Importance of desorption process from Abukuma River's suspended particles in increasing dissolved 137Cs in coastal water during river-flood caused by typhoons

Abstract

Desorption of radiocesium (¹³⁷Cs) from riverine particles into seawater strongly influences ¹³⁷Cs concentrations in coastal seawater. This process is important for quantifying the input of radionuclides to marine environments. Here we quantify the particulate ¹³⁷Cs flux from the Abukuma River, Japan, during typhoon Hagibis and following typhoons in 2019 and estimate the resulting increased dissolved ¹³⁷Cs levels in coastal seawater. Particulate ¹³⁷Cs export flux, 1.1 × 10¹² 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 ¹³⁷Cs 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 × 10¹²Bq, and its daily flux to the surrounding coastal seawater (1.5-3.0 × 10¹⁰ 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× 10⁸-1.0 × 10⁹ Bq/d). Simulated results suggest that the massive influx of riverine particles and subsequent desorption of ¹³⁷Cs increased dissolved ¹³⁷Cs 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.

Keywords: Desorption; Dissolved Cs; Riverine suspended particle; Typhoon.