Biogeosciences, 10, 6215–6223, 2013 Open Access www.biogeosciences.net/10/6215/2013/ doi:10.5194/bg-10-6215-2013 Biogeosciences © Author(s) 2013. CC Attribution 3.0 License. Export of 134 Cs and 137 Cs in the Fukushima river systems at heavy rains by Typhoon Roke in September 2011 S. Nagao1, M. Kanamori2, S. Ochiai1, S. Tomihara3, K. Fukushi4, and M. Yamamoto1 1Low Level Radioactivity Laboratory, Institute of Nature and Environmental Technology, Kanazawa University, Nomi, Ishikawa 923-1224, Japan 2Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan 3Aquamarine Fukushima, Obama, Iwaki, Fukushima 971-8101, Japan 4Division of Earth Dynamics, Institute of Nature and Environmental Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan Correspondence to: S. Nagao ([email protected]) Received: 31 December 2012 – Published in Biogeosciences Discuss.: 15 February 2013 Revised: 19 July 2013 – Accepted: 27 July 2013 – Published: 2 October 2013 Abstract. At stations on the Natsui River and the Same River gen explosions (Japanese Government, 2011; Chino et al., in Fukushima Prefecture, Japan, effects of a heavy rain event 2011). Surface deposition of 134Cs and 137Cs shows consid- on radiocesium export were studied after Typhoon Roke dur- erable external radioactivity in a zone extending northwest ing 21–22 September 2011, six months after the Fukushima from the NPP, about 20 km wide and 50–70 km long inside Dai-ichi Nuclear Power Plant accident. Radioactivity of the 80 km zone of the NPP (MEXT, 2011; Yoshida and Taka- 134Cs and 137Cs in river waters was 0.009–0.098 Bq L−1 in hashi, 2012). Moderate radioactivity (100–600 kBq m2) was normal flow conditions during July–September 2011, but it also detected in the Naka-dori region. The deposition pattern increased to 0.85 Bq L−1 in high flow conditions because of is explained by emission rates of 134Cs and 137Cs coupled heavy rains occurring with the typhoon. The particulate frac- with wind direction and precipitation (Morino et al., 2011). tions of 134Cs and 137Cs were 21–56 % of total radiocesium A major part of radiocesium deposited on the ground sur- in the normal flow condition, but were close to 100 % after face is present at the surface to 5 cm depth (MEXT, 2012a; the typhoon. These results indicate that the pulse input of Koarashi et al., 2012). Chemical extraction of 134Cs and radiocesium associated with suspended particles from land 137Cs from selected soil samples has revealed that both to coastal ocean occurred because of the heavy rain event. radionuclides in the soil are only slightly water-soluble. Export flux of 134Cs and 137Cs attributable to the heavy rain Even the fraction extracted with 1 M ammonium acetate was accounts for 30–50 % of the annual radiocesium flux from in- only approximately 10 % (Matsunaga et al., 2013). How- land to coastal ocean region in 2011. Results show that rain ever, 134Cs and 137Cs have been transported from contam- events are one factor contributing to the transport and disper- inated watersheds to rivers in Fukushima Prefecture since sion of radiocesium in river watersheds and coastal marine the Fukushima Dai-ichi NPP accident (MEXT, 2012a; Sak- environments. aguchi et al., 2012). Similar outcomes were observed for the Pripyat River and Dnieper River in Ukraine after the Chernobyl accident in 1986 (IAEA, 2006a). The migration of 137Cs has decreased markedly over time in river wa- 1 Introduction ters from Ukraine (UHMI, 2004; IAEA, 2006b) and Finland (Saxén and Ilus, 2001). The radioactivity of 137Cs shows A nuclear accident at the Fukushima Dai-ichi Nuclear Power little change from upstream to downstream of the exclu- Plant (NPP) occurred after the 2011 Tohoku¯ earthquake and sion zone in the Pripyat River of the Chernobyl area (IAEA, tsunami. About 15 PBq of both 134Cs and 137Cs was released 2006a). An increase in radioactivity of 134Cs and 137Cs in from the NPP as a result of venting operations and hydro- Published by Copernicus Publications on behalf of the European Geosciences Union. 6216 S. Nagao et al.: Export of 134 Cs and 137 Cs river waters was also found in the Chernobyl area during a spring flood event (IAEA, 2006a, b) and in northwestern Italy from a delayed release in summer during ice and snow melt- ing in mountain areas (Spezzano et al., 1994). The transport of materials generally depends on watershed conditions such as vegetation, slope, soil types, and spring snow-melting. Fukushima Daiichi NPP ◆ Clarifying the migration behavior of radiocesium and its con- trolling factors is important for future prediction of its disper- sion in Fukushima Prefecture, Japan. To elucidate the short-term to long-term impacts of the Fukushima Dai-ichi NPP accident on riverine and coastal marine ecosystems, the Japanese Government has been mon- itoring the radioactivity of 134Cs and 