HR: 11:20h
AN: H42A-04    [Abstracts]
TI: Natural Arsenic Pollution of Groundwater in Mining Zones of Mexico
AU: * Armienta, M A
EM: victoria@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico. Instituto de Geofisica, Circuito exterior, C.U., Mexico, DF 04510, Mexico
AU: Rodriguez, R
EM: rrdz@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico. Instituto de Geofisica, Circuito exterior, C.U., Mexico, DF 04510, Mexico
AU: Villasennor, G
EM: mgvc@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico. Instituto de Geologia, C.U., Mexico, DF 04510, Mexico
AU: Romero, F
EM: fmrch@hotmail.com
AF: Universidad Nacional Autonoma de Mexico. Instituto de Geologia, C.U., Mexico, DF 04510, Mexico
AU: Talavera, O
EM: talavera@geo.arizona.edu
AF: Universidad Autonoma de Guerrero, Taxco el Viejo, Taxco, GRO , Mexico
AU: Ceniceros, N
EM: nora@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico. Instituto de Geofisica, Circuito exterior, C.U., Mexico, DF 04510, Mexico
AU: Aguayo, A
EM: ale@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico. Instituto de Geofisica, Circuito exterior, C.U., Mexico, DF 04510, Mexico
AU: Cruz, O
EM: olicruz@geofisica.unam.mx
AF: Universidad Nacional Autonoma de Mexico. Instituto de Geofisica, Circuito exterior, C.U., Mexico, DF 04510, Mexico
AB: 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.
DE: 1831 Groundwater quality
SC: Hydrology [H]
MN: 2007 Joint Assembly