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 pie (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 (Cs-137, Th-230, and Pu-239,Pu-2 40) with depth in a mature saltmarsh core were analysed in order to investi gate historical discharge trends and waste-dispersal mechanisms. Core sampl es 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 Cs -137 and Pu-239,Pu-240 With activities as high as 4500 Bq kg(-1) for Cs-137 and 600 Bq kg(-1) for Pu-239,Pu-240. Thorium-230 exhibited complex activit y profiles with depth, specific activities ranging between 200 and 2400 Bq kg(-1). The vertical distributions of Sellafield-derived radionuclides (Cs- 137 and Pu-239,Pu-240) in mature saltmarsh deposits reflect the time-integr ated discharge pattern from Sellafield, implying a transport mechanism that has involved the mixing of sediment labelled with radioactivity from recen t discharges and sediment labelled from historical discharge events before deposition. A mechanism involving the transport of contaminated silt theref ore seems to dominate. The vertical distribution of Springfields-derived 23 0Th in, the same areas reflects the annual gross-alpha discharge pattern fr om BNFL Springfields. In contrast to the Sellafield-derived radionuclides, a fairly rapid transport mechanism from source to sink is implied, with lit tle or no time for mixing with radionuclides discharged years earlier. (C) 1999 Elsevier Science Ltd. All rights reserved.