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

Experimentally observed activity-depth profiles of fallout radionuclides in the soil frequently exhibit a comparatively fast moving tail in soil layer s below the peak concentration (tailing). Monte Carlo calculations on the b asis of the convection-dispersion model show that this phenomenon can be ex plained by assuming that either the hydraulic properties of the soil (chara cterised by the diffusion/dispersion coefficient and pore water velocity) o r the sorption properties of the soil (characterised by the distribution co efficient K-d), or both, exhibit a horizontal variability according to a lo g-normal distribution. Modifications of the activity-depth profile due to a K-d value which decreases linearly with depth were examined by using a ran dom walk approach, based also on the convection-dispersion model. In this c ase, however, a pronounced tailing effect of the activity-depth profile did not result. Interpretation and realistic modelling of an experimentally ob served activity-depth profile which exhibits a tailing effect is thus not u nambiguously possible without any additional information on the spatial var iability of the hydraulic parameters and, independently, also for the sorpt ion properties.