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Accueil du site → Doctorat → Allemagne → 2006 → Hydrogeological characterization of the Upper Drâa catchment : Morocco

Rheinischen Friedrich-Wihelm-Universität Bonn (2006)

Hydrogeological characterization of the Upper Drâa catchment : Morocco

Cappy, Sébastien

Titre : Hydrogeological characterization of the Upper Drâa catchment : Morocco

Auteur : Cappy, Sebastien

Université de soutenance : Rheinischen Friedrich-Wihelm-Universität Bonn

Grade : Doktorgrade (Dr. rer. nat) 2006

Résumé partiel
The Drâa valley located in the south of Morocco is a typical semi-aridarea facing water scarcity problems. This Ph.D. thesis is part of the interdisciplinaryresearch project IMPETUS (integrated approach to the efficient management ofscarce freshwater resources in West Africa), which is a project of the universitiesof Bonn and Cologne (Germany). The IMPETUS project is part of the GLOWA program(global change of the water cycle) funded by the German BMBF (Federal Ministryof Education and Research) and focuses on the development of integrated strategiesfor an efficient and sustainable freshwater resource management. Groundwateris one of the primordial components of the hydrological cycle. Therefore theaim of this work is to characterize and quantify the hydrogeological propertiesof the aquifer system prevailing in the Upper Drâa catchment down to the Mansour Eddahbi reservoir (15,200 km2). Investigations were carried out between 2001 and 2005 following various approaches(e.g. geological surveys, water balance estimations, groundwater hydrogeochemicalclassification etc.) at different scales on the three main hydrogeologicalunits of the Upper Drâa catchment : the High Atlas, the Basin of Ouarzazateand the Anti-Atlas domain. The various aquifers of the Upper Drâa catchmentare characterized in respect to their dimensions and their hydrodynamical characteristics.The origin of groundwater is assessed by natural labelling using both, thegroundwater geochemistry such as major ions and trace elements and the isotopicsignatures (δ18O, δ2H). A Local MeteoricWater Line (LMWL ; δ2H= 8 δ18O + 10.6) is defined, which lies slightly abovethe Global Meteoric Water Line (GMWL). Based on the weighted δ18Omean values of the precipitation and the isotopic composition of selected springs,a clear altitude gradient is elucidated (δ18O = -0.002 ”altitude” - 3.0). Tritium and carbon-14 analyses completed by the calculation of saturationindexes for selected minerals allow the estimation of the groundwater residencetime. In order to simulate the alluvial aquifer system of the Basin of Ouarzazate,a three dimensional finite-difference groundwater flow model (PMWIN5) is used. While the Anti-Atlas domain shows poor groundwater storage capacity and lowgroundwater renewability, the folded Liassic limestones and dolomites of theHigh Atlas, covering 20 % of the Upper Drâa catchment, represent themain aquifer system in regard to its aquifer qualities, its volume and itsrecharge. Triassic formations underlying the karstified Liassic aquifer actas an aquitard. With tritium ages younger than 10 years the recharge of theLiassic aquifer system in the High Atlas by recent precipitation is efficient.This important renewability of the groundwater resources in the High Atlasis additionally confirmed by water balance calculations on two different scalesgiving groundwater recharge rates ranging between 4 % (Assif-n’-Ait Ahmed catchment:100 km2) and 11 % (Ifre catchment : 1,240 km2) of thetotal precipitation. The “SouthernAtlas Marginal Zone” (SAMZ), which is a tectonic compressional zone,acts as the decisive hydrogeological barrier between the aquifers of the HighAtlas Mountains and the Basin of Ouarzazate. Therefore, the discharge of theLiassic aquifer of the High Atlas occurs mainly by the surface drainage ofthe various wadis and the shallow subsurface flow in their adjacent alluvialaquifers. Those wadis have consequently a primordial role for the rechargeprocesses of the aquifer system of the Basin of Ouarzazate. The isotopic signaturesof most of the groundwater samples from the alluvial aquifers in the Basinof Ouarzazate reflect mean altitudes of the groundwater recharge area between2,400 to 2,900 m a.s.l., which localise the recharge area within the High AtlasMountains. Therefore the recharge by precipitation within the basin is notsignificant. Additionally, the steady state simulation with MODFLOW of thealluvial aquifer system of the Basin of Ouarzazate proves an efficient rechargeof the alluvial aquifers by river bed infiltration over the whole model domainaveraging 85 % of the total recharge ( 27 Mio m³/a). Only the remaining15% of the total recharge are provided by the infiltration of precipitation. The hydrochemical characterization of the groundwater of local aquifers developedin the Mio-Pliocene multi-layer aquifer in the Basin of Ouarzazate shows variousorigins according to their hydraulic connections with the overlying alluvialaquifers. But, the recharge of those small deeper aquifers is not effectiveas proved by groundwater ages older than 50 years. The confined aquifer ofInfracenomanian in the Tikirt area shows a heterogeneous character in regardto the groundwater chemistry due to some local leakage from the alluvial aquiferof the Imini Wadi. This confined aquifer is mainly recharged in the High Atlasas confirmed by the isotopic composition. Due to the fact that the volume ofthis confined aquifer is small and that the tritium ages reveal groundwaterolder than 50 years, the productivity of this aquifer is not suitable for exploitation.Precambrian rocks in the Skoura Mole located in the northern part of the basinembody old groundwater dated between 500 to 2,800 years with the carbon-14method. The unproductive character of those fissured rocks is explained bya low rate of infiltration and a low permeability confirming the aquitard roleof these formations.

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