Oct. 3 (UPI) -- Scientists have discovered a surprising new source of radioactive cesium some 60 miles from the Fukushima nuclear plant that was damaged by the 2011 earthquake and tsunami.
A team of researchers from the Woods Hole Oceanographic Institution in Massachusetts and Japan's Kanazawa University found unusually high levels of radioactive cesium-137 in the groundwater beneath several beaches.
Because the levels are higher than samples from other radioactive sources, including samples collected from the harbor of the nuclear power plant, scientists hypothesize the cesium-137 was carried by currents and absorbed by sands shortly after the 2011 meltdown.
Another recent survey suggests Fukushima has continued to leak low levels of cesium into the ocean over the past few years, but the amounts are too small to account for the heightened levels found in the groundwater beneath local beaches.
"No one expected that the highest levels of cesium in ocean water today would be found not in the harbor of the Fukushima Dai-ichi nuclear power plant, but in the groundwater many miles away below the beach sands," WHO researcher Virginie Sanial said in a news release.
Sanial and her colleagues hypothesize that brackish water and layers of sand have kept the concentrations of cesium-137 hidden beneath the beaches. Researchers discovered the unusual radioactive source while drilling and testing sediment cores from nearby beaches.
According to their analysis -- detailed in a new paper published this week in the journal PNAS -- the sand and water are beginning to release the cesium-137 back into the ocean.
In brackish and freshwater, the cesium-137 atoms cling to sand grains. But saltier water causes the atoms to be released. Scientists believe the radioactive atoms are losing their grasp on the sand particles as fresh saltwater currents mix with the brackish groundwater beneath the beaches.
"It is as if the sands acted as a 'sponge' that was contaminated in 2011 and is only slowly being depleted," said WHO scientist Ken Buesseler.
"Only time will slowly remove the cesium from the sands as it naturally decays away and is washed out by seawater," added Sanial.