[M-AG34] Radioisotope migration: Development from Environmental Dynamics Research on the F1NPS Accident

[MAG34-04] Eluchidation and Modeling of the Effects of Forest Management on Air Dose Rates

*Yusei Uehara¹, Yuichi Onda², Junko Takahashi², Miyu Nakanishi¹, Yupan Zhang², Shiori Takamura¹ (1.Program of Geoscience, University of Tsukuba, 2.Center for research in radiations, isotopes, and earth system sciences, University of Tsukuba)

Keywords:air dose rates, forest management, thinning, effective rainfall, soil moisture content

Approximately 69% of the Cs-137 deposited on land due to the accident at the Fukushima Daiichi Nuclear Power Plant caused by the Tohoku-Pacific Ocean Earthquake on March 11, 2011 was deposited in forested areas. Currently, attempts are being made to reduce air dose rates in forested areas using thinning, but it is not known how much forest management specifically affects air dose rates, and optimal forest management methods to reduce air dose rates have not been proposed. It has been suggested that thinning forests may increase solar radiation and precipitation in the forest, which may promote decomposition of organic matter in the soil and transfer radionuclides deeper into the soil, resulting in a reduction in air dose rates. There are many models for predicting air dose rates in forests, but few of them focus on a single radiation source location and soil moisture content. Therefore, it is a challenge to develop a forest air dose rate prediction model that takes these factors into account. In a previous study (Nakanishi et al., 2023), a model was presented to predict air dose rates in each forest from precipitation based on the relationship between soil moisture content and air dose rates and precipitation and soil moisture content, but the relationship between precipitation and soil moisture content could not express the increase during precipitation due to hysteresis. However, the models were not able to represent the increase during precipitation due to hysteresis effects. Therefore, in this study, rain gauges, soil moisture content gauges, and air dose rate gauges were installed in the study area where thinning had been conducted to observe the effects of thinning on these values, and the best parameters for the model were obtained from these measured data, and the model was created and compared with the measured values. Comparison of the measured data showed that precipitation was higher in the thinned area than in the natural forest area. Correspondingly, soil moisture content was higher in the thinned area. As for the air dose rate, the results showed that the control area condition was lower. However, the correlation between soil moisture content and air dose rate was higher in the thinned area, and the contribution of soil moisture content to air dose rate was also higher in the thinned area. As for the air dose prediction model, the relationship between precipitation and soil water content, which was a particular issue in the previous study, showed a better relationship between effective rainfall and soil water content by changing the half-life, using the effective rainfall method as in the previous study. Furthermore, by changing the time rainfall method to daily rainfall to predict the model over a longer period, we were able to estimate temporal changes in air dose rate over a four-month period. The effect of thinning on air dose rate was confirmed by the increase in rainfall in the forest due to the decrease in throughfall and the associated effect of soil moisture content on air dose rate.