Groundwater, usually, is meteoric water stored in rock reservoirs. From their presence in atmosphere until their discharge (natural or artificial) and during stays in the reservoir, the water inherits the mineralogical and isotopic characteristic of the crossed environments (Mustafa et al., 2016, Mouna et al., 2017) . The groundwater mineralization is affected by many factors; the natural processes (atmospheric input, the dissolution/precipitation, geology of the reservoir, groundwater recharge source and climate) and anthropogenic activities (agriculture, urbanization, industry) (Mouna et al., 2017). These factors lead to influence the initial chemical composition and hydrogeochemical groundwater facies (Ameur et al., 2015; Ameur et al., 2015; Hassen et al., 2015 and 2016, Alharbi et al., 2017; Jarray et al., 2017).
Increasing water demand, in arid and semi-arid regions due to agricultural, urban and industrial development, greatly stressed the water resources. The utility of water in drinking water, agriculture, domestic and industrial supply is related at the water quality (Ameur et al., 2015; Alharbi et al., 2017; Houatmia et al., 2015).
Groundwater in the south Mediterranean country is sensitive to climatic and anthropogenic hazards essentially the preponderance of the agricultural needs (Hassen et al., 2015 and 2016, Jarray et al., 2017, Ameur et al., 2015). In Tunisia, water resources were evaluated in 2000 to 4825 Mm3, with 2700 Mm3 of surface water and 2125 Mm3 of groundwater (Jellalia et al., 2015). Indeed, Nadhour Sisseb El Alem region is characterized by a mean annual temperature of 20°C and a potential evapotranspiration of 1500 mm/year (INM, 2016). The mean annual precipitation is 350 mm indicating therefore a semi-arid climate (Souei, 2019). Study area is characterized by an absence of high mountains (Fig. 1). The study basin is considered among the important water reservoirs of the Central Tunisia (Fig.1). Water resources are used to satisfy the water demands of the Nadhour, Sisseb and El Alem regions and some cities of Sahel region, since the 70's.
The large expansion of irrigated area and the unfavorable climatic conditions increase the demand for groundwater. Indeed the deep aquifer is considered the main source of freshwater for different purposes (Houatmia et al., 2016; Souei et al., 2018).
In the last decades the Nadhour Sisseb El Alem deep aquifer system has been subjected to overexploitation, caused by development of illicit well, consequently, a piezometric drop of 20 m apparent (Souei, 2019; Souei et al., 2018). In addition to the natural recharge, a process of artificial recharge by water from the dam is carried out to compensate the piezometric drop (Houatmia et al., 2015, Hamdi et al., 2017; Ibn Ali et al., 2017; Souei, 2019). Piezometric drop causes a perturbation of groundwater flow in the basin, reducing the communication between the aquifer units from upstream to downstream (Souei et al. 2018; Souei and Zouaghi 2018). In addition, the shallow aquifer has been abandoned due to the high salinity and the deepening of the water level (Souei et al., 2018; Souei, 2019). The freshwater resources of this basin are threatened by natural agents and human activities
Water management to conserve its quantity and quality has recognized supplemental attention in the last decades in the Nadhour Sisseb El Alem basin (Houatmia et al., 2015, Souei et al., 2016, Souei et al., 2018, Souei and Zouaghi, 2018). To investigate hydrochemical processes guiding the groundwater chemistry, ionic relationships plots, water facies diagrams, and Statistics analysis are used (Hamzaoui-Azaza et al., 2013; Tlili et al., 2013; Isawi et al., 2016).
Therefore, the chemical composition of the water and the mineralization process are affected by climate change and human activity. Study of the hydrochemical of water in the Nadhour-Sisseb-El Alem basin has numerous interests, in particular decrease the financial managment costs (annual cost of deepening wells, consumption increasingly important energy), prevent the water degradation and soil salinization, and improve the specific flow rates of wells. Knowledge of the hydrochemical process of groundwater in this area improved the use of groundwater and guiding the sustainable development of water resources and effective management.
