Distribution patterns for stable Cs-133 and their implications with respect to the long-term fate of radioactive Cs-134 and Cs-137 in a semi-natural ecosystem

Stable Cs-133 was measured in soil and vegetation samples taken from a coni ferous forest in Bavaria, Germany. This site has been under continuous inve stigation since 1987, mainly with respect to radiocaesium. Soil profiles fo r stable Cs-133, expressed in mg kg-l dry weight, and radioactive Cs-134 an d Cs-137, expressed in Bq kg(-1) dry weight, were established for 1993, 199 5, and 1996, and compared. The profile for Chernobyl Cs-134 shows a clear m aximum in the Of and Oh horizon. The profiles for weapons fallout Cs-137 an d stable Cs-133 are similar for the organic layers including the Ah horizon . Both profiles exhibit a gradual increase from the L to the Ah horizon, in contrast to the profile for Chernobyl Cs-134. For four edible fungal speci es (Clitocybe nebularis (Batsch: Fr.) Kummer, Xerocomus badius (Fr.) Kuhner ex Gilbert, Hydnum repandum L. ex Fr., and Russula cyanoxantha (Schaeffer ex Fries)) and one plant species (leaves of bilberry [Vaccinium myrtillus L .], berries edible), transfer factors for stable Cs-133 were calculated on the basis of its concentration in those soil layers, from which the corresp onding species takes up radiocaesium. Using this definition, transfer facto rs for radioactive Cs-134 agree within a factor of 2 or better with the cor responding transfer factors for stable Cs-133. Since the stable Cs-133 is e xpected to have attained a dynamic equilibrium within the forest ecosystem, the similar bioavailability for stable Cs-133 and radiocaesium indicates t hat the current distribution of stable Cs-133 within the organic soil layer s and understory vegetation represents the equilibrium distribution which i s expected for decay-corrected radioactive Cs-134 and Cs-137 several decade s after deposition. It is concluded that the transfer factors for radiocaes ium will not change with time from several years after deposition onwards, at least for species which take up radiocaesium predominantly from the orga nic soil layers. (C) 1999 Elsevier Science Ltd, All rights reserved.