Evaluation of radon emanation from soil with varying moisture content in a soil chamber

Abstract

The EPA chamber (2 × 2 × 4 m long) was constructed to study convective and diffusive soil gas movement under known conditions of 226Ra and 222 Rn concentration, moisture, density, soil constituent, and physical response to pressure variation. The radon emanation rates of soil are known to depend strongly on the moisture content of the soil. Because the moisture content varies greatly with depth in the EPA's soil chamber (from saturated at the bottom to nearly dry at the top), it is not possible to fully understand the radon distribution within the chamber without knowing the emanation rate as a function of moisture. Soil radon concentrations vary in the chamber from 7.4 kBq m −3, near the soil surface, to 86.2 kBq m −3, at the chamber bottom. This paper describes measurements of the emanation coefficient and diffusion of radon in soil contained in the chamber, using a wide range of moisture contents. In addition, equal amounts of well-mixed oven-dried soil were placed in 20 L aluminized gas-sampling bags, and, after approximately 1 month of in-growth, radon samples were taken, after which water was added, and another period of in-growth and sampling followed. The emanation coefficients and radon concentrations in the gas bag experiment were observed to increase with increasing moisture content and then decrease before reaching saturated conditions. The emanation and diffusion effects on the radon concentration soil gradient were identified for this sandy soil having approximately 200 Bq kg −1 radium and a soil density of 1682 kg m −3.