Modelling Radon Entry into Houses with Basements: Model Description and Verification

We model radon entry into basements using a previously developed three‐dimensional steady‐state finite difference model that has been modified in the following ways: first, cylindrical coordinates are used to take advantage of the symmetry of the problem in the horizontal plane, thereby increasing resolution and computing eficiency without signifiant loss of generality; second, the configuration of the basement has been made m e realistic by incorporating the concrete fmtm which sup ports the basement walls and floor; third, a quadratic relationship between the pressure and flow in the L‐shaped gap between slab, footer, and wall has been employed; and fourth, the natural convection of the soil gas which follows from the heating of the basement in winter has been taken into account. The temperature field in the soil is determined fiom the equation of energy consmation, using the basement, surface, and deep‐soil temperatures as boundary conditions. The pressure field is determined from Darcy's law and the equation of mass conservation (continuity), assuming that there is nofIow across any boundary except the soil surface (atmospheric pressure) and the opening in the basement shell (fixed pressure), Since the energy conservation equation includes both heat advection and conduction, the temperature and pressure equations must be coupled. After the pressure and temperature fields have been obtained, the velocity field is found fiom Darcy's h. Finally, the radon concentration field is found from the equation of mass‐transport, assuming that diffusive entry through openings may be neglected. The convective radon entry rate through the opening or openings is then calculated. In this paper we describe the modified model, compare the predicted radon entry rates with and without the consideration of thermal convection, and compare the predicted rates with rates determined from data from seven houses in the Spokane River valley of Washington and Idaho. Although the predicted rate is much lower than the mean of the rates determined from measurements, er‐TOTS in the measurement of soil permeability and variations in the permeability of the area immediately under the basement slab, which has a signifiant influence on the pressure field, can account for the range of entry rates inferredfiom the data.