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.