In March 2011, large amounts of radionuclides were released into the environment as a result of an accident at Tokyo Electric Power Company’s Fukushima Daiichi Nuclear Power Plant (FDNPP). Approximately 13–15 PBq of caesium-137 (hereinafter, 137Cs, physical half-life of 30 years) was released from the FDNPP accident1,2, and as a result, 137Cs became a major source of radiation contaminating the environment. Since 137Cs has high bioavailability (i.e. absorption and transfer rates) due to having chemical characteristics similar to those of the monovalent cation potassium, there has been concern that 137Cs will accumulate in wildlife over the long-term3. Indeed, 137Cs has been detected in many wildlife species (e.g. insects4, frogs5, fishes6,7,8, birds9, mammals10,11) since the FDNPP accident.
After the FDNPP accident, serious radionuclide fallout occurred over extensive areas of north-eastern Japan. As most of this region is covered by forests (approximately 70%), the long-term 137Cs contamination of forestry ecosystems has been a concern12. Due to its long-term retention (ca. 10–100 years) at high concentrations, radiocaesium that has been deposited in forests is considered to have a long ecological half-life13,14. The dynamics of 137Cs in forestry ecosystems can be roughly divided into two stages: the “early” stage, in which the distribution of radiocaesium within the system occurs promptly between the soil and trees, and the “steady state” stage, in which there are long-term changes in the subsequent distribution of radiocaesium among organisms14,15. Generally, immediately after deposition in the soil following release into the atmosphere by an accident, a high proportion of the radiocaesium released exists in an ionic state, which is easily absorbed by plants. However, after physical and chemical changes in the soil following deposition, radiocaesium binds to soil particles over time and becomes less easily dissolvable16. Therefore, radiocaesium in soil can generally be separated into: (1) an exchangeable fraction; (2) a bound to organic matter fraction; and (3) and a particle-bound fraction (also called a “strongly bound fraction”). Radiocaesium in the exchangeable fraction is substitutable with monovalent cations, which have an ionic radius similar to that of caesium and are absorbed by negatively charged sites in organic matter and soil particles. Radiocaesium in the bound to organic matter fraction is bound to organic matter, whereas radiocaesium in the strongly bound fraction is a specific fraction for caesium strongly bound to clay mineral layers, and is difficult to elute17. Thus, radiocaesium exists as different types, or physicochemical fractions, in the environment. These physicochemical fractions also change gradually over time in the environment, from fractions in which radiocaesium can be transferred relatively easily to those in which it is relatively difficult to transfer; for example, the rate of radiocaesium transfer to plants decreases over time18. These changes over time in the physicochemical fractions of radiocaesium are considered to influence the rates of transfer of radioactive nuclides to wildlife. Consequently, to clarify radiocaesium transfer from the environment to organisms, both the concentration and physicochemical fractions of radiocaesium in the environment need to be determined.
In this study, we examined the relationship between the physicochemical fractions of 137Cs in the environment and the bioavailability of 137Cs in wild boars (Sus scrofa), which mainly inhabit the forests and countryside surrounding the FDNPP, and for which investigational data on the transfer of 137Cs in the environment have been accumulated. Monitoring results in Fukushima Prefecture after the FDNPP accident showed that wild boar tended to have particularly higher activity concentrations of 137Cs in muscle tissue than did other wildlife species10,19. Studies have also shown that wild boar captured in areas with similar levels of soil contamination exhibit extremely large inter-individual variations in the activity concentrations of 137Cs19. Studies conducted after the Chernobyl Nuclear Power Plant accident also reported that wild boar had high activity concentrations of 137Cs, which enters the muscles after the digestion of food, and that the accumulation of 137Cs occurred over extended periods20,21,22,23. Regarding the activity concentration of radiocaesium in wild boar after the FDNPP accident, a monitoring survey conducted by the Fukushima Prefectural Government in 2017 detected animals in which the fresh mass (hereinafter, “FM”) activity concentration of radiocaesium was several thousand to a ten thousand Bq kg–1 at more than 6 years after the accident24. In addition, seasonal changes have been reported in the activity concentration of 137Cs in wild boar muscle;19,25 however, the factors that affected these fluctuations in wild boar after the FDNPP accident remain unclear.
Internal exposure to anthropogenic radioactive materials in animals is typically via the ingestion of radiation-contaminated food23,26. Our previous study found a significant positive relationship between the activity concentration of 137Cs in the muscle and stomach contents of wild boars, clearly indicating that ingested material influences the activity concentration of 137Cs in wild boar muscle. Wild boars are omnivores that feed mainly on plants (e.g. leaves, roots, subterranean stems), as well as earthworms, insects and other small animals27. It is likely that wild boars also passively ingest soil while eating these foods. While the activity concentration of 137Cs in soil is typically several orders of magnitude higher than that in plants, the elutability of 137Cs in water is very limited because much of the 137Cs that exists in soil is in the strongly bound fraction. In wild animals, it is unlikely that all of the 137Cs contained in the orally ingested material is absorbed; rather, only the 137Cs that is eluted from the ingested material is absorbed and distributed in the body. In terms of studying radiation dynamics, assessing internal radiation exposure and identifying the factors responsible for variable levels of radionuclides in wildlife, it is important to study wildlife species with high 137Cs levels, such as wild boar; specifically, it is important to clarify the fractions of 137Cs that are absorbed from the ingested material and the degree of their influence on the activity concentration of 137Cs in the body. Therefore, this study aimed to analyse the physicochemical fractions of 137Cs contained in the diet of wild boars by examining their stomach contents and clarifying the physicochemical fractions of 137Cs that could be eluted from these contents.
Wild boar utilize most of their annual home range in a day28,29, and utilize the same area every day28. Consequently, their stomach contents reflect individual food habits over a relatively long period (Nemoto et al., in prep.). Stomach contents are thus a valuable tool for evaluating the food habits of wild boar. For these reasons, we examined the relationships between the concentration of 137Cs physicochemical fractions in the stomach contents and seasonal variations in the activity concentrations of 137Cs in muscle.
Source: Ecology - nature.com
