Effect of soil ageing on radionuclide mobility: experimental approach and first results

The behaviour of radionuclides in soil is governed by several mechanisms whose intensity, kinetics and reversibility vary significantly according to the specific reactivity of each element, its speciation and the intrinsic properties of soils. The prediction of the behaviour of radionuclides (mobility, evaluation of their toxicity, speciation) in the terrestrial ecosystem depends on our understanding of the involved dominant processes. The purpose of this work, performed under controlled conditions, is to determine if the soil organic matter ageing could modify Se mobility in soils. It focuses on the variation with time of the Se solid partition between the different soil components (organic matter and minerals) and of the main parameters that controlled its distribution. To perform this study, different soils with the same mineralogical origin but diverse contents of organic matter are initially contaminated with selenium (spiked with 75Se) and incubated at a constant temperature and moisture during three years. The time's effect of incubation is studied by characterizing the evolution of the bio-physico-chemical properties of the whole system and the distribution of selenium between phases. In addition, Se availability to transport is addressed by carrying out batch and soil column experiments. A set of operational analytical tools is developed to relate physical soil parameters (such as the aggregates size and their stability), soil solution chemical parameters (dissolved, particulate and solid carbon, major ions and dissolved organic acids) and microbiological state of the soil, to the distribution of Se between phases (soil, soil solution and atmosphere) and its transport ability. The observed OM transformations are compared with predicted data using RothC (organic matter cycle model), and correlated with the evolution of the Se mobility. The global experimental approach and results from the first six months will be presented. (author)