Diagnóstico geofísico da percolação de drenagem ácida de mina em aquífero fraturado
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Data
2023-04-12
Autores
Buchi, Fernanda Miranda de Siqueira
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Universidade Estadual Paulista (Unesp)
Resumo
As atividades mineiras provocam profundas mudanças ambientais, devido à mobilização de milhares de toneladas de solo e rocha. O material estéril consiste em solo ou rocha ausente de minerais econômicos ou com teores não econômicos, produzido durante a lavra ou beneficiamento de minérios, que pode ser disposto em bacias de decantação, em pilhas ou bota-foras. Tanto a disposição dessas pilhas como sua interação com o ambiente consistem num dos maiores problemas ambientais relacionados à indústria mineira. Um passivo ambiental frequentemente 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, ocorre a geração de águas com baixos valores de pH e que favorecem a dissolução 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. Desde a década de 1995, a mina está em fase de descomissionamento e enfrenta sérias dificuldades relacionadas à geração de DAM, tanto nos bota-foras quanto na cava. A fim de auxiliar a tomada de decisões que visem a remediação do passivo ambiental, o presente trabalho aplicou o método geofísico da Eletrorresistividade na interface entre uma pilha de rejeitos conhecida como BF-04 e rochas fraturadas a jusante, com o intuito de diagnosticar o fluxo subterrâneo local e averiguar a eventual percolação de drenagem ácida de mina para o aquífero fraturado. Para tanto, foram definidas 9 linhas de aquisição geofísica em arranjo Schlumberger e Dipolo-Dipolo, através da técnica de tomografia elétrica, além da análise de dados hidroquímicos de poços de monitoramento de modo a complementar as interpretações geofísicas referentes ao ambiente hidrogelógico local. Os resultados foram apresentados em seções 2D e 3D de resistividade e mostram o contraste de zonas saturadas livres de contaminação e zonas afetadas pela DAM, apontadas nos modelos como regiões com valores de resistividade da ordem de 40Ω.m. As interpretações geofísicas sugerem que há conectividade hidrogeológica entre a pilha de rejeito e o leito rochoso fraturado sotoposto, o que promove a infiltração das águas ácidas para o aquífero regional e dispersão dos efluentes para além das dependências da mina. Os dados hidroquímicos corroboram as interpretações geofísicas, e evidenciam a percolação de efluentes ácidos no sistema fraturado, o que demonstra a necessidade da implementação de ações que tornem a geração de DAM restrita e com características químicas próximas dos padrões naturais, para que seja possível o descomissionamento do empreendimento mineiro.
Mining activities are known for the deep changes they cause in the environment due to the mobilization of thousands of tons of soil and rock. The sterile material consists of soil or rock without economic minerals or with non-economic contents, produced during the mining or beneficiation of ores, which can be disposed of in decantation basins or in piles. Both the disposal of these piles and their interaction with the environment are one of the biggest environmental problems related to the mining industry. A frequent environmental liability generated in these enterprises is acid mine drainage (AMD), 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. Since the 1990s, the mine has been decommissioning and faces serious difficulties related to the generation of AMD, both in the tailings piles and pit. In order to assist in decision-making aimed at remediating the environmental liability, the present work applied the geophysical method of Electroresistivity at the interface between a tailing pile known as BF-04 and closely fractured rocks, with the aim of diagnosing the local underground water flow and verify possible percolation of acid mine drainage into the fractured aquifer. For this purpose, nine lines of geophysical acquisition in the Schlumberger and Dipolo-Dipolo arrangements were defined using the electric tomography technique, in addition to the analysis of hydrochemical data from monitoring wells in order to complement the geophysical interpretations related to the local hydrogeological environment. The results were presented in 2D and 3D sections of resistivity and show the contrast between saturated zones free of contamination and zones affected by the AMD, indicated in the models as regions with resistivity values of the order of 40Ω.m. The geophysical interpretations suggest that there is hydrogeological connectivity between the tailings pile and the fractured bedrock below, which promotes the infiltration of acidic waters into the regional aquifer and the dispersion of effluents beyond the mine facilities. The hydrochemical data corroborates the geophysical interpretations, and show the percolation of acid effluents in the fractured system, which demonstrates the need to implement actions that make the generation of DAM restricted and with chemical characteristics close to natural standards, so that the decommissioning of the mining enterprise is possible.
Mining activities are known for the deep changes they cause in the environment due to the mobilization of thousands of tons of soil and rock. The sterile material consists of soil or rock without economic minerals or with non-economic contents, produced during the mining or beneficiation of ores, which can be disposed of in decantation basins or in piles. Both the disposal of these piles and their interaction with the environment are one of the biggest environmental problems related to the mining industry. A frequent environmental liability generated in these enterprises is acid mine drainage (AMD), 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. Since the 1990s, the mine has been decommissioning and faces serious difficulties related to the generation of AMD, both in the tailings piles and pit. In order to assist in decision-making aimed at remediating the environmental liability, the present work applied the geophysical method of Electroresistivity at the interface between a tailing pile known as BF-04 and closely fractured rocks, with the aim of diagnosing the local underground water flow and verify possible percolation of acid mine drainage into the fractured aquifer. For this purpose, nine lines of geophysical acquisition in the Schlumberger and Dipolo-Dipolo arrangements were defined using the electric tomography technique, in addition to the analysis of hydrochemical data from monitoring wells in order to complement the geophysical interpretations related to the local hydrogeological environment. The results were presented in 2D and 3D sections of resistivity and show the contrast between saturated zones free of contamination and zones affected by the AMD, indicated in the models as regions with resistivity values of the order of 40Ω.m. The geophysical interpretations suggest that there is hydrogeological connectivity between the tailings pile and the fractured bedrock below, which promotes the infiltration of acidic waters into the regional aquifer and the dispersion of effluents beyond the mine facilities. The hydrochemical data corroborates the geophysical interpretations, and show the percolation of acid effluents in the fractured system, which demonstrates the need to implement actions that make the generation of DAM restricted and with chemical characteristics close to natural standards, so that the decommissioning of the mining enterprise is possible.
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Palavras-chave
Passivo ambiental, Mina de urânio, Pilha de rejeito, Aquífero, Eletrorresistividade, Environmental liability, Uranium mine, Tailing pile, Aquifer, DC resistivity