On March 11, 2011, the magnitude 9.0 Tokuku earthquake struck off northeastern coast of Japan. The earthquakes, its aftershocks, and the powerful tsunami it triggered led to the deaths and injuries of thousands of people. Furthermore, damage caused by the earthquake and tsunami disabled the power supply and cooling mechanisms of the Fuhushima Daiichi nuclear reactors. Water decomposition caused by contact with hot fuel rods created an accumulation of hydrogen gas that exploded and destroyed three units of the plant. As a result, high volatility fission products including radioactive iodine, strontium, and cesium, were discharged into the environment through the air and deposited in the soil and water.  This major release of radioactive elements into the environment has had a lasting impact on the environment.
One of the primary isotopes that was released by the meltdown of the nuclear reactor was Cs-137. Initially discharged into the atmosphere, Cs-137 was brought to the surface by dry and wet deposition. Cs-137 has a half-life of 30.1 years, so Cs-137 absorbed in the top soil layer can remain there for many years.  This contamination of the soil is especially concerning due to its deleterious effect on stock farming and agriculture, and its potential impacts on human health. The soils around Fukushima have been contaminated with depositions of more than 100,000 MBq km-2.  In Japan, the limit for cesium concentrations in soil is 5,000 Bq kg-1. For food production, the upper limit for cesium in the soil is 2,500 Bq kg-1, so food production in eastern Fukushima prefecture was severely impaired.  The total Cs-137 deposition and distribution in the environment has been estimated by integrating daily observations of Cs-137 deposition. This has shown that the soils in large areas of eastern and northeastern Japan have been strongly contaminated by Cs-137, while western regions of Japan have been sheltered by mountain ranges. 
The waters around Fukushima were also subject to nuclear contamination. In addition soil, Cs-137 in the atmosphere was also deposited in the Pacific Ocean. Furthermore, seawater was used to help cool the damaged reactors following the earthquake, and the release of this highly contaminated water led to radioactive discharges directly into the sea. Not only were radioactive isotopes released into the ocean, other nonvolatile activation products and fuel rod materials were also released as they made contact with water.  A study on levels of Cs-137 and other radioactive isotopes in the ocean water near the discharge point has shown that not only were there significantly elevated concentrations, but the timing of the peak of the concentrations was approximately one month after the earthquake, which corresponds with the decision to use seawater to cool the reactors.  In terms of radionuclide activities in the surface of the ocean, Fukushima exceeds Chernobyl.  Cs-137 and Cs-134 have been detected throughout waters up to 600 km offshore from Fukushima. Fukushima-derived cesium isotopes were also detected in zooplankton and mesopelagic fish.  Despite this significant increase in cesium isotope levels in the waters off of japan, their risk is below those generally considered harmful to marine animals and human consumers. 
The Fukushima Daiichi nuclear reactor meltdown caused the largest accidental source of radionuclides in the ocean, and atmospheric and soil contamination comparable only to the Chernobyl nuclear power plant disaster.  Studies on the extent of this contamination, such as those described above, have been useful for the establishment and monitoring of regulatory measures, and for guiding of activities to lessen the impact of the contamination. Furthermore, this event has led to international collaborations for comprehensive field measurements to better understand the extent and impact of nuclear contamination, and hopefully better guard against potential accidents in the future.
© Catherine Dong. 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.
 K. O. Buesseler et al., "Fukushima-Derived Radionuclides in the Ocean and Biota Off Japan," Proc. Nat. Acad. Sci. (USA) 109, 5984 (2012).
 T. J. Yasunari et al., "Cesium-137 Deposition and Contamination of Japanese Soils Due to the Fukushima Nuclear Accident," Proc. Nat. Acad. Sci. (USA) 108, 19830 (2011) [Correction, ibid. 110, 7525 (2013)].
 K. Buesseler, M. Aoyama, and M. Fukasawa, "Impacts of the Fukushima Nuclear Power Plants on Marine Radioactivity," Environ. Sci. Technol. 45 9931 (2011).
 N. Kinoshita et al., "Assessment of Individual Radionuclide Distributions from the Fukushima Nuclear Accident Covering Central-East Japan," Proc. Nat. Acad. Sci. (USA) 108, 19526 (2011).