Investigation of sorption behavior of 137Cs in a river–sea system boundary area after the Fukushima Dai-ichi nuclear power plant accident

The radiocesium (¹³⁷Cs) distribution between dissolved and particulate phases was examined in river water and coastal seawater as a function of the ¹³⁷Cs sorption behavior on suspended particles. Dissolved ¹³⁷Cs activity concentrations in the Tomioka River (salinity <0.1), about 10 km south of Fukushima Dai-ichi Nuclear Power Plant, and in coastal seawater at Tomioka fishery port (salinity >30), Fukushima Prefecture, from June 2019 to October 2021 were 3.6–20 Bq/m³ (geometric mean 11 Bq/m³) and 2.4–86 Bq/m³ (13 Bq/m³), respectively. Although the suspended particle concentration was lower in the river than in seawater, the mean ¹³⁷Cs activity on suspended particles was 11,000 Bq/kg-dry in the river versus 3200 Bq/kg-dry in seawater. Proportions of ion-exchangeable, organically bound, and refractory fractions of ¹³⁷Cs on suspended particles were determined by sequential extraction. The ion-exchangeable fraction accounted for 0.3–2.0% (average: 1.2%) and 0.4–1.3% (0.8%) at the river and port sites, respectively. The organically bound fraction accounted for 0.3–4.8% (1.8%) and 0.1–5.5% (2.1%) at the river and port sites, respectively. In both areas, the refractory fraction accounted for >90% of ¹³⁷Cs. Therefore, the small labile ¹³⁷Cs fraction on suspended particles in coastal seawater indicates that the mobility of ¹³⁷Cs to marine biota is quite low.

Synopsis

This study is the first to examine radiocesium sorption forms on suspended particles in coastal seawater near the Fukushima Dai-ichi Nuclear Power Plant. It suggests immobility of ¹³⁷Cs in suspended particles being incorporated to marine biota.