Discharge from the river catchment area is a major transport process of earth surface materials (clastic material and organic matter) to the ocean. The natural and anthropogenic environmental changes may affect discharge of these materials to downstream and coastal areas. These processes are also related to geomorphological development and sediment disaster in river watersheds. It is therefore important to clarify the transport of earth surface materials to achieve proper management and prediction of river watersheds and coastal areas. To investigate the transport processes of earth surface materials, we focus on the atmospheric radionuclide tracers (7Be, 210Pb, and 137Cs) in the river catchment systems. These radionuclides have strong adsorbility with earth surface materials and therefore can be used as the effective tracers. Their different half-lives (210Pb: 22.3 years, 7Be: 53 days) can provide time scale of erosion and transport processes in the river watershed. We introduce two observation results in (1) the small reservoir-catchment system and (2) Kumaki River catchment system in the Noto Peninsula located in the central part of Japan.
Temporal changes in atmospheric deposition fluxes of 210Pb and 7Be and their sedimentation fluxes in the irrigation reservoir (Shin-ike Reservoir) were observed in the northern area of Noto Peninsula since June 2016. Deposition fluxes of 210Pb and 7Be from the atmosphere ranged 0.33-7.7 Bq/m2/day and 1.1-31 Bq/m2/day, respectively, and showed clear seasonal fluctuation with high deposition flux during winter and low during summer. Sedimentation flux of 7Be observed with sediment traps on the reservoir bottom was almost same level with deposition flux from the atmosphere, suggesting that the contribution of the catchment-derived 7Be is negligible. On the other hand, sedimentation flux of 210Pb was much larger than 210Pb deposition flux from the atmosphere, suggesting that 210Pb eroded from the catchment or resuspension of old 7Be-deplated sediment from the bottom was major source to reservoir sediment. These differences may be associated with the half-lives of 210Pb (22.3 years) and 7Be (53 days) and the residence time in the reservoir-catchment system.
To investigate the transport processes of suspended solids (SS) in the Kumaki River located in the central area of the Noto Peninsula, the atmospheric radionuclides (210Pbex, 7Be, and 137Cs) of SS samples were observed. The SS samples were obtained with a continuous centrifuge of the river water (100 L) collected at the three sampling points (upstream, midstream, and downstream) in Kumaki River every month since April 2016. The 7Be/210Pbex activity ratio of SS sample, which is related to the residence time of these radionuclides in the catchment area, showed clear positive correlation with the water discharge and it was high during winter season with higher precipitation in each sampling point. This result indicates that SS was transported in a shorter time scale during rainfall event and winter season. On the other hand, 7Be/210Pbex ratio exceptionally decreased during heavy rainfall events with the higher water discharge over the threshold value, implying that older stored sediment and subsurface soil were eroded and transported under these conditions. Temporal and spatial changes in 137Cs concentration and 137Cs/210Pbex ratio of SS sample showed different fluctuation in each sampling point, suggesting that changes in discharge amount and source of SS were affected by the subcatchment-scale local hydrological environments and land use conditions.