Étude et modélisation de la formation des dépressions piézométriques en Afrique sahelienne

Parmi les différentes théories généralement considérées pour expliquer la formation et l'entretien des dépressions piézométriques régionales : surexploitation ; drainage profond ; tectonique subsidente ; réajustement du niveau marin ; reprise évapotranspiratoire, seule la dernière semble pouvoir s'appliquer à l'ensemble des cas décrits en zone sahélienne. Les études hydrogéologiques en cours dans cette région, et notamment les résultats des isotopes de l'environnement, mettent en effet en évidence le rôle prépondérent des transferts verticaux sur les écoulements latéraux.Le schéma conceptuel présenté est basé sur les hypothèses suivantes: considérant une nappe libre alimentée par ses bordures, en milieu à faible perméabilité ; (1) il existe, en tout point de la nappe, un déficit entre l'infiltration saisonnière et la reprise évapotranspiratoire ; (2) infiltration, évapotranspiration et déficit décroissent de façon exponentielle avec la profondeur de la nappe ; (3) la piézométrie actuelle est le résultat d'une évolution commencée depuis la fin d'une période humide correspondant au remplissage maximun de la nappe (dernier pluvial Holocène pour les grandes dépressions sahéliennes).Dans un premier temps, la validité de ce schéma est testée à l'aide d'un modèle analytique simple prenant en compte les caractéristiques climatiques (évolution temporelle du déficit hydrique), les propriétés hydrodynamiques (perméabilité, coefficient d'emmagasinement) et le « pouvoir évaporatoire » du sol (« facteur de forme » k). Ce dernier, expression principalement de la texture du terrain, apparaît logiquement comme le paramètre le plus sensible pour le modèle.Un modèle transitoire unidimensionnel est ensuite présenté afin de simuler la baisse piézométrique entre deux limites à potentiel constant. La surface piézométrique obtenue semble en accord avec les données observées sur le terrain.The discovery of the large piezometric depressions in the Sahelian zone dates back to the 1950s, when the early hydrogeological studies took place in West Africa (DEGALLIER,1954 ; ARCHAMBAULT, 1960). Since then, numerous examples have been described throughout the region (aquifers of Trarza in Mauritania ; Ferlo in Senegal ; Gondo, Nara, Azaouad in Mali ; Kadzell in Niger ; Yaere in North Cameroon...), raising what was, certainly, the major enigma in Sahelian hydrogeology (fig. 1).So far, none of the different theories put forward to explain this phenomenon has met with general agreement : (1) Overexploitation cannot be involved due to the insignificant pumping in comparison to the reserves. (2) Geological subsidence due to neotectonic causes would never be active enough in this stable part of the African shield. (3) Drainage to a deeper aquifer is impossible to - consider in this particularly flat area. (4) Changes in sea water level may explain the piezometric evolution of the coastal aquifers (DIENG, 1987) but cannot be considered for the whole Sahelian zone. (5) Evaporation and transpiration losses were often considered as insufficient to generate deep piezometric depressions. However, only this last hypothesis seems to fit the whole of the hydrogeologic and climatic Sahelian conditions and will therefore be considered as the governing factor in this paper.Evidence of a vertical water deficitThe major role of evaporation and transpiration processes in the formation of the piezometric depressions is deduced from various data : (1) the geographical distribution of the « depressed