Abstract

GUTIERREZ-RUIZ, Margarita; ROMERO, Francisco M.  and  GONZALEZ-HERNANDEZ, Galia. Soils and sediments affected by the dispersion of metal sulfide from inactive tailings in the Santa Barbara mining area, Chihuahua, Mexico. Rev. mex. cienc. geol [online]. 2007, vol.24, n.2, pp.170-184. ISSN 2007-2902.

The inactive tailings of the Santa Bárbara mining unit in northern Mexico were studied in order to evaluate the environmental impact on surrounding soils and sediments. Total concentrations as well as the concentrations of water-soluble, potentially toxic elements were analyzed to assess the potential for leaching and contamination. The near-neutral pH values of the oxidized tailings throughout the impoundments suggest that the dissolution of minerals with neutralizing potential was sufficient to consume the acidity released by metal sulfide oxidation. The tailings contain high total concentrations of Zn (1,762-21,059 mgkg^-1), Pb (972-16,881 mgkg^-1), As (410-3,281 mgkg^-1) and Cu (472-2,415 mgkg^-1). The total concentrations of the other elements studied were relatively low (Ba: 62-586 mgkg^-1; Cd: 10-239 mgkg^-1; and Se: 1-10 mgkg^-1) and even not detected by the analytical techniques in the case of the Cr, Ni, Hg and Ag. However, the concentrations of water-soluble, potentially toxic elements were low (Zn < 2.0 mg l^-1, Pb < 0.07 mgl ^-1, As <0.05 mgl ^-1, Cu < 0.02 mgl ^-1, Cd < 0.2 mgl ^-1 and Fe < 0.25 mgl^-1). Considering the near-neutral pH values and the low concentrations of potentially toxic elements available for leaching, we concluded that the potential for net acid generation was very low. The total concentrations of Zn, Pb, As, Cu and Cd in samples of soils and sediments located around the inactive tailing were greater than the natural background values determined for this study (background values: 974 mgkg^-1 Zn; 947 mgkg^-1 Pb; 190 mgkg^-1 Cu; 342 mgkg^-1 As; and 28 mgkg^-1 Cd ). These results indicate that the soils and sediments have been contaminated by dispersed tailings. However, the concentrations of water-soluble, potentially toxic elements in all samples of soils and sediments were low: Cu < 0.16 mgl ^-1 and Cd < 0.005 mgl ^-1. For the case of Pb, in 39 samples of a total of 47 (included superficial and subsuperficial samples), concentrations below the detection limit (0.07 mgl ^-1) were registered, and only eight samples had water soluble Pb concentrations between 0.1 and 0.89 mgl ^-1. The concentrations of water soluble As varied between 0.0005 and 0.65 mgl ^-1, and those of Zn varied between 0.02 and 2.85 mgl ^-1. The low potential for leaching of potentially toxic elements from the contaminated soils and sediments indicate that they do not represent a serious environmental risk under present conditions. This is corroborated by the low concentration of potentially toxic elements found in shallow groundwater, values that did not exceeded the limits for drinking water.

Keywords : tailings; background values; soils; sediments; contamination; leaching potential.