Subarctic-scale transport of ¹³⁴Cs to ocean surface off northeastern Japan in 2020

We studied the spatiotemporal variations in ¹³⁴Cs, ¹³⁷Cs, and ²²⁸Ra concentrations at the sea surface off southeastern Hokkaido, Japan (off-Doto region) from 2018 to 2022 using low-background γ-spectrometry. The ¹³⁴Cs concentrations in the off-Doto region, decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, exhibited wide lateral variation each year (e.g., 0.7-1.1 mBq/L in 2020). By studying the ²²⁸Ra concentrations and salinity, this variation was explained based on the current mixing patterns. Furthermore, the ¹³⁴Cs concentrations in the waters highly affected by the Oyashio Current (OYC) gradually increased from 2018 to 2020, and subsequently decreased in 2022. This implies that the water mass maximally contaminated with ¹³⁴Cs was transported back to the side of the Japanese islands 10 years after the FDNPP accident along with counter-clockwise currents (e.g., the OYC) in the northern North Pacific Ocean. The ¹³⁴Cs concentrations in the OYC-affected waters in the off-Doto region in 2020 were ~ 1/6 times those in the ¹³⁴Cs-enriched core of waters off the western American Coast in 2015, which can be ascribed to dilution via spatial dispersion during subarctic current circulation. Overall, we elucidated the ocean-scale subarctic current systems in the northwestern North Pacific Ocean, including water circulation timespans.