Tritium and radiocarbon in the western North Pacific waters: post-Fukushima situation

Impact of the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident on tritium (3H) and radiocarbon (14C) levels in the water column of the western North Pacific Ocean in winter 2012 is evaluated and compared with radiocesium (134,137Cs) data collected for the same region. Tritium concentrations in surface seawater, varying between 0.4 and 2.0 TU (47.2-236 Bq m−3), follow the Fukushima radiocesium trend, however, some differences in the vertical profiles were observed, namely in depths of 50-400 m. No correlation was visible in the case of 14C, whose surface Δ14C levels raised from negative values (about −40‰) in the northern part of transect, to positive values (∼68‰) near the equator. Homogenously mixed 14C levels in the subsurface layers were observed at all stations. Sixteen surface (from 30 in total) and 6 water profile (from 7) stations were affected by the Fukushima tritium. Surface and vertical profile data together with the calculated water column inventories indicate that the total amount of the FNPP1-derived tritium deposited to the western North Pacific Ocean was 0.7 ± 0.3 PBq. No clear impact of the Fukushima accident on 14C levels in the western North Pacific was observed.