Speciation of 129 I and 127 I in Seawater and Implications for Sources and Transport Pathways in the North Sea XIAOLIN HOU,* ,† ALA ALDAHAN, ‡ SVEN P. NIELSEN, † GO ¨ RAN POSSNERT, § HARTMUT NIES, | AND JIM HEDFORS ‡ Risø National Laboratory, NUK-202, Technical University of Denmark, DK-4000 Roskilde, Denmark, Department of Earth Sciences, Uppsala University, SE-752 36 Uppsala, Sweden, Tandem Laboratory, Uppsala University, SE-751 21 Uppsala, Sweden, and Bundesamt fuer Seeschifffahrt und Hydrographie, D-22589 Hamburg, Germany Surface seawater samples collected from the North Sea and English Channel were analyzed for total 129 I and 127 I, as well as for iodide and iodate. Relatively high 129 I concentrations (2-3 × 10 11 atoms/L) were observed in the northern part of the English Channel and in the southeastern North Sea. The atomic ratio of 129 I/ 127 I decreases from the eastern (1.0-1.9 × 10 -6 ) to the western (4-6 × 10 -8 ) parts of the North Sea and from the northeastern (1.5 × 10 -6 ) to southwestern (1-5 × 10 -8 ) parts of the English Channel. The ratios of iodide to iodate are 0.1-0.5 and 0.5-1.6 for 127 I and 129 I, respectively, in open seawaters, whereas these ratios range from 0.6 to 1.3 and 0.8 to 2.2, respectively, in coastal waters. The results suggest that (1) imprints of the La Hague facility dominates the 129 I distribution in the surface water of the North Sea, (2) reduction of iodate to iodide is relatively fast during the transport to the European continental coast, (3) oxidation of newly produced 129 I - to 129 IO 3 - is insignificant during water exchange between the coastal area and open sea, (4) reduction of iodate and oxidation of iodide in the open sea seems to be a slow process. Introduction Iodine exists in the ocean surface waters predominantly as dissolved iodate, iodide, and a minute amount of organic iodine (1). Iodide is a thermodynamically unfavorable species in oxygenated water, so its formation through the reduction of iodate cannot occur spontaneously by chemical means alone. Although iodate is a thermodynamically favorable species of iodine in seawater, the kinetic barrier prevents the direct oxidation of iodide to iodate (1). Numerous studies have been carried out to investigate the origin of iodide, the conversion of iodine between different species, and the marine geochemical cycle of iodine by determination of the concentrations of various species of iodine in seawater in certain areas (1-4). However, the mechanism of conversion among iodine species is still not clear because of the difficulties associated with distinguishing the origin of newly produced and converted iodine species. Isotopic tracers are an excellent tool for the distinction and detection of the source of chemical species. Laboratory research on the conversion of different chemical species of iodine using short-lived isotopes of iodine has been carried out (5-6), but the results are only qualitative because of inadequate simulation of the real seawater environment and consideration of complex interactions among a variety of minor and trace components in seawater. 129 I(T1/2 ≈ 15.7 Ma) is a naturally produced long-lived radioisotope of iodine, which has a natural atomic ratio ( 129 I/ 127 I) of about 10 -12 in the ocean (7). Releases from human nuclear activities dominate the present 129 I level in the environment (8-11), where the nuclear reprocessing facilities at Sellafield (U.K.) and La Hague (France) are responsible for about 90% of the anthropogenic releases (9, 10, 12). These sources and their rapid increase since 1990 (Figure S-1) provide a unique temporal and spatial field tracer for the investigation of the iodine marine geochemical cycle by chemical speciation of 129 I combined with that of stable iodine. The occurrence of a relatively huge anthropogenic 129 I input in marine waters has been used to trace ocean currents and water transport in the North Atlantic and Arctic Oceans and related seas (9-10, 13-17). However, none of these studies have used the chemical speciation of iodine to further quantify effects on the mixing of water masses and finger- printing of transport mechanisms. Schwehr et al. (18) showed a potential application of 129 I and 127 I speciation in the estuarine surface waters of Galveston Bay for tracing ter- restrial organic carbon. A main objective of this study was to investigate the source of iodide in the coastal water and interconversion process of iodide and iodate by chemical speciation of 129 I and 127 I in surface seawater collected from the English Channel and the North Sea. A second objective of this study was to investigate the distribution and transport pathways of different species of 129 I and 127 I in the North Sea surface waters. Such knowledge will provide significant information about the use of 129 I as an environmental tracer. Experimental Section Surface water was collected from 42 sites in the English Channel and the North Sea in August-September 2005 (Table S-1 and Figure 1). The samples were filtered through a Φ 0.45 μm membrane (Sartorius AG, Go ¨ttingen, Germany) on site and then tightened and stored in clean polyethylene con- tainers under normal laboratory conditions until analysis. A modified method of Hou et al. (19-20) was used for the separation of iodine species. Wet AG1-×4 resin (Bio-Rad laboratories) in NO3 - form was packed in column of φ 10 × 200 mm. A 50-100 mL of seawater spiked with 50 Bq of 125 I - and 125 IO3 - tracers was loaded onto the column, and the column was washed with 30 mL of deionized water and then 50 mL of 0.2 M KNO3. The effluent and the washes were combined for the determination of iodate. Iodide on the column was eluted by 150 mL of 2.0 M KNO3. One milliliter of separated iodide, iodate solution, or original seawater was diluted to 20 mL with 0.15 M NH4OH, and Cs + as (CsCl) was added as an internal standard. The concentration of iodine ( 127 I) was determined using an X Series II ICP-MS (Thermal Electron Corporation). The detection limit, calculated as 3SD of blanks, was 0.27 nM. * Corresponding author fax: +45 4677 5347; e-mail: xiaolin.hou@ risoe.dk. † Risø National Laboratory, NUK-202, Technical University of Denmark. ‡ Department of Earth Sciences, Uppsala University. § Tandem Laboratory, Uppsala University. | Bundesamt fuer Seeschifffahrt und Hydrographie. Environ. Sci. Technol. 2007, 41, 5993-5999 10.1021/es070575x CCC: $37.00  2007 American Chemical Society VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 5993 Published on Web 08/04/2007