Fluvial discharges of radiocaesium from watersheds contaminated by the Fukushima Dai-ichi Nuclear Power Plant accident, Japan.

A large amount of radiocaesium, (134)Cs and (137)Cs, was released to the atmosphere and Pacific Ocean from the Tokyo Electric Power Company's Fukushima Dai-ichi Nuclear Power Plant (FDNPP) that was damaged by the tsunami caused by the Tohoku earthquake on 11 March 2011. Radiocaesium deposited on the ground is now the most important consideration in assessing the risk to people in the vicinity of the FDNPP and remediating the contaminated area. Transfer of radiocaesium from the ground through rivers is an important factor in the downstream contamination of irrigation waters, paddy fields, lakes, and the sea. We estimated the transport of radiocaesium through two small rivers, the Hiso River and Wariki River, that traverse mountainous areas in Fukushima Prefecture. Areal depositions of radiocaesium in their watersheds (catchments) were high (1-3 MBq m(-2)). Water samples were collected from the rivers twice during each of two baseflow and two flood stages in 2011 and analysed for radiocaesium in particulate and dissolved forms. The radiocaesium concentrations depended strongly on the rates of water discharge. Maximum activities of radiocaesium in the samples from the Hiso River and Wariki River when there was precipitation or flooding (July and September) were 25 ± 0.31 and 35 ± 0.25 Bq L(-1), respectively. Particulate radiocaesium during periods of flooding contributed over 90% of the total radiocaesium activity in the samples. The discharge of radiocaesium from the catchments during 2011 was estimated to be 0.5% and 0.3% of the total amount of radiocaesium deposited on the catchments of the Hiso River and Wariki River, respectively. It is considered that the most of the radiocaesium deposited in the catchment remains on the soil surface.