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Caractérisation et modélisation hydrogéologique d'un aquifère en milieu de socle fracturé : cas du site expérimental de Sanon (région du plateau central au Burkina Faso)
Abstract and Figures
Le territoire burkinabè est constitué à plus de 80% de formations de socle. Ces formations renferment des ressources en eau souterraine qui constitue la seule source en eau potable de certaines populations. Cependant, le quart des forages réalisés en vue d’exploiter ces eaux sont négatifs. Une bonne compréhension du fonctionnement hydrogéologique et hydrodynamique des aquifères de socle qui sont très hétérogènes peut aider à résoudre ce problème. L’objectif de la thèse est de proposer un modèle conceptuel hydrogéologique en milieu de socle fracturé soudano-sahélien en combinant différentes méthodes hydrogéophysiques et hydrogéologiques classiques. Le site expérimental de Sanon (environ 14 km2) a été choisi comme site d’étude en raison de sa géologie qui est représentative de certaines formations de socle de l’Afrique de l’Ouest. Les résultats indiquent que le profil d’altération de Sanon est cohérent avec d'autres modèles proposés ces dernières années, suggérant que la conductivité hydraulique des aquifères de socle est une conséquence des processus d'altération plutôt que de la fracturation tectonique. Concernant les propriétés hydrodynamiques, une corrélation linéaire positive d’environ 80% a été obtenue entre les transmissivités et les paramètres RMP. La présence d’un dôme piézométrique dans la vallée centrale a été observée en toute saison. La recharge se fait directement au niveau de ce dôme piézométrique (11 et 13% de la pluie annuelle) et l’alimentation des zones périphériques se ferait plutôt par des transferts latéraux. L’ensemble des informations géologiques et hydrogéologiques a permis de faire une simulation numérique qui reproduit l’écoulement observé.
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... In this sense, the alterite aquifer is defined as heterogeneous, as it is a complex structure due to the presence of discontinuities [32][33][34][35]. In fact, the flow exists on the basis of the presence of fractures in these media dividing the space into a series of permeable masses, known as porous matrices [36,37]. The existing watercourse interacts through the exchange of flows. ...
... The calibration procedure consists in reducing the bias between the observed and simulated values through the adjustment of model parameters, here the hydraulic conductivity and groundwater recharge [36,38,[46][47][48]. In this study, a "trial and error" procedure is used, in which values are manually assigned to the various parameters to achieve an optimal calibration. ...
... This refers first of all to the quantification of the pollutant and its progression from the surface to underground percolation. In this work the steady state hydrodynamic model is defined and coupled to the transport model [36,52]. The preservation of the resource requires a good understanding of the processes of transfer or transport of pollutants. ...
In Burkina Faso, human activities around water points in rural areas affect groundwater resources, which become unfit for consumption. Nearly 33.5% of boreholes are subject to point source pollution. The assessment of the evolution of such pollution should be monitored to assess groundwater quality. In addition, withdrawals for irrigation alone are estimated at 85%, i.e. 46% of the water demand, heightening the deterioration in quality while creating depression zones further leading to an increase in recharge. It is therefore critical to understand the evolution and fate of the transfer of pollutants in such environments. In this study, we aimed to model the transfer of pollutant and predict the future state of pollution using the MT3D-USGS Groundwater Solute Transport Simulator code through the Groundwater Modelling Software (GMS) over the period 2012-2062 (50 years). A mathematical model is further developed through inferential statistics and used as a surrogate model for comparison. The results showed that deterioration in water quality was more attributable to withdrawals, especially for Cyanide (Cn) and Arsenic (As). A rather slow degradation is reported for Lead (Pb), which extends over 22 km, and Fluoride (F), which extends from 4 to 10 km due to localized recharge. A faster degradation for Cn over a distance of 2 to 16 km and as from 3 to 11 km is also observed because of the geological setting of the subsoil. These results might assist decision-makers for the quantitative and qualitative management of groundwater resources, and the management of the basement aquifer in the area through the establishment of protection zones.
