Using Natural Radionuclides to Trace Sources of Suspended Particles in the Lower Reaches of the Yellow River

Natural radionuclides are powerful tools for understanding the sources and fate of suspended particulate matter (SPM). Particulate matter with different particle sizes behaves differently with respect to adsorption and desorption. We analyzed the activities and distribution characteristics of multiple natural radionuclides (²³⁸U, ²²⁶Ra, ⁴⁰K, ²²⁸Ra, ⁷Be and ²¹⁰Pb_ex) on size-fractionated SPM at the Lijin Hydrographic Station (Huanghe or Yellow River) every month over a one-year period. Results showed that medium silt (16 – 32 µm) was the main component. As expected, the activity of each radionuclide decreased with an increase of particle size. We examined the sources of SPM with different particle sizes using activity ratios of ²²⁶Ra/²³⁸U, ²²⁸Ra/²²⁶Ra, ⁴⁰K/²³⁸U and ⁷Be/²¹⁰Pb_ex, and concluded that SPM with different particle sizes originated from different sources. Our results indicate that fine SPM (< 32 µm) was mainly from the erosion of soil along the lower reaches of the Yellow River, while coarse SPM (> 32 µm) was mainly derived from resuspension of riverbed sediment. During high runoff periods, the concentration of SPM increased significantly, and the proportion of fine particles originating upstream increased. Naturally occurring radioactive isotopes, especially on size-fractionated particles, are therefore seen as useful tracers to understand the sources and behaviors of riverine particles transported from land to sea.