Spatial and Temporal Fluctuations of Nuclear Accident-Derived Tritium Concentrations in the River Waters of Eastern Fukushima, Japan

Tritium (3H, T) released into the atmosphere due to the accident at the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) in March of 2011 contaminated the terrestrial environment around the plant via precipitation and direct deposition, though there was no human radiation exposure (internal doses) to tritium. Most tritium exists in the chemical form of HTO and migrates in river water, groundwater, and evaporated water as media. After the accident, the temporal variation of 3H concentrations in the river waters of paddy fields and forests in Fukushima Prefecture from 2011 to 2020 was investigated and was approximately 5–6 times higher in both land-use areas than the background level in 2011. Afterward, high concentrations persisted for 2 years and had mostly settled to background levels after 3 years. The effective half-life of 3H in forest-type river waters was ~1.3 times longer than that in paddy-type waters, depending upon the different water retention capacities of the watersheds. Moreover, the 3H concentration in river water was significantly correlated with the accumulation of 137Cs in the catchment area, suggesting that the deposition of 137Cs and 3H is closely related.