Lingering radioactivity at the Bikini and Enewetak Atolls

Ken O Buesseler,Matthew A Charette,Steven M Pike,Paul B Henderson,Lauren E Kipp

doi:10.1016/j.scitotenv.2017.10.109

Ken O Buesseler, Matthew A Charette + Show 3 more

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https://doi.org/10.1016/j.scitotenv.2017.10.109

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Abstract

We made an assessment of the levels of radionuclides in the ocean waters, seafloor and groundwater at Bikini and Enewetak Atolls where the US conducted nuclear weapons tests in the 1940's and 50's. This included the first estimates of submarine groundwater discharge (SGD) derived from radium isotopes that can be used here to calculate radionuclide fluxes in to the lagoon waters. While there is significant variability between sites and sample types, levels of plutonium (239,240Pu) remain several orders of magnitude higher in lagoon seawater and sediments than what is found in rest of the world's oceans. In contrast, levels of cesium-137 (137Cs) while relatively elevated in brackish groundwater are only slightly higher in the lagoon water relative to North Pacific surface waters. Of special interest was the Runit dome, a nuclear waste repository created in the 1970's within the Enewetak Atoll. Low seawater ratios of 240Pu/239Pu suggest that this area is the source of about half of the Pu in the Enewetak lagoon water column, yet radium isotopes suggest that SGD from below the dome is not a significant Pu source. SGD fluxes of Pu and Cs at Bikini were also relatively low. Thus radioactivity associated with seafloor sediments remains the largest source and long term repository for radioactive contamination. Overall, Bikini and Enewetak Atolls are an ongoing source of Pu and Cs to the North Pacific, but at annual rates that are orders of magnitude smaller than delivered via close-in fallout to the same area.

Highlights

The US conducted 66 nuclear weapons tests at the Bikini and Enewetak Atolls between 1946 and 1958 accounting for N 50% of the global fallout during that time period and 20% of global fallout by the time atmospheric testing ended
Our overall findings are in agreement with several prior studies regarding the levels and sources of artificial radionuclides at the Bikini and Enewetak Atolls, and include some important new insights into the magnitude of ongoing sources
Pu is found at orders of magnitude higher activity levels here than elsewhere in the world's ocean and seafloor sediments, whereas Cs is only slightly higher and exceeded today by comparison, in the ocean off Japan near the Fukushima Dai-ichi nuclear power plants

Summary

Introduction

The US conducted 66 nuclear weapons tests at the Bikini and Enewetak Atolls between 1946 and 1958 accounting for N 50% of the global fallout during that time period and 20% of global fallout by the time atmospheric testing ended (based upon yields; Buesseler, 1997; Hamilton, 2004). Unlike most of the global fallout that was distributed widely from high altitude testing, the majority of tests were conducted either on the surface of the atoll lagoons, on the islands, or underwater This resulted in “close-in” fallout that contaminated the local islands and lagoon sediments, and larger areas of the North Pacific Ocean where close-in fallout accounts for 60% of the total fallout Pu (Bowen et al, 1980). Activities greater than background fallout levels were found for many radionuclides in lagoon waters due to their continued remobilization, presumably from marine sediments This issue persists today, though estimated doses to humans from consumption of marine foods is thought to be a small source of the total radiological dose, for example b 0.1% at Enewetak Atoll (Robison and Noshkin, 1999). While monitoring of the ocean has largely ended and ground level radiation doses are at background (Bordner et al, 2016), there are still ongoing health studies of the populations that left the atoll islands and of the roughly 500 remaining inhabitants that have returned to live on Enewetak Island (https:// marshallislands.llnl.gov/)

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