The use of a numerical mass-balance model to estimate rates of soil redistribution on uncultivated land from 137Cs measurements

A numerical mass-balance model is developed which can be used to estimate rates of soil redistribution on uncultivated land from measurements of bombderived 137Cs inventories. The model uses a budgeting approach, which takes account of temporal variations in atmospheric fallout of 137Cs, radioactive decay, and net gains or losses of 137Cs due to erosion and deposition processes, combined with parameters which describe internal 137Cs redistribution processes, to estimate the 137Cs content of topsoil and the 137Cs inventory at specific points, from the start of 137Cs fallout in the 1950s to the present day. The model is also able to account for potential differences in particle size composition and organic matter content between mobilised soil particles and the original soil, and the effect that these may have on 137Cs concentrations and inventories. By running the model for a range of soil erosion and deposition rates, a calibration relationship can be constructed which relates the 137Cs inventory at a sampling point to the average net soil loss or gain at that location. In addition to the magnitude and temporal distribution of the 137Cs atmospheric fallout flux, the soil redistribution rates estimated by the model are sensitive to parameters which describe the relative texture and organic matter content of the eroded or deposited material, and the ability of the soil to retain 137Cs in the upper part of the soil profile.