Time series of¹²⁹I and ¹²⁷I speciation in precipitation from Denmark

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 ¹²⁹I 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 ¹²⁹I L^-1, and the annual deposition flux of ¹²⁹I is (1.25 ( 0.30) ± 1012 atoms m-2. Increased ¹²⁹I levels in precipitation and ¹²⁹I/ ¹²⁷I 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.