The behaviour of radionuclides in the Ribble Estuary, NW England.
The main aim of this project was to consider the geochemical behaviour of a suite of
radionuclides (137Cs, 241 Am, isotopes of Pu, Th and U and 234mpa)in the Ribble
estuarine environment. Controls on the vertical distribution of radionuclides in
sediment deposits were considered and the fluxes of sediment and radionuclides at
sites close to the river channel were investigated.
Vertical activity distributions were studied by taking cores from various intertidal
sediment deposits. Sediment/activity fluxes were studied by installing sediment traps.
All samples were analysed by gamma spectrometry. Selected samples were analysed
for total organic carbon, mineralogy (XRD), major and trace-metals (XRF), grainsize
distribution (laser granulometry) and alpha-emitting radionuclides (alpha
spectrometry). The geochemical phase associations of radionuclides were
investigated using sequential extraction experiments.
Sellafield-derived radionuclides exhibited distinct subsurface maxima (up to 4 785 ±
42 Bq kg" 137Cs, 618 ± 14 Bq kg" 239,240pUand 868 ± Bq kg" 241Am) in mature
saltmarsh sediment deposits. Thorium-230 exhibited more complex depth profiles
(maximum = 2383 ± 36 Bq kg"). Variations in grain-size were low and therefore not
important in controlling the specific activity variation with depth at these sites. The
effects of early diagenesis on the specific activity profiles of 137Cs, 241 Am, 239,240pU
and 230Th were small. The vertical distribution of Sellafield-derived radionuclides
reflected the cumulative discharge pattern from Sellafield implicating a sediment
transport mechanism that has involved the integration of contamination from
previous discharge events. The vertical distribution of 230Th reflected the annual
discharge pattern from BNFL Springfields implicating a transport pathway that
involves little mixing of sediment.
Levels of Springfields-derived 234mpaand 234Th were highly variable in time and
space «0.5 - 495 kBq kg' recorded at Penwortham over the course of the study).
The antecedent river discharge regime appears to be the dominant influence on the
specific activity of these radionuclides at upstream sites.