Temporal and spatial variations of 134Cs and 137Cs levels in the Sea of Japan and Pacific coastal region: Implications for dispersion of FDNPP-derived radiocesium.

To investigate the dispersion of Fukushima Dai-ichi Nuclear Power Plant (FDNPP)-derived radiocesium in the Sea of Japan and western Pacific coastal region and determine the sources of radiocesium in these areas, we examined the temporal and spatial variations of 134Cs and 137Cs concentrations (activities) during 2011-2016 in seawaters around the western Japanese Archipelago, particularly in the Sea of Japan. In May 2013, the surface concentration of 134Cs was ∼0.5 mBq/L (decay-corrected to March 11, 2011), and that of 137Cs exceeded the pre-accident level in this study area, where the effects of radiocesium depositions just after the FDNPP accident disappeared in surface waters in October 2011. Subsequently, radiocesium concentrations gradually increased during 2013-2016 (∼0.5-1 mBq/L for 134Cs), exhibiting approximately homogeneous distributions in each year. The temporal and spatial variations of 134Cs and 137Cs concentrations indicated that FDNPP-derived radiocesium around the western Japanese Archipelago, including the Sea of Japan, has been supported by the Kuroshio Current and its branch, Tsushima Warm Current, during 2013-2016. However, in the Sea of Japan, the penetration of 134Cs was limited to depths of less than ∼200 m during three years following the re-delivery of FDNPP-derived radiocesium.

Simple 40K removal by acidified water leaching for estimating low levels of radiocesium in fishery products following Fukushima Dai-ichi Nuclear Power Plant accident.

We developed a simple method for 40K removal by acidified water leaching combined with ammonium phosphomolybdate coprecipitation, for estimating low levels of radiocesium in Japanese flounder. 40K removal from ashed samples was approximately 98%, with the radiocesium yield of ~98%. The treatment reduced the γ-spectral baseline to ~1/5 at ground and ~3/100 at underground levels (1.04 and 0.11 cph/keV, respectively). Faint signal of 134Cs, which is embedded in the Compton continuum of 40K in an untreated sample, was detected.

Migration of the FDNPP-derived 134Cs and 137Cs along with 226Ra and 228Ra concentrations across the northwestern North Pacific Ocean.

We examined lateral distributions of 134Cs, 137Cs, 226Ra, and 228Ra in the surface seawaters around the Kuril Islands and the Kamchatka Peninsula in the northwestern North Pacific Ocean during June 2014. The sampling area included three water current areas, the Oyashio Current, the current from the Okhotsk Sea, and the coastal current along the east Kamchatka Peninsula. 226Ra and 228Ra distributions differed along the three currents. Low levels of 134Cs were detected in the surface waters of the Oyashio Current (0.09-0.35 mBq/L), but it was <∼0.1 mBq/L at the surface along the other two currents. This indicates that the distribution of Fukushima Dai-ichi Nuclear Power Plant (FDNPP)-derived radiocesium in surface waters off the Kamchatka and along the Kuril Islands is predominantly governed by the Oyashio current system.

Delivery mechanism of (134)Cs and (137)Cs in seawater off the Sanriku Coast, Japan, following the Fukushima Dai-ichi NPP accident.

To assess the delivery mechanism of radiocesium emitted from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), we examined vertical profiles of (134)Cs, (137)Cs, and (228)Ra concentrations and the (228)Ra/(226)Ra ratio in the water columns off the Sanriku Coast in the northwestern Pacific Ocean, in July 2012, along with their surface lateral variations in July 2009. Radiocesium concentrations exhibited maximum peaks (3-5 mBq/L for (134)Cs) at depths of 100-200 m, accompanied by high (228)Ra concentrations (0.6-0.8 mBq/L) in comparison with shallower depths (∼0.4 mBq/L). Taking the circulation patterns of currents in the area into account, it was inferred that radioactive depositions were supplied to the (228)Ra-rich Tsugaru Warm Current Water (TWCW) in the offshore area of the Sanriku Coast following the FDNPP accident, and that after the spring of 2011, this water (∼26.5σθ) was covered by lower density surface water, which helped intrude its way to depths of 100-200 m.

228Ra/226Ra ratio and 7Be concentration in the Sea of Japan as indicators for water transport: comparison with migration pattern of Fukushima Dai-ichi NPP-derived 134Cs and 137Cs.

To assess the migration patterns of radiocesium emitted from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), we analyzed (228)Ra/(226)Ra ratios and (7)Be concentrations and compared them with (134)Cs and (137)Cs concentrations in seawater samples collected within the Sea of Japan before and after the FDNPP accident (i.e., during the period 2007-2012) using low-background γ-spectrometry. The (228)Ra/(226)Ra ratios in surface waters exhibited lateral and seasonal variations, reflecting the flow patterns of surface water. This indicates the transport patterns of the FDNPP-derived radiocesium by surface water. Cosmogenic (7)Be (half-life: 53.3 d) exhibited markedly high concentrations (5-10 mBq/L) at depths shallower than 50 m, with concentrations decreasing steeply (0.2-2 mBq/L) at depths of 50-250 m. The distribution of (7)Be concentrations suggests that the downward delivery of the FDNPP-derived radiocesium to below 50 m depth was negligible for a few months prior to its removal from the Sea of Japan.

Lateral variation of 134Cs and 137Cs concentrations in surface seawater in and around the Japan Sea after the Fukushima Dai-ichi Nuclear Power Plant accident.

