Subsurface dispersion path and travel time of radiocesium from Fukushima by Mode Water

Abstract

The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident on March 11, 2011 resulted in the release of immense amounts of radioactive materials into the ocean. However, the dispersion of radioactive materials in the subsurface has not yet been clarified due to the spatiotemporal limitation of observations. Thus, herein, a tracer experiment was implemented using a three-dimensional numerical model to estimate the dispersion path of 137Cs released directly from the FDNPP and its travel time in the subsurface of the North Pacific Ocean from 2011 to 2020. The results show that the subsurface 137Cs spreads by two mode waters, namely, Sub-Tropical Mode Water (STMW) and Central Mode Water (CMW). Subsurface 137Cs primarily spreads clockwise in the sub-tropical region, while a portion driven by STMW is dispersed southward. The clockwise dispersion path of 137Cs released into the ocean by STMW is relatively shallower and inward than that by CMW. The 137Cs that was spread clockwise reached the east of Taiwan and the Philippines via STMW and CMW nine years after the accident, respectively. The model described in this study is applicable for estimating the water path and travel time of tritium water planned to be discharged from Fukushima.

Quantifying the Water Contribution of Subtropical Mode Water and Related Isopycnal/Diapycnal Water Mixing in the Western Pacific Boundary Current Area Using Radiocesium: A Significant Nutrient Contribution From Subtropical Pacific Gyre to the Marginal Region

“…However, it should be noted that the temperature (15°C–18°C) of samples in the WPBCA is slightly lower compared with the original STMW (16°C–19°C) in the northern NPSG. This suggests that the water with high Cs activity concentration may be influenced during the formation of the Cs maximum layer, especially during the water subduction near the Kuroshio extension region (Oka et al., 2015), and is a large portion of original STMW in the NPSG (Lee et al., 2023). The formation processes of the Cs maximum water in the STMW are out of the scope of this study, and could be further discussed in future research.…”

A study on the pathways and their interannual variability of the Fukushima-derived tracers in the northwestern Pacific

“…In a recent study using a 3-dimensional hydrodynamic model (Lee et al, 2023), it was reported that a radioactive tracer originating from the FDNPP took approximately 8-9 years to reach the Taiwan and Philippine islands and followed a clockwise trajectory; however, there may have been limited southward movement that dissipated rapidly. This main propagation indicates a relatively longer travel time compared to previous studies (Behrens et al, 2012;Aoyama et al, 2018;Inomata et al, 2018;Wang et al, 2022) because they consider the limitation associated with the minimum concentration of 0.01 Bq m -3 of the tracer dyne method.…”

“…Subducted radioactive materials within the NPSTMW spread into the subtropical region of the North Pacific (Men et al, 2015;Cedarholm et al, 2019;Lee et al, 2023). Men et al (2015) reported that 134 Cs was found at 21.50°N, 125.00°E at a depth of 200 m near Taiwan Island after monitoring the area between 2011 and 2014.…”

“…Using high-resolution reanalysis data from March 2011 to April 2013 and a backward particle tracking simulation, Cedarholm et al (2019) demonstrated that the subsurface pathway, reaching the deepest observed cesium location in 2013, runs along the KE to ~165°E, where it turns sharply southward on the 26.5s q isopycnal surface. Using an eddypermitting (1/4°horizontal resolution) numerical ocean model to estimate the dispersion path and travel time of the 137 Cs released directly from the FDNPP from 2011 to 2020, Lee et al (2023) reported that subsurface 137 Cs spread clockwise in the subtropical region, while a portion driven by the NPSTMW was dispersed southward.…”

Sociological and Ecological Vulnerabilities from Nuclear Waste Discharge: A Case Study of Fukushima's Marine Ecosystem

“…This demonstrates the widespread dispersal of pollutants and their insertion into the marine food chain, raising concerns about the long-term ecological consequences. The ocean's capacity to neutralize and dissipate radionuclides does not completely mitigate environmental risks (Lee et al, 2023). These substances might gather in speci c areas due to oceanic processes, resulting in irradiation "hotspots" that may harm local marine life.…”