ИСТИНА

Although previous authors attempted to elucidate 137Cs washoff processes in different river catchments during high-flow events, investigations on large scale river catchments are still rare. This chapter presents observations of riverine 137Cs dynamics during two high-flow events in 2018 on the Abukuma river. River waters were collected at Kuroiwa site, locating at midstream of Abukuma river, during two high-flow events on 5th to 8th July 2018 (JUL18) and 30th Sep to 3rd Oct 2018 (OCT 18). River water samples of 20-40 L in JUL18 and OCT18 were collected for 5 times and 7 times, respectively. The samples water was subjected to measurements of 137Cs activity concentration in suspended sediment (Bq kg-1) and dissolved 137Cs activity concentration (mBq L-1). Mean 137Cs activity concentration in suspended sediment during JUL18 and OCT18 were 1570 and 1200 Bq kg-1 , respectively. Mean dissolved 137Cs activity concentrations during JUL18 and OCT18 were 5.5 and 2.2 mBq L-1 , respectively. Total 137Cs wash-off during JUL18 and OCT18 were estimated as 8.6 × 109 and 5.5 × 1010 Bq, respectively. Dissolved 137Cs activity concentrations decreased with time during both of JUL18 and OCT18 and the temporal trends were similar to those of concentrations of major cations (Na+, K+, Ca2+, Mg2+). Maximum 137Cs activity concentration in suspended sediment was observed in peak discharge phases during both of JUL18 and OCT18. Total carbon contents, specific surface area and speciation of suspended sediments appeared not to explain variations in 137Cs activity concentration in suspended sediments during the events. In future, comparative studies in various catchments should improve our understanding of riverine 137Cs dynamics and predictability