Modeling of radon transport in unsaturated soil

Abstract

This study applies a recently developed model, LEACHV, to simulate transport of radon through unsaturated soil and compares calculated soil radon activities against field‐measured values. For volatile and gas phase transport, LEACHV is modified from LEACHP, a pesticide version of LEACHM, a well‐documented one‐dimensional model for water and chemical movement through unsaturated soil. LEACHV adds consideration of air temperature changes and air flow driven by barometric pressure change to the other soil variables currently used in LEACHP. It applies diurnal barometric pressure and air temperature changes to reflect more accurately the typical field conditions. Sensitivity analysis and simulated results have clearly demonstrated the relative importance of barometric pressure change, rainfall events, changes in water content, gas advection, and radon source term in radon transport process. Comparisons among simulated results illustrated that the importance of barometric pressure change and its pumping phenomenon produces both fluctuation in soil gas radon activities and an elevation of the long‐term average radon activity in the shallow soil. Barometric pressure pumping was found to produce an effect on radon activity in shallow soils of an equal magnitude to the distributed source parameter. Comparison between measured and simulated soil radon activities showed that LEACHV can provide realistic estimates of radon activity concentration in the soil profile.