Riverine 129I dynamics during high-flow events on the Abukuma River in Fukushima

This study presents the riverine ¹²⁹I concentrations and fluxes during high-flow events. The river water samples obtained by a previous study were subjected to ¹²⁹I analyses. River water samples were collected at the midstream of the Abukuma River for five and seven times during high-flow events in July 2018 and October 2018, respectively. Suspended solids and filtered water samples were measured for ¹²⁹I/¹²⁷I ratios using an accelerator mass spectrometer system and for ¹²⁷I concentrations using ICP-MS. Aggregated mean values of dissolved ¹²⁹I concentration and ¹²⁹I concentrations in suspended solids were 0.11 μBq/L (n = 12) and 0.60 mBq/kg (n = 12), respectively. Corresponding values of ¹²⁹I/¹²⁷I ratios were in 2.2 × 10⁻⁹ (n = 12) and 3.7 × 10⁻⁹ (n = 12), respectively. These concentrations and ratios were comparable to those at the background level before the Fukushima Dai-ichi nuclear power plant accident. Positive correlations with dissolved ¹³⁷Cs and Cl⁻ concentrations suggested that the dissolved ¹²⁹I concentration decreased due to dilution of the river water during the events. A positive correlation with total carbon content suggested that organic matter contents determine ¹²⁹I concentrations in suspended solids. The total ¹²⁹I fluxes during JUL18 and OCT18 were 9.9 × 10³ and 2.2 × 10⁴ Bq, respectively. Although dissolved ¹²⁹I was predominant under low-flow conditions, the particulate ¹²⁹I flux increased by one or two orders of magnitude during the peak water discharge phases. These results underline the importance of particulate ¹²⁹I dynamics during high-flow events for quantitative evaluation of the ¹²⁹I cycle.