Radiocesium accumulation in Lake Kasumigaura by riverine input and migration following the Fukushima Dai-ichi nuclear power plant accident.

Vertical radiocesium concentration profiles and inventories in sediments were measured in Lake Kasumigaura following the 2011 Fukushima Dai-ichi Nuclear Power Plant accident. Further measurements of radiocesium concentrations in suspended solids (SS) have been conducted since September 2012 in the Koise and Sakura rivers inflowing into the lake. Cesium-137 (Cs) accumulated intensively near the inflow outlets in the lake. At the lake center, the Cs inventory in sediments increased during 2011-2014; however, few changes were observed during 2014-2016. The Cs surface concentration and inventory decreased considerably in Tsuchiura-iri Bay until 3 years after the accident, indicating Cs migration. However, the rate of decrease subsequently slowed due to the Cs supply from the river. The Cs concentration in river SS declined during 2012-2015; however, it remained 1-2 orders of magnitude above its pre-accident level. The entrainment coefficient of particulate Cs in the inflows was initially higher in the Koise River but decreased exponentially more rapidly in the Koise River than in the Sakura River until 2015. Therefore, in the future, the difference in Cs concentrations will be smaller. The Cs concentration in the Koise River will continue to decrease; thus, the difference in the Cs inventory between the northern and southern parts of the lake will decrease. Total estimated amounts of Cs in the entire lake were 3.72 × 10 Bq in December 2012 and 4.18 × 10 Bq in August 2016. The accumulated amount of Cs in the entire lake based on sediment analysis was similar to the riverine input of particulate Cs based on riverine SS analysis from December 2012‒;August 2016, confirming the high trapping performance of the lake for particulate matter provided by the basin. Moreover, the amount of Cs accumulated in the lake in 2016 may have originated from comparable rates of atmospheric deposition and riverine input. These findings provide useful insights for future prediction and management of radiocesium contamination and the effects of riverine inputs in general shallow lakes.