Radon entry into houses: the importance of scale-dependent permeability

Soil permeability to air can increase substantially with measurement length scale. We tested the hypothesis that the scale effect could resolve large model underpredictions of radon and soil-gas entry into two experimental basement structures located in natural sandy-loam soil at a field site in Ben Lomond, CA. Previously, the model input for permeability at the site had been assessed based on 0.5-m scale measurements. After determining the soil-structure interaction scale (system scale) to be approximately 3 m, the model input was changed to reflect 3-m scale permeability measurements. This adjustment reduced unacceptably large model underpredictions, of a factor of 3 to 5, to a range near that of acceptable experimental error, 20 to 40%. The permeability scale effect may explain large and persistent model underestimates of radon entry into real houses. The results argue strongly for determining permeability at a length scale consistent with that of the system under study.