The contribution of Cs export flux from the Tone River Japan to the marine environment.

The contribution of Cs transport to the marine environment via the Tone River, Japan was investigated. This river has the largest discharge among rivers on the North Pacific side of eastern Japan. The sampling site was located upstream near the river mouth and dissolved and particulate Cs in the river water was measured during 2014-2015, three years after the Tokyo Electric Power Corporation Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Annual fluxes of total (dissolved and particulate) Cs with considering desorption of Cs from riverine particles by change of salinity from the Tone River were similar in both years (78-107 × 10 Bq/y), indicating that about 0.03-0.06% of the estimated total amount of Cs deposited in the catchment (1.9-2.8 × 10 Bq) was transported to the marine environment each year. Although the annual flux was about one order of magnitude lower than the daily direct discharge into the ocean from the FDNPP (800 × 10 Bq/y) during the corresponding period, continuous monitoring of rivers in the southern coastal area of east Japan on the North Pacific side are needed for the effect of Cs release via the rivers in the Kanto area over the long-term.