R&D for environmental restoration
Fig.1 Study area, method and result
After the accident at TEPCO’s Fukushima Daiichi Nuclear Power Station (FDNPS), “Dynamic radioactive cesium (137Cs) studies” in the sea area have been conducted to address concerns about the transfer of radioactive materials to marine products. Even now, 12 years after the accident, a small amount of 137Cs is being transported from land to sea via rivers. In particular, in the coastal region, there exists a turbid area (high-turbidity layer) near the seabed. This layer is formed by the resuspension sediment and river flow. The 137Cs present in such a highly turbid layer may affect small organisms living on the seabed and the benthic fish that prey on these organisms. Hence, there is a need to elucidate the supply mechanism. The high-turbidity layer is easily transported by waves and currents and is thought to play an important role in radioactive material transport. Understanding the relationship between the high-turbidity layer in coastal areas and the amount and timing of 137Cs inflow from rivers is important for predicting future impacts on benthic fish. In this study, we conducted a survey in a coastal area to understand the actual situation of the effect of 137Cs in the high-turbidity layer on the river.
The observation site was set at the Ukedo River estuary, which has a large basin area around the FDNPS, as shown in Fig.1(a). We installed two mooring systems, namely, a water quality system (WQ system) and a sediment trap system (ST system), as shown in Fig.1(b). The ST system incorporated a ST to measure the amount of particles forming the high-turbidity layer (sinking particles). The WQ system is equipped with an instrument to measure the variation in turbidity and current in the high-turbidity layer. The index of particle migration (shear stress) was calculated from the current and publicly available wave data obtained from the WQ system.
The variation in high-turbidity layer (variation of sinking particles) can be estimated from three factors: current, waves (resuspension of seabed sediment), and river inflow (sediment and other suspended matter from rivers). The percentage contribution of the three factors to total turbidity was calculated by multiple regression analysis.
Using the percentage of each factor to distinguish the sinking particles, the percentage of resuspension averaged 79% for St. 1 and 83% for St. 2 (Fig.1(c)). Thus, it is shown that the particles containing 137Cs in the high-turbidity layer are predominantly attributed to resuspension, and the influence of newly supplied 137Cs by river inflow is small. In the future, we plan to calculate the offshore transport in the high-turbidity layer to contribute to basic data for predicting the transfer of 137Cs to benthic fish.
(Toshiharu Misonou)
(295 kB)
