The vertical distribution of radionuclides in a Ribble Estuary saltmarsh: transport and deposition of radionuclides

Routine discharges of low-level liquid radioactive waste by British Nuclear Fuels plc (BNFL) at Sellafield and Springfields have resulted in enhanced levels of radionuclides in sediments of the Ribble Estuary, NW England, UK. Variations in radionuclide concentrations (137Cs, 230Th, and 239240Pu) with depth in a mature saltmarsh core were analysed in order to investigate historical discharge trends and waste-dispersal mechanisms. Core samples from Longton/Hutton Marsh were analysed by gamma-spectrometry and alpha-spectrometry for radionuclides and by laser granulometry to establish grain-size variations with depth. Distinct subsurface maxima were present for 137Cs and 239240Pu with activities as high as 4500 Bq kg-1 for 137Cs and 600 Bq kg-1 for 239240Pu. Thorium-230 exhibited complex activity profiles with depth, specific activities ranging between 200 and 2400 Bq kg-1. The vertical distributions of Sellafield-derived radionuclides (137Cs and 239240Pu) in mature saltmarsh deposits reflect the time-integrated discharge pattern from Sellafield, implying a transport mechanism that has involved the mixing of sediment labelled with radioactivity from recent discharges and sediment labelled from historical discharge events before deposition. A mechanism involving the transport of contaminated silt therefore seems to dominate. The vertical distribution of Springfields-derived 230Th in the same areas reflects the annual gross-alpha discharge pattern from BNFL Springfields. In contrast to the Sellafield-derived radionuclides, a fairly rapid transport mechanism from source to sink is implied, with little or no time for mixing with radionuclides discharged years earlier.