Environ. Sci. Technol. 2009, 43, 6522–6528
Time Series of 129I and 127I Speciation in Precipitation from Denmark X I A O L I N H O U , * ,†,| A L A A L D A H A N , ‡,⊥ SVEN P. NIELSEN,† AND ¨ RAN POSSNERT§ GO Risø National Laboratory for Sustainable Energy, NUK-202, Technical University of Denmark, DK-4000 Roskilde, Denmark, Department of Earth Sciences, Uppsala University, SE-758 36 Uppsala, Sweden, Tandem Laboratory, Uppsala University, SE-751 21 Uppsala, Sweden, SKLLQG, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an, China, and Department of Geology, United Arab Emirates University, Al Ain, UAE
Received April 29, 2009. Revised manuscript received July 1, 2009. Accepted July 3, 2009.
Environmental 129I mainly released from reprocessing plants at La Hague (France) and Sellafield (UK) provides a unique atmospheric and environmental tracer. This study deals with 129 I and 127I speciation in precipitation collected in Denmark during 2001-2006 that indicates many new findings. The concentrations of total 129I in precipitation vary from 0.28 to 5.63 × 109 atoms 129I L-1 with an average of (2.34 ( 1.43) × 109 atoms 129 -1 I L , and the annual deposition flux of 129I is (1.25 ( 0.30) × 1012 atoms m-2. Increased 129I levels in precipitation and 129I/ 127 I ratio are attributed to the releases of 129I from the reprocessing plants at La Hague and Sellafield. Iodide is the major specie of 129I, which accounts for 50-99% of total 129I. The concentrations of total 127I vary from 0.78 to 2.70 µg iodine L-1 with an average of 1.63 ( 0.47 µg iodine L-1, and annual deposition flux of 0.95 ( 0.26 mg m-2. Unlike 129I, iodate is the major specie of 127I, which accounts for 43-93% of total 127I. The 129I/127I atomic ratios for total iodine vary from 5.04 to 76.5 × 10-8 with an average of (30.1 ( 16.8) × 10-8. These values are 10 times lower for iodate with an average of (2.95 ( 3.13) × 10-8. Seasonal variations of 129I/127I values and 129I concentrations are associated with highs in spring and lows in summer-autumn periods. Re-emission of 129I from the surface water of the English Channel, Irish Sea, North Sea, and Norwegian Sea, especially from the European continental coast areas, is evidently the major source of 129I in the precipitation, while stable 127I in the precipitation has multiple sources, i.e., marine, as well as terrestrial emission. This work shows that data on speciation of iodine isotopes can provide thorough indications about the sources and geochemical cycle despite the complicated atmospheric chemistry of iodine. * Corresponding author e-mail:
[email protected]; tel: +45 4677 5357; fax: +45 4677 5347. † Risø National Laboratory for Sustainable Energy, Technical University of Denmark. ‡ Department of Earth Sciences, Uppsala University. § Tandem Laboratory, Uppsala University. | SKLLQG, Institute of Earth Environment, Chinese Academy of Sciences. ⊥ Department of Geology, United Arab Emirates University. 6522
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 17, 2009
Introduction Iodine is an essential element to humans and other mammals. Insufficient intake of iodine from foodstuff and water causes iodine deficiency disorder, which is attributed to the low concentration of iodine in soil and agricultural products. Oceans are the main pool of iodine on the Earth’s surface (1), and it has been generally accepted that iodine in the terrestrial environment, especially in soil, originates mainly from the oceans through transport by clouds and aerosols and subsequent deposition (2-5). Low concentrations of iodine in soil are attributed to long distances of the soil locations from marine areas, as a consequence of low transport and deposition of marine-derived iodine to the soil. However, there is conflicting evidence about this issue showing less correlation between iodine deposition flux on the soil and its distance from the ocean (6) and relative high emission of iodine from terrestrial plants and soil (7, 8). For some years it has been accepted that iodine is mainly emitted to the atmosphere from the surface of the oceans as methyl iodide and other alkyl iodides of biological origin (9). Recent experiment showed that molecular iodine (I2) is released from macroalgae and a high concentration of I2 was observed in coastal areas of the ocean (10, 11). It has also been shown that atmospheric iodine chemistry plays an important role in ozone destruction, particulates formation, and cloud condensation nuclei formation (10-13). The complicated atmospheric chemistry of iodine ultimately feeds into the general iodine geochemical cycle through precipitation (3). Therefore, speciation of iodine in precipitation will provide important information about the geochemical cycle of iodine and related atmospheric chemical process. Presently, most iodine-129 (1.57 × 107 years) in environment is dominated by the releases from human nuclear activities. Up until the year 2006, about 4990 kg 129I had been discharged to the marine system from two European nuclear fuel reprocessing plants (NRPs) at La Hague (France) and Sellafield (U.K) (14). As a consequence, the 129I/127I ratio in the Irish Sea, English Channel, North Sea, and Nordic Seas has significantly increased to 10-8-10-5 (15-19) compared to preanthropogenic values (10-12). These two reprocessing plants have also released 75 and 180 kg of 129I to the atmosphere, respectively; another European NRP at Marcoule (France) has released a comparable amount of 129I (145 kg) to the atmosphere during its operation (1956-1997) (15). As a consequence, the 129I/127I levels in the vicinity of these three NRPs have increased to 10-6-10-4 (19-21). Despite the nutritional importance of stable iodine and the potential radiotoxic effects of 129I, only a few studies on the speciation of 127I in precipitation have been reported (2-5, 22). Of them, only scattered samples, or short timeseries (
