Investigação geofísica em pilha de estéril afetada por processo de drenagem ácida, Mina Osamu Utsumi, Poços de Caldas (MG)
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Data
2019-11-29
Autores
Büchi, Fernanda Miranda de Siqueira
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Universidade Estadual Paulista (Unesp)
Resumo
As atividades mineiras são conhecidas pelas profundas mudanças que provocam no meio ambiente e envolvem a geração de uma enorme quantidade de rejeitos, material sem interesse econômico. O material estéril, proveniente das atividades de lavra, é disposto em bota-foras, que podem funcionar como aquíferos porosos artificiais. Tanto a disposição dessas pilhas como sua interação com o meio consistem um dos maiores problemas ambientais relacionados à indústria mineira. Um frequente passivo ambiental gerado nesses empreendimentos é a drenagem ácida de mina (DAM), devido à exposição de materiais que contêm sulfetos, principalmente pirita, a condições oxidantes. Como resultado, há produção de águas com baixos valores de pH e que favorecem a mobilização de elementos químicos tóxicos, como metais pesados, associados às matrizes rochosas. Nesse contexto, está inserida a mina de urânio Osamu Utsumi, localizada no município de Caldas – MG. Geologicamente, a região faz parte do Maciço Alcalino de Poços de Caldas, considerado o maior complexo alcalino da América Latina e composto principalmente por rochas nefelína-sieníticas. Desde a década de 1990, a mina está em fase de descomissionamento, e enfrenta sérias dificuldades relacionadas à geração de DAM, tanto nos bota-foras quanto na cava. Nas últimas décadas, a geofísica tem sido utilizada como uma importante ferramenta na avaliação de impactos ambientais, devido a sua eficácia e custos relativamente baixos. Com o objetivo de entender a dinâmica hidrogeológica na pilha de estéreis denominada BF-8 da mina, o presente trabalho aplicou o método da Eletrorresistividade para diagnóstico do fluxo subterrâneo presente no local. Foram definidas 7 linhas de aquisição geofísica em arranjo Schlumberger, através da técnica de tomografia elétrica. Os resultados mostraram que mesmo no período de estiagem da região de estudos ocorre geração de DAM, produto da interação de zonas de acúmulo existentes no interior da pilha com regiões em que há concentração de pirita, que ocorre de maneira disseminada no bota-fora. A análise das seções 2D permitiu a diferenciação de zonas de acúmulo de águas naturais de zonas de acúmulo de águas de alta salinidade (<40 Ω.m), consideradas águas ácidas. Também foi possível a constatação de que há constante contribuição do aquífero poroso, constituído pelo material que compõe o BF-8, para o aquífero fraturado, formado pelo embasamento cristalino.
Mining activities are known for the deep changes they cause in the environment, and involve the generation of a huge amount of tailings, material without economic interest, to the detriment of a few grams of ore. The sterile material from the mining activities is disposed of in piles, which can function as an artificial porous aquifer. Both the disposal of these piles and their interaction with the environment constitute one of the biggest environmental problems related to the mining industry. A frequent environmental liability generated in these enterprises is acid mine drainage (DAM), due to the exposure of sulfide-containing materials, mainly pyrite, to oxidizing conditions. As a result, there is production of water with low pH values, which favors the dissolution of toxic chemical elements, such as heavy metals, associated with rocky matrices. In this context, the Osamu Utsumi uranium mine, located in the city of Caldas - MG, is inserted. Geologically, the region is part of the Poços de Caldas Alkaline Massif, considered the largest alkaline complex in Latin America and composed mainly of nepheline-syenitic rocks. Since the 1990s, the mine has been decommissioning and faces serious difficulties related to the generation of DAM, both in the tailings piles and pit. In recent decades, geophysics has been used as an important tool in environmental impact assessment due to its effectiveness and relatively low costs. In order to understand the hydrogeological dynamics in the BF-8 sterile pile of the mine, the present work applied the DC Resistivity method to diagnose the underground flow present at the site. Seven lines of geophysical acquisition in the Schlumberger arrangement were defined using the electric tomography technique. The results showed that even during the drought period of the study region, there is a generation of DAM, a product of the interaction of accumulation zones existing inside the pile with regions where there is a concentration of pyrite, which occurs in a scattered manner in the BF-8. The analysis of the 2D sections allowed the differentiation of natural water accumulation zones from high salinity water accumulation zones (<40 Ω.m), considered acid waters. It was also possible to find that there is a constant contribution of the porous aquifer, constituted by the material that composes the BF-8, to the fractured aquifer, formed by the crystalline basement.
Mining activities are known for the deep changes they cause in the environment, and involve the generation of a huge amount of tailings, material without economic interest, to the detriment of a few grams of ore. The sterile material from the mining activities is disposed of in piles, which can function as an artificial porous aquifer. Both the disposal of these piles and their interaction with the environment constitute one of the biggest environmental problems related to the mining industry. A frequent environmental liability generated in these enterprises is acid mine drainage (DAM), due to the exposure of sulfide-containing materials, mainly pyrite, to oxidizing conditions. As a result, there is production of water with low pH values, which favors the dissolution of toxic chemical elements, such as heavy metals, associated with rocky matrices. In this context, the Osamu Utsumi uranium mine, located in the city of Caldas - MG, is inserted. Geologically, the region is part of the Poços de Caldas Alkaline Massif, considered the largest alkaline complex in Latin America and composed mainly of nepheline-syenitic rocks. Since the 1990s, the mine has been decommissioning and faces serious difficulties related to the generation of DAM, both in the tailings piles and pit. In recent decades, geophysics has been used as an important tool in environmental impact assessment due to its effectiveness and relatively low costs. In order to understand the hydrogeological dynamics in the BF-8 sterile pile of the mine, the present work applied the DC Resistivity method to diagnose the underground flow present at the site. Seven lines of geophysical acquisition in the Schlumberger arrangement were defined using the electric tomography technique. The results showed that even during the drought period of the study region, there is a generation of DAM, a product of the interaction of accumulation zones existing inside the pile with regions where there is a concentration of pyrite, which occurs in a scattered manner in the BF-8. The analysis of the 2D sections allowed the differentiation of natural water accumulation zones from high salinity water accumulation zones (<40 Ω.m), considered acid waters. It was also possible to find that there is a constant contribution of the porous aquifer, constituted by the material that composes the BF-8, to the fractured aquifer, formed by the crystalline basement.
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Palavras-chave
Geofísica, Mineração a céu aberto, Monitoramento ambiental, Eletrorresistividade, Passivo ambiental, Mina de urânio, Bota-fora, Aquífero, DC resistivity, Environmental liability, Uranium mine, Tailing pile, Aquifer