Contamination of Alpine snow and ice at Colle Gnifetti, Swiss/Italian Alps, from nuclear weapons tests
Research highlights
► High-resolution profile of atmospheric 239Pu deposition from Alpine ice core. ► Direct ICP-SFMS determination without preconcentration and purification. ► 239Pu profile reflects the three main periods of atmospheric nuclear weapons testing. ► The data presented are in very good agreement with 239Pu profiles previously obtained.
Introduction
Plutonium is present in the environment as a consequence of the atmospheric nuclear tests carried out since the 1950s as well as the production of nuclear weapons and industrial releases over the past 60 years. Plutonium is not naturally present on Earth, and was first artificially produced and isolated in 1940 by deuteron bombardment of uranium in the cyclotron of Berkeley University. It exists as five main isotopes, 238Pu, 239Pu, 240Pu, 241Pu, 242Pu, derived from civilian and military sources (weapons production and detonation, nuclear reactors, nuclear accidents), of which 239Pu is the most abundant in the environment. It was estimated that approximately 6 tons of 239Pu were released into the environment as a result of 541 atmospheric weapon tests (Carter and Moghissi, 1977). In 1961–62 nearly 180 above-ground nuclear tests were carried out with a total explosive yield of over 260 Mt TNT equivalent (Harley, 1980), more than 50% of the total explosive yield of all atmospheric blasts (440 Mt TNT equivalent). On 30th October 1961, over the Arctic, the Soviet Union detonated the 50 Mt TNT equivalent “Tsar Bomb”, the most powerful nuclear weapon ever built, about 4000 times more powerful than the bombs detonated in Hiroshima and Nagasaki. In 1963 the USSR and USA signed the “Limited Test Ban Treaty” in which they committed themselves to stop all above-ground nuclear tests. From then, until 1980, only 64 above-ground nuclear explosions occurred, all of them carried out by France (41) and China (23) (Lawson, 1998). As of September 2009, it was estimated that there are still over 23,000 nuclear devices worldwide, primarily in the USA and Russia (FAS, 2009).
Radioactive debris from atmospheric detonations of nuclear weapons are partitioned in the troposphere and stratosphere, according to particle size, explosive power of the device and altitude of detonation. The subsequent fallout occurs on time scales of minutes to years. The maximum tropospheric residence time for fine fallout aerosol of a small yield test (<100 kt TNT) was estimated to be 70 days (Norris and Arkin, 1998). For large tests (>500 kt TNT) the typical residence time is 15–18 months (Zandler and Araskog, 1973).
Due to the high biological toxicity and long half-lifes of the relevant Pu isotopes (e.g. 24.2 × 103 y for 239Pu, 373 × 103 y for 242Pu and 81 × 106 y for 244Pu), which are α-emitters, there has been an increase in public concern regarding past environmental releases, as well as current safety measures for storage and handling of this material.
Nuclear Pu fallout has been studied in various environmental archives, such as soil (Cizdziel et al., 2007, Turner et al., 2003), sediments (Saito-Kokubu et al., 2007, Schertz et al., 2006), cryoconitic material from Alpine glaciers (Tieber et al., 2009), peat bogs (Testa et al., 1999), oceanic water (Yamada et al., 2006), corals (Buesseler, 1997) and herbarium grass (Warneke et al., 2002). Polar and mid-latitude mountain glaciers are particularly important for studying the anthropogenic impact on the environment during the last decades because they allow highly resolved temporal records (Barbante et al., 2011, Gabrieli et al., 2010, Schwikowski, 2004). The extremely low concentrations of Pu in glacial ice require demanding analytical techniques with very sensitive instrumentation and/or large sample amounts for accurate determination (Olivier et al., 2004). Koide et al., 1982, Koide et al., 1985 presented the first Pu record in an ice core from Greenland. Mid-latitude ice cores have been studied as well, on Mont Blanc in the Western Alps (Warneke et al., 2002) and on Belukha Glacier in the Siberian Altai mountains (Olivier et al., 2004). In Olivier et al. (2004), Pu was analyzed by Accelerator Mass Spectrometry (AMS) using an optimized sample preparation procedure. As high levels of uranium can interfere with the mass spectrometric measurement of 239Pu, a purification step was required. Analyses were carried out using a 14 MV tandem accelerator to determine the concentrations of 239Pu and 240Pu. This method gives very accurate results but is time-consuming and requires expensive instrumentation and demanding procedures for sample preparation. Moreover, the large sample quantity required for pre-concentration (2–3 kg) precludes this procedure for high resolution analysis of ice-cores.
We present here a high-resolution profile of atmospheric 239Pu deposition reconstructed from an ice core collected on Colle Gnifetti in the Monte Rosa massif (Swiss/Italian Alps, 4450 m asl). 239Pu was analyzed directly by Inductively Coupled Plasma – Sector Field Mass Spectrometry (ICP-SFMS) equipped with a desolvation system without need for any pre-concentration or sample clean-up steps.
Section snippets
Materials and methods
The sample information and procedures described here are identical to those previously reported in detail by Gabrieli et al. (2010), so only a brief summary is included here.
239Pu profile
In Fig. 2, the 239Pu profile inferred from the Colle Gnifetti ice core and the records of atmospheric and underground nuclear tests are reported. Pu is first detectable in Colle Gnifetti ice in 1954–1955 while activities are constantly less than the instrumental detection limit in earlier samples. The first 239Pu activity peak occurs in 1955/56 with a maximum value of 3.9 ± 1.8 mBq kg−1. A slight decrease followed, with activities about 2.9 ± 1.7 mBq kg−1, until a second activity peak of 5.2 ± 0.9 mBq kg−1
Acknowledgments
This work was supported by the Agenzia Regionale per la Prevenzione e Protezione Ambientale del Veneto (ARPAV) – Department of Belluno, the Agence de l’Environnement et de la Maitrise de l’Energie (ADEME), and the Università Italo-Francese (UIF) and the European Union Marie Curie IIF Fellowship (MIF1-CT-2006-039529, TDICOSO). The authors are grateful to Alberto Luchetta and Fabio Decet (ARPAV) for their cooperation, and ELGA LabWater in providing the PURELAB system. We acknowledge the
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Now at: Centre for Ice and Climate, Niels Bohr Institute, University of Copenhagen, 2100 Copenhagen, Denmark.