Science of the Total Environment 621 (2018) 1185–1198 Contents lists available at ScienceDirect Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv Lingering radioactivity at the Bikini and Enewetak Atolls Ken O. Buesseler ⁎, Matthew A. Charette, Steven M. Pike, Paul B. Henderson, Lauren E. Kipp Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA HIGHLIGHTS GRAPHICAL ABSTRACT • Both 239,240Pu and 137Cs remain elevated today at Bikini and Enewetak Atolls. • These sites remain a small long term source of Pu and Cs to the North Pacific. • About half of the Pu in the waters at Enewetak Lagoon originates near Runit Dome. • Only a small fraction of Pu and Cs from Runit Dome is delivered via groundwater. article info abstract Article history: We made an assessment of the levels of radionuclides in the ocean waters, seafloor and groundwater at Bikini Received 4 August 2017 and Enewetak Atolls where the US conducted nuclear weapons tests in the 1940's and 50's. This included the Received in revised form 5 October 2017 first estimates of submarine groundwater discharge (SGD) derived from radium isotopes that can be used here Accepted 12 October 2017 to calculate radionuclide fluxes in to the lagoon waters. While there is significant variability between sites and Available online 30 October 2017 sample types, levels of plutonium (239,240Pu) remain several orders of magnitude higher in lagoon seawater 137 Editor: D. Barcelo and sediments than what is found in rest of the world's oceans. In contrast, levels of cesium-137 ( Cs) while relatively elevated in brackish groundwater are only slightly higher in the lagoon water relative to North Pacific Keywords: surface waters. Of special interest was the Runit dome, a nuclear waste repository created in the 1970's within the Marshall Islands Enewetak Atoll. Low seawater ratios of 240Pu/239Pu suggest that this area is the source of about half of the Pu in Runit dome the Enewetak lagoon water column, yet radium isotopes suggest that SGD from below the dome is not a signif- Plutonium icant Pu source. SGD fluxes of Pu and Cs at Bikini were also relatively low. Thus radioactivity associated with sea- Cesium floor sediments remains the largest source and long term repository for radioactive contamination. Overall, Bikini Radium and Enewetak Atolls are an ongoing source of Pu and Cs to the North Pacific, but at annual rates that are orders of Nuclear weapons tests magnitude smaller than delivered via close-in fallout to the same area. © 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction time atmospheric testing ended (based upon yields; Buesseler, 1997; Hamilton, 2004). Unlike most of the global fallout that was distributed The US conducted 66 nuclear weapons tests at the Bikini and widely from high altitude testing, the majority of tests were conducted Enewetak Atolls between 1946 and 1958 accounting for N50% of the either on the surface of the atoll lagoons, on the islands, or underwater. global fallout during that time period and 20% of global fallout by the This resulted in “close-in” fallout that contaminated not only the local islands and lagoon sediments, but also larger areas of the North Pacific ⁎ Corresponding author. Ocean where close-in fallout accounts for 60% of the total fallout Pu E-mail address: [email protected] (K.O. Buesseler). (Bowen et al., 1980). https://doi.org/10.1016/j.scitotenv.2017.10.109 0048-9697/© 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 1186 K.O. Buesseler et al. / Science of the Total Environment 621 (2018) 1185–1198 The fate of radionuclides at these islands was evaluated starting with between the islands and the lagoons, including between the Runit the first atomic tests, but became more extensive after testing ended in Dome and Enewetak Lagoon. To our knowledge, these are the first direct the 1960s and again in the 1970s in preparation for resettlement of the estimates of groundwater-surface water exchange rates for artificial islands (see review by Robison and Noshkin, 1999). Activities greater radionuclides at the site of the Marshall Islands US nuclear weapons than background fallout levels were found for many radionuclides in testing program. Water column ratios of short- to long-lived Ra isotopes lagoon waters due to their continued remobilization, presumably from provide an estimate of the lagoon flushing times (Charette et al., 2013), marine sediments. This issue persists today, though estimated doses to useful for calculating artificial radionuclide fluxes from the lagoons to humans from consumption of marine foods is thought to be a small the Pacific Ocean. source of the total radiological dose, for example