Radiocesium (Cs-137) migration from headwater forested areas to downstream rivers has been investigated in many studies since the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, which was triggered by a catastrophic earthquake and tsunami on 11 March 2011. The accident resulted in the release of a huge amount of radioactivity and its subsequent deposition in the environment. A large part of the radiocesium released has been shown to remain in the forest. The dissolved Cs-137 concentration and its temporal dynamics in river water, stream water, and groundwater have been reported, but reports of dissolved Cs-137 concentration in soil water remain sparse.
In this study, soil water was sampled, and the dissolved Cs-137 concentrations were measured at five locations with different land-use types (mature/young cedar forest, broadleaf forest, meadow land, and pasture land) in Yamakiya District, located 35 km northwest of FDNPP from July 2011 to October 2012. Soil water samples were collected by suction lysimeters installed at three different depths at each site. Dissolved Cs-137 concentrations were analyzed using a germanium gamma ray detector. The dissolved Cs-137 concentrations in soil water were high, with a maximum value of 2.5 Bq/L in July 2011, and declined to less than 0.32 Bq/L by 2012. The declining trend of dissolved Cs-137 concentrations in soil water was fitted to a two-component exponential model. The rate of decline in dissolved Cs-137 concentrations in soil water (k(1)) showed a good correlation with the radiocesium interception potential (RIP) of topsoil (0 -5 cm) at the same site. Accounting for the difference of Cs-137 deposition density, we found that normalized dissolved Cs-137 concentrations of soil water in forest (mature/young cedar forest and broadleaf forest) were higher than those in grassland (meadow land and pasture land). (C) 2017 Elsevier Ltd. All rights reserved.