137 Cs Trapped by Biomass within 20 km of the Fukushima Daiichi Nuclear Power Plant Akio Koizumi,* ,† Tamon Niisoe, † Kouji H. Harada, † Yukiko Fujii, † Ayumu Adachi, † Toshiaki Hitomi, † and Hirohiko Ishikawa* ,‡ † Department of Health and Environmental Sciences, Graduate School of Medicine, Kyoto University, Kyoto 606-8501, Japan ‡ Research Division of Atmospheric and Hydrospheric Disasters, Disaster Prevention Research Institute, Kyoto University, Uji 611-0011, Japan * S Supporting Information ABSTRACT: Analysis of 137 Cs trapped in biomass in highly contaminated zones is crucial in predicting the long-term fate of 137 Cs following the explosion at the Fukushima Daiichi Nuclear Power Plant. We surveyed forest 20-50 km from the plant in July and September 2011 to evaluate 137 Cs trapped in biomass within 20 km of the plant. We determined the ambient dose rate and collected forest soils and twigs at 150 sampling points. Removability from the canopy was evaluated by washing leaves and branches with water and organic solvents. The biomass of the forest canopy was then calculated. 137 Cs fallout was simulated with an atmospheric transport model. The modeled dose rate agreed with observations (n = 24) (r = 0.62; p < 0.01). Washing experiments demonstrated that unremovable portions accounted for 53.9 ± 6.4% of 137 Cs trapped by deciduous canopy (n = 4) and 59.3 ± 13.8% of 137 Cs trapped by evergreen canopy (n = 10). In total, it was estimated that 74.5 × 10 12 Bq was trapped by canopy in the forest within the no-go zone, with 44.2 × 10 12 Bq allocated to unremovable portions, and that 0.86% of the total release was trapped in biomass as of September 2011. ■ INTRODUCTION Following the Tohoku earthquake and tsunami of March 11, 2011, there were explosions at the Fukushima Daiichi Nuclear Power Plant (DNPP) on March 15, which released massive quantities of radionuclides into the atmosphere. The total amount of 137 Cs released into the atmosphere has been estimated to be approximately 8.6 × 10 15 Bq. 1 More than 70% of Fukushima Prefecture (Figure 1) in northern Japan is covered by forest, and the majority of radioactivity fell on this forest. The fates of radionuclides in forest are complicated and not well-understood. Dynamics of 137 Cs in forest can be divided into two phases: acute (0-3 years) and late (3+ years) phases. 2 In the acute phase, radionuclides are captured by the forest canopy. In the late phase, radionuclides enter the recycling process through absorption from soil, redistribution in the forest ecosystem, and immobilization in tree biomass. Once 137 Cs has entered the recycling process of the forest ecosystem, 137 Cs disappears only at the rate of its radioactive decay with a half-life of 30 years, despite radionuclides in the soil slowly departing the ecological system through heavy precipitation and runoff. 2 After the explosions at the DNPP, the Japanese government set up a no-go zone; i.e., an area within a 20 km radius of the DNPP. It has been speculated that massive amounts of radionuclides have fallen in this area and the land will be Received: August 2, 2012 Revised: July 18, 2013 Accepted: July 26, 2013 Published: July 26, 2013 Figure 1. Locations of nine sampling sites. The red “×” indicates the Fukushima DNPP. A total of 29 blue boxes and 50 open circles represent model grid cells and monitoring sites by the Japanese government 15 inside the 20 km zone. Details of the 9 sampling sites and 50 monitoring sites are listed in Tables S1 and S2 of the Supporting Information. Article pubs.acs.org/est © 2013 American Chemical Society 9612 dx.doi.org/10.1021/es401422g | Environ. Sci. Technol. 2013, 47, 9612-9618