Soil radon (Rn-222) monitoring in a forest site in Fukushima, Japan

Soil radon (Rn-222) 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 Rn-222 activity level, variability and factors controlling Rn-222 concentration in soil air using data obtained from August to December 2013. Time series of Rn-222 activity concentration showed depth-dependent variability with an equilibrium value (Rn-222(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 Rn-222. Transport mechanism of Rn-222 in soil air was considered to be diffusion-controlled with data sets on changing Rn-222 concentration with time in selected cases that showed decreasing (or increasing) Rn-222 concentration with time at every depth. Important factors affecting soil Rn-222 variability are meteorological parameters, low-pressure front passing over the site, and subsequent precipitation. Time lags of decreasing Rn-222 concentration at different depths after rain indicate a certain relationship of Rn-222 level with moving water (and water vapor) in soil. The findings obtained in this study are important to evaluate the fate of fallout nuclides (radiocesium) in contaminated forest sites using soil radon as a tracer of moving soil air