地中ラドン輸送モデルを用いた気象要素のラドン散逸への影響

Abstract

The radon exhalation from soil surface varies with various meteorological and soil parameters. As the observed radon exhalation rates are simultaneously affected by various parameters, it is difficult to estimate an effect of each parameter on radon exhalation. The effect of each parameter is studied based on the time-dependent one-dimensional radon transport model we developed. The model simulates the radon exhalation rate by using various meteorological and soil parameters under the realistic environmental conditions. The effect of the precipitation on the radon exhalation is explained by change of water porosity in the soil. Under the loamy soil condition in Kanto area, the radon exhalation was found to stop during the precipitation more than 5 mm. It is well known that the radon exhalation closely depend on the change rate of atmospheric pressure. From our simulation results, we can know the mechanism of pressure effect on radon exhalation. The change of atmospheric pressure is propagated to the deep soil layer from the surface. The time lag causes the pressure difference between the surface and the deep soil layer and produces the change of radon exhalation. In addition, it is found that atmospheric pressure dependence of radon exhalation is not affected by the variation of permeability in the range of three orders. The reason of remarkable calculation results is explained in this paper. The radon exhalation increases with increasing wind speed and may rise double by the wind speed exceeding 10 m s-1. The wind effect has a threshold value, about 5 m s-1 in this study and is ignored in the wind speed below the value. Therefore, we can conclude that contribution of the wind speed to the radon exhalation is relatively small.