Behaviour of radioactive substances in the soil under long-term conditions

Within the frame of the project SR 2220 'Safety in the Post-Operation Phase of a Final Disposal of Radioactive Wastes' investigations of the behaviour of radioactive substances in the soil under long-term conditions were carried out on behalf of the Federal Minister for the Environment, Nature Conservation and Nuclear Safety (BMU). The investigations have led to the following results: Following contamination of the soil by irrigation with ground water that has been polluted through a repository, radionuclides migrate downwards because of the activities of burrowing and digging animals; in the case of very mobile radionuclides also the flushing out with rain and irrigation water (bulk flow) into the deeper soil layers plays an important role. Radionuclides absorbed by clay particles are literally freighted in the process of clay displacement into deeply lying soil layers. The radionuclides are brought back out of the lower soil layers and into the upper layers due to the activities of animals. Radionuclides which are readily taken up by plants migrate preferentially upwards through the plants. In the long run, equilibrium occurs between the concentration in the topsoil and in the subsoil. At the same time, a final decrease in soil radionuclide concentration through washout into the ground water and dilution with unpolluted soil takes place due to deposition of atmospheric dust and the weathering of local stone. Equilibrium conditions between inflow and outflow of soil radionuclides are reached over a time-span of up to ca. 30,000 years; the last interim periods between ice ages, which are suitable to overhead irrigation practise, lasted almost long enough to permit the establishment of equilibrium for most of the radionuclides. It is therefore proposed that the concentration in a soil layer of 1 m be calculated for equilibrium conditions. Transfer coefficients as a function of both the Kd-value and transfer factor are given for the migration of selected radionuclides between soil layers and into groundwater. (orig.)