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.
Mining activities are known for the profound changes they cause in the environment, especially those related to environmental liabilities generated by these enterprises. One major concern of this sector relies on the acid mine drainage (AMD) process, which is formed by the exposure of sulfide minerals to oxidizing conditions. As a result, the production of low-pH saline waters favors the mobilization of heavy metals to the environment. In the last decades, the characterization and monitoring of contamination plumes have become more feasible due to the application of geophysical tools, more precisely the DC resistivity geophysical method. Thus, the aim of the present study is the application of electrical resistivity tomography to understand the hydrogeological dynamic of a waste rock pile in the Osamu Utsumi uranium mine, with the identification of preferred groundwater and AMD flow. The analysis of the 2D geophysical products allowed the differentiation of high salinity water accumulation zones (< 40 Ω m) in the interior of the pile. Likewise, the evaluation of the pseudo-3D model generated by the interpolation of 2D sections was crucial for the recognition of low resistivity zones within the bedrock, possibly controlled by fractures that work as recharge zones between the waste rock pile and the fractured aquifer. The identification of discharge and recharge zones related to the fractures system is a fundamental step for planning and actions to prevent water-sulfides interaction process, which is responsible for the generation of AMD, and also the installation of groundwater pumping systems.
ABSTRACT.In São Paulo State, there are hundreds of contaminated areas due to leakage of hydrocarbons in gas stations, industry or transportation. Chemicalcompounds transportation is a high-risk activity in function of the unpredictability of accidents. The integration between geophysical data and soil/groundwater chemicalanalyzes provides a comprehensive and appropriate view to the management and monitoring of contaminated areas. This paper brings a set of geochemical data andgeophysical diagnosis acquired in a diesel-contaminated area due to a railroad accident in 2002, during the transport of fuels in Cerquilho, São Paulo State. Furthermore,the site was still affected by the contamination when the geophysical acquisition was performed. DC Resistivity and Induced Polarization were performed, using electricaltomography technique. High chargeability values (2.97 mV/V) were associated with the neoformation of sulfides and hydroxides minerals in the unsaturated zone dueto hydrocarbon degradation. Furthermore, the resistivity data indicated the presence of residual phase in the unsaturated zone by a heterogeneous pattern of anomalies.In addition, low resistivity values (<12Ω.m) in the saturated zone indicated a dissolved phase of the contaminant. The geophysical diagnosis of residual phases inhydrocarbon contamination are used as a subsidy to the proper planning of remediation techniques in aquifer systems.Keywords:leakage; hydrocarbon, residual phase, DC resistivity; induced polarization; remediation.RESUMO.No Estado de São Paulo existem centenas de áreas contaminadas por vazamento de hidrocarbonetos em postos de combustíveis, indústrias ou transporte,esta última de alto risco devido à imprevisibilidade dos acidentes. A integração de dados geofísicos com análises químicas proporciona uma visão abrangente eadequada ao estudo e monitoramento de áreas contaminadas. Este trabalho reúne um conjunto de dados geoquímicos e de diagnóstico geofísico, adquiridos numa áreacontaminada por diesel resultante de acidente ferroviário em 2002 no município de Cerquilho/SP, cuja contaminação ainda estava presente no período da investigaçãogeofísica. Foram realizados levantamentos de Eletrorresistividade e Polarização Induzida, por meio da técnica de tomografia elétrica. Altos valores de cargabilidade(2,97 mV/V) foram relacionados à neoformação de sulfetos e hidróxidos em ambiente insaturado, produtos da degradação dos hidrocarbonetos. Por outro lado, osdados de resistividade indicaram a presença de fase residual em zona insaturada através de um padrão heterogêneo, além de baixos valores de resistividade (<12Ω.m)em zona saturada do aquífero, o que indica uma fase dissolvida do contaminante. O diagnóstico geofísico de fases residuais em contaminações por hidrocarbonetospode servir de subsídio a planejamento adequado de técnicas de remediação em sistemas aquíferos.Palavras-chave:vazamento; hidrocarboneto; fase residual; eletrorresistividade; polarização induzida; remediação
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