Cesium, iodine and tritium in NW Pacific waters – a comparison of the Fukushima impact with global fallout

[摘要] Radionuclide impact of the Fukushima Dai-ichi nuclear power plant accident onthe distribution of radionuclides in seawater of the NW Pacific Ocean iscompared with global fallout from atmospheric tests of nuclear weapons.Surface and water column samples collected during the Ka'imikai-o-Kanaloa(KOK) international expedition carried out in June 2011 were analyzed for¹³⁴Cs, ¹³⁷Cs, ¹²⁹I and ³H. The ¹³⁷Cs, ¹²⁹I and³H levels in surface seawater offshore Fukushima varied between0.002–3.5 Bq L⁻¹, 0.01–0.8 μBq L⁻¹, and0.05–0.15 Bq L⁻¹, respectively. At the sampling site about 40 kmfrom the coast, where all three radionuclides were analyzed, the Fukushimaimpact on the levels of these three radionuclides represents an increase abovethe global fallout background by factors of about 1000, 50 and 3,respectively. The water column data indicate that the transport of Fukushima-derivedradionuclides downward to the depth of 300 m has already occurred.The observed ¹³⁷Cs levels in surface waters and in the water column arecompared with predictions obtained from the ocean general circulation model,which indicates that the Kuroshio Current acts as a southern boundary for thetransport of the radionuclides, which have been transported from theFukushima coast eastward in the NW Pacific Ocean. The ¹³⁷Cs inventory inthe water column is estimated to be about 2.2 PBq, what can be regarded as alower limit of the direct liquid discharges into the sea as the seawatersampling was carried out only in the area from 34 to 37° N, and from142 to 147° E. About 4.6 GBq of ¹²⁹I was deposited in the NWPacific Ocean, and 2.4–7 GBq of ¹²⁹I was directly discharged as liquidwastes into the sea offshore Fukushima. The total amount of ³H releasedand deposited over the NW Pacific Ocean was estimated to be 0.1–0.5 PBq.These estimations depend, however, on the evaluation of the total¹³⁷Cs activities released as liquid wastes directly into the sea, whichshould improve when more data are available. Due to a suitable residencetime in the ocean, Fukushima-derived radionuclides will provide usefultracers for isotope oceanography studies on the transport of water massesduring the next decades in the NW Pacific Ocean.

[学科分类] 地球化学与岩石