ИСТИНА

Redistribution of radiocesium derived from the accident of Fukushima Dai-ichi Nuclear Power Plant through river system pose potential risks on residents in the downstream area of river basin. Niidagawa river basin (265 km2) on north eastern part of Fukushima prefecture is indeed a case in point. The river basin has highly contaminated area on its upstream part and urban area on its downstream part. Quantitative evaluation of Cs-137 redistribution associated with sediment is required for mitigating radiological risk on residents and ocean environment. Previous studies on the environmental monitoring of various contaminants, as well as radiocesium, suggested that large portion of discharge of such contaminants should occur during intensive rainfall events. This study aims to quantify radiocesium discharge in the Niidagawa river Basin with employing instruments grasping sediment discharge even during intensive rainfall events. Niida river basin includes area highly affected by Fukushima Nuclear Power Plant accident. For quantifying radiocesium discharge, we installed sets of integrated suspended sediment sampler, turbidity sensor and water level sensor on three points in summer 2014. The points are named as Sakegawa-bashi (N37°38’33”, E141°00’20”), Notegami-kita (N37°39’16”, E140°47’47”) and Warabi-daira (N37°36’49”, E140°48’04”). Specific intial Cs-137 deposition are 752, 810, 1462 kBq/m2, respectively. We have collected suspended sediments trapped in the samplers and served them for radiocesium measurements with HPGe gamma detectors in laboratory. Radiocesium concentrations (Bq/kg) of suspended sediment samplers were converted into radiocesium discharge (Bq) with multiplying time-series of turbidity (mg/L) and water discharge (m3/s). Mean Cs-137 concentration of suspended sediment (n = 3) at Sakegawa-bashi, Notegami-kita and Warabi-daira were 13000, 14000, and 31000 Bq/kg, respectively. Those concentrations appeared to agree with initial Cs-137 deposition for each point basically. No common temporal trend of Cs-137 concentration was found among Cs-137 concentrations of these three points. Estimated total Cs-137 discharges at these points were in 1011 Bq order. Estimated Cs-137 wash-off rates (%), calculated by dividing specific Cs-137 discharge (Bq/m2) with initial Cs-137 inventory, were in 10-1 order. For evaluating Cs-137 discharge during intensive rainfall events, we focused on three large rainfall events during 8-11th August (59 mm), 5-8th October (109 mm) and 13-16th October (69.5 mm). Ratios of Cs-137 discharges during these events to total Cs-137 discharge through all observation period approximately were approximately 0.3 at Sakegawa-bashi and approximately 0.7 at both of Notegami-kita and Warabi-daira. Observation points with smaller catchment area appeared to have observed high discharge Cs-137 during intensive rainfall events. Our observation has quantified Cs-137 discharges as well as the discharge during intensive rainfall events from a river basin. The results suggested that total Cs-137 discharge in the basin is very small with comparing to total Cs-137 deposition. It was found that most parts of Cs-137 are discharged particularly from area of smaller catchment during intensive rainfall events. Continuous observations will provide information on seasonal variation of Cs-137 transfer and detailed processes of its discharge during extreme rainfall events.