Long-Term Effects of 90Sr and 239+240Pu Contaminated on the Seafloor Sediment on the Transfer through Marine Food Web: Sensitivity to the Physical Property- and Sediment-Related Parameters

Abstract

This study, as a follow-up to the <sup>137</sup>Cs study of Kim et al. (2023), performs a set of sensitivity experiments on ocean physical properties and sediment-related parameters based on initial bottom contamination of <sup>90</sup>Sr and <sup>239+240</sup>Pu in the upper sediment layer. Both studies utilizes the Extended BURN model proposed by Bezhenar et al. (2016). The initial contamination levels were set to 3 and 0.4 Bq/kg, respectively, based on observations off the coast of Fukushima. The sensitivity experiments were divided into EXP-WOHAD, which examined the effects of the vertical diffusion coefficient and vertical sediment flux, and EXP-WHAD, which investigated the effects of ocean currents. The results of the EXP-WOHAD experiments showed that both radionuclides are more sensitive to sediment fluxes than to vertical diffusion coefficients. For both <sup>90</sup>Sr and <sup>239+240</sup>Pu, transport was dominated by transport to the intermediate sediment layer rather than to seawater. Specifically, for <sup>239+240</sup>Pu, transport to seawater was insignificant due to its strong sorption properties in sediments. In terms of bioaccumulation, <sup>90</sup>Sr accumulated most in fish and invertebrates, while <sup>239+240</sup>Pu accumulated most in invertebrates, followed by zooplankton. Notably, <sup>239+240</sup>Pu showed high bioaccumulation despite its low initial sediment contamination. The changes in radioactivity concentrations in the EXP-WHAD experiments were similar to those in the EXP-WOHAD experiments, and the bioaccumulation trends in biota were also comparable. This study confirms that the accumulation of radionuclides in seawater, intermediate sediments, and biota is sensitive to the marine physical environment and varies by nuclide. When the sediment flux increased by a factor of 100, the remaining radioactivity levels of <sup>90</sup>Sr and <sup>239+240</sup>Pu in biota after 100 years showed a significant reduction, decreasing by more than 900 and 30,000 times, respectively; it highlighting the importance of parameterization according to the physical environment of the area of interest.