Environmental and anthropogenic tritium in surface waters of Fukushima Prefecture, Japan

Tritium (3H or T) radionuclide with a half-life of 12.32 years is generated by cosmic rays in the upper atmosphere and becomes tritiated water (HTO) existing in all water cycle compartments across the globe. In Japan, the anthropogenic tritium was released to the atmosphere on March-April 2011 due to the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident and to the ocean with start of the Advanced Liquid Processing System (ALPS)-treated water discharge from 24th August 2023 prompting tritium monitoring in Fukushima waters. On 12-14th October 2023, 19 nested headwater catchments were sampled during baseflow for tritium measurements in this study and one river water sampling site located west of Fukushima city overlapped with the Fukushima Prefectural tritium monitoring programme, which have been sampling surface water sites twice per year to measure tritium without enrichment. In addition, monthly precipitation was collected at IER roof for tritium analysis. Using our measured tritium with a lump-parameter model, we used the long-term time-series of tritium in monthly precipitation scaled from the Tokyo to Fukushima area and combined the environmental tritium, which is generated by anthropogenic activities and cosmic rays in the upper atmosphere, with the FDNPP anthropogenic tritium release to simulate mean transit times (MTTs) from 1 to 200 years. Using available tritium measurements without enrichment allowed to exclude the short MTTs due to the FDNPP tritium input reducing the MTT range and estimated groundwater storage volume. As a result, we demonstrate that environmental tritium radioisotope is a useful tracer with developed tritium time-series in precipitation and surface water measurements to evaluate terrestrial water cycle in Fukushima. We also recommend the continuation of tritium monitoring is needed to disentangle natural levels with the ongoing tritium-related FDNPP activities for tritium measurements without and with electrolytic enrichment.