[M-AG34] Radioisotope migration: New development for radioisotope migration study related to 1F NPP accident

[MAG34-P04] Novel estimation method using rainfall data reveal changes in air dose rate due to soil moisture content in the Fukushima Prefecture Forest

*Nakanishi Miyu¹, Yuichi Onda², Hiroaki Kato², Junko Takahashi² (1.Master’s Program in Geosciences, University of Tsukuba, 2.Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba)

Keywords:air dose rate, shielding effect, effective rainfall, hysteresis

Radionuclides released and deposited as a result of the 2011 Fukushima Daiichi Nuclear Power Plant accident caused an increase in air dose rates in forests in Fukushima Prefecture. While air dose rates are generally reported to increase during rainfall, they were found to decrease in forests in Fukushima Prefecture. This is due to the shielding of radioactive cesium released from the soil by increased soil moisture. Few studies have described the shielding effect of soil moisture in forests affected by radionuclides in the past, and it is necessary to capture changes in air dose rates due to changes in soil moisture to evaluate air dose rates excluding the effect of soil moisture changes. The objective of this study is to develop a method for estimating changes in air dose rates due to rainfall even in the absence of soil moisture data. To this end, we not only investigated soil moisture and air dose rates in forests in Fukushima Prefecture, but also developed a simple method for estimating soil moisture and air dose rates by calculating effective rainfall based on rainfall data. In the forests of Fukushima Prefecture, the correlation between soil moisture content and air dose rate was extremely high, and it was found that changes in soil moisture content had a significant effect on air dose rate values. When estimating soil moisture in forests from effective rainfall, it was found necessary to use effective rainfall combined with half-life, to take into account the existence of hysteresis that shows different dynamics between soil absorption and drainage, and that soil moisture does not increase in dry soil due to the existence of water repellency. Field measurements showed that when rainfall occurs after a period of no rainfall, the soil moisture content may not increase in the initial stage, suggesting the presence of water repellency. This case was characterized by a tendency toward low soil water content at the onset of rainfall in addition to negligible rainfall. The air dose rate remained almost the same at this time. When rainfall continued for a long time, the water repellency disappeared regardless of the soil water content at the start of rainfall, resulting in a decrease in the air dose rate. In order to estimate air dose rates using rainfall data, it is important to estimate soil water content from effective rainfall based on the dynamics of soil moisture. Among them, capturing hysteresis and water repellency is necessary for estimating soil moisture content.