A total of 82 surface seawater samples was collected in the Japan Sea and the southwestern Okhotsk Sea before and after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. Analysis of (134)Cs and (137)Cs concentrations using low-background γ-spectrometry revealed that the (137)Cs concentration of the samples collected in June 2011 was 1.5-2.8mBq/L, which is approximately 1-2 times higher than the pre-accident (137)Cs level, while the (134)Cs concentration was less than detectable to 1mBq/L. In addition to (134)Cs being clearly detected (∼1mBq/L), (137)Cs concentration in water samples from the northeastern Japan Sea (2-2.8mBq/L) was also higher than that from the coast in the southwestern Japan Sea (∼1.5mBq/L). These higher concentrations in the northeastern Japan Sea could be ascribed to the atmospheric transport of nuclides from the FDNPP as aerosols and subsequent transport and dilution after delivery to the sea surface.

134Cs and 137Cs activities in coastal seawater along Northern Sanriku and Tsugaru Strait, northeastern Japan, after Fukushima Dai-ichi Nuclear Power Plant accident.

A total of 37 seawater samples were collected at 10 sites along the coastline of the Northern Sanriku and Tsugaru Strait, 250-450 km north of the Fukushima Dai-ichi Nuclear Power Plant in April-December 2009 and May-June 2011, and analyzed for (134)Cs and (137)Cs activities using low-background γ-spectrometry. The (134)Cs and (137)Cs activities measured in these samples in May 2011 were found to be 2-3 mBq/L and 2.5-4 mBq/L, respectively. By June, these values had decreased by 25-45%/month and 5-30%/month, respectively. These results can be plausibly explained by surface infusion of these isotopes into the sea by atmospheric transport from Fukushima and their subsequent reduction by water migration to off-shore and deeper regions.

Seasonal variations in 228Ra/226Ra ratio within coastal waters of the Sea of Japan: implications for water circulation patterns in coastal areas.

In this study, low-background gamma-spectrometry was used to determine the (228)Ra/(226)Ra ratio of 131 coastal water samples from various environments around Honshu Island, Japan (mainly around Noto Peninsula) at 1-3 month intervals from April 2003 until September 2005. Spatial variation in (228)Ra/(226)Ra ratios was also assessed by analyzing 34 coastal water samples from five areas within the Sea of Japan during May and June 2004. The (228)Ra/(226)Ra ratio of coastal water from all sites around Noto Peninsula shows seasonal variation, with minimum values during summer ((228)Ra/(226)Ra=0.7) and maximum values during autumn-winter ((228)Ra/(226)Ra=1.7-2). This seasonal variation is similar to that recorded for coastal water between Tsushima Strait and Noto Peninsula. The measured lateral variation in (228)Ra/(226)Ra ratios within coastal water between Tsushima Strait and Noto Peninsula is only minor (0.5-0.7; May-June 2004). Coastal waters from two other sites (Pacific shore and Tsugaru Strait, north Honshu) show no clear seasonal variation in (228)Ra/(226)Ra ratio. These measured variations in (228)Ra/(226)Ra ratio, especially the temporal variations, have important implications for seasonal changes in patterns of coastal water circulation within the Sea of Japan.

Seasonal variation of 228Ra/226Ra ratio in seaweed: implications for water circulation patterns in coastal areas of the Noto Peninsula, Japan.

To examine water circulation patterns of coastal water, 72 seaweed (Sargasso) samples and 27 coastal water samples were collected from coastal areas of the Noto Peninsula, Japan, during the period from December 1998 to June 2002. The (228)Ra and (226)Ra activities of those samples were measured by low-background gamma-ray spectrometry. There was a wide range of activities of (228)Ra (0.5-2Bq/kg-fresh) and (226)Ra (0.5-1.2Bq/kg-fresh) in the Sargasso samples. The (228)Ra/(226)Ra activity ratio of Sargasso samples exhibited seasonal variation with minimum values in June ((228)Ra/(226)Ra= approximately 1) and maximum values in December (1.5-2.5), which was mainly governed by changes in (228)Ra activity. It is also notable that the seasonal variation of the (228)Ra/(226)Ra ratio of Sargasso is in approximate agreement with that of the ambient coastal water. Sargasso samples appear to have retained the (228)Ra/(226)Ra ratio of the ambient coastal waters, and the temporal variations in that ratio provide insight into seasonal changes in water circulation in the Noto Peninsula coastal area.

Residence times of global weapons testing fallout 237Np in a grassland soil compared to 239 + 240Pu, 241Am, and 137Cs.

The vertical distribution of weapons testing fallout 237Np has been determined in an undisturbed grassland soil (Alfisol). By using a compartmental model for multi-layered soils, the mean residence half-times of 237Np in each soil layer were calculated and compared with results on weapons fallout 239 + 240Pu, 241Am and 137Cs in the same soil. The results show that the mobility of 237Np was in most soil horizons either equal or slightly enhanced as compared to that of Pu, Am, and radiocesium.

Breeding cycles and fecal gonadal steroids in the brown dipper Cinclus pallasii.

Breeding cycles and fecal gonadal steroids were investigated in free-living brown dippers, Cinclus pallasii. The brown dippers were marked with colored leg bands, and their behavior was observed by a field scope. The breeding season was divided into six stages according to the behavior observed; winter territory-defending, nest-building, copulation, incubation, nestling, and fledgling stages. Fecal pellets on the stones in their territories were collected and the levels of immunoreactive estradiol-17 beta (E2) and testosterone (T) in 10 mg feces were measured by radioimmunoassay. Levels of fecal E2 were high only during the copulation stage, which lasted for a few weeks, in all females of the three pairs observed. In contrast, T levels in the feces of males increased in late winter when intensive territory-defending behavior was observed, and remained high until the end of the breeding season. T levels during the nest-building stage and the copulation stage were similar to those during other nesting stages. When the birds breed only once a year, the duration of high levels of T was shorter than that of the pair with the second clutch. T levels in the feces of females showed a similar pattern to E2 levels.