Significance of Fukushima-derived radiocaesium flux via river-estuary-ocean system.

The environmental dynamics of Fukushima-derived radiocaesium from land to ocean and the impact of its flux on the marine environment are matters of concern because radiocaesium will be continually transported to the open ocean for the next several decades, or possibly more than one hundred years. In order to assess the distribution and flux of radiocaesium in a river-estuary-ocean system, we investigated the activity concentration of radiocaesium in Matsukawa-ura Lagoon, the largest lagoon in Fukushima, where it is very easy to carry out observations with a wide salinity gradient. Activity concentrations of dissolved 137Cs are elevated in seawater of low to intermediate salinity. It can thus be inferred that radiocaesium desorbs from suspended particles in an estuarine area. The porewater activity concentration of 137Cs in lagoon sediment was about 10 times higher than that in the overlying lagoon water. This direct measurement indicates that a significant amount of radiocaesium in sediment desorbs into porewater. From the results of a mass balance model, dissolved 137Cs flux from the lagoon's bottom is 15.3 ± 3.7 times greater than the riverine input, including desorption from particles. In the case of the whole Pacific coast of northeastern Japan (Miyagi, Fukushima, and Ibaraki Prefectures), dissolved 137Cs flux into the open ocean, including diffusion of porewater, is estimated to be up to 1.5 times greater than the sum of riverine input and the ongoing release from the Fukushima Dai-ichi Nuclear Power Station's harbor. Consequently, our results suggest that radiocaesium is transported to the open ocean under the control of various processes, not only by desorption from particles but also, for example, by the diffusion of porewater.

Behavior of Radiocesium in Sediments in Fukushima Coastal Waters: Verification of Desorption Potential through Pore Water.

Concentrations of 137Cs in seawater, seabed sediment, and pore water collected from the area around Fukushima were investigated from 2015 to 2018, and the potential of coastal sediments to supply radiocesium to the bottom environment was evaluated. The 137Cs concentration in the pore water ranged from 33 to 1934 mBq L-1 and was 10-40 times higher than that in the overlying water (seawater overlying within 30 cm on the seabed). At most stations, the 137Cs concentrations in the overlying water and the pore water were approximately proportional to those in the sediment. The conditional partition coefficient between pore water and sediment was [0.9-14] × 102 L kg-1, independent of the year of sampling. These results indicated that an equilibrium of 137Cs between pore water and sediment has been established in a relatively short period, and 137Cs in the pore water is gradually exported to seawater near the seabed. A simple box model estimation based on these results showed that 137Cs in the sediment decreased by about 6% per year by desorption/diffusion of 137Cs from the seabed.

Discovery and biogeochemistry of asphalt seeps in the North São Paulo Plateau, Brazilian Margin.

An initial multiple biogeochemical dataset was acquired from the first discovered asphalt seeps in the Brazil margin during deep-sea dive surveys in 2013 using a manned submersible. These surveys were conducted on the outer escarpment of the North São Paulo Plateau. Sediment cores taken from the submersible were processed for pore water and sediment biogeochemistry. The silica concentration, as a chemical geothermometer, showed a steep gradient in the pore water, which indicates the possibility of an active brine system operating in the seepage area. Rare earth elements were used as powerful tracers of chemical processes. Low rare earth element concentrations in both asphalt and Fe-Mn oxyhydroxide-phase sediments suggests that rare earth elements were released during the oil fractionation and biodegradation processes and further depleted under the reducing environment. The main bacterial communities of the sediment were Proteobacteria in the asphalt sites, while at non-asphalt sites, the main bacterial communities of sediment were Firmicutes. Stable carbon and nitrogen isotopes were used to determine the food sources of the heterotrophs, and results suggest that asphalt probably provides a carbon source for these benthic animals. This study may provide useful information to clarify the impact of heavy hydrocarbon seepage on the marine ecosystem.

Spatial assessment of radiocaesium in the largest lagoon in Fukushima after the TEPCO Fukushima Dai-ichi Nuclear Power Station accident.

Radionuclides deposited on land by global fallouts and nuclear power station accidents spread over coastal environments through estuarine areas connecting land to ocean. In this study, we monitored activity concentration of radiocaesium in surface sediment and re-suspended particles in Matsukawa-ura lagoon, the largest lagoon in Fukushima, after the TEPCO Fukushima Dai-ichi Nuclear Power Station accident. Radiocaesium distribution in surface sediment varied spatiotemporally and irregularly due to the effect of tidal waves. The effective half-life was significantly shorter than physical half-life, suggesting some system of radiocaesium discharge in the lagoon. Sediment trap observation revealed re-suspended particles from sediment were transported to the ocean. For these reasons, it is suggested that re-suspension of particles in the lagoon and their transportation to the ocean by the seawater exchange process are important processes of radiocaesium discharge. Moreover, our results show that seawater exchange process contributes to the dispersion of radiocaesium in the ocean.