aquifers », which is in accordance with the regional isohyets, suggesting that climatic factors are compulsory conditions ; (2) the piezometric measurements (figs.2, 3), which show a deficit between direct aquifer recharge by precipitation and evapotranspiration losses during the dry season ; (3) the environmental isotopic results, which demonstrate that vertical water movements prevail over lateral transfers (fig.4). Furthermore, evaporation effects and plant uptake may still be noticeable at relatively great depth : the presence of root activities of Acacia was, for instance, recently proven at a depth of 35m in North Senegal (DUPUIS et DREYFUS, 1989).According to theoretical and experimental results from different approaches (isotopic interpretation of unsaturated soil profiles ; calculation of unsaturated soil suction profiles ; piezometric measurements and mathematical simulations) it can be assumed that steady-state exfiltration from an unconfined aquifer decreases exponentially with water table depth (ARANYOSSY, 1991).Conceptual schemeThe conceptual model presented here (fig. 5) is based on the following assumptions : (1) there is, everywhere, a water deficit (D) between the seasonal infiltration and the evapotranspiration losses; (2) Infiltration, evapotranspiration and water deficit follow an exponential decrease with respect to the water table depth ; (3) the present piezometric profile is the result of an evolution that started at the end of a past period corresponding to a total replenishment of the aquifer (last pluvial Holocene period for the Sahelian region). Furthermore, it is considered that the climatic evolution towards arid conditions is accompanied by a parallel increase of the water Deficit (for simplification, D is assumed to be a linear function of time).Considering a low permeability, unconfined aquifer system, the lateral water movement coming from the imposed constant head recharge zones on the boundaries (lake, river or sea) is not important enough to counterbalance evaporation losses. The water table therefore progressively drops, decreasing the evaporation losses, up to a quasi-steady state where, finally, lateral transfers just compensate the vertical water deficit (ARANYOSSY, 1988).Mathematical simulationIn a first step, a simple analytical model is introduced to check the validity of this hypothesis. Parameters included depend on climatic conditions (evolution of the water deficit D) ; on hydrodynamic characteristics (permeability, storativity) and on the soil aptitude for evaporation (k factor). It follows that : (1) the water table decreases very rapidly during the first millenium and then slows down to a quasi stationary state ; (2) the maximum calculated water table depths correspond to those observed in the field ; (3) the most sensitive parameter is the « k factor » - the value of which depends mainly on the soil texture (fig. 6).A transient numerical model is finally presented to depict, in one dimension, the evolution of an unconfined aquifer between two constant head boundaries. Simulation on an 8 000 year time scale generates a piezometric profile (fig.7) in conformity with the configuration observed in the field.In conclusionThe formation of the Sahelian piezometric depressions results from the conjunction of several compulsory factors : (1) climatic conditions : evolution from a humid to an arid climate accompanied by a vertical water deficit ;(2) geological conditions : low permeability sedimentary formations ;(3) geographical conditions : constant recharge zones on the boundaries of the aquifer