... The section of electrical resistivity makes it possible to perceive a very resistant environment characterized by electrical resistivities greater than 400 ohm.m, in line with the presence of laterite from the surface. In the geological context of crystalline basement, beyond 400 ohm.m, we have to deal with the presence of massive rocks of relatively low porosity and whose alteration is not deep [19][20]. At the east of this profile, however, there are formations of lower resistivity below 400 ohm.m which are more characteristic of alterites. ...
... The application of Electrical Resistivity Tomography in Burkina Faso has already proven itself in the sense that it made it possible to image geological formations and to explain the behavior of aquifers in a crystalline basement environment [21][22][23]. The combination of the Wenner α and Wenner β devices for electrodes spaced 5 m apart and used by Soro [19]; Outoumbe has allowed to observe the presence of discontinuities over an investigation depth of 60 m, and to well describe the alteration profile of hard rock aquifers [22]. Geological conceptual models have been proposed on this basis facilitating the comprehension of such media. ...
... The ratio of outliers after data processing is less than 1% for all electrical resistivity sections: the data can be considered good at the end of this step. Furthermore, the RMS values obtained ranging from 1.83% to 4.37% are low compared to those obtained by Soro [19] which vary between 4.8% and 13.2%. Such values attest the quality of the models obtained. ...
Throughout Burkina Faso's road network, some roadways have experienced subsidence or collapse following pipe ramming works conducted for the laying of pipes such as drinking water supply networks. When such works are conducted, it is difficult to make a diagnosis of the properties of the formations underlying the road embankment because a destructive sounding would lead to expensive and tedious repairs. In this present study, a geophysical method, namely electrical resistivity tomography has been used to image the structure and the geometry of these formations so as to anticipate potential disasters. Four electrical resistivity profiles were conducted near the insertion and receiving pits, parallel to the national road N°4, at the exit of the capital Ouagadougou. The strategy of prospection has allowed to image down to an investigation depth of approximately 10 m. The study showed that at an average depth of 2 m, an environment of very low electrical resistivity (about 50 ohm.m) is observed in a very resistant environment. This conductive environment corresponds to the presence of a porous and very wet material which extends laterally and in depth under the roadway, and which can lead to a subsidence or a collapse of this roadway on the surface. ARTICLE HISTORY
... The thicknesses of the alterites are relatively significantly thicker in the central valley (30 to 50 m) and progressively decrease towards the catchment ridges [45]. [48] and Soro et al. [39]). S1CN, S1CNP, S1, S2, S11, and SaG are observation wells. ...
... The Sanon experimental site is equipped with 16 functional observation wells (Figure 1d), 7 of which (S1CNP, S2, S2BIS, S3, SA, S18, S19) capture only the saprolite (shallow reservoirs), 5 (S1CN, S1, S5, S11, S11P) capture only the fractured layers (deep reservoirs), and 4 (S1Bis, S8, S16, SaG) both reservoirs. The [48] and Soro et al. [39]). S1CN, S1CNP, S1, S2, S11, and SaG are observation wells. ...
... Over the CM and CG plots, the cultivation practices are flat plowing perpendicular to the slope (to retain surface runoff). Such practice is predominant in the entire watershed [48]. The observed differences could be due to both the effect of land use and location on the hydrographic network. ...
Groundwater is a vital water supply for local populations and ecosystems globally. With the continuous population growth, the anthropic pressure on groundwater is ever increasing, thus reducing the amount of available water resource. Yet, estimating the impact of anthropogenic activities on aquifer recharge is still a significant challenge for research, especially in basement aquifers. This study aims to improve the actual knowledge of deep drainage and deep aquifer recharge pathways and dynamics in the basement as affected by land use/land cover (LULC). The methodology used in this study accounted for hydraulic processes in soil layers within both unsaturated and saturated zones in an integrated approach. An experimental setup consisting of three (3) experimental plots, respectively under natural vegetation (NV), cropped millet (CM) and cropped groundnut (CG) on which deep drainage was monitored during the years 2020 and 2021. The results show significant differences between the LULC types after two years of implementation. Deep drainage is improved under CM and CG plots located in the central valley, as compared to the NV plot located in the ridge zone. Deep drainage is estimated at 8%, 24% and 25% of the annual rainfall, respectively for NV, CM and CG. The ratio between the recharge value obtained by the water table fluctuation (WTF) method and the deep drainage tends to 1 for the CM and CG plots, highlighting a rapid water transfer between unsaturated and saturated zones. The central valley, which seems to be a preferential recharge pathway, provides promising insights under specific conditions for the implementation of artificial recharge infrastructures.
