Abstract. Cruise SoSo 5 Rivers took place during October 2014 off the coast of Fukushima Prefecture shortly after the passage of two typhoons. Detection of dissolved 134Cs and 137Cs in all samples reflected contamination caused by accidental releases of radiocaesium from the Fukushima Dai-ichi Nuclear power plant (FNPP1) accident. The dissolved activities were generally higher at coastal sites and decreased with distance from shore, and they were higher in the surface than in the bottom water. The tendency of 137Cs activities to decrease with distance from the coast reflected mixing of coastal water and open-ocean water of which 137Cs activity concentration was ~1.5 Bq m−3. At stations very close to the coast, we observed high particulate 137Cs activity concentration that exceeded dissolved 137Cs activity concentration. 137Cs activities were generally 1–2 orders of magnitudes lower in organic particles than in dissolved form, and the ratios of 137Cs activity concentration in organic particles to 137Cs activity concentration in dissolved form ranged from 0.01 ± 0.00 to 0.12 ± 0.01. The ratio of 137Cs to 134Cs activity concentrations in organic particles did not change with distance from shore or with 137Cs activity concentration and generally remained around 1, even in samples collected far from the coast. This pattern indicated that the organic particles had come from rivers or a source very close to the coast. The 137Cs / 134Cs activity ratio in dissolved form north of FNPP1 region was estimated to be 1.074 ± 0.015, a ratio that is in good agreement with the 137Cs / 134Cs activity ratio in the core of Unit 1 of the FNPP1 while the 137Cs / 134Cs activity ratio at Tomioka port which located south of FNPP1 was 0.998 ± 0.017. Therefore we can conclude the source of radiocaesium in seawater in the coastal region north of FNPP1 was deposited radiocaesium released from the core of Unit 1 of FNPP1, while the source of radiocaesium observed in the coastal region south of FNPP1 was a mixture of deposited radiocaesium released from the core of Unit 2 and the core of Unit 1 of FNPP1. During September–October of each year, the typhoon season in Japan, the 137Cs activity concentration generally increased at Ukedo port, Tomioka port, FNPP1, and Iwasawa beach, and showed a good relationship with the 7-day modified antecedent precipitation index (API) while there is less correlation between the modified API and 137Cs activity concentration near the outlet of canal from unit 5 and 6 of FNPP1 to the sea.
Impact of typhoons on particulate and dissolved <sup>137</sup>Cs activities in seawater off the Fukushima Prefecture: results from the SOSO 5 Rivers cruise (October 2014)