The distribution of fallout 137Cs and 210Pb in undisturbed and cultivated soils

The affinity of bomb-derived fallout 137Cs and naturally-occurring fallout 210Pb for soil and sediment particles make them valuable sediment tracers, and they have been used in a wide range of environmental investigations. A knowledge of their behaviour and distribution in soils is vital for understanding their movement within the environment and therefore for interpreting the information that they provide as sediment tracers. The study reported in this paper combines both empirical evidence and theoretical reasoning to develop an improved understanding of the distribution of fallout 137Cs and 210Pb in undisturbed and cultivated soils. Results from field experiments suggest that the initial distribution of these radionuclides in topsoils is approximately exponential. The primary factors influencing the post-depositional redistribution of these radionuclides in stable undisturbed soils have been represented as effective diffusion and convection processes, and a one-dimensional transport model has been employed to describe temporal changes in their vertical distribution in the soil profile. Cultivation and soil erosion are the dominant processes controlling their vertical distribution in cultivated soils. The information obtained is essential for exploiting fully the potential for using these fallout radionuclides as tracers in studying soil erosion, sediment delivery and sediment deposition, and associated sediment budgets.