Anthropogenic and natural tritium radioisotope in terrestrial water cycle of Fukushima, Japan

Environmental tritium (³H) radioisotope with a half-life of 12.32 years is naturally generated in the upper atmosphere by cosmic rays and enters the water cycle in the troposphere as the water molecule (HTO) to become a useful tracer in Japan and other countries. In 2011, anthropogenic ³H entered the terrestrial water cycle due to the Fukushima Daiichi Nuclear Power Plant (FDNPP) atmospheric release and discharged in Advanced Liquid Processing System (ALPS) treated water from the FDNPP site to the Pacific Ocean in 2023 raising concerns internationally. In Japan, ³H measurements in monthly precipitation have been conducted by the Government and Universities while many surface water sites were sampled twice per year across Fukushima Prefecture accumulating a decade-long record of ³H measurements. However, there are no ³H measurements in precipitation during the FDNPP accident requiring atmospheric numerical modeling to quantify anthropogenic ³H in Fukushima. To utilize ³H as a tracer in Fukushima, we combine simulated anthropogenic ³H released by the FDNPP in 2011 with the long-term time-series of ³H in precipitation from 1950 to present in the Tokyo area, which was scaled to Fukushima area. Using annual ³H in precipitation is 2.86 TU-3.70 TU with an average of 3.37 TU from 2016 to 2021 lead to the scaling factor from Tokyo area to Fukushima city between 1.30 and 1.61. For Fukushima surface water sites, measured ³H concentrations are at low levels of natural ³H concentrations and lead to insignificant doses due to drinking water exposure. In addition, we sampled several headwater catchments near Fukushima city in October 2023 for measuring ³H and estimated tritium-tracer mean transit time and subsurface water storage volume after the ALPS-treated water discharge. As a result, we demonstrate that environmental ³H radioisotope is a useful tracer with developed ³H time-series in precipitation and surface water measurements to evaluate terrestrial water cycle in Fukushima.