The Contamination of Water and Soil from the Dissolution of As-Bearing Mineral Waste in Matehuala, Mexico

Extremely high concentrations of arsenic (As) in groundwater have been reported in central Mexico related to leachates from metallurgical wastes from an abandoned smelter. At the site, contaminated groundwater has been extensively used for maize cultivation for a long time resulting in very high soil pollution. However, the As-containing minerals’ identity, concentration, and solubility remain unresolved. In the present work, highly contaminated soil samples from the area were studied to determine total As concentrations in shallow (0–5 cm) and deepsoils (5–30 cm) and to identify and characterize As-bearing minerals and their solubility behavior. Results showed that soil samples contained up to 4.2% As and the mineralogy consisted mainly of calcite, gypsum, and quartz. Identified arsenic minerals included pharmacosiderite, bukovskýite, scorodite, beudantite, clinoclase, sodium arsenate, adamite, arsenolite, arsenopyrite, orpiment, and a mixture of calcium arsenates (guerinite, haidingerite and pharmacolite). Additionally, As was adsorbed on ferrihydrite. Soil fractionation analysis showed that up to 74% of total As was present in the most mobile fractions, e.g., soluble, exchangeable, phosphate absorbable, and slightly reducible. Furthermore, As solubility in water accounted for up to 60 mg/L at pH 7, explaining the high pollution observed in groundwater and highlighting the risk to humans and ecosystems. According to saturation indices calculations, As may derive from the dissolution of adamite, arsenolite, pharmacolite, and haidingerite, while clinoclase and ferrihydrite may precipitate, counteracting As solubility and mobilization. The results of this study increase knowledge on the identification and solubility of As-bearing minerals in calcicxerosols and semi-arid climatic conditions where considerable contamination is observed in groundwater from As-containing waste disposal.