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

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.

The contribution of 137Cs export flux from the Tone River Japan to the marine environment.

The contribution of 137Cs transport to the marine environment via the Tone River, Japan was investigated. This river has the largest discharge among rivers on the North Pacific side of eastern Japan. The sampling site was located upstream near the river mouth and dissolved and particulate 137Cs in the river water was measured during 2014-2015, three years after the Tokyo Electric Power Corporation Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Annual fluxes of total (dissolved and particulate) 137Cs with considering desorption of 137Cs from riverine particles by change of salinity from the Tone River were similar in both years (78-107 × 109 Bq/y), indicating that about 0.03-0.06% of the estimated total amount of 137Cs deposited in the catchment (1.9-2.8 × 1014 Bq) was transported to the marine environment each year. Although the annual flux was about one order of magnitude lower than the daily direct discharge into the ocean from the FDNPP (800 × 109 Bq/y) during the corresponding period, continuous monitoring of rivers in the southern coastal area of east Japan on the North Pacific side are needed for the effect of 137Cs release via the rivers in the Kanto area over the long-term.

Appearances of Fukushima Daiichi Nuclear Power Plant-Derived 137Cs in Coastal Waters around Japan: Results from Marine Monitoring off Nuclear Power Plants and Facilities, 1983-2016.

Monitoring of 137Cs in seawater in coastal areas around Japan between 1983 and 2016 yielded new insights into the sources and transport of Fukushima Daiichi Nuclear Power Plant (FDNPP)-derived 137Cs, particularly along the west coast of Japan. Before the FDNPP accident (1983-2010), the activity concentrations of 137Cs, mainly from fallout, were decreasing exponentially. Effective 137Cs half-lives in surface seawater ranged from 15.6 to 18.4 yr. After the FDNPP accident (March 2011) 137Cs activity concentrations in seawater off Fukushima and neighboring prefectures immediately increased. Since May/June 2011, 137Cs activity concentrations there have been declining, and they are now approaching preaccident levels. Along the west coast of Japan remote from FDNPP (i.e., the Japan Sea), however, radiocesium activity concentrations started increasing by 2013, with earlier (May/June 2011) increases at some sites due to airborne transport and fallout. The inventory of 137Cs in the Japan Sea (in the main body of the Tsushima Warm Current) in 2016 was calculated to be 0.97 × 1014 Bq, meaning that 0.44 × 1014 Bq of FDNPP-derived 137Cs was added to the estimated global fallout 137Cs inventory in 2016 (0.53 × 1014 Bq). The net increase of 137Cs inventory in the Japan Sea through the addition of FDNPP-derived 137Cs accounts for approximately 0.2% of the total 137Cs flux from the plant to the ocean from the accident.

The Contribution of Sources to the Sustained Elevated Inventory of (137)Cs in Offshore Waters East of Japan after the Fukushima Dai-ichi Nuclear Power Station Accident.

We have evaluated the contribution of sources of (137)Cs to the inventory of radiocesium in waters (surface area: 6160 km(2), water volume: 753 km(3)) off Fukushima Prefecture and neighboring prefectures from May 2011 to February 2015. A time-series of the inventory of (137)Cs in the offshore waters revealed a clearly decreasing trend from May 2011 (283.4 TBq) to February 2015 (1.89 TBq). The (137)Cs inventory about four years after the accident was approximately twice the background inventory of 1.1 TBq. The magnitudes of the (137)Cs influxes from sources into offshore waters for periods of 182-183 days were estimated from the first period (1 October 2011 to 31 March 2012: 15.3 TBq) to the last period (1 October 2014 to 31 March 2015: 0.41 TBq). We assumed that three sources contributed (137)Cs: continuous direct discharge from the Fukushima Dai-ichi Nuclear Power Station (FNPS) even after the massive discharge in late March 2011, desorption/dissolution from sediments, and fluvial input. Quantification of these sources indicated that the direct discharge from the FNPS is the principal source of (137)Cs to maintain the relatively high inventory in the offshore area.

Plutonium isotopes concentration in seawater and bottom sediment off the Pacific coast of Aomori sea area during 1991-2005.

A radioactivity survey was launched in 1991 to determine the background levels of ²³9+²4°Pu in the marine environment off a commercial spent nuclear fuel reprocessing plant before full operation of the facility. Particular attention was focused on the ²4°Pu/²³9Pu atom ratio in seawater and bottom sediment to identify the origins of Pu isotopes. The concentration of ²³9+²4°Pu was almost uniform in surface water, decreasing slowly over time. Conversely, the ²³9+²4°Pu concentration varied markedly in the bottom water and was dependent upon the sampling point, with higher concentrations of ²³9+²4°Pu observed in the bottom water sample at sampling points having greater depth. The ²4°Pu/²³9Pu atom ratio in the seawater and sediment samples was higher than that of global fallout Pu, and comparable with the data in the other sea area around Japan which has likely been affected by close-in fallout Pu originating from the Pacific Proving Grounds. The ²4°Pu/²³9Pu atom ratio in bottom sediment samples decreased with sea depth. The land-originated Pu is not considered as the reason of the increasing ²³9+²4°Pu concentration and also decreasing the ²4°Pu/²³9Pu atom ratio with sea depth, and further study is required to clarify it.