Assessment of accumulation of radioactive cesium in lake and dam reservoir

Lake and dam reservoir in Gunma Prefecture in eastern Japan was contaminated by the radiocesium released during the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident. There was radiocesium peak in the sediment surface only by the FDNPP accident in natural lake such as Lake Akagi onuma and Lake Haruna. The inventory in sediment in Lake Ozenuma was 7,500 Bq/m² by the radiocesium deposits after the FDNPP accident. The sedimentation rate of Lake Ozenuma was estimated 3.4mm by the global fallout in 1960's. In dam reservoir of Lake Umeda and Lake Akaya, there were several times peak of radiocesium because of the flooding from watershed. Total radiocesium deposits in dam reservoir increased about 3.3 fold of initial radiocesium deposits in 2015. The sedimentation rate of Pond Kondo was 37mm of very fast estimated by the fallout from the FDNPP accident. The ¹³⁷Cs in interstitial water of lake sediment in Lake Onuma was high level about parts per thousand of ¹³⁷Cs in sediment. (author)