Groundwater mixing and geochemical assessment of low-enthalpy resources in the geothermal field of southwestern Tunisia

Abstract

In southern Tunisia, the lack of permanent surface water, associated with drought conditions, leads to an endless abstraction from the limited low renewable multi-layered aquifer system of Système Aquifère du Sahara Septentrional containing two principal aquifers, the complex terminal and the Continental Intercalaire (CI) multi-layered systems. The deepest aquifer embodies the low-enthalpy thermal resources of Southern Tunisia. The thermo-mineral surface manifestations of the conductively heated waters reveal the complexity of the geotectonic history of the region and create oases systems in net contrast to the desertic landscape of the Chotts region. As agriculture constitutes the principal economic activity in the studied area and as it is currently threatened by thermal water quality degradation, the present hydrogeological investigation aims for a characterization of the CI thermal resources on the basis of a critical quality evaluation. Thus, different geothermometry approaches have been applied to characterize the low-enthalpy thermal field showing a temperature ranging from 27 to 72 °C. The application of cations geothermometers owns to different temperature estimations ranging from 28 to 202 °C. Based on Na + -Mg 2+ -K + , K + -Mg 2+ -Ca 2+ and HCO 3 − -Cl − -SO 4 2− geoindicators, the CI thermal resources refer to immature waters characterized by MI varying from − 0.5 to 0.19. The assessment of groundwater chemical composition based on a Durov diagram proves that CI thermal water is governed by mixing between different water bodies with relative abundance of Na + , Cl − , and SO 4 2− elements. According to D'Amore genetic diagram and meteoric genesis indices, the analyzed groundwater derived from deep circulation in sandy formations. The water quality can change along fluid migration pathways expressed by scattered isotopic and chemical data and a number of mixing ratios. Most changes in the background CI composition result from continuous pumping that modifies natural flows and increase quality degradation.

Climate impact on surface and groundwater in North Africa: a global synthesis of findings and recommendations

“…Also, this trans-boundary Algero, Tunisian basin suffers from drought and the wastewater/waste sediment of phosphate Company (CPG/GCT-Gafsa-Tunisia and FERPHOSBir Elatter/Tebessa, Algeria). According to recent studies, droughts are endemic in both southern Europe and northern Africa (Pavel 2003;Khelifi et al 2014Khelifi et al , 2017Mhamdi et al 2014;Hamed 2017Hamed , 2018El Gayar and Hamed 2017).…”

“…In this regard, different approaches have been used in the evaluation of the groundwater quality for particular uses (Hamed et al 2012(Hamed et al , 2013Besser et al 2017bBesser et al , 2018Hadji 2017). Previous works proved that these waters are often unsuitable for both domestic uses and agricultural activities.…”

“…The marine intrusion has affected almost the entire coastal zone and essentially areas with strong agricultural or industrial/tourist activities (Annaba, Tunis, Hammamet, and Gabès Gulfs). Currently, following these unfavorable conjunctures, which are the seat of an imbalance on a socio-economic level, it is time to reflect on the project of management and mobilization east-west and north-south surface water (rainwater of dams), moving towards the transfer of seawater mainly to the Gafsa mining area (under study project, Hamed et al , b, 2018, also towards the recycling of wastewater, etc. (Fig 5).…”

“…5 Location of surface and groundwater transfer, intrusion marine, and the recent project in Tunisia (Hamed et al , b, 2018 Page 9 of 15 25…”

“…For example, in the Gafsa mining basin, it is necessary to avoid the washing of phosphate by fresh water and also in the oil field. It is also necessary to bring sea water from the Gulf of Gabés (Hamed et al , b, 2018 (Fig. 5).…”