Suspended sediment from different geological sources in a watershed determined by natural radionuclides

The geological setting is essential for occurrence of slope failure and landslide so that the geology may control the suspended sediment yield with different magnitude. In the Saru River watershed of central Hokkaido, northern Japan, the typhoon Etau in August 2003 brought heavy rainfall, causing the slope failure and landslide across the areas of various geologies and the highest sediment yield since 1960s. Prolonged sediment runoff has caused the serious problems in association with turbid water, sedimentation in the reservoir, and their impacts on fishery and ecology downstream. To clarify the suspended sediment sources within the Nukabira River watershed, a tributary of Saru River, hydrological monitoring of discharge and turbidity and fingerprinting technique using natural radionuclide were conducted during a heavy rainfall event in August 2010. GIS analysis for slope failure and landslide areas was also conducted to investigate the distribution of potential suspended sediment sources. The activity of radionuclides, including U-series, Th-series, cesium-137 and potassium-40 were determined by gamma ray spectrometry. Statistical analysis showed the best composite fingerprints of Pb-212, Ac-228 and K-40 to classify the suspended sediment sources into six geological units, which were sedimentary rock, plutonic rock, metamorphic rock and three types of cretaceous accretionary complex consist of sedimentary rock, basalt block and volcanic rock. The contribution of source group to suspended sediment was calculated according to the assumption that the Mahalanobis distance in tracer properties between sources and suspended sediment can represent relative contribution of the source. During the rainfall event on August 11, 2011, dominant source of suspended sediment was found to be the areas consist of metamorphic rock (31%), sedimentary rock (30%) and accretionary sedimentary rock (24%). GIS analysis showed the spatial distribution of slope failure and landslide within the watershed and the high density in accretionary basalt block and metamorphic rock in slope failure and in metamorphic rock in landslide. The sediment yield of source group, calculated from the amount of suspended sediment and contribution of source group, was found to be the highest in metamorphic rock (4.7x10³ t/km²), in consistence with the dense distribution of slope failure and landslide. Active landslide can provide the significant amount of sediment from hillslope to stream channel, resulting in the source of fine sediment. In the area of accretionary basalt, there was still inconsistency between the sediment yield and the density of slope failure and landslide. The low suspended sediment yield may be attributable to the lack of fine sediment on the bare slope along the stream channel and riverbed, due to the large particle size and hardness of sediment. These results indicate that the natural radionuclides can be available to elucidate the suspended sediment sources and sediment yield of various geological units within a watershed.