... Water is an essential element, even primordial to life on earth. The history of water and that of men are closely linked [1]. Just like air, drinking water is vital; having it in sufficient quantity and quality contributes to maintaining health and directly contributes to the quality of the environment [2]. ...
... The calculation of the ionic balance (IB) is done by checking the scale of anions and cations. In theory, the sum of anions expressed in meq/L must correspond exactly to the sum of cations also expressed in meq/L [1]. A perfectly balanced ionic balance is rarely obtained. ...
... According to [20], in practice, the choice of geophysical methods depends on: ...
... In the context of groundwater research, the parameters sought concern the reservoir: its position, its geometry and the quality of the water it contains [20]. ...
in English
Clean drinking water supply is a major concern to the population of the urban municipality of Boké, Republic of Guinea. This study is aimed to investigate the favourable geological structure for the accumulation of groundwater in Boké. Apparent resistivity data was collected by using the Schlumberger-type vertical electrical sounding technique. The apparent resistivity values were obtained on a bi-logarithmic scale in which the distances AB/2 were plotted on the abscissa and the resistivities are on the ordinate. It found the number of terrains and their characteristics (resistivity and thickness) in the area and the behaviour of the current in the soil through a curve. The results show that the structures favourable to the accumulation of groundwater were fractured dolerites, cracked shales and cracked or crushed sandstones. The sandstones were the most important in terms of the amount of water. They are located at a depth of more than 100 metres. It was also found that dolerites and shales are located at shallower depths (less than 100 m).
... Because of the sparse water infrastructure these areas have, combined with highly erratic rainfall, aquifers have become increasingly relied upon for agricultural, domestic and pastoral use [4,5]. Therefore, monitoring the spatiotemporal dynamics of groundwater levels becomes crucial for managing water policies and for planning in anticipation of water shortages [6,7]. In crystalline basement environments, such as those derived from the Pan-African basement, aquifers are discontinuous and primarily controlled by secondary porosity resulting from weathering, fracturing, and structural discontinuities [8,9,10]. ...
This study models the piezometric surface of Pan-African basement aquifers in the Sudano-Sahelian zone of Cameroon using a combined 2D and 2.5D geostatistical approach based on kriging. The integrated analysis of the thalweg surface (Zr) and the piezometric surface (Zp) provides insight into groundwater flow dynamics in a fractured basement context. Results indicate a preferential groundwater flow direction from southwest to northeast; both regional topography and underlying structural features influence this. The comparison between Zp and Zr reveals critical hydrogeological patterns: areas where Zp closely matches Zr (ΔZ < 5 m) indicate discharge zones or resurgence areas, typically near thalwegs. In contrast, sectors with large Zp–Zr differentials (> 40 m) represent deep recharge or long-term storage zones. For instance, in upstream areas, Zp reaches 680 m while Zr remains around 630 m, yielding saturated thicknesses exceeding 50 m. The maximum observed piezometric gradient is approximately 26%, confirming sharp vertical transitions linked to structural controls. A strong empirical correlation (Rp² = 0.9961) between Zp, Zr, and the digital elevation model (DEM) enabled the formulation of a predictive equation for estimating theoretical piezometric levels. The kriged 2.5D surface further reveals local discontinuities, interpreted as fractures or lithological contacts affecting vertical and lateral flows. This modeling framework represents a robust decision-support tool for sustainable groundwater resource management. It helps in identifying optimal drilling targets in structurally favorable zones and helps monitor vulnerable areas at risk of seasonal depletion or contamination. The approach strengthens hydrogeological planning in tropical basement environments increasingly impacted by anthropogenic and climatic pressures.
