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Etude Hydrogéologique, Hydrochimique in situ et Modélisation Hydrodynamique du Système Aquifère du Bassin Sédimentaire Côtier de la Région de Pointe-Noire

Abstract : The Pointe-Noire region is located in the large hydrogeological complex of the coastal sedimentary basin of Congo Brazzaville, with a surface area of 6000 km2 . This area is essentially made up of loose rocks with little or no consolidation and interstitial porosity. It consists of a hydrogeological complex, including an aquifer system composed of several superimposed aquifer layers with hydraulic continuity. The present study has allowed to represent and schematize the reservoir functions and the hydrodynamic behavior of the AQ-2 aquifer. It has also allowed to define the aquifer system in a first approximation according to the geological conditions and to obtain a first estimate of its mode of supply and its flow direction. A hydrogeological information system (HIS) of the AQ-2 aquifer was established from a historical reconstruction of the piezometric loads of the drillings and piezometers of different measurement campaigns carried out with the help of geographic information systems (GIS). Two hydrological zones linked to the topography and/or relief of the study area were identified: a coastal plain zone characterized by protrusions and a high depth of the piezometric surface with a hydraulic gradient of 0.03 revealing a lower flow rate and an average permeability that reflects a weaker drainage than recharge; and a plateau zone characterized by a type of hydrogeological structure called hydraulic threshold reflecting a drop in the piezometric surface and hydraulic gradient (0.005 and 0.006) that can be explained by flow rate or permeability variations. This morphology is imposed by two phenomena, on the one hand, the inflow of water from the Hinda plateau and on the other hand, the general axial drainage revealed by the presence of semi-permeable facies. The hydrochemistry of the groundwater shows a low mineralization and a difference in the spatial distribution of major ions that could be explained in part by the nature of the soils of the coastal sedimentary basin and the possible influence of the immediate environment of the structures in probable relation to their poor protection or their proximity to the ocean. The modeling of water flows in an unsaturated environment allowed us to evaluate the recharge of the aquifer in the different ecosystems occupying the recharge zone (Eucalyptus: 400 mm/year and savanna: 520 mm/year). Numerical modeling of groundwater flow in the AQ-2 controlled by the distribution of rock permeabilities and by the configuration of the piezometric surface, which depends on the topography and is controlled by the climate, is established using the modeling program Processing Modflow for Windows (PMWIN). This flow model, which is based on the "hydrogeological environment", gave satisfactory results. In general, the results of the numerical modeling of groundwater flow in the AQ-2, where all assumptions were taken into account, are satisfactory. The permeability calibration led to the identification of three permeability zones related to the different geological horizons. The average recharge of the AQ-2 is estimated around 56.3 mm.year-1
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Submitted on : Tuesday, August 3, 2021 - 8:37:48 AM
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Guy Moukandi N’kaya. Etude Hydrogéologique, Hydrochimique in situ et Modélisation Hydrodynamique du Système Aquifère du Bassin Sédimentaire Côtier de la Région de Pointe-Noire. Hydrologie. Université Marien N'Gouabi (Congo), 2012. Français. ⟨tel-03312879⟩



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