Mobilisation of 137Cs and 90Sr from sediments: potential sources to arctic waters

Tracer studies (134Cs and 85Sr) have been used to investigate the kinetics of sorption and mobilisation of radionuclides from marine sediments. Distribution coefficients (KD ml g−1) for 134Cs in seawater-sediment systems varied with contact time, mixing conditions, sediment type, and salinity, and appeared to be correlated with cation exchange capacity. After 1 year contact time under static conditions, Kd for 134Cs ranged from 740 to 1680 ml g−1. Extraction studies showed that the majority of 134Cs was strongly bound to sediment components, although a small fraction (1–10%) was easily displaced and remained in dynamic equilibrium with soluble species. Kinetic modelling using a simple box model indicated that the uptake of ionic Cs to sediments could be described by a two-component function. The relatively fast component probably reflects diffusion through the sediment-water microlayer followed by surface sorption to sediment (ion exchange). Rate constants for transfer between seawater and the exchangeable fraction were similar for the different sediments, 0.23 ± 0.03 d−1 for uptake and 0.12 ± 0.04 d−1 for removal. The slow component is thought to represent strong fixation within the mineral lattice; rate constants for fixation showed greater variation, ranging from 0.01 to 0.1 d−1. Ionic 85Sr was rather conservative in seawater, showing little transfer to sediments: Kd was 11 ± 2 ml g−1. Furthermore, 85Sr bound to sediments by ion exchange was easily displaced. Sequential extraction of Irish Sea and Stepovogo sediments indicated, however, that 90Sr was rather less mobile than the ionic tracer. This might reflect the presence of different 90Sr species in the discharge (i.e. non-ionic) or a different water-sediment interaction mechanism in the natural environment. For both radionuclides, KD in freshwater was a factor of 100 higher than in seawater, indicating that 137Cs and 90Sr can be mobilised if freshwater sediments are transferred to the marine environment.