Migration of fallout-radionuclides in the soil:effect of non-uniformity of the sorption properties on the activity-depth profiles

摘要

Experimentally observed activity-depth profiles of fallout radionuclides in the soil frequently exhibit, a comparatively fast moving tail in soil layers below the peak concentration (tailing). Monte Carlo calculations on the basis of the convection-dispersion model show that this phenomenon can be explained by assuming that either the hydraulic properties of the soil ( characterised by the diffusion/dispersion coefficient and pore water velocity) or the sorption properties of the soil (characterised by the distribution coefficient K_d) or both, exhibit a horizontal variability according to a log-normal distribution. Modifications of the activity-depth profile due to a K_d value which decreases linearly with depth were examined by using a random walk approach, based also on the convection-dispersion model. In this case, however, a pronounced tailing effect of the activity-depth profile did not result. Interpretation and realistic modelling of an experimentally observed activity-depth profile which exhibits a tailing effect is thus not unambiguously possible without any additional information on the spatial variability of the hydraulic parameters and, independently, also for the sorption properties.