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The implementation of open pit mines promotes alterations on geological and hydrogeological processes, mainly on natural streamflow patterns. Drainage system tries to reach its equilibrium due to modification on slope profiling water flows through new pathways contributing to erosion and leaching processes. One of the impacts related to mining is the generation of acid mine drainage (AMD), which occurs as a product of sulfide minerals exposure to oxidizing environment and, in contact to water, favors the formation of sulfuric acid. Thus, a low pH promotes a higher mobility of heavy metals and radionuclides, which become a source of contamination. In order to understand the hydrogeological dynamic in rock masses that contribute to AMD production, this paper aimed to subsidize mitigation programs in fractured aquifers using a structural analysis and geophysical survey for the reduction of acid drainage generation. The study was carried at the four mining fronts that compose the mine pit of Osamu Utsumi Mine, named according to its cardinal position (NE, SE, SW and NW). Local structural survey indicated that fractures attitudes are mainly N20E/80NW and N55 W/75NE, with intersections between them. Evidence of water flows, like whitish kaolinite stains and small vegetation growth, was identified in fronts NE, SW and NW, in addition to water springs at the base of the slopes. The flows arise mainly in areas that coincide to intersection between fracture systems in an orthogonal arrangement and the fracturing pattern indicated that at all fronts most of the fracture planes project into the open pit area, which favor the water channeling to the center of the mine as a water catchment basin. Moreover, the relationship between the persistence and spacing among the discontinuities enables a good hydraulic conductivity within the rock masses and the water upwelling on the slopes surface. The geophysical data corroborated with the structural survey, which identified the fracture planes as linear structures associated with low resistivity zones. DC resistivity method showed a strong contrast between saturated zones, differing natural from acidic water. Delimitation of linear features in the inversion models indicated that water flows are channeled through fracture planes and promote an expressive weathering process inside the rock masses, observed at depths of up to 70 m.

ABSTRACT. Among the potential environmental impacts in mining activities, acid mine drainage (AMD) is a relevant problem caused by reactive minerals, such as sulfides, due to their exposure to the surface conditions. This is the context of contamination of a waste pile (BF-04) at Osamu Utsumi mine, closed in 1995 after years of physical and chemical processing of the uranium ore and currently under decommissioning plan. This study is based on the application of the combined geophysical methods of Electrical Resistivity and Induced Polarization and the analysis of previous geochemical data, aiming the evaluation of zones related to the generation of AMD and groundwater flux into the waste pile BF-04, containing sulfide minerals and uranium. The association of high resistivity and high chargeability zones is related to disseminated sulfide minerals in rock with silica cement. Infiltration of meteoric water through those zones induces the oxidation of sulfides, high sulfur concentration in groundwater followed by a drop in pH values, which results in a higher leaching capacity and solubility of ions and heavy metals. In addition, high chargeability zones (higher than 10 mV=V) represent portions of generation of AMD, while the low resistivity anomalies (under 70W:m) are related to preferential flow zones of the contaminated groundwater.Keywords: minning, sulfides, uranium, electrical resistivity, chargeability.RESUMO. Dentre os potenciais impactos ambientais em mineração, a drenagem ácida de mina (DAM) é um problema grande onde ocorrem minerais instáveis como os sulfetos, outrora isolados em subsuperfície. É neste contexto de contaminação do meio físico que está inserido uma pilha de rejeitos (BF-04) pertencente à mina de urânio Osamu Utsumi, cujas atividades de mineração, processamento físico e processamento químico foram encerradas em 1995, seguidas pela etapa de descomissionamento até os dias atuais. Neste estudo foram aplicados os métodos geofísicos da Eletrorressitividade e Polarização Induzida combinados com dados geoquímicos prévios, com o intuito de avaliar zonas de geração e fluxo de drenagem ácida de mina na pilha de rejeitos de mineração BF-04 com sulfetos e urânio. A combinação de zonas de alta resistividade e alta cargabilidade revela sulfetos disseminados em rochas com cimento silicático. A infiltração de águas meteóricas nestes locais induz a oxidação de sulfetos, liberação de enxofre nas águas subterrâneas seguida pela queda no pH, que resulta num efluente com alta capacidade de lixiviação e solubilidade de sais e metais. Neste sentido, zonas de alta cargabilidade (acima de 10 V=V) representam locais de geração de drenagem ácida de mina, enquanto que zonas de baixa resistividade (abaixo de 70 W:m) revelam zonas preferenciais de fluxo do efluente.Palavras-chave: mineração, sulfetos, urânio, resistividade, cargabilidade.

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Acid mine drainage (AMD) has been identified as the main cause for outflow of acid water and radioactive/non-radioactive contaminants. AMD encompasses pyrites oxidation when water and oxygen are available. AMD was identified in uranium waste rock piles (WRPs) of Indústrias Nucleares do Brasil-Caldas facility (Brazilian uranium mine), resulting in high costs for water treatment. AMD reduction is the main challenge, and scientific investigation has been conducted to understand oxygen and water transportation within WRPs, where 222Rn is used as natural tracer for oxygen transportation. The study consists of soil radon gas mapping in the top layer of WRP4 using active soil gas pumping, radon adsorption in active charcoal and 222Rn determination using high-resolution gamma-ray spectrometry. A sampling network of 71 points was built where samples were collected at a depth of 40 cm. Soil radon gas concentration ranged from 33.7 to 1484.2 kBq m(-3) with mean concentration of 320.7±263.3 kBq m(-3).