Distribution of heavy metals polluting the soil near an abandoned mine in Northwestern Mexico

Abandoned mine wastes pollute the soil in their vicinities and threaten the health of livestock and human beings. This is the situation around San Felipe de Jesús in northwestern Mexico. We surveyed 900 ha of agricultural land to assess and map the concentrations of toxic elements in the topsoil to discover where pollution is serious, what its source might be and to decide whether remediation is needed. The total concentrations of Pb, As, Zn, Cu and Mn plus Fe and Ca were analysed by X-ray fluorescence spectrometry. We found that all of the first five elements listed were concentrated near the tailings pile as a ‘hot spot’ and where the concentrations of Pb, As and Zn exceed national and international standards. Iron and Ca, in contrast, are evenly spread throughout the region. The elements Pb, Zn and Mn gradually decrease in concentration from the tailings pile toward the Sonora River, probably because they have spread in dust or by water during storms. Arsenic and Cu also decrease in concentration from the the tailings pile towards the centre of the region, but they increase again in the soil on the river flood plain, most likely as the result of spills in the catchment north of the river basin. These results will serve to assess the risks incurred in the use of the land for agriculture and to define policies for that use and possible remediation.