⁹⁰Sr and ⁸⁹Sr in seawater off Japan as a consequence of the Fukushima Dai-ichi nuclear accident

The impact of the earthquake and tsunami on the east coast of Japan on 11 March 2011 caused a loss of power at the Fukushima Dai-ichi nuclear power plant (NPP) that resulted in one of the most important releases of artificial radioactivity into the environment. Although several works were devoted to evaluating the atmospheric dispersion of radionuclides, the impact of the discharges to the ocean has been less investigated. Here we evaluate the distribution of Fukushima-derived ⁹⁰Sr (n = 57) and ⁸⁹Sr (n = 19) throughout waters 30-600 km offshore in June 2011. Concentrations of ⁹⁰Sr and ⁸⁹Sr in both surface waters and shallow profiles ranged from 0.8±0.2 to 85±3 Bqm⁻³ and from 19±6 to 265±74 Bqm⁻³, respectively. Because of its short half-life, all measured ⁸⁹Sr was due to the accident, while the ⁹⁰Sr concentrations can be compared to the background levels in the Pacific Ocean of about 1.2 Bqm⁻³. Fukushima-derived radiostrontium was mainly detected north of Kuroshio Current, as this was acting as a southern boundary for transport. The highest activities were associated with near-shore eddies, and larger inventories were found in the closest stations to Fukushima NPP. The data evidence a major influence of direct liquid discharges of radiostrontium compared to the atmospheric deposition. Existing ¹³⁷Cs data reported from the same samples allowed us to establish a ⁹⁰Sr / ¹³⁷Cs ratio of 0.0256±0.0006 in seawater off Fukushima, being significantly different than that of the global atmospheric fallout (i.e., 0.63) and may be used in future studies to track waters coming from the east coast of Japan. Liquid discharges of ⁹⁰Sr to the ocean were estimated, resulting in an inventory of 53±1 TBq of ⁹⁰Sr in the inshore study area in June 2011 and total releases of ⁹⁰Sr ranging from 90 to 900 TBq, depending upon the reported estimates of ¹³⁷Cs releases that are considered