Depth distribution of Chernobyl derived 137Cs: From monitoring to modeling

It is common in the literature to confront the measured 137Cs depth profile with the analytical solution of 1D Advection-Dispersion Equation (ADE) calibrated for the time of sampling which is a pure transport model useful for undisturbed soils. The degree of accuracy is strongly associated to the soil texture. It has been proven that soil with high clay content can retain radiocesium better. Moreover, the effect of biological processes and earthworm population activities in topsoil may result in redistribution of 137Cs concentration, as well. In this research, a 3D numerical transport model of 137Cs Chernobyl fallout including main physicochemical reactions of 137Cs transport in soil was presented. The numerical model was constructed on the basis of recently reported Chernobyl derived 137Cs depth profile on some selected soil of Guilan Province of Iran. In the course of simulation process, the proper assumptions about 137Cs fallout scenario (pulse deposition) and soil parameters (forest soil) have been considered. It has been shown that the predicted depth profile is in good agreement with the experiment especially in the location of activity peak. The latter might be related to the high clay mineral content of the soil which strongly has been slowed down the downward migration. It however almost visually two different patterns appears on the left side of 137Cs peak between theoretical and experimental results. In order to understand it, we have employed a fractional distribution plot to compare these profiles in more detail. It is demonstrated that the total accumulated activities in topsoil behind the peak almost equally unchanged in theory and experiment. It might be associated to the biological, earthworm population activities in topsoil and surface vegetation. Moreover, we would observe that the maximum absolute difference of simulated-experiment data at each soil layer is as much as about 10 percent and disappears in deeper layers.