Export of ¹³⁴ Cs and ¹³⁷ Cs in the Fukushima river systems at heavy rains by Typhoon Roke in September 2011

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 ¹³⁴Cs and ¹³⁷Cs 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 ¹³⁴Cs and ¹³⁷Cs 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 ¹³⁴Cs and ¹³⁷Cs 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.