Leaked tritium reveals the source of 137Cs from the Fukushima Daiichi Nuclear Power Plant to the ocean

Minimizing the leakage of radioactive nuclides from nuclear facilities is essential for decommissioning and remediation following accidents. Countermeasures implemented following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident significantly reduced groundwater outflow from the site, which had been a primary pathway for ¹³⁷Cs discharge to the ocean. Nevertheless, ¹³⁷Cs continues to be released from the site, and since 2016, unexplained seasonal variations in ¹³⁷Cs concentrations have been observed in the marine environment. This study investigates a drainage channel named the "K discharge channel (Channel-K)," at the FDNPP, which receives both rainfall runoff and groundwater inflow. It is a potential pathway for land-to-sea ¹³⁷Cs transport, as it exhibits seasonal variations similar to those in coastal seawater. Using the leaked ³H from storage tanks containing treated radioactive water as a tracer, we applied a watershed hydrological tracer approach and the rainfall method to quantify the fractions of discharge for surface runoff (including overland flow and roof drainage) and baseflow entering the channel. The results indicate that roof drainage passing through the reactor building is the primary source of ¹³⁷Cs, accounting for 53 % of the flux entering the Channel-K. Seasonal variations in ¹³⁷Cs concentrations are driven by temperature-dependent baseflow and rainfall-driven surface flow. This study demonstrates the applicability of ³H-based tracer hydrology for identifying contaminant pathways and provides new insights into source characterization for sustainable remediation of nuclear and legacy-contaminated sites.