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98 The Legacy of Abandoned Mines Na primeira daquelas situações, é comum o recurso a canais de drenagem de águas limpas que garantam o escoamento dos terrenos envolventes às áreas mineiras contaminadas, ou que colectem as águas das chuvas que caem sobre as áreas já recuperadas e impermeabilizadas. Na segunda daquelas vertentes,a criação de bacias, pode constituir-se numa medida de carácter provisório ou permanente, associadas a sistemas de tratamento, que podem ser do tipo activo ou do tipo passivo. São também adoptadas frequentemente soluções mistas, complementadas ou não com barreiras de materiais inorgânicos, que permitem estabelecer condições especiais para a precipitação de contaminantes específicos contidos na água. No terceiro caso, a inundação controlada das explorações mineiras subterrâneas é, sempre que possível, uma solução a privilegiar. Embora numa fase inicial possa conduzir a uma ligeira deterioração da qualidade da água, na maioria das situações, a médio/longo prazo, inibe a progressão da contaminação. DESVIO DE ÁGUAS LIMPAS O desvio de águas limpas em torno das áreas mineiras degradadas constitui, pela sua importância, uma das primeiras fases de actuação, sendo determinante para a eficácia das demais medidas a implementar. A título de exemplo refira-se o caso de Algares, em Aljustrel, em que águas limpas de vertente de encosta se misturavam ao longo de mais de um quilómetro com águas contaminadas que percolavam pelos elevados volumes de “safrões” (stockwork), depositados ao longo dos anos naquela área mineira. Um outro exemplo, o de Terramonte, as linhas de água, mais ou menos permanentes, ao atravessarem a zona mineira muito contaminada, perdiam qualidade inviabilizando a sua utilização para o consumo humano ou até agrícola. A criação de um canal de desvio da ribeira, desde montante até jusante da área mineira, assim como a drenagem de águas de flanco de encosta, constitui-se solução eficaz no desvio de águas limpas. Vestígios do canal periférico à zona mineira de Aljustrel. Traces of the channel peripheral to the Aljustrel mining area. 1) Divert clean waters originating from outside the mining area in order to avoid that these enter into contact with any waste, whether from mineral extraction or its treatment; 2) Create basins for containing and/ or treatment of contaminated waters, whether originating from the mines or from run-offs through materials deposited in the mining area, in order to avoid the contamination of aquifers, water lanes, soils and sediments; 3) Flood the mining works, creating anoxic conditions that hamper the progressive oxidation of the sulphides contained in the encasing rocks and in the mineral. When in contact with normal atmosphere these change progressively, and produce sulphuric acid that leads to the leaching of the heavy metals contained therein. The solutions to be adopted in the projects must generally follow the above principles. In the first of those situations, the most common solution is the recourse to clean water drainage channels that will guarantee the seeping of the terrains surrounding the contaminated mining areas, or that will collect the rain waters that fall on the already recovered and impermeable covers. In the second instance, the creation of basins may become a provisional or permanent measure. These are associated to treatment systems that can be either active or passive. Mixed solutions are also frequently adopted, complemented or not with barriers made up of inorganic materials, which allow establishing special conditions for the precipitation of specific contaminants contained in the water. Thirdly, the controlled flooding of underground mining operations is a preferential solution. Although in an initial stage it could lead to a slight deterioration in the quality of water, it inhibits the progress of contamination in the medium/long term. DIVERSION OF CLEAN WATERS The diversion of clean waters surrounding the degraded mining areas is, due to its relevance, one of the primary action stages, and is decisive for the remaining measures to be implemented. An instance of this is the case of Algares, in Aljustrel, in which the clean waters running across the embankment were mixed along a length of more than one kilometre with contaminated waters that percolated through the large volumes of stockwork deposited throughout the years in that mining area.
A Herança das Minas Abandonadas 99 ALJUSTREL (ALGARES) (desvio de águas limpas) ALJUSTREL (ALGARES) (clean water diversion) 1 Another instance is that of Terramonte, where the more or less permanent water channels, when crossing the highly contaminated mining area, ceased to have the required quality and thus rendering them impractical for human or even agricultural use. The creation of channels to divert the stream, from upstream to downstream of the mining area, as well as the drainage of waters from the lateral surface of the embankment, are an effective solution for the diversion of clean waters. Canal de desvio de águas limpas, em Algares, Aljustrel – Situação ante e pós- remediação. 1 Antes remediação. 2 Após remediação. Clean water diversion channel, in Algares, Aljustrel – Pre and post remedial action. 1 Contaminating waters that are mixed in the channel. 2 A section of the Aljustrel clean water peripheral channel. 2
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A HERANÇA DAS MINAS ABANDONADAS A
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The “Legacy of the Abandoned Mine
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REFLEXÕES REFLECTIONS A Herança d
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NOTES 1. The Post-Mining Alliance i
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