LONG-TERM RADIOCESIUM BEHAVIOR 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 Belg ian Ardenne spruce stand) and in controlled conditions (spruces growin g in contaminated lysimeters and in nutrient media). Experiments in co ntrolled conditions show that deposited radiocesium (thermogenetated a erosols) is washed out very quickly; the short ecological half-lives a re confirmed by the post-Chernobyl observations in the NPP contaminate d zone. Moreover, field observations reveal that spruce contamination levels remain more or less stable, with seasonal variations (transloca tion of cesium-137 during the spring and storage in the wood trunk and in the old needles during the winter). Studies conducted on throughfa ll 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 radioc esium movements in trees are closely related to the potassium cycle an d to the tree physiological status. Five years after the accident, for ests seem to be at a steady state without any significant decontaminat ion, throughfall losses being compensated by root absorption. On the o ther hand, field experiments conducted on spruce litter show that in B elgium, litter decomposition is a very slow process: K and Cs losses a re quite negligible after 400 days. These results emphasize the radiol ogical role of the litter. The relative distribution of deposited radi oactivity in the different compartments of the forest ecosystem is giv en, the soil compartment being the main reservoir of radioactivity (pa rticularly the upper Of and OAh organic soil horizons). Soil-tree tran sfer factors obtained in lysimeters contaminated by cesium-137 (calcul ated on the soil solution basis) range from 4 to 40 depending on the s oil solution concentration of the different soil layers exploited by t he root system. Cesium exchangeable fractions (CaCl2 and NH4Ac) repres ent up to 15% of the total soil activity. In our spruce forest soils ( brown acid), a geochemical barrier is present (OAh horizon) where radi ocesium presents a quite complete fixation.