Spatial distribution and migration of cadmium in contaminated soils associated with a geochemical anomaly: a case study in southwestern China

The karst terrain of Guizhou in southwestern China is ecologically fragile, but has undergone severe heavy metal contamination. To assess such contamination, the spatial distribution of cadmium (Cd) within soils was studied in a lead (Pb)-zinc (Zn) smelting area, coal mining area, Pb-Zn mining area, sewage irrigation area, and an uncontaminated area. Cd concentrations were highest in topsoil, with the highest value of 23.36 mg/kg in the Pb-Zn mining area and lowest value of 0.46 mg/kg in the uncontaminated area. Cd content decreased from 0 to 0.8 m depth, then sharply increased, reflecting Cd precipitation within the contaminated soil profiles. Migration of Cd within the soil was affected by organic content in the Pb-Zn smelting area (R² = 0.99**), coal mining area (r = 0.72*), and Pb-Zn mining area (r = 0.73*). In contrast, Cd accumulated within a clay horizon in the uncontaminated area, where the correlation between Cd and specific surface area was 0.78**; Cd concentrations reached 2.11 mg/kg within this horizon. Reducible, oxidizable, and acid-exchangeable fractions accounted for 60-80% of total Cd in soils having pH values of 5.05-6.86. This indicates that Cd could easily transfer from soil to food or water, leading to human health and environmental risks.