Radiological assessment of the Ribble Estuary—III. Redistribution of radionuclides

The Ribble Estuary in the UK is contaminated by radionuclides derived from BNFL Sellafield and Springfield. Following the temporary shutdown of the Springfields plant, beta dose and gamma air kerma rates were measured at a range of sites in the estuary. Sediment surface samples were analysed for 137Cs, 234mPa, 234Th and 241Am activity concentrations by gamma spectrometry, 230Th and Pu isotopes by alpha spectrometry and sediment grain size (percentage < 63 μm) by wet sieving. Temporal variations in Springfieldsderived radionuclides are explained by variations in discharges by radionuclides and the distribution of fine grained sediments in the estuary which is controlled mainly by the spring-neap tidal cycle and river runoff. Radionuclide dynamics, modelling variations in the 234Th/137Cs activity concentration ratio and calculating environmental beta dose rate and 234Th half-lives allow the estuary to be split into three areas: (a) upper estuary where fine grained sediments tend to be deposited during periods of low and average river flow; (b) middle reaches of the estuary which encompass most of the main channel sites where fine grained sediments are probably deposited and removed on most tides; (c) outer estuary including large areas of salt marshes which tend to accumulate fine grained sediment. This is confirmed by similar results derived from the 234Th/239,240Pu ratio.