RELATIVE EFFECTIVENESS OF SUB-SLAB PRESSURIZATION AND DEPRESSURIZATION SYSTEMS FOR INDOOR RADON MITIGATION - STUDIES WITH AN EXPERIMENTALLYVERIFIED NUMERICAL-MODEL

The performance of sub-slab-ventilation (SSV) systems has been paramet rically studied with a numerical model that was earlier compared succe ssfully with experiment (Bonnefous et al., 1992). The model distinguis hes between the sub-slab gravel and the underlying soil. It is used to examine system performance for the following system parameters: the p ermeability of the soil and of the sub-slab gravel, the magnitude of p ressurization (or depressurization) applied by the SSV system, and the mode of SSV application (i.e. pressurization (SSP) or depressurizatio n (SSD)). The mechanisms contributing to the successful performance of SSP and SSD systems are identified. For SSD systems, the mechanisms a re (1) the inversion of the pressure gradient across the basement slab , and (2) the reduction of the radon concentration in the soil. For SS P systems, the mechanisms are (1) the elimination of convective flow o f soil-gas from the soil into the sub-slab gravel by pressurization of the sub-slab region, (2) the reduction of the radon concentration in the soil, and (3) the suppression of diffusion of soil-gas from the so il into the sub-slab gravel by advective flow of air from the gravel b ed into the subgravel soil. Numerical modeling demonstrates that place ment of a sub-slab gravel layer substantially improves the SSV system performance. Except in the case of highly permeable soils, SSD systems are predicted to perform better than SSP systems. This prediction is consistent with reported field experience. The numerical model is used to elucidate the reasons for this difference in performance.