137Cs in river systems in Fukushima Prefecture (MEXT, 2012b). Japanese rivers have short lengths, high riverbed slopes, and high river regime co- efficients (ratio of maximum / minimum discharge; Suetsugu, 2005). Annual mean precipitation is generally high (e.g., Natsui River ▽ 1718 mm during 1971–2000) because of tsuyu (the rainy sea- son in Japan), typhoons, and snow-melting events in spring (MLTI, 2012). Matsunaga et al. (1991) reported that the ra- 137 dioactivity of Cs derived from fallout increased in the ○ ▽ Kuji River during high flow conditions caused by rain events. Nagano et al. (2003) pointed out that variations of suspended Same River form and dissolved form concentrations of elements in the Kuji River waters occurred as a function of the water dis- charge rates. It is important that we evaluate rain event effects Fig. 1. Natsui River and the Same River sampling locations. Closed on radiocesium export from land to ocean in the Fukushima circles represent sampling stations. Open circles show a station for area. monitoring precipitation at Onahama. Open inverted triangles rep- This study was conducted to investigate the transport of resent water level observatory sites. 134Cs and 137Cs in river systems in Fukushima Prefecture after rain events. This report describes monitoring results of radioactivity of 134Cs and 137Cs in river waters at two (<100 kBq m2: MEXT, 2011). The Natsui River watershed rivers after a heavy rain event, a typhoon, in September 2011. area is 749 km2. That of the Same River is 600 km2. The Field experiments were conducted at the Natsui River and the Natsui River length is 67 km. That of the Same River is Same River in the southern part of Fukushima Prefecture, 58 km. The annual mean water discharge data in 2011 were Japan. We examined the transport behavior of radiocesium 17.6 m3 s−1 for the Natsui River and 21.4 m3 s−1 for the and estimated its export flux from inland to coastal areas. Same River (Fukushima Prefectural Government, 2012). The water discharge data are presented in Fig. 2. River water samples (10–20 L) were collected at normal flow conditions 2 Materials and methods on 12 and 27 July, 13 September, 24 November, 6 Decem- ber, and 22 September in high-flow conditions after Typhoon Typhoon Roke (T1115) struck Japan on 21 September and Roke in 2011. Sampling was conducted at the Iwaki-bashi subsequently weakened to an extra-strong tropical cyclone bridge over the Natsui River and the Eguri-Ohashi bridge on 22 September 2011 (JMA, 2011). The typhoon precipi- over the Same River. tated more than 400 mm of rain daily in parts of eastern and In river waters after the heavy rain event with the typhoon, western Japan (JMA, 2011). Fukushima Prefecture recorded particles were separated using centrifugation and filtration rainfall of 100–200 mm during 15–22 September. The daily with No. 5A (approx. pore size of 7 µm) filters and 0.45 µm rainfall on 21 September was 137 mm because of the impact pore size membrane filters. In this study, suspended solids us- of Typhoon Roke at Onahama of Iwaki city, located in the ing centrifugation are designated as “fraction 1” (FR1). The southern coastal region of Fukushima and in a watershed of suspended solids on the filters are designated as “fraction 2” the Natsui River. This value is about one-tenth of the annual (FR2) for those filtered with No. 5A filters and “fraction 3” mean rainfall (1409 mm for 1981–2010; JMA, 2012). (FR3) for those collected with 0.45 µm filters. The sampling locations are presented in Fig. 1. This The radioactivity of 134Cs and 137Cs in the filtered study investigated Natsui River and Same River, each flow- river waters was measured as dissolved forms of radio- ing to the Pacific coast through less-contaminated areas cesium with gamma-ray spectrometry using ammonium Biogeosciences, 10, 6215–6223, 2013 www.biogeosciences.net/10/6215/2013/ S. Nagao et al.: Export of 134 Cs and 137 Cs 6217 10 1 Cs radioactivity (Bq/l) 0.1 137 Total 0.01 Fig. 2. Water discharge of the Natsui River and the Same River in J A S O N D 2011. Data were referred from the Fukushima Prefectural Govern- Date (month) ment. Fig. 3. Total radioactivity of 137Cs in water samples from the Natsui River (◦) and the Same River (•) during July–December 2011. molybdophosphate (AMP)/Cs compound method (Tanaka et al., 2006). The 134Cs and 137Cs were measured using gamma-ray spectrometry with a low background Ge detector 3 Results and discussion at the Low Level Radioactivity Laboratory and the Ogoya 3.1 Radioactivity of 134 Cs and 137 Cs Underground Laboratory of Kanazawa University for 1–3 days (Hamajima and Komura, 2010). The gamma lines were Total radioactivity measurements of 134Cs and 137Cs in used for the activity calculation at 605 keV and 795 keV for the river waters are shown in Table 1 and Fig.