The aim of this study was the determination of hydrochemical processes of groundwater, to clarify the relationship between the hydrogeological unit of the study area, to assess the effect of anthropogenic activity, and develop a conceptual model to support the management and development of water resources in the Nadhour Sisseb El Alem region.
The study area is characterized by a semi-arid Mediterranean climate, long dry periods, with an irregularity rainfall. The precipitation (2008-2009) show rainfall varies between a minimum of 150mm characterizes the SW part of the basin and a maximum of 330mm record at the center-east part of the basin (Fig. 1). Generally, rainfalls not exceed 400 mm/year at the northern zone and 300 mm/year at the southern zone.
The basin is characterized by insufficient sporadic flows and floods, produce rare torrential and turbid flows that lead generally to endorheic systems, lose themselves in often saline depressions. In the case of low flows, runoff water ends their cycle by evaporation at the El Alem, Bled Sadiaa and Bled El Ktifa plains and at Sebkhet El Kelbia, saline depression, when the flows are important (Fig. 1). The watershed contains many Surface water resources; four hill dams (Ogla, Sehel, Kseb and Saidaine), the great Nebhana Dam and more than 30 hilly lakes (Fig. 1). These resources are intended for aquifer recharge and irrigation. A large part of Nebhana dam is mobilized to supply irrigation water to some regions of the Sahel. The artificial recharge is done by spreading in the wadi bed.
Nadhour Sisseb-El-Alem basin includes multilayered aquifers (Figs. 2 and 3), composed by phreatic and deep groundwater stored in clastic Formations; Oligocene (Fortuna Formation), Miocene (Beglia and Saouaf Formation), Mio-Pliocene (Ségui Formation) and Quaternary filling (Figs. 2 and 3).
Geology and structural framework
The geology of the study area (Figs. 2 and 3) shows, that the basin is formed essentially by two collapsed structures.
The Nadhour-Saouaf syncline has N30 to N40 direction is occupied by quaternary deposits (Figs. 2 and 3). The central part of the SE flank is filed. The Mountains of the flanks of synclinal are formed by the Oligo-Miocene series, crossed by multi directional faults (Figs. 2 and 3). This structure is limited by several anticlines with a principal direction N040 (Fig. 1). The anticlines are occupied by Jurassic and Cretaceous series crossed by multi directional faults. Dextral and sinistral strike-slip faults limit these structures (Saadi, 1997).
The El Alem structure forms the southern area of the study basin (Figs. 2 and 3). It is characterized by a thick series of quaternary filling (Fig. 3). It is limited to the east by the Draa Es Souatir monoclinal (Fig. 1). This monoclinal is formed by Oligo-Miocene deposit, show an East dip (Fig. 3). The latter is covered by quaternary deposits, limited to the West by a NS sub-surface fault (Souei et al., 2018; Souei and Zouaghi, 2018). The Sbikha monoclinal is the western limit of El Alem basin, has submeridian direction (Fig. 2). Hydrological and petroleum drilling data show the presence of subsurface highest structure in the Sisseb region (Souei et al., 2018; Souei, 2019) (Fig. 3). A fled anticline structure set at the west of the Draa Es Souatir structure (Fig. 3) limits the basin to the east (Souei et al., 2018; Souei and Zouaghi, 2018).
The studied region contains outcrops corresponds from the Triassic (Rhetian) to Quaternary (Fig. 2). These show important thicknesses and lithologic variation. The felling is very thick in collapsed structures (Souei et al., 2018) (Figs. 2 and 3). The Triassic deposit is locally exposed in the massif of Jebel Fkirin and Jebel Zarss (Salaj et Stranik, 1970a,b; Mencik et al., 1978) (Fig. 2). The Triassic series are composed by clays, sandstone, gypsum and limestone facies (Mencik et al., 1978). Jurassic Formations are outcropping at Jebels Bent Saidaine, Fkirin and Zarss (Fig. 2). It composed of calcero-dolomitic series (Soussi, 2000).
