Lateral variations of 134Cs and 228Ra concentrations in surface waters in the western North Pacific and its marginal sea (2018–2019): Implications for basin-scale and local current circulations

•We examined lateral 134Cs distribution off eastern Hokkaido in 2018 and 2019.•Surface 134Cs concentrations were 0.2–0.9 mBq/L on the FDNPP accident date.•FDNPP radiocesium re-entered until 2018 via basin-scale current circulations.•134Cs, 228Ra, and salinity data clarified current circulation and mixing patterns. Lateral variations in 134Cs, 137Cs, and 228Ra concentrations (activities) in surface waters off the northeastern coast of Japan, including the subarctic Oyashio Current, subtropical Kuroshio Current, East Sakhalin Current (ESC), and Soya Warm Current (SWC), were examined from July to October in 2018 and 2019. The 134Cs concentrations in the surface seawater samples, decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, were in the range of 0.2–0.9 mBq/L, whereas those transported to the study area with basin-scale currents varied between 0.2 and 1.3 mBq/L; this included the East Kamchatka Current (EKC) for 8 years following the FDNPP accident. Combined with the 228Ra (0.1–1.4 mBq/L) and 137Cs (1.0–1.7 mBq/L) concentrations and the salinity (32.4–34.6), the distribution of 134Cs concentrations indicated that FDNPP-derived radiocesium in this study area was partially transported with the EKC from the northeastern North Pacific. Based on a simple three-component (SWC, ESC, and EKC) mixing model that uses the salinity and 134Cs concentration, the fractions of the EKC showed a maximum value of ~ 0.4 off the southeastern Hokkaido, Japan.