Early-warning ecological risk assessments of multi-element pollution in the surface soil of karst basins in southwest China based on the delayed geochemical hazard model

Soil potentially toxic element (PTE) pollution, especially in karst regions, poses significant ecological risks due to the unique geological features and environmental conditions. This study focuses on the delayed geochemical hazard (DGH) model to assess the progressive risks of cadmium (Cd) and lead (Pb) contamination in the surface soils of karst regions in southwestern China. The study found that Pb and Cd pollution in karst areas presents ecological risks, with the region's high porosity and alkaline soils facilitating the transformation of pollutants from stable to mobile forms. The analysis revealed that Pb underwent multidirectional transformations, with 28.3% of the soil showing a transition from carbonate-bound Pb (Pb_C) to exchangeable Pb (Pb_E). However, Cd primarily transformed into carbonate-bound (Cd_C) and oxide-bound (Cd_O) forms, with 3.77% of samples exhibiting the highest outbreak probability for the pathway from sum Cd (Cd_E+C+F+O+R) to Cd_C+O. In Pb-Cd co-contamination, Cd predominates, altering risk pathways by triggering transformations at iron-manganese oxide and organic matter binding sites, thereby increasing pollutant mobility. The most prominent combined risk pathway in Pb-Cd contamination was the shift from M_E+O to M_C (outbreak probability: 5.66%), indicating enhanced mobility of both PTEs. New pathways also emerged, such as the transition from M_E+C+F+O to M_E+C, suggesting that Pb and Cd interactions accelerate the risk evolution, favoring highly mobile forms. These findings highlight the importance of not only considering total PTE concentrations but also their speciation and potential transformations in risk assessments. The DGH model effectively predicts evolutionary risks in co-contaminated karst areas, providing insights for early warning of multi-element pollution, particularly in vulnerable karst landscapes.