Long-term radiocesium behaviour in spruce and oak forests

This paper summarizes the data obtained during a period of 3 years in ‘in situ’ (distribution of the Chernobyl radiocesium fallout in a Belgian Ardenne spruce stand) and in controlled conditions (spruces growing in contaminated lysimeters and in nutrient media). Experiments in controlled conditions show that deposited radiocesium (thermogenetated aerosols) is washed out very quickly; the short ecological half-lives are confirmed by the post-Chernobyl observations in the NPP contaminated zone. Moreover, field observations reveal that spruce contamination levels remain more or less stable, with seasonal variations (translocation of cesium-137 during the spring and storage in the wood trunk and in the old needles during the winter). Studies conducted on throughfall water pointed out a high seasonally significant correlation between K and cesium-137. Studies of the cycles of stable K and Cs in needles and throughfall water sampled monthly for a year, confirm that radiocesium movements in trees are closely related to the potassium cycle and to the tree physiological status. Five years after the accident, forests seem to be at a steady state without any significant decontamination, throughfall losses being compensated by root absorption. On the other hand, field experiments conducted on spruce litter show that in Belgium, litter decomposition is a very slow process: K and Cs losses are quite negligible after 400 days. These results emphasize the radiological role of the litter. The relative distribution of deposited radioactivity in the different compartments of the forest ecosystem is given, the soil compartment being the main reservoir of radioactivity (particularly the upper Of and OAh organic soil horizons). Soil-tree transfer factors obtained in lysimeters contaminated by cesium-137 (calculated on the soil solution basis) range from 4 to 40 depending on the soil solution concentration of the different soil layers exploited by the root system. Cesium exchangeable fractions (CaCl 2 and NH 4Ac) represent up to 15% of the total soil activity. In our spruce forest soils (brown acid), a geochemical barrier is present (OAh horizon) where radiocesium presents a quite complete fixation.