Mineralogy and Geochemistry of Rural Road Dust and Nearby Mine Tailings: A Case of Ignored Pollution Hazard from an Abandoned Mining Site in Semi-arid Zone

Abandoned mine tailings are considered as one of the main sources of potentially toxic elements. Because of the lack of supervision, particularly from small-scale or artisanal mining, abandoned tailings have become part of the natural landscape, especially in rural areas from developing countries. Abandoned mine tailings represent a latent problem in terms of the possible affectations to human health and the environment. An example of this is the small-sized (similar to 200 x similar to 300 m) abandoned mine tailings located similar to 500 m south of San Felipe de Jesus town, Sonora, in northwestern Mexico. The mineralogy determined in mine tailings samples consists of divalent hydrated metal sulfates (rozenite, starkeyite, kieserite, szomolnokite and epsomite), trivalent hydrated metal sulfates (coquimbite) and divalent-trivalent hydrated sulfates (copiapite), which are highly soluble efflorescent minerals associated with acid mine drainage. Rozenite was detected in road dust samples, evidencing that dust is dispersed and transported from abandoned residues. In order to assess the possible impact of the tailings (un-oxidized, oxidized, efflorescent minerals), concentrations of potentially toxic elements (total and soluble fractions) in samples from mine tailings, unpaved road soils and road dust from San Felipe de Jesus were determined. Average concentrations (ppm) of potential toxic elements in mine tailings samples ranged from 16,756-1306 (As), 665-98 (Cd), 5691-338 (Cu), 14,162-832 (Pb), 492-82 (Sb), 176,219-8285 (Zn). Enrichment factors determined in mine tailings, agricultural soils and road dust exhibit similar patterns, differing only in level of enrichment, which also confirms the dispersion of potentially toxic elements toward surroundings. Contamination Index (CI) and Hazard Average Quotient (HAQ) were calculated in mine tailings to assess potential contamination associated with potentially toxic elements dispersed by aeolian and/or hydric processes, respectively. The CI values suggest that mine tailing materials have a high potential for polluting soils and sediments. Semi-arid conditions of the region favor the suspension and transport of contaminants, potentially affecting surrounding agricultural fields and population. The HAQ values from efflorescence minerals and mine tailings indicate that potential of toxicity is very high, and might affect the quality of water (groundwater and surficial) in the region. CI and HAQ can provide a good estimation of pollution hazards associated with the abandoned mine tailings in the San Felipe de Jesus area.