Effects of changing water content and atmospheric pressure on radon flux from surfaces of five soil types

A computer code, Rn3D, was used to study the effects of varying the water content of five homogeneous soil types (clay, silt, loam, sand, and gravel) and atmospheric pressure on the transport of radon from soil surfaces. Temperature (20C) and radium content were assumed to be the same for all soils. Surface fluxes and soil pore space concentrations were computed for steady-state diffusion only, steady-state diffusion with steady pressure gradients, and sinusoidal (e.g., diurnal) changes in atmospheric pressure. Pressure gradients drive advective radon transport. A steady-state pressure gradient of -0.5 Pa/m enhanced the total radon surface flux over the diffusive flux from 0.01% for clay to 1,000% for gravel at 0% saturation. At 90% saturation the enhancements were one-tenth as much. The degree of enhancement was approximately proportional to the gradient along the soil column. A net enhancement of surface flux over steady diffusive flux (up to 6%) for sinusoidal surface pressure changes was observed for all five soil types. The study reveals that radon flux is affected as much by varying soil water content as by varying soil type