By Quirin Schiermeier of Nature magazine
Radiation released by the tsunami-struck Fukushima Daiichi nuclear power plant could have long-lasting consequences for the natural environment in the vicinity of the damaged plant.
Scientists estimate that in the first 30 days after the accident on 11 March, trees, birds and forest-dwelling mammals were exposed to daily doses up to 100 times greater-and fish and marine algae to doses several thousand times greater - than are generally considered safe.
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Radioecologists with the French Institute of Radioprotection and Nuclear Safety (ISRN) in Cadarache converted concentrations of radioisotopes measured in the soil and seawater into the actual doses that various groups of wildlife were likely to have received. Their results are published this week in Environmental Science & Technology.
The soil samples used for the analysis came from a contaminated forest area 25-45 kilometers northwest of Fukushima. The seawater samples were taken from a region close to the reactor site. Both were measured in late March.
The French team reckons that about 50 radioisotopes have been released, with iodine-131 and caesium-137 being the most abundant (see 'Radiation release will hit marine life'). At the end of March, concentrations of caesium-137, which has a half life of 30 years, reached 47,000 becquerel per liter in seawater, and 72,900 becquerels per kilogram in soils. A becquerel is defined as one radioactive decay per second.
The team then plugged those concentrations into a piece of software called ERICA (Environmental Risk from Ionizing Contaminants) to calculate the radiation dose that various groups of wildlife would have received. ERICA accounts for factors that are known to affect the rate at which organisms absorb radioisotopes, such as a species' cellular characteristics and metabolism. The dose rate (measured in milligrays per day) specifies how much radiation is absorbed per kilogram of organic tissue per day, a more biologically meaningful indication of how organisms are affected by exposure to radioactivity.
"Even so, it's just a rough assessment," says Thomas Hinton, a co-author of the study. "We need many more samples before we can try to determine the full extent of Fukushima's ecological effects."
The team found that flatfish, mollusks, crustaceans and brown seaweed offshore of Fukushima received radiation doses that, according to known dose-effect relationships, are likely to markedly increase mortality.
Terrestrial organisms are somewhat better off. However, the dose rates were still high enough to reduce the reproductive success of birds, rodents and trees-in particular pine and spruce.
"The reported values are not written in stone but they're definitely plausible," says Nick Beresford, a radioecologist at the Centre for Ecology & Hydrology in Lancaster, UK. "But even though it's preliminary, this is a very useful ecological assessment."
'Good' timing
Many land species, says Hinton, may get off relatively lightly because the accident happened early in the flowering season. Had it occurred in mid-spring, the harm would probably have been much more severe, especially for plants.
Radiation effects on egg hatching and the survival of newborn mammals still need to be surveyed, he cautions.
For all the desolation it has caused, the Fukushima event could help scientists to gain a better understanding of the effects of nuclear radiation on wildlife and the environment.
There is wild disagreement, for example, over how radiation affects the fitness of birds and invertebrates. A recent study that reports reduced survival in barn swallows (Hirundo rustica) in the Chernobyl exclusion zone, where dose rates are now barely above natural values, has met with sharp criticism.
"Some researchers are reporting-possibly biased--results downright contrary to established paradigms of radioecology," says Hinton. "So what's going on? Long-term surveys in the Fukushima forest zone will hopefully help us find out."
Radioecologists regret that the few ecological studies done after the Chernobyl reactor meltdown 25 years ago missed out on many research opportunities and hope that the Fukushima area will become the natural observatory site that Chernobyl has not, owing mainly to political circumstances. The contaminated zone should ideally be thoroughly surveyed at least twice a year, says Hinton.
Amid more pressing priorities, the Japanese government is preparing an environmental monitoring program that involves around 300 experts from across the country.
"They have all the expertise, no doubt," says Beresford, "and the sooner the work starts the more useful it will be."
This article is reproduced with permission from the magazine Nature. The article was first published on May 27, 2011.