Taoudeni Basin Report
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Integrated and Sustainable Management of Shared Aquifer Systems and Basins of the Sahel Region RAF/7/011 TAOUDENI BASIN 2017 INTEGRATED AND SUSTAINABLE MANAGEMENT OF SHARED AQUIFER SYSTEMS AND BASINS OF THE SAHEL REGION EDITORIAL NOTE This is not an official publication of the International Atomic Energy Agency (IAEA). The content has not undergone an official review by the IAEA. The views expressed do not necessarily reflect those of the IAEA or its Member States. The use of particular designations of countries or territories does not imply any judgement by the IAEA as to the legal status of such countries or territories, or their authorities and institutions, or of the delimitation of their boundaries. The mention of names of specific companies or products (whether or not indicated as registered) does not imply any intention to infringe proprietary rights, nor should it be construed as an endorsement or recommendation on the part of the IAEA. INTEGRATED AND SUSTAINABLE MANAGEMENT OF SHARED AQUIFER SYSTEMS AND BASINS OF THE SAHEL REGION REPORT OF THE IAEA-SUPPORTED REGIONAL TECHNICAL COOPERATION PROJECT RAF/7/011 TAOUDENI BASIN COUNTERPARTS: Mr Adnane Souffi MOULLA (Algeria) Mr Abdelwaheb SMATI (Algeria) Ms Ratoussian Aline KABORE KOMI (Burkina Faso) Mr Alphonse GALBANE (Burkina Faso) Mr Sidi KONE (Mali) Mr Aly THIAM (Mali) Mr Brahim Labatt HMEYADE (Mauritania) Mr Sidi Haiba BACAR (Mauritania) EXPERT: Mr Jean Denis TAUPIN (France) Reproduced by the IAEA Vienna, Austria, 2017 INTEGRATED AND SUSTAINABLE MANAGEMENT OF SHARED AQUIFER SYSTEMS AND BASINS OF THE SAHEL REGION INTEGRATED AND SUSTAINABLE MANAGEMENT OF SHARED AQUIFER SYSTEMS AND BASINS OF THE SAHEL REGION Table of Contents 1. INTRODUCTION 1 2. STUDY SITES 4 2.1. General presentation of involved countries 4 2.2. Rainfall and hydrology 7 2.3. Geology and hydrogeology 9 3. DATA ACQUISITION AND METHODOLOGY USED 20 24 4. RESULTS AND INTERPRETATION 4.1. Results and discussion: Mauritania study zone 24 4.2. Results and discussion: Mali study zone 30 4.3. Results and discussion: Burkina Faso study zone 55 5. CONCLUSIONS 77 80 6. REFERENCES 85 ACRONYMS 87 ANNEXES INTEGRATED AND SUSTAINABLE MANAGEMENT OF SHARED AQUIFER SYSTEMS AND BASINS OF THE SAHEL REGION 1. INTRODUCTION The Taoudeni Basin ( 1,500,000 km 2) is a major geological sedimentary formation in West Africa named after the Taoudeni village in northern Mali. It covers large parts of the West African Craton in Mauritania, Mali and the south-western part of Burkina Faso (Fig. 1). Figure 1: The Taoudeni basin. The study zones included in the IAEA-supported project RAF/7/011 are presented as red ellipses. The Taoudeni is the largest sedimentary syncline basin in north-west Africa, formed during the mid-late Proterozoic. It continued to subside until the mid-Palaeozoic-age, when Hercynian deformation and uplift occurred. It contains up to 6,000 m of Late Precambrian and Palaeozoic sediments. It is only sparsely covered by thin Mesozoic–Cenozoic continental deposits including Quaternary dunes and lacustrine systems. These last deposits show a spatial continuity between different basins, Taoudeni and Tanezrouft basin in the northern part and Taoudeni and the Iullemeden basin in eastern part. Water reserve in Taoudeni basin are very important, but the current climatic condition from Sahel-Sudanese climate (annual precipitation between 600 to 1000 mm/y) in the southern part to arid climate (< 150 mm/y) in central and northern part does not allow a complete replenishment of water extracted by 1 INTEGRATED AND SUSTAINABLE MANAGEMENT OF SHARED AQUIFER SYSTEMS AND BASINS OF THE SAHEL REGION human activities. If in the northern part of Taoudeni the population density is low and consumption of water limited, in the central part, and especially in southern part, the population growth rate is about 3% by year, which involves an increasing demand of water, resulting a decrease of water table in numerous aquifers and a water quality degradation due to human activities and the lack of environmental protection. Two study zones has been investigated for the purpose of this IAEA-supported project RAF/7/011, involving three countries, Mauritania, Mali and Burkina Faso with a limited action for Mali respecting to the initial project due to security problems in northern part (Fig.1). The study zone is shared between Mali and Burkina Faso in the southern part (SE Mali-SW Burkina Faso) and corresponds to the limit east of Taoudeni sedimentary basin where Infra Cambrian deposit infill unconformable onto the Paleo-Proterozoic crystalline basement at the surface. The total area is 9°30'-15°5' latitude and 1°30'-8°30'W longitude. Previous studies on isotopes and hydrochemistry in the target area Concerning existing data or project in Mauritanian Taoudeni basin study zone, accessible bibliography by internet and scientific data does not refer to other studies. Also, some geological elements (map, cross section, stratigraphic profile was given by