Abstract
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.
Highlights
The Tohoku earthquake and subsequent tsunami on 11 March 2011 led to damages at the Fukushima Daiichi Nuclear Power Plant (F1-NPP) on the east coast of Japan
Global and local fallout from atmospheric nuclear weapons tests conducted in the 20th century was the major source of 137Cs in the North Pacific Ocean (Aarkrog, 2003)
In 2000, measured and estimated surface seawater 137Cs activities across the North Pacific were in the range of 1.7– 2.8 Bq m−3 (Hirose and Aoyama, 2003a)
Summary
The Tohoku earthquake and subsequent tsunami on 11 March 2011 led to damages at the Fukushima Daiichi Nuclear Power Plant (F1-NPP) on the east coast of Japan. Significant amounts of radionuclides were released to the atmosphere and, by direct discharge or leakage, to the ocean. Radioactive isotopes were released to the atmosphere from 12 March 2011 onward with a peak on 15 March (e.g., Stohl et al, 2012). On the Pacific Islands (e.g., Hawaii, Guam, Commonwealth of the Northern Mariana Islands), an increase in gross beta activity in aerosols and trace amounts of radioactive iodine, cesium, and tellurium on air filters were identified between 19 and 23 March 2011 (EPA, 2011)
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