Hydrodynamic and Hydro-geochemical Processes in the Catchment Area of Lake Retba and Their Implications in Relationship between Groundwater, Lake and Ocean

Mansour GUEYE^1, , Abdoul Aziz GNING¹, Fatou NGOM DIOP¹ and Raymond MALOU¹

¹Département de Géologie, Faculté des Sciences et Techniques/Université Cheikh Anta Diop, Dakar, Sénégal

Cite this paper

Mansour GUEYE, Abdoul Aziz GNING, Fatou NGOM DIOP and Raymond MALOU. Hydrodynamic and Hydro-geochemical Processes in the Catchment Area of Lake Retba and Their Implications in Relationship between Groundwater, Lake and Ocean. American Journal of Water Resources. 2016; 4(4):91-101. doi: 10.12691/AJWR-4-4-3

Abstract

The drought that began in the Sahel in the late 70s has led to a drying up of most surface water bodies and a decrease of ground water level. In the north coast of Cap-Verde, the Retba lake is one of the few depressions that have preserved its water and experienced all the same, a very important salinization. This saline ecosystem plays an important role in the economy of the area due to the exploitation of salt deposits. However, it is now threatened by a significant reduction of the water surface. Its backup requires a good knowledge of the environment and the interactions between the different reservoirs, namely the lake, groundwater and ponds. To study the relationship between different parts of the hydro system, a network, monitoring the groundwater table including 41 village wells across the watershed, has been established. All observation points and the level of the lake were leveled with the reference 0 IGN. Water samples were taken at the wells, lake and ponds and source for chemical analysis. The results of the altimetric study indicated that the lake is below sea level, at odds of -5.26 m and a good part of the watershed is under the level of the sea. Hence its vulnerability of being invaded by the ocean water due to the difference in altitude. Water table monitoring shows different movements from one point to another due to the interference of several factors in the process of charging and discharging of the water table. Thus, some wells are only under the influence of climate parameters (rain and evaporation), while others are fed or drained by lateral flow. Water chemistry is dominated by two chemical facies: calcic and chloride facies (Ca-cl) and sodium chloride-facies (Na-Cl). Na and Cl ions control mainly the mineralization water and cationic exchange is the main factor controlling water chemistry even if other processes such as anthropic pollution could occur. These cationic exchanges reflect the intrusion of seawater into the sand aquifers. These results also show that in the late dry season, water ponds have a chemical signature very close to seawater. This shows that the ponds that are in the north of the lake is an outcrop of the Sea that feeds the lake permanently. The lake water is a mixture of seawater and groundwater whose evaporation causes sedimentation of salt exploited by local people. Monitoring of the groundwater level, the lake level and the chemical analysis shows a connection between the lake, the groundwater, the ocean and ponds.

Keywords

lake retba, ponds, water chemistry, fluctuation, water table, mineralization, seawater intrusion, chemical facies, altimetry

