Heavy metal and arsenic dispersion in a copper-skarn mining district in a Mexican semi-arid environment: sources, pathways and fate

The Cu–Au rich ores of the Concepción del Oro mining district, located on the semi-arid Mexican Altiplano, have been mined for over 400 years. The residues of these activities were piled on the banks of the main stream that drains the area. The tailings piles are neither treated nor protected, so the waste material has been dispersed over the surroundings by seasonally occurring heavy rains and winds. A small town with 5,000 inhabitants has grown up around the mining operations. Mineralogical observations indicate that sulphates are the first products of alteration of the primary sulphides and that Fe hydroxides are the stable minerals that are formed after longer periods of time. At the present time, an area of about 40 km² along both sides of the stream has being severely contaminated with Cu–As–Zn-rich tailings, indicating that the seasonal heavy rains are indeed the main dispersion factor. Contaminated soils reach Cu-concentrations of up to 100 times, and As-concentrations of up to 20 times the local natural values. Factor analysis and geostatistical methods helped to identify pollutant origins and sources. Extraction tests on polluted agricultural soils using acetic acid mobilized 5–10 % of the total As content, 0.5–2.75 and 1–4.5 % of Cu and Zn, respectively, indicating the potential of the biological activity of the soil to enhance the mobility of the elements mentioned, and, in this way, to get into the food chain and/or groundwater. Stabilization measures of the tailings heaps must be urgently undertaken in order to minimize the risk for the inhabitants of the region and to stop the pollution of a broader area.