Operations of a pumped storage power plant attenuated radioactive Cs concentration in kokanee stocked in the upper reservoir.

This study investigates the seasonal variation in radioactive cesium (sum of '134Cs and '137Cs; hereafter radioactive Cs) concentrations in kokanee (Oncorhynchus nerka) within a pumped-storage power plant reservoir system, with particular focus on the marked decline in radioactive Cs levels during plant operation. Radioactive Cs transfer pathways to kokanee were examined in relation to water residence time, vertical temperature profiles, and water quality under operational and non-operational conditions. Two primary pathways originating from the contaminated forest surrounding the reservoir were identified. The first involves suspended detritus, primarily decomposed forest litter, which accumulates at the thermocline and facilitates Cs transfer to kokanee via interactions with plankton. The second pathway involves detritus deposited on the lakebed, consumed by chironomids, which serve as prey for kokanee. Radioactive Cs concentrations in chironomids were higher than those in plankton, suggesting that seasonal dietary shifts in kokanee contribute to observed variations in radioactive Cs accumulation. Power plant operations can transport part of the contaminated detritus accumulated at the thermocline out of the lake, thereby inhibiting its sedimentation on the lake bottom, reducing radioactive Cs availability to lower trophic organisms, and resulting in a marked reduction of radioactive Cs concentrations in kokanee. These findings highlight the influence of anthropogenic operations on Cs dynamics in aquatic ecosystems and have important implications for ecological risk assessment and the management of radiological contamination.