Evaluation of semi-mechanistic models to predict soil to grass transfer factor of 137Cs based on long term observations in French pastures

International audienceThe aim of this study was to evaluate and improve the accuracy of the semi-mechanistic models used in regulatoryexposure assessment tools, to describe the transfer factors of137Cs from pasture soils to grass observed indifferent grazing areas of France between 2004 and 2017. This involved a preliminary parameterization step ofthe dynamic factor describing the ageing of radiocesium in the root zone using a Bayesian approach. A data setwith mid-term (10 years about) and long term (more than 20 years) field and literature data from 4 Europeancountries was used. A double kinetics of the bioavailability decay was evidenced with two half-life periods equalto 0.46 ±0.11 yr and 9.57 ±1.12 yr for the fast and slow declining rates respectively. We, then, tested a fewexisting alternative models proposed in literature. The comparison with field data showed that these modelsalways underestimated the observations by one to two orders of magnitude, suggesting that the solid-liquidpartition coefficient (Kd) was overestimated by models. The results suggest that semi mechanistic modelsmight fail in the long-term prediction of the radionuclide transfer from soil-to-plant in the food chain. Theyhighlight the need to calculate Kd using easily exchangeable137Cs (i.e. labile fraction) rather than total soil137Cs