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

Abstract

The impact of the earthquake and tsunami on theeast coast of Japan on 11 March 2011 caused a loss of powerat the Fukushima Dai-ichi nuclear power plant (NPP) thatresulted in one of the most important releases of artificialradioactivity into the environment. Although several workswere devoted to evaluating the atmospheric dispersion of ra-dionuclides, the impact of the discharges to the ocean hasbeen less investigated. Here we evaluate the distribution ofFukushima-derived90Sr (n=57) and89Sr (n=19) through-out waters 30–600 km offshore in June 2011. Concentrationsof90Sr and89Sr in both surface waters and shallow profilesranged from 0.8±0.2 to 85±3 Bq m−3and from 19±6 to265±74 Bq m−3, respectively. Because of its short half-life,all measured89Sr was due to the accident, while the90Srconcentrations can be compared to the background levels inthe Pacific Ocean of about 1.2 Bq m−3. Fukushima-derivedradiostrontium was mainly detected north of Kuroshio Cur-rent, as this was acting as a southern boundary for transport.The highest activities were associated with near-shore ed-dies, and larger inventories were found in the closest stationsto Fukushima NPP. The data evidence a major influence ofdirect liquid discharges of radiostrontium compared to theatmospheric deposition. Existing137Cs data reported fromthe same samples allowed us to establish a90Sr /137Cs ratioof 0.0256±0.0006 in seawater off Fukushima, being signif-icantly different than that of the global atmospheric fallout(i.e., 0.63) and may be used in future studies to track waterscoming from the east coast of Japan. Liquid discharges of of90Sr to the ocean were estimated, resulting in an inventory of53±1 TBq of90Sr in the inshore study area in June 2011 andtotal releases of90Sr ranging from 90 to 900 TBq, dependingupon the reported estimates of137Cs releases that are consid-ered.