Soil weathering-water environment-ecological risks in Hanjiang River Basin, China

Abstract

The objective of this study is to assess water environment and ecological risks from heavy metals of soils in Hanjiang River Basin, China. We carried out investigations on the changing patterns of spatiotemporal distribution of soil chemical elements, and further identified the changing behaviors, geochemical process and driving forces of heavy metals in soil by integrated analysis of natural environmental factors and human activities. In order to achieve these goals, we used field investigation, laboratory testing and GIS spatial simulation. The following findings were achieved. Firstly, general soil weathering in the Hanjiang River Basin basically evolves from Ca and Na stages to K stage. Ca and Na decreased rapidly, and silicate weathering evolved gradually. Major ions are subject to carbonate dissolution. About 2/3 of the soils in the basin are alkaline (pH > 7) which provide good conditions for mineralization of organic nitrogen. Secondly, specific soil weathering processes differ in spatial patterns and element transport, driven by integrated interior forces (e.g., mineral materials) and external forces (e.g., temperature, precipitation, wind, and gravity). Original mineral materials are changed into soils with different physical properties (soil texture, mechanical composition) and chemical components (organic matter). Upstream original mineral materials have been transported, accumulated, and deposited downstream, driven by forces of gravity, water transport, glacial effects, and wind. Thirdly, key ecological factors of plant growth change greatly. Increase of soil humus mitigates pollution hazards and enhances soil self-purification. Land use and cover is one of the key contributors to determine geochemical patterns of soil organic matter. Intensity of mineral weathering decreases with the decrease of biomass. The comprehensive pollution index and ecological risk index increased from northwest to southeast, corresponding to increased erosive rainfall and rainstorms more than human activities.