Field-Based Modeling of Cesium-137 Adsorption/Desorption Reactions in Fukushima River bottom Sediment

The 2011 Fukushima Daiichi Nuclear Power Plant (FDNPP) accident released significant amounts of radioactive Cesium-137 (¹³⁷Cs). Both dissolved and suspended forms of ¹³⁷Cs contribute to river runoff. Although the Ministry of the Environment has monitored ¹³⁷Cs in Fukushima river-bottom sediments since 2011, identifying long-term trends is challenging. Existing adsorption/desorption models focus on ideal lab conditions and lack real-world field data. This study uses long-term monitoring data to examine changes in ¹³⁷Cs concentration in river sediments. Sediment samples were collected every four months, while river water and suspended sediment were sampled quarterly. The model simulates adsorption/desorption reactions between river water and sediments, incorporating fast and slow reaction sites. By adjusting reaction rates, measured and modeled ¹³⁷Cs concentrations were optimized.　Results show ¹³⁷Cs concentrations in particle-size-corrected sediments aligned with suspended forms within a year. An increase in ¹³⁷Cs concentration at 45 sites was observed, a phenomenon not seen in dissolved or suspended forms. The distribution coefficient (Kd) of river bottom sediments fluctuated significantly in the first three years. Slow adsorption dominated ¹³⁷Cs accumulation within six months, while its effect on suspended concentrations was negligible. These findings highlight that slow adsorption/desorption processes are critical for the long-term behavior of 137Cs in river-bottom sediments.