Six-year monitoring study of 137Cs discharge from headwater catchments after the Fukushima Dai-ichi Nuclear Power Plant accident

Since headwater catchments are the source areas of Cs-137 for downstream river systems, Cs-137 discharge from headwater areas needs to be evaluated. Dissolved form (Dissolved), coarse organic matter (Org), and suspended sediments (SS) were sampled and Cs-137 concentrations were measured from June 2011 to November 2016 in four headwater catchments in Yamakiya District, located 35 km northwest of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP). The data up to September 2013 (2.5 y after the accident) have been already published (Iwagami et al., 2017a, b). The data up to November 2016 (5.7 y after the accident) are newly reported in the present paper together with data at a new sampling site. The whole data from June 2011 to November 2016 is discussed. The normalized Cs-137 concentrations (Cs-137 concentrations normalized by the average deposition density of each catchment) in Dissolved, Org, and SS were in the order of 10(-6) m(2)/L, 10(-2)m(2)/kg, and 10(-1)m(2)/kg, respectively, before 2013 and declined to around 10(-8) m(2)/L, 10(-4)m(2)/kg, and 10(-2) m(2)/kg, respectively, in 2016. As a result of the decontamination program, the discharge of SS increased, whereas Cs-137 concentrations in SS declined significantly and the total flux of Cs-137 decreased. Although the clear effect of land use on decline trend in normalized Cs-137 concentrations in Dissolved was not found, more data are necessary for elucidating the relation between them.