EOS7Rn - A new TOUGH2 module for simulating radon emanation and transport in the subsurface

A new fluid property module, EOSRn, was developed for TOUGH2 to simulate the transport of the radon gas (Rn) in saturatedunsaturated soils. It is an enhanced version of the EOS7R module for radionuclide transport, with a source term added in the transport equation to model radon generation by emanation from radioactive decay of the soil radium (Ra) content. We implemented physical properties of this gas component in two-phase (liquid-gas) porous media, as a function of soil moisture and/or soil temperature, such as those of the diffusion coefficient, the emanation factor, the adsorption coefficient and Henry’s law coefficient. To ensure that temporal and spatial numerical discretization of this nonlinear source term is effective and has been well implemented in TOUGH2, we performed comparative studies between EOS7Rn and an exact analytical solution at steady-state isothermal unsaturated conditions for many numerical experiments of one-dimensional radon transport in homogeneous and layered soil columns. We found that the radon activity concentration profiles and flux densities calculated by EOS7Rn were in good agreement with the analytical solution for all the studied numerical experiments. Consequently, relative errors for calculated TOUGH2’s radon mass balance and flux densities were very negligible. Like most other sister modules, EOSRn can simulate nonisothermal multiphase flow and fully coupled three-dimensional transport in fractured porous media. It will help in predicting the radon exhalation from highly radium-contaminated soils and underground cavities to outdoor and indoor environments.