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

We have evaluated the contribution of sources of 137Cs to the inventory of radiocesium in waters (surface area: 6160 km2, water volume: 753 km3) off Fukushima Prefecture and neighboring prefectures from May 2011 to February 2015. A time-series of the inventory of 137Cs in the offshore waters revealed a clearly decreasing trend from May 2011 (283.4 TBq) to February 2015 (1.89 TBq). The 137Cs inventory about four years after the accident was approximately twice the background inventory of 1.1 TBq. The magnitudes of the 137Cs 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 137Cs: 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 137Cs to maintain the relatively high inventory in the offshore area.