Migration of radiocesium in two forest soils as obtained from field and column investigations.

Depth profiles of radiocesium were measured in a podsolic parabrown earth of a spruce stand and in a podsol of a pine stand up to 3 years after the Chernobyl accident. At the same sites undisturbed soil columns of 20 cm diameter and 40 cm length were taken, transferred to the laboratory and irrigated intermittently with synthetic rainwater containing initially a known amount of radiocesium. The resulting migration of radiocesium in the columns under unsaturated conditions was determined as a function of time up to 3 years with a scanner technique. The depth profiles of radiocesium observed in the field and in the columns were evaluated with a compartment model to obtain the residence half-times of this radionuclide in the various soil horizons. The field observations yielded a residence half-time in the organic layer of both soils of approximately 4-6 years for Chernobyl-derived cesium, and of 10-15 years for cesium from the global fallout of weapons testing. In the mineral soil (0-5 cm), under spruce the residence time of Chernobyl-derived cesium was 15 years, that of cesium from the global fallout (present in the soil since approximately 30 years) was 50 years. Under pine, the residence time in the mineral soil was 4 years for Chernobyl-derived cesium and 11 years for global fallout cesium. Obviously, in each layer of both soils cesium becomes less available for migration with time. The residence times of radiocesium evaluated from the column experiments were in good agreement with those obtained from the field observations. Due to the comparatively short duration of the column experiments, however, the long-term increase of the residence time of radiocesium in the soil was not yet unambiguously observable.