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

A new fluid property module, EOS7Rn, was developed for TOUGH2 to simulate the transport of radon gas (222Rn) in saturated–unsaturated 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 (226Ra) content. We implemented variable physical properties (diffusion coefficient, emanation factor, adsorption coefficient, and Henry's law coefficient) of this gas component in two-phase (liquid–gas) porous media as a function of soil moisture and/or soil temperature. Numerical trials have been carried out to ensure that temporal and spatial numerical discretization of this nonlinear source term are effective and have properly been 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 (1D) radon transport in homogeneous and layered soil columns typical of Uranium Mill Tailings (UMT) landfill soils.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. Relative errors in radon mass balance and flux densities were determined to be negligible. For the second verification of EOS7Rn for transient nonisothermal radon transport problems, two simulation examples are presented to demonstrating the importance of the radon emanation and thermal effects on radon transport in a geothermal reservoir. Like most other sister modules, EOS7Rn 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.