The influence of the Fukushima event on the 129-Iodine content of western Pacific waters

The unfortunate events resulting in damage to the Fukushima Dai-ichi nuclear facility in March 2011 caused the release of many anthropogenic radionuclides, including 129-Iodine into the environment. Characterization of the released 129-I is afforded by a dedicated cruise off eastern Japan in June of 2011. These measurements were compared with samples intended to reflect pre-Fukushima background that were collected during a May 2011 transect of the Pacific by a commercial container vessel. The confluence of the Kuroshio and Oyashiro currents create a near standing eddy that effectively constrained the bulk of the 129I from migrating south into the Kuroshio current. In surface waters, we observed peak 129I concentrations of ~300 μBq/m3 which represents an elevation of nearly three orders of magnitude compared to pre-Fukushima backgrounds. "Pre-event" 129-I was ≤3 x 10-11 (129/127-I). We coupled our 129I results with 137Cs measurements from the same cruise and derived an average 129I/137Cs activity ratio of 0.442x10-6 for Fukushima effluent and a ratio of 58.9x10-6 for pre-Fukushima Pacific Ocean water. The majority of the seawater samples collected in June record direct release of 129-I with minimal atmospheric deposition.