The role of channels in the transport of radon from the soil

Abstract

Experimental measurements of permeability as a function of scale as well as direct detection of flow indicate macroscopic channels are present at a soil site. A model for heterogeneous media is developed that provides a dimensionless number S, an estimate of the ratio of the exhalation from the channels to the exhalation from the soil matrix, to indicate whether or not channels are important for transport of radon to the atmosphere. The model considers the effects of flow and transverse diffusion in the channels, flow and diffusion in the soil matrix, and coupling between the channels and matrix due to diffusive exchange. Specific predictions depend on soil properties, the geometry of the channels, and the pressure gradient. The model indicates that channels are important for the present soil at times of higher pressure gradients such as can occur with fronts, wind, and atmospheric turbulence. The radon concentration exhausted for steady state upward flow is greater than that exhausted for transient upward flow. On a time‐averaged basis, the contribution of channels to overall exhalation of radon appears moderate. The modeling suggests channeling in soils could be a factor enhancing exhalation of radon to the atmosphere at other locations. The distance of subsurface radon transport is increased by significant channeling, but transverse diffusion into the surrounding matrix is a strong mitigating influence.