Transport of Particulate ¹³⁷Cs in the Coastal Area off Fukushima Based on Long-Term Continuous Measurement

Since the accident at the Fukushima Daiichi Nuclear Power Station in March 2011, numerous studies have examined the behavior of radioactive cesium (¹³⁷Cs) in the ocean. Recent studies suggest that large amounts of particulate ¹³⁷Cs deposited on land are transported to coastal waters via rivers, becoming a major source of coastal ¹³⁷Cs input. Although numerical simulations and conceptual studies indicate that particulate ¹³⁷Cs entering coastal waters can be transported offshore through sedimentation, resuspension, and lateral transport, long-term, high-frequency observational studies remain limited. In this study, we evaluated the transport of particulate ¹³⁷Cs in coastal area off Fukushima using one year of continuous measurement data from multiple moored systems.

Moored systems were deployed at three sites near the mouth of the Ukedo River, where current velocity, current direction, and turbidity were continuously measured from February 2017 to February 2018. These data were combined with regularly collected measurements of suspended solid concentrations (mg/L) and particulate ¹³⁷Cs concentrations (Bq/L) to estimate hourly lateral fluxes of particulate ¹³⁷Cs (Bq/h). The study area is influenced by ¹³⁷Cs inputs transported via the Ukedo River, and the relationships between particulate ¹³⁷Cs fluxes and seasonal variability, meteorological conditions (waves, precipitation, and wind), and river discharge were analyzed. Furthermore, by focusing on differences in fluxes among the observation sites, we examined the factors controlling riverine input and transport variability from the coastal area toward offshore.

This study uses long-term monitoring data off Fukushima to improve understanding of particulate ¹³⁷Cs transport processes in coastal waters and to provide observational constraints for future numerical modeling studies.