In March 2011, a magnitude 9.0 earthquake rattled Japan, triggering a tsunami and the catastrophic failure of three reactors at the Fukushima Daiichi nuclear power plant. The National Atmospheric Deposition Program, part of the Illinois State Water Survey within the Prairie Research Institute at the University of Illinois, collected rainwater samples from its nationwide network in the days following the nuclear incident to test for radioactive fallout. Together with researchers from the United States Geological Survey, NADP researchers published a paper in the journal Environmental Science and Technology detailing their findings. In an interview with News Bureau physical sciences editor Liz Ahlberg, NADP coordinator David Gay and researcher Christopher Lehmann talked about radioactive fallout and the study.
See related video here.
What role does precipitation play in radioactive fallout?
Precipitation (that is, both rain and snow) captures radioactive particles in the upper atmosphere and carries it to Earth's surface. This process is called "wet deposition." Radioactive particles can also fall directly to Earth's surface as dry deposition, but the process that carries radioactivity long-distance (such as from Japan to the United States), is likely dominated by wet deposition.
How could particles from Japan rain down in the U.S.? What did this study teach us about how radioactive pollution travels in the atmosphere?
The tsunami caused by the March 11, 2011, earthquake led to catastrophic failure among the six nuclear reactors at the Fukushima Daiichi power plant. The resulting explosions sent radioactive materials from the reactor high into Earth's upper atmosphere. The radioactive particles were then transported around the globe by Earth's normal west to east circulation patterns. We used NOAA's HYSPLIT model to evaluate the path of the radioactive material from Japan to the United States. The model's predicted path agreed reasonably well with measurements from our network. Thus, the impact of the Fukushima disaster agreed well with our predictions, namely that the radiation entered the upper atmosphere, was transported across the Pacific Ocean, and was deposited to the U.S. in rain and snow.
What, exactly, did you test for?
Samples were collected across the U.S. from our NADP network of more than 250 sites. Samples were prepared at our laboratory at the Illinois State Water Survey and sent to the USGS National Reactor Facility in Denver. Along with our partners at the USGS, we tested the samples for radioisotopes of iodine (specifically, I-131) and cesium (Cs-134 and Cs-137).
What radioactive elements did you find in precipitation, and where?
Our partners at the USGS detected all three radioactive elements in the precipitation samples. Radioactive I-131 has a half-life (that is, the amount of time it takes to lose half of its radioactive energy) of about eight days. By the time we analyzed the samples, we were able to detect I-131 at only five of the 167 locations tested: at two sites in Washington state, two sites in California and one site in Colorado. Radioactive Cs-134 and Cs-137 have longer half-lives - 2.1 years and 30.2 years, respectively - and we were able to detect those species at many more sites. Radioactive cesium was detected at widely distributed sites across the U.S., with the highest Cs-134 activities detected in Alaska, and the highest Cs-137 activities detected in California.
Did the levels of radioactive particles in precipitation pose a threat to health or the environment?
We certainly have heard and read news reports about the tragic, but localized, impact of the radiation released in the Fukushima disaster. We did not expect (nor have there been reported) health impacts on the U.S. population. The purpose of this study was to record the distribution and magnitude of radioactive fallout using our existing national network. Determining the direct health impact on the U.S. population is more complicated, as it must take into account duration of exposure to radiation, and other factors. The U.S. Environmental Protection Agency routinely monitors the potential radioactive exposure of the U.S. population as part of its "RadNet." According to the U.S. EPA, the short-term exposure of the U.S. population is unlikely to have any significant health impact.