Speciation of ¹²⁹I and ¹²⁷I in seawater and implications for sources and transport pathways in the North Sea

Surface seawater samples collected from the North Sea and English Channel were analyzed for total ¹²⁹I and ¹²⁷I, as well as for iodide and iodate. Relatively high ¹²⁹I concentrations (2-3 × 10¹¹ atoms/L) were observed in the northern part of the English Channel and in the southeastern North Sea. The atomic ratio of ¹²⁹I/¹²⁷I 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 ¹²⁷I and ¹²⁹I, 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 ¹²⁹I 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 ¹²⁹I^- to ¹²⁹IO 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.