Concentration of radiocesium in the wild Japanese monkey (Macaca fuscata) over the first 15 months after the Fukushima Daiichi nuclear disaster

Abstract

Following the massive earthquake that struck eastern Japan on March 11, 2011, a nuclear reactor core meltdown occurred at the Fukushima Daiichi Nuclear Power Plant, operated by Tokyo Electric Power Company, and was followed by the release of large amounts of radioactive materials. The objective of this study was to measure the concentration of radiocesium (134)Cs and (137)Cs in the muscle of Japanese monkeys (Macaca fuscata) inhabiting the forest area of Fukushima City and to determine the change in concentration over time as well as the relationship with the level of soil contamination. Cesium concentrations in the muscle of monkeys captured at locations with 100,000-300,000 Bq/m(2) were 6,000-25,000 Bq/kg in April 2011 and decreased over 3 months to around 1,000 Bq/kg. However, the concentration increased again to 2,000-3,000 Bq/kg in some animals during and after December 2011 before returning to 1,000 Bq/kg in April 2012, after which it remained relatively constant. This pattern of change in muscle radiocesium concentration was similar to that of the change in radiocesium concentration in atmospheric fallout. Moreover, the monkeys feed on winter buds and the cambium layer of tree bark potentially containing higher concentrations of radiocesium than that in the diet during the rest of the year. The muscle radiocesium concentration in the monkeys related significantly with the level of soil contamination at the capture locations.

Figure 1. Map showing the location of Fukushima City, where the present investigation was conducted.

Figure 2. The soil contamination levels by radiocesium concentration (Bq/m²) and the distribution of monkey troops (irregular outlines) in Fukushima Prefecture.

This map was made according to the soil contamination map created by the Ministry of Education, Culture, Sports, Science and Technology (converted to the values of July 2, 2011) and the distribution map of Japanese monkeys by Fukushima Prefecture . Fukushima City, the study area, is indicated by a thick outline.

Figure 3. Change in muscle radiocesium concentration in Japanese monkeys over time.

Individual data from animals captured in each area with soil radiocesium contamination are shown.

Figure 4. Change in muscle radiocesium concentration in Japanese monkeys captured during and after June 2011.

Individual data from monkeys captured in the area with a soil radiocesium contamination of 100,000–300,000 Bq/m² are shown.

Figure 5. Changes in the ¹³⁴Cs/¹³⁷Cs concentration ratio in muscle in Japanese monkeys over time.

Figure 6. Change in monthly total concentration of radiocesium in atmospheric fallout in Fukushima City.

Data published by the Ministry of Education, Culture, Sports, Science and Technology are plotted .