FIXATION OF CS-137 BY SOILS AND SEDIMENTS IN THE ESK ESTUARY, CUMBRIA, UK

Enhanced levels of Cs-137 are observed on the Cumbrian coast (UK) due to the discharge of low level waste by BNFL (British Nuclear Fuels Ltd) at Sellafield. Sorption of this radionuclide onto estuarine sediments is of considerable strategic importance as these areas represent a secondary source of transfer to inland areas. The study reported here was undertaken to determine, both quantitatively and qualitatively, the characteristics of the sorption and fixation of Cs-137 in sediments and soils sampled from within the Esk estuary. This was carried out by means of a detailed set of laboratory analyses on samples collected from the area. These involved a sequential solvent extraction to determine the relative proportions of Cs-137 associated with exchangeable, reducible, oxidiseable and residual fractions of various substrates; a series of batch sorption experiments was also carried out to determine the sorption characteristics of a selection of the substrates sampled. To complement these investigations grain size determinations coupled with X-ray diffraction analyses were employed to determine the mechanical and mineralogical properties of each sample collected. Cs-137 was most strongly associated with the residual phase as defined by the sequential leaching scheme. The more consolidated, clay rich substrates appeared to sorb the greatest quantities of Cs-137 within the residual phase although there was no obvious correlation between the quantity of radiocaesium associated with this phase and the relative proportion of illite and <2 mum material within the substrate. Constants derived from the batch sorption isotherms, however, correlated well with the sequential leaching data and it is suggested that the Langmuir model may be useful as a simple predictive model of the capacity of substrates within the study area to fix Cs-137. Data from the batch sorption study also suggested that caesium binding was either biphasic or monophasic and that, to some extent, this was controlled by the relative proportions of sand and clay-sized material within the samples.