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 |
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| Authors: | A Mennani B Blavoux M Bahir Y Bellion M Jalal M Daniel |
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| Institution: | a Laboratoire d'Hydrogéologie, Départment de Géologie, Faculté des Sciences Semlalia, Université Cadi Ayyad de Marrakech, BP 2390, Marrakech 40000, Maroc;b Laboratoire d'Hydrogéologie, Faculté des Sciences, Université d'Avignon, 33 Rue Louis Pasteur, 84000 Avignon, France |
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| Abstract: | The Essaouira synclinal zone is one of the Moroccan semi-arid zones with annual rainfalls not exceeding 300 mm yr−1 and very high potential evapo-transpiration of about 920 mm yr−1. This syncline with a Senonian axial zone is bordered by two diapiric structures of Triassic deposits: the Tidzi Diapir that outcrops in the east and south, and the hidden Essaouira diapir in the west, which was found by geophysics. This syncline contains two main superimposed aquifers. (i) The Plio-Quaternary aquifer consists of sands, sandstone and conglomerates and provides the main part of the water supply. This free-water table flows out towards the northwest and its surface is affected by significant piezometric variations. (ii) The calcareous dolomitic Turonian is a confined aquifer under the Senonian marls in the and in direct contact with the Plio-Quaternary. For a few years, the drinking water supply to the town of Essaouira has come from deep drillings.These two aquifers were sampled in June 1995 and in Januray 1996 after exceptional rainfalls. All waters have the same geochemical profile. The interpretation of the total dissolved solids and chloride content of Plio-Quaternary aquifers makes it possible to specify their origins. It emphasises, in particular, the source from the Ksob Wadi in the northeast and the role of the hidden Essaouira diapir. Nitrate levels were raised excessively, increasing at the same time as chloride concentrations during the rains of the winter of 1996, and underline the wells vulnerability to pastoral and domestic activities.The interpretation of O- and H-isotopes distinguishes two contrasting Plio-Quaternary and Turonian aquifers with an Atlantic origin for the rain recharge. A specific campaign was varried out in November 1996 to date water from the Turonian aquifer by 14C. Two boreholes draw water of several thousands years old. |
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