Apport de la géologie, de l’hydrogéologie et des isotopes de l’environnement à la connaissance des «nappes en creux» du Grand Yaéré (Nord Cameroun)

La carte piézométrique de la nappe du Logone-Chari-Tchad met en évidence des anomalies piézométriques interprétées comme des « nappes en creux ». Les informations de l’hydrogéologie et des isotopes de l’environnement conduisent à remettre en question les grandes profondeurs des niveaux statiques observées par certains auteurs dans ces dépressions piézométriques. Les données hydrogéologiques démontrent que dans la zone déprimée de la surface piézométrique, l’aquifère est de type bicouche. Par ailleurs, la distribution des teneurs en isotopes stables (oxygène-18 et deutérium) et en tritium confirme le cloisonnement des aquifères :La relation δ 2H vs. δ 18O montre que les effets d’enrichissement par évaporation lors de la recharge des nappes ne sont très marqués que dans les eaux des nappes superficielles dont les niveaux statiques ne dépassent pas 20 m de profondeur. Les dépressions fermées dont les points les plus bas atteignent 60 m sous la surface du sol s’interprètent difficilement dans l’hypothèse d’une reprise évaporatoire.Il résulte de cette étude que l’absence de dépendance nette entre les niveaux piézométriques superficiels et les niveaux profonds place le problème des anomalies piézométriques du Grand Yaéré dans un contexte totalement différent de celui des anciennes interprétations qui s’appuyaient sur l’hypothèse d’une nappe libre généralisée monocouche. À l’avenir, la construction de la carte piézométrique de la nappe du Logone-Chari-Tchad devra tenir compte de la structure des deux nappes superposées.Piezometric depressions, common in sub-Saharan Africa, are major hydrogeological anomalies manifested by closed curves, pronounced hollows and dips attaining several tens of meters below the regional water table level. The Logone-Chari-Chad piezometric map reveals piezometric anomalies that have been interpreted as depressed aquifers. The depth of the water table is 60 m in the Tagawa-Am Talia axis, 40 m between Louba-Louba and Andirni and 30 m around Yagoua. Factors linked to evaporation are generally thought to be responsible for these depressed zones.The objective of this study (based on the saturated zone) was: 1) to place the Logone-Chari-Chad piezometric anomalies in their hydrogeological settings, and 2) to evaluate the use of environmental isotopes to explain their formation processes. To achieve our goal, 27 water supply points (8 boreholes and 19 wells) were selected from the borders and centre of the Logone-Chari-Chad depression. Samples were collected between 1989 and 1991. Measurements performed in the field involved static water levels, whereas the laboratory analyses 18O, 2H and 3H were performed at the International Atomic Energy Agency (IAEA) laboratory in Vienna, within the framework of the project RAF/8/012 funded by IAEA.The new geological and hydrogeological data demonstrate that in the depressed zone of the piezometric surface, the aquifer has two layers. In contrast, the Logone-Chari-Chad piezometric map was previously drawn considering the aquifer as a single-layer. From a hydrochemical point of view, the groundwater in the Logone-Chari-Chad aquifer is stratified: calcium bicarbonate type water was found at the surface (shallow groundwater), whereas sodium carbonate type water was found at depth (deeper groundwater).Seasonal piezometric fluctuations of 1.5 to 3 m have been observed in the shallow groundwater. In the deeper groundwater, they range from 0.20 to 0.30 m. The difference in the values of water table fluctuation leads not only to variations in the mode of groundwater circulation, but also to variations in the hydrodynamic properties of aquifers, such as transmissivity.The distribution in stable isotope contents (18O, 2H and 3H) confirmed the compartmentalization of aquifers. The correlation between 3H and 18O showed that there are two water types, with different recharge modes and episodes. On the border of the depression, shallow groundwater pinches out on the semi-permeable substratum, resulting in a tritium content greater than 4 UT. In the depression axis, there is deeper groundwater with a tritium content below 4 UT.The relationship between 2H and 18O shows that the enrichment effects of evaporation at the time of recharge are very pronounced only in the shallow groundwater, where the static level does not exceed 20 m below the soil surface. The closed piezometric depressions, whose deepest point attains 60 m below the soil surface, cannot be explained by the presence of intense evaporation. The variation in tritium content with respect to the static level shows that in the depressed zone, the first 20 m are characterized by a tritium content greater than 4 UT, whereas at depths of 30 m or more, tritium contents are lower than 4 UT.The absence of dependence between shallow and deep piezometric levels invalidates the interpretation of great water depths proposed in previous studies of the piezometric depression of the Logone-Chari-Chad water table. Thus, the hypothesis that the Logone-Chari-Chad is a single-layer system should be abandoned. The future construction of the piezometric map of the Logone-Chari-Chad water table should take into account the structure and lithology of the two superimposed layers

Cellular Barcoding Identifies Clonal Substitution as a Hallmark of Local Recurrence in a Surgical Model of Head and Neck Squamous Cell Carcinoma

Local recurrence after surgery for head and neck squamous cell carcinoma (HNSCC) remains a common event associated with a dismal prognosis. Improving this outcome requires a better understanding of cancer cell populations that expand from postsurgical minimal residual disease (MRD). Therefore, we assessed clonal dynamics in a surgical model of barcoded HNSCC growing in the submental region of immunodeficient mice. Clonal substitution and massive reduction of clonal heterogeneity emerged as hallmarks of local recurrence, as the clones dominating in less heterogeneous recurrences were scarce in their matched primary tumors. These lineages were selected by their ability to persist after surgery and competitively expand from MRD. Clones enriched in recurrences exhibited both private and shared genetic features and likely originated from ancestors shared with clones dominating in primary tumors. They demonstrated high invasiveness and epithelial-to-mesenchymal transition, eventually providing an attractive target for obtaining better local control for these tumors