Contamination Potential of an Urban Mine Tailings Deposit in Central Mexico ?A Preliminary Estimation

Abstract

This study consisted of a geochemical analysis of the Dos Carlos tailings? deposit located in the Mining District of Pachuca-Real del Monte in the state of Hidalgo, Central Mexico. The goal of the study was to determine the potential effects of this deposit on the environment and health of the population of the metropolitan area of Pachuca. Sampling was conducted from the top to the base of two raised sections at opposite ends of the deposit, and macroscopic features (profiles A and B) of these deposits were evaluated. Subsequently, mineralogical analyses of the collected samples were performed using X-ray diffraction and physico-chemical analysis of the leachates. The results were compared with the maximum permissible limits established by different national and international standards for drinking water and hazardous waste. In addition, geochemical modeling was conducted using PHREEQC to calculate the distribution of aqueous species, ionic activities and saturation indices. Of the 35 samples of leachate analyzed for profile A, two samples had two nearly three times the national permissible maximum limits of Cr6+ in drinking water, four samples had 3 NO? levels that exceeded these limits and 19 samples had 2 NO? levels that exceeded these limits. Of the 10 samples analyzed from profile B, six samples had 3 NO? levels that were up to fourteen times higher than the limit for human consumption, and six samples had levels of 2 NO? that were up to 28 times higher than the permissible limit. The predominant minerals found in the Dos Carlos tailings? deposit were quartz, feldspar, clay from the kaolinite group, and calcite and or thopyroxene (enstatite-ferrosilite series). The samples from profile A indicated that minerals containing Agmetallic, Cd2+, Cu+, Ni2+, Pb2+ and Zn2+ were dissolved, whereas minerals containing Fetotal, Mn2+, Pb(OH)3 and Zn2+ were in a state of precipitation. In profile B, the dissolution of minerals containing Agmetallic, As5+, Cd2+, Cu+, Mn2+, Ni2+, Pb2+ and Zn2+ was observed only in a superficial sample, whereas Agmetallic, Fetotal, Mn2+, Pb3(PO4)2 and PbHPO4 were found in a precipitated state. The above-mentioned species that occurred in a state of dissolution pose an environmental threat to the groundwater in the study area; indeed, the tailings? deposit provides the conditions necessary for these minerals to leach and infiltrate into the subsoil or be transported by wind, deposited in the soil of the surrounding area and inhaled by the nearby population, potentially compromising respiratory health.