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The effectiveness of a passive subslab ventilation system in reducing radon concentrations in an occupied home was investigated by measuring radon concentrations and pressure differentials during a 1-year period when a passive subslab ventilation system was being cycled on and off. Radon concentrations in the house were 30% lower during periods when the stack was open to the atmosphere. This effect was most pronounced when the home was unoccupied and during the winter and spring months. Furnace use and wind speed were the best predictors of transient changes in basement radon concentrations, whether the stack was open or closed. Pressure differential measurements show that subslab depressurization occurs when the stack is open during the winter and spring months due to bouyancy-driven air flow up the stack, but not during the summer. Numerical simulations of gas flow and radon transport into the house from the surrounding soil were calibrated to observed pressure differentials and radon concentrations. The model predicts that peak radon concentrations caused by furnace use will be reduced by flow out of the stack. However, the model is unable to account for the reduction in average radon concentrations observed while the stack is open in the winter. * Corresponding author telephone: (509) 372-6132; fax: (509) 372-6328; e-mail address: dj holford@pnl.gov.FIGURE 1. Effectiveness of passive stack in lowering radon concentrations in basements of 21 homes.

A computer code, Rn3D, was used to study the effects of varying the water content of five homogeneous soil types (clay, silt, loam, sand, and gravel) and atmospheric pressure on the transport of radon from soil surfaces. Temperature (20°C) and radium content were assumed to be the same for all soils. Surface fluxes and soil pore space concentrations were computed for steady‐state diffusion only, steady‐state diffusion with steady pressure gradients, and sinusoidal (e.g., diurnal) changes in atmospheric pressure. Pressure gradients drive advective radon transport. A steady‐state pressure gradient of −0.5 Pa/m enhanced the total radon surface flux over the diffusive flux from 0.01% for clay to 1000% for gravel at 0% saturation. At 90% saturation the enhancements were one‐tenth as much. The degree of enhancement was approximately proportional to the gradient along the soil column. A net enhancement of surface flux over steady diffusive flux (up to 6%) for sinusoidal surface pressure changes was observed for all five soil types. The study reveals that radon flux is affected as much by varying soil water content as by varying soil type.

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