Lead (Pb) is a non-essential toxic heavy metal, and soil Pb pollution has been a significant global environmental concern. Elevated Pb concentrations have been reported in the world''s largest continuous karst region, located in southwestern China. However, the speciation and origin of Pb in alluvial soils within this area remain poorly understood. In this study, a total of 122 alluvial agricultural soil samples and 319 carbonate rock samples were collected to settle these problems. The total Pb concentrations and chemical speciations were determined using chemical and BCR sequential extraction analysis. Potential sources of Pb were identified through Pb isotope analysis. Results showed that Pb contents in karst alluvial soils varied from 11.1 to 1386 ppm, with an average of 54.6 ppm and a median of 29.1 ppm. Among the samples, 9.02% exceeded the risk screening threshold (70 ppm), and only two samples surpassed the risk control threshold (400 ppm). Both Enrichment Factor (EF) and Geo-accumulation Index (Igeo) assessments indicated low levels of Pb contamination. BCR analysis revealed that Pb was predominantly associated with reducible and residual fractions, accounting for 51.87 and 38.68% of the total Pb, respectively. The acid-soluble fraction was minimal (average 1.12%), suggesting a low potential ecological risk. Significant differences were observed in Pb isotopic compositions between potential natural and anthropogenic sources within the study area. Binary mixing model calculations indicated that natural sources contributed approximately 75.26% of the total Pb. Integrated analysis of BCR speciation and Pb isotopes suggested that anthropogenic Pb was primarily associated with non-residue phases. From carbonate rocks to soils, substantial losses of Ca, Mg, CO2 were observed, while Fe, Mn and Al were significantly enriched. The average Pb content increased from 2.32 ppm in carbonate rocks to 54.6 ppm in soils. These findings suggest that the significant loss of soluble elements and the fixation of Pb by Fe-Mn (hydro) oxides are the main mechanisms driving Pb enrichment in karst soils. Additionally, the extreme Pb enrichment in a few samples is likely linked to mining and smelting activities. Although Pb levels in most karst alluvial soils are generally low, the potential ecological risks should not be overlooked.
[1]Guilin Univ Technol, Sch Earth Sci, Guilin 541004, Peoples R China.
[2]Chinese Acad Geol Sci, Inst Mineral Resources, Beijing 100037, Peoples R China.
[3]Guangxi 204 Geol Team, Hezhou 542899, Peoples R China.
[4]Beijing Remediat Environm Co Ltd, Beijing 100080, Peoples R China.
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