Friday, March 22, 2013

北大平洋に見られる福島からの汚染 Contamination from Fukushima found in North Pacific

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 1.5 yr after the Fukushima Dai-ichi nuclear power plant accident to monitor dispersion patterns of these radioisotopes towards the Hawaiian Islands.
要約: 福島第一原発の事故から1年半後、ハワイ島に向かう放射性核種の拡散パターンをモニタリングするため、北太平洋の中央と西の海でセシウム134と137を含む水面の海水を収集しました。

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 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 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 Bq m−3 (Station Aloha, 17 values).
ハワイの海水表面の18ヶ月の期間の分析からは、平均で1立方あたり1.46 ± 0.06bqで、季節による変異性や傾向は見られませんでした。(ステーションアロハ17件-表の青色ひし形点)

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 1.5 yr at the main Hawaiian Islands due to the transport patterns across the Kuroshio and Kuroshio extension currents.

Simulations on the long-term dispersal of 137Cs released into the Pacific Ocean off Fukushima by Helmholtz Centre for Ocean Research


 Our study deals with one of the consequences of the disastrous earthquake and subsequent tsunami that hit Japan in March 2011.

Caused by the explosions and meltdowns in the Fukushima Dai-ichi nuclear power plants, large amounts of radioactive material, including long-living isotopes such as caesium-137, were released into the environment. Contaminated waters entered the Pacific Ocean by atmospheric deposition and direct discharge of coolant in the weeks after the catastrophe.


In this study we investigate the long-term fate of these waters in the Pacific Ocean, using numerical model experiments. The model simulates the ocean currents with very high resolution; it thus captures much of the intense eddying flows along the Kuroshio.


To simulate the dispersal of the contaminated water by the ocean currents, we use an idealized tracer, behaving like a dye with a half live of 30 years, which we injected into the coastal waters off Fukushima. We want to emphasize that our approach is restricted to the physical spreading, and does not account for possible biological effects, for example, adhesion of isotopes to sinking organic matter. Neither does our model assimilate any ocean data, therefore we do not account for the actual state of the local currents at the time of the release. However the initial state turned out to be of minor importance for the tracer spreading after two to three years.


The following animation shows the spreading of our dye at the Ocean's surface. The color represents the concentration of the tracer, relative to the initial concentration in the coastal area off Fukushima at the end of the injection period.

The tracer invades the highly energetic Kuroshio regime within the first weeks after the release and starts spreading meridionaly due to the eddies dominating the flow field in this area.


After one year, maximum concentrations are diluted by two orders of magnitude and already crossed the dateline, further heading east.


The southern edge of the tracer cloud reaches the Hawaiian islands after about two years, while the northern edge has begun to enter the Bering Sea.


Meanwhile, concentrations near Japan have been dropping significantly due to replacement by less contaminated waters through the Kuroshio, separating the main cloud into a northern and a southern part.

After four years, the maximum concentration has dropped by three orders of magnitude, and the entire northern Pacific is filled with a certain amount of tracer.


When the peak concentration reaches the North American coast after about five to six years, it has been diluted by four orders of magnitude.

The concentration levels from now on tend to homogenize over the entire basin.


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