Leaching potential of heavy metals and metalloids in mining-impacted soils of central Mexico: Accelerated column tests and geochemical modeling

Abstract

The Guanajuato mining district has been hosting mining operations for over 500 years. It was one of the most important Ag-Au producers in Mexico. Due to the naturally high concentration of heavy metals and metalloids (HM&Ms) and different mineral recovery methods employed over time, the region shows high levels of HM&Ms in stream sediments and soil, representing a potential environmental risk. This study aims to assess the HM&Ms concentration in soils and investigate the mobility and leaching potential of these in soils affected by mining activities in the Guanajuato River basin (GRB), using accelerated leaching column tests and geochemical modeling. The research employed X-ray diffraction and inductively coupled plasma optical emission spectroscopy for soil characterization and assessed the geochemical interactions using PHREEQC modeling. Soil samples exhibited Cu, Ni, Pb, and Zn levels exceeding the global soil averages, with contamination indices indicating moderate to significant contamination. Accelerated column leaching tests showed that soils in GRB have the potential to release Zn > Cu > Pb > Ni in low concentrations (<1 mg/L) due to the stability of mineral phases and alkaline pH. However, soil leachates can precipitate clays and oxyhydroxides, which, at alkaline conditions, enhance their retention capacity and control HM&Ms leachability. These findings highlight the importance of soil mineralogy, pH, time, and environmental interactions in the mobility, transport, and fate of HM&Ms within the basin. This understanding can inform strategies to reduce contaminant migration, mitigate environmental impacts, and guide practical remediation efforts in the basin.