Problématique de l’accès à l’eau potable dans la ville de Ngaoundéré (Centre Nord-Cameroun)

Des enquêtes de terrain menées dans la ville de Ngaoundéré ont permis de constater que plus de 70% de personnes sur une population estimée à environ 200 000 habitants, n’ont pas accès à l’eau potable à travers le réseau de distribution de la Société Nationale des Eaux du Cameroun (SNEC). La situation est particulièrement préoccupante dans les quartiers périurbains dominés par l’habitat spontané. Ces quartiers, de plus de 5000 habitants, devraient bénéficier de réseaux d’adduction d’eau de la SNEC en lieu et place des solutions de type hydraulique rurale. En effet, les sources et les puits peu profonds y sont prédominants. Ils sont traditionnellement mis en œuvre malgré leur inadaptation et la très mauvaise qualité de l’eau qui y est associée. Sur 397 personnes enquêtées dans les quartiers périurbains, 243 (soit plus de 60%) affirment souffrir régulièrement de maladies d’origine hydrique telles que la typhoïde, la dysenterie amibienne, la diarrhée.... Quinze échantillons d’eau analysés présentent des pH acides (en moyenne 4), des concentrations en matières en suspension variant de 0,65 à 3,42 mg/l et des turbidités comprises entre 1,7 et 43,15 N.T.U

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

Influences of lithology on water quality: a study of the Ngam and Mou watersheds in West Cameroon (Central Africa)

The Ngam and Mou Rivers belong to the river system of the central part of the Cameroon Volcanic Line. The Ngam River watershed comprises 51 Ma old basalts, while the Mou River watershed comprises younger, \u3c 5 Ma pyroclastic basalts. The physico-chemical characteristics of water samples from both watersheds indicated pH between 4.2 and 8.2, and 4.2 to 7.5, and very low mineralization between 11 and 246 μS/cm and 45 to 165 μS/cm, respectively, for the Ngam River and the Mou River. Chemical data showed that waters from old, highly weathered plateau basalts (Ngam River watershed) were dominated by calcium and sodium (signatures of feldpars from syenites and orthogneisses), while waters in the Mou River watershed were dominated by magnesium (signature of olivines and pyroxenes from basalts). Mineralizations in the two watersheds are thus primarily controlled by lithology rather than anthropic activities. These results are promising for identifying potential potable water resources in these areas

LITHOFACIES AND DEPOSITIONAL ENVIRONMENT OF VOLCANO-SEDIMENTARY DEPOSITS OF SE MAYO OULO BASIN

International audienceThe study of volcano-sedimentary deposits of Mangbai in SE Mayo Oulo basin-Cameroon leads to the identification of five major distinct lithofacies types. From the coarset to the finest lithofacies, there are: Gb-clast supported by massive coble and boulder, Gg-granule to massive gravel, Sm-massive sand, Shhorizontal layered sandstones, Fml-massive laminated claystones. Coarse facies (Gb, Gg, Sm) are cemented with volcanic matrix while the finest facies (Sh, Fml) are normal sedimentary compacted deposits. The coarsest facies distribution is identified on the top SE of the mountain while the medium to finest facies deposits are located at the base of the sequence. The formation of the volcano-sedimentary deposits of Mangbai are associated to the development and establishment of Africa major rift system while sedimentary deposits in the Mayo Oulo basin are more recent and date from Lower Cretaceous

Le réseau Ampacs : "Analyse multi-échelle de la pression anthropique et du climat du Sahel".

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