... To this we can add that the method used as the Schlumberger can lead to discrepancies. Indeed, this is confirmed by the results of the synthetic modeling which concludes that the Wenner device is more suitable and produces less deviation [16]. However, it has limitations in estimating the depth of fissured zones and its implementation. ...
... Compared to neighboring sedimentary aquifers, the conceptualization of the recharge pathways at local and regional scales in the weathered rock aquifer of the Sahel is more challenging as these aquifers are discontinuous and heterogeneous. From recent results, a typical weathering profile of weathered rock aquifer in the Sahel and other tropical regions is composed of the fissured or fractured horizon overlain by a clayey saprolite layer (Dewandel et al. 2006;Lachassagne et al. 2014Lachassagne et al. , 2021Soro 2017). The saprolite is often overlain by a laterite layer with variable hydraulic conductivity ranging from 10 -7 to 4 × 10 -3 m/s (Bonsor et al. 2014). ...
Groundwater recharge rate for weathered rock aquifers in the Sahel of West Africa is sparse due to insufficient knowledge of the recharge pathways. This study was initiated in the Tougou ephemeral river catchment (37 km2) to improve understanding of recharge pathways to a weathered rock aquifer in the dryland of Burkina Faso. It is based on field investigations conducted in 2016–2020 on a piezometric transect and large-diameter wells, perpendicular and longitudinal to the river, respectively. Geophysical measurements, lithologs analysis, permeability tests, groundwater and river level monitoring, and rainfall measurements were undertaken. The results showed a laterally continuous saprolite aquifer (10–22 m thick) overlain by residual soils beneath the river and discontinuous laterite on the banks. Below the river, the saprolite overlays the weathered schist aquifer with an upward or downward hydraulic gradient regardless of the seasons. The saprolite is replenished by focused recharge from the river and diffuse recharge from the rainfall on the banks, according to information from the groundwater hydrographs, groundwater tracing results, and cross-correlation between the groundwater levels and the two potential recharge sources (rainfall and river water). The low hydraulic conductivity, electrical resistivity tomography profiles and lithologs indicate restriction of diffuse and focused recharge through soil matrix infiltration, suggesting there are preferential recharge pathways. For the 2018–2020 period, the water-table fluctuation method provides recharge estimates of 14–51 mm/year on the northern bank, 10–36 mm/year on the southern bank and 148–194 mm/year in the riverbed.
L’objectif de ce travail est de produire la carte de fracturation de la zone étudiée et identifier les failles porteuses de minéralisation d’eau à travers l’usage couplé de télédétection et des SIG. Pour ce faire, l’image Landsat OLI et SRTM sont utilisées pour visualiser, modéliser le bassin versant et ressortir les structures linéamentaires à travers la combinaison de la méthode automatique et l’extraction manuelle par filtrages directionnels. A l’issue des différentes opérations ayant conduit à la validation de la carte de fracturation, il ressort que le bassin versant amont Bénoué est une zone fortement fracturée avec une densité de fracturation comprise entre 3,03 km/Km2 et 8,63 km/Km2. Une campagne d’identification et de collecte des fiches des ouvrages hydrauliques a permis de recenser et géolocaliser 63 puits et 40 forages. Les opérations de traitement ont été effectuées sur les données des ouvrages et 12 forages ont retenus l’attention dans cette étude. A cela s’ajoute les opérations effectuées sur des images pour une meilleure connaissance des aquifères du bassin. Les fiches techniques des ouvrages sont exploitées pour déterminer les principaux paramètres hydrauliques essentiels et ont servi dans l’identification des failles productives en eau. Le calcul de débits d’exploitation et la transmissivité des ouvrages, superposés sur les fractures ou les zones de densité de fracturation élevée ont permis d’identifier, de cartographier et de classer les structures par ordre d’importance. Les résultats de ces calculs a permis de constater que les transmissivités du bassin oscillent entre 0,017 et 0,178 m2/h. Elles illustrent à souhait que les aquifères du socle soudano sahélien du Cameroun ont une forte fonction conductrice. This work aims to produce a fracturing map of the study area and identify faults carrying water mineralization through the coupled use of remote sensing and GIS. To achieve this, Landsat OLI and SRTM imagery were used to visualize and model the watershed and highlight lineamentary structures, through a combination of automatic methods and manual