Model estimation of 137Cs concentration change with time in seawater and sediment around the Fukushima Daiichi Nuclear Power Plant site considering fast and slow reactions in the seawater-sediment systems

Significance of the great protection of the Yangtze River: Riverine input contributes primarily to the presence of PAHs and HMs in its estuary and the adjacent sea

The Yangtze River protection strategies are expected to improve the water quality and ecological function of the Yangtze River Estuary (YRE). The concentrations of 16 polycyclic aromatic hydrocarbons (PAHs) and 6 heavy metals (HMs) in the YRE were measured and the riverine fluxes were calculated subsequently. In particular, the concentrations of low molecular weight PAHs (LMW-PAHs), arsenic (As) and mercury (Hg) in seawater decreased over time, while those of other studied pollutants did not change a lot. In sediments, the concentration changes for all the pollutants were insignificant. For the present pollutants, the river input is the dominant source, and the flux decreased after the protection. The contribution of the discharge from wastewater treatment plants (WWTPs) was quantified. Its influence cannot be ignored. The seafood quality remained stable and the risk via diet was insignificant. Long-term monitoring is necessary, and the positive impact of the Protection Strategy is gradually emerging.

Research on Radionuclide Diffusion Mechanism in the Ocean and Emergency Response under Oceanic Radioactive Events

On March 11, 2011, a serious radionuclide leakage accident occurred at Fukushima Daiichi nuclear power plant, and a large number of radionuclides were released, causing serious pollution to the ocean environment. On August 25, 2021, Japan announced the overall plan for the discharge of radioactive sewage from the Fukushima Daiichi nuclear power plant into the ocean, and the discharge will begin around the spring of 2023. All operational and under-construction nuclear power plants in China are distributed in coastal areas presently. In case of a nuclear leakage accident, radionuclides will diffuse through the ocean and pollute the ecological environment. The study of radionuclide diffusion mechanism in the ocean and emergency response plays an important role in accident mitigation under oceanic radioactive events. A radionuclide diffusion model in the ocean was established and the radionuclide diffusion mechanism in the ocean was analyzed. And then a prediction and monitoring system of radionuclide diffusion in the ocean was proposed. The results show that the short-term radionuclide diffusion is mainly influenced by the source term, flow field and decay of 131I, and the degree of influence decreases in turn. On the whole, influences of the flow field and 131I decay are weakened during the long-term diffusion. At the same time, the influence of 137Cs decay begins to be obvious and the influence of suspended matter is increasing. The influence of ocean organisms is always small. Problems of scientific prediction and protection were analyzed, and the emergency response scheme was given. It is of great significance to improve the capacity of emergency response for oceanic radioactive events.

Migration processes of radioactive cesium in the Fukushima nearshore area: Impacts of riverine input and resuspension

It is essential to evaluate secondary migration caused by riverine input and resuspension from seabed sediments to estimate the future distribution of radioactive cesium (¹³⁷Cs) in the coastal area off Fukushima Prefecture. In particular, the inflow from rivers cannot be ignored because most of the ¹³⁷Cs inflow from rivers is deposited on the coast without elute into seawater. Two mooring systems were installed near the Ukedo River's mouth (Fukushima Prefecture) from February 2017 to February 2018. The first contained a sediment trap system, collecting sinking particles during the period. The second comprised a turbidity sensor and a current sensor. The contribution of resuspension and inflow from the river to the mass flux was quantitatively evaluated using multiple regression equations. The results showed that resuspension caused 79%-83% of secondary ¹³⁷Cs migration in nearshore areas, whereas the influence of riverine ¹³⁷Cs input on the sediment was only 7% per year.

Viewpoint on the Integration of Geochemical Processes into Tracer Transport Models for the Marine Environment

Two types of models are used to describe the interactions of tracers dissolved in the ocean with marine sediments: equilibrium and dynamic models. A brief description of these models is given in this opinion paper, and some examples are presented to show that preference should be given to the dynamic descriptions in modern pollutant transport models to be used in emergencies as well as in the case of chronic tracer releases to the sea.

