Hydrogeologic effects on the quality of water in the Oued Issen watershed, western Upper Atlas Mountains, Morocco |
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| Authors: | S Boutaleb L Bouchaou Y Hsissou J Mudry J Mania P Chauve |
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| Institution: | Applied Geology Laboratory, University Ibnou Zohr, Faculty of Sciences, BP 28/S, 80000 Agadir, Morocco, MA
Equipe d'Accueil 2642 “Stress, Strain, Flow” Faculty of Sciences, Leclerc Square, 25030 Besan?on Cedex, France Fax: +33-3-81665794 e-mail: jacques.mudry@univ-fcomte.fr, FR
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| Abstract: | The study of spatial and temporal variations of some hydrochemical properties in the Oued Issen watershed, Morocco, has revealed
their close relation to the area's lithologic and geologic characteristics and to variations in hydroclimatological cycles.
High concentrations of sodium chloride are measured during flood periods, when the outcrops of the basin are leached by streams
draining the rather dense hydrographic network.
These variations depend also on the relative abundance of tributaries on both sides of the Oued, their respective contributions
being hydrochemically very different: (1) northern-side tributaries, which mainly leach evaporitic terranes dating from the
Late Triassic and Late Liassic periods and which are rich in gypsum and halite, increase the mineralization of the Oued Issen
waters flowing toward the Abdelmoumen Dam; (2) the low salinity of water observed in the upstream portion of the watershed
is due to dilution by waters from the southern-side tributaries, which are derived from snowmelt and the leaching of the high
Paleozoic massif; (3) in the middle part of the basin, the increase in mineralization of the Oued Issen becomes substantial
between the Abdelmoumen Dam and the Dkhila Dam, the next dam downstream, due to the absence of dilution by waters from the
southern-side tributaries, which are sparse in this part of the basin. In addition, during flood periods, the outflow from
the first dam is stopped. Thus, the main contribution of NaCl to the Oued is from the very salty Tirkou spring, which is situated
downstream from the first dam.
The geological characteristics of the basin have induced the development of high-salinity zones, which are particularly evident
in the southwestern part of the basin in the vicinity of the confluence of the Oued Boulebaz with the Oued Issen. The discharge
of very salty springs is controlled by faults that offset the formations that underlie the hydrographic network and degrade
the quality of the Oued Issen. This water later recharges the unconfined aquifer along the Oued where it flows on the Oued
Souss Plain.
Received, July 1998/Revised, July 1999, September 1999/Accepted, November 1999 |
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| Keywords: | salinization hydrochemistry evaporites contamination Morocco |
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