Hydrogeological Characterization and Hydrodynamic Behaviour of the Overexploited Diass Aquifer System (Senegal) Inferred from Long Term Groundwater Level Monitoring

Diakher Hélène MADIOUNE^{1, 2,} , Moctar DIAW¹, Ibrahima MALL¹, Philippe ORBAN², Serigne FAYE¹ and Alain DASSARGUES²

¹Department of Geology, Cheikh Anta DIOP University, Dakar, Senegal

²Hydrogeology & Environmental Geology, Urban & Environmental Engineering, University of Liege, Belgium

Cite this paper

Diakher Hélène MADIOUNE, Moctar DIAW, Ibrahima MALL, Philippe ORBAN, Serigne FAYE and Alain DASSARGUES. Hydrogeological Characterization and Hydrodynamic Behaviour of the Overexploited Diass Aquifer System (Senegal) Inferred from Long Term Groundwater Level Monitoring. American Journal of Water Resources. 2020; 8(3):104-117. doi: 10.12691/AJWR-8-3-1

Abstract

Dakar, the Capital city of Senegal concentrates about 23.2% (about 3 millions inhabitants) of the total population [1] and a large proportion of the industrial activities. Water supply is ensured by surface water pumped and piped from the Guiers Lake (250 km distant from the capital) and from groundwater resources. Among these latter, the Diass aquifer system contributes to a substantial proportion (31% in 2019) of the total water supply distribution due to growing demand induced by the rapid demographic growth (about 2.5%). The Diass horst aquifer system located 50 km east of Dakar (Senegal) is exploited with two main aquifers covered by a sandy superficial aquifer: the confined/unconfined Palaeocene karstic limestone and the confined Maastrichtian sandstone aquifer underneath. This system has experienced intensive groundwater abstraction during the last 60 years to meet the increasing water demand. Abstraction for urban drinking water occurs in nine pumping fields with a rate reaching 174,000 m³/d in 2019. This high yield together with the drought conditions since the 1970s is likely to affect groundwater imbalance and change the flow regime. The objective of the study is to improve our understanding of the system dynamic with regards to the high pumping rate in order to build a conceptual scheme for further hydrogeological modeling of the system. In this study, we use monitored pumping rates, piezometric level from 1960s to 2019 and rainfall data from 1931 to December 2016 together with the hydrogeological configuration to infer the dynamics of the aquifer system. The high abstraction rate during the period 1958-2019 which vary from 16,000 to 174,000 m³/d has caused a continuous groundwater level decline (up to 30 m), a modification of the flow patterns and to some extent a quality deterioration through salinization processes as shown in a few boreholes in Sébikotane and Mbour. The piezometric levels which were above the sea level prior 1959 exhibit now negative values and can even reach -40 m in the vicinity of the pumping fields creating therefore piezometric depressions and convergent flow pattern. The hydrodynamic of the system derived from the results show that the reservoir acts as a multilayer aquifer system with interconnected compartments by faults that allow flux exchanges except the confining Ponty and Sébikotane faults. Overexploitation inducing important drawdown has induced an increase of the drainance fluxes between those different compartments [2]. In order to foster more appropriate and sustainable groundwater abstraction in the complex hydrogeological system with regards to demand and water quality conservation, it is important to assess the main system behavior.

Keywords

aquifer overexploitation, hydraulic head, groundwater pumping, flow exchange, recharge, Palaeocene, Maastrichtian, Diass horst

Copyright

This work is licensed under a Creative Commons Attribution 4.0 International License. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

