Factors of the slow decline in the activity concentration of ¹³⁷Cs in eutrophic lake water

We investigated the activity concentrations of dissolved 137Cs, particulate 137Cs, and total 137Cs (the sum of dissolved and particulate 137Cs) in Lake Inba, Japan, from 2019 to 2022 to predict the long-term decline in the 137Cs contamination. The effective half-lives of dissolved and total 137Cs were 2.9 and 18.1 years, respectively. We also observed no decreasing trend of particulate 137Cs. These results suggested that the more gradual decrease in the total 137Cs is caused by the small decline in particulate 137Cs. To decrease the total 137Cs, particulate 137Cs must be decreased. We found that the particulate 137Cs concentrations were correlated with the suspended solid (SS) and volatile suspended solid (VSS) concentrations and that the SS concentrations were positively correlated with the chlorophyll a concentrations. Phytoplankton, which are the chlorophyll a host, are classified into the particulate fraction, and thus, particulate 137Cs were contained in phytoplankton that had accumulated 137Cs. We think that phytoplankton is also an important component of particulate 137Cs in eutrophic lakes although the main factor determining particulate 137Cs concentration is inorganic SS. Since phytoplankton is a food resource for fish, it is necessary to examine in detail the relationship of 137Cs contamination of fish with particulate 137Cs and phytoplankton.