Hydrogeological characterization of the Upper Drâa catchment: Morocco

Abstract

The Drâa valley located in the south of Morocco is a typical semi-arid area facing water scarcity problems. This Ph.D. thesis is part of the interdisciplinary research project IMPETUS (integrated approach to the efficient management of scarce freshwater resources in West Africa), which is a project of the universities of 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 Ministry of Education and Research) and focuses on the development of integrated strategies for an efficient and sustainable freshwater resource management. Groundwater is one of the primordial components of the hydrological cycle. Therefore the aim of this work is to characterize and quantify the hydrogeological properties of the aquifer system prevailing in the Upper Drâa catchment down to the Mansour Eddahbi reservoir (15,200 km²). <br /> Investigations were carried out between 2001 and 2005 following various approaches (e.g. geological surveys, water balance estimations, groundwater hydrogeochemical classification etc.) at different scales on the three main hydrogeological units of the Upper Drâa catchment: the High Atlas, the Basin of Ouarzazate and the Anti-Atlas domain. The various aquifers of the Upper Drâa catchment are characterized in respect to their dimensions and their hydrodynamical characteristics. The origin of groundwater is assessed by natural labelling using both, the groundwater geochemistry such as major ions and trace elements and the isotopic signatures (δ¹⁸O, δ²H). A Local Meteoric Water Line (LMWL; δ²H = 8 δ¹⁸O + 10.6) is defined, which lies slightly above the Global Meteoric Water Line (GMWL). Based on the weighted δ¹⁸O mean 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 saturation indexes for selected minerals allow the estimation of the groundwater residence time. In order to simulate the alluvial aquifer system of the Basin of Ouarzazate, a three dimensional finite-difference groundwater flow model (PMWIN5) is used.<br /> While the Anti-Atlas domain shows poor groundwater storage capacity and low groundwater renewability, the folded Liassic limestones and dolomites of the High Atlas, covering 20 % of the Upper Drâa catchment, represent the main aquifer system in regard to its aquifer qualities, its volume and its recharge. Triassic formations underlying the karstified Liassic aquifer act as an aquitard. With tritium ages younger than 10 years the recharge of the Liassic aquifer system in the High Atlas by recent precipitation is efficient. This important renewability of the groundwater resources in the High Atlas is additionally confirmed by water balance calculations on two different scales giving groundwater recharge rates ranging between 4 % (Assif-n'-Ait Ahmed catchment: 100 km²) and 11 % (Ifre catchment: 1,240 km²) of the total precipitation. The “Southern Atlas Marginal Zone” (SAMZ), which is a tectonic compressional zone, acts as the decisive hydrogeological barrier between the aquifers of the High Atlas Mountains and the Basin of Ouarzazate. Therefore, the discharge of the Liassic aquifer of the High Atlas occurs mainly by the surface drainage of the various wadis and the shallow subsurface flow in their adjacent alluvial aquifers. Those wadis have consequently a primordial role for the recharge processes of the aquifer system of the Basin of Ouarzazate. The isotopic signatures of most of the groundwater samples from the alluvial aquifers in the Basin of Ouarzazate reflect mean altitudes of the groundwater recharge area between 2,400 to 2,900 m a.s.l., which localise the recharge area within the High Atlas Mountains. Therefore the recharge by precipitation within the basin is not significant. Additionally, the steady state simulation with MODFLOW of the alluvial aquifer system of the Basin of Ouarzazate proves an efficient recharge of the alluvial aquifers by river bed infiltration over the whole model domain averaging 85 % of the total recharge (~ 27 Mio m³/a). Only the remaining 15% of the total recharge are provided by the infiltration of precipitation. <br /> The hydrochemical characterization of the groundwater of local aquifers developed in the Mio-Pliocene multi-layer aquifer in the Basin of Ouarzazate shows various origins according to their hydraulic connections with the overlying alluvial aquifers. But, the recharge of those small deeper aquifers is not effective as proved by groundwater ages older than 50 years. The confined aquifer of Infracenomanian in the Tikirt area shows a heterogeneous character in regard to the groundwater chemistry due to some local leakage from the alluvial aquifer of the Imini Wadi. This confined aquifer is mainly recharged in the High Atlas as confirmed by the isotopic composition. Due to the fact that the volume of this confined aquifer is small and that the tritium ages reveal groundwater older 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 basin embody old groundwater dated between 500 to 2,800 years with the carbon-14 method. The unproductive character of those fissured rocks is explained by a low rate of infiltration and a low permeability confirming the aquitard role of these formations. <br /> The groundwater in the Upper Drâa catchment presents a high variability in quality, which often do not meet the international standards for drinking water due to a high mineralization. This high mineralization results only from the leaching of evaporitic minerals (gypsum, halite and sylvite) naturally present in the various geological formations (Trias, Cretaceous, Eocene and Mio-Pliocene). This is confirmed by the isotopic signatures of the groundwater, which do not show any evaporation effect. In the High Atlas, chemical processes such as dedolomitization associated with the leaching of gypsum influence the groundwater chemistry. In the Basin of Ouarzazate, the exhalation of CO₂ along faults in the Precambrian rocks has also local impact on the composition of the groundwater. <br /> This study provides specific hydrogeological data for the interdisciplinary research. Thus, it supports tasks such as hydrological modelling, anthropological investigations, development of an integrated information system, and the future implementation of Decision Support Systems in the IMPETUS project.