Speciation of ¹²⁹I in sea, lake and rain waters

Concentrations of the very long-lived fission product ¹²⁹I and stable iodine ( ¹²⁷I) in the Baltic Sea and lake and rain waters from Finland, were measured as well as their occurrence as iodide (I ^-) and iodate (IO ₃ ^-). The highest concentrations of both ¹²⁷I and ¹²⁹I occurred in sea water, on average 11.1±4.3μg/l and 3.9±4.1×10 ^-9 at/l. In rain and lake waters the concentration of ¹²⁹I was more or less identical and almost one order of magnitude lower than in sea water. Based on these observations, and data from the literature, it is assumed that the source of ¹²⁹I in lakes is precipitation and the major source in the Baltic Sea is the inflow of sea water from the North Sea through the Danish Straits. The concentration of ¹²⁹I in the Baltic Sea has increased by a factor of six during ten years from 1999. In all studied water types the main chemical form of both iodine isotopes was iodide; in sea and lake waters by 92-96% and in rain water by 75-88%. Compared to ¹²⁷I the fraction of iodide was slightly higher in case of ¹²⁹I in all waters.