[M-AG34] Radioisotope migration: New development for radioisotope migration study related to 1F NPP accident

[MAG34-07] Tracer study using ³H in groundwater by post-accident applied to runoff analysis at the Fukushima Daiichi Nuclear Power Plant

*Hikaru Sato¹, Yuichi Onda², Daisuke Tsumune³ (1.Faculty of Life and Environmental Sciences, University of Tsukuba, 2.Center for Research in Isotopes and Environmental Dynamics, 3.Central Research Institute of Electric Power Industry)

Keywords:Cs-137, H-3 tracer, surface runoff, base flow, Fukushima Daiichi Nuclear Accident

Tritium (³H) is often used as a tracer to estimate the age of groundwater and the interaction between surface water and groundwater flow. ³H from the tank area originated in the post-Fukushima Daiichi Nuclear Accident in 2013 and 2014 migrated downstream through groundwater and was detected in wells and drainage channels. The drainage channel collects rainwater from the site and discharges it into the port. This causes higher caesium-137 (¹³⁷Cs) concentrations in seawater near drainage channels outlet and understanding the factors that form the ¹³⁷Cs flux in drainage channels is urgent. The ³H concentration in the channel water is high and shows similar levels of concentrations with weighted averaged them in the wells during low rainfall, and tends to decrease during rainfall. Therefore, this study calculated the fractions of discharge by surface runoff and base flow flowing into the drainage channel utilizing this relationship and ³H in groundwater as a tracer. Furthermore, we attempted to evaluate the estimated ¹³⁷Cs concentration in surface water using the calculated fractions.
The main data used in the analysis for the period 2015-2020 were obtained from the website of Tokyo Electric Power Company Holdings. The concentrations of ³H, ¹³⁷Cs, and β in the drainage canal water showed that the water was a mixture of two types of water. Thus, a binary mixing model using ³H concentrations and discharge rates could be used to calculate the fractions of surface runoff and base flow. Although the concentration of ¹³⁷Cs in the drainage channel tended to increase as the contribution of surface flow increased, the estimated concentration in the surface flow was found to be independent of the contribution of surface flow. In addition, the estimated concentrations in surface flow, which were several hundred Bq/L until 2016, decreased to tens of Bq/L in 2020, however, no decrease in concentrations was observed after 2017. Thus, ³H originating in the post-accident can be applied as a tracer to the runoff analysis at the site. This is expected to lead to the further development of studies on the dynamics of radionuclide concentration.