Scientists in Japan have found a new source of radiation derived from the 2011 Fukushima Dai-ichi nuclear disaster, leaching into the ocean more than 95 kilometres from the crippled power plant.

The newly identified reservoir represents the highest concentration of the isotope caesium-137 discovered outside the actual boundaries of the plant. The researchers stress that it does not constitute a public health hazard “because no one is exposed to, or drinks, these waters.”

In a paper published in the journal Proceedings of the National Academy of Sciences, a team led by Virginie Sanial of the Woods Hole Oceanographic Institution in Massachusetts report the discovery of the radiation in brackish groundwater stored under a beach.

The scientists call the finding “unexpected” and suggest that the caesium was originally deposited on sand suspended within the groundwater by the action of waves and tides in the weeks following the Fukushima meltdowns.

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In the comparatively fresh water beneath the beach, the caesium “stuck” to the minerals, staying in place. However, tidal action in recent times has reintroduced salt to the groundwater, unfastening the radioactive material and flushing it back out into the sea.

Caesium has a half-life of just over 30 years, so Sanial and her colleagues acknowledge that the caesium-137 they discovered might also be left over from nuclear weapons tests in the 1960s. However, sampling also turned up significant quantities of another isotope, caesium-134, which can only have come from Fukushima.

The researchers conclude that the discovery of the new radiation source should be heeded by authorities in charge of the 440 operational nuclear reactors around the world.

“This new unanticipated pathway for the storage and release of radionuclides to ocean should be taken into account in the management of coastal areas where nuclear power plants are situated,” they write.