Plutonium and 137Cs in the western North Pacific: estimation of residence time of plutonium in surface waters

Long-term measurements of plutonium and 137Cs in the western North Pacific have been carried out. Plutonium concentrations in surface waters, for which the meridional distribution is high in the mid-latitude and low in the tropical region, decreased gradually during the period 1979 to 1987. The 137Cs concentration showed no decrease in the same period though surface 137Cs changed temporally and spatially. To determine the scavenging rate of plutonium from the mixed layer, we apply a simple box model to the long-term results of the Pu/137Cs activity ratio in surface waters, in which an atmospheric fallout data set of plutonium and 137Cs on Tokyo is used. Numerical calculations give a rate constant of 0.25 y-1 for the scavenging of plutonium from the mixed layer, which is lower than the typical values for thorium isotopes. The scavenging behaviour of these radionuclides in the mixed layer can be understood with a complexation-scavenging model, which is consistent with the reversible scavenging model. (author)