A comparative study of radiocesium mobility measurements in soils and sediments from the catchment of a small upland oligotrophic lake (Devoke Water, U.K.)

The mobility of radiocesium was studied in a number of soil and sediment cores collected from the Devoke Water catchment. Samples were subjected to a range of techniques, which are currently used in countries from western Europe and the former Soviet Union and which were developed on the premise that radionuclides are present in different forms which influence their mobility in the environment. The techniques were found to provide complimentary information and are very useful in the prediction of the mobility of post-Chernobyl radionuclides in the aquatic environment. Radiocesium was found to be much more mobile in the organic soils of the Devoke Water catchment than in the mineral soils. A simple model, based on the similarities in the ion-exchange behavior of radiocesium and potassium, was successfully used to predict the concentration of radiocesium in runoff water and in the lake. The results suggest radiocesium in the peat soils to be mainly bound to organic exchange sites. In-situ K(D) values for radiocesium in Devoke Water sediments, which varied over one order of magnitude with depth in the sediment, were accurately predicted using an ion-exchange model based on the premise that radiocesium was sorbed exclusively on the frayed edges of clay minerals. It is shown that 'exchangeable'-K(D)'s more accurately reflect the fraction of particulate radiocesium that is available for exchange with the solution and most closely correspond to the assumptions of the ion-exchange theory. Therefore, this parameter is recommended for use in radionuclide transport models.