Heavy metal levels, health risks, and potential sources in karst soils of southwestern China.

Distinguishing natural geochemical variability from anthropogenic influences is important for interpreting heavy metals in karst soils. This study evaluated As, Cr, Cu, Ni, Pb, and Zn in surface soils of Longli County, Guizhou Province, using the Nemerow pollution index, Monte Carlo-based health risk assessment, and Positive Matrix Factorization (PMF). Mean concentrations were 17.2, 103, 19.9, 23.9, 23.0, and 82.7 mg/kg, respectively. Five metals had mean concentrations below the corresponding Guizhou provincial background values, while Cr was only slightly higher than its background value (95.9 mg/kg) and did not differ significantly from it (p = 0.155). All mean concentrations were below the applicable agricultural soil risk-screening values. The Nemerow index was 1.03, formally corresponding to light contamination, although this classification was primarily driven by the marginal Cr/background ratio. Non-carcinogenic risks remained below the screening benchmark, with children showing greater vulnerability. Under the conservative speciation assumptions used in the assessment, the calculated lifetime carcinogenic risk was higher for adults; however, the Cr- and As-related components may be overestimated because total Cr and total As concentrations were evaluated using cancer toxicity factors specific to Cr(VI) and inorganic As, respectively. PMF suggested contributions from geogenic-anthropogenic processes and potential anthropogenic influences associated with agricultural, traffic, and historical mining or mineral-processing activities. Overall, the soils showed generally low heavy metal contamination, while localized variability and uncertainties in source attribution and carcinogenic-risk estimates support targeted monitoring and further species-specific assessment rather than broad remediation.