extraction using directional filtering. At the end of the various operations leading to the validation of the fracturing map, it emerged that the upstream Bénoué watershed is a highly fractured zone, with a fracture density ranging from 3.03 km/Km2 to 8.63 km/Km2. A campaign to identify and collect data on hydraulic structures identified and geolocated 63 wells and 40 boreholes. Processing operations were carried out on the data from the structures, and 12 boreholes received special attention in this study. In addition, image processing was carried out to gain a better understanding of the basin's aquifers. Borehole data sheets were used to determine the main hydraulic parameters and to identify water-bearing faults. Calculations of operating flow rates and transmissivity of structures, superimposed on fractures or zones of high fracture density, enabled the structures to be identified, mapped, and ranked in order of importance. The results of these calculations show that transmissivities in the basin range from 0.017 to 0.178 m2/h. They illustrate that the aquifers of the Sudano-Sahelian basement of Cameroon have a strong conductive function.
Dryland hydrology in Mediterranean regions - a review
The West African Monsoon gives agricultural activities timetable in the whole West Africa; this is shorter when one is going northward, as well in duration as in rainfall mean annual amount. After a long drought (1968-1995), West Africa experiences since the end of the 20th century a coming back to better rainfall conditions; mean annual rainfall amount of sudano-sahelian belt are similar, in average and in interannual variability, to those observed during the 1900-1950 period. The objective of this study is to determine whether the recent rainfall evolution explains the hydrological and agronomical dynamics observed in West Africa, mostly the increased occurrence of floods and the few recovery of crop yields, although rainfall has sensitively increased. Simple statistical methods are used here in two sub-regions, Senegambia and the Middle Niger River Basin, to highlight the 1950-2013 evolution of the monsoon characteristics which have a hydrological or agronomical interest (annual rainfall, extreme rainfall, date of onset and end and duration of the rainy season). Maybe could we consider that 1900-1950 and 1995-2015 periods should be the “normal range of rainfall”, the 1951-1967 and 1968-1995 being respectively humid and dry periods. Otherwise, an increase in the number of extreme rainfall events, higher than the increase in rainfall amount, is observed. Finally, although the rainy season is nowadays longer than during the dry period (1968-1995), a recent increase in the occurrence of “bad” agronomical rainy seasons is noticed.
L’importance croissante des problématiques environnementales entraîne un regain d’intérêt pour les sols. Les nombreux aléas auxquels ils sont soumis les fragilisent et affectent leurs propriétés. Une gestion et une protection optimales des sols passe par une connaissance approfondie de leur variabilité spatiale, y compris l’identification de zones vulnérables. L’objectif des travaux présentés est d’exploiter les données issues de la prospection électrique, afin d’exploiter le pouvoir discriminant de la résistivité électrique vis-à-vis des différents horizons d’une couverture pédologique, en vue d’en caractériser les propriétés géométriques.Tout d’abord, une modélisation de mesures de résistivité électrique multi-profondeurs a été réalisée en vue 1)de définir une stratégie de prospection optimale, et 2)d’évaluer les performances et limites de la méthode électrique. Cette étape a permis d’évaluer les dimensions minimales détectables d’une hétérogénéité pédologique ainsi que l’impact, sur sa détection, de différents paramètres : espacement inter-profils utilisé lors des mesures, profondeur et épaisseur de l’hétérogénéité, contraste de résistivité entre celle-ci et l’encaissant.L’étude expérimentale d’une tranchée ouverte sur plusieurs mètres, couplant prélèvements de sols et mesure de résistivité, a permis de caractériser la variabilité verticale et latérale des horizons de la couverture pédologique. Des relations entre résistivité électrique et propriétés pédologiques telles que teneur en eau massique, teneur en eau volumique ou masse volumique apparente de sol ont également été établies.Enfin, une méthode originale d’exploitation de données électriques issues d’une prospection multi-profondeurs a été proposée. Basée sur une taxonomie géophysique et pédologique, elle a permis, par la cartographie de profils verticaux de résistivité apparente, de retrouver des délinéations concordantes avec celles d’une carte des sols. Cette approche constitue un outil précieux pour l’étude de la variabilité spatiale de la couverture pédologique.
