An investigation into the upward transport of uranium-series radionuclides in soils and uptake by plants

Abstract

The upward migration of radionuclides in the 238U decay series in soils and their uptake by plants is of interest in various contexts, including the geological disposal of radioactive waste and the remediation of former sites of uranium mining and milling. In order to investigate the likely patterns of behaviour of 238U-series radionuclides being transported upward through the soil column, a detailed soil–plant model originally developed for studying the behaviour of 79Se in soil–plant systems has been adapted to make it applicable to the 238U series. By undertaking a reference case simulation and a series of sensitivity studies, it has been found that a wide variety of behaviour can be exhibited by radionuclides in the 238U decay chain in soils, even when the source term is limited to being a constant flux of either 238U or 226Ra. Hydrological conditions are a primary factor, both in respect of the overall advective flow deeper in the soil, which controls the rate of upward migration, and in the influence of seasonally changing flow directions closer to the soil surface, which can result in the accumulation of radionuclides at specific depths irrespective of changes in sorption between the oxic and anoxic regions of the soil. However, such changes in sorption can also be significant in controlling the degree of accumulation that occurs. This importance of seasonally varying factors in controlling radionuclide transport in soils even in very long-term simulations is a strong argument against the use of annually averaged parameters in long-term assessment models.With a water table that was simulated to fluctuate seasonally from a substantial depth in soil to the surface soil layer, the timing of such variations in relation to the period of plant growth was found to have a major impact on the degree of uptake of radionuclides by plant roots.In long-term safety assessment studies it has sometimes been the practice to model the transport of 226Ra in soil, but to assume that both 210Pb and 210Po can be treated as being present in secular equilibrium with the 226Ra. This simplification is not always appropriate. Where geochemical conditions are such that the 226Ra migrates upward in the soil column faster than 210Pb and 210Po, disequilibrium is not a significant issue, as the 226Ra supports 210Pb and 210Po at concentrations somewhat below those estimated on the basis of assumed secular equilibrium. However, for low, but realistic, values of the distribution coefficients for 210Pb and 210Po and high, but realistic, distribution coefficients for 226Ra, the 210Pb and 210Po can reach the surface soil in high concentrations that are not locally supported by 226Ra. This means that models based on the assumption of secular equilibrium should not be employed without a careful consideration of the hydrological and hydrochemical situation of interest.