科研之友
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摘要
To study why Rn-220 is commonly ubiquitous in soil gas but, in contrast to Rn-222, not detectable in groundwater, we conducted targeted laboratory experiments. In these experiments, we used a special Rn-220 productive sand and analyzed the behavior of Rn-220 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 Rn-220 in the gas phase. Under saturated conditions, we analyzed the dependence of Rn-220 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 Rn-220 through the pore space is limited by water menisci between the grains, acting as barriers for Rn-220. Under saturated conditions, the observed dependency of Rn-220 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 Rn-220 to the flowing water phase. Extrapolating the findings to common natural conditions, the results explain why Rn-220 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 Rn-220 in subsurface fluids.
- 出版日期2013-1-6
