Abstract. At stations on the Natsui River and the Same River in Fukushima Prefecture, Japan, effects of a heavy rain event on radiocesium export were studied after Typhoon Roke during 21-22 September 2011, six months after the Fukushima Dai-ichi Nuclear Power Plant accident. Radioactivity of 134 Cs and 137 Cs in river waters was 0.009-0.098 Bq L −1 in normal flow conditions during July-September 2011, but it increased to 0.85 Bq L −1 in high flow conditions because of heavy rains occurring with the typhoon. The particulate fractions of 134 Cs and 137 Cs were 21-56 % of total radiocesium in the normal flow condition, but were close to 100 % after the typhoon. These results indicate that the pulse input of radiocesium associated with suspended particles from land to coastal ocean occurred because of the heavy rain event. Export flux of 134 Cs and 137 Cs attributable to the heavy rain accounts for 30-50 % of the annual radiocesium flux from inland to coastal ocean region in 2011. Results show that rain events are one factor contributing to the transport and dispersion of radiocesium in river watersheds and coastal marine environments.

Effects of a heavy rain event on radiocesium export were studied at stations on the Natsui River and the Same River in Fukushima Prefecture, Japan after Typhoon Roke during 21–22 September 2011, six months after the Fukushima Daiichi Nuclear Power Plant accident. Radioactivity of ¹³⁴Cs and ¹³⁷Cs in river waters was 0.011–0.098 Bq L⁻¹ at normal flow conditions during July–September in 2011, but it increased to 0.85 Bq L⁻¹ in high flow conditions by heavy rains occurring with the typhoon. The particulate fractions of ¹³⁴Cs and ¹³⁷Cs were 21–56% in the normal flow condition, but were close to 100% after the typhoon. These results indicate that the pulse input of radiocesium associated with suspended particles from land to coastal ocean occurred by the heavy rain event. Export flux of ¹³⁴Cs and ¹³⁷Cs by the heavy rain accounts for 30–50% of annual radiocesium flux in 2011. Results show that rain events are one factor controlling the transport and dispersion of radiocesium in river watersheds and coastal marine environments

Total radioactivity of 137 Cs in water samples from the Niida River was 0.025-4.18 Bq/L during May 2011-November 2012. Higher radioactivity was found during May-September 2011 and after a high flow condition caused by a rain event in 2012. The particulate phase of 134 Cs and 137 Cs was 47-48 % for May-September 2011, 75-93 % in normal flow conditions and 86-91 % in high flow conditions after December 2011. These results indicate that the supply of 134 Cs and 137 Cs from the watershed differs from the early stage up to September 2011 and from the last stage.

Cs from the watershed to the river waters occurred mainly as particulate forms and that their radioactivity depends on the levels of radioactivity at the watersheds of the respective river systems.

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