Elevated cadmium in alluvial agricultural soils in the world's largest continuous karst areas, southwestern China: concentration, speciation, origin and risk assessment

Cadmium (Cd) pollution in soils is a global environmental concern, particularly in karst soil environments where high Cd concentrations are frequently observed. However, the origin, speciation, and associated risks of Cd in alluvial agricultural soils within the world's largest continuous karst terrain remain poorly understood. To address this knowledge gap, 123 uniformly distributed alluvial agricultural soils and 319 carbonate rocks were collected in the continuous karst region southwestern China. The concentration and speciation of Cd were determined using the four-step sequential extraction BCR method. The lead (Pb) isotope tracing method was employed to identify potential Cd sources. The results indicate that the mean and median Cd concentrations are 1464 ng/g and 837 ng/g, respectively. Both values significantly exceed the background level for Chinese agricultural soils and Cd concentrations reported globally in other typical karst areas. The mean Cd concentration in the local carbonate rocks is 231 ng/g, which is approximately 6.6 times higher than the global average. Meanwhile, the median Cd concentration in alluvial agricultural soils is 8.72 times higher than that in the corresponding carbonate rocks, indicating substantial Cd enrichment during carbonate rock weathering. Therefore, the hyper-accumulation of Cd is influenced by both pedogenic processes and the high Cd content in parent rocks. Given that the study area is located within the world's largest low-temperature mineralisation zone, the elevated Cd levels in the soils may be associated with to contamination from local mining and smelting activities. Lead isotope analysis further confirmed that both anthropogenic activities and natural weathering of Cd-rich parent rocks contributed to the elevated Cd contents. BCR analysis showed that Cd in alluvial karst soils is predominantly present in the residual phase (59.5 %), followed by the reducible (19.3 %), acid-soluble (17.2 %), and oxidizable (4 %) phases. Notably, the proportion of the residual phase decreases as the total soil Cd concentration increases, while the acid-soluble and reducible fractions increase. This trend indicates that Cd tends to accumulate in more bioavailable forms in areas affected by industrial or mining activities. Elevated Cd levels represent a high potential health risk for children, with ingestion as the primary exposure pathway. This study highlights the underestimated ecological and health risks of Cd in alluvial agricultural soils within karst regions. • Cd background is as high as 837 ng/g in alluvial agricultural karst soils, southwestern China. • Elevated Cd background is attributed to the weathering of Cd-enriched carbonate rocks. • Acid-soluble Cd increases with increasing of total Cd content. • Potential ecological and health risks of Cd in alluvial soils should not be ignored.