Transport of 2011 Fukushima Dai-ichi Nuclear Power Plant accident-derived 137Cs to the tropical North Pacific

The impact and transport patterns of the 2011 Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident-derived radiocesium in the tropical North Pacific remain poorly understood. In this study, we investigated the latitudinal and vertical distributions of the concentrations (activities) of 137 Cs, 226 Ra, and 228 Ra in this tropical area (8°–10°N) in April–May 2025. We observed low 228 Ra concentrations in surface and mixed layers (< 0.5 mBq/L), indicating that the influence of seawater from the coastal area was not dominant, even near the eastern coast of the Philippines. 137 Cs concentrations at the surface in the tropical area exhibited moderate variations (1.0–1.2 mBq/L) with the highest concentrations observed at a depth of 200 m (1.7–2.3 mBq/L). Additionally, the surface and 200 m depth fractions of FDNPP accident-derived 137 Cs were 0.25–0.4 and 0.35–0.6, respectively. By 2025, FDNPP accident-derived 137 Cs was partially transported across the tropical North Pacific by westward currents (e.g., the North Equatorial Current). These findings can be used to elucidate current radionuclide dynamics in tropical areas.