Analysis of cadmium sources in surface soil in typical karst regions: Anthropogenic input contribution and...

In karst regions, soil cadmium (Cd) enrichment is commonly attributed to underlying carbonate bedrock weathering. However, the quantitative contribution of anthropogenic Cd in areas remote from urban and industrial centers is poorly understood, limiting effective pollution management. This study aimed to identify and quantify the sources of Cd in a typical karst region. We investigated Cd pollution and sources in the topsoil of mountainous and farmland areas by integrating high field strength elements (HFSEs), rare earth elements (REEs), the positive matrix factorization (PMF) model, and Cd stable isotope analysis. The median Cd concentration in the studied topsoil was 7.88 mg/kg (average 8.07 mg/kg), which is 13.13 times greater than China's soil risk control standard (0.60 mg/kg, pH 6.50--7.50), with Cd isotopic compositions (δ^114/110Cd) ranging from -0.233 ‰ to 0.159 ‰. The HFSEs, REEs and PMF results indicated substantial anthropogenic influence. The Cd isotope results revealed that the Cd in mountain soils mainly originated from bedrock weathering (55.0 %), with contributions from metal smelting (19.8 %), traffic (18.8 %), and coal combustion (6.4 %). In agricultural soils, anthropogenic sources dominated (72.9 %), primarily comprising emissions from metal smelting and traffic (each 21.3 %) and coal combustion (14.0 %), followed by fertilizer application (16.3 %), with bedrock contributing 27.1 %. These findings demonstrate that atmospheric particulate transport efficiently delivers Cd to soils far from pollution centers, highlighting the critical need to include long-range atmospheric transport in the risk assessment and management of Cd pollution in karst regions. The integrated geochemical and isotope approach provides a robust, transferable framework for source apportionment and for developing effective heavy metal pollution control strategies in similar environments worldwide.