Litho-structural conditioning in the exploration of fractured aquifers: a case study in the Crystalline Basement Aquifer System of Brazil

Productive regions in the Crystalline Basement Aquifer System (CBAS) in Paraná state, Brazil, were identified qualitatively and quantitatively through spatial correlation of wells and geological conditions such as lineaments, hydrography, aeromagnetometry and lithology. Different methods applied in Precambrian metamorphic and igneous aquifers around the world and some Brazilian states were integrated and applied in CBAS with the aim of understanding the best scale and approach to productivity. The median productivity of the 224 wells analyzed is 0.29 m³/h/m. Under a multi-scale regional assessment, the results showed that the best condition is associated with the distance of 350 m from the lineaments (1:100,000), especially those with directions N40W, N10E and N70E. Considering the hydrolithological units, gneisses are the most productive, especially where lineaments coincide with regional structures such as shear zones, foliations and Cenozoic tectonic reactivations. Quartzites, granitoids, schists, phyllites, and rhyolites were also favorable when close to important rivers and not necessarily coinciding with regional lineaments, the high density of lineaments, or with vertical fractures. The areas of intersection of the lineaments and the weathering mantle did not serve as a discriminatory parameter. As the median depths of the water wells reach 90 m from the surface, the extraction of oriented magnetic gradients referred to magnetic sources down to 800 m depth, thus corroborating surface mapped lineaments and new nonoutcroppable structures. Considering the complexity of the environment and the global use of groundwater from fractured aquifers, this work contributed by discriminating geospatial parameters to decrease the exploratory risk in CBAS.