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]	Hardie, L. A., Smoot, J. P., and Eugster, H. P., “Saline lakes and their deposits: a sedimentological approach.,” in Modern and Ancient Lake Sediments. vol. Special Publication, A. Matter and M.E. Tucker, Eds., International Association of Sedimentologists. 1978, 7-41.
[2]	Langbein, W. S., “Salinity and hydrology of closed lakes,” Geological Survey Professional paper 412 20 pp.1961.
[3]	Yechieli, Y. and Wood, W. W, “Hydrogeologic processes in saline systems: playas, sabkhas, ans saline lakes,” Earth-Science Reviews, vol. 58. 343-365. 2002.
[4]	Carn, J., Garnier, J.M., and Maglione, G.F., Données préliminaires sur les possibilités d’installation d’une saline à l’emplacement du lac Retba, Cap-Vert, Sénégal. Rapport ORSTOM, Dakar, 1976.
[5]	Sarr, R. Debenay, J. P. and Sow, E. “Sea level changes recorded by foraminiferal assemblages in the Upper Holocene of the Retba Lake (Senegal).” Revue de micropaléontologie, vol. 52. 31-41. 2009.
[6]	Sarr, R. Sow, E. and Bodergat, A. M. “Holocene marine incursions in Lac Retba (Cap Vert, Senegal) evidenced by ostracod faunas,” Geobios, vol. 42. 381-395. May-Jun 2009.
[7]	Garnier, J.M., “Evolution géochimique d'un milieu confiné: le lac Retba (Cap-Vert) Sénégal,” Revue de Géographie Physique et de Géologie Dynamique, vol. XX(2). 13-58. 1978.
[8]	Sarr, F. O. “Apports de la télédétection et des systémes d'information géographique (S.I.G) au suivi des zones côtières: Exemple du Lac Retba.,” Mémoire d'Ingénieur Géologue, Institut des Sciences de la Terre, Université Cheikh Anta Diop, Dakar, 1997.
[9]	Elouard, P. Evin, J. and Hébrard, L. “Observations et résultats de mesures au radiocarbone sur les cordons littoraux coquilliers du lac Retba - Sénégal,” Bulletin de Liaison de l’ASEQUA, vol. 46. 15-19. 1975.
[10]	Elouard, P. Faure, H. and Hébrard, L. “Variations du niveau de la mer au cours des 15 000 dernières années autour de la presqu’île du Cap-Vert, Dakar - Sénégal,” Bulletin de Liaison de l’ASEQUA, vol. 50. 29-49. 1977.
[11]	Sow, E. Compère, P. and Sarr, R. “Diatomées fossiles du lac Retba (Sénégal, Afrique de l’Ouest). Aperçu paléo-écologique.,” Algological Studies, vol. 120. 65-82. 2006.
[12]	Sow, E. Fofana, C. A. K. AW, C. and Sarr, R. “Mise en evidence d'une période d'isolement du lac Retba (Sénégal) entre 1200 et 250 B.P. par l'abondance de Paralia sulcata (EHRENBERG) Cleve,” Journal des sciences, vol. 8. 52-58. 2008.
[13]	Martin, A. “Etude géochimique des eaux souterraines de l’Afrique de l’Ouest Presqu’ile du Cap-Vert (Sénégal),” BRGM DAK 67, 1967.
[14]	Martin, A. “Les nappes de la presqu’île du Cap-Vert (République du Sénégal): Leur utilisation pour l’alimentation en eau de Dakar, Notice et cartes hydrogéologiques 1/50 000,” BRGM, France.1970.
[15]	Diouf, M.B. Contribution à l'étude des beach-rock sénégalais: Révision et nouvelles données. Département de Géologie, Université de dakar, rmémoire de D.E.A, 1984, 11-12.
[16]	Vallet, P., Approvisionnement en eau et assainissement de Dakar et ses environs: Etude des eaux souterraines, Tome II: Etude hydrogéologique de la nappe des sables Quaternaires. Rapport OMS projet Sénégal 3201 1972.
[17]	Richter B.C., and Kreitler, C.W. Geochemical techniques for identifying sources of grounwater salinization. United States of America: Library of Congress Cataloging-in-publication Data, 1993.
[18]	Bricquet, J.P., Bamba, F., Mahe, G., Toure, M., and Olivry, J.C, “Evolution récente des ressources en eau de l’Afrique atlantique.,” Revue des sciences de l’eau, vol. 10(3). 321-337. 1997.
[19]	Malou, R. Ngom. F.D. and Dacosta, H. “Impact du changement climatique sur les ressources en eau au Sénégal,” in 5ème Conférence Inter-Régionale sur l'Environnement et l'Eau Valorisation et Gestion Durable des Systèmes Hydriques pour une Santé et une Productivité Améliorée dans les Régions Chaudes, Ouagadougou, EIER-ETSHER-CIGR. 56-65.
[20]	Ngom, F.D., Caractérisation des transferts hydriques dans le bassin de la Néma au Sine Saloum. Doctorat de 3ème Cycle, Département de Géologie, Université Cheikh Anta Diop, Dakar, 2000.
[21]	Moon, S., Woo, N.C., and Lee, K.S, “Statistical analysis of hydrographs and water-table fluctuation to estimate groundwater recharge,” Journal of Hydrology, vol. 292. 198-209. 2004.
[22]	Schoeller, H., Les eaux souterraines. Paris: Eds. Masson, 1962.
[23]	Kortatsi, B.K. “Hydrochemical characterization of groundwater in the Accra plains of Ghana,” Environ Geol vol. 50. 299-311. 2006.
[24]	Kattan, Z., “Characterization of surface water and groundwater in the Damascus Ghotta basin: hydrochemical and environmental isotopes approaches,” Environ Geol, vol. 51. 173-201. 2006.
[25]	Garcia, M.G., Hidalgo, M.V., and Blesa, M.A, “Geochemistry of groundwater in the alluvial plain of Tucumàan province, Argentina,” Hydrogeology Journal, vol. 9. 597-610. 2001.
[26]	Demetriades, A., “General ground water geochemistry of Hellas using bottled water samples,” Journal of Geochemical Exploration vol. 107. 283-298. 2010.
[27]	Diédhiou, M., Approche multitraceur géochimique et isotopique à l'identification des sources de la pollution nitratée et des processus de nitrification/dénitrification dans la nappe de Thiaroye. Doctorat Hydrogéologie, Univ Cheikh Anta Diop, Dakar. 2011.
[28]	Magaritz, M., Nadler, A., Koyumdjiski, H., and Dan, J., “The Use of Na/Cl Ratios to Trace Solute Sources in a Semiarid Zone,” Water Resources Research, vol. 17( 3). 602-608. 1981.
[29]	Trabelsi, R., Zairi, M., Smida, H., and Ben Dhia, H., “Salinisation des nappes côtières : cas de la nappe nord du Sahel de Sfax, Tunisie,” C. R. Geoscience vol. 337. 515-524. 2005.
[30]	Appelo C.A.J., and Postma, D., Geochemistry, Groundwater and Pollution, 2nd eds. Rotterdam: A.A. Balkema, 2005.
[31]	Kouzana, L., Mammou, B.A., and Felfoul, S. M., “Seawater intrusion and associated processes: Case of the Korba aquifer (Cap-Bon, Tunisia),” C.R. Geoscience, vol. 341. 21-25. 2009.
[32]	Pulido-Leboeuf, P., Pulido-Bosch, A., Calvache, M.L., Vallejos, A., and Andreu, J.M, “Strontium, SO42-/Cl- and Mg2+/Ca2+ ratios as tracers for the evolution of seawater into coastal aquifers: The example of Castell de Ferro aquifer (SE Spain),” C.R. Geoscience, vol. 335. 1039-1048. 2003.
[33]	Kouzana, L., Mammou, B.A., and Gaaloul, N., “Intrusion marine et salinisation des eaux d'une nappe phréatique côtière (Korba, Cap-Bon, Tunisie),” Geo-Eco-Trop, vol. 31. 57-70. 2007.
[34]	Trabelsi, R., Zairi, M., Smida, H., and Dhia, B.H., “Salinisation des nappes côtières : cas de la nappe Nord du Sahel de Sfax, Tunisie.,” C.R. Geoscience, vol. 337. 515-524. 2004.