K. Zouari
1, M. Megribi
2, N. Chkir
1, R. Trabelsi
1, B. Ben Baccar
3and P. Aggarwal
41Laboratory of Radio-Analysis and Environment of the National School of Engineers of Sfax - Tunisia
2General Water Authority, Tripoli, Libyan Arab Jamahiriya
3Direction Générale des Ressources en Eau, Ministère de l’Agriculture et des Ressources en Eaux, Tunisa
4Hydrology Section, International Agency of Atomic Energy, Vienna, Austria
Groundwater recharge assessment in arid and semi-arid areas is difficult due to the low amount and variability of recharge. A multitracer approach investigation based on a linear mixing model for oxygen-18 and a ‘well-mixed’ reservoir model for C-14 activities allows direct investigation of relatively long-term renewal rates of an aquifer. The recharge process of the three layered Djeffara system, was investigated using isotopic approach. This study investigates the whole basin of the Djeffara from the Gabès in Tunisia to Jebel Nefouza in Libya. Over this basin, recharge is highly heterogeneous with different origins, from rainfall to fossil water through interconnection between underlying Continental Intercalaire aquifer mainly through geological features (faults) but as well as by vertical leak-age. Recent recharge mainly occurs in the Libyan basin through water-bearing for-mations outcrops. Heterogeneity of the recharge is reflected through the wide variation of oxygen-18 content of the groundwater. The carbon-14 activities range falls between 0 and 100 pmc showing pre and post-aerial thermonuclear test recharge. A well-mixed reservoir model has been applied to estimate renewal rates taking in account recent and fossil contribution to input water signature. This model gives relatively low renewal rates for the area. Using carbon-14, mean annual rates of groundwater renewal range from 0.004 to 2.5 ‰ but with relatively significant differences for each layer of the Djeffara system indicating different recharge mechanisms.
Groundwater; Recharge; Isotopes; Semi-arid environment; Djeffara; Tunisia, Libya
Keywords
Abstract
heavy anthropogenic stress. Djeffara plain is located in the most populous and intensively agricultural region of Libya and in southeastern Tunisia. In order to supply the ever increasing water demand, the Djeffara aquifer system is witnessing intense exploitation which causes an excessive depletion of the fresh ground-water resources. The situation is being exacer-bated by the lack of adequate recharge to replenish the water withdraw from the various aquifers of the Djeffara.
A number of major hydrological studies and project have been carried out on this aquifer system. The regional organisation, the Saharan and Sahel Observatory (OSS), is involved in the overall monitoring of the aquifer system and is currently responsible for implementing the UNEP/GEF North-western Sahara Project. How-ever, the calibration of the model elaborated for Djeffara aquifer system is constrained by major uncertainties in the model input parameters such as recharge rate, evaporative water loss, presence and amount of leakage between aquifers, etc. Isotopic investigations carried out under the IAEA TC project RAF8/035 attempt to provide an assessment of such parameters based on stable (deuterium, oxygen 18, car-bon 13) and radioactive (tritium, carcar-bon 14)
tracers. The aim of this paper is to quantify the renewal rate of the Djeffara aquifer system using isotopic tools.
2. Hydrogeological setting
The Djeffara plain is limited by the Skhira region, in the north, El-Hamma faults, and Dahar mountains in the west and southwest, and by Jebel Nefousa in Libya in the south. It is submitted to arid climate conditions with rain-fall mainly decreasing from 150 to less than 20 mm·yr–1. Humidity and temperature depend on the climate regime, but generally the mean annual temperature is higherthan 20°Cin the major part of the region. Daytime temperature may reach 40-50°Cin the desert (Dubief, 1959, Edmunds et al., 1997).
The Djeffara plain is underlied by a multilay-ered aquifer system characterized by a lithos-tratigraphic and structural complexity. It is composed by a succession of marine and con-tinental sedimentary deposits whose lithology and thickness varies from the north to the south. Lithologically, it represents the lateral
TUNISIA
Limit of study area Sabkhas
Mediterranean Sea
Figure 1. Location map of the Djeffara plain
continuation of the Continental Terminal (CT) or in other places the Continental Intercalaire (CI).