Cretaceous Formations are outcropping all along the fold structure of the basin (Fig. 2). Lower Cretaceous are formed by four Formations corresponding to the Berriasian-Valanginian Sidi Khalif Formation (clays, marl and limestones alternations), the Valanginian-Barremian M’cherga Formation (Alternations of clays, marl and limestones) (Mencik et al., 1978), the Barremian-Aptian Serdj Formations (limestone and marl) and the Aptian-Albian Fahdene Formations (shale and marl with some limestone) (Meddeb, 1986). The Cenomanian is composed of clays, marl and limestones alternations corresponding to Bahloul Formation (Burollet, 1956; Turki et al., 2002; Saadi, 1997). Kef Formation (Turonian-Campanian) is composed of clay (Soussi, 2000). The late cretaceous (Campanien-Maastrichtien) is composed by the limestone bar of the Abiod Formation. The late Maastrichtian–Paleocene is represented by the El Haria formation which composed by marls and clays (Mencik et al., 1978). Eocene is subdivided into tow lithostratigraphic formations corresponding to the Ypresian Bou Dabous Formation (limestones) and the Lutetian Souar Formation formed by the marl, clay and limestone alternations (Burollet, 1956 ; Saadi, 1990 ; Lajnef et al., 2005) (Fig. 2).
The Oligocene–Aquitanian series (Fortuna Formation) is represented by sandstone, sand and clay with silt (Yaich, 1992) (Fig. 2). The Burdigalian characterized by pedogenic red silte with root. The Langhian formed by the lumachellic calcareous bar of the Ain Grab Formation (Fig. 2).
The Mahmoud Formation (Langhien supérieur) is composed by green clays. The Middle to Late Miocene series are composed by the Beglia (Serravallien moyen à supérieur) and the Saouaf (Serravallien -Tortonien) Formations (Fig. 2). Beglia Formation is composed of coarse sands with quartz dragees (Saadi, 1997). Saouaf Formation is represented by thick detrital series with sands, clays and lignites , gypsum, marls and salts at the top) (Ben Moktar and Mannaï-Tayech, 2012). The Messinian-Pliocene (Segui Formation) starts with a polygenic conglomerate progressively transforming into clays and coarse sand finished by sandy-conglomerates bar (Salaj and Stranik, 1970 a, b; Mencik et al., 1978; Abbès and Boukadi, 1988) (Fig. 2).
From a hydrogeological point of view, the basin is formed by deep and phreatic water tables, (Houatmia et al., 2016; Souei, 2012; Souei et al., 2018; Souei, 2019) subjected to intense overexploitation. The aquifer system is composed of four hydrogeological units: Nadhour, Sisseb, El Alem and Etrabelsia (Souei et al., 2018). They are independent but interconnected (Souei et al., 2018; Souei, 2019). The Mio-Plio-Quaternary aquifer flows from the northwest to the southeast (Fig. 2) (Souei et al. 2018). To the north, in Nadhour region, the isopièze curves are tight indicating a high hydraulic gradient of 0.021. In Bled Sisseb and Etrabelsia regions, the spacing of isopièze curves is very important (Fig. 2). The hydraulic gradient is 0.001. Towards the south, in El Alem region, the curves become more spacing with a hydraulic gradient of 0.006.
The water table stored in the Mio-Plio-quaternary deposit is characterized by transmissivity between 2.2 10-3 m²/s and 8.5 10-3 m²/s. The permeability values are between 0.34 and 3.34 10-4 m/s. Aquifers are pumped up frequently with a flow rate between 5 and 20 l/s. In particular, it is pumped up in the Zouagha, region, with a flow rate 55 l/s. The rate of groundwater extraction increased from 11Mm3 in 1985 to 25Mm3 in 2012. Groundwater is extracted from deep borehole tapping the Mio-Plio-quaternary aquifer, ranging in depth from 100 to 400 m. It is mainly consumed by the agricultural sector based on irrigated cultivation which consumes more than (89%) of total pumped water (DGRE, 2013). 11% of pumped water is oriented to the drinking water supply (DGRE, 2013). The industrial sector consumes only 0.02 Mm3/year (DGRE, 2013).