Vertical distributions of 137Cs and Pu in sediments of the Lake Inba after the Fukushima Daiichi Nuclear Power Plant accident

We collected two sediment cores on 11 August 2011 (Core-1) and 30 July 2012 (Core-2) at the same position in Lake Inba to study the vertical distributions of 137Cs and Pu in sediments after the Fukushima Daiichi Nuclear Power Plant accident. Radiocesium (134Cs, 137Cs) and Pu isotopes (239Pu, 240Pu) were analyzed. The 134Cs/137Cs activity ratios of upper layers in the two sediment cores were approximately 1 (decay corrected to 15 March 2011), which suggested the FDNPP accident-derived radionuclides have contaminated the samples. For Core-1, 137Cs inventory was calculated to be 32300 ± 660. The 137Cs inventory of the surface sediments (0-2 cm) accounted for about 34% of total inventories of Core-1. The 137Cs inventory was around 37500 ± 330 Bq/m2 in Core-2. Meanwhile, the 137Cs inventory of surface sediment (0-2 cm) in core-2 accounted for 17% of the total inventory. The activities of 239+240Pu ranged from 0.007 to 0.298 Bq/kg, which was comparable with that of freshwater lake and marine sediments around Japan. The atom ratios of 240Pu/239Pu around 0.18 indicated that global fallout was the main source for Pu contamination in the lake sediments