Versão online: http://www.lneg.pt/iedt/unidades/16/paginas/26/30/185
Comunicações Geológicas (2014) 101, Especial II, 651-653
IX CNG/2º CoGePLiP, Porto 2014
ISSN: 0873-948X; e-ISSN: 1647-581X
Recharge and hydrogeochemical evolution groundwater in semi-arid zone (Essaouira Basin, Morocco) Recarga e evolução hidrogeoquímica numa zona semiárida (Bacia Essaouira, Marrocos) M. Bahir1*, R. El Moukhayar1, H. Chamchati1, N. Youbi1,2, P. Carreira3, N. Chkir4 Artigo Curto Short Article © 2014 LNEG – Laboratório Nacional de Geologia e Energia IP
Abstract: Arid and semi-arid areas are threatened by desertification, and thus the degradation due to various factors, including drought, of course, but also the exploitation of resources. They are, in fact, already in a state of stress (resources less than 500 m3 per year per capita) with important disorders induced by climate change. These make an impact on the hydrological cycle, with longer droughts and increased flooding. These disorders will strengthen the already observed degradation of ecosystems, which are often overexploited. Essaouira synclinal area is part of the semi-arid areas of Morocco that are subject to the impact of climatic and human pressures. In the case of this coastal area, which includes two main aquifers superimposed; the Plio-Quaternary and Turonian, the resulting vulnerability is compounded by the risk of infiltration navy. The Rainfall in the area does not exceed 300 mm year-1, the average temperature hovers around 20°C, the piezometric map of Essaouira synclinal basin was made, different water samples have been collected from October 2009 after exceptional rainfall, all waters are sodium-chloride facies, interpretation of mineralization indicates power by the Ksob Wadi in the northeast and increasing levels of chlorides in the central part generated by the Essaouira diaper hidden. Excessive levels of nitrates have been identified, as well as chlorides after rains of winter 2009. The electrical conductivity and concentrations of 18O and 2H were measured, a local meteoric water line was determined according to the Atlantic origin of precipitation. However, the water Turonian, characterized by significant resources, demonstrates a very low charging current; its vulnerability would be more related to human pressure than changes in climatic conditions. Keywords: Basin of Essaouira, Aquifer, Semi-arid regions, Drought, Hydro geochemistry, Stable isotopes, Recharge. Resumo: As áreas áridas e semi-áridas estão ameaçadas pela desertificação, e, assim, pela degradação devido a vários fatores, incluindo a seca, é claro, mas também a exploração dos recursos. Estes estão, de fato, já em estado de estresse (recursos inferiores a 500 m3 por ano per capita) com distúrbios importantes induzidos pela mudança climática, que acarretam um impacto sobre o ciclo hidrológico, com secas mais longas e aumento das inundações. Esses distúrbios irão agravar a degradação dos ecossistemas já observada, que muitas vezes são sobre-explorados. A área sinclinal Essaouira é parte das áreas semiáridas de Marrocos que estão sujeitas ao impacto de pressões climáticas e humanas. No caso desta zona costeira, que inclui dois principais aquíferos sobrepostos, o Plio-Quaternário e o Turoniano, a vulnerabilidade resultante é agravada pelo risco de infiltração marinha. As chuvas na área não ultrapassam 1300 mm ao ano, a temperatura média gira em torno de 20°C, o mapa piezométrico da bacia sinclinal Essaouira foi feita, diferentes amostras de água foram coletadas a partir de outubro de 2009, após chuvas excepcional, todas as águas são de fácies sódio-cloreto, a interpretação da mineralização indica poder pelo KSOB Wadi no nordeste e aumento dos níveis de cloretos na parte
central gerada pela fralda Essaouira escondido. Os níveis excessivos de nitratos foram identificados, bem como os cloretos, após as chuvas de Inverno de 2009. A condutividade elétrica e as concentrações de 18O e 2 H foram medidos, uma linha de água meteórica local, foi determinada de acordo com a origem atlântica da precipitação. A condutividade eléctrica, temperatura e pH foram determinados in situ e amostras de água colhidas para determinação da composição isotópica (2H,18O,3H) A recta das águas meteóricas local foi determinada e comparada com a recta das águas meteóricas global (GMWL). As idades aparentes em radiocarbono das águas subterrâneas foram calculadas. A recarga dos aquíferos principais da região estudada foi analisada. O estudo efectuado demonstrou que a taxa de recarga no sistema aquífero profundo Turoniano é bastante baixa. Esta situação poderá vir a causar problemas de armazenamento de água e consequentemente de fornecimento de água à cidade de Essaouira e regiões limítrofes, uma vez que este sistema é responsável pelo abastecimento de mais de 50% de água potável a esta cidade e áreas rurais limítrofes, pelo que deverá ser objecto de uma gestão racional recursos hídricos subterrâneos. Palavras-chave: Bacia de Essaouira, Aquífero, Regiões semi-áridas, Seca, Hidrogeoquímica, Isótopos estáveis, Recarga. 1
