On the fate of ~(220)Rn in soil material in dependence of water content: Implications from field and laboratory experiments

摘要

To study the potential of ~(220)Rn as a groundwater tracer, we analyzed different groundwater systems with a laboratory-proven radon-in-water detection system. However, with one single exception, no ~(220)Rn was detected in the groundwater, although ~(222)Rn was always present at high concentrations. Field observations of ~(220)Rn and ~(222)Rn in soil gas revealed soil water content to be the crucial control for ~(220)Rn 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 ~(220)Rn. This finding was confirmed by the results of laboratory experiments with monazite pebbles and manganese sand, which both are ~(220)Rn sources. Besides the water content, the laboratory experiments also identified the water flow (turbulent in the experiment versus laminar in groundwater) to control the ~(220)Rn emanation. The laminar flow condition in groundwater, together with the soil water content, set a conceptual frame to explain why ~(220)Rn can be detected in unsaturated soil but not in groundwater.