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 (IO3−). The highest concentrations of both 127I and 129I occurred in sea water, on average 11.1 ± 4.3 μg/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.

► Concentrations of 129I in Finnish lakes are equal with the concentrations in rain water.
► The source of 129I in lakes is the precipitation.
► Concentrations of 129I in the Baltic Sea are at least ten times higher than in lakes.
► In all water types iodide is the major chemical species of iodine.
► 129I occurs slightly more in iodide form than stable iodine (127I).