Eco-Issues
Assessing water–sediment processes for metals in rivers polluted by mining
to predict environmental impacts in developing countries
M. AURORA Armienta1, AZUCENA Dótor1, FLOR E. Arcega-Cabrera2,
OSCAR Talavera3, ALEJANDRA Aguayo1, NORA Ceniceros1 & OLIVIA Cruz1
1 Instituto de Geofísica, Universidad Nacional Autonoma de Mexico,
Circuito Exterior, CU, México 04510 DF, México
victoria@geofisica.unam.mx
2 Posgrado en Ciencias del Mar y Limnología,
Universidad Nacional Autonoma de Mexico, CU, México 04510 DF, México
3 Escuela Regional de Ciencias de la Tierra, UA de Guerrero, Taxco El Viejo, Guerrero, México
Abstract Water–sediment geochemical processes influencing As, Pb, Zn, Cu and Fe mobilization were investigated in a river within the historic mining area of Taxco, México. Tailings wash off and acid mine drainage have led to concentrations of metals in sediments well above the regional background. Zinc and lead associate with the carbonate fraction, followed by Fe oxi-hydroxides. Arsenic and Fe concentrate in the residual fraction, followed by organic matter/sulphide and Fe oxi-hydroxides. Metals are mainly transported as particulates in the water. Calcium, silicate and iron minerals are oversaturated in the river water. Limestones play a key role in the contaminant behaviour. Principal components analysis indicated that Zn, Cu and Pb may be released to the water following small changes in physico-chemical and/or hydrogeological conditions. Application of straightforward analytical techniques jointly with geochemical and statistical evaluation, is an affordable alternative to clarify the environmental impact of mining wastes in developing countries.
Key words mining; contamination; metals; river; sediments; Mexico; hydrogeochemistry;
principal components; limestones; Taxco
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H42A-04
Natural Arsenic Pollution of Groundwater in Mining Zones of Mexico
Arsenic concentrations exceeding drinking-water standards have been measured in groundwater of various areas of Mexico. This is a relevant public health problem since groundwater supplies most drinking water of the country. Although a natural source has been proposed as the cause of water contamination at most sites, the specific processes releasing As have only been identified in a few aquifers. The geological characteristics of Mexico including volcanic, geothermal, and highly mineralized zones constitute favorable environments for As occurrence. Furthermore, As-abundance in bedrock has lead Mexico to be one of the major world As-producers. As-bearing minerals like arsenopyrite, scorodite, mimetite, adamite, tennantite and nickeline can be found in several zones. Besides, arsenic may be a minor component of Fe, Ag, Cu, Pb, Zn, and Au ores. While thousands of people have been chronically exposed to As, arsenic-related health effects have been documented only for residents at some Mexican locations, like Comarca Lagunera, Zimapan, and Acambaro. Water-rock interactions may release As to water in mining areas, but ore extraction and processing produce surface wastes that can also release As to groundwater. Investigations developed in two historical mining zones revealed different As contents in groundwater. At Zimapan, a semi-arid area about 250 km NE of Mexico City, abundant arsenopyrite and hydrogeological conditions produced high As concentrations in deep wells exploited for drinking water supply. Oxidation and dissolution of As-bearing minerals mainly arsenopyrite, scorodite and tennantite released As to the fractured deep limestone aquifer. In addition, mining operations polluted shallow wells. In contrast, low levels of As were detected in wells near mine tailings in the warm sub-humid zone of Taxco, Guerrero. To explain those differences, the mineralogy and the geochemical processes occurring in tailings at both areas were studied. Results showed that besides As levels in processed ore, mineralogy of the ore deposits is one of the main causes of different degrees of As release. Precipitation of secondary minerals like gypsum, goethite, K-jarosite and hematite produced a cemented layer that retains As in the active oxidation zone of the tailings at Taxco. Formation of beundantite also reduces As dispersion. At Zimapan, the relative proportion of pyrite and calcite leads to differences on As mobility, evidenced by mineralogy and geochemical fractionation of As in tailings. Geology of the substrate under the tailings also affects the pollution levels of shallow wells. Granular alluvial material permits a greater mobilization of As at Zimapan than in Taxco, where presence of limonites and calcareous lutites results in low As groundwater contents. Economically and technical affordable treatment methods have been tested to clean the water. At Zimapan, addition of local limestone proved to remove a high proportion of As; flocculation with iron salts was also suitable to treat polluted water.
