The Fukushima Dai-ichi Nuclear Power Plant (FNPP) is located slightly north of the Kuroshio separation point on the northeast coast of Japan (37°25’N, 142°2’E). In March 2011, the offshore earthquake, and resulting tsunami and aftershocks caused a loss of power at the FNPP. Soon thereafter, destabilization led to an explosive release of radioactive materials into the atmosphere and leakage of coolant water directly discharged into the ocean.
All the samples were analyzed onshore in the lab using one of two resin-based methods: ammonium molybdophosphate on an organic polymer polyacrylonitrile or potassium-nickel hexacyanoferrate (II) [Šebasta and Štefula, 1990; Kamenik et al., 2013; Pike et al., 2013]. Two cesium radioisotopes, 134Cs (half-life of 2.06 years) and 137Cs (half-life of 30.07 years) are used in this study to examine radionuclides derived from Fukushima. The 137Cs isotope is known to have preexisted in the global oceans prior to the FNPP accident as a result of the weapons testing experiments that peaked in the 1960s [Bowen et al., 1980; United Nations, 2000]. By 2011, after decades of ocean mixing and dispersion, the distribution of surface weapons-based 137Cs concentration was nearly uniform throughout the North Pacific, with values of 1.5–2.0 Bq/m3 [Aoyama and Hirose, 2004; Aoyama et al., 2011; Inomata et al., 2009]. The 134Cs isotope, on the other hand, is unique to the FNPP release with no significant anthropogenic 134Cs remaining in the ocean prior to the 2011 accident due to its short half-life. As it is known that the ratio of 134Cs/137Cs from the FNPP contamination was close to 1.0 [Buesseler et al., 2012], any detectable level of 134Cs allows us to distinguish the Fukushima-origin 137Cs from preexisting sources.