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Boletín de la Sociedad Geológica Mexicana

Print version ISSN 1405-3322


SANTOS-JALLATH, José E.; CORIA-CAMARILLO, Jhonnatan; HUEZO-CASILLAS, José de Jesús  and  RODRIGUEZ-CRUZ, Geovanni. Influence of mine waste dams on the Maconí river, Queretaro, and assessment of the natural attenuation process by dispersion. Bol. Soc. Geol. Mex [online]. 2013, vol.65, n.3, pp.645-660. ISSN 1405-3322.

La Negra mine is located in the state of Querétaro, Mexico, and since mining operations started all wastes resulting from the mill process have been stored in five tailings dams that were built near the mine in riverbeds that converge with the Maconí River which, in turn, converges with the Moctuzuma River, 5 km downstream. The study target was to evaluate physical dispersion of tailings from the dams and their influence over the Maconí River, and also to identify existence of a physical phenomenon of natural attenuation by water drag. Tailings samples were taken in order to determine total concentration of arsenic (As), lead (Pb), cadmium (Cd), copper (Cu) and zinc (Zn), and also their concentration in the water soluble fraction. Sediments were also sampled from the riverbed where the tailings dams are located and also from the Maconí River; sediments samples from riverbeds located outside of the dams' influence were also included to serve as reference values. A granulometric analysis was conducted on these samples, pH was measured and As, Cd, Pb, Cu and Zn total concentrations were determined; also water and sodium bicarbonate extraction tests were conducted in order to determine the soluble and interchangeable fraction concentration of these elements, respectively. Selected samples were studied by optical microscopy and X-ray diffraction. Total concentrations in tailings are: As (1643 to 13459 mg/kg), Cd (5 to 64 mg/kg), Pb (469 to 4583 mg/kg), Cu (439 to 745 mg/kg) and Zn (1937 for 4316 mg/kg). Concentrations of As, Cd and Pb in the water soluble fraction is below the Mexican environmental regulations criteria (NOM-141), indicating that the tailings are not dangerous due to toxic element mobility. It was observed that tailings dispersion has occurred and these accumulate near the dams; in tailings of dam number 3, the influence of the tailings is present at a distance of up to 437 meters, while in dam number 5 they reached 956 meters. In the river, total concentration averages were 461 mg/kg for As, 140 mg/kg for Pb, 66 mg/kg for Cu, and 290 mg/kg for Zn, whereas Cd was not detected. These concentrations are of the same order as the reference samples: As (434 mg/kg), Pb (122 mg/kg), Cu (77 mg/kg), Zn (288 mg/kg) and Cd below the detection limit. These results show that there is a natural contribution of As and heavy metals due to the orebody outcrop in the area (given the results of the reference samples), and that water drag causes natural dispersion in the Maconí River, since the total concentration of these elements decreases considerably downstream. The arsenic concentration in the water-soluble fraction exceeds 0.5 mg/L (permissible level indicated in NOM-147) only in sediments near dam number 3 and in the riverbed of dam number 5, whereas in the Maconí River soluble fraction concentrations are below the mentioned level. The interchangeable fraction in sediments, which was extracted with sodium bicarbonate, shows As solubility that increases relative to the water soluble fraction.

Keywords : mining waste; tailings dispersion; natural attenuation; river sediments; environmental impact.

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