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In forest ecosystems, forest litter is considered an active medium for radiocesium ( 137 Cs). To understand discharge mechanisms of highly bioavailable dissolved 137 Cs from forests to river systems, we investigated the characteristics of 137 Cs leaching from forest litter as observed from litterbag experiments. Leaching experiments with conifer needle and deciduous broadleaf litters were then conducted. After soaking conifer needles and broadleaf litters for 20 min, 140 min, and 1 day, the mean values of the 137 Cs leaching ratios were 0.13-2.0% and 0.81-6.6%, respectively, indicating that 137 Cs leaching ratios are different between forest litter types. To elucidate the factors affecting 137 Cs leaching from forest litter, a multi-regression analysis of 137 Cs leaching ratios was conducted against antecedent mean precipitation and temperature before sampling the litterbag and accumulated temperature during the litterbag experiments. The 137 Cs leaching ratios showed a negative correlation to the antecedent mean precipitation for both litters and the accumulated temperature for broadleaf litters, whereas it exhibited a positive correlation with the antecedent mean temperature for both litters and the accumulated temperature for conifer needle litters. It was proposed that the fraction of 137 Cs in labile sites in forest litter increased/decreased due to litter decomposition by antecedent/accumulated temperature, and that this fraction can be washed off by the antecedent precipitation. The different effects of accumulated temperature on 137 Cs leaching from conifer needles and broadleaf litters could be due to their different decomposition rates. Our results contribute further the understanding of the mechanisms associated with dissolved 137 Cs discharge from forested catchments., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier Ltd. All rights reserved.)

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 137 Cs concentration in two rivers which located surrounding the FDNPP. The result revealed a declining trend for the dissolved and particulate 137 Cs concentration in river water from four to seven years after the FDNPP accident. The dissolved and particulate 137 Cs concentrations for both rivers had similar temporal patterns and showed declining trends with time. However, the dissolved 137 Cs concentration had longer half-life than the particulate 137 Cs concentration and large seasonal variations related to water temperature. The environmental half-life for the dissolved 137 Cs concentration was longer than previous reported values within three years after the accident, suggesting that the declining trend for the dissolved 137 Cs concentration is gradually decreasing with time. The temperature dependency of the dissolved 137 Cs concentration became weaker year by year. From the D 10 equation we proposed, the dissolved 137 Cs 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., (Copyright © 2018 The Authors. Published by Elsevier Ltd.. All rights reserved.)