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In a semi-arid to arid climate context, dependency on groundwater resources may lead to overexploitation and deterioration of water quality. The Continental Intercalaire (CI) aquifer is one such continentalscale aquifer (more than a million of km(2)), which is mainly confined, poorly recharged but intensely abstracted. To date, the management of this resource relies on hydrogeological modelling and key parameters such as recharge/discharge rate and groundwater dynamics. We use a combination of residence time indicators (C-14, Cl-36, He-4) and stable isotopes of water (H-2 and O-18) to give greater constraint on the groundwater residence time in the CI. In previous studies, C-14 measurements and steady state modelling indicate a residence time of less than 100 ka whereas in others, Cl-36 measurements and transient scenarios modelling suggest a longer residence time (>500 ka). In this study, most of the C-14 measurements are below the limit of detection, establishing residence times greater than 40 ka and confirming the necessity of strict sampling protocols to exclude all air and AMS measurements when low C-14 concentrations are expected. In the Tunisian recharge area, detectable C-14 indicate sporadic recharge episodes (3-7 ka and 29-43 ka), whereas He-4 and Cl-36 concentrations in central areas suggest very old (

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Le Sahel d’Oualidia fait partie du littoral atlantique. Il borde la lagune d’Oualidia, avec un climat dominant de type semi-aride. Dans cette zone, les aquifères carbonatés de grès calcaire plio-quaternaire et du calcaire de Dridrate (Hauterivien supérieur), sont en relation directe avec l’océan atlantique. Ces aquifères subissent des prélèvements par pompage destinés à l’irrigation de cultures maraîchères et à l’alimentation de la population rurale. Par conséquent, ils sont menacés par des problèmes de surexploitation et de salinisation par intrusion marine. De nombreuses questions sont actuellement posées sur l’importance des ressources souterraines exploitées et sur leur état de salinité. Dans le cadre de cette étude, l’utilisation des isotopes stables 18O et 2H, a permis de mettre en évidence la recharge des aquifères qui se fait par des pluies d’origine atlantique. Les eaux souterraines sont localement affectées par des intrusions d’eau océanique. La datation de certains échantillons par le 14C a permis de révéler des âges anciens. D’autres analyses seront réalisées pour vérifier ces hypothèses. El Sahel de Oualidia se encuentra en el litoral atlántico. Está en el borde de la laguna de Oualidia con un clima predominantemente de tipo semi-árido. En esta zona, los acuíferos carbonatados de las areniscas calcáreas plio-cuaternarias y de la caliza de Dridrate (Hauteriviense superior) están relacionados directamente con el Océano Atlántico. Estos acuíferos están sometidos a descarga por bombeos utilizados para el riego de los cultivos de las marismas y para el abastecimiento de la población rural. Por ello, están amenazados por problemas de sobreexplotación y de salinización por intrusión marina. Se plantean actualmente numerosas cuestiones sobre la importancia de los recursos subterráneos explotados y sobre su estado de salinización. Dentro del marco de este estudio, la utilización de isótopos estables de 18O y 2H permite poner en evidencia la recarga de los acuíferos que se efectúa por las lluvias de origen atlántico. Las aguas subterráneas se ven localmente afectadas por intrusiones de agua oceánica. La datación de algunas muestras por el 14C demuestra la existencia de edades antiguas. Se realizarán otros análisis para verificar estas hipótesis.

