Observation of atmospheric radioactivity in East Japan over 1 year after Fukushima nuclear plant accident

A huge amount of radioactive matters were emitted and extensively diffused from nuclear plant facilities by the accident in Fukushima Dai-ichi Nuclear Power Plant (FDNPP) after the great earthquake and tsunami in east Japan on 11 March, 2011. The emission has caused serious environmental pollution, affecting various civil and industrial activities, including agricultural and fishing industry. Soon after the FDNPP accident, Japanese researchers started monitoring radioactive nuclides in various environmental fields to understand extent and magnitude of radioactive pollution. Investigators belonging to The Japanese Geoscience Union, The Geochemical Society of Japan and The Japan Society of Nuclear and Radiochemical Sciences have started joint, systematic, and extensive monitoring of radioactive nuclides in atmosphere, precipitation, soil, and underground water at various places in eastern Japan, in spite of confusion and difficulties due to the earthquake. Some samplings were also conducted in western Japan. As a significant part of this activity, a monitoring network of atmospheric radioactive nuclides has been constructed to understand the atmospheric radioactivity density over Japan. During late March and early April in 2011, aerosol sampling was started at more than 20 sites in Japan. The sampling was conducted at 11 sites, Fukushima (city), Sendai, Koriyama, Hitachi, Kashiwa, Yokohama, Nagano, Niigata, Yuri-Honjyo, Kazo, and Mito, surrounding FDNPP at about 80 km and 200 km after June 2011, and it continued at Fukushima, Marumori, Koriyama, and Hitachi since October, 2011. In this paper, variation of atmospheric radioactivity and its causes are presented. In the average, atmospheric concentrations of radionuclides decreased gradually after the FDNPP accident. However, increase of those of factor 10 – 100 is often observed. These increase showed the influence of advection of radionuclides emitted at FDNPP until September 2011. Even when the advection from FDNPP was negligibly small, significant amount of atmospheric radioactive Cs has observed. It was higher where radioactive Cs deposition on soil was higher, indicating re-suspension contributed significantly. After December 2011, atmospheric concentrations of radioactive Cs often increased ~ 10-3 Bq/m3 and it probably due to re-suspension from soil and vegetation.