Distribution and inventories of some artificial and naturally occurring radionuclides in medium to coarse-grained sediments of the channel

Concentrations of artificial (60Co, 137Cs, 238Pu and 239,240Pu) and naturally occurring radionuclides (40K, 212Pb and 214Pb, daughter nuclides of the 232Th and 238U series) in bottom sediments of the Channel are reported. They are grain-size modulated but usual grain-size normalisation methods fail due to the strong heterogeneity of the sediment admixture and/or the occurrence of rock debris in the area of concern. When plotted versus distance from Cap La Hague, 60Co and Pu isotope concentrations display a maximum in the Central Channel, but 137Cs do not. This is further explained by the contribution of the releases from the La Hague plant relative to other radionuclide inputs, especially Atlantic inflow and direct atmospheric fallout. Apparent transit times from Cap La Hague are derived from Pu isotopic ratios and yield average sediment velocities ranging from some kilometres to some tens of kilometres per year. Sediment inventories of artificial radionuclides show that a significant part of the input of 60Co and Pu isotopes is immobilised in the Channel seabed while most of the 137Cs input has been evacuated by water mass circulation.