Le Sahel d’Oualidia fait partie du littoral atlantique. Il borde la lagune d’Oualidia, avec un climat dominant de type semi-aride. Dans cette zone, les aquifères carbonatés de grès calcaire plio-quaternaire et du calcaire de Dridrate (Hauterivien supérieur), sont en relation directe avec l’océan atlantique. Ces aquifères subissent des prélèvements par pompage destinés à l’irrigation de cultures maraîchères et à l’alimentation de la population rurale. Par conséquent, ils sont menacés par des problèmes de surexploitation et de salinisation par intrusion marine. De nombreuses questions sont actuellement posées sur l’importance des ressources souterraines exploitées et sur leur état de salinité. Dans le cadre de cette étude, l’utilisation des isotopes stables 18O et 2H, a permis de mettre en évidence la recharge des aquifères qui se fait par des pluies d’origine atlantique. Les eaux souterraines sont localement affectées par des intrusions d’eau océanique. La datation de certains échantillons par le 14C a permis de révéler des âges anciens. D’autres analyses seront réalisées pour vérifier ces hypothèses.El Sahel de Oualidia se encuentra en el litoral atlántico. Está en el borde de la laguna de Oualidia con un clima predominantemente de tipo semi-árido. En esta zona, los acuíferos carbonatados de las areniscas calcáreas plio-cuaternarias y de la caliza de Dridrate (Hauteriviense superior) están relacionados directamente con el Océano Atlántico. Estos acuíferos están sometidos a descarga por bombeos utilizados para el riego de los cultivos de las marismas y para el abastecimiento de la población rural. Por ello, están amenazados por problemas de sobreexplotación y de salinización por intrusión marina. Se plantean actualmente numerosas cuestiones sobre la importancia de los recursos subterráneos explotados y sobre su estado de salinización. Dentro del marco de este estudio, la utilización de isótopos estables de 18O y 2H permite poner en evidencia la recarga de los acuíferos que se efectúa por las lluvias de origen atlántico. Las aguas subterráneas se ven localmente afectadas por intrusiones de agua oceánica. La datación de algunas muestras por el 14C demuestra la existencia de edades antiguas. Se realizarán otros análisis para verificar estas hipótesis.

International audience; Past variations of the recharge is estimated in the Atlas Mountains, the main recharge area of the Continental Intercalaire (CI) aquifer, one of the major Saharan aquifers, over the last 775 kyr. In this Mediterranean climatic context, continental archives generally record single humid events, grouped in hypothetical time interval and do not offer continuous chronicles of precipitation. We propose to use spatially distributed image data as a temporal constraint, to infer the past recharge in the Atlas area. Based on a simplified but robust climatic scenario, assuming a piston model, we apply a Markov Chain Monte Carlo (MCMC) inversion approach to attribute a specific recharge to the last nine interglacial periods and an undifferentiated recharge to the glacial periods. The interglacial recharge values vary from a few mm yr−1 to more than 60 mm yr−1. Glacial recharge is less than 1 mm yr−1. These values are then analyzed in terms of intensity and allow questioning some initial hypothesis, especially the generally accepted value of the initial image value (around image). Our analysis suggests a higher value, around image. This approach allows us to bring out paleoclimatic information retained in these continental archives. Especially, computed recharges provide reliable evidence that Marine Isotope Stage (MIS) 5 was more humid than the Holocene period in agreement with marine archives that document wet condition in the North Sahara during that period.

International audience; L’aquifère de la plaine de Oussja-Ghar El Melah en Tunisie (P = 500 mm/an), est constitué d’une cuvette côtière fluviatile Plio-Quaternaire, enserrée entre montagnes, zone marécageuse et mer. Durant les dernières décennies, urbanisation et activités agricoles se sont intensifiées, induisant une baisse de la piézométrie, processus amplifié par les années sèches, et une détérioration de la qualité de l’eau. Les premiers résultats chimique et isotopique (18O, 2H, 3H) des eaux ont montré une hétérogénéité des faciès salins ainsi que différentes origines possibles dans l’acquisition de la minéralisation. La dégradation de la qualité des eaux est plus importante dans les zones les plus irriguées marquées par de fortes teneurs en nitrates. Les teneurs en tritium montrent un renouvellement récent des eaux différenciant deux modes de recharge, rapide liée à l’infiltration des eaux d’irrigation et des eaux de ruissellement au niveau des rivières et plus diffuse constituée par l’infiltration directe des précipitations.

Past variations of the recharge is estimated in the Atlas Mountains, the main recharge area of the Continental Intercalaire (CI) aquifer, one of the major Saharan aquifers, over the last 775 kyr. In this Mediterranean climatic context, continental archives generally record single humid events, grouped in hypothetical time interval and do not offer continuous chronicles of precipitation. We propose to use spatially distributed Cl-36/Cl data as a temporal constraint, to infer the past recharge in the Atlas area. Based on a simplified but robust climatic scenario, assuming a piston model, we apply a Markov Chain Monte Carlo (MCMC) inversion approach to attribute a specific recharge to the last nine interglacial periods and an undifferentiated recharge to the glacial periods. The interglacial recharge values vary from a few mm yr (1) to more than 60 mm yr (1). Glacial recharge is less than 1 mm yr (1) . These values are then analyzed in terms of intensity and allow questioning some initial hypothesis, especially the generally accepted value of the initial Cl-36/Cl value (around 133 x 10(-15) at at(-1)). Our analysis suggests a higher value, around 175 x 10(-15) at at(-1). This approach allows us to bring out paleoclimatic information retained in these continental archives. Especially, computed recharges provide reliable evidence that Marine Isotope Stage (MIS) 5 was more humid than the Holocene period in agreement with marine archives that document wet condition in the North Sahara during that period.

