Acid mine drainage, soil pollution, and carcinogenic risk of geogenic contaminants in artisanal and small-scale gold mining areas of Geita, Tanzania.

This study examines two hypotheses: (i) Geogenic metal(loid)s in the Geita region contribute to high cancer rates. (ii) Poor waste management in artisanal and small-scale gold mining (ASGM) activities increases the mobility of carcinogenic metal(loid)s, thereby increasing health risks. Using pollution and ecological risk indices, this study evaluated the contamination of toxic metal(loid)s and assessed their associated carcinogenic risks. The findings reveal that As, Cr, and Ni pose significant threats, with Cr exceeding the unacceptable carcinogenic risk level (1 × 10-4) for both adults and children. The mean concentrations of As and Ni in both topsoil and subsoil exceed Tanzania's agricultural soil standards. Moreover, pollution indices, including the enrichment factor, geo-accumulation index, and contamination factor, confirm the high pollution levels of these metal(loid)s. The results further reveal that Cr and Ni are geogenic, originating from the weathering of mafic rocks within the Geita Greenstone Belt, while As is predominantly associated with gold ores. One of the five analyzed gold ores had a pH of 3.01 and As concentration of 99.3 mg/kg, while some tailings exhibited a pH of 9.3 and As concentration of 11.2 mg/kg. Under neutral conditions, As concentrations ranged from 0.05 mg/kg to 2.39 mg/kg, indicating a significant variation. This suggests that some gold ores in Geita are acid-forming, while certain tailings are alkaline. Consequently, the mobility of toxic metal(loid)s is enhanced by both acid and alkaline drainage. These findings support both hypotheses. Assessing acid mine drainage in ASGM sites should be prioritized globally to reduce associated health risks.