Radiological assessment of the ribble estuary—I. Distribution of radionuclides in surface sediments

Surface sediment samples were collected from the estuary of the River Ribble, UK, which is contaminated by radionuclides derived from BNFL Sellafield and Springfields. Activity concentrations were measured by gamma and alpha-spectrometry for 137Cs, 238Pu, 239,240Pu, 241Am (Sellafield-derived) and 228Th, 230Th, 232Th, 234Th, 234mPa, 234U and 238U (Springfields-derived). Activity concentrations for Sellafield-derived and Springfields-derived radionuclides, with the exception of 234Th and 234mPa, showed significant relationships with sediment grain size with higher levels in finer-grained sediments at upstream sites in the estuary. For 234Th and 234mPa, the relationship with grain size was obscured by the effect of the short half-life for 234Th. Their highest activity concentrations were also found at upstream sites with fine-grained sediments but were more likely to show variability at any given grain size. Repeated surveys showed that activity concentrations fluctuated over relatively short periods of time due to sediment redistribution throughout the estuary.