Trend of 137Cs concentration in river water in the medium term and future following the Fukushima nuclear accident

It is a critical to examine the migration behavior of radiocesium derived from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in river systems to predict the future contamination status and propose effective countermeasures to reduce exposure. We conducted a three-year-long observation (April 2015-March 2018) of the Cs-137 concentration in two rivers which located surrounding the FDNPP. The result revealed a declining trend for the dissolved and particulate Cs-137 concentration in river water from four to seven years after the FDNPP accident. The dissolved and particulate Cs-137 concentrations for both rivers had similar temporal patterns and showed declining trends with time. However, the dissolved Cs-137 concentration had longer half-life than the particulate Cs-137 concentration and large seasonal variations related to water temperature. The environmental half-life for the dissolved Cs-137 concentration was longer than previous reported values within three years after the accident, suggesting that the declining trend for the dissolved Cs-137 concentration is gradually decreasing with time. The temperature dependency of the dissolved Cs-137 concentration became weaker year by year. From the D-10 equation we proposed, the dissolved Cs-137 concentration will likely remain at the same level for several decades. The results of the present study promote our understanding of both the medium- and long-term impacts of the FDNPP accident on river systems. (C) 2018 The Authors. Published by Elsevier Ltd.