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 Cs-137 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 Cs-137 in the offshore waters revealed a clearly decreasing trend from May 2011 (283.4 TBq) to February 2015 (1.89 TBq). The Cs-137 inventory about four years after the accident was approximately twice the background inventory of 1.1 TBq. The magnitudes of the Cs-137 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 Cs-137: 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 Cs-137 to maintain the relatively high inventory in the offshore area.