Why Are Dissolved ¹³⁷Cs Concentrations Lower in Fukushima Rivers? A Comparative Study with European Catchments

Nuclear accidents contaminate large terrestrial areas with long-lived radionuclides, and river systems play a key role in their redistribution. The concentration of dissolved radiocaesium (¹³⁷Cs) in river water is influenced by catchment-scale physical and geochemical characteristics. After the Chernobyl accident, environmental radionuclide concentrations generally declined over time; however, systematic inter-river comparisons remain limited, and the key factors controlling long-term differences in dissolved ¹³⁷Cs concentrations are still poorly understood.

In this study, we investigated the environmental behavior of dissolved ¹³⁷Cs in river systems affected by the Fukushima Daiichi Nuclear Power Plant accident and compared it with long-term observations from major European rivers impacted by the Chernobyl accident. In Fukushima, river water samples were seasonally collected between 2021 and 2024 from headwater catchments in the Yamakiya and Kuchibuto River basins. Samples were filtered through 0.22 µm membranes, and dissolved ¹³⁷Cs was measured using high-purity germanium detectors. Major ions (K⁺, NH₄⁺), stable ¹³³Cs, and dissolved organic carbon (DOC) were also analyzed. Univariate and multivariate regression analyses were applied to identify dominant release mechanisms. Catchment land cover, topographic gradients, and precipitation were analyzed using GIS, and groundwater residence times were estimated. These results were compared with long-term monitoring data and additional field measurements from nine European river catchments in Ukraine, Finland, Austria, and Italy, incorporating climatic, vegetation, and anthropogenic factors into an international comparison framework.

In Fukushima headwater catchments, dissolved ¹³⁷Cs concentrations increased from summer to autumn, coinciding with rising temperatures, enhanced organic matter decomposition, and increased K⁺ availability. Multiple regression analysis identified ¹³³Cs and K⁺ as significant explanatory variables, indicating that ion exchange plays a key role in ¹³⁷Cs mobilization. In contrast, DOC showed only a weak relationship with ¹³⁷Cs in Fukushima rivers. Comparative analysis of dissolved ¹³⁷Cs trends since 1986 revealed that European rivers have maintained higher concentrations over longer periods. Correlation analysis demonstrated that DOC and ¹³³Cs were significant scaling factors controlling dissolved ¹³⁷Cs concentrations across European river systems, whereas K⁺ and NH₄⁺ contributed little to concentration variability.

These results indicate that differences in the long-term behavior of dissolved ¹³⁷Cs between Fukushima and European rivers are associated with contrasting DOC- and ¹³³Cs-related controls at the catchment scale. This study suggests that accounting for regional variability in biogeochemical controls should be useful for long-term river environment and also can inform environmental modeling of radionuclide transport under nuclear emergency conditions, contributing to improved preparedness and long-term risk assessment.