220Rn/222Rn Isotope Pair as a Natural Proxy for Soil Gas Transport

Radon (Rn) is a naturally occurring radioactive noble gas, which is ubiquitous in soil gas. Especially, its long-lived isotope Rn-222 (half-life: 3.82 d) gained widespread acceptance as a tracer for gas transport in soils, while the short-lived Rn-220 (half-life: 55.6 s) found less interest in environmental studies. However, in some cases, the application of Rn-222 as a tracer in soil gas is complex as its concentrations can be influenced by changes of the transport conditions or of the Rn-222 production of the soil material. Due to the different half-lives of Rn-220 and Rn-222, the distances that can be traveled by the respective isotopes before decay differ significantly, with Rn-220 migrating over much shorter distances than Rn-222. Therefore, the soil gas concentrations of Rn-220 and Rn-222 are influenced by processes on different length scales. In laboratory experiments in a sandbox, we studied the different transport behaviors of Rn-220 and Rn-222 resulting from changing the boundary conditions for diffusive transport and from inducing advective gas movements. From the results gained in the laboratory experiments, we propose the combined analysis of Rn-220 and Rn-222 to determine gas transport processes in soils. In a field study on soil gases in the cover soil of a capped landfill we applied the combined analysis of Rn-220 and Rn-222 in soil gas for the first time and showed the feasibility of this approach to characterize soil gas transport processes.