Background activity concentrations of radionuclides and internal dose rates in the wild mouse and shrew mole in forests of Rokkasho, Aomori Prefecture, Japan.

Understanding baseline internal radiation exposure in small terrestrial mammals is essential for evaluating potential radiological impacts associated with nuclear facilities. We quantified naturally occurring and artificial radionuclides in two representative forest-dwelling species-the small Japanese field mouse (Apodemus argenteus) and the Japanese shrew mole (Urotrichus talpoides)-collected from three forest types in Rokkasho, Aomori Prefecture, Japan, prior to the operation of a commercial spent nuclear fuel reprocessing plant. Activity concentrations of 14C, 40K, 210Po, 210Pb, 226Ra, 232Th, 238U, and 137Cs were measured in whole-body composite samples or in 11 organs/tissues. The shrew mole exhibited markedly higher 210Po activity concentrations-approximately one order of magnitude greater than those of the mouse-likely reflecting its carnivorous feeding habits. Seasonal variations were observed in several organs, particularly for 210Po. Internal dose rates were estimated using the ERICA Tool, yielding mean values of 0.25 ± 0.08 μGy h-1 for the mouse and 4.1 ± 1.4 μGy h-1 for the shrew mole, with 210Po contributing 70-98% of the total dose rate. These results provide essential baseline data for future environmental radiological assessments and for interpreting radiation effects in wildlife inhabiting regions affected by nuclear accidents, such as Fukushima. Continued monitoring of naturally occurring radionuclides, particularly 210Po, will be essential for accurate ecological risk assessments and for distinguishing background variability from accident-derived contamination.