H065-0014
Trapping efficiency of fine sediments in reservoir lakes by using suspended radiocesium using the dataset from 29 monitoring sites in Fukushima rivers

Wednesday, 9 December 2020
Poster
Keisuke Taniguchi1, Yuichi Onda2, Hugh Smith3, Willium Blake4, Yukio Takeuchi5, Kazuki Fujita5 and Kazumi Katsuno5, (1)University of Tsukuba, Tsukuba, Japan, (2)University of Tsukuba, Center for Research in Isotopes and Environmental Dynamics, Tsukuba, Japan, (3)Landcare Research NZ, School of Environmental Sciences, Lincoln, New Zealand, (4)Plymouth University, Plymouth, United Kingdom, (5)Fukushima Prefectural Centre for Environmental Creation, Miharu, Fukushima, Japan
Abstract:
The spatial variation of riverine suspended solids (SS) is influenced by the flow conditions from the headwater to the river mouth. Installation of wide-spread and long-term monitoring networks will be beneficial to evaluate the variations quantitively. After the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011, we have observed the water discharge and the concentration and flux of SS at 29 monitoring sites in the nine river systems in 80km from FDNPP until now. The water level and turbidity in every 10 minutes have been recorded at each site and collected SS every few weeks to months by a SS sampler (Taniguchi et al., 2019, 2020).

In total, there are 34 dams in the nine river systems. The storage volumes range from 1.7×10-5 to 1.5×10-1 km3, while the catchment areas are distributed from 1.0×10-1 to 2.9×103 km2. The local residence time Δτ, the ratio between the storage volume and inflow of the reservoir, was from 4.6×10-4 to 5.5 year-1 (average 0.042). Estimating the fine sediment trapped in these reservoirs were important to evaluate the downstream effect of the contaminant.

137Cs flux normalized by the watershed area and initial deposition was from 3.4×10-2 to 2.5×10-4 in 2013 and 2014. The value in the site with the dam(s) in the watershed tended to be smaller. In this study, the following method using 137Cs as a tracer was used to evaluate the trapping efficiency of fine sediments by dams. 1) Data from 14 sites less affected by dams were selected, and multiple regression equations for 137Cs fluxes were calculated using land use coverages in the watershed as variables (eq. 1). 2) Using data from all sites, we calculated a multiple regression equation that includes the effect of the dam (eq.2). We adopted Δτ as the parameter for the dam effect. 3) The difference between the predictions of the two equations corresponds to the amount of fine sediment trapped in the dam(s).

The declining rate between the predictions in Eqs. 1 and 2 (i.e., TE for fine sediment) showed a negative correlation with the catchment area ratio of the dam and site. Assuming that the area ratio for 100%, the rate of decline was about 70%, similar to the TE given by Brune (1953).

Taniguchi et al. (2019) Environ. Sci. Technol. 53, 12339-12347.

Taniguchi et al. (2020) Doi: 10.34355/Fukushima.Pref.CEC.00015

Brune (1953) Trans. Am. Geophys. Union 34 (3), 407–418.

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