Measurement of Radioactive Fallout from the March 2011 Fukushima Nuclear Reactor Incident

The U.S. Geological Survey (USGS) collaborated with the National Atmospheric Deposition Program in an effort to monitor North American precipitation samples for the presence of nuclear fallout in response to the Japan Fukushima Dai-ichi Nuclear Power Station incident that occurred on March 11, 2011. Specifically, excess precipitation and filtrate (insolubles suspended in precipitation) from National Trends Network sites and selected Mercury Deposition Network sites were analyzed for the presence of radiological fallout for samples collected during the March 8th to April 5th, 2011 sampling period. NADP samples were provided to the USGS's TRIGA Nuclear Facility in Denver Colorado, where the radiological determinations were made.

bacK trajectory figure

Atmospheric back trajectories from sites where radioactive fallout was measured in NADP wet deposition samples. NOAA's HYSPLIT model was used for this analysis (click to enlarge).

This project was implemented to add information about radioactive fallout from the March 12-14, 2011 incident and to test the capabilities of the NADP to respond to an unexpected atmospheric release. This is the second time samples from the NADP network have been used to measure radioactive fallout, the first being after the Chernobyl reactor failure in 1986.

The study found concentrations (activity) and fallout (deposition) of radioactive iodine and radioactive cesium in significant number of samples. Detectable quantities of Iodine-131, Cesium-137, and Cesium-134 were observed at 21% of the 167 tested locations. Concentrations of I-131 detected in 5 samples ranged from 29.6 to 1090 picocuries per liter (pCi/L). Concentrations of Cs-134 detected in 23 samples ranged from 0.4 to 55 pCi/L. Concentrations of Cs-137 in 33 samples ranged from 0.70 pCi/L to 39 pCi/L.

Detections and measurable fallout from wet deposition was observed primarily at NADP sites located along the West Coast of the US, the central Rocky Mountain region and northern Great Plains, the central and upper Mississippi River Valley and eastern mountainous regions ranging from Virginia northward through Vermont. Deposition was also observed at NADP sites in Alaska (see figure).

deposition figure

NADP sites where USGS measured 137Cs in precipitation samples. Dot size represents relative deposition amounts (0.8 – 240 Bq/m²) (click to enlarge).

While the USGS does not assess human health risks from exposure to radioactive fallout, the United States Environmental Protection Agency's RadNet confirms that radiation levels in the United States were far below the level of concern for human health impact. More information on EPA's findings is available here

As is the standing policy of the USGS and the NADP, data from the study is freely available for others to use. The observations of radiological deposition from this project are available at the USGS website, dedicated to the Fukushima incident. Data are tabulated by NADP site and week ( ). Full results from the sampling and analysis were published in the USGS Open File report, which is available online:

Fission Products in National Atmospheric Deposition Program Wet Deposition Samples Following the Fukushima Dai-Ichi Nuclear Power Station Incident, March 8 - April 5, 2011, U.S. Geological Survey Open- File Report 2011-1277, 2011, 34p., Gregory A. Wetherbee, Timothy M. Debey, Mark A. Nilles, David A. Gay, and Christopher M.B. Lehmann.

Additionally, the analysis was reported and published in Environmental Science and Technology:

Wet Deposition of Fission-Product Isotopes to North America from the Fukushima Dai-ichi Incident, March 2011, Environmental Science and Technology, doi:10.1021/es203217u, March 6, 2012, Gregory A.Wetherbee, David A. Gay, Timothy M. Debey, Christopher M.B. Lehmann, and Mark A. Nilles.

A University of Illinois news release and video interview about the study is available at

The study demonstrated the flexibility and ability of the NADP's network to respond to an international tragedy. The samples routinely collected by NADP at locations across the U.S. can be utilized by scientists to detect many different and additional types of pollutants in the precipitation of North America that are not routinely measured by NADP

For more information about the study, please contact one of the following:

NADP Coordinator
NADP Assistant Coordinator
USGS Principal investigator/lead author
USGS Atmospheric Deposition Program Manager/coauthor
USGS Nuclear Engineer Manager-Reactor Research Facility/coauthor