Penetration process model for 137Cs depth distribution in uncultivated soil without erosion

Based on 137Cs diffusion and migration processes in soil, a penetration process model is developed for 137Cs depth distribution in uncultivated soil without erosion. By using the model, the changes of the 137Cs depth distribution with time for the 137Cs reference inventory profiles at Kaixian, Chongqing and Zichang, Shaanxi, are tentatively reconstructed. The 137Cs depth distribution in the Kaixian profile (2004) has an exponential decrease shape with depth. The highest 137Cs concentration occurs in the surface horizon, and the coefficient for 137Cs depth distribution shape is h0 = 0.0997 cm-1, with the related depth moving coefficient D = 0.8006 cm2·a-1. The 137Cs depth distribution in the Zichang profile (2001) has a non-exponential decrease shape with depth. The highest 137Cs concentration occurs in the sub-surface horizon, and the coefficient for 137Cs depth distribution shape of the whole profile is h0=0.0825 cm-1, with the related depth moving coefficient D = 1.24 cm2·a-1. The higher 137Cs concentration in the subsurface horizon than in the surface horizon, which is represented by the non-exponential decrease shape of 137Cs depth distribution with a negative value of D, is probably attributed to the fact that either porosity or moisture penetration of soil decreases with the depth's increase in the surface and sub-surface horizons, thus leading to a sudden decline of the 137Cs penetration rate.