Large amounts of water to cool the fuel rods was supplied and this procedure increased the amount of water contaminated with radionuclides, and the contaminated water leaked onto the land and into the ocean. By early April (end of May), the estimated total amount of accidentally and deliberately emitted 137 Cs was 2.6 (3.5 ± 0.7) PBq (Tsumune et al., 2011). The maximum 137 Cs concentrations at the north drain outlet of the plant and at 30 km off Fukushima were 68,000 Bq L-1 (on 30 March: TEPCO, 2011a) and 186 Bq L-1 (on 15 April: MEXT, 2011), respectively. Thereafter, these concentrations have been decreasing, but the level of radioactivity in seawater is still higher than the background level due to the atmospheric nuclear weapons test before 11 March (MOE, 2009). Moreover, about 0.11 million tons of radioactivity-contaminated water (total radioactivity, 720 PBq, including 137 Cs, 140 PBq) had

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