counterpart in different IAEA meeting concerning the IAEA-supported project RAF/7/011, with a zone covering 15°-17° N ; 6-12°W. Regarding existing data or project in the study zone, few geochemical and isotopic data are available for the central and western part. However, the eastern part has been the subject of numerous studies and projects in hydrogeology and water quality, since 70's with the decrease of rainfall in Sahelian Sudanian zone: -The "Mali sud" project (SOGREAH, 1988) which has determined transmissivity parameters on western area of BKF, -The "Bilan d'eau" project in BKF (Iwago, 1989) which enabled to classify aquifers in three classes depending on the type of porosity, 2 INTEGRATED AND SUSTAINABLE MANAGEMENT OF SHARED AQUIFER SYSTEMS AND BASINS OF THE SAHEL REGION -The ERES program (close to Bobo Dioulasso) a detailed study of groundwater hydrodynamic in the Kou Basin- 1500 km 2 (SOGREAH, 1994) according to the diversity of Infracambrian sedimentary layers. -The RESO (1994) and BAD (1999) projects which are an extension of ERES project to develop an extensive data bank on wells location. The first significant work was done by Ouedrago (1994) with a geological synthesis of BKF part of Taoudeni basin and a description of local groundwater in terms of hydrodynamic and geochemistry. Dakouré (2003) and Huneau et al. (2011), have completed the work of Ouedrago with geochemical and isotopic studies of groundwater on the same zone using different detailed geochemical and isotopic analyses performed over the area since the 1980s through the IAEA-supported Technical Cooperation projects, in particular two projects in Burkina Faso (BKF/8/002 and BKF/8/003) and one in Mali (MLI/8/002). Geochemical and isotopic data from 110 sampling points were selected, among which 98 points for tritium and 39 points for radiocarbon groundwater residence time calculation. The isotopic signature of the recharge was calculated using data from GNIP (IAEA/WMO, 2006) of Bobo-Dioulasso, Barogo and Bamako stations. The main results of this study were to provide a fine hydrodynamic and the geochemical state of the aquifer system and to propose a first model of a single aquifer flowing through the different sedimentary formations in spite of hydraulic local discontinuities. The author brings to light and quantify the decrease of level water (30 m/y in BKF) with respect to increase human consumption and decrease of rainfall. The geochemistry study shows mainly Ca–Mg–HCO 3 type which can shift towards a Na–K– HCO 3 type that indicates developed interactions between groundwater and clay minerals related to the residence time or towards a Cl–NO 3–SO 4–HCO 3 type indicating the anthropogenic influence on groundwater related to the poor sanitary conditions observed around wells. Isotopic study shows in general, over the zone aquifer, weak or null current renewable with locally recent recharge predominant. The sampling of the IAEA-supported project RAF/7/011 corresponds to the same zone that the study of Dakouré. The last work (Koussobe, 2013) was focused on hydrogeology, hydrochemistry and isotopic study of Gondo plain (Sourou basin) with hydrodynamic modelling. This work is located a little further north to RAF/7/011 project, studying the hydrogeology of Infra Cambrian, Continental Terminal on top of the Infra Cambrian deposit and crystalline basement, geological formation identified in the Gondo plain. The main results has allowed to define the geometric and piezometric characterization of the different aquifers, 3 INTEGRATED AND SUSTAINABLE MANAGEMENT OF SHARED AQUIFER SYSTEMS AND BASINS OF THE SAHEL REGION to show a recent recharge (superficial aquifers) or deep aquifer on the border of Gondo plain and recharge before 1950 for other deep aquifers below the Continental Terminal (no carbon- 14 was analysed to refine dating but one can assume older age as in the Dakouré study, showing stable isotopes more depleted than current rainfall).Due to difference of knowledge level according to the study zone of Mauritania and Mali-BKF, the defined objectives are different. Objectives of the IAEA-supported project RAF/7/011 in the target aquifer The objectives of the IAEA-supported project RAF/7/011 in Mauritania Taoudeni basin are to: produce a first evaluation of groundwater potentiality; a first geochemical characterization of spatial variability to linked to geological information; define the flow direction with a potentiometric map showing if there is a relation between superficial and deeper aquifers and showing if current recharge occurs with water stable isotope and tritium. The main difficulty is to compare stable isotopes in groundwater and stable isotope in rainfall, indeed data are very scarce, including limited data in North Senegal (Travi et al., 1987) and recent data (2013-2015) in the Mauritanian coastal section Rosso-Tiguint-Boutilimit-Nouakchott, and with climate conditions different than the RAF/7/011 study zone. The objectives of the project RAF/7/011 are different in western and eastern study zone of Mali.