On the fate of ²²⁰Rn in soil material in dependence of water content: Implications from field and laboratory experiments
2012
Huxol, Stephan | Brennwald, Matthias S. | Hoehn, Eduard | Kipfer, Rolf
To study the potential of ²²⁰Rn as a groundwater tracer, we analyzed different groundwater systems with a laboratory-proven radon-in-water detection system. However, with one single exception, no ²²⁰Rn was detected in the groundwater, although ²²²Rn was always present at high concentrations. Field observations of ²²⁰Rn and ²²²Rn in soil gas revealed soil water content to be the crucial control for ²²⁰Rn release from soil grains to soil pores. We identified water films around and water menisci between the soil grains to impede the diffusive transport of ²²⁰Rn. This finding was confirmed by the results of laboratory experiments with monazite pebbles and manganese sand, which both are ²²⁰Rn sources. Besides the water content, the laboratory experiments also identified the water flow (turbulent in the experiment versus laminar in groundwater) to control the ²²⁰Rn emanation. The laminar flow condition in groundwater, together with the soil water content, set a conceptual frame to explain why ²²⁰Rn can be detected in unsaturated soil but not in groundwater.
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