Interpreting and predicting in situ distribution coefficients of radiocaesium in aquatic systems

Abstract: This study presents an overview of our post-Chernobyl investigations of the mobility of radiocaesium in sediments, as expressed in terms of its sediment/water distribution coefficients or K_d values, and focuses on the two major factors controlling radiocaesium mobility: the quantity of selective binding sites and the competition for these sites by other cations. In situ K_d values have been measured in a number of very different western European lake sediments and show a variation of over three orders of magnitude. This variability can be quantitatively interpreted in terms of a competitive ion-exchange process between radiocaesium and ammonium ions on highly selective binding sites at the frayed particle edges of illitic clay minerals. These findings allow the radiocaesium K_d in freshwater sediments to be predicted, within acceptable limits, on the basis of the quantity of highly selective exchange sites and the pore-water NH₄ concentration. The implications of these findings for the exchangeability of sediment-bound radiocaesium are discussed.