Solid phases controlling the mobility of potentially toxic elements and the generation of acid drainage in abandoned mine gold wastes from San Antonio–El Triunfo mining district, Baja California Sur, México

In many countries of Latin America (Mexico included), As and other toxic elements in water and soils may have a natural geogenic origin; however, the anthropogenic activities (specially in mining areas) trigger the metal(loid)s mobilization (mainly by acid mine drainage) aggravating the environmental pollution problem. In San Antonio–El Triunfo mining district (SA-ET MD), Baja California Sur State, Mexico, 200 years of intermittent mining activities have left around 800,000 tons of mine waste scattered across 400 km² of the SA-ET area. Some environmental studies in the area assume mining wastes as the main source of contamination of groundwater (not considering the geothermal source). Moreover, recent studies have shown that the pollution in the region has occurred as consequence of seasonal erosion caused by winds and irregular episodes of heavy rainfall. Nevertheless, the actual potential to release As and other potentially toxic elements as well as the hazardousness derived from this mine waste and the geochemical-mineralogical mechanism that influence such mobilization has not been assessed yet. Through the detailed physicochemical and mineralogical characterization (X-ray diffraction and scanning electron microscopy), this novel study shows that jarosite and arsenolite are the key mineral phases controlling the potential release and arsenic mobilization and therefore the hazardousness of the mining wastes. What is more, from pH and acid base accounting results, it has been found that 35 % of the analyzed samples maintain basic to near-neutral conditions due to the neutralizing potential of aluminosilicates (feldspar, muscovite, etc.), while in the remaining 65 % of analyzed samples, the mineral composition is not enough to neutralize the generated acidity; hence, those samples present the potential of generating acid mine drainage, and consequently, they are considered hazardous.