Transport Processes of Suspended and Riverbed Sediments Inferred from Atmospheric Radionuclides in the Kumaki River, Japan

Abstract

Sequential sampling and analyses of suspended and riverbed sediments with atmospheric radionuclides ⁷Be (T1/2=53 days), ²¹⁰Pb (T1/2=22.3 years), and ¹³⁷Cs (T1/2=30.2 years) make clear the sediment transport processes in the Kumaki River, central Japan. The ⁷Be/²¹⁰Pbex (excess ²¹⁰Pb) activity ratio of suspended and riverbed sediments revealed the following relation: atmospheric deposition (rainwater) > suspended sediment ≥ riverbed sediment. This relation implies that the riverbed sediment is older than the suspended sediment and it was formed by the accumulation of the suspended sediment during the normal flow condition and mixing with old sediments. The ¹³⁷Cs concentration in the riverbed sediment in the upstream area was clearly higher than that in the midstream area, while the concentration in the surface soil of the forest was much higher than that in the paddy field. This suggests that the riverbed sediment in the upstream area may be attributed to the forest soil and the paddy field contributes more to the midstream area, reflecting the land use difference in the Kumaki River catchment.