Studies of 137Cs absorption-desorption processes in sediments, using 7-ray spectroscopy T. J. Mertzimekis, K. G. Ioannides, C. A. Papachristodoulou, K. C. Stamoulis and D. T. Karamanis Nuclear Physics Laboratory, The University of Ioannina, GR-451 10 Ioannina, Greece Abstract The description of 137 Cs behaviour during water to sediment transport generally involves the characterization of the sorption process in a particular hydro-geological system by certain prescribed kinetics parameters, hi the present work the kinetics of radiocesium sorption and desorption and the corresponding radiocesium distribution profiles in lake sediment cores were investigated. Radiocesium reaches rapidly (< than 5 d) at a depth which does not exceed 4 cm. The desorption of radiocesium was found to depend upon cation concentrations. Empirical laws were derived both for cesium adsorption and desorption. 1 Introduction To study the migration of a radionuclide in a aquatic environment, it is nec- essary to quantify the behaviour of a particular solute species relative to the carrier lake or river due to physico-chemical sorption processes. Since in most of the cases it is impractical to study the sorption processes in-situ, we resort to the results of laboratory simulation experiments and then extrapolate to predict the behavior of the real system. In the aquatic environment, radioisotopes are carried by water. The migration of radionuclide is influenced by various processes, predominantly by reversible and irreversible fixation to sediments. Thus sediments are an important com- ponent of the aquatic environment, functioning either as a sink or as tempo- rary repository for radionuclides. Of the most important nuclides for radiation protection is radiocesium ( 137 Cs). Once in water, 137 Cs migrates to bottom sed- iments by direct sorption or by sorption on suspended solids that later deposit