Modelling of radon-222 as a proxy for vapour phase transport in the unsaturated zone

This study uses naturally occurring Rn-222 as a tracer in the unsaturated zone to better understand volatile organic compounds (VOC) release, transport and interaction. Using the STELLA Research software package, a two-dimensional model of Rn-222 transport and generation in the unsaturated zone was produced. The model was validated using a laboratory facility comprising of a 2 x 1 x 0.2 m layered chamber. In these experiments transport by diffusion is measured for a homogeneous column of material. The model described the Rn-222 concentrations satisfactorily for both simple and complex experiments. Sensitivity analysis and simulated results demonstrate the importance of moisture content in the transport of Rn-222. The simulations show excellent comparison with experimental data and provide good estimation of Rn-222 activity and VOC transport in the unsaturated zone.