Radiocesium and 210Pb in sediments, soils and surface waters of a high alpine catchment:: A mass balance approach relevant to radionuclide migration and storage

The behaviour of particle-reactive metals in natural environments is linked to adsorption, particle erosion, transport and sedimentation. In the field these processes are best studied and quantified on small, local scales, where the number of variables remains manageable. The Muttsee watershed, situated above 2500 m in the north-eastern Swiss Alps, represents such a 'simple' natural test site, a self-contained bedrock-soil-water-sediment system that allows the analysis of radioactive metals in order to study their interaction with water, bedrock, soil and sediment and to quantify radionuclide and particle transfer. A small lake (Muttsee) forms a natural trap for eroded material and similar to 80% of the radionuclides Pb-210 and Cs-137,Cs-134 deposited in the catchment area. Radionuclide-dated sediment cores from the lake allow the determination of sedimentation rates, which increased from 0.30 cm year(-1) during the period 1963-1986 to 0.55 cm year(-1) during 1986-1992. The related increase in erosion rate (10 to 19 cm kyrs(-1)) may be related to a landslide that occurred during the early 1960s, when sections of plant stabilized soil were covered by rock debris. The data, which can be evaluated using mass balance calculations, allow assessment of the role of a soil cover in controlling radionuclide residence times in the drainage basin, and also allow estimation of changes in the extent of soil cover during the last 40 years. This changing extent of soil cover makes it impossible to use soil profiles for an assessment of atmospheric radionuclide fluxes. Radionuclide measurements in soils serve as a pedological tool, allowing in cases recognition of soil creep and transient cover.