Some of highest levels of cesium-137 contamination are in groundwater under nearby sandy beaches.
Six years after the Fukushima nuclear reactor disaster in Japan, radioactive material is leaching into the Pacific Ocean from an unexpected place. Some of the highest levels of radioactive cesium-137, a major by-product of nuclear power generation, are now found in the somewhat salty groundwater beneath sand beaches tens of kilometers away, a new study shows.
Scientists tested for radioactivity at eight different beaches within 100 kilometers (62 miles) of the plant, which experienced three reactor meltdowns when an earthquake and tsunami on March 11, 2011, knocked out its power.
Oceans, rivers and fresh groundwater sources are typically monitored for radioactivity following a nuclear accident, but several years following the disaster, those weren’t the most contaminated water sources. Instead, brackish groundwater underneath the beaches has accumulated the second highest levels of the radioactive element (surpassed only by the groundwater directly beneath the reactor), researchers report October 2 in the Proceedings of the National Academy of Sciences.
Cesium-137 is a radioactive isotope of cesium (a soft, silvery-gold metal) that’s formed by nuclear fission and potentially fatal to humans when exposed to high concentrations.
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The scientists who led the study, Virginie Sanial of Woods Hole Oceanographic Institution and Seiya Nagao of Kanazawa University, say the levels of radiation “are not of primary concern” to public health, but that this new and unanticipated source “should be taken into account in the management of coastal areas where nuclear power plants are situated.”
Some cesium stuck to the sand and, over time, percolated down to the brackish groundwater beneath. Now, the radioactive material is steadily making its way back into the ocean. The groundwater is releasing the cesium into the coastal ocean at a rate that’s on par with the leakage of cesium into the ocean from the reactor site itself, Sanial’s team estimates.
But with about half of the world’s nuclear power plants located on coastlines, such areas are potentially important contamination reservoirs and release sites to monitor after future accidents.
Indeed, approximately half of the 440 operational nuclear reactors in the world are situated on a coastline.
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After the 2011 accident, scientists monitored leaking radiation as it entered the atmosphere or trickled into rivers, but the Fukushima plant — damaged by a devastating earthquake and tsunami — is the first major incident to happen along such a large water body, namely the Pacific Ocean. This new PNAS study is now the first to consider subterranean pathways for the storage and release of radioactive contaminants following a nuclear disaster.
The researchers sampled eight beaches between 2013 and 2016, all within 100km of the plant.
They plunged seven-foot-long tubes into the sand, pulling up sand and groundwater samples for analysis. The cesium levels in the brackish groundwater — a combination of fresh water and salt water — was ten times higher than what’s currently being detected in the waters swirling around Fukushima’s harbor, while cesium was tracked in the sand up to a depth of three feet.
But here’s the thing — cesium loses its “stickiness” when it’s exposed to salt water.
So, with each passing wave and tide, the cesium is slowly getting released back into the ocean. The amount of radioactive waste detected by the researchers in the adjoining water is roughly equal to the amounts drifting in from the other two known sources: ongoing releases and runoff from the plant itself, and overflow from rivers that carry cesium from the fallout on land.
Cesium has long half-life, so “only time will slowly remove the cesium from the sands as it naturally decays away and is washed out by seawater,” said Sanial in a release.