Iodine-129 and iodine-127 in European seawaters and in precipitation from Northern Germany

In order to obtain a comprehensive survey on the consequences of the marine 129I discharges from the European reprocessing plants La Hague and Sellafield, the distribution of 129I and 127I in surface waters of the North Sea, the English Channel, the Irish Sea, and the Northeast Atlantic was studied using accelerator mass spectrometry for 129I and ICP-MS for 127I. Samples of seawater were taken in the German Bight in May, September, and November 2005 and in the entire North Sea and the English Channel in August 2005. Further samples were obtained from the Irish Sea in June and August 2006 and from Arctic waters between Spitsbergen and Southern Norway in September 2005. 129I is a conservative tracer in seawater. The concentrations of 127I are relatively constant with exceptions of coastal areas with high biological activity and of areas influenced by influx from rivers and the Baltic Sea. The variability of the 129I/127I isotopic ratios is exclusively determined by admixture of 129I released from the reprocessing facilities Sellafield and La Hague to the seawater. The 129I/127I ratios were between 4×10−9and 3×10−6: at least 3 orders of magnitude higher than the natural equilibrium isotopic ratio 1.5×10−12. 129I/127I ratios of a few times 10−10 were only found in seawater from the Indian Ocean and from the Pacific at Hawaii. Comparison of the results obtained for seawater with those of a measurement of airborne iodine species and with iodine isotopes in precipitation in Northern Germany demonstrates the transfer of 129I and 127I from the sea into the atmosphere and the dominating role of the marine discharges for the atmospheric fallout of 129I in Western Europe. The results are discussed with the goal to estimate the relevance of the marine discharges for the contamination of the continental areas. ► I-129 from European reprocessing plants drastically changed its natural abundances. ► I-129 in seawater demonstrates the complex transport processes in Europe. ► I-129 in precipitation over Northern Germany is dominated by the releases into the sea. ► I-129 is an indicator of the long-term human impact on the environment. ► I-129 serves as a radioactive tracer of the environmental iodine cycle.