Soil gas 222Rn concentration in northern Germany and its relationship with geological subsurface structures

Abstract

222Rn in soil gas activity was measured across the margins of two active salt diapirs in Schleswig-Holstein, northern Germany, in order to reveal the impact of halokinetic processes on the soil gas signal. Soil gas and soil sampling were carried out in springtime and summer 2011. The occurrence of elevated 222Rn in soil gas concentrations in Schleswig-Holstein has been ascribed to radionuclide rich moraine boulder material deposits, but the contribution of subsurface structures has not been investigated so far. Reference samples were taken from a region known for its granitic moraine boulder deposits, resulting in 222Rn in soil gas activity of 40 kBq/m3. The values resulting from profile sampling across salt dome margins are of the order of twice the moraine boulder material reference values and exceed 100 kBq/m3. The zones of elevated concentrations are consistent throughout time despite variations in magnitude. One soil gas profile recorded in this work expands parallel to a seismic profile and reveals multiple zones of elevated 222Rn activities above a rising salt intrusion. The physical and chemical properties of salt have an impact on the processes influencing gas migration and surface near radionuclide accumulations. The rise of salt supports the breakup of rock components thus leading to enhanced emanation. This work provides a first approach regarding the halokinetic contribution to the 222Rn in soil gas occurrence and a possible theoretical model which summarizes the relevant processes was developed.