The trends are promising, but there is still cause for concern.
Radiation levels across the Pacific are returning to normal 5 years after a meltdown at the Fukushima nuclear plant released radioactive materials into the sea, but the waters surrounding the plant remain highly contaminated.
An international team conducted a review of 20 studies examining radioactive cesium levels measured in the ocean from Japan’s coast, across the Pacific, to North America. Their findings were published in the Annual Review of Marine Science.
Cesium is a byproduct of nuclear power and is highly soluble in water, making it ideal for measuring radioactive material in the ocean originating from the stricken Fukushima plant. Immediately following the earthquake and tsunami that sparked the meltdown, radiation levels off the coast of Japan were tens of millions of times higher than normal.
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Radioactive materials are by nature unstable and decline in concentration over time. Ocean currents also have the ability to dilute radiation. Meanwhile, radiation from the site continues to contaminate underground water, and sediments on the seafloor act as a long-term reservoir of contaminants for bottom-dwelling marine life.
The researchers found that in 2011, about half of the fish sampled in coastal waters off Fukushima contained unsafe levels of radioactive material. By 2015, fewer than 1 percent of fish in the area had concentrations above the unsafe limit. Ocean currents have also dispersed radioactive material across the Pacific Ocean, all the way to North America, diluting them in the process.
Based on trends observed over the past 5 years and simulations, the researchers predict that radiation levels across the ocean will return to the background levels prior to the disaster by 2020.
But for some, the damage has already been done. The World Health Organization released a report in 2013 estimating an increased risk of certain cancers for populations living in the areas worst affected by the disaster.
The researchers also express concern that the seafloor and harbor near the Fukushima plant are still highly contaminated, according to a press release. Despite this persisting radioactivity, there has been a lack of support for continued monitoring of the area.
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