Ten-year radiocesium fluvial discharge patterns from watersheds contaminated by the Fukushima nuclear power plant accident.

Abstract After the Fukushima Dai-ichi nuclear power plant accident in March 2011, the fluvial discharge of 137Cs from watersheds to rivers was analyzed between 2011 and 2021. The concentrations of dissolved and particulate 137Cs were measured in river water samples collected from two rivers (the Hiso and Wariki rivers, mainly draining farmlands and forests, respectively) draining approximately 4–7 km2 watersheds in a montane area (the areal deposition of 134+137Cs; 1–3 MBq·m−2 in March 2011) of Iitate Village, Fukushima. Over the 10 yr analysis, the particulate 137Cs concentrations in the Hiso and Wariki rivers decreased by 70 and 50 times, respectively, and that of the dissolved form decreased by 150 and 130 times, respectively. Moreover, the apparent Kd (distribution coefficient) of 137Cs for water samples from both rivers have increased gradually over these periods. In 2011, the 137Cs discharge rates through the Hiso and Wariki rivers were 0.63% and 0.46% per year of the total amount of 137Cs deposited in their catchments, respectively; however, by 2021, these rates had decreased to 0.09% and 0.03% per year. The cumulative 137Cs discharge rates over 10 yr in the farmland- and forest-dominated rivers were 1.95% and 1.33%, respectively. These rates of the farmland-dominated river were ∼1.4-fold greater than those of the forest-dominated river. Moreover, ∼90% of the of the discharge occurred in particulate form while the remaining ∼10% was in the dissolved form. Thus, 137Cs deposited within these watersheds due to the accident was gradually discharged by the rivers over the 10-yr period; however, the majority remains stored in soils and litters etc. of the catchment area. These results indicate that 137Cs outflow from land-to-ocean will be limited in the future, as the river export of 137Cs is expected to continue decreasing.