Role of mesoscale eddies in transport of Fukushima-derived cesium isotopes in the ocean

Abstract

We present the results of in-situ measurements of 134 Cs and 137 Cs released from the Fukushima Nuclear Power Plant (FNPP) collected at surface and different depths in the western North Pacific in June and July 2012. It was found that 15 month after the incident concentrations of radiocesium in the Japan and Okhotsk seas were at background or slightly increased level, while they had increased values in the subarctic front area east of Japan. The highest concentrations of 134 Cs and 137 Cs up to 13.5 ± 0.9 and 22.7 ± 1.5 Bq m −3 have been found to exceed ten times the background levels before the accident. Maximal content of radiocesium was observed within subsurface and intermediate water layers inside the cores of anticyclonic eddies (100 -500 m). Even slightly increased content of radiocesium was found at some eddies at depth of 1000 m. It is expected that convergence and subduction of surface water inside eddies are main mechanisms of downward transport of radionuclides. In situ observations are compared with the results of simulated advection of these radioisotopes by the AVISO altimetric velocity field. Different Lagrangian diagnostics are used to reconstruct the history and origin of synthetic tracers imitating measured seawater samples collected in each of those eddies. The results of observations are consistent with the simulated results. It is shown that the tracers, simulating water samples with increased radioactivity to be measured in the cruise, really visited the areas with presumably high level of contamination. Fast water advection between anticyclonic eddies and convergence of surface water inside eddies make them responsible for spreading, accumulation and downward transport of cesium rich water to the intermediate depth in the frontal zone.

Lagrangian simulation and tracking of the mesoscale eddies contaminated by Fukushima-derived radionuclides

“…It originated in the beginning of February 2012 with the center at around 41.9 • N, 148 • E and captured some contaminated water from the HE. The TsE was sampled in the R/V Professor Gagarinskiy cruise on 5 July, 2012, and found to have concentrations of 137 Cs and 134 Cs over the background level at the surface and at intermediate depths (Budyansky et al, 2015). All these eddies will be studied in this section from the Lagrangian point of view in order to simulate and track by which transport pathways they could have gained water masses from the Fukushima area or from other origins and to compare qualitatively the simulation results with in situ measurements.…”

“…The Kumamoto et al, 2014) with measured radiocesium concentrations at the background level (the green diamonds) and with higher concentration levels (the magenta diamonds). (b-d) The Lagrangian maps show evolution of the Tsugaru eddy (TsE), which originated on 4 February 2012 (a) after splitting of the HE and was sampled by Budyansky et al (2015) at station 84 on 5 July 2012 and shown to have increased radiocesium concentrations (the magenta diamond in d).…”

“…137 Cs and 134 Cs isotopes with 30.17 and 2.06 years half-life, respectively, were detected over a broad area in the western North Pacific in 2011 and 2012 (Honda et al, 2012;Buesseler et al, 2012;Inoue et al, 2012a, b;Tsumune et al, 2012Tsumune et al, , 2013Kaeriyama et al, 2013;Oikawa et al, 2013;Aoyama et al, 2013;Kameník et al, 2013;Kumamoto et al, 2014;Kaeriyama et al, 2014;Budyansky et al, 2015). 137 Cs concentration levels off Japan before the accident were estimated at the background level to be 1-3 mBq kg −1 , while 134 Cs was not detectable.…”

“…Identification of such structures in the ocean would help to predict, for short and medium-length periods of time, where a contaminant will move even without a precise solution of the Navier-Stokes equations. This approach has been successfully used in simulating propagation of oil in the Gulf of Mexico (Mezić et al, 2010;Huntley et al, 2011;Olascoaga and Haller, 2012) and propagation of Fukushima-derived radionuclides in the Pacific ocean (Prants et al, 2011b;Budyansky et al, 2015;Prants et al, 2014).…”

“…The present authors have developed a set of Lagrangian tools for tracking the origin, history and fate of water masses advected by analytic, altimetric and numerical velocity fields generated by eddy-resolved regional circulation models (Budyansky et al, 2009;Prants et al, 2011a, b;Prants, 2013;Prants et al, 2013;Prants, 2014;Budyansky et al, 2015). Each elementary volume of water can be attributed to physico-chemical properties (temperature, salinity, density, radioactivity, etc.)…”