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 se diment cores from Esthwaite Water, Cumbria, UK in order to test models to d escribe the simultaneous uptake of radiocaesium from the water column and d iffusion of activity within the sediment. A new, simplified method of simul ating time dependent diffusion of tracers in sediments is;developed and tes ted. Comparison of experimental with field measurements showed that rates o f diffusion of activity which had been introduced by diffusion across the s ediment-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 di stribution coefficient (K-d(t))) values in the experiments were ca. 2000 1 kg(-1), more than one order of magnitude lower than those measured in situ( ca. 7 x 10(4) 1 kg(-1)). Much better agreement, however, was observed betwe en the exchangeable distribution coefficients (K-d(e)), measuring 'mobile' Cs, which were ca. 2000 1 kg(-1) in the experiments and ca. 4900 1 kg(-1) i n the field. Modelling the removal of Chernobyl radiocaesium from Esthwaite Water showed that the majority of the activity was transported to the sedi ments 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 signif icantly different mobility of these two fractions of Cs-137 in sediments. M odel sensitivity analyses showed that removal of activity across the sedime nt-water interface is more strongly influenced by diffusion within the sedi ments than by the thickness of the benthic boundary layer. (C) 2000 Elsevie r Science Ltd. All rights reserved.