Distribution and transit times of plutonium-bearing particles throughout the channel

The distribution and dispersive behaviour of Pu isotopes in the sediments of the Channel is presented and elucidated. The Pu concentrations vary by a factor of 70 and 1000, for 239,240Pu and 238Pu, respectively, which is consistent with the relative contributions of source terms (fallout versus industrial) and expected grain-size modulations. The Pu isotopic ratios (238Pu/239,240Pu) increase from the western approaches of the Channel (0.05 or lower: atmospheric fallout) to 0.9 in the Central Channel (industrial input), then slightly decrease in the Eastern Channel. A relation between the Pu isotopic ratios and the passing time at Cherbourg enables us to estimate the transit time of sediments from Cherbourg to any sampling station. Average velocities derived from these data range between some km a-1 to some tens of km a-1. They are ca 2 orders of magnitude lower than residual tide currents responsible for the dispersion of conservative elements (1000 km a-1), thus suggesting that the sediment movements occur only significantly during paroxysmic events. This approach gives access to the dynamics of the dispersion of Pu associated to these sediments and set new constraints to numerical models. (author)