Mechanism of cadmium (Cd) enrichment in the soil of karst areas with high geochemical background in Southwest China

Karst areas are formed from the dissolution of carbonate rocks and are present worldwide. The soil found in such areas is enriched in heavy metals, such as cadmium (Cd), lead (Pb) and Arsenic (As), and has geological high-background characteristics. However, this enrichment is anomalous depending on the type of bedrock, and the mechanism of enrichment has not yet been comprehensively elucidated. To explore the mechanisms of Cd enrichment in these soils, we investigated the distribution patterns and migration characteristics of Cd during the weathering and pedogenesis of carbonate rocks, and the causes of Cd enrichment and associated ecological risks were determined. The conclusions were as follows: (1) In the process of natural evolution, the boundary between the dissolution of carbonate rocks and the weathering of insoluble substances is unclear, resulting in the overlap of and interaction between the processes of dissolution-accumulation-weathering and pedogenesis. (2) In addition to the enrichment effects caused by the dissolution of carbonate rocks and the inheritance, the readsorption of insoluble substances and the retention of secondary carrier minerals are the main driving forces of Cd enrichment. (3) During the late stage of weathering of insoluble materials, soil acidification significantly promotes the Cd activation process. When the proportion of active Cd significantly increases, the ecological risk potential increases, especially in geological high-background areas rich in Cd.