Diffusion of radionuclides in soil through fallout studies

Madagascar soil undergoes irreversible damage caused by erosion. To tackle this problem, Malagasy researchers use nuclear technology developed by IAEA member countries. The diffusion coefficient of radionuclide fallout, such as ¹³⁷Cs and ²¹⁰Pb, allows to study the vertical distribution of the radionuclide and to estimate the rate of soil loss in the study area. We studied the distribution of ¹³⁷Cs and ²¹⁰Pb excess from soil samples taken from five references sites. The sample cores were collected, stored and conditioned. For this work, we used an ORTEC gamma spectrometry system. The ¹³⁷Cs activity was calculated from the peak at 661.65 keV and that of the total ²¹⁰Pb from the peak at 46.5 keV. The activity of supported ²¹⁰Pb is calculated from the ²¹⁴Bi peak at 609.5 keV. Lead-210 activity in excess was determined as the difference between the total ²¹⁰Pb activity and supported ²¹⁰Pb activity. To calculate the diffusion coefficient and the migration rate, we used the method developed by D.Walling. This method provides an approximate value of diffusion-migration coefficient without going through the resolution of the differential diffusion equation. We used MS Excel spreadsheet to fit the depth profiles from the surface. Most profiles obtained have a high diffusion coefficient. We can say that the quality of fitting influences the accuracy and precision of the diffusion coefficient. ¹³⁷Cs profiles obtained give migration coefficients of between 0.38 and 0.55 kg.m^-2.a^-1. These values are coherent with the diffusion and migration model. The quality of the smoothing has little influence on the value of the migration coefficient. In conclusion, the results obtained from the diffusion model are generally acceptable, and they helped to make a database on migration-diffusion parameters.