Department of Geology, Cadi Ayyad University, Marrakech, Morocco. Centro de Geologia da Universidade de Lisboa (CeGUL), Faculdade de Ciências (FCUL),Lisboa, Portugal. 3 Instituto Tecnológico e Nuclear (ITN) - Sacavém, Portugal. 4 Département de Géographie, Faculté des Lettres et Sciences Humaines de Sfax, Tunisia. * Corresponding author / Autor correspondente:
[email protected] 2
1. Introduction The relative scarcity of water resources in the Essaouira Basin, their fragility and their uneven distribution give rise to a greater risk of shortage that is growing continually with the demographic pressures and the needs of the socioeconomic growth. In the Western High Atlas, Essaouira synclinal area is part of the Essaouira Basin, with an area of 200 km2, bounded by the Ksob Wadi in the north, Tidzi Wadi in the south, the Tidzi Diapir in the East and the Atlantic Ocean to the west. In coastal regions, salinization and pollution in groundwater systems is generally associated to the effects of seawater and on the other hand by the anthropogenic
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activities such as domestic wastes, agriculture and industry. This sedimentary basin is filled with Mesozoic and Cenozoic materials, which are overlain with superficial PlioQuaternary terrains. One main aquifer was identified, supplying water for drinking and for agricultural activities. The prevailing climate is semi-arid with highly variable rainfall averaging 300 mm year-1. 2. Geological and hydrogeological data In geological terms, the Essaouira synclinal zone is less rugged, with a lower relief, characterized by low hills and shaped by a sparse water system. The Plio-Quaternary and Turonian are the main reservoirs of groundwater in the Essaouira Basin. (Mennani et al., 2001) The PlioQuaternary, with a matrix of sandstone or limestone marine dune has a hydraulic conductivity primary porosity and contains a large free surface whose wall is formed in the synclinal structure, by the Senonian marls, peel off the anti-Pliocene and it shows that the Plio-Quaternary can be in direct contact with the Triassic and Cretaceous other levels (Lozes, 1959; SCP, 1959) (Duffaud et al., 1966). It is operated in rural areas and provides drinking water, domestic needs and to a lesser extent irrigation for farmland. (Bahir et al., 1999, 2000). The Turonian contains a layer quickly captivated by the Senonian marls in the synclinal structure and probably in direct contact with the Pliocene-Quaternary on the edges of this structure to the north to Ksob Wadi. 3. Groundwater geochemical characterization In spite of the occurrence of calcareous and dolomitic levels, all Essaouira basin waters are of Na-Cl-type. This chemical signature should be the result of the preferential recharge area that is located in the Tidzi diapir. A high correlation coefficient was found between electrical conductivity, chlorides and sodium contents, suggesting the large contribution of these elements to the groundwater chemical load. Nevertheless, groundwater is highly bicarbonate as a result of the presence of carbonate compounds in the reservoirs matrix. (Bahir et al., 2013). 4. Principal component analysis Water resources assessment and management require a multidisciplinary approach involving chemical, physical and geological data. Considering the numerous species involved in the chemical analyses, the sources of salinization were sought using Principal Component Analysis (PCA). The application of the PCA method to Essaouira groundwater data allows the identification of 3 factors. − Factor 1 represents 41% to the total variance of the groundwater system with a positive correlation between chloride, sodium, calcium and magnesium. − Factor 2 with a relevance of 19% to the groundwater characterization represents the sulphates content. These values could have natural or anthropogenic origins.
