The behaviour of bomb-derived caesium-137 fallout in catchment soils

The use of Cs-137 measurements for investigating soil erosion and sediment redistribution within the landscape is now ell established. However, for this potential to be fully realised, and to interpret the results obtained from such investigations in a meaningful manner, it is important to understand how bomb-derived Cs-137 fallout interacted with particles at the soil surface. Furthermore, as it is the surface soil which is mobilised by most wind and water erosion processes, it is important to determine how the Cs-137 content at the surface has changed over time. This paper reports a series of field and laboratory experiments which were carried out to provide empirical information on the fate of Cs-137 applied to different soil types to simulate fallout during individual storm events and over a 10 month period. the results suggest that most of the Cs-137 was contained in the top few millimetres of the soil profile, with activity concentrations declining approximately exponentially with depth. A field sampling programme was carried out to examine the net longer-term effect of internal soil processes and of continuous Cs-137 fallout on depth distributions and surface activity concentrations of radiocaesium by examining the Cs-137 profiles associated with a representative range of soil types subject to different land use. The results show that in uncultivated soil must Cs-137 is still contained in the top surface layers, while in cultivated soils the Cs-137 is mixed throughout the plough layer. This information is then used to formulate a mathematical description of the behaviour of Cs-137 fallout in topsoils that can be used to estimate Cs-137 activity concentrations at the surface of uncultivated and cultivated fields from the start of atmospheric fallout to the present day. Copyright (C) 1996 Elsevier Science Limited