Discontinuous Nature of Phreatic Aquifers in Granitic Rocks at Watershed Scale: A Stratiform Model from Perennial Streams and Well Data

The study was carried out within 4,000 km2 segment of Sassandra watershed located in Soubré, south-west of Côte d’Ivoire where coffee and cocoa production are preponderant. This work aimed to provide a general methodology in order to draw the flow directions and map the potential productive aquifer in hard-rocks. Drilling data, remote sensing, and geomorphic data in a context of weathered plutonic and metamorphic Precambrian were used. The regional water table was modeled through a linear relationship between the topographic surface of the digital elevation model and the base surface of the perennial streams thalwegs. As result, a map of regolith thickness obtained has been compared with the geology to emphasize their relationship. Furthermore, other correlations have been found between the hydraulic data and geomorphological features to get more precise stratification model. Concealed by the regolith, the hard-rock aquifer is made up of three layers. From top to bottom we have the saprolite (< 10 m), the weathering induced fissured layer (35 m mean thickness) below the base of the saprolite and finally, the unweathered with very low hydraulic conductivity. Each layer is characterized by a constant density of water-bearing fissures. This shows the impact of a stratiform weathering profile on the layering of the aquifer. High variability of observing yields is mainly due to thickness heterogeneity of the higher-storage regolith and underlying higher-conductive fissured saprock. Also, the wells tapping water under a regolith with medium or high thickness were the most productive justifying that regolith thickness mapping is a first rank tool in hydrogeology prospecting. The thickness map obtained from the interpolated base surface of the regolith and the DEM represents a useful tool for groundwater management.