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SOIL GAS MEASUREMENTS BELOW FOUNDATION DEPTH IMPROVE INDOOR RADON PREDICTION

H.U. Johner and H. Surbeck
Federal Office of Public Health, Environmental Radioactivity Survey, CH-1700 Fribourg, Switzerland
Tel.: **41 26 300 91 62, Fax: **41 26 300 9743, E-mail: hans-ueli.johner@bag.admin.ch

A soil gas measurement method developed earlier [1] was applied to boreholes drilled to below
foundation depth. Radon concentration and permeability were measured at 50 cm intervals. In
radon prone areas permeability showed to increase with depth over several orders of magnitude,
indicating a low permeability top layer with a thickness of 0.5 m and more. A radon availability
index (RAI) was empirically defined and the maximum RAI of each boring proved to be a reliable
indicator for radon problems in nearby houses. The permeability of the top layer also proved to be
an important factor for a better understanding of soil gas transport and the influence of rain.
Implications for radon mitigation are derived.
Key words: Permeability, soil gas, radon, top layer, radon availability index, mitigation.