Time Series of I-129 and I-127 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.