A hydrodynamic survey carried out in semiarid southwest Niger revealed an increase in the unconfined ground water reserves of approximately 10% over the last 50 years due to the clearing of native vegetation. Isotopic samplings (3H, 18O, 2H for water and 14C, 13C for the dissolved inorganic carbon) were performed on about 3500 km2 of this silty aquifer to characterize recharge. Stable isotope analyses confirmed the indirect recharge process that had already been shown by hydrodynamic surveys and suggested the tracers are exclusively of atmospheric origin. An analytical model that takes into account the long-term rise in the water table was used to interpret 3H and 14C contents in ground water. The natural, preclearing median annual renewal rate (i.e., recharge as a fraction of the saturated aquifer volume) lies between 0.04% and 0.06%. For representative characteristics of the aquifer (30 m of saturated thickness, porosity between 10% and 25%), this implies a recharge of between 1 and 5 mm/year, which is much lower than the estimates of 20 to 50 mm/year for recent years, obtained using hydrological and hydrodynamic methods and the same aquifer parameters. Our study, therefore, reveals that land clearing in semiarid Niger increased ground water recharge by about one order of magnitude.

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A mathematical model is applied to the Aoste Valley (Italy), which is a good example of the hydrological workings of alluvial rock basin aquifers in mountainous regions. The course of the Dorea Baltea follows the valley, which between the altitudes of 500 and 600 m is dominated by summits reaching more than 3000 m. The lacustrine- and fluviatile-type sedimentary deposits are accompanied by several alluvial cones that, due to their varying nature, confer very diverse hydrodynamic characteristics to the land. In this particular context, the source and drainage of the Dorea control the hydrodynamics of the aquifers and influence the spatial dispersion of the physicochemical properties of the groundwater. The model also led to quantify and determine the zones and types of exchanges with the river. Simulation of lowering the water table in the river showed the variable sensitivity of the water table piezometry with various spatial impacts. Finally, the stream–aquifer impact is highlighted by the spatial evolution of the sulphate contents coming from deep lateral sources. Taking into account the stream–aquifer exchanges in an underground-flow mathematical model gives a better understanding of the workings of the valley aquifers and thus their management, especially for the development of watercourses in mountainous regions.[Journal translation]

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