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− Factor 3 represents 17% of the total variance and it is probably the result of the influence of bedrock in the water mineralization; factor 3 shows a high correlation with potassium. The higher concentrations located in the central part of the area are the result of Tidzi diapir influence on the water recharge, leading to high contents in sodium and chloride, according to the main flow direction. The evolution of these parameters does not indicate the main flow direction probably as related to the Ksob wadi recharge and the occurring dilution effect. − Anthropogenic influence on the aquifers Contamination. − Mass Balance Simulation. Component of initial water + reactant phase → component of final water + product phase. 5. Isotopes In the studied zone, the hydrodynamic behaviour is influenced by the structure of the ground (folds and faults) which steers the flowing water and determines the outputs. In such a context, stable isotopes constitute a good tool for determining the origin and history of waters, recharge surfaces and relationships between the aquifers. In the Essaouira Basin, the hydrodynamic behaviour is strongly influenced by runoff (Chkir et al., 2008). Analyses were performed at the Technological Institute of Lisbon Sacavém Department of Environmental and Analytical Chemistry in the context of the Integrated Action between the universities of Lisbon and Marrakesh and funded jointly; the results of these tests are grouped. In the Essaouira Basin, the isotope content of water Plio-Quaternary is between -3.72 and -4.56‰ vs. d18O SMOW. These waters are the cloth Turonian between 4.17 and -4.55‰ vs. d18O SMOW. Tritium analysis was performed for thirteen water points supplied by the different aquifers of the Essaouira coastal zone. Tritium values vary from a minimum value smaller than 0.8 UT and a maximum value of 4.2 UT. The 14C radioisotope was utilized for dating ancient waters with very low tritium concentrations (Table 1). To assess initial 14C activities of underground waters of the coastal zone of Essaouira many models were tested. 6. Conclusion The economic and social activities in the Essaouira basin depend heavily on groundwater. In this basin, the very high evaporation and scarce precipitation trigger the processes of salinization and contamination. In this context, rainfall initiates the remobilization of residual chemical species and contaminants (sodium, chloride, and nitrate) trapped and stored in unsaturated zone during the long period of drought. This clearly shows the small potential of natural attenuation by precipitation that is likely to occur in semi-arid regions. According to the climatic and environmental
Recharge and hydrogeochemical evolution of groundwater in semi-arid zone
conditions as well as to social and economical factors, it is clear that each of the referred basins requires specific management measures to protect and to preserve the groundwater resources and to ensure water supply. Rajagopal & Graham (1989) suggest three recommendations that might be useful for Essaouira basin, where few management schemes are implemented: − The use of the water from Ksob wadi for artificial recharge of the aquifers. In fact, the most significant outflows of the wadi are related to occasional floods in winter. A small part feeds the aquifers and the rest is lost to the Atlantic Ocean. − The improvement of drilling techniques as well as a better protection and construction of wells. − The raise of public awareness and public participation in water management. Table 1. Data obtained for the radioisotope concentrations for the underground waters. Tabela 1. Os dados obtidos para as concentrações de radioisótopos para as águas subterrâneas.
It has been shown by this study that the use of stable and radioactive isotopes is a good tool for determining the origin, story and recharge zones of the main aquifers of the studied area. Indeed, potable waters supplying the Essaouira city and its neighbouring rural agglomerations is presently based on exploiting underground waters, notably those of the Plio-Quaternary aquifer. This aquifer is
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submitted to several constraints. Less deep, this aquifer is sensitive to drought episodes, more frequent in Morocco; the most severe one was happened in 1995. It has been shown by this study that the recharge rate of the deep Turonian aquifer of the studied region is too low. Acknowledgments This work is funded by the bilateral project: FCT Portugal CNRST Morocco. The authors would like to thank the reviewers for their constructive remarks. References Bahir, M., Chamchati, H., El Moukayar, R., Youbi, N., Chkir, N., 2013. Managment and protection of groundwater resources in semiarid zones: Application of hydrochemical methodologies to Essaouira synclinal basin (Morocco). International Journal of Hydrology Science and Technology, 3, 160-175. Bahir, M., Jalal, M., Mennani, A., Fekri, A., 1999. Contribution à l’étude hydrogéologique et hydrochimique de la zone synclinale d’Essaouira. Congrès International sur l’eau et la désertification. Le Caire. Bahir, M., Mennani, A., Jalal, M., Youbi., N., 2000. Contribution à l’étude des ressources hydriques du bassin synclinal d’Essaouira (Maroc). Estudios Geologicos, 56, 3-4. Chkir, N., Trabelsi, R., Bahir, M., Hadji Ammar, F., Zouaei, K., Chamchati, H., Monteiro, J.P., 2008. Vulnérabilité des ressources en eau des aquifères côtiéres en zones semi-arides-étude comparative entre les bassins d’Essaouira (Maroc) et la Jeffara (Tunisie). Comunicações Geológicas, 95, 107-121. Duffaud, F., Brun, L., Planchot, B., 1966. Bassin du sud-ouest marocain (SW Morocco basin). In: Reyre (Ed.). Bassin sédimentaire du littoral africain. Association des Services Géologiques Africains. 1ère partie, Paris, 5-12. Lozes, A., 1959. Rapport de synthèse sur les études sismiques dans la zone côtière d’Essaouira. Rapport inédit, Office National de la Recherche Pétrole, Ref. 20048, 25. Mennani, A., Blavoux, V., Bahir, M., Bellion, Y., Jalal, M., Daniel, M., 2001. Apports des analyses chimiques et isotopiques à la connaissance du fonctionnement des aquifères Plio-quaternaire et turonien de la zone synclinale d’Essaouira (Maroc occidental). Journal of African Earth Sciences, 9, 4-32. Rajagopal, R., Graham, T., 1989. Expert opinion and groundwater quality protection. The case of nitrate in Drinking water. Groundwater, 27(6), 835–847. SCP - Société Chérifienne de Pétrole, 1959. Note sur la structure dite de Mogador. Rapport inédit, Office National de la Recherche Pétrole, 31, 104 p.
