Zonal and vertical transports of Fukushima-derived radiocesium in the subarctic gyre of the North Pacific until 2014.

The Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident in March 2011 resulted in serious radiocesium contamination of the North Pacific Ocean. Most of the radiocesium was dissolved in seawater and transported by surface currents and subduction of mode waters. Within several years after the accident, a high-concentration water plume of the FNPP1-derived radiocesium at the sea surface had been transported from Japan to the North American continent across the subarctic gyre of the North Pacific Ocean. We measured vertical profiles of dissolved radiocesium along the nominal 47°N zonal line across the North Pacific subarctic gyre twice, in summer 2012 and summer 2014. Using these data and published data, we quantitatively discussed the zonal and vertical transports of the water plume until 2014. The FNPP1-derived radiocesium remained in the surface layer shallower than 200 m, which is the approximate winter mixed-layer depth in the western subarctic gyre. The mean penetration depth did not change between 2012 and 2014. The highest concentration was observed at 180°W in 2012 and at 151°W in 2014, which suggests that the zonal transport speed of the water plume in the eastern subarctic gyre was about 3.8 cm s-1. By combining the data from the zonal line in 2014 and a nominal 152°W meridional line in 2015, we elucidated the three-dimensional size of the high-concentration water plume in summer 2014. The total inventory of the FNPP1-derived radiocesium in the subarctic North Pacific Ocean, decay-corrected to the accident date, was estimated to be 12.0 ± 2.4 PBq.

Spatiotemporal change of cesium-137 in the Pacific coast of Tohoku, Japan: The mussel watch approach.

We measured radiocesium in mussel tissue collected from the Pacific coast of Tohoku from 2011 to 2015 to investigate the temporal and spatial dynamics of radiocesium in the coastal area. Radioactive 137Cs was detected in all the samples collected in 2011, but it was not found in samples from localities north of Sendai after 2012. In contrast, 137Cs was detected in many sites in the Fukushima area even from 2012 to 2015. The fluctuation of 137Cs concentration in mussel tissue seems to reflect the 137Cs concentration in suspended particles in the seawater, suggesting that there was an influx of soil deposition and resuspension of seabed sediment. These results suggest that the 137Cs concentration in mussel tissue sensitively indicates the 137Cs concentration in the environment, and that the "mussel watch" approach is an effective way to understand the dynamics of radiocesium concentrations in coastal areas.

Observation of Dispersion in the Japanese Coastal Area of Released 90Sr, 134Cs, and 137Cs from the Fukushima Daiichi Nuclear Power Plant to the Sea in 2013.

The March 2011 earthquake and tsunami resulted in significant damage to the Fukushima Daiichi Nuclear Power Plant (FDNPP) and the subsequent release of radionuclides into the ocean. Here, we investigated the spatial distribution of strontium-90 (90Sr) and cesium-134/cesium-137 (134, 137Cs) in surface seawater of the coastal region near the FDNPP. In the coastal region, 90Sr activity was high, from 0.89 to 29.13 mBq L-1, with detectable FDNPP site-derived 134Cs. This indicated that release of 90Sr from the power plant was ongoing even in May 2013, as was that of 134Cs and 137Cs. 90Sr activities measured at open ocean sites corresponded to background derived from atmospheric nuclear weapons testing fallout. The FDNPP site-derived 90Sr/137Cs activity ratios in seawater were much higher than those in the direct discharge event in March 2011, in river input, and in seabed sediment; those ratios showed large variability, ranging from 0.16 to 0.64 despite a short sampling period. This FDNPP site-derived 90Sr/137Cs activity ratio suggests that these radionuclides were mainly derived from stagnant water in the reactor and turbine buildings of the FDNPP, while a different source with a low 90Sr/137Cs ratio could contribute to and produce the temporal variability of the 90Sr/137Cs ratio in coastal water. We estimated the release rate of 90Sr from the power plant as 9.6 ± 6.1 GBq day-1 in May 2013 on the basis of the relationship between 90Sr and 137Cs activity (90Sr/137Cs = 0.66 ± 0.05) and 137Cs release rate.

Radiocesium in North Pacific coastal and offshore areas of Japan within several months after the Fukushima accident.

We measured activity concentrations of radiocesium (134Cs and 137Cs) in seawater samples collected in North Pacific coastal and offshore areas of Japan within several months after the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident in March 2011, including archived seawater samples whose radiocesium concentrations were previously reported to be below detection limits. By merging 329 new data with published results, we succeeded in reconstructing the temporal changes in activity concentrations and inventories of FNPP1-derived radiocesium in the coastal and offshore areas within several months after the accident for the first time. 137Cs directly-discharged from the FNPP1 was transported eastward within the coastal area about 250 km from the FNPP1 during two months after the accident due to complex movements of coastal surface currents. The eastward speed was calculated to be about 5 cm s-1. Eastward transport of 137Cs to the offshore area more than 600 km away from the FNPP1 along the north flank of the Kuroshio Extension Current was faster (about 9 cm s-1) and probably more dominant in the eastward transport. The total inventory of directly-discharged 137Cs in early April 2011 was estimated to be 3-6 PBq approximately, which agrees with the smaller estimates in previous studies (2-6 PBq).

Fukushima-derived radiocesium in the western North Pacific in 2014.

