Soil radon ( 222 Rn) has been monitored since August 2013 at three different soil depths on a campus forest of Fukushima University in Japan where a large amount of fallout nuclides were released by the accident of Fukushima Daiichi Nuclear Power Plant in March 2011. The primary purpose of this study is to evaluate 222 Rn activity level, variability and factors controlling 222 Rn concentration in soil air using data obtained from August to December 2013. Time series of 222 Rn activity concentration showed depth-dependent variability with an equilibrium value ( 222 Rn eq ) during this observation period; 7.5, 14 and 23 kBq m -3 at 0.3, 0.6 and 1.0 m in depth, respectively. Two typhoons passing over the site had a great influence on soil radon level, which was practically used for evaluating effective diffusion coefficient of 222 Rn there. Transport mechanism of 222 Rn in soil air was considered to be diffusion-controlled with data sets on changing 222 Rn concentration with time in selected cases that showed decreasing (or increasing) 222 Rn concentration with time at every depth. Important factors affecting soil 222 Rn variability are meteorological parameters, low pressure front passing over the site, and subsequent precipitation. Time-lags of decreasing 222 Rn concentration at different depths after rain indicate a certain relationship of 222 Rn level with moving water (and water vapor) in soil. The findings obtained in this study are important to evaluate fate of fallout nuclides (radiocesium) in contaminated forest sites using soil radon as a tracer of moving soil air. IntroductionRadon ( 222 Rn) in the environment has been extensively studied to evaluate dose levels due to inhaling the radioactive gas into the body, to estimate its flux from the ground surface to the atmosphere (exhalation) as a tracer of air movement in the lower atmosphere, and to predict seismic activity in tectonically active areas (Dorr et al. 1983;Zahorowski et al. 2004;Vaupotič et al 2010). Another important and basic aspect exists in the study of 222 Rn in soil air to elucidate mechanisms of its migration in soil and transportation to the ground surface (Nazaroff 1992;Neznal and Neznal 2005;Fujiyoshi et al. 2010Fujiyoshi et al. , 2013. Soil radon monitoring has shown that 222 Rn activity concentration varies to a great extent depending on geological, meteorological and hydrological factors, and that its migration through the soil and rock is controlled not only by diffusion but convection It is widely recognized that both liquid and gaseous water movements are fundamental factors controlling many processes in soil. Soil water dynamics are strongly linked to temperature variations and then biological activities. These processes, complicated due to interrelations among controlling factors, have not been clarified thoroughly (Wells et al. 2007;Bittelli et al. 2008). Understanding radon transportation in soil is useful for evaluating soil air movement in the surface soil layers, because radon is chemically inert and radioactive...

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