Evaluation of sources and seasonal variations of dissolved ¹³⁷Cs in stream water in forested catchments

The Fukushima Daiichi Nuclear Power Plant accident released substantial radiocesium into terrestrial environments in 2011. Understanding the sources of dissolved radiocesium and the factors controlling its seasonal variation in stream water is crucial for assessing and mitigating environmental radioactive contamination. From 2015 to 2021, we investigated dissolved ¹³⁷Cs concentrations and dissolved organic carbon (DOC) in stream water, groundwater (just before groundwater spring), groundwater, litter leachate, soil pore water, and infiltrated water. In the headwater area, dissolved ¹³⁷Cs concentrations increased just after groundwater spring compared to the concentrations in the groundwater. During approximately three years of stream water monitoring, dissolved ¹³⁷Cs concentrations correlated with water temperature, concentrations of DOC, and K⁺, revealing that in a forested catchment, the influence of organic matter is the primary factor in the mechanism of the relationship between water temperature and dissolved ¹³⁷Cs concentration, rather than the influence of K⁺. A three-component mixture model revealed that stream water composition is derived mainly from groundwater, leaching from forest litter/surface soil, and soil pore water. The soil pore water particularly influenced summer stream water, likely due to the water table fluctuations with precipitation. The leaching water also influenced the summer stream water compared to the winter stream water. These findings suggest that seasonal variations in dissolved ¹³⁷Cs are driven by temperature-dependent leaching from surface forest litter/soil and increased contributions from soil pore water. This study is the first to identify the sources and seasonal drivers of dissolved ¹³⁷Cs in forested headwater streams.