In 2014, we measured activity concentration of radiocesium in the western North Pacific Ocean. In the north of Kuroshio Front high activity concentration of Fukushima-derived radiocesium in surface mixed layer in 2012 had been transported eastward by 2014. In the south of the front we found a radiocesium subsurface maximum in 200-600 m depth, which was similar to that observed in 2012. The subsurface maximum spread southward from 18°N to 15°N between 2012 and 2014, which suggests spreading of Fukushima-derived radiocesium into the whole western subtropical area by 2014 due to formation and subduction of the subtropical mode water.

Determination of strontium-90 from direct separation of yttrium-90 by solid phase extraction using DGA Resin for seawater monitoring.

It is important for public safety to monitor strontium-90 in aquatic environments in the vicinity of nuclear related facilities. Strontium-90 concentrations in seawater exceeding the background level have been observed in accidents of nuclear facilities. However, the analytical procedure for measuring strontium-90 in seawater is highly demanding. Here we show a simple and high throughput analytical technique for the determination of strontium-90 in seawater samples using a direct yttrium-90 separation. The DGA Resin is used to determine the abundance of strontium-90 by detecting yttrium-90 decay (beta-emission) in secular equilibrium. The DGA Resin can selectively collect yttrium-90 and remove naturally occurring radionuclides such as (40)K, (210)Pb, (214)Bi, (238)U, and (232)Th and anthropogenic radionuclides such as (140)Ba, and (140)La. Through a sample separation procedure, a high chemical yield of yttrium-90 was achieved at 95.5±2.3%. The result of IAEA-443 certified seawater analysis (107.7±3.4 mBq kg(-1)) was in good agreement with the certified value (110±5 mBq kg(-1)). By developed method, we can finish analyzing 8 samples per day after achieving secular equilibrium, which is a reasonably fast throughput in actual seawater monitoring. By processing 3 L of seawater sample and applying a counting time of 20 h, minimum detectable activity can be as low as 1.5 mBq kg(-1), which could be applied to monitoring for the contaminated marine environment. Reproducibility was found to be 3.4% according to 10 independent analyses of natural seawater samples from the vicinity of the Fukushima Daiichi Nuclear Power Plant in September 2013.

Depth profile and mobility of (129)I and (137)Cs in soil originating from the Fukushima Dai-ichi Nuclear Power Plant accident.

The (129)I derived from the FDNPP accident were clearly identified near the surface and showed a trend of rapid decrease with depth. The FDNPP (129)I and (137)Cs was 51.6 ± 1.7 mBq cm(-2) and 88.2 ± 27.1 kBq cm(-2) (average of four cores inventory) respectively. On average, 91% of the FDNPP (129)I existed within the top 5 g cm(-2) and 98% within the top 10 g cm(-2) and average of 100% of the FDNPP (137)Cs existed within the top 5 g cm(-2). From the observation of the temporal variation of depth profiles from the same upland field (Kawauchi village, 20 km away from the FDNPP to the southwest direction), downward migration rates of 0.81 ± 0.32 g cm(-2) yr(-1) for the FDNPP (129)I and 0.19 ± 0.17 g cm(-2) yr(-1) for the FDNPP (137)Cs were estimated. A simple diffusion model was introduced to evaluate the downward mobility of the FDNPP-derived (129)I and (137)Cs. The apparent diffusion coefficients D of 0.0086 ± 0.0034 and 0.0011 ± 0.0010 g(2) cm(-)(4) d(-)(1) were obtained for (129)I and (137)Cs, respectively. These values might be representative for Haplic Gray lowland soils in near the steady state under humid temperate climate.

Determination of picomolar beryllium levels in seawater with inductively coupled plasma mass spectrometry following silica-gel preconcentration.

A robust and rapid method for the determination of natural levels of beryllium (Be) in seawater was developed to facilitate mapping Be concentrations in the ocean. A solid-phase extraction method using a silica gel column was applied for preconcentration and purification of Be in seawater prior to determination of Be concentrations with inductively coupled plasma mass spectrometry (ICP-MS). Be was quantitatively adsorbed onto silica gel from solutions with pH values ranging from 6.3 to 9, including natural seawater. The chelating agent ethylenediamine tetraacetic acid was used to remove other ions in the seawater matrix (Na, Mg, and Ca) that interfere with the ICP-MS analysis. The reproducibility of the method was 3% based on triplicate analyses of natural seawater samples, and the detection limit was 0.4 pmol kg(-1) for 250 mL of seawater, which is sufficient for the analysis of seawater in the open ocean. The method was then used to determine the vertical profile of Be in the eastern North Pacific Ocean, which was found to be a recycled-type profile in which the Be concentration increased with depth from the surface (7.2 pmol kg(-1) at <200 m) to deep water (29.2 pmol kg(-1) from 3500 m to the bottom).

Assessment of individual radionuclide distributions from the Fukushima nuclear accident covering central-east Japan.

A tremendous amount of radioactivity was discharged because of the damage to cooling systems of nuclear reactors in the Fukushima No. 1 nuclear power plant in March 2011. Fukushima and its adjacent prefectures were contaminated with fission products from the accident. Here, we show a geographical distribution of radioactive iodine, tellurium, and cesium in the surface soils of central-east Japan as determined by gamma-ray spectrometry. Especially in Fukushima prefecture, contaminated area spreads around Iitate and Naka-Dori for all the radionuclides we measured. Distributions of the radionuclides were affected by the physical state of each nuclide as well as geographical features. Considering meteorological conditions, it is concluded that the radioactive material transported on March 15 was the major contributor to contamination in Fukushima prefecture, whereas the radioactive material transported on March 21 was the major source in Ibaraki, Tochigi, Saitama, and Chiba prefectures and in Tokyo.