Soil properties and weather conditions mask a potential tectonic contribution to radon concentrations measured in soil air – a case study from the northern Upper Rhine Graben

Within the framework of the project Neotectonics in the Northern Upper Rhine Graben (NeoNORG), the relationship between fault zones in a sedimentary basin and associated radon anomalies is investigated. The area of interest is located west of Darmstadt near the village Wolfskehlen. Radon levels in the Quaternary strata of the Upper Rhine Graben are generally low or moderate. However, tectonic fault zones could represent pathways of increased gas permeability and advective gas transport which would result locally in elevated soil radon concentrations. 

A multi-method geophysical approach was chosen to visualise the subsurface structure. Each method has different advantages in terms of penetration depth and resolution (i.e., electrical resistivity tomography, ground penetrating radar and seismics of different wave types). The combination of these different geophysical investigation methods allows to trace the fault zones from the crystalline basement of the sedimentary basin at a depth of 2 km to several metres below the earth's surface.

To investigate the relationship between radon concentration and fault zones, soil gas measurements were carried out at the surface along several profiles. In total 800 soil gas measurements were conducted, in which 600 active short-term measurements were conducted by soil gas sampling and 200 passive long-term measurements (three-week exposure period) were conducted using exposimeters. In addition, parameters such as soil material, weather conditions and soil permeabilities were recorded.

The evaluation of the measurements indicates no direct influence of the fault zones on the measured radon levels. Instead, there are very distinct correlations with the soil substrate and weather conditions. The preliminary results suggest that the migration of radon or the accumulation of primordial radionuclides along fault zones is superimposed by stronger signals such as weather and soil material in the study area of the Northern Upper Rhine Graben.