18th SWIM, Cartagena (Spain), 31 May - 3 June 2004

Isotopic and hydrochemical investigation in the origins of groundwater salinisation in the Nefzauoa Oases (Southern )

Tobias El-Fahem1; Tobias Siegfried2; Wolfgang Kinzelbach3; Stefan Peiffer4; Brahim Ben Baccar5;

1 - Institute of Geology, GIB, University of Bonn, Germany, E-Mail: [email protected]; 2 - Institute of Hydromechanics and Water Resources Management, IHW, ETH Zurich, Switzerland, E-Mail: [email protected]; 3 - Institute of Hydromechanics and Water Resources Management, IHW, ETH Zurich, Switzerland, E-Mail: [email protected]; 4 - Department of Hydrology, University of Bayreuth, Germany; E-Mail: [email protected]; 5 - Ministry of Agriculture of the Tunisian Republic, Direction Générale des Ressources en Eaux (DGRE), Section: Kébili Governorate, Tunisia

Keywords: arid zone, drainage water, hydrochemistry, salinisation, stable isotopes

The region of the Nefzaoua Oases in the south of the Tunisian Republic is an important region with regards to its production and it's tourist value. The System of Oases covers approx. 3000 km² and is limited in the west by the biggest Sahara salt lake, the Chott El , and in the south by the Sahara desert. Precipitation is seasonal and irregular distributed. In all sectors demand for water is provided by groundwater resources. Until the beginning of 1970 water had been supplied by artesian wells screened in the deep aquifer systems of the Complèxe Terminal and the Continental Intercalaire. However, intensification of both local and regional abstraction caused a widespread decrease of hydraulic head in these aquifers. For the last twenty years salinisation of the Complexe Terminal water has been observed locally. This development has led to increasing concerns about the implications of today’s groundwater intakes. Mainly two areas in the Nefzaoua Oases are affected by this development towards prohibitively high salinity levels that would prevent this water from being used efficiently in future. One area can be identified in the north of the oases along the -Kébili fault, whereas the other one is located to the south of the town of . The hydrogeological situation of the Nefzauoa Oases needed testing various hypotheses that existed with regards to an explanation of this quality changes. Therefore, a sample campaign was launched in 2002. It involved hydrochemical and stable isotopes investigations as well as analyses of tritium, CFC and water temperature. As a result, any contamination due to attraction of highly saline Chott waters can be ruled out. The hydrochemistry shows clearly differing diverse of salinisation processes in each of the two areas. In combination with stable isotopes and information on water temperature, two possible sources of salinisation could be identified. In the north, salinisation is caused by deep geothermal groundwater ascension from the Continental Intercalaire aquifer as proven by a distinct increase of water temperature. The region to the south of Douz, however, does not show an influence of saline water from the deeper aquifer. Rather, chemical and isotopic signatures point towards pollution by agricultural drainage waters, concentrated on the surface due to evaporation. This water seeps away from the surface through the zones lacking confinement into the Complèxe Terminal aquifer. These findings can manage future groundwater as well as oases irrigation systems to counteract further deterioration of groundwater quality.

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