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

Influence of precipitation on the daily beryllium-7 (7Be) activity concentration in the atmospheric surface layer

The paper presents the monitoring results of the temporal variability of daily ⁷Be activity concentration in the atmospheric surface layer at Sevastopol in 2011-2020. The ⁷Be activity concentration in individual samples of atmospheric aerosols varies from 0.1 to 13.3 mBq m^-3 and averages 4.0 ± 2.0 Bq m^-3. Higher ⁷Be activity concentrations have been observed during the period from May to August while lower concentrations have been measured from December to January. Quantitative estimates of the influence of precipitation (amount, duration, intensity) on the temporal variability of ⁷Be activity concentration have been obtained. It has been found that daily ⁷Be activity concentration decrease by 2-82% on the first day with precipitation. It has been shown that an increase in precipitation duration and a decrease in its intensity lead to a more significant decrease in the daily ⁷Be activity concentration. The estimates of the scavenging coefficient have been obtained; the average value is 0.6 ± 1.0 h^-1. An increase in the precipitation intensity or amount is accompanied by a decrease in the ⁷Be scavenging coefficient. Mean 0.5-folding and residence times of ⁷Be activity concentration in the atmosphere during a moderate rain event are 2.9 ± 2.2 and 15.4 ± 13.6 h, respectively. The recovery of ⁷Be activity concentration in the atmosphere after precipitation has been investigated. It has been found that this process takes 1-2 days. The mean value of the reload coefficient is 0.94 ± 0.34 d^-1. The relationship between the value of the reload coefficient and local meteorological parameters (air temperature, relative humidity, atmospheric pressure, wind speed, surface net solar radiation flux) has been investigated. No statistically significant correlations at a 95% confidence level between the reload coefficient and the considered meteorological parameters have been revealed. Parameterization describing a decrease in the daily ⁷Be activity concentration in the atmosphere due to precipitation and its recovery during the precipitation-free period has been introduced.

Circulation paths of 134Cs in seawater southwest of Japan in 2018 and 2019

The spatial variations of low-level ¹³⁴Cs concentrations (activities) in seawater off the Japanese Archipelago, particularly in the eastern East China Sea (ECS), in 2018 and 2019 were examined. The ¹³⁴Cs concentrations, decay-corrected to the date of the Fukushima Dai-ichi Nuclear Power Plant accident, in seawaters were 0.5-2.0 mBq/L. High ¹³⁴Cs concentrations (1.1-2.0 mBq/L) of the Kuroshio Current subsurface water (densities of 25-26σ_θ) in the eastern ECS could indicated the contribution of the subtropical mode water from the Pacific Ocean side, and total column inventories were 330-426 Bq/m². In contrast, as indicated by the same ¹³⁴Cs concentration level at the surface of the eastern ECS and Sea of Japan, larger portions of the subsurface waters remained in the ECS and Yellow Sea side in response to the existence of the shallow Tsushima Strait.

Vertical profiles of Fukushima Dai-ichi NPP-derived radiocesium concentrations in the waters of the southwestern Okhotsk Sea (2011-2017)

We examined the vertical ¹³⁴Cs and ¹³⁷Cs concentration profiles in the southwestern Okhotsk Sea in 2011, 2013, and 2017. In June 2011, atmospheric deposition-derived ¹³⁴Cs from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) was detected at depths of 0-200 m (0.06-0.6 mBq/L). In July 2013, ¹³⁴Cs detected at depths of 100-200 m (∼0.05 mBq/L) was ascribed to the transport of low-level ¹³⁴Cs-contaminated water and/or the convection of radioactive depositions (<0.03 mBq/L at depths of 0-50 m). In July 2017, ¹³⁴Cs was detected in water samples at depths above 300 m (0.03-0.05 mBq/L), and the inventory, decay-corrected to the FDNPP accident date, exhibited its maximum value (85 Bq/m²) during this period. Combining temperature-salinity data with the concentrations of global fallout-derived ¹³⁷Cs led to a plausible explanation for this observation, which is a consequence of re-entry of FDNPP-derived radiocesium through the Kuril Strait from the northwestern North Pacific Ocean to the Okhotsk Sea and subsequent mixing with the south Okhotsk subsurface layer until 2017.

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.

Fukushima-derived radioactivity measurements in Pacific salmon and soil samples collected in British Columbia, Canada

Despite the many studies that have shown minimal health risks to individuals living outside of Japan following the Fukushima Nuclear Accident, there are persisting concerns regarding the consumption of Pacific seafood that may be contaminated with radioactive species from Fukushima. To address these concerns, the activity concentrations of anthropogenic 134Cs and 137Cs, as well as naturally occurring 40K, were measured in Pacific salmon collected from Kilby Provincial Park, British Columbia (BC), in 2013 and from the Quesnel River, BC, in 2014 using low-background gamma-ray spectroscopy. In addition, soil samples and a single roof-debris sample were collected and analysed to provide a record of Fukushima-derived contamination in BC. Cesium-134 was not detected in the salmon samples. Cesium-137 was not detected in any of the sockeye or chum samples, although it was detected in all of the Chinook samples. The weighted average (±1σ) 137Cs activity concentration in the Chinook salmon collected in 2013 and 2014 was 0.23 (3) and 0.20 (3) Bq/kg fresh weight, respectively. A conservative annual dose estimate for an adult who consumes the average Canadian quantity of seafood per year, contaminated with radiocesium at the maximum concentrations measured in this campaign, was calculated to be 0.054 μSv per year. Cesium-134 was detected in all but two of the soil samples. A weak positive correlation was observed between presence of 134Cs and of 7Be suggesting that the 134Cs arrived via atmospheric deposition. Cesium-137 was present in every soil sample, although the total radiocesium activity concentrations measured were significantly less than action levels set by Health Canada.

Spatial variations of 226Ra, 228Ra, 134Cs, and 137Cs concentrations in western and southern waters off the Korean Peninsula in July 2014

We examined the spatial distributions of ²²⁶Ra, ²²⁸Ra, ¹³⁴Cs, and ¹³⁷Cs concentrations (activities) in seawater off the western and southern Korean Peninsula in July 2014. Radium-228 (and ²²⁶Ra) concentrations in water samples varied widely from 5 to 14 mBq/L (2-4 mBq/L), showing a negative correlation with salinity, particularly at the surface off the western Korean Peninsula. This indicates that the seawaters in this area are fundamentally comprised of ²²⁸Ra-poor and high-saline Kuroshio Current water and ²²⁸Ra-rich and low-saline water (e.g., continental shelf water), with various mixing ratios. Although Fukushima Dai-ichi Nuclear Power Plant (FDNPP)-derived ¹³⁴Cs was below the detection limit (<0.08 mBq/L) in waters off the western Korean Peninsula, low level ¹³⁴Cs (0.1-0.2 mBq/L) was detected in waters off the southern Korean Peninsula accompanied by higher ¹³⁷Cs concentrations (1.6-1.9 mBq/L) relative to that off the western Korean Peninsula. Combined with the lower radium concentrations, the detection of ¹³⁴Cs is explained by mixing of FDNPP-derived radiocesium-contaminated Kuroshio Current water.

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.