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THE INFLUENCE OF METEOROLOGICAL AND SOIL PARAMETERS ON RADON EXHALATION

H.Kojima and K.Nagano
Dep.Liberal Arts, Faculty of Science and Technology, Science University of Tokyo, Noda Chiba,
278, JAPAN

To clarify the mechanism of radon transport from soil to surface, long term continuous measurements
of radon exhalation were conducted at one site in combination with measurements of radon
concentration in soil gas, soil temperature, pressure difference, soil moisture and meteorological
parameters. Based on the results of measurements over a two year period, the influence of
meteorological and soil parameters on the exhalation was estimated. From the long-term data, it was
found that the main influencing factors on the variation of the hourly exhalation data were the water
content in the soil and the pressure difference between the surface and the soil air.
A time-dependent radon transport model was numerically solved as a function of the pressure
difference and the water content profile in the soil. The predicted exhalation rates from the model
agreed quite well with the observed ones.
To clarify the mechanism of radon transport from soil to surface, long term continuous measurements
of radon exhalation were conducted at one site in combination with measurements of radon
concentration in soil gas, soil temperature, pressure difference, soil moisture and meteorological
parameters. Based on the results of measurements over a two year period, the influence of
meteorological and soil parameters on the exhalation was estimated. From the long-term data, it was
found that the main influencing factors on the variation of the hourly exhalation data were the water
content in the soil and the pressure difference between the surface and the soil air.
Radon is emanated from solid grains and transports in soil pore filled with air and water. The
emanated radon further is distributed between the water and gas phases in the soil pores. Also, the
radon transports primarily by diffusion and advection mechanisms in soil pores. So, multi-phase radon
generation and transport model is required to simulate radon transport under realistic conditions. Also,
most of the model has included time-varying parameters such as atmospheric pressure and soil
moisture. Therefore, data obtained in measurements cannot be adequately explained without time-dependent
modeling. In this study, a multi-phase, time-dependent radon transport model was
numerically solved as a function of the pressure difference and the water content profile in the soil.
The predicted exhalation rates from the model agreed quite well with the observed ones.