Cesium-134 and 137 activities in the central North Pacific Ocean after the Fukushima Dai-ichi nuclear power plant accident.

Surface seawater ¹³⁴Cs and ¹³⁷Cs samples were collected in the central and western North Pacific Ocean during the 1.5 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 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 Bqm^-3) were in the range of expected preexisting levels. Some samples north of the Hawaiian Islands (1.6-1.8 Bqm^-3) were elevated above the 18-month baseline established in surface seawater in Hawaii indicating that those might carry atmospheric fallout. The 18-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 Bqm^-3 (Station Aloha, 17 values). In contrast, samples collected between Japan and Hawaii contained ¹³⁴Cs activities in the range of 1-4 Bqm^-3 and ¹³⁷Cs levels were about 2-3 times above the pre-existing 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 1.5 yr at the main Hawaiian Islands due to the transport patterns across the Kuroshio and Kuroshio extension currents. [ABSTRACT FROM AUTHOR]