Experimental and in situ study of radiocaesium transfer across the sediment-water interface and mobility in lake sediments

Experiments have been carried out to study the uptake of radiocaesium by sediment cores from Esthwaite Water, Cumbria, UK in order to test models to describe the simultaneous uptake of radiocaesium from the water column and diffusion of activity within the sediment. A new, simplified method of simulating time dependent diffusion of tracers in sediments is developed and tested. Comparison of experimental with field measurements showed that rates of diffusion of activity which had been introduced by diffusion across the sediment–water interface (as in the present experiments) were approximately one order of magnitude greater than those determined from in situ Chernobyl and weapons test activity–depth profiles. Measured total solids–aqueous distribution coefficient (K_d^t) values in the experiments were ca. 2000 l kg⁻¹, more than one order of magnitude lower than those measured in situ (ca. 7×10⁴ l kg⁻¹). Much better agreement, however, was observed between the exchangeable distribution coefficients (K_d^e), measuring ‘mobile’ Cs, which were ca. 2000 l kg⁻¹ in the experiments and ca. 4900 l kg⁻¹ in the field. Modelling the removal of Chernobyl radiocaesium from Esthwaite Water showed that the majority of the activity was transported to the sediments by attachment to and settling of suspended particles. It is concluded that activity deposited on particulates (forming the majority of activity observed in situ) was more strongly bound to sediments than that introduced by direct diffusion across the sediment–water interface, leading to significantly different mobility of these two fractions of ¹³⁷Cs in sediments. Model sensitivity analyses showed that removal of activity across the sediment–water interface is more strongly influenced by diffusion within the sediments than by the thickness of the benthic boundary layer.