La caractérisation non-destructive de la variabilité spatiale et temporelle de la teneur en eau pour des milieux hétérogènes superficiels est un enjeu de taille pour la compréhension du fonctionnement hydrogéologique de certaines zones critiques.Pour répondre à ces besoins, une candidate potentielle est la méthode de Résonance Magnétique des Protons (RMP), technique géophysique récente qui permet d'imager la teneur en eau de la subsurface de façon directe. Ses déclinaisons opérationnelles en 2-D et en 3-D sont émergentes et nécessitent une étude exhaustive afin d'en comprendre les possibilités et les limitations.Le caractère intégrateur de la mesure RMP, en conjonction avec l'erreur expérimentale qui la contamine, limite la résolution de la méthode. Conséquence : la traduction d'un jeu de mesures sous la forme d'une distribution spatiale de teneur en eau n'est pas unique. Certaines sont plus désirables que d'autres, aussi des aprioris géométriques sont-ils utilisés pour limiter l'espace des solutions possibles. De ce fait, l'estimation qui est faite de la teneur en eau est biaisée (aprioris) et entachée d'incertitudes (bruit), jamais quantifiées à ce jour à plus d'une dimension.Afin de mettre à jour les processus qui vont influencer la résolution spatiale et l'estimation du volume d'eau, les propriétés du problème direct de l'imagerie RMP sont analysées au moyen d'outils non-biaisés telles que les corrélations.Le problème d'imagerie RMP inverse étant non-linéaire, ce travail de thèse propose une méthodologie de reconstruction d'images de la teneur en eau qui offre l'accès aux analyses des incertitudes, axée sur un algorithme d'échantillonnage inverse de Monte-Carlo (Metropolis-Hastings). Une étude intensive des biais introduits par l'apriori sur la reconstruction de différents volumes 3D est effectuée.Enfin, les possibilités d'imageries RMP sont illustrées sur deux cas d'applications réels en milieux hétérogènes karstiques ou thermo-karstiques, validés par comparaison avec d'autres sources d'informations. Le premier cas est une imagerie 2-D du conduit karstique de Poumeyssens, dont la cartographie est connue.Le deuxième cas est l'imagerie 3-D d'une poche d'eau sous-glaciaire dans le glacier de Têre-Rousse laquelle est explorée par de nombreux forages et d'autres méthodes géophysiques.
Models commonly used for the analysis of hydraulic test data are generalized by regarding the dimension of the flow to be a parameter which is not necessarily integral and which must be determined empirically. Mathematical solutions for this generalized radial flow model are derived for the standard test conditions: constant rate, constant head, and slug tests. Solutions for the less common, sinusoidal test are contained within the general solutions given. Well bore storage and skin are included and the extension to dual-porosity media outlined. The model is presented as a model of fractured media, for which it is most likely to find application because of the problem of choosing the appropriate flow dimension.
The term hard rock was perhaps originally coined by drillers to indicate poor drillability of these rocks. Hard rocks are characterized by insignificant primary (intergranular) porosity and primary permeability. However, weathering and fracturing can impart secondary porosity and permeability to varying extent. Usually, crystalline (igneous and metamorphic) rocks are included in this group. As the hydraulic properties of these rocks are mainly controlled by fracturing, these are also referred as fractured rocks. Unlike sedimentary rocks, the hard rocks generally represent anisotropic and heterogeneous media.