Elevated radioactive contamination from the Fukushima nuclear power plant accident in aquatic biota from a river with a lake in its upper reaches

Abstract

Five years have passed since the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident occurred. Forests, streams, and lakes remain radioactively contaminated, with slight sign of convergence. The radiocesium concentration of brown trout (Salmo trutta) in Lake Chuzenji (160 km from the FDNPP) still exceeds the Japanese regulatory limit of 100 Bq·kg−1, likely due to elevated contamination in Lake Chuzenji. In this study, the concentration of 134Cs and 137Cs in algae, litter, sand substrate, and aquatic insects in a river originating from Lake Chuzenji (Daiya site) and in a nearby river (Watarase site) from 2013 to 2015 were compared. At the Daiya site, 134Cs and 137Cs concentrations of algae and aquatic insects were high (e.g., 137Cs in algae: 160 Bq·kg−1 at the Watarase site and 320 Bq·kg−1 at the Daiya site) and still increasing in some groups such as Perlodidae and Heptageniidae, though the mean air dose rate (0.05 μSv·h−1) was lower than that at the Watarase site (0.11 μSv·h−1). We attributed this to high flow out of higher 134Cs and 137Cs concentration originating from Lake Chuzenji. Thus, lower reaches fed by contaminated headwaters will likely experience prolonged contamination.