Groundwater pollution by arsenic and other toxic elements in an abandoned silver mine, Mexico

This study evaluated the impact of an abandoned Ag mine on the quality of surface and groundwater. The mining site of Huautla is in south Morelos State, central Mexico. Ag-Pb-Zn and Au-Cu sulfide ores were mined in the district. The ores were characterized by the presence of Ag, galena (PbS), sphalerite (ZnS), and stromeyerite (CuAgS). Ag was the metal of greater interest. Other metals included Cu, Pb, Zn, and Au. Mining activities stopped in the early 1990s when the market price of Ag decreased; the abandoned mines then were flooded by rising groundwater levels. Because of the urgent demand for water by the inhabitants in the area, this water has been used as drinking water and as waterholes for livestock. Water sampling points included abandoned mines (Am,rica, Pajaro, Santiago, Tlachichilpa, and San Francisco), dams, and dug wells. The greatest concentrations of As and other toxic chemical elements (Fe, Mn, Pb, Cd, F) were detected in groundwater samples from flooded mines. The presence of these elements was related to the rock-water interaction process. The oxidation of sulfides appears to be the cause of increased metal concentrations in groundwater samples from flooded mine. Other possible water-rock interaction processes that can control the presence of arsenic in groundwater were the adsorption of arsenic in iron oxyhydroxides, the adsorption in carbonates, and/or coprecipitation with calcite. In the case of the San Francisco and Am,rica mines, the oxidation conditions, low correlation of As with SO4 (2-) and Fe2+, and concentrations of silica indicate that the presence of As in the groundwater could be due also to competition for adsorption sites.