|
|
| Fig. 1: Path to Togakushi Shrine line with sugi trees, or Japanese red cedar in English. (Source: Wikimedia Commons) |
On March 11, 2011, Japan was hit by a major earthquake, which created a tsunami that together severely damaged the Fukushima Daiichi nuclear power plant. Of note, the tsunami disabled cooling systems at the power plant, resulting in nuclear meltdown. This led to the dispersion of radioactive material throughout the surrounding regions. Radiocesium, or cesium-137, is a particularly concerning type of radioactive material that is known to have significant adverse effects on populations, causing harm via various environmental vectors. [1] This report examines two specific cases of radiocesium contamination in the environment caused by the Fukushima Daiichi disaster.
A major consequence of the Fukushima Daiichi disaster was radiocesium contamination of surrounding forest ecosystems. Forests actually retain large proportions of radiocesium in the organic soil layer, which is on the surface of forest floors and largely composed of decaying plant matter. Overtime, radiocesium is transferred to mineral soil which is mostly composed of sand, silt and clay where it is fixed and no longer poses a significant risk. However, the type of forest significantly influences how long radiocesium is retained in organic soil after initial contamination. Koarashi et al. sought to measure the retention properties of Japan's two most dominant forest types, deciduous broad-leaved forests (oak, maple, beech) and evergreen coniferous forests (pine, cedar). They conducted studies at two sites near Fukushima, one populated with Japanese oak and the other populated with Japanese cedar (shown in Fig. 1). Koarashi et al. then observed that the Cs-137 (radiocesium) inventory in the organic soil layers for the deciduous broad-leaved site was on average 10.0 kBq per square meter (kBq stands for kilobecquerel, a unit of radioactivity). The measurement for the coniferous site was on average 23.0 kBq per square meter. Additionally, the proportion of radiocesium measured that migrated below the organic layer was roughly 78% in the deciduous broad-leaved forest and roughly 46% in the coniferous site. Thus, the coniferous forest site retained a significantly greater proportion of radiocesium in the organic layer than the deciduous broad-leaved site. The researchers reasoned this was due to the timing of the Fukushima nuclear disaster, as in March deciduous broad-leaved forests have no leaves while coniferous forests still do. This means contaminated leaves had later fallen and been integrated into the organic soil as litter, increasing the amount of radiocesium available to the cycle of trees taking up radiocesium in the soil and later redepositing it as litterfall, before it can eventually migrate to the mineral soil. Thus, different seasonal properties of these two forest types resulted in increased radiation risk for longer in evergreen coniferous forests. [2]
 |
| Fig. 2: The small cabbage white butterfly Pieris rapae. (Source: Wikimedia Commons) |
Possible consumption of vegetation grown in soil contaminated with radiocesium is another consequence from Fukushima, which can have significant effect on creatures in Japan's ecosystem. According to researchers Taira et al., "high morphological abnormality and mortality rates" were reported in the pale grass blue butterfly, which is common in Japan. [3] They examined the biological effects of consuming cabbage grown in contaminated soil with the cabbage white butterfly (pictured in Fig. 2) to better understand what affected the pale grass blue butterfly. In their experiments, they reared cabbage white butterfly larvae from Okinawa in a lab. Miniature cabbage was grown from seeds, cultivated in soils collected from Okinawa (control) and Fukushima (experimental). After cultivation, cabbage leaf samples underwent radioactivity concentration measurements to verify that cabbage grown in Fukushima soil had significant levels of radiocesium while cabbage grown in Okinawa soil did not. After an initial 5 days of eating normal cabbage, certain larvae were randomly selected to be fed contaminated cabbage. Taira et al. observed the development of multiple physical abnormalities in the experimental group, which included deformed wings, thorax, mouth parts and reproductive organs. There was also a significant decrease in granulocytes, or a type of white blood cell. They believe these findings roughly parallel what the pale blue grass butterfly experienced and suggest this is possibly generalizable to other similar species, but recognize genetic variation and conditions in the wild prohibit these observations to truly generalize. In terms of the human population, Taira et al. hesitate to speculate how consuming radiocesium contaminated cabbage would affect people especially due to the low likelihood of contaminated vegetables ending up in grocery stores. However, the adverse biological effects of consuming vegetation grown in contaminated soil cannot be ignored. [3]
The dispersion of radioactive material can have various unintended consequences on surrounding wildlife. Koarashi et al. have shown how radiation risk from radiocesium concentrations can persist in forest ecosystems near Fukushima, with certain forest types posing greater risk than others for an extended period of time. Taira et al. investigated the biological effects of consuming radiocesium contaminated cabbage on a particular species of butterfly, demonstrating how ecosystems affected by radioactive material suffer from ripple effects down the food chain. These specific environmental consequences are a result of manmade technology and our inability to properly safeguard the environment against possible flaws. Thus, while human safety is an important factor to consider when approaching nuclear energy and power plants, we must also consider environmental safety and anticipate ecological consequences that may result from future mistakes.
© Cameron Cruz. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] T. J. Yasunari et al., "Cesium-137 Deposition and Contamination of Japanese Soils Due to the Fukushima Nuclear Accident," Proc. Natl. Acad. Sci. (USA) 108, 19530 (2011).
[2] J. Koarashi et al., "Forest Type Effects on the Retention of Radiocesium in Organic Layers of Forest Ecosystems Affected by the Fukushima Nuclear Accident," Sci. Rep. 6, 38591 (2016).
[3] W. Taira et al., "Developmental and Hemocytological Effects of Ingesting Fukushima's Radiocesium on the Cabbage White Butterfly Pieris rapae," Sci. Rep. 9, 2625 (2019).
