## 4 Discussion
### Transport of Radiocesium along a River  
The mechanism of the radiocesium transport from upstream to downstream along a river system has been investigated by many authors. [PERSON] et al. [10] reviewed the fundamental process of radiocesium transfer from hillslopes to the Pacific Ocean, and pointed out that soil erosion is  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**River**} & \multicolumn{2}{c}{**Sediment**} & \multicolumn{3}{c}{**River water**} \\ \cline{2-7}  & \(A_{\textbf{dw}}\) (**Bq\(\cdot\)kg\({}^{-1}\)**) & \(\lambda_{s}\) (y\({}^{-1}\)) & \(T_{e}\) (y)** & \(A_{\textbf{dw}}\) (**mBq\(\cdot\)L\({}^{-1}\)**) & \(\lambda_{w}\) (y\({}^{-1}\)) & \(T_{e}\) (y)** \\ \hline Odaka & 1990 & 0.15 \(\pm\) 0.04 & 4.7 \(\pm\) 1.3 & 121 & 0.19 \(\pm\) 0.09 & 3.7 \(\pm\) 1.7 \\ Ohta & 8020 & 0.46 \(\pm\) 0.12 & 1.5 \(\pm\) 0.4 & 387 & 0.34 \(\pm\) 0.10 & 2.1 \(\pm\) 0.6 \\ Niida & 3272 & 0.39 \(\pm\) 0.13 & 1.8 \(\pm\) 0.6 & 401 & 0.66 \(\pm\) 0.12 & 1.0 \(\pm\) 0.2 \\ Mano & 1103 & 0.33\(\pm\) 0.03 & 2.1 \(\pm\) 0.2 & 221 & 0.78 \(\pm\) 0.07 & 0.9 \(\pm\) 0.1 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Estimation of initial deposition \(A_{0}\), annual decrease rate \(\lambda\) and effective half-life \(T_{e}\) of sediment and river water in the Minami-Soma City.  
an important process for transporting radiocesium. Most radiocesium is attached to fine particles, and the most significant transfer occurs during major rainfall and runoff events. [PERSON] et al. [15] performed simulation calculations of sediment transfer on the Abukuma River and showed an accumulation of \({}^{137}\)Cs over floodplains, suggesting that \({}^{137}\)Cs supplied from upstream by the washload would mostly reach the ocean directly. [PERSON] et al. [16] performed detailed investigations of radiocesium deposition along the Odaka and the Ukedo Rivers. They reported high radiocesium concentrations over the floodplains and proposed a mechanism for \({}^{137}\)Cs transport and deposition.  
In the present work, it has been shown that high concentrations of radiocesium in sediments at the upstream area in the Minami-Soma area were rapidly transported to the ocean with effective half-lives of 1.5-2.1 y. On the other hand, accumulation of \({}^{137}\)Cs over floodplains in the downstream occurred as a secondary contamination. The air dose rates were about 1.0 \(\upmu\)Sv\(\cdot\)h\({}^{-1}\) over the floodplains on the Odaka river, whereas the air dose rates at the land surface were about 0.1-0.2 \(\upmu\)Sv\(\cdot\)h\({}^{-1}\), as shown in Figure 8. According to the \({}^{137}\)Cs depth profiles of soil cores measured at right and left side floodplains, the surface \({}^{137}\)Cs concentration reached to 20,000 Bq\(\cdot\)kg\({}^{-1}\) in muddy soil, whereas \({}^{137}\)Cs concentration in sediments in the river channel was about 10 Bq\(\cdot\)kg\({}^{-1}\).  
The accumulation of radiocesium over the floodplain is caused by floods due to heavy rainfall during the typhoon and rainy seasons in Japan. Flooding occurs several times every year in the Minami-Soma area, and the water level rises to cover the floodplains where tall plants are growing. The flood velocity is decelerated over the floodplains and the plants help to retain the clay or mud over the floodplains. After the flood, the water level returns to normal and the radiocesium included in sand or mud remains on the floodplains. Therefore, most of radiocesium supplied in the river from the contaminated upstream area are transported to ocean directly, but accumulation occurs over the floodplains in the downstream where initial contamination was low.