DOWNSTREAM TRANSPORT OF RADIOCESIUM IN FUKUSHIMA RIVERBANKS: USING RELIEF RATIO AS A PREDICTOR ALONG PACIFIC-CONNECTED RIVERS

Abstract

Radioactive materials, notably radiocesium, were released into the environmentfollowing the Fukushima Daiichi Nuclear Power Plant accident in 2011. This study investigates the distribution and downstream movement of radiocesium in riverbank soils of Fukushima Prefecture, with a particular focus on the effects of relief ratio and watershed area on erosion and redeposition processes driven by tidal fluctuations. Air dose rates were systematically monitored by the Japanese Ministry of Environment at 97 sites. The research tested the predictive value of relief ratio and watershed areas for changes in air dose rates before and after water level events. However, statistical analysis revealed no significant correlation between these variables and observed fluctuations in air dose rate, either holistically or at the individual watershed scale. While individual site analysis held higher correlations, results were not statistically significant. Additionally, a comparison of observed and physical decay rates from 2015 to 2024 indicated persistent differences four years after the accident. Overall, the findings suggest that neither relief ratio nor watershed area reliably predict radiocesium behavior in the studied riverbanks.