References

[1]	Agence Nationale de la Statistique et de la Démographie (ANSD). Recensement Général de la Population et de l’Habitat, de l’Agriculture et de l’Elevage en 2013 rapport définitif Ministère de l’Economie, des Finances et du Plan.2014
[2]	Madioune, D.H. Etude hydrogéologique du système aquifère du horst de Diass en condition d’exploitation intensive (bassin sédimentaire sénégalais) : apport des techniques de télédétection, modélisation, géochimie et isotopie. Thèse de Doctorat en sciences de l’Ingénieur/Université de Liège et docteur es sciences/ Faculté des Sciences et Techniques (FST)/Univ Cheikh Anta Diop 325p. October2012
[3]	Mahé, G.,Oliviry, J.C.Variations des écoulements en Afrique de l’Ouest et Centrale de 1951 à 1989.Sécheresse1(6), 109-117. 1995.
[4]	Paturel, J.E., Servat, E., Delattre, M.O., Lubes-Niel, H. Analyse de séries pluviométriques de longue durée en Afrique de l’Ouest et Centrale non sahélienne dans un contexte de variabilité climatique. Hydrological Sciences Journal. 43(6), 937-946. 1998.
[5]	Lebel, T.,Ali,A. Recent trends in the Central and Western Sahel rainfall regime (1990–2007). Journal of Hydrology. 375, 52-64. 2009.
[6]	Aguiar, L.A. A., Garneau, M., Lézine,A.M., Maugis, P. Evolution de la nappe des Sables quaternaires dans la région des Niayes du Sénégal (1958–1994):relation avec le climat et les impacts anthropiques. Sécheresse 21(2), 97-104. April-May. June 2010.
[7]	Sircoulon, J.H.A.Variation des débits des cours d’eau et des niveaux des lacs en Afrique de l’Ouest depuis le début du 20èmesiècle. The Influence of Climate Change and Climate Variability on the Hydrologic Regime and Water Resources. In: Proceedings of theVancouver Symposium, August 1987. IAHS Publ. No.168. 1987.
[8]	Hubert, P., Bader, J.C., Bendjoudi, H. Un siècle de débits annuels du fleuve Sénégal. Hydrological Sciences Journal. 52(1), 68-73. 2007.
[9]	Mahé, G., Diello, P., Paturel, J.E., Barbier, B.,Karambiri, H., Dezetter, A., Dieulin, C., Rouché, N. Baisse des pluies et augmentation des écoulements au Sahel: Impact climatique et anthropique sur les écoulements du Nakambeau Burkina Faso.Sécheresse. 21(1e), 1-6. 2010.
[10]	Noble. J., Nair, A.R., Sinha, U.K., Joseph, T.B., Navada,S.V.Isotope studies to understand the dynamic changes due to long term exploitation of groundwater systems in western Rajasthan, India inIsotopic Assessment of Long Term Groundwater Exploitation; Proceedings of a final research coordination meeting held in Vienna, 12-16 May 2003; IAEA (2006), vol 37(1); Issue 49(1) 55-72. 2006.
[11]	Zouari, K.., Kamel, S., Chkir,N. Long term dynamic isotope and hydrochemical changes in the deep aquifer of “Complexe Terminal” (Southern Tunisia) in Isotopic Assessment of Long Term Groundwater Exploitation; Proceedings of a final research coordination meeting held in Vienna, 12-16 May 2003; IAEA (2006), vol 37(1); Issue 49(1) 127-156. 2006.
[12]	Al-Momani. M.R., Amro. H., Kilani. S., El-Naqa. A., Rimawi. O., Katbeh. H., Tuffaha, R.Isotope response to hydrological systems for long term exploitation, case of Azraq Basin, Jordan inIsotopic Assessment of Long Term Groundwater Exploitation; Proceedings of a final research coordination meeting held in Vienna, 12-16 May 2003; IAEA (2006), vol 37(1); Issue 49(1) 177-211. 2006.
[13]	Lashkaripour, G.R., Ghafoori, M. The Effects of Water Table Decline on the Groundwater Quality in Aquifer of Torbat Jam Plain, Northeast Iran,International Journal of Emerging. Sciences, 1(2), 153-163, June 2011.
[14]	Jiansheng,S., Zhao, W., Zhaoji, Z., Yuhong, F., Yasong, L., Feng’e, Z., Jingsheng, C., Yong, Q.,Assessment of deep groundwater over-exploitation in the North China Plain Geoscience Frontiers2(4)(2011) 593-598. 2011.
[15]	Madioune,D.H., Faye.S., Orban.P., Brouyère, S., Dassargues, A., Mudry,J., Stumpp, C., Maloszewski, P.Application of isotopic tracers as a tool for understanding hydraulic behavior for the highly exploited Diass aquifer system Journal of Hydrology. 511 (2014). 443-459. 2014.
[16]	Allen, R.G., Pereira, L.S., Raes, D., Smith, M. Crop evapotranspiration guidelines for computing crop water requirements. FAOIrrig.Drain.Pap. 56, 300p. 1998.
[17]	Martin, A. Les nappes de la presqu’île du Cap-Vert. République du Sénégal. Leur utilisation pour l’alimentation en eau de Dakar.Thèse 3èmecycleDoc. BRGM. docteur-es-sciences, Dépt de Géologie, Fac.des Scien.EtTechn., Univ.C.A.Diop de Dakar, 56p. 1970.
[18]	Fall, M. Contribution à l’étude hydrogéologique des calcaires paléocènes de Pout et du Lac Tanma.Mem DEA Département de Géologie Faculté des Sciences et Techniques (FST)/Univ Cheikh Anta Diop. Rapport N°9 Nouvelle série. 43. 1981.
[19]	Faye, A. Contribution à l’étude géologique et hydrogéologique du horst de Ndiass et de ses environs (Sénégal occidental). Thèse 3ème cycle Université de Dakar. 160p. 1983.
[20]	Roger, J., Barusseau, J.P., Castaigne, P., Duvail,C., Noël, B. J., Nehlig,P., Serrano, O., Banton,O., Comte, J-C., Travi, Y., Sarr, R., Dabo, B., Diagne, E., Sagna, R. Programme d’Appui au Secteur Minier cartographie géologique du bassin sédimentaire. Projet 9 ACP SE 009. 2009.
[21]	Géohydraulique& OMS. Approvisionnement en eau et assainissements de Dakar et ses environs. Etude des eaux souterraines Projet Sénégal 3201 (EX 22) Tome II et III. 1972.
[22]	Arlab. Alimentation en eau des I.C.S. Etude complémentaire du Maastrichtien. Rapport technique Arlab 183/83. Arch. DEH. 1983.
[23]	BRGM. Etude hydrogéologique pour le renforcement de l’alimentation en eau potable de la Petite côte. République du Sénégal, Rapport, SONEES Dakar, république du Sénégal. Vol. 2 : annexes, 88 Dakar 013 3E, 19p. 1988.
[24]	Sarr, B.Contribution à l’étude hydrogéologique des aquifères de l’Ouest du bassin du Sénégal. Thèse Docteur 3éme cycle. Fac Scien . UCAD. 128 p + annexes. 2000.
[25]	DGPRE Etude hydrogéologique complémentaire du horst de Diass, rapport de synthèse des investigations hydrogéologiques et hydrochimiques - version définitive 122p. 2018.