Sunday, 9 July 2006

A Simplified Diffusion Process Model for 137Cs Redistribution in Undisturbed Soil.

X. Zhang, He Xiubing, Fu Jiexiong, Qi Yongqing, and Long Yi. Institute of Mountain Hazards and Environment, China

137Cs is an artificial radionuclide with a half-life of 30.12 years which released into the environment as a result of atmospheric testing of thermo-nuclear weapons primarily during the period of 1950s-1970s with the maximum rate of 137Cs fallout from atmosphere in 1963. 137Cs fallout is strongly and rapidly adsorbed by fine particles in the surface horizons of the soil, when it falls down on the ground mostly with precipitation. Its subsequent redistribution is associated with movements of the soil or sediment particles, The 137Cs nuclide tracing technique has been used for assessment of soil losses for both undisturbed and cultivated soils. For undisturbed soils, a simple profile-shape model was developed in 1990 to describe the 137Cs depth distribution in profile, where the maximum 137Cs occurs in the surface horizon and it exponentially decrease with depth. The model implied that the total 137Cs fallout amount deposited on the earth surface in 1963 and the 137Cs profile shape has not changed with time. The model has been widely used for assessment of soil losses on undisturbed land. However, temporal variations of 137Cs depth distribution in undisturbed soils after its deposition on the ground due to diffusion and migration processes are not considered in the previous simple profile-shape model. Thus, the soil losses are overestimated by the model. A simplified diffusion process model is developed to describe the post redistribution of 137Cs fallout in an undisturbed soil profile under the same assumption that the total 137Cs fallout amount deposited on the earth surface in 1963 as the previous simple profile-distribution model in this paper. The change of 137Cs depth distribution due to diffusion processes in an undisturbed soil profile with time after its deposition on the ground can be generally described as following: 137Cs concentration in the surface horizons decreases with time while 137Cs distribution depth increases with time. However, the 137Cs concentration still always decreases exponentially with depth and the maximum 137Cs concentration occurs in the surface horizon of the profile. Although the 137Cs inventory at an eroded site is less than the reference value, the soil profile has same 137Cs depth distribution shape as the profile at the reference site if the two profiles have same uniform soil textures. The soil losses related to different 137Cs loss proportions of the reference inventory at the Kaixian site of the Three Gorge Region, China, are estimated by the two models. As to 20% -60% of 137Cs loss proportions of the reference inventory, the annual soil loss depths estimated by the new simplified diffusion process model are only 21.9%~28.4% of the values estimated by the simple profile shape model.

Keywords: 137Cs redistribution; undisturbed soil; simplified diffusion process model.

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