Freshwuter und Estuurine Rudioecokogy Edited by G. Desmet et 01. 0 1997 Elsevier Science B.V. All rights reserved 173 Long-term kinetics of radiocesium fixation by soils A. Konopleva, A. Bulgakova, J. Hiltonb, R. Comans‘ and v. popova aZnstitute of Experimental Meteorology, SPA “Typhoon”, Lenin au. 82, 249020 Obninsk, Kaluga Region, Russian Federation bZnstitute of Freshwater Ecology, East Stoke, Wareham, Dorset, BH20 6BB, UK ‘Netherlands Energy Research Foundation ECN, Westerduinweg 3, P. 0. Box 1, 1755 ZG Petten, The Netherlands ABSTRACT Results of a one-year laboratory experiment on the kinetics of 137Cs fixation on mineral (MS) and organic (OS1 and OS2) soils are presented. The soils were collected from different areas contaminated after the Chernobyl accident (Ukraine, Russia, UK). Three phases of fixation are identified: rapid sorption on highly selective sites, redistri- bution from non-selective to highly selective sites and solid-state diffusion of 137Cs into mineral particles. A model of cesium distribution processes is proposed and parameter values are evaluated. For modelling short term transport processes the use of an exchangeable 137Cs distribution coefficient is recommended. 1. INTRODUCTION Cesium fixation and remobilization in soils and sediments are the key proc- esses determining redistribution of this radionuclide in the aquatic environ- ment. Although cesium sorption on minerals, soils and sediments has often been reported, the mechanisms and kinetics of underlying processes remain unclear. Also, the adsorption process is usually studied using very short (-1 day) or relatively short (days-weeks) equilibration times, even though the real equilibration time can be much longer (months) 111. In the present paper the results of a long-term investigation of 137Cs fmation in soils are presented, possible mechanisms of underlying processes and mathematical modelling of cesium chemical speciation in soils are discussed.