From 1946 to 1958, the U.S. tested 67 nuclear weapons in the Marshall Islands, a remote constellation of atolls in the Pacific Ocean that was then a U.S. trust territory. Two atolls, Bikini and Enewetak, were used as ground zero for the tests, which caused unprecedented environmental contamination. According to new research, presented in three papers in the Proceedings of the National Academy of Sciences, radiation levels in some regions of the islands are still far higher than in areas affected by the Chernobyl and Fukushima nuclear accidents.

The U.S. performed nuclear testing on Bikini and Enewetak Atolls in the northern Marshall Islands between 1946 and 1958.

On March 1, 1954, the U.S. military detonated its largest thermonuclear weapon on an island located in the northwestern rim of Bikini Atoll. The weapon, code-named Castle Bravo, released an energy equivalent to 15 million tons of TNT (15 megatons).

The Marshall Islands have experienced rapid growth since the 1960s. Most of the nation’s residents live on two crowded islands and are unable to return to their home islands because of nuclear contamination.

“Our three studies showed that the concentration of nuclear isotopes on some of the islands was well above the legal exposure limit established in agreements between the U.S. and Republic of the Marshall Islands,” said lead authors Emlyn Hughes and Malvin Ruderman and their colleagues from Columbia University.

In the first study, the researchers assessed current radiological conditions on four affected atolls (Enewetak, Bikini, Rongelap, and Utirik) in the northern Marshall Islands.

They measured external gamma radiation on nine islands and concentrations of americium-241, cesium-137, plutonium-238, and plutonium-239/240 in the soil samples from 11 islands.

In the second study, the scientists measured concentrations of plutonium-239/240, plutonium-238, americium-241, bismuth-207, and cesium-137 in cores from the Bravo bomb crater site.

“We found radiation levels orders of magnitude above background for plutonium-239/240, americium-241, and bismuth-207 in the top 25 cm of sediment across the entire Bravo crater, the location of the largest aboveground U.S. nuclear weapons test,” they said.

In the third study, the team determined the levels of cesium-137 contamination for over 200 fruits, primarily coconuts and pandanus, from 11 islands on four atolls.

They found that contamination remains above limits set by international safety standards in some measured fruits.

“Based upon our results, we conclude that to ensure safe relocation to Bikini and Rongelap Atolls, further environmental remediation appears to be necessary to avoid potentially harmful exposure to radiation,” the researchers said.

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Maveric K.I.L. Abella et al. Background gamma radiation and soil activity measurements in the northern Marshall Islands. PNAS, published online July 15, 2019; doi: 10.1073/pnas.1903421116

Emlyn W. Hughes et al. Radiation maps of ocean sediment from the Castle Bravo crater. PNAS, published online July 15, 2019; doi: 10.1073/pnas.1903478116

Carlisle E.W. Topping et al. In situ measurement of cesium-137 contamination in fruits from the northern Marshall Islands. PNAS, published online July 15, 2019; doi: 10.1073/pnas.1903481116