The uranium content and 234U/238U ratios have been measured in more than thirty groundwater samples over the Tunisian part of the Continental Intercalaire (CI) aquifer. In this large artesian aquifer, uranium behavior is mainly controlled by redox front systems. The uranium concentrations vary by 5 orders of magnitude from 10-4 to 20 ppb and the (234U/238U) activity ratio from 1 to 10. Close to CI outcrops in the Dahar Mountains, the stepwise decrease in U concentrations combined with a significant increase of the (234U/238U) ratio up to 10 ("augmenting regime") is followed, downflow by a (234U/238U) decrease up to 3 ("decaying regime"), clearly indicating the occurrence of a redox front. We will discuss the significance of 234U-238U disequilibria in terms of water residence times and its potential to trace water-rock interaction mechanisms.

Niger is the largest nation in West Africa (with 1.26 million km2), located between the Sahara and sub- Saharan regions without coast line. The majority of the country is covered by desert. Low precipitation (151 mm per year in 2009, World Bank 2010) and recurring droughts have made groundwater an essential water source. In the Diffa region (ca. 150 000 km2), located in south-eastern Niger and in the north-west of Lake Chad, groundwater is an important water source for the local population, together with surface water from Lake Chad and less importantly from the Komadougou Yobe River. A decreasing trend in rainfall, and hence shrinking of the area around Lake Chad, and irrigation development proposed by Nigerian authorities to meet growing food demand have led to actual and potential increases in groundwater exploitation. Moreover, water from Lake Chad has high salt content, limiting its usage. Groundwater resources in the region are found in several aquifer systems: (a) the Manga Quaternary aquifer covering nearly the entire Diffa region at 20 m to more than 50 m depth, except in a central part of the region (less than 10 m depth); (b) the alluvial aquifer located in the Komadougou valley and in topographic depressions of the region at a shallow depth (less than 10 m); (c) the Pliocene aquifer in the Pliocene sands and silts, covering the entire region at about 300 m depth in most of the region or at about 200 m in the central part; (d) the Continental Intercalary/Hamadien aquifer, covering the entire region at 40 to 60 m depth with a very low or zero recharge rate. To improve groundwater resource use and management of the potable water supply in the Diffa region, Technical Cooperation project NER8011 was conducted from 2009- 2012 to characterize the Manga Quaternary aquifer system. The project was entitled Studying the Recharge Process and the Status of Mineral Deposits Within the Manga Aquifer (Diffa region). The project was implemented by le Ministere de L'Hydraulique et de l'Environnement, Direction des Ressources en Eau of Niger

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The Merguellil catchment (central Tunisia), which is typical of the Mediterranean situation, has undergone rapid hydrological changes over the last decades. The most visible signs are a marked decrease in surface runoff in the upstream catchment and a complete change in the recharge processes of the Kairouan aquifer downstream. Fluctuations in rainfall have had a limited hydrological impact. Much more important have been the consequences of human activities, such as soil and water conservation works, small and large dams, and pumping for irrigation. Several independent investigations of the catchment were implemented, based on hydrodynamics, thermal surveys, and geochemistry including isotopes. These helped to identify the different terms of the regional water balance and to characterize their changes over time. However, major uncertainties remain and our results may contradict previous interpretations or calculations. Conservation works, now covering more than a quarter of the upstream catchment, drastically reduce the runoff production from rain events of less than 40 mm. Wadi Merguellil now ends in the big El Haouareb Reservoir, which loses more than half of its water by infiltration through karstic fissures and 30% by evaporation, the rest being pumped or released. El Haouareb Dam was built in 1989 and the reservoir has often dried up in the last decade. The major modifications in groundwater flow resulting from construction of the dam are observed in the geochemical tracers in the first seven kilometres downstream from it. Temperature measurements confirmed the recent invasion of new water. The rest of the Kairouan plain aquifer retains the signature of older recharge, but the whole aquifer is affected by the decrease in the water table (about 1 m per year), the consequence of the ever increasing pumping for irrigation.