediment into a non-radioactive water. During the transfer from water to chironomids the larvae radioactivity increased steadily (figure 3). Conversely, the transfer from the sediment to larvae did not seem to depend on the contact time. The transfer from water to carp was regular without any steady state during the 63 days of the experiment (figure 3). Then the fish concentration factor was less than 5. For 42 days, the transfer factor from sediment to carp was 3.6.10-3. The retention factor from food to carp was 0.03 when fishes were fed with daphnids and 0.13 with chironomids. An experiment showed that the various ways of 137Cs transfer could have an added impact. Thus the carp radioactivity was the sum of the separate transfers. Water was responsible for 4 % of the fish 137 Cs concentration, sediment for 45 % and chironomids for 51 %.It is possible to include the different kinetic equations in a mathematical model. If the radioactivity of one of the components is known, the nuclide concentration can be computed in others, as a relation of the contact time, the quantity and quality of ingested food, etc... This model gives a concentration factor for juvenile carp of 1000 in 180 days and 500 for 3-year old fish. Considering the field conditions (e.g. seasonal nutritive cycles) the computed concentration factor in fish was between 200 and 350. For a 1 mBq.l-1 137Cs concentration in water, the model gave a concentration of 0.2 to 0.35 Bq.kg-1 WW in carp, which was the 137Cs radioactivity level measured in the Rhone fish before the Chernobyl accident.During the 60Co accumulation phase, the mean weight of the fish in the three groups increased exponentially and the resultant relative weight gain was 52-59 % after 63 days (table 5).The 60Co accumulation kinetics showed that the steady state should be reached after 165 days for fish exposed to 60Co in food, 92 days for fish exposed to radiocobalt in water and 120 days for fish exposed to 60Co in both sources (figure 4). According to the 60Co concentration in the fish in the three treated groups, the accumulation from water accounted for 75 % of the total radioactivity and the accumulation of the radionuclide from both water and food was in addition.Depuration of 60Co from carp was a relatively intensive process reflecting a high Co turnover. Biological half-lives for loss from the long-lived compartment ranged from 35d in fish previously contaminated by food, to 87d in fish previously contaminated by food, to 87d in fish previously contaminated by water (figure 5).137Cs and 60Co are the most concentrated radionuclides in liquid wastes of the pressurised water reactors, and they are often measured in the aquatic ecosystem components. Though it accounts for the highest fraction of total radioactivity in the liquid wastes, 60Co cobalt is not the most concentrated radionuclide in fish. Experimental studies show that it is primarily transferred from the water so it is logical that its concentration in fish remains at a low level. Conversely the 137Cs has a low concentration in water but as it is transferred simultaneously from water, sediment and food, its concentration in fish is still important. Moreover its 30 years half-life means that the cesium contamination of fish is a long and important process, all the more so as the source terms can add their own effects during time and space