Although the Djeffara appears to be a continu-ation of the CT, the two systems are hydrogeo-logically independent (recharge mechanism, groundwater flow, recharge sources, chem-istry). The basin is strongly affected by tectonic deformations and faults in different directions, which induce a distinct lateral compartmental-isation. El Hamma fault is one of the prominent features that bound the Djeffara aquifer from the north-west. The water bearing formations in the area are classified in five aquifers: Conti-nental sands of Mio- Pliocene and Quaternary, Miocene maritime sands, Senonian limestone, Triassic sands and Dolomites, and limestone and dolomites of Upper Jurassic (Fig. 2). These
aquifers are hydraulically connected through the existing faults. They are structured accord-ing to three main layer separated by aquitards (Fig. 3) and have been used as a structure of the numerical groundwater flow model (Besbes et al., 2005). The three aquifers, from top to bottom, are as follows:
•
TheUpper Aquifer (UA),which includes, in Libya, the thick and productive Mio-Plio-Quaternary formations of the Tripoli-Sabrata-Swani-Ben Ghashir-Qarabolli region.On the central region, the Upper Aquifer shows Quaternary plating, generally of reduced thickness. On the south the UA is lied to the Al Azizah limestone formation and the Bir al Ghanam gypsum formation
Legend :
Quaternary Miocene Senonian Turonian
Cenomanien Albo-Aptian Neocomien Jurassic Trias 0
500
100 0
1500
ahdiB le telmeZ Draa Oudref Oudref 19 Ghannouch 4 girZ niA 2 aluomT niA 4 anatteK Zerkine 1bis 1-eniruoG 1-ararG uoB Gargabia hcuoahC.diS Ben Gardane-1 272/76 49/76 A1/38 282/81 222/76 222/76 48/79 DW-5
NW
TUNISIA LIBYA
Figure 2. Coastal hydrogeological cross section in Djeffara plain (in Besbes et al., 2005).
Aquifère supérieur (Upper aquifer) Aquifère moyen (Middle aquifer)
Aquifère du Trias (Triassic aquifer)
Figure 3. General structure of the Djeffara hydrogeological conceptual model – hydrogeological cross section from inland to the Sea (in Besbes et al., 2005).
outcrops. At the Jebel Nefousa foothill, but only in the SW direction, the UA is in hydraulic continuity with the Mesozoic Kikla formation (CT) outcrops, constituted by con-tinental sands. In Tunisia, the UA groups the known shallow aquifers of the region, namely: Gabes, Djerba, Zarzis, Ben Gardane, El Hamma. In the SW area, the UA joins the outcrops of the Continental Intercalaire of the eastern Dahar. Where the UA constitutes the main resource within Tripoli region, and where it is exploited by relatively deep wells, in Tunisia the UA is exclusively developed by means of shallow wells.
•
TheDeep Coastal Aquifer(Middle Aquifer, MA), which groups a number of permeable for-mations which can be referred to as Miocene sands (Lower Miocene aquifer, Zarzis Vindobonian sands), constantly pres-ent through the whole coastal region, and which represent the main formations of this aquifer. The MA also groups the Mio-Pliocene formations of North Gabes, the Senonian limestones of South Gabes and the Upper Triassic gres of Abu Shayba within the middle-oriental Djeffara.•
The Triassic Aquifer (TA), located within the Triassic permeable formations, basically the dolomitic formations of Al Aziziyah in cen-tral Djeffara, the sand-type formations of Ras Hamia in Libya and Kirchau in Tunisia, which are present everywhere but absent in the north of Tebaga Mountain (Gabes region).The three layers of Djeffara aquifer system show a general SW-NE flow, from the Dahar Mountains and El Hamma faults towards the Mediterranean Sea, in accordance with the plain structure.
Under the RAF/8/035 project, a total of about 400 water samples were collected from the Djeffara aquifer levels (Fig. 4).
Groundwater were analysed for chemical and isotopic compositions. Measurement of pH, Conductivity, TDS and alkalinity were made in the field. Major elements were determined in the local laboratories using standard protocols.
Oxygen 18, deuterium, carbon 13, carbon 14, and tritium, were measured using standard procedures thanks to IAEA support.
Figure 4. Spatial distribution of sampled points for Djeffara aquifer system