b 0.1% at Enewetak Our primary goal was to make an independent reassessment in 2015 Atoll (Robison and Noshkin, 1999). While monitoring of the ocean has of the levels of the major fallout radionuclides that remain in the lagoon largely ended and ground level radiation doses are at background waters and seafloor of Bikini and Enewetak Atolls, some 60–70 years (Bordner et al., 2016), there are still ongoing health studies of the after nuclear testing ended. We accomplished this by comparing our populations that left the atoll islands and of the roughly 500 remaining data within the lagoons to values measured earlier at these sites and inhabitants that have returned to live on Enewetak Island (https:// as found elsewhere globally. The levels of radionuclides from these marshallislands.llnl.gov/). nuclear tests are also compared to the world's largest accidental release During a January 2015 expedition to the Bikini and Enewetak Atolls, of radionuclides to the ocean in 2011 associated with the Fukushima we sought to determine if there have been significant changes in radio- Dai-ichi nuclear power plants. A secondary goal was to address for the nuclide levels in the water column, sediments, and groundwater. Of first time the relative importance of Pu contamination to the Enewetak special interest was a possible ongoing source associated with a waste lagoon that originates from the Runit Dome area, either via submarine disposal site commonly known as the Runit Dome in Enewetak Atoll, groundwater discharge or from surrounding lagoon sediments. where close to 100,000 cubic meters of radioactive debris were buried in a 100 m diameter nuclear testing crater in the late 1970s and covered with a concrete cap (Davisson et al., 2012; Hamilton, 2013; Noshkin and 2. Sampling and methods Robison, 1997). The debris was a combination of contaminated Enewetak soils, coral materials, scrap metal, excess equipment and 2.1. Physical setting some Pu-bearing nuclear material that was explosively scattered over the island of Runit during a test in 1958 (Quince) and other debris The Bikini and Enewetak Atolls are among the 29 atolls that make up from low-yield tests in the Runit Island area. the Republic of the Marshall Islands, located in the equatorial Pacific Plutonium isotope ratios from individual tests are known to vary some 4000 km west of Hawaii. Geographically, the Bikini Atoll is cen- based upon the nuclear weapons design and fission and fusion yields, tered at 11° 35′ N 165° 23′ E and consists of 26 islands (23 remaining with higher yields resulting in the production of higher mass Pu iso- after weapons testing) with an area of 8.8 km2 that form a roughly cir- topes. The Quince test did not result in any nuclear yield or fission, but cular reef that surrounds a lagoon of 630 km2 area with a mean depth of deposited weapons grade Pu in the lagoon sediments near Runit Island, 46 m. Enewetak is centered on 11° 30′ N 162° 15′ E with a ring of 42 some of which was buried under the dome with a distinct and lower islands (39 remaining) of 6.9 km2 area and a lagoon of 932 km2 area 240Pu/239Pu atom ratio than from other tests. Other sites of interest in- and mean depth of 48 m. The collective land area of the 1000's of cluded Bravo Crater in Bikini Atoll and Mike Crater in Enewetak Atoll, small islands that make up the Marshall Islands is only equivalent to where the largest US thermonuclear (hydrogen bomb) tests were con- the area of Washington DC (180 km2), yet they are spread across an ducted. These two high yield tests resulted in the largest deposition of ocean area that exceeds the size of Alaska (1,900,000 km2). Both atolls close-in fallout, and high 240Pu/239Pu ratios that are characteristic of are low lying with an average elevation of 2–3 m, thus in addition to the North Pacific today relative to other ocean regions that received the radiological concern, sea level rise will have immediate conse- only global fallout (Buesseler, 1997; Diamond et al., 1960; Kelley et al., quences for these atolls. 1999). These two remote atolls were selected for US nuclear testing and be- In this study we sampled seawater, groundwater and lagoon sedi- came known as the western part of the US Pacific Proving Grounds 239 ments at selected sites with an emphasis on Pu isotopes ( Pu t ½ = (PPG). Given the different designs of the nuclear devices and range of 240 137 24,100 yr.; Pu t ½ = 6560 yr.) and radiocesium ( Cs t ½ = yields, there is in effect a mosaic of different radionuclides and contam- 30.1 yr.). These artificial radionuclides have been widely studied in the ination levels associated with particular test sites within the lagoons.