Influence of variable stress on underground radon concentrations

Stresses applied to rocks of the Earth’s crust cause local deformation of the crust. In order to monitor a radon signal due to variable pressures and rock stresses we use a natural laboratory under a reservoir of the Vianden (Luxembourg) pumping storage power plant. Depending on energy demands, the artificial reservoirs at the top of a hill experience daily variations in water levels of up to 16 meters, thus exerting variable pressures on the underlying rocks. Radon concentrations are continuously measured in boreholes drilled into the bedrock under water reservoir. We observe some very strong variations in transient radon concentrations induced by variations of water level. The reservoir consists of two independent reservoirs that were emptied individually for some period of time. The observed radon pattern depends on the location of the boreholes under the reservoir and is different if both basins are working together, or if one of the reservoirs is empty. The observed patterns can be accounted for by variable pathways of fluids, fluid overpressure and a dynamic flow in cracks.