Soil water effects on concentration profiles and variations of 222Rn in a vadose zone

Concentration profiles of 222Rn and their variations were studied to understand the vertical motion in the ground caused by fluctuations of meteorological parameters, such as precipitation, ground-water level and atmospheric pressure. A method of trapping 222Rn was developed to measure the concentration profiles by circulating and bubbling soil gas in porous cups into toluene. For locations where the water table is close to the surface, observations showed a peak concentration in the vertical profile of 222Rn at about a 40-cm depth and revealed that the concentration in soil gas decreases as soil moisture increases near the water table. Soil gas in a buried vessel also was circulated continuously to monitor 222Rn in the ground air near the surface using an ionization chamber. A rising of the water table following precipitation causes a burst of 222Rn concentration by upflow in the ground, though the concentration in the soil gas soon decreases with the intake of air above the surface due to increasing atmospheric pressure following precipitation. The concentration in soil gas again increases via the phenomenon accompanying the recovery of equilibrium in soil water between 222Rn and 226Ra.