Structural analysis and geophysical survey for hydrogeological diagnosis in uranium mine, Poços de Caldas (Brazil)

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.