Cesium-134 and 137 activities in the central North Pacific Ocean after the Fukushima Dai-ichi Nuclear Power Plant accident

Abstract. Surface seawater ¹³⁴Cs and ¹³⁷Cs samples were collected in the central and western North Pacific Ocean during the 2 yr after the Fukushima Dai-ichi Nuclear Power Plant accident to monitor dispersion patterns of these radioisotopes towards the Hawaiian Islands. In the absence of other recent sources and due to its short half-life, only those parts of the Pacific Ocean would have detectable ¹³⁴Cs values that were impacted by Fukushima releases. Between March and May 2011, ¹³⁴Cs was not detected around the Hawaiian Islands and Guam. Here, most ¹³⁷Cs activities (1.2–1.5 Bq m^–3) were in the range of expected preexisting levels. Some samples north of the Hawaiian Islands (1.6–1.8 Bq m^–3) were elevated above the 23-month baseline established in surface seawater in Hawaii indicating that those might carry atmospheric fallout. The 23-month time-series analysis of surface seawater from Hawaii did not reveal any seasonal variability or trends, with an average activity of 1.46 ± 0.06 Bq m^–3 (Station Aloha, 18 values). In contrast, samples collected between Japan and Hawaii contained ¹³⁴Cs activities in the range of 1–4 Bq m^–3, and ¹³⁷Cs levels were about 2–3 times above the preexisting activities. We found that the southern boundary of the Kuroshio and Kuroshio extension currents represented a boundary for radiation dispersion with higher activities detected within and north of the major currents. The radiation plume has not been detected over the past 2 yr at the main Hawaiian Islands due to the transport patterns across the Kuroshio and Kuroshio extension currents.