Suspended Particle–Water Interactions Increase Dissolved ¹³⁷Cs Activities in the Nearshore Seawater during Typhoon Hagibis

Abstract

Distributions of 137 Cs in dissolved and particulate phases of the downstream reaches of seven rivers and adjacent nearshore and offshore waters as far as ∼60 km south of the Fukushima Dai-ichi nuclear power plant (FDNPP) were studied during the high-river-flow period (June−September 2019) and during the period of October 2019 after typhoon Hagibis. Dissolved 137 Cs activities in nearshore water were higher than those in rivers and offshore waters, and this distribution was more intensified after the typhoon, indicating the desorption of 137 Cs from riverine suspended particles in addition to the ongoing release of contaminated water from the FDNPP and re-entry of radiocesium via submarine groundwater discharge. This scenario is also supported by the reduction of distribution coefficient (K d ) from a geometric mean value of 5.5 × 10 5 L/kg in rivers to 9.8 × 10 4 L/kg in nearshore water. The occupation of desorbed 137 Cs to the dissolved activity of this nuclide in nearshore water was estimated to be 0.7%−20% (median: 9.7%) during the high-river-flow period, increasing to 1.4%−66% (32.3%) after the typhoon, suggesting that the desorption during the flood period such as typhoons further contributes to the increase in dissolved 137 Cs levels in nearshore water.

Spread of Fukushima‐derived radiocesium over the coastal ocean in response to typhoon‐induced flooding in September 2011

“…That is, it is reasonable to expect that a considerable portion of the radiocesium was in particulate form at the inshore stations, whereas it was mainly in dissolved form at the offshore stations. The reason for the presence of particulate radiocesium at the inshore stations is that, as mentioned above, the large amount of river runoff stirred up and resuspended radiocesium that had already accumulated on the shallow sea bottom near the river mouths (Charette et al 2013; Otosaka and Kobayashi 2013; Otosaka and Kato 2014; Buesseler et al 2015; Sanial et al 2017; Uchiyama et al 2017; Takata et al 2020).…”

“…The probability of this type of discharge will increase if a large runoff event occurs just before a water‐sampling survey, which was the case in this study (as will be discussed in the subsection of the Precipitation and river discharge section). Moreover, it is plausible that the large amount of river runoff stirred up and resuspended radiocesium that had already accumulated on the shallow sea bottom near the river mouths; high levels of radiocesium had been deposited onto seabed sediments in coastal areas (Charette et al 2013; Otosaka and Kobayashi 2013; Otosaka and Kato 2014; Buesseler et al 2015; Sanial et al 2017; Uchiyama et al 2017; Takata et al 2020).…”

“…In particular, Takata et al (2020) found that the radioactivity of cesium was higher in nearshore waters than in river and offshore waters during the typhoon season (October 2019), indicating the desorption of radioactive cesium from riverine suspended particles in the nearshore waters. However, the typhoon‐induced riverine discharge of radioactive cesium has not been thoroughly investigated, especially for further spread over the coastal ocean: in Takata et al (2020), the oceanic study area was located within 20 km from the coast.…”

Visualization of Radiocesium Distribution in Surface Layer of Seafloor Around Fukushima Daiichi Nuclear Power Plant

“…Anomalies were observed along a cliff in these at areas, 5 km from the river mouth. Radiocesium is transported to sea in combination with ne particles smaller than 10 mm 9,15 . The distribution characteristics in the contour maps showed that such ne particles owing along the rivers were carried by approximately 5 km from the river mouth and accumulated in the hollow of a cliff.…”

“…Otosaka (2017) estimated that > 43% of 137 Cs in the surface sediment around the FDNPP was transported to deeper sediment layers via vertical mixing of sediments from March 2011 to the end of 2015 8 . Moreover, radiocesium, which ows from rivers to seas, increases during ooding events, such as typhoons 9 . These evaluation values are based on limited monitoring data, such as grab sampling and short core sampling data of several dozens.…”