One of the most prevalent anthropogenic sources of metal contaminants in soils is mining activity. Mining has been historically important for Mexico's economic growth and development; however, an undesirable outcome of mining is the generation of large quantities of solid waste material which can result in contamination of our natural environment. The potential for continuous contamination of soils and water bodies in Mexico concerns researchers working on environmental problems. One of the main concerns is that the quantity of polluted soils resulting from mining activity is unknown, but acknowledged to be significant. There are estimates of 10,000 to 50,000 abandoned or inactive mine sites and, prior to December 2004, there were 21,705 current mining claims. Additional concerns are the lack of ordinances dictating safe metal and metalloid contamination levels, and the lack of federal programs aimed at the restoration of metal polluted sites.

Speciation Pb and Zn in soil samples around a tailing dam (TD) were studied using single and sequential extraction procedures as well as columns of both soil samples and tailings. Single extractions were intended to recover metals occupying soluble (deionized water pH 5.5), available (0.1 M NaNO₃), and exchangeable (1 M Mg(NO₃)₂) positions, and sequential extractions were intended to recover metals bonded to organic matter (OM). The soil samples we studied have been exposed to tailings, transported by wind and/or rain from a TD in central Mexico, for at least 30 years.  As a consequence, soils had highly variable total concentrations of Pb (110-1600 mg kg^-1) and Zn (280-2900 mg kg^-1). Yet, < 1% of the total metal concentration was recovered by extraction with deionized water at pH 5.5 (soluble metal) or by extraction with 0.1 M NaNO₃ (available metal); in both cases, more Zn was recovered as compared to Pb. Low concentrations of soluble and available Zn and Pb seem to correspond to low occupancy of cation exchangeable positions (13.2 ± 17.2 % Pb and 4.7 ± 5.13 % Zn). Zinc (34.1%) and Pb (42.5%) complexation with organic matter (estimated by 0.1 M Na₄P₂O₇.10H₂O pH 10 and H₂O₂ extractions) appears to play the most important role in the attenuation of Pb and Zn leaching from the tailings. According to current Mexican rules, and with the procedures used in this study, only a few samples qualify as hazardous to the environment (Figure 1); however, Mexican regulations are less strict than in other countries.

Our work contributes to the generation of data for regular geochemical environmental evaluations.  This type of data is needed for the building of a census of mining sites for future restoration programs and for the establishment and/or reevaluation of ordinances dictating safe metal contamination levels of soils in Mexico.