On the one-year anniversary of the nuclear accident in Fukushima radiation in the ocean yet to reach Hawaii

 

by Henrieta Dulaiova, Jan Kamenik and Kamila Stastna

Department of Geology and Geophysics, University of Hawaii, Honolulu, HI 96822

E-mail: hdulaiov@hawaii.edu

 

 

It has been one year since the great Tohoku earthquake and subsequent tsunami on March 11, 2011. These events resulted in power failures that prevented proper cooling of several reactors at the Fukushima Daichi nuclear power plant. Three units experienced partial core meltdown and four had hydrogen explosions that damaged the reactor containments. Significant amounts of radionuclides were released to the atmosphere and by direct discharge or leakage to the ocean. It is estimated that  over the couple of weeks after the accident 1 to 4 x10¹⁶ Bq cesium-137 was released to the environment making it the second largest accidental release of radioactivity after Chernobyl (http://www.sciencedirect.com/science/article/pii/S0265931X1100289X).

 

For atmospheric releases it was only a matter of days before they dispersed in the northern hemisphere. Fukushima-related radionuclides in the atmosphere were registered by air- and rainwater monitoring systems (RadNet, epa.gov) across the US, in particular in Alaska, California, Guam, Hawaii, Idaho, Nevada, Saipan, Northern Mariana Islands, Washington and even on the east coast in Massachusetts and Pennsylvania. As early as on March 22, 2011 the US Environmental Protection Agency reported that " As of 6:30pm (EDT) preliminary monitor results in Hawaii detected minuscule levels of an isotope that is consistent with the Japanese nuclear incident. This detection varies from background and historical data in Hawaii." (www.epa.gov/japan2011).

 

Figure 1, left panel: EPA’s RadNet monitoring network does regular year-round gross beta radiation measurements on air filters in Hilo and Honolulu (http://www.epa.gov/narel/radnet/). An increase in beta activity was observed in Guam, the Commonwealth of Northern Marianna Islands and Hawaii after the Fukushima accident. Right panel: CNMI and Guam observed half as large increase in gross beta radiation than the station in Kahuku, HI. The windward side of Oahu (Kahuku) had nine times the increase in beta radiation than the leeward side (Honolulu). The EPA also deployed samplers to analyze specific radionuclides and found ¹³⁷Cs, ¹³⁴Cs, ¹³²Te, ¹³²I and ¹³¹I, which are indicative of Fukushima releases. Table 1 shows the maximum detected levels of cesium for some of the Pacific Islands. These radiation levels do not pose a health risk for the US population.

Most of atmospheric releases were blown towards the Pacific Ocean rather than depositing on Japan but experts are still working on estimating how much of radiation was released and deposited on land and in the ocean.

Our group at the Department of Geology and Geophysics, University of Hawaii in Manoa started monitoring seawater radiocesium (¹³⁴Cs and ¹³⁷Cs) concentrations immediately after the accident. We have been collecting samples at Waikiki and the HOT cruise graciously provided us monthly water samples from the offshore station Aloha (http://hahana.soest.hawaii.edu/hot/) starting April 2011. So far no Fukushima related radiation has been found in the ocean at these two locations.

In addition we received samples from Guam collected by colleagues at the University of Guam and the Midway Islands that were collected by a SEA education cruise (http://www.sea.edu/?gclid=COj-yKys4K4CFWMGRQodXGb9ZQ) in March and April, 2011. These samples would indicate radiation from atmospheric fallout since it was too early for the ocean releases to travel that far, though no Fukushima radiation was detected.

We can say that no Fukushima related radiation was found, but there is plenty of radioactivity in the ocean from both natural and man-made sources. Cesium isotopes are produced in nuclear reactors and have radioactive half-lives ¹³⁷Cs = 30 years and ¹³⁴Cs = 2 years. Cesium behaves chemically very similarly to potassium, it is a conservative element that attaches to particles very little and does not take part in any major chemical reactions in the surface ocean. This element therefore works best as a tracer of Fukushima releases because it stays dissolved in the ocean water and is carried by currents. As opposed to other elements such as actinides (plutonium, americium) that attach to particles (plankton, dust) and are readily removed from the surface ocean by settling.

Based on results collected in the International Atomic Energy Agency’s MARIS database the Central Pacific had pre-Fukushima ¹³⁷Cs activities of about 1.2 - 1.4 Bq/m³. This is leftover after nuclear weapons’ testing and the Chernobyl accident. The half-life of this isotope is 30 years so it stays around for a couple of decades after its release. ¹³⁴Cs on the other hand, has a half-life of 2 years and in about 5 half-lives so 10 years 97% of it decays. Fukushima releases had both of these in a ratio of 1:1. Our methods can detect as little as 0.2 Bq/m³ of cesium, so in every sample we could see 1.2-1.5 Bq/m^{3 137}Cs, but no detectable ¹³⁴Cs.

To put these activities in perspective, we can compare the cesium to naturally occurring potassium, which has a radioactive isotope ⁴⁰K. Its activity in seawater is 11,000 Bq/m³. In some parts of the ocean near Japan activities approached this level, but not around Hawaii.

People in Hawaii are also concerned about the potential contamination of the debris released by the tsunami. It is hard to track the debris field in the ocean (http://www.oceanrecov.org/tsunami-debris/maps.html) and certainly it is beyond our means to survey it all for radioactivity. While it is possible that atmospheric fallout impacted some of the debris, the largest releases of radioactive waters to the ocean happened the first week of April, 2011 after the debris has mostly moved offshore.

We participated on a scientific cruise organized by scientists from the Woods Hole Oceanographic Institution funded by the Moore foundation. The University of Hawaii research vessel R/V Kaimikai-O-Kanaloa was used to sail between Yokohama, Japan and 147 degrees longitude east along the northern side of the Kuroshio current (http://www.whoi.edu/page.do?pid=67796). On the way back to Hawaii surface water samples were collected (black dots and red crosses on the maps below) which we then analyzed for ¹³⁷Cs and¹³⁴Cs. The cruise track intersected the southern edge of the modeled debris field (bottom panel of figure 2). While we found Fukushima related radionuclides, the levels were rather low (< 12 Bq/m³ of each isotope). However, we know based on results from other researchers that in some places radioactivity was much higher, about 1,000 times the normal. We did not have a chance to measure the radioactivity of the debris.

Figure 2: Map of sampling locations in the East and Central Pacific Ocean. The Fukushima- Honolulu transect was sampled in June 2011, Guam and Midway in March and April, 2011. Coastal Oahu and station Aloha have been sampled since late March. 20-100 L of surface water is collected and processed through an AMP-PAN resin, which selectively sorbs cesium. The resin is then counted by gamma-spectrometry for ¹³⁷Cs & ¹³⁴Cs. The chemical recovery is determined using stable cesium via ICP-MS analysis.

The bottom panel shows the modeled tsunami debris field by Nikolai Maximenko and Jan Hafner from the University of Hawaii International Pacific Research Center (http://www.soest.hawaii.edu/iprc/news/marine_and_tsunami_debris/debris_news.php).

 

This material is based upon work supported by NSF under grant No. RAPID OCE-1137412, the Gordon and Betty Moore Foundation and the School of Ocean Earth Science and Technology, UH.  Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the funding agencies.