A review on cesium desorption at the freshwater-seawater interface

Understanding the processes governing the behavior of radiocesium in the sea is still essential to make accurate assessments of its potential impacts on marine ecosystems. One of the most important of this process is the desorption that may occur at the river-sea interface due to changes in physico-chemical conditions, including ionic strength and solution composition. It has been the subject of many studies using field measurements or laboratory experiments, but there was no global interpretation of these works and their results. The present review summarizes relevant laboratory experiments studying desorption of Cs (stable or radioactive) from particles in sea or brackish waters. To date, 32 experimental studies have been carried out on 68 Cs-bearing samples since 1964. A wide range of desorbed fraction (0-86%) was observed, partly depending on the experimental design. For particles containing radiocesium issued from a contamination in the environment, the desorption ranges from 0 to 64% of the particulate activity, with a median at only 3%. Particles contaminated in laboratory show a range between 6 and 86% with a multimodal distribution. The desorption initiates at low salinity (3-4) and rapidly reaches a threshold around 10-15. Laboratory experiments show that two first-order reactions govern the kinetics of the process, with half-life reaction times of 1 h and a few days. These two reactions are probably linked to the adsorption of Cs onto different particles sites. Also, the dynamic of Cs desorption depends on its initial distribution on these different sites, in relation with the history of its contamination and an aging effect.

Temporal Changes in 137Cs Concentrations in Fish, Sediments, And Seawater off Fukushima Japan

We analyzed publicly available data of Fukushima ¹³⁷Cs concentrations in coastal fish, in surface and bottom waters, and in surface marine sediments and found that within the first year of the accident pelagic fish lost ¹³⁷Cs at much faster rates (mean of ∼1.3% d^-1) than benthic fish (mean of ∼0.1% d^-1), with benthopelagic fish having intermediate loss rates (mean of ∼0.2% d^-1). The loss rates of ¹³⁷Cs in benthic fish in the first year were more comparable to the decline of ¹³⁷Cs concentrations in sediments (0.03% d^-1), and the declines in pelagic fish were more comparable to the declines in seawater. Retention patterns of ¹³⁷Cs in pelagic fish were comparable to that in laboratory studies of fish in which there were no sustained ¹³⁷Cs sources, whereas the benthopelagic and benthic fish species retained ¹³⁷Cs to a greater extent, consistent with the idea that there is a sustained additional ¹³⁷Cs source for these fish. These field data, based on 13 511 data points in which ¹³⁷Cs was above the detection limit, are consistent with conclusions from laboratory experiments that demonstrate that benthic fish can acquire ¹³⁷Cs from sediments, primarily through benthic invertebrates that contribute to the diet of these fish.

Consideration on the Long Ecological Half-Life Component of (137)Cs in Demersal Fish Based on Field Observation Results Obtained after the Fukushima Accident

Radiocesium concentrations in most marine fish collected off the coast of Fukushima and surrounding prefectures have decreased with time, and four years after the Fukushima Daiichi Nuclear Power Plant accident occurred, radiocesium concentrations have generally fallen below the detectable level (ca. < 10 Bq kg(-1)-raw). Only in some demersal fish species have detectable concentration levels still been found, and even these species have showed slow radiocesium decreases. The food web was considered as the major factor causing this phenomenon; however, slow elimination rates of radiocesium from these fish species also could be the cause. The latter effect was examined by considering that the (137)Cs concentration decreasing trend in fish could be fit with a set of three exponentially decreasing components; that is, having short, intermediate, and long biological half-lives. The long ecological half-life component was calculated using a 400-1500 d period of monitoring results for Japanese rockfish (Sebastes cheni) and compared with previous reported laboratory results for biological half-life. The obtained ecological half-lives ranged from 274-365 d, and these values agreed with the biological half-life of this fish species. This result implied that the long biological half-lives of radiocesium in some demersal fish species made their radiocesium contamination periods longer.

Fukushima-derived radionuclides in sediments of the Japanese Pacific Ocean coast and various Japanese water samples (seawater, tap water, and coolant water of Fukushima Daiichi reactor unit 5)

We investigated Ocean sediments and seawater from inside the Fukushima exclusion zone and found radiocesium (¹³⁴Cs and ¹³⁷Cs) up to 800 Bq kg⁻¹ as well as ⁹⁰Sr up to 5.6 Bq kg⁻¹. This is one of the first reports on radiostrontium in sea sediments from the Fukushima exclusion zone. Seawater exhibited contamination levels up to 5.3 Bq kg⁻¹ radiocesium. Tap water from Tokyo from weeks after the accident exhibited detectable but harmless activities of radiocesium (well below the regulatory limit). Analysis of the Unit 5 reactor coolant (finding only ³H and even low ¹²⁹I) leads to the conclusion that the purification techniques for reactor coolant employed at Fukushima Daiichi are very effective.