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

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.

Establishing historical 90Sr activity in seawater of the China seas from 1963 to 2018

Historical ⁹⁰Sr activity in seawater was established in the China seas from 1963 to 2018. Based on the exponential decrease in ⁹⁰Sr activity in seawater, the effective half-life (EHL) of ⁹⁰Sr was quantified to be 11.5 ± 1.6 a, 16.5 ± 2.4 a, 27.2 ± 6.2 a, and 26.7 ± 4.3 a in the Bohai Sea, Yellow Sea, East China Sea, and South China Sea, respectively. We found contrasting patterns in the EHL of ⁹⁰Sr and ¹³⁷Cs in the marginal seas and open oceans that were closely related to the subtly different pathways of ⁹⁰Sr and ¹³⁷Cs in marine environment. Additionally, we demonstrated that Fukushima-derived ⁹⁰Sr (<0.01 Bq/m³) would be difficult to identify in the China seas. Our study not only provided the key parameter of the EHL in marine models for predicting the ⁹⁰Sr activity in the China seas in the post-Fukushima era but also enhanced our understanding of ⁹⁰Sr behavior and its fate in marine environments.

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.