Reservoir sediments generally act as a sink for radionuclides derived from nuclear accidents, but under anaerobic conditions, several radionuclides remobilize in bioavailable form from sediments to water columns, which may contribute to the long-term contamination of aquatic products.
This study systematically investigated the ¹³⁷Cs activities of sediment-pore water, providing a direct evidence of the remobilization of bioavailable ¹³⁷Cs from sediments in a highly contaminated reservoir (Ogaki Dam Reservoir) affected by the TEPCO’s Fukushima Daiichi Nuclear Power Station accident.

Fig.1　Vertical distribution of ¹³⁷Cs and water quality in sediment-pore water
Vertical distribution of (a) ¹³⁷Cs activity concentrations, (b) distribution coefficient (K_d) values for ¹³⁷Cs in sediment-pore water and (c) water quality in the Ogaki Dam Reservoir in July 2019.

We observed that the dissolved ¹³⁷Cs activity concentration of pore water (3.0−65.8 Bq/L) was one to two orders of magnitude higher than that of reservoir water (Fig.1(a)). The distribution coefficient (K_d) values for the ¹³⁷Cs of sediment-pore water ranged from 2.6×10³ L/kg to 1.2×10⁴ L/kg and decreased with depth (Fig.1(b)). The major ¹³⁷Cs competing cation NH₄⁺ (0.24-1.22 mmol/L) was significantly higher than the K⁺ concentration (0.04-0.11 mmol/L) (Fig.1(c)).

Fig.2　Correlations between the K_d values for the ¹³⁷Cs and NH₄⁺ concentration in pore water
The blue color line indicates the exponential regression line of the Ogaki Dam Reservoir. The orange color line denote the previously reported exponential regression line in European lakes (Comans, R.N.J. et al.*).

Moreover, the K_d values were significantly and negatively correlated with the NH₄⁺ concentration and the exponential regression lines follow a straight line with an approximate slope close to −1 (Fig.2).

These results strongly indicate that the competitive ion exchange process between ¹³⁷Cs and NH₄⁺ through a highly selective interaction with the FESs of clay minerals is the major reason for the variability of K_d values between sediments and pore water. However, when compared under similar NH₄⁺ concentrations, the K_d values in Ogaki Dam Reservoir were higher than in some European lakes, suggesting that the sediments of Ogaki adsorb ¹³⁷Cs strongly and are less likely to remobilize (Fig.2).

Our findings provide important parameter values for mid- and long-term assessments of the radiation impact of radionuclide discharges to freshwater environments.

* Comans, R.N.J. et al., Interpreting and Predicting in Situ Distribution Coefficients of Radiocaesium in Aquatic Systems, Studies in Environmental Science, vol.68, 1997, p.129−140.

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Reference

1. Funaki, H. et al., Remobilisation of Radiocaesium from Bottom Sediments to Water Column in Reservoirs in Fukushima, Japan, Science of The Total Environment, vol.812, 2022, 152534, 10p.