To obtain a better understanding of recent tritium concentration and its seasonal cycle in Japan, monthly precipitation samples were collected in Hokkaido, Gifu and Okinawa prefectures from June 2014 to December 2017. The arithmetic mean ( ± standard deviation) of tritium concentrations in precipitation samples from Hokkaido, Gifu and Okinawa were estimated to be 0.62 ± 0.27 Bq L−1, 0.32 ± 0.12 Bq L−1 and 0.13 ± 0.05 Bq L−1, respectively. These results indicate that the concentrations increase with latitude. In addition, the highest and the lowest concentrations appeared in spring and summer, respectively. To clarify the origins and sources of these cycles, further analyses of chemical compositions of precipitation and meteorological conditions are needed.

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In this study, to estimate the recent tritium concentration and its variation with latitude and time in Japan, environmental water samples were taken monthly from June 2014 to October 2016 in Okinawa Island, subtropical region of Japan. The inland water samples were taken from two springs and the drop water samples were taken in a limestone cave. The samples were distilled to remove impurities and then electrolysed using electrolytic enrichment system. Each of the enrichment samples was mixed with the liquid scintillation cocktail, and the tritium concentration was measured with a low background liquid scintillation counter. Arithmetic mean ± standard deviation for the tritium concentration of Morinokawa (spring water), Kakinohanahikawa (spring water) and Gyokusendo (cave drop water) samples were estimated to be 0.13 ± 0.04 Bq L-1 , 0.12 ± 0.03 Bq L-1 and 0.13 ± 0.03 Bq L-1 , respectively. The comparison between these results and reported data suggested that the latitude effect is one of factors in the relatively low tritium concentration observed in Okinawa Island.