Long-term studies on the temporal change of radiocesium in wild rodents and insectivores in Nihonmatsu City, Fukushima Prefecture, Japan

The radiocesium (¹³⁴Cs and ¹³⁷Cs) released by the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011 has a relatively long physical half-life and may remain in the natural environment for a long period of time and continue to be transferred to wildlife. In this study, we conducted a fixed-point investigation of radiocesium activity concentrations in stomach contents and skeletal muscles samples of wild rodents (Apodemus speciosus, Apodemus argenteus and Microtus montebelli) and insectivores (Urotrichus talpoides) from 2012 to 2021. Two sampling sites in Mt. Kuchibutoyama and Mt. Hayama were selected in forested areas in Fukushima Prefecture. In Mt. Kuchibutoyama, a decreasing trend in ¹³⁷Cs activity concentrations was observed for stomach contents of wild rodents and insectivores as a whole and for skeletal muscle of A. speciosus and A. argenteus. The effective half-life (T_ef) of ¹³⁷Cs activity concentration was estimated to be 7.6 years in the stomach contents, and 5.4 years and 7.4 years in skeletal muscle of A. speciosus and A. argenteus, respectively, with corresponding ecological half-life (T_ec) of 10 years in the stomach contents, and 6.6 years (A. speciosus) and 9.8 years (A. argenteus) in the skeletal muscle. In Mt. Hayama, there was a slight upward trend in the ¹³⁷Cs activity concentration in the stomach contents of wild rodents and insectivores, and no significant T_ef could be obtained. Statistically significant T_ef and T_ec could not also be estimated for ¹³⁷Cs radioactivity levels in skeletal muscle of A. speciosus and A. argenteus. These results suggest that ¹³⁷Cs concentration in insects and plants eaten by wild rodents and insectivores in Mt. Hayama has remained at a certain level for a long time. In November 2021, ¹³⁷Cs concentrations above 1.0 kBq/kg were still detected in skeletal muscle of A. argenteus captured in Mt. Kuchibutoyama and A. speciosus and A. argenteus captured in Mt. Hayama. It is expected that ¹³⁷Cs concentration levels in the stomach contents and skeletal muscles of these wild rodents and insectivore will remain for a long time to come, especially at Mt. Hayama. Furthermore, we also collected soil and plant samples when sampling wild rodents and insectivores in this study. It is important as a future task to study temporal changes in radiocesium activity concentrations in the habitat of wild small mammals and to examine the relationship between these changes and temporal changes in radiocesium activity concentrations in wild small mammals.