Fluence rate of radon from soil: effect of sorption barriers, moisture content, and temperature.

The effects of activated carbon (AC) and mordenite (termed additives here) on the fluence rate of Rn gas from soil over a range of moisture contents and temperatures were examined in an effort to develop mitigation strategies for Rn in indoor environments. It is possible that an additive--an effective sorbent for Rn--could be mixed with the layer of soil placed adjacent to the substructure of buildings during their construction. The following variables and levels within each variable were examined: additive--0, 25, and 50 wt% mixed with the top 20% of the soil in columns; degree of water saturation, S-5, 30, and 55%; and temperature--5, 13, and 21 degrees C. At any level, mordenite was not effective in decreasing the Rn fluence rate from soil. On the other hand, when AC was present at the 25% level (at S = 30% and temperature = 13 degrees C), the fluence rate was about 5 mBq m-2 s-1 compared to approximately 12 mBq m-2 s-1 for the soil with no additive or one containing mordenite. The fluence rate was even less for levels of AC greater than 25%. AC effectively reduced the Rn fluence rate over all moisture contents and temperatures that were examined. The fluence rate increased with increasing moisture content; this was likely due to an increase in the emanation coefficient for Rn with increasing moisture content. The fluence rate increased or decreased with increasing temperature depending on the amount of AC present in the soil. The results indicate the AC may be an effective additive to soil backfill materials to limit the migration of Rn from the soil into buildings.