Elevated thallium enrichment, source, and health risks in soils surrounding a lead–zinc smelter, Southwest China

• Thallium levels in and around Asia's largest lead-zinc smelter were reported for the first time, with the highest levels in the area of capacity restoration areas. • Heavy metals showed strong spatial heterogeneity. • Pb and Cd are the key ecological risk factors. • Mining activities dominated ecological risk sources. While soil pollutants such as As, Pb, Zn, and Cd are well studied, knowledge of the distribution, source identification, and risks of highly toxic thallium (Tl) remains insufficient. Analysis of 601 soil samples across 175 sites at a Pb–Zn smelter in southwestern China revealed an average Tl concentration of 3.91 mg/kg (range: 0.31-71.60 mg/kg). The highest Tl content (71.6 mg/kg) occurred at 0-0.5 metres depth in capacity restoration areas, exceeding crustal abundance and Chinese soil background values by 159.11-fold and 122.60-fold, respectively. The capacity restoration areas experienced peak contamination, followed by Metallurgical Plant II. Principal component analysis indicated that mining activities (contribution rate: 38.95 %), atmospheric deposition, transport (contribution rate: 30.59 %), and rainfall (contribution rate: 25.93 %) constitute the primary sources of pollution. The pollution indicators and risk assessments revealed Pb, Cd, and Zn as the main cumulative contaminants, with higher risk indicators than the other heavy metals (HMs). Probabilistic health risk assessments indicate that carcinogenic and noncarcinogenic risks primarily arise from exposure pathways, including oral ingestion, dermal contact, and inhalation. The average HIs for adults and children were 9.35E+06 and 4.98, respectively. The average TCR was 9.78E-04 for adults and 3.26E-03 for children. Unacceptable noncarcinogenic and carcinogenic risks from HMs were detected for both children and adults, with Pb and Cd posing greater risks than Zn and Tl. This study offers critical support for controlling soil pollution, especially Tl, in Pb–Zn smelting areas.