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

Abstract

The use of 137Cs measurements for investigating soil erosion and sediment redistribution within the landscape is now well 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 137Cs 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 137Cs 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 137Cs 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 137Cs 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 137Cs fallout on depth distributions and surface activity concentrations of radiocaesium by examining the 137Cs profiles associated with a representative range of soil types subject to different land use. The results show that in uncultivated soil most 137Cs is still contained in the top surface layers, while in cultivated soils the 137Cs is mixed throughout the plough layer. This information is then used to formulate a mathematical description of the behaviour of 137Cs fallout in topsoils that can be used to estimate 137Cs activity concentrations at the surface of uncultivated and cultivated fields from the start of atmospheric fallout to the present day.