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.

日本东北沿海近海地表水中¹³⁴ Cs，¹³⁷ Cs和²²⁸ Ra浓度（活性）的横向变化，包括北极北极亚矢潮，亚热带黑潮洋流，东萨哈林海流（ESC）和大豆暖流（SWC）分别在2018年和7月至10月和2018年7月进行了检查。经福岛第一核电站事故（FDNPP）事故日期的衰减校正后，地表海水样品中的¹³⁴ Cs浓度在0.2-0.9范围内。 mBq / L，而那些流域规模流向研究区的物质在0.2至1.3 mBq / L之间变化；在FDNPP事故发生后的8年中，包括东部堪察加半岛海流（EKC）。与²²⁸结合Ra（0.1–1.4 mBq / L）和¹³⁷ Cs（1.0–1.7 mBq / L）浓度和盐度（32.4–34.6），¹³⁴ Cs浓度的分布表明，该研究区中FDNPP衍生的放射性铯与来自东北太平洋的EKC。基于使用盐度和¹³⁴ Cs浓度的简单三组分（SWC，ESC和EKC）混合模型，EKC的馏分在日本北海道东南部显示最大值约为0.4。