Factors controlling dissolved Cs activities in coastal waters on the eastern and western sides of Honshu, Japan.

The distributions of dissolved Cs in river, nearshore, and offshore waters on the east and west coasts of the Japanese island of Honshu were studied in 2018-2021, 7-10 years after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. On the east side along the north western North Pacific (Fukushima Prefecture), estuarine processes, including desorption from riverine particles and dissolution into pore water from riverine particles that had settled to the seafloor, contributed to the maintenance of high dissolved Cs activities in nearshore and offshore waters. A survey and mass-balance calculation in a semi-enclosed estuarine area, the Matsukawa-ura, in the northern part of Fukushima, provided convincing evidence that rivers contributed to the influx of Cs to coastal waters. In contrast, the extremely low activities of dissolved and particulate Cs in the Tedori River of Ishikawa Prefecture on the western side of Japan along the Japan Sea suggested that inputs of riverine Cs made a negligible contribution to the increase of dissolved Cs activities in the nearshore and offshore waters. The relatively high dissolved Cs activities observed in the offshore waters of the Japan Sea were due to movement of FDNPP-derived Cs into the Japan Sea via the Tsushima Warm Current. Mechanisms controlling the distributions of Cs activities in coastal waters of the eastern and western sides of Japan therefore differ.