Radon generation and transport in soils

Soil-gas Rn, soil moisture, and soil temperature have been monitored for one year or more at five sites in Pennsylvania which have been characterized for soil bulk density, porosity, diffusivity and permeability. Radon-222 and 222Rn vary in an annual, approximately sinusoidal pattern having an amplitude of 2- to 10-fold at all five sites. Since Rn partitioning between gas and water is temperature sensitive, and because soil moisture and temperature change in annual cycles, much of the variability in 222Rn occurs in annual cycles. Therefore knowledge of regional and temporal soil moisture and temperature patterns allows estimates of 222Rn in soil gas. These estimates suggest Rn is least elevated by moisture in cold, arid soils, and temporal variability will be small in arid soils. In areas where the soil substrate has lower Rn, emanation coefficient, or bulk density than the soil, soil-gas Rn can diffuse into the substrate. Under extreme conditions the Rn concentration can have a gradient towards the rock. Soil cores were used to measure the fraction of Rn exhaling to the gas phase over a range of moisture tensions. The greatest exhaling fraction generally occurred at intermediate moisture tension. The low exhaling fraction in dry soil is attributed to lodging of recoiling Rn in adjacent soil grains. The low exhaling fraction in wet soil is attributed to waterinhibited diffusion. This core method provides reasonable estimates of the combined effects of emanation and diffusion on Rn in soils. Bulk diffusion coefficients and permeability coefficients generally decrease by nearly 1 and 2 orders of magnitude respectively from the eluvial to the illuvial horizons in typical soils