Quantifying groundwater-derived 137Cs fluxes to surrounding coastal waters using radium isotope

Large amounts of radioactive cesium (¹³⁴Cs and ¹³⁷Cs) were released as a result of the Fukushima Daiichi Nuclear Power Plant (F1NPP) accident. Even 15 years after the accident, ¹³⁷Cs concentrations in the marine environment have not returned to pre-accident levels, indicating that leakage from areas outside of the F1NPP site may still be ongoing.[1] Although ¹³⁷Cs concentrations in sandy beach groundwater outside the F1NPP site have been reported to be higher than those in seawater, suggesting groundwater as a major leakage pathway, no observational data are available from areas in close proximity to the plant.[2] Based on these considerations, this study aims to estimate discharge flux of ¹³⁷Cs originating from groundwater in the coastal waters surrounding the F1NPP.

Groundwater-derived ¹³⁷Cs discharge flux (Bq day^-1) was estimated by dividing the inventory (Bq) by the residence time of groundwater (day). Residence times following groundwater discharge to the coastal ocean were estimated using changes in the short-lived radium isotope activity ratios (²²³Ra/²²⁴Ra) between groundwater and seawater. Radium isotopes were selected as groundwater tracers for three reasons: (i) they were scarcely released from the F1NPP, such that the influence of the accident on Ra isotopes can be considered negligible[3]; (ii) radium isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra, and ²²⁸Ra) exhibit pronounced concentration differences between groundwater and seawater; and (iii) the wide range of half-lives and multiple isotopes enables their application to the estimation of water residence times as well as to the quantification of nutrient fluxes transported via submarine groundwater discharge.[4] In addition, the spatial area representative of ¹³⁷Cs leakage for inventory estimation was defined based on the variability of Ra isotopes and ³H. The mean ¹³⁷Cs concentration within the target domain was determined using seawater sampling data of ¹³⁷Cs concentrations conducted by Tokyo Electric Power Company Holdings, Inc. (TEPCO HD). The ¹³⁷Cs inventory (Bq) was then calculated by multiplying the mean ¹³⁷Cs concentration (Bq m⁻³) by the volume (m³) of the target domain.

The calculated discharge flux is from 2.1×10⁹ to 8.6×10⁹ (Bq day⁻¹). These values are comparable to the flux required to sustain coastal ¹³⁷Cs concentrations (2.0 × 10⁹ Bq day⁻¹)[1], indicating that submarine groundwater discharge may explain why ¹³⁷Cs concentrations in the vicinity of the FDNPP have not returned to pre-accident levels.

 

[1]Tsumune et al., J Environ Radioact , 2024

[2]Sanial et al., Proc Natl Acad Sci, 2017

[3]Buesselar et al., Ann Rev Mar Sci , 2017

[4]Garcia-Orellana et al., Earth-Science Review, 2021