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

[摘要] The impact of the earthquake and tsunami on the east coast of Japan on 11March 2011 caused a loss of power at the Fukushima Dai-ichi nuclear powerplant (NPP) that resulted in one of the most important releases of artificialradioactivity into the environment. Although several works were devoted toevaluating the atmospheric dispersion of radionuclides, the impact of thedischarges to the ocean has been less investigated. Here we evaluate thedistribution of Fukushima-derived ⁹⁰Sr (n = 57) and ⁸⁹Sr (n = 19)throughout waters 30–600 km offshore in June 2011. Concentrations of⁹⁰Sr and ⁸⁹Sr in both surface waters and shallow profiles rangedfrom 0.8 ± 0.2 to 85 ± 3 Bq m⁻³ and from 19 ± 6 to265 ± 74 Bq m⁻³, respectively. Because of its short half-life,all measured ⁸⁹Sr was due to the accident, while the ⁹⁰Srconcentrations can be compared to the background levels in the Pacific Oceanof about 1.2 Bq m⁻³. Fukushima-derived radiostrontium was mainlydetected north of Kuroshio Current, as this was acting as a southern boundaryfor 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 ofradiostrontium compared to the atmospheric deposition. Existing ¹³⁷Csdata reported from the same samples allowed us to establish a⁹⁰Sr / ¹³⁷Cs ratio of 0.0256 ± 0.0006 in seawater offFukushima, being significantly different than that of the global atmosphericfallout (i.e., 0.63) and may be used in future studies to track waters comingfrom the east coast of Japan. Liquid discharges of ⁹⁰Sr to the oceanwere estimated, resulting in an inventory of 53 ± 1 TBq of ⁹⁰Srin the inshore study area in June 2011 and total releases of ⁹⁰Srranging from 90 to 900 TBq, depending upon the reported estimates of¹³⁷Cs releases that are considered.

[学科分类] 地球化学与岩石