In this work we discuss a geophysical-hydrogeological strategy proposed to explore rationally the groundwater stored in the semi-arid terrains, such as those of the upper Curaçá river basin, Bahia. Three main subjects were considered and solved in this study. The first one was to develop a conceptual geohydrologic model to represent the water storage and transmission within the aquifer zones of the area. Based on surface and aero-geophysical information and on geological data obtained in deep cored mining-wells we propose the model to consist of: (i) a thick layer of alluvial deposits and weathered rocks (saprolites) having up to 30 m thickness, covering almost continuously the Curaçá river plain and those of their tributaries; and (ii) large and wide belts of densely fractured rocks, laterally and vertically alternating with zones of impervious crystalline basement. These belts of open, subvertical fracturing may reach more than 500 m in width and depths larger than 300 m. The second was to optimize a geoelectrical procedure capable of imaging, with good spatial resolution, these complexes underground reservoirs. Thus, a two-dimensional scheme of IP-resistivity imaging was performed with a closely spaced association of deep vertical electrical soundings. The third was to search and explain the mechanisms and processes responsible for the high water salinization in the wells drilled in this system. Based on a simplified model for a steady-state, two-dimensional flow of water in a water-table homogeneous and isotropic aquifer, we suggest this salt concentration process is mainly due to the intense and almost continuous evapo-transpiration, that characterizes the semi-arid lands. This process acts over a hydraulic circulating system controlled by the terrain topography. The circulation cells have different sizes and depth of propagation, the salt accumulation being concentrated in the smaller cells.
crystalline aquifers; geoelectrical structure; saltiness
