Effects of lithology and acid mine drainage on Cd concentration and isotope distribution in a large riverine system, Guangxi Province, South China

Abstract

Cadmium is highly toxic to plants, animals, and humans. When assessing its risk, it is critical to distinguish anthropogenic activities from natural weathering processes. Specific Cd isotopes have been used to track the source and fate of Cd in the environment. However, Cd isotopic signatures in soils and sediments and its fractionation in large river systems, especially in high-population areas and within complex Cd source regions, remain poorly understood. Using chemical and isotope analyses, this study identifies Cd sources in the Xijiang riverine system, China, as being the result of natural weathering and acid mine drainage. Cadmium isotopic fractionation between stream sediment and topsoil was uniform, with Δ114/110Cdstream sediment–topsoil values averaging 0.50‰ ± 0.04‰ through natural weathering processes and fractionation being controlled mainly by the Cd isotopic composition of acid mine drainage. During the natural weathering of carbonate sediments, Ca2+ and HCO3− in soils tend to drain into the Xun River with leachate, resulting in a weakly alkaline water column with increasing Cd contents in stream sediments. In mining regions, H+, Cd2+, Zn2+, and other metal ions are prone to release from sphalerite into solution under supergene conditions, ultimately resulting in high Cd contents in sediments of the Meng River. This study highlights the use of Δ114/110Cdstream sediment–topsoil values as a tool for distinguishing the effects of anthropogenic activities from those of natural weathering processes in river systems at risk of Cd pollution.