Radioactive contamination in the Tokyo metropolitan area in the early stage of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident and its fluctuation over five years

RESEARCH ARTICLE Radioactive contamination in the Tokyo metropolitan area in the early stage of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident and its fluctuation over five years a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 Masanobu Ishida*, Hideo Yamazaki Graduate School of Science and Engineering, Kindai University, Higashiosaka, Osaka, Japan * Abstract OPEN ACCESS Citation: Ishida M, Yamazaki H (2017) Radioactive contamination in the Tokyo metropolitan area in the early stage of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident and its fluctuation over five years. PLoS ONE 12(11): e0187687. https://doi.org/10.1371/journal.pone.0187687 Editor: Tim A. Mousseau, University of South Carolina, UNITED STATES Received: May 10, 2017 Accepted: October 24, 2017 Published: November 14, 2017 Copyright: © 2017 Ishida, Yamazaki. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. The activities of radionuclides are available from: http://www.gesui.metro.tokyo.jp/english/ oshi/ Radioactive contamination in the Tokyo metropolitan area in the immediate aftermath of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident was analyzed via surface soil sampled during a two-month period after the accident. 131I, 134Cs, and 137Cs were detected in these soil samples. The activity and inventory of radioactive material in the eastern part of Tokyo tended to be high. The 134Cs/137Cs activity ratio in soil was 0.978 ± 0.053. The 131 137 I/ Cs ratio fluctuated widely, and was 19.7 ± 9.0 (weighted average 18.71 ± 0.13, n = 14) in the Tokyo metropolitan area. The radioactive plume with high 131I activity spread into the Tokyo metropolitan area and was higher than the weighted average of 6.07 ± 0.04 (n = 26) in other areas. The radiocesium activity and inventory surveyed in soil from a garden in Chiyoda Ward in the center of Tokyo, fell approximately 85% in the four months after the accident, and subsequently tended to rise slightly while fluctuating widely. It is possible that migration and redistribution of radiocesium occurred. The behavior of radiocesium in Tokyo was analyzed via monitoring of radiocesium in sludge incineration ash. The radiocesium activity in the incineration ash was high at wastewater treatment centers that had catchment areas in eastern Tokyo and low at those with catchment areas in western Tokyo. Similar to the case of the garden soil, even in incineration ash, the radiocesium activity dropped rapidly immediately after the accident. The radiocesium activity in the incineration ash fell steadily from the tenth month after the accident until December 2016, and its half-life was about 500 days. According to frequency analysis, in central Tokyo, the cycles of fluctuation of radiocesium activity in incineration ash and rainfall conformed, clearly showing that radiocesium deposited in urban areas was resuspended and transported by rainfall run-off. Funding: This work was supported by JSPS KAKENHI (B) Grant Numbers 24310014, 26289155, and 26303004 and Grants-in-aid for JSPS Fellow Grant Number 15J10109. Competing interests: The authors have declared that no competing interests exist. PLOS ONE | https://doi.org/10.1371/journal.pone.0187687 November 14, 2017 1 / 18 Tokyo metropolitan pollution Introduction The tsunami triggered by the East Japan Great Earthquake Disaster of March 11, 2011 cut off all electric power to Fukushima Daiichi Nuclear Power Plant (FDNPP). No. 1, No. 2, and No. 3 reactors lost their cooling capability, their nuclear fuel went into melt down and their reactor vessels were destroyed between March 12 and 15 [1–6]. As a result, part of the radioactive material discharged into the atmosphere was deposited across eastern Japan [7–11]. Based on monitoring data from each location, it is assumed that radioactive material in the plume that spread into the Tokyo metropolitan area on March 16 and 22 was deposited on the ground via rainfall. The timing and route of the spread of the radioactive plume have been variously hypothesized, but have not been clarified in detail as of yet [12–17]. When we started our measurements, almost no public information about the radioactive contamination in the Tokyo metropolitan area and Kanto district had been shared. The world has experienced nuclear power plant accidents before, including nuclear fuel melt down at Three Mile Island (1979) [18] and Chernobyl (1986) [19,20]. Six years after the FDNPP accident in 2011, much about the behavior of radioactive material in the environment has been clarified [1,21]. The FDNPP accident caused the nuclear fuel meltdown simultaneously in three reactors, discharging a large quantity of radioactive material into the environment [6–11]. One characteristic of the FDNPP accident is that a large quantity of radioactive material was discharged into the topographically and meteorologically complex natural environment of Japan. Because many of the urban areas are formed in the plains at the foot of steep mountainous areas, they are frequently affected by heavy rains such as typhoons. Another major characteristic is that the plume containing a large amount of radioactive material drifted into the Tokyo metropolitan area [16,21–24]. The capital, Tokyo, where over 30 million residents live, is located south of Kanto Plain, about 230 km southwest of FDNPP. Conditions and radioactive contamination shortly after the FDNPP accident have been reported in a variety of publications [2–5]. However, these include almost no discussion of the fact that radioactive materials were carried into the Tokyo metropolitan area, which is a densely populated urban area and the socio-economic and cultural core of Japan, nor of any evaluation of the fluctuations in the radioactive material in Tokyo’s urban districts. This study surveyed the distribution of radioactive materials in soil to evaluate the fluctuation of radioactive contamination in the Tokyo metropolitan area and the Kanto district in the immediate aftermath of FDNPP accident. Changes in radionuclide activities and inventories in soil between the time of the accident and the present time in the central part of Tokyo were monitored to clarify the behavior of the radioactive materials in the urban distritcts. Moreover, the behavior of radionuclides in the urban environment, which is covered by concrete and asphalt, was analyzed based on changes in the radiocesium activity in incineration sludge ash discharged from Tokyo’s wastewater treatment centers [25]. Sampling and methods Soil sampling Most soil samples were taken from the private site. These collections were approved by the owner of the land. In the case of collection at publ (...truncated)