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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 134 Cs and 137 Cs 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 134 Cs was ∼0.5 mBq/L (decay-corrected to March 11, 2011), and that of 137 Cs 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 134 Cs), exhibiting approximately homogeneous distributions in each year. The temporal and spatial variations of 134 Cs and 137 Cs 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 134 Cs was limited to depths of less than ∼200 m during three years following the re-delivery of FDNPP-derived radiocesium. [ABSTRACT FROM AUTHOR]

We investigated lateral profiles of 228 Ra (half-life; 5.75 years) activity and 228 Ra/ 226 Ra (1600 years) activity ratio using 241 surface water samples collected in/around the Sea of Japan and the East China Sea (ECS) during June–October of 2009–2014. In the ECS, the 228 Ra/ 226 Ra ratio in the surface waters exhibited markedly wide variation (<0.05–3.5) in June, predominantly reflecting the mixing between the 228 Ra-rich continental shelf water and the 228 Ra-depleted Kuroshio Current water. In July, the surface waters of the central Sea of Japan (135–138°E) became separated into three currents: the Offshore Branch of the Tsushima Warm Current (OBTWC) ( 228 Ra/ 226 Ra =0.7–1.2) at 39–41°N, the Coastal Branch of the TWC (CBTWC) (~0.7) on the southern side, and sub-Arctic Current (~0.7) on the northern side. From the central to northeastern Sea of Japan, the 228 Ra/ 226 Ra ratio at the surface (0.8–1.0) was within a range between that of the CBTWC and OBTWC. The fraction of continental shelf water in the CBTWC, OBTWC, and in their combined current was estimated to be 11–16%, ~8%, and 10–11%, respectively. [ABSTRACT FROM AUTHOR]

Lateral and temporal profiles for concentrations of Cs and Cs were examined in marine sediments obtained from coastal and offshore areas on the Sea of Japan-side of central Honshu. In 2013, concentrations declined sharply from an estuarine (30 Bq/kg-dry for Cs) to offshore areas. In the offshore Sado Basin, the concentration of short-lived Cs, which was not detected (<0.3 Bq/kg-dry) in 2010, was 1-15 Bq/kg-dry in 2011-2013. Discharged riverine materials affected by radiocesium derived from the Fukushima Dai-ichi Nuclear Power Plant have been delivered to this offshore basin. [ABSTRACT FROM AUTHOR]

We analyzed a total of 25 seawater samples collected from 2 columns within the Sea of Japan for Ra (half life 1,600 years) and Ra (half life 5.75 years) by employing low-background γ-spectrometry. The Ra activity level in the deep proper water (DPW) (~0.13 mBq/L) was found to be higher than that of deep waters from the Pacific and Atlantic Oceans (<0.1 mBq/L), indicating its continuous delivery to the DPW by active water circulation before complete radioactive decay. Using a steady state box model of Ra and Ra, the turn-over time in the DPW was calculated to be ~80 years. [ABSTRACT FROM AUTHOR]

Abstract: 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 134Cs and 137Cs concentrations using low-background γ-spectrometry revealed that the 137Cs 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 137Cs level, while the 134Cs concentration was less than detectable to 1mBq/L. In addition to 134Cs being clearly detected (∼1mBq/L), 137Cs 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. [Copyright &y& Elsevier]