SEDIMENTATION AND DISTRIBUTION OF GAMMA-EMITTING RADIONUCLIDES IN BOTTOM SEDIMENTS OF SOUTHERN LAKE PAIJANNE, FINLAND, AFTER THE CHERNOBYL ACCIDENT

The rates of sedimentation of fallout nuclides were determined by means of sediment traps during 28 months after the Chernobyl accident in southern Lake Paijanne, Finland. The spatial distribution of the radionuclide content of the lake bottom was studied on 35 sediment cores in winters 1987/88 and 1988/89. The results were compared with simultaneous observations of the radionuclide content of the water and seston. The role of different transfer mechanisms in the elimination of radionuclides from the water column is discussed. The values recorded for the flux to the lake sediments were on average of the same order of magnitude as the initial deposition on the lake surface (Cs-137 65 kBqm-2). The radionuclide flux to the sediments was rapid during the first months after the accident. After that the elimination of dissolved nuclides from the water mass became significantly slower. The highest flux rate was that of Ce-144 and the lowest that of Rh-106 (Ru-106). Of the radiocesium, about half of the initial inventory was transferred to the sediments after the first observation year. The content of radionuclides showed considerable spatial variation on the lake bottom (Cs-137 7-280 kBqm-2). Direct adsorption of radiocesium explains unexpectedly high concentrations on shallow erosion bottoms. There was a clear tendency for the concentrations to increase with depth, as a result of the focusing effect. Sediment resuspension had a significant impact on the total flux of radionuclides to deepwater sediments. Estimates were made of the resuspended flux of radionuclides.