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

After the Fukushima Dai-ichi nuclear power plant accident in March 2011, the fluvial discharge of Cs-137 from watersheds to rivers was analyzed between 2011 and 2021. The concentrations of dissolved and particulate Cs-137 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 km(2) watersheds in a montane area (the areal deposition of Cs134+137; 1-3 MBq.m(-2) in March 2011) of Iitate Village, Fukushima. Over the 10 yr analysis, the particulate Cs-137 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 K-d (distribution coefficient) of Cs-137 for water samples from both rivers have increased gradually over these periods. In 2011, the Cs-137 discharge rates through the Hiso and Wariki rivers were 0.63% and 0.46% per year of the total amount of Cs-137 deposited in their catchments, respectively; however, by 2021, these rates had decreased to 0.09% and 0.03% per year. The cumulative Cs-137 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 similar to 1.4-fold greater than those of the forest-dominated river. Moreover, similar to 90% of the of the discharge occurred in particulate form while the remaining similar to 10% was in the dissolved form. Thus, Cs-137 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 Cs-137 outflow from land-to-ocean will be limited in the future, as the river export of Cs-137 is expected to continue decreasing.

Automated summary

The study analyzed the discharge of Cs-137 from two rivers in the Fukushima area over a period of 10 years following the nuclear accident in 2011. It was found that the concentrations of both dissolved and particulate Cs-137 in the rivers decreased significantly, while the distribution coefficient of Cs-137 gradually increased. Furthermore, the discharge rates of Cs-137 from the rivers decreased over time, indicating a limited future outflow of Cs-137 from land to the ocean.