Comparison of two numerical modelling approaches to a field experiment of unsaturated radon transport in a covered uranium mill tailings soil (Lavaugrasse, France) Zakaria Sa ^ adi * ,J  er ^ ome Guillevic Institut de Radioprotection et de Sûret e Nucl eaire (IRSN), PRP-DGE/SEDRAN/BRN, 31 Avenue de la Division Leclerc, Fontenay-aux-Roses, 92262, France article info Article history: Received 25 November 2014 Received in revised form 6 March 2015 Accepted 13 March 2015 Available online xxx Keywords: Uranium mill tailings Radon Diffusion Exhalation Modelling Uncertainty abstract Uncertainties on the mathematical modelling of radon ( 222 Rn) transport in an unsaturated covered uranium mill tailings (UMT) soil at field scale can have a great impact on the estimation of the average measured radon exhalation rate to the atmosphere at the landfill cover. These uncertainties are usually attributed to the numerical errors from numerical schemes dealing with soil layering, and to inadequate modelling of physical processes at the soil/plant/atmosphere interface and of the soil hydraulic and transport properties, as well as their parameterization. In this work, we demonstrate how to quantify these uncertainties by comparing simulation results from two different numerical models to experi- mental data of radon exhalation rate and activity concentration in the soil-gas measured in a covered UMT-soil near the landfill site Lavaugrasse (France). The first approach is based on the finite volume compositional (i.e., water, radon, air) transport model TOUGH2/EOS7Rn (Transport Of Unsaturated Groundwater and Heat version 2/Equation Of State 7 for Radon; Sa^ adi et al., 2014), while the second one is based on the finite difference one-component (i.e., radon) transport model TRACI (Transport de RAdon dans la Couche Insatur ee; Ferry et al., 2001). Transient simulations during six months of variable rainfall and atmospheric air pressure showed that the model TRACI usually overestimates both measured radon exhalation rate and concentration. However, setting effective unsaturated pore diffusivities of water, radon and air components in soil-liquid and gas to their physical values in the model EOS7Rn, allowed us to enhance significantly the modelling of these experimental data. Since soil evaporation has been neglected, none of these two models was able to simulate the high radon peaks observed during the dry periods of summer. However, on average, the radon exhalation rate calculated by EOS7Rn was 34% less than that was calculated by TRACI, and much closer to the measured one for physically-based soil radon diffusion models. Unlike TRACI, EOS7Rn was able to simulate qualitatively seasonal variations of both radon exhalation and concentration. These results show that EOS7Rn produces less numerical errors than TRACI, and can be considered as a promising model for predicting radon transport in the landfill, if soil evaporation is modelled and its numerical inversion for parameter estimation is realized. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Few modelling studies have been reported in the literature for simulating in-situ experiments of transient two-phase (watereair) flow and radon transport in covered uranium mill tailings soils at landfill sites. Data-model comparison is very complicated because of the need to characterize double porosity media (e.g., macropores) and to construct the local history of climate, hydrology and bio-geo-chemistry of an UMT-landfill site. First transient simulations of unsaturated radon transport in UMT-landfill soils began with the works of Gee et al. (1984), Mayer et al. (1981), Mayer and Gee (1983), and Simmons and Gee (1981) who showed the importance of the long-term moisture content in multidimensional numerical simulations for predicting the long- term radon activity concentration in the soil-gas phase and exha- lation from the long-term climatic history of the site. In their works, however, Richards' approximation for the two-phase flow problem has been considered and no data-model comparison has been made. * Corresponding author. Tel.: þ33 158358898; fax: þ33 146290284. E-mail address: zakaria.saadi@irsn.fr (Z. Sa^ adi). Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad http://dx.doi.org/10.1016/j.jenvrad.2015.03.019 0265-931X/© 2015 Elsevier Ltd. All rights reserved. Journal of Environmental Radioactivity xxx (2015) 1e12 Please cite this article in press as: Sa^ adi, Z., Guillevic, J., Comparison of two numerical modelling approaches to a field experiment of unsaturated radon transport in a covered uranium mill tailings soil (Lavaugrasse, France), Journal of Environmental Radioactivity (2015), http://dx.doi.org/ 10.1016/j.jenvrad.2015.03.019