The problem of soil degradation through alkalinization/salinization in an irrigated area with a semi-arid climate was examined in the inner delta of the Niger River, Mali, by the study of groundwater hydraulics and hydrochemistry in an area recharged by irrigation water. On the basis of data analysis on various scales, it is concluded that the current extent of the surface saline soils is due to a combination of three factors: (1) the existence of ancient saline soils (solonchaks) resulting from the creation of a broad sabkha west of the former course of the Niger River, now called the Fala of Molodo. These saline crusts were gradually deposited during the eastward tilting of the tectonic block that supports the Niger River; (2) the irrigation processes during the recent reflooding of the Fala of Molodo (river diversion in 1950). These used very poorly mineralized surface water but reintroduced into the alluvial groundwater system -generally of a low permeability (Kp10 -6 m s -1 ) -salts derived from the ancient solonchaks; and (3) the redeposition of the dissolved salts on the surface due to the intense evapotranspiration linked to the present Sahelian climate. In this context, only efficient artificial draining of subsurface alluvial groundwater can eliminate most of the highly mineralized flow and thus reduce the current saline deposits.Résumé En périmètres irrigués sous climat semi aride (delta intérieur du fleuve Niger, Mali), la dégradation des sols par alcalinisation/salinisation est abordée par l'analyse hydrodynamique et hydrogéochimique des eaux souterraines sous-jacentes. A partir d'informations recueillies à différentes échelles, nous proposons un mode de mise en place actuelle des salants en surface sur la base de trois concepts: (1) l'existence de salants anciens résultant de la création d'une vaste zone de sebkhra à l'Ouest du cours ancien du Niger (Fala de Molodo). Cette mise en place se serait faite progressivement au cours du basculement vers l'Est du bloc tectonique supportant le fleuve; (2) au cours de la remise en eau récente du Fala (1950), les processus d'irrigation à partir d'eau de surface très peu minéra-lisée, réintroduisent dans la nappe alluviale globalement peu perméable des sels provenant des mises en solution des salants anciens; (3) par phénomène d'évapotranspiration intense lié au climat actuel, ces sels se redéposent en surface. Dans un tel contexte, seul un drainage artificiel efficace des eaux souterraines de sub-surface (nappe alluviale) permettrait de soustraire le maximum de flux très minéralisé et, en conséquence, de limiter les dépôts salins actuels.Resumen En perimetros irigados bajo un clima arida (delta interior del rio Niger, Mali), la degradacion de los suelos (alcalinisacion/salinisacion) es analizada con respecto a las aguas subterraneas. A partir de recoleccion de dadas en hidrodinamica, geofisica y hidroquimica, proponemonos un modo de formacion actuale de los salinos segun tres episodios sucesivos: (1) salinos antiguos que se deducen de la formacion de una Sebkh...
In Madagascar, evidence of a rift neotectonics oriented north–south is shown and analyzed on microtectonic, kilometric, and regional scales. This phase, reported for the first time, influences significantly the water flow patterns and the following modern processes: directional control of the hydrographic system; local changes in base water levels; development of zones of active erosion and sedimentation; opening and (or) reopening of drainage fractures in karstic systems; typological diversification of some depressions (lowland floodings); hydraulic transfer between surface and subsurface controlled by preferential major axes of underground water drainage. [Journal Translation]
. Le caractère bicouche de l'aquifère est précisé, en particulier par la mise en évidence de drainances verticales dont le sens dépend de l'époque de la recharge. L'infiltration des eaux de pluie sur les interfluves altéritiques est rapide, sans evaporation, la réalimentation de la nappe étant très tamponnée par la zone non saturée. Durant les trois cycles climatiques, le suivi piézo-métrique dans des forages montre que les réponses du milieu souterrain au signal pluie sont perceptibles pour des précipitations cumulées inférieures à 100 mm.Groundwater circulation in a weathered crystalline medium: an example in a subtropical humid region at high elevation (Madagascar) Abstract The regime of circulation of groundwater in a weathered crystalline medium (high elevation subtropical humid area) is analysed on a large scale during three successive climatic phases: (a) by performing a hydraulic study of the medium (pumping and piezometric tests) under natural flow; and (b) by collecting isotopic data ( ls O and 2 H). The bilayered structure of the aquifer is ascertained. In particular, it is shown that there are vertical drainages running upwards or downwards depending upon the time of the recharge. Rain water percolation through the alteritic interfluves is fast, without evaporation, the saturated zone playing the role of a buffer regarding recharge of the water table. During the three climatic cycles, monitoring of the water level by piezometric measurement was carried out, and shows that the underground medium responds to rainfall for precipitations less then 100 mm.
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