Importance of desorption process from Abukuma River's suspended particles in increasing dissolved Cs in coastal water during river-flood caused by typhoons.

Desorption of radiocesium (Cs) from riverine particles into seawater strongly influences Cs concentrations in coastal seawater. This process is important for quantifying the input of radionuclides to marine environments. Here we quantify the particulate Cs flux from the Abukuma River, Japan, during typhoon Hagibis and following typhoons in 2019 and estimate the resulting increased dissolved Cs levels in coastal seawater. Particulate Cs export flux, 1.1 × 10 Bq, from the Abukuma River during the 4-day period of typhoon Hagibis (12-15 October 2019) equaled two-thirds of the annual flux during 2012-2015, the period of high Cs levels following the Fukushima Daiichi Nuclear Power Plant accident. The flux of the desorbed fraction from the Abukuma River during typhoon Hagibis was 0.061-0.12 × 10Bq, and its daily flux to the surrounding coastal seawater (1.5-3.0 × 10 Bq/d) was one to two orders of magnitude greater than the estimated input to the coastal seawater during the pre-typhoon period (1.3× 10-1.0 × 10 Bq/d). Simulated results suggest that the massive influx of riverine particles and subsequent desorption of Cs increased dissolved Cs levels in the coastal seawater by an order of magnitude, from 3.3 mBq/L (pre-typhoon level) to 45-126 mBq/L. This found pathway opens up new scenarios involving radionuclide dynamics in the boundary area of river-sea system.