Factors controlling dissolved 137Cs activities in coastal waters on the eastern and western sides of Honshu, Japan

Abstract

The distributions of dissolved ¹³⁷Cs 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 ¹³⁷Cs 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 ¹³⁷Cs to coastal waters. In contrast, the extremely low activities of dissolved and particulate ¹³⁷Cs in the Tedori River of Ishikawa Prefecture on the western side of Japan along the Japan Sea suggested that inputs of riverine ¹³⁷Cs made a negligible contribution to the increase of dissolved ¹³⁷Cs activities in the nearshore and offshore waters. The relatively high dissolved ¹³⁷Cs activities observed in the offshore waters of the Japan Sea were due to movement of FDNPP-derived ¹³⁷Cs into the Japan Sea via the Tsushima Warm Current. Mechanisms controlling the distributions of ¹³⁷Cs activities in coastal waters of the eastern and western sides of Japan therefore differ.

Keywords: Desorption; Dissolution from sediments; Riverine Cs input; Semi-closed estuarine system.