Remobilisation of radiocaesium from bottom sediments to water column in reservoirs in Fukushima, Japan.

Reservoir sediments generally act as a sink for radionuclides derived from nuclear accidents, but under anaerobic conditions, several radionuclides remobilise in bioavailable form from sediments to water columns, which may contribute to the long-term contamination of aquatic products. This study systematically investigated the ¹³⁷ Cs activities of sediment-pore water, providing a direct evidence of the remobilisation of bioavailable ¹³⁷ Cs from sediments in two highly contaminated reservoirs affected by the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. We observed that the dissolved ¹³⁷ Cs activity concentration of pore water (3.0-65.8 Bq L ^-1 ) was one to two orders of magnitude higher than that of reservoir water. Moreover, the distribution coefficient (K _d ) values for the ¹³⁷ Cs of sediment-pore water (2.6-14 × 10 ³  L kg ^-1 ) decreased with depth. The K _d values were significantly and negatively correlated with the concentration of the major ¹³⁷ Cs competing cation NH ₄ ⁺ . Our results strongly indicate a competitive ion exchange process between ¹³⁷ Cs and NH ₄ ⁺ via a highly selective interaction with the frayed edge sites of phyllosilicate minerals, which is the major reason for the variability of K _d values of sediment-pore water, even in the Fukushima case. Additionally, the sediment accumulation rates were relatively high, and the annual depositional rate of exchangeable ¹³⁷ Cs prevailed over the annual diffusive flux of ¹³⁷ Cs from the sediment to the overlying water. This finding indicates that even after 10 years since the FDNPP accident, the bioavailable ¹³⁷ Cs is still continuously supplied from the catchment covered by mountainous forests, and reservoir sediments are a long-term important source of bioavailable ¹³⁷ Cs in the riverine system. Our findings provide important parameter values for mid- and long-term assessments of the radiation impact of radionuclide discharges to freshwater environments.
Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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