79Se is a high-life fission product presents in high-activity nuclear wastes. Selenium is often taken into account for health and environmental risk assessments as it is usually considered as a mobile and a toxic element. The terrestrial ecosystems are thought to be a potential reservoir of selenium. In this study, we specially studied how the associations between selenium and soil organic matter and/or microorganisms modify soils selenium mobility and partition (solid, liquid and gazeous). A particular attention was paid to the temporal evolution of the retention parameters,which remains one of the weak points of modeling especially for long term simulations (several tens-hundreds of thousand years).
Methodology developed consists on a regular characterization of 3 soils samples pre-contaminated at a very weak selenium concentration (2.2x10-9gSe.g-1soil) and incubated during 1.5 year in controlled conditions. The evolution of the organic status of the soil samples was established and well modeled (Roth C). Alternatively, the selenium-Organic Matter associations did not present any significant modification with time affecting selenium mobility. The detailed study of selenium mobility showed that selenium sorption processes was poorly reversible (sorption Kd : 20-50 L.kg-1) ; desorption Kd :100-500 L.kg-1), whatever the soil organic status. In addition, numerous transitory phenomenons were able to disturb the "basic" behavior of selenium by increasing the amount of soluble selenium. We specially established that : (i) the use of phosphate fertilizers constituted the most important way of selenium remobilization leading to the release of 30-40% of sorbed selenium in some extreme cases; (ii) the weak selenium volatization (~ 0.2% for 1.5 year of incubation) may constitute a signification biotic way of 79Se dispersion as soon as very long time scales are considered; (iii) the colloidal transport due to the soil microorganisms could also constituted a source of selenium mobilization however this extend of this process has still to be quantified.