Discussions on the driving mechanism of postdepositional migration of 241Am and 137Cs in organomineral sediments (Lake Krugloe, Tomsk region, Russia).

Abstract

A core of bottom sediments from Lake Krugloe located within the 30km influence zone of the Siberian Chemical Plant (located in the city of Seversk "Tomsk-7") was studied to determine scales and rates of migration of artificial radionuclides 137Cs and 241Am in organomineral sediment. It was found that the main portion of 137Cs and 241Am was contained in the sediment interval above 10cm. This means that the horizon of 10cm corresponds to 1950-the time of the start of widespread tests of nuclear weapons. The 210Pbex dates also confirm that this particular horizon was formed in the 1950s. Pore waters in the core above the 10cm horizon are in oxidizing conditions. The depth of the oxidized/reduced boundary was determined from the distribution of redox-sensitive elements Fe and U dissolved in the pore solution. The core distribution of 137Cs is a slightly sloping step, with the lower edge at the 10cm level. The smearing of the lower boundary of this distribution showing the scale of 137Cs migration made it possible to estimate the diffusion mobility of 137Cs. Its diffusion coefficient turned out to be of the order of 10-8cm2s-1. As shown by measurements, the scale of migration of 241Am and the scale of migration of 137Cs have similar values. Theoretical analysis carried out in this work shows that the most probable mechanism of 137Cs and 241Am migration in the pore solution of lake sediment is the migration of colloidal particles to which these radionuclides are strongly bound. Calculation of the diameter of such particles by the Stokes-Einstein formula shows that they have submicron dimensions (d≈ 400nm). No evidence was found that the change in redox conditions in the sediment had an effect on migration of Am ions in pore solution.