en moyenne 7 m en dessous du sol lorsque l'on prend comme référence la surface de base des thalwegs. La surface piézométrique est corrélée à la surface topographique. La superposition de la carte piézométrique à celle du réseau hydrographique montre qu'à l'échelle de l'analyse réalisée, la circulation de l'eau souterraine est calquée sur celle des eaux de surface ; bassins versants hydrogéologique et topographique se superposent, montrant ainsi le lien entre les eaux de surface et les eaux souterraines. Ce résultat pourra aider à la gestion intégrée des ressources en eau souterraine et de surface surtout face à la vulnérabilité aux pesticides qui sont fréquemment utilisés dans la zone étudiée. Ce document est protégé par la loi sur le droit d'auteur. L'utilisation des services d'Érudit (y compris la reproduction) est assujettie à sa politique d'utilisation que vous pouvez consulter en ligne. L'interprétation mathématique des résultats a permis, au travers des équations établies, de calculer directement le niveau piézométrique en tout point de la zone d'étude. La carte piézométrique ainsi obtenue montre que le niveau piézométrique est en moyenne 7 m en dessous du sol lorsque l'on prend comme référence la surface de base des thalwegs. La surface piézométrique est corrélée à la surface topographique. La superposition de la carte piézométrique à celle du réseau Modélisation piézométrique dans le bassin versant du Sassandra 106hydrographique montre qu'à l'échelle de l'analyse réalisée, la circulation de l'eau souterraine est calquée sur celle des eaux de surface ; bassins versants hydrogéologique et topographique se superposent, montrant ainsi le lien entre les eaux de surface et les eaux souterraines. Ce résultat pourra aider à la gestion intégrée des ressources en eau souterraine et de surface surtout face à la vulnérabilité aux pesticides qui sont fréquemment utilisés dans la zone étudiée.Mots clés : Aquifère de socle, eau souterraine, surface piézo-métrique, modèle numérique d'altitude, thalwegs, modéli-sation. ABSTRACTThe piezometric levels in hard-rock from Côte d'Ivoire are poorly understood because of the complexity of the geological substratum. This work aims to model the piezometric level of aquifers located in crystalline and crystallophyllian basement, particularly in the Sassandra Watershed (south-western Côte d'Ivoire). By exploiting, on the one hand the method of linear correlation between the elevation of ground surface and base surface of thalwegs, and on the other hand the difference in elevation between the piezometric surface and the base surface thalwegs, we have established equations to determine groundwater levels and thereby compensate gaps in our knowledge. This study is based on field data, drilling datasheet, maps (topographic and geological) and an extract of the Digital Elevation Model (DEM) in south-western Cote d'Ivoire.The mathematical interpretation of the results, using established equations, helped us to calculate directly the piezometric level at any point in the study area. The pi...

The aims of this work were a better knowledge of hydraulic properties of fractured reservoirs in the Oumé area and to estimate the transmissivity of highly fractured hard rock using geostatistical approaches. The boreholes available tapping the fractured reservoirs and have a total depth oscillate between 39 to 93m. The inflows are met between 1 to 63m from the top of crystalline basement. Aquifers of crystalline rocks are more productive that those in the schists. Geostatistical approach is used to perform an assessment of transmissivity on Oumé area. The variographic analysis of logT showed a spatial structuring with a small nugget effect witch represented 17 % of the total dispersion. Cross-validation results showed that conditions of unbiasedness and optimality are met. The actual and estimated transmissivity values spread respectively on four and two orders of magnitude. This result was explained by missing of data and the kriging smoothing. Actual transmissivity values and kriging estimated values were comparables in statistical viewpoint. Estimated transmissivity values can be accepted and transmissivity map obtained will be used for the aquifer modelling. Geostatistical approach used in this study has supplied satisfactory results in the knowledge of this important parameter on the Oumé area.

The study was carried out within 4,000 km 2 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.

Randomness of fracture networks still makes channelized flow a challenge to track in hard-rock aquifers. While not underestimating geological and hydrological criteria that are also handled here through mapping exercises, this study raises an issue of water quality encountered in lifelong boreholes. Chemical classification checked against a recent conceptual model of bedrock aquifers gives birth to a new typology of groundwater in a complex granitic aquifer system located in the SW of Ivory Coast (West Africa). Major ion chemistry, borehole completion data, digital elevation model and satellite images are used to interpret the geochemical water facies as an expression of connexions between the saprolite and the saprock, or transient insulation. From major ions ratios, cumulate mineralization, carbonate equilibrium, stable isotopes, the maturation of ground waters and mixing between bedrock layers are described at seasonal and local scales. The results highlight some vertical feeding of the water table into the main saprock aquifer owing to shortcuts through the saprolite, along with the existence of dead-ends in the hydraulically active fracture network. Also, some influence of fault zones, either drain or barrier, is confirmed on the (Ca, Mg) bicarbonate water facies within the saprock.