Shipboard determination of radiocesium in seawater after the Fukushima accident: results from the 2011–2012 Russian expeditions to the Sea of Japan and western North Pacific Ocean

• 88 seawater samples from the western North Pacific and Sea of Japan were collected.
• Radiocesium was concentrated from the samples and determined on the board of a ship.
• In 2011–2012, 134Cs activity for 21 sampling stations ranged from <0.2 to 29 Bq m−3.
• 137Cs/134Cs activities ratio in seawater is estimated to be 0.99 on 15 March 2011.
• The Oyashio current provides clean water to the Kuroshio–Oyashio Interfrontal Zone.

A total of 88 seawater samples were collected during two Russian research expeditions (April–May 2011 and August–September 2012) to the Sea of Japan, the Oyashio Current region near Kuril Islands and the Kuroshio–Oyashio transition area in the western North Pacific Ocean. The observations were made aboard the R/V Pavel Gordienko and Akademik Shokalsky in order to study the impact of the Fukushima accident on radioactive contamination of the marine environment. On the board of a ship, the water samples were passed through filters to retain particles with the size of >1 micron. Cesium was extracted from the large volumes (100–3000 L) of the filtrated water using a selective fiber chemisorbent impregnated with copper ferrocyanide. Measurements of 134Cs and 137Cs activities in 83 samples of sorbents and 21 samples of filters were performed in the ship-based laboratory with a semiconductor HP-Ge detector. The quantified activity concentrations of dissolved radiocesium ranged from 1 Bq m−3 to 34 Bq m−3 for 137Cs and from 0.2 Bq m−3 to 29 Bq m−3 for 134Cs. Activity concentrations of 137Cs and 134Cs were strongly correlated with each other (r = 0.993, n = 59). The 137Cs/134Cs activities ratio in the Fukushima-derived radiocesium inventory for the study areas was deduced to be 0.99 ± 0.03 (on 15 March 2011) and the pre-Fukushima background level of 137Cs in seawater was estimated as 1.3 ± 0.3 Bq m−3. The lowest activities of both isotopes were determined in the western part of the Sea of Japan near the Russian coast, while the maximal levels were observed in the open Pacific Ocean, some 500–800 km offshore the Fukushima Dai-ichi Nuclear Power Plant. Contamination with 134Cs at a level of 0.3–2.6 Bq m−3 was registered in seawater samples collected in 2011 near the Kuril Islands and Kamchatka in the Oyashio Current region. During the period from April–May 2011 to August–September 2012, activity concentrations of 137Cs and 134Cs in surface waters had decreased for all seven stations repeatedly sampled in the study. A detailed observation of radiocesium distribution within the water column down to the depth of 200 m at nine stations from the Kuroshio–Oyashio Interfrontal Zone and Kuroshio Extension in 2012 revealed maximal activity concentrations of both cesium radionuclides in the 100–200 m depth layer. The average inventory of Fukushima-derived 137Cs in the top 200 m of the water column for the nine stations was estimated as 1.19 kBq m−2 (decay corrected to 15 March 2011) which is 4.6 times higher than the background value of 0.26 kBq m−2 expected for this depth. The monitoring results obtained in the study and relevant data published by others show that following the Fukushima accident, the Oyashio current acts as a provider of low-contaminated subarctic waters to the heavily contaminated Kuroshio–Oyashio mixed water region.

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