Processes controlling 220Rn concentrations in the gas and water phases of porous media

To study why 220Rn is commonly ubiquitous in soil gas but, in contrast to 222Rn, not detectable in groundwater, we conducted targeted laboratory experiments. In these experiments, we used a special 220Rn productive sand and analyzed the behavior of 220Rn in the gas and water phases of the sand under unsaturated and saturated conditions. To simulate changing water contents of soils under unsaturated conditions, we slowly flooded and drained a box filled with the sand and analyzed the resulting dynamics of 220Rn in the gas phase. Under saturated conditions, we analyzed the dependence of 220Rn concentrations in the water phase on water flow by extracting water at different pumping rates from the saturated sandbox and a flow tank filled with the same saturated sand. The results revealed that under unsaturated conditions the migration of 220Rn through the pore space is limited by water menisci between the grains, acting as barriers for 220Rn. Under saturated conditions, the observed dependency of 220Rn concentrations in water on the induced water flow velocity implies that fast flowing water in porous media is able to disturb commonly immobile water layers around the grains and, therefore, stimulate the emanation of 220Rn to the flowing water phase. Extrapolating the findings to common natural conditions, the results explain why 220Rn can be detected in unsaturated soil but not in groundwater. In addition, general conclusions to small scale dynamics of soil gas and groundwater are drawn from the dynamics of 220Rn in subsurface fluids.

► Both, 220Rn and 222Rn occur in soil gas, and only 222Rn is detectable in groundwater. ► Laboratory studies on 220Rn concentrations in unsaturated and saturated soils ► Hysteric behavior of 220Rn-in-soil gas concentration due to flooding and draining ► High flow velocities enhance 220Rn concentrations in groundwater. ► Slow natural groundwater flow limits 220Rn emanation to negligible level.