Magnetic properties of surface sediments from Liuxi River, southern China and their environmental significance

Purpose Magnetic minerals within fluvial sediments are affected by a variety of natural and anthropogenic factors, limiting the capability of sediment magnetism in tracing provenance, assessing pollution, and other environmental or geological fields. Methods Twenty-three samples were collected from Liuxi River, a river influenced by complex natural and human processes, in southern China to conduct the analysis of magnetism, particle size, and geochemical elements. The micromorphology of magnetic particles was directly observed by scanning electron microscope and energy dispersive spectrometer. Results The following are the factors affecting the sediment magnetic properties: (1) There is a sampling site in the upper reach with over 100 times magnetic susceptibility than other sites. This sampling site is close to the areas having documented iron ore mining and smelting activities in the past few hundred years, and the sample contains the very high concentrations of metals (e.g., Fe, Mn, Ni, Pb, Sn, V, Cr, Cu, and Bi), implying the ancient mining activity could pose environmental risks in modern sediments. (2) In a downstream direction, magnetic concentration shows a decreasing tendency and magnetic grain size becomes finer. The contributions of superparamagnetic particles and high-coercivity minerals (e.g., hematite) to the total magnetism increase, and technogenic magnetic particles appear downstream of urban areas. In general, the upstream magnetic minerals are controlled by weakly weathered coarse-grained particles produced by granite denudation, while the downstream magnetic minerals are affected by the sedimentary rocks and the human activity. (3) Magnetic minerals become coarse after the river flows over barrages in response to the hydrodynamic changes. Conclusions The investigation reveals the natural and anthropogenic influences according to the analysis of the sediments in Liuxi River, and further research